Gabapentin enhances the analgesic effect of morphine in healthy volunteers

Hydrocodone, Oxycodone, and Morphine Metabolism and Drug-Drug Interactions

Awareness of drug interactions involving opioids is critical for patient treatment as they are common therapeutics used in numerous care settings, including both chronic and disease-related pain. Not only do opioids have narrow therapeutic indexes and are extensively used, but they have the potential to cause severe toxicity. Opioids are the classical pain treatment for patients who suffer from moderate to severe pain. More importantly, opioids are often prescribed in combination with multiple other drugs, especially in patient populations who typically are prescribed a large drug regimen. This review focuses on the current knowledge of common opioid drug-drug interactions (DDIs), focusing specifically on hydrocodone, oxycodone, and morphine DDIs. The DDIs covered in this review include pharmacokinetic DDI arising from enzyme inhibition or induction, primarily due to inhibition of cytochrome p450 enzymes (CYPs). However, opioids such as morphine are metabolized by uridine-5'-diphosphoglucuronosyltransferases (UGTs), principally UGT2B7, and glucuronidation is another important pathway for opioid-drug interactions. This review also covers several pharmacodynamic DDI studies as well as the basics of CYP and UGT metabolism, including detailed opioid metabolism and the potential involvement of metabolizing enzyme gene variation in DDI. Based upon the current literature, further studies are needed to fully investigate and describe the DDI potential with opioids in pain and related disease settings to improve clinical outcomes for patients. SIGNIFICANCE STATEMENT: A review of the literature focusing on drug-drug interactions involving opioids is important because they can be toxic and potentially lethal, occurring through pharmacodynamic interactions as well as pharmacokinetic interactions occurring through inhibition or induction of drug metabolism.

Functional Impairment and Cognitive Symptoms Among People with HIV Infection on Chronic Opioid Therapy for Pain: The Impact of Gabapentin and Other Sedating Medications

Gabapentin is associated with dizziness, falls, and somnolence yet commonly prescribed to people with HIV (PWH) treated with chronic opioid therapy (COT). Physical function and cognition are understudied when prescribed together. Among PWH on COT, we evaluated whether co-prescribed gabapentin is associated with (a) functional impairment; (b) trouble thinking clearly; and (c) difficulty controlling drowsiness using logistic regression models adjusted for prescribed opioid dose, other (non-gabapentin) sedating medication, substance use disorder, and mental/physical health indicators in a cross-sectional study. Among 166 participants, 40% were prescribed gabapentin, 41% reported functional impairment, 41% trouble thinking clearly, and 38% difficulty controlling drowsiness. Gabapentin co-prescribed with COT was significantly associated with trouble thinking clearly but not with functional impairment or difficulty controlling drowsiness. Clinicians should be cognizant of potential problems with thinking clearly when co-prescribing gabapentin and opioid medication.

Risk of major adverse events associated with gabapentinoid and opioid combination therapy: A systematic review and meta-analysis

Background: The use of opioid-gabapentinoid combinations has increased, raising several safety concerns. However, meta-analysis studies focusing on this issue are limited. Objective: To evaluate the risk of central nervous system (CNS) depression, gastrointestinal (GI) adverse events, and mortality of combination therapy compared with those of opioid therapy and to explore the differences in the results according to study design and indications. Methods: Relevant studies were selected (published before 30 January 2022) by searching the MEDLINE, Embase, and CENTRAL databases. The pooled odds ratios (OR) with 95% confidence intervals (CI) of the outcomes were estimated using the Mantel-Haenszel method. Subgroup and meta-regression analyses were performed according to study characteristics. Quality assessment was conducted using the Risk of Bias 2 tool for randomized controlled trials (RCTs) and Cochrane Collaboration's Risk of Bias in non-RCTs tool for non-randomized trials. Results: Adverse events were reported in 26 RCTs and 7 non-RCTs, and mortality was reported in 10 non-RCTs. Compared to opioid therapy, dizziness, cognitive dysfunction, and respiratory depression in combination therapy significantly increased in non-RCTs (OR 3.26, 95% CI 1.82-5.85; OR 3.13, 95% CI 1.51-6.50; OR 1.71, 95% CI 1.31-2.24, respectively), and a similar trend for dizziness and cognitive dysfunction was also identified in the RCT analysis, although the difference was not significant. Combination therapy for cancer pain was associated with the highest risk of sedation in subgroup analysis. Combination therapy significantly decreased the risk of GI adverse events, including nausea, vomiting, and constipation. The mortality risk associated with combination therapy was higher than that associated with opioid therapy (OR 2.76, 95% CI 1.26-6.05). Conclusion: Opioid-gabapentinoid combination therapy could be associated with an increased risk of CNS depression and mortality, despite tolerable GI adverse events. These data suggest that combination therapy requires close monitoring of CNS depression, especially in cancer patients. Caution is needed in interpreting the clinical meanings owing to the lack of risk difference in respiratory depression in the RCT-only analysis and the absence of RCT or prospective studies investigating mortality.

Pharmacokinetic Drug-Drug Interactions among Antiepileptic Drugs, Including CBD, Drugs Used to Treat COVID-19 and Nutrients

Anti-epileptic drugs (AEDs) are an important group of drugs of several generations, ranging from the oldest phenobarbital (1912) to the most recent cenobamate (2019). Cannabidiol (CBD) is increasingly used to treat epilepsy. The outbreak of the SARS-CoV-2 pandemic in 2019 created new challenges in the effective treatment of epilepsy in COVID-19 patients. The purpose of this review is to present data from the last few years on drug–drug interactions among of AEDs, as well as AEDs with other drugs, nutrients and food. Literature data was collected mainly in PubMed, as well as google base. The most important pharmacokinetic parameters of the chosen 29 AEDs, mechanism of action and clinical application, as well as their biotransformation, are presented. We pay a special attention to the new potential interactions of the applied first-generation AEDs (carbamazepine, oxcarbazepine, phenytoin, phenobarbital and primidone), on decreased concentration of some medications (atazanavir and remdesivir), or their compositions (darunavir/cobicistat and lopinavir/ritonavir) used in the treatment of COVID-19 patients. CBD interactions with AEDs are clearly defined. In addition, nutrients, as well as diet, cause changes in pharmacokinetics of some AEDs. The understanding of the pharmacokinetic interactions of the AEDs seems to be important in effective management of epilepsy.

Tilidine and dipyrone (metamizole) in cold pressor pain: A pooled analysis of efficacy, tolerability, and safety in healthy volunteers

The cold pressor test (CPT) is widely implemented and offers a simple, experimental acute pain model utilizing cold pain. Previous trials have frequently paired the CPT with opioids in order to investigate the mechanisms underlying pharmacological analgesia, due to their known analgesic efficacy. However, opioid side effects may lead to unblinding and raise concerns about the safety of the experimental setting. Despite the established clinical efficacy of dipyrone (metamizole), its efficacy, tolerability, and safety in cold pressor pain has not been systematically addressed to date. This pooled analysis included data of 260 healthy volunteers from three randomized, placebo‐controlled, double‐blind substudies using the CPT following a pre‐test‐post‐test‐design. These substudies allow for comparing a single dose of 800 mg dipyrone with two different doses of the opioid tilidine/naloxone (50/4 mg and 100/8 mg, respectively). Outcomes included pain intensity ratings, pain tolerance, medication‐attributed side effects, as well as changes of blood pressure and heart rate. We demonstrate that both opioid doses and dipyrone had a comparable, significant analgesic effect on cold pressor pain. However, dipyrone was associated with significantly less self‐reported adverse effects and these were not significantly different from those under placebo. These results indicate that the combination of dipyrone and the CPT provides a safe, tolerable, and effective experimental model for the study of pharmacological analgesia. In combination with a CPT, dipyrone may be useful as a positive control, or baseline medication for the study of analgesic modulation.

Human surrogate models of central sensitization: A critical review and practical guide

Background

As in other fields of medicine, development of new medications for management of neuropathic pain has been difficult since preclinical rodent models do not necessarily translate to the clinics. Aside from ongoing pain with burning or shock‐like qualities, neuropathic pain is often characterized by pain hypersensitivity (hyperalgesia and allodynia), most often towards mechanical stimuli, reflecting sensitization of neural transmission.

Data treatment

We therefore performed a systematic literature review (PubMed‐Medline, Cochrane, WoS, ClinicalTrials) and semi‐quantitative meta‐analysis of human pain models that aim to induce central sensitization, and generate hyperalgesia surrounding a real or simulated injury.

Results

From an initial set of 1569 reports, we identified and analysed 269 studies using more than a dozen human models of sensitization. Five of these models (intradermal or topical capsaicin, low‐ or high‐frequency electrical stimulation, thermode‐induced heat‐injury) were found to reliably induce secondary hyperalgesia to pinprick and have been implemented in multiple laboratories. The ability of these models to induce dynamic mechanical allodynia was however substantially lower. The proportion of subjects who developed hypersensitivity was rarely provided, giving rise to significant reporting bias. In four of these models pharmacological profiles allowed to verify similarity to some clinical conditions, and therefore may inform basic research for new drug development.

Conclusions

While there is no single “optimal” model of central sensitization, the range of validated and easy‐to‐use procedures in humans should be able to inform preclinical researchers on helpful potential biomarkers, thereby narrowing the translation gap between basic and clinical data.

Significance

Being able to mimic aspects of pathological pain directly in humans has a huge potential to understand pathophysiology and provide animal research with translatable biomarkers for drug development. One group of human surrogate models has proven to have excellent predictive validity: they respond to clinically active medications and do not respond to clinically inactive medications, including some that worked in animals but failed in the clinics. They should therefore inform basic research for new drug development.

Dual-trajectories of opioid and gabapentinoid use and risk of subsequent drug overdose among Medicare beneficiaries in the United States: a retrospective cohort study

BACKGROUND AND AIMS

Little is known about opioid and gabapentinoid (OPI-GABA) use duration and dose patterns' associations with adverse outcome risks. We examined associations between OPI-GABA dose and duration trajectories and subsequent drug overdose.

DESIGN

Retrospective cohort study.

SETTING

US Medicare.

PARTICIPANTS

Using a 5% sample (2011-16), we identified 71 005 fee-for-service Medicare beneficiaries with fibromyalgia, low back pain, neuropathy and/or osteoarthritis initiating OPIs and/or GABAs [mean age ± standard deviation (SD) = 65.5 ± 14.5 years, female = 68.1%, white = 76.8%].

MEASUREMENTS

Group-based multi-trajectory models identified distinct OPI-GABA use patterns during the year of OPI and/or GABA initiation, based on weekly average standardized daily dose (i.e. OPIs = morphine milligram equivalent, GABAs = minimum effective daily dose). We estimated models with three to 12 trajectories and selected the best model based on Bayesian information criterion (BIC) and Nagin's criteria. We estimated risk of time to first drug overdose diagnosis within 12 months following the index year, adjusting for socio-demographic and health factors using inverse probability of treatment weighted multivariable Cox proportional hazards models.

FINDINGS

We identified 10 distinct trajectories (BIC = -1 176 954; OPI-only = 3, GABA-only = 3, OPI-GABA = 4). Compared with OPI-only early discontinuers (40.6% of the cohort), 1-year drug overdose risk varied by trajectory group: consistent low-dose OPI-only users [16.6%; hazard ratio (HR) = 1.47, 95% confidence interval (CI) = 1.19-1.82], consistent high-dose OPI-only users (1.8%; HR = 4.57, 95% CI = 2.99-6.98), GABA-only early discontinuers (12.5%; HR = 1.39, 95% CI = 1.09-1.77), consistent low-dose GABA-only users (11.0%; HR = 1.44, 95% CI = 1.12-1.85), consistent high-dose GABA-only users (3.1%; HR = 1.43, 95% CI = 0.94-2.17), early discontinuation of OPIs and consistent low-dose GABA users (6.9%; HR = 1.24, 95% CI = 0.90-1.69), consistent low-dose OPI-GABA users (3.4%; HR = 2.49, 95% CI = 1.76-3.52), consistent low-dose OPI and high-dose GABA users (3.2%; HR = 2.46, 95% CI = 1.71-3.53) and consistent high-dose OPI and moderate-dose GABA users (0.9%; HR = 7.22, 95% CI = 4.46-11.69).

CONCLUSIONS

Risk of drug overdose varied substantially among US Medicare beneficiaries on different use trajectories of opioids and gabapentinoids. High-dose opioid-only users and all consistent opioid and gabapentinoid users (regardless of doses) had more than double the risk of subsequent drug overdose compared with opioid-only early discontinuers.

α2δ-1-Bound N-Methyl-D-aspartate Receptors Mediate Morphine-induced Hyperalgesia and Analgesic Tolerance by Potentiating Glutamatergic Input in Rodents

BACKGROUND

Chronic use of μ-opioid receptor agonists paradoxically causes both hyperalgesia and the loss of analgesic efficacy. Opioid treatment increases presynaptic N-methyl-D-aspartate receptor activity to potentiate nociceptive input to spinal dorsal horn neurons. However, the mechanism responsible for this opioid-induced activation of presynaptic N-methyl-D-aspartate receptors remains unclear. α2δ-1, formerly known as a calcium channel subunit, interacts with N-methyl-D-aspartate receptors and is primarily expressed at presynaptic terminals. This study tested the hypothesis that α2δ-1-bound N-methyl-D-aspartate receptors contribute to presynaptic N-methyl-D-aspartate receptor hyperactivity associated with opioid-induced hyperalgesia and analgesic tolerance.

METHODS

Rats (5 mg/kg) and wild-type and α2δ-1-knockout mice (10 mg/kg) were treated intraperitoneally with morphine twice/day for 8 consecutive days, and nociceptive thresholds were examined. Presynaptic N-methyl-D-aspartate receptor activity was recorded in spinal cord slices. Coimmunoprecipitation was performed to examine protein-protein interactions.

RESULTS

Chronic morphine treatment in rats increased α2δ-1 protein amounts in the dorsal root ganglion and spinal cord. Chronic morphine exposure also increased the physical interaction between α2δ-1 and N-methyl-D-aspartate receptors by 1.5 ± 0.3 fold (means ± SD, P = 0.009, n = 6) and the prevalence of α2δ-1-bound N-methyl-D-aspartate receptors at spinal cord synapses. Inhibiting α2δ-1 with gabapentin or genetic knockout of α2δ-1 abolished the increase in presynaptic N-methyl-D-aspartate receptor activity in the spinal dorsal horn induced by morphine treatment. Furthermore, uncoupling the α2δ-1-N-methyl-D-aspartate receptor interaction with an α2δ-1 C terminus-interfering peptide fully reversed morphine-induced tonic activation of N-methyl-D-aspartate receptors at the central terminal of primary afferents. Finally, intraperitoneal injection of gabapentin or intrathecal injection of an α2δ-1 C terminus-interfering peptide or α2δ-1 genetic knockout abolished the mechanical and thermal hyperalgesia induced by chronic morphine exposure and largely preserved morphine's analgesic effect during 8 days of morphine treatment.

CONCLUSIONS

α2δ-1-Bound N-methyl-D-aspartate receptors contribute to opioid-induced hyperalgesia and tolerance by augmenting presynaptic N-methyl-D-aspartate receptor expression and activity at the spinal cord level.

Pain management after total knee arthroplasty: A prospective randomized study

INTRODUCTION

Total knee arthroplasty (TKA) is a common procedure for improving mobility and quality of life in patients with osteoarthritis. Postoperative pain control management after TKA is still a concern as it relates to patients satisfaction and functional recovery.Many anesthetic regimens and techniques have been explored to decrease postoperative pain and enhance the fast recovery after TKA. The aim of this study was to evaluate the best anesthetic treatment in pain control after TKA.

METHODS

51 patients were included in a randomized prospective study and distributed in three groups. The first group (CG) in which no analgesic protocol was implemented (control group). The second group (LIA group) received an intraoperative local infiltration anesthesia (LIA) (60 ml mixture of two ropivacaine 75mg/10 mL + adrenaline 100μg/10 mL + physiological solution). The third group (FNB group) had only a femoral nerve block (FNB). Continuous outcomes including visual analogue scale (VAS) at 5,24,48 h and at 1 week, morphine consumption and range of motion (ROM) at 1,2,7 days.

RESULTS

There was significant difference between all groups (p < 0,001) in terms of the VAS score: at 5h after surgery (4.55,2.15,1.82); at 24h (4.15,2.65,3.36); at 48h (3.85,2.45,2.73); at 1 week (2.95,1.80, 1.64), respectively for groups CG, LIA, FNB.ROM was better in LIA and FNB groups than CG: at 1 die after surgery (44°,50°,54°); at 3 dies (69°,70°,71°); at 7 dies (91°,98°,98°), respectively for groups CG, LIA, FNB (p < 0,001).

DISCUSSION

LIA and FNB groups both showed a significant reduction at VAS score, better range of motion and less morphine consumption than CG (control group). LIA group has obtained a constant pain control in the postoperative days; FNB group had a good pain control in the hours after surgery, with a decrease in efficacy in the following days.

CONCLUSION

Further studies are still needed in order to define LIA as the reference pain management in TKA.

Complex encephalopathy arising from the combination of opioids and gabapentin

The interactions between opioids and gabapentin are more clinically relevant than ever. Prescriptions dispensed for gabapentin increased from 39 million in 2012 to 64 million in 2018 in the USA and are ever increasing. Authors present a challenging case of these interactions. A 58-year-old man presented to the emergency department with acute respiratory failure and altered mental status. He was on high dose opioids and gabapentin as prescription medications. Despite full intensive care support and resolution of his respiratory failure with non-invasive positive pressure ventilation, the patient did not regain consciousness. After ruling out other causes, the diagnosis of gabapentin withdrawal was considered. Gabapentin was administered by a nasogastric tube that quickly resulted in a reversal of his symptoms. We concluded that severe gabapentin withdrawal should be considered in patients on higher doses of gabapentin when it is stopped abruptly. In such patients, gabapentin should be replaced. As most patients are unable to swallow in this situation and intravenous formulation is not available, nasogastric tube can be used for replacement.

Complex encephalopathy arising from the combination of opioids and gabapentin

The interactions between opioids and gabapentin are more clinically relevant than ever. Prescriptions dispensed for gabapentin increased from 39 million in 2012 to 64 million in 2018 in the USA and are ever increasing. Authors present a challenging case of these interactions. A 58-year-old man presented to the emergency department with acute respiratory failure and altered mental status. He was on high dose opioids and gabapentin as prescription medications. Despite full intensive care support and resolution of his respiratory failure with non-invasive positive pressure ventilation, the patient did not regained consciousness. After ruling out other causes, the diagnosis of gabapentin withdrawal was considered. Gabapentin was administered by a nasogastric tube that quickly resulted in a reversal of his symptoms. We concluded that severe gabapentin withdrawal should be considered in patients on higher doses of gabapentin when it is stopped abruptly. In such patients, gabapentin should be replaced. As most patients are unable to swallow in this situation and intravenous formulation is not available, nasogastric tube can be used for replacement.

The effect of pre-emptive gabapentin on anaesthetic and analgesic requirements in patients undergoing rhinoplasty: A prospective randomised study

Background and Aims:

Hypotensive anaesthesia is necessary in rhinoplasty for better visualisation of surgical field and reduction of surgery time. Gabapentin is a new generation anticonvulsant with anti-hyperalgesic and anti-nociceptive properties. We aimed to investigate the effect of pre-operative administration of oral gabapentin (1200 mg) on anaesthetic requirements and post-operative analgesic consumption and its role in hypotensive anaesthesia for rhinoplasty.

Methods:

Seventy adult patients undergoing rhinoplasty, were randomly allocated to two groups. Group I (G I) (n = 35) received gabapentin 1.2 g and Group II (G II) (n = 35) received oral placebo capsules 2 h before surgery. General anaesthesia was maintained with sevoflurane in oxygen-nitrous oxide to maintain bispectral index value between 40 and 60, and remifentanil infusion to keep mean arterial pressure (MAP) at 55–60 mmHg. End-tidal sevoflurane concentration, intra-operative remifentanil consumption and time to intended MAP were recorded. Visual analogue scale (VAS) scores, post-operative analgesic requirements and side effects for the first 24 h were recorded.

Results:

G I required significantly lower intra-operative remifentanil (G I = 0.8 ± 0.26 mg and G II = 1.7 ± 0.42 mg; P = 0.001) and end-tidal sevoflurane concentration, with reduced doses of post-operative tramadol and diclofenac sodium. Time to the intended MAP was significantly less in G I than G II (59.1 ± 12.3 vs. 73.6 ± 16.4, respectively, with P = 0.001).

Conclusion:

Pre-operative oral gabapentin significantly reduced intra-operative remifentanil and sevoflurane requirements during hypotensive anaesthesia along with decreased post-operative analgesic requirement.

The efficacy of combination treatment of gabapentin and electro-acupuncture on paclitaxel-induced neuropathic pain

Paclitaxel, a chemotherapeutic drug, induces severe peripheral neuropathy. Gabapentin (GBT) is a first line agent used to treat neuropathic pain, and its effect is mediated by spinal noradrenergic and muscarinic cholinergic receptors. Electro-acupuncture (EA) is used for treating various types of pain via its action through spinal opioidergic and noradrenergic receptors. Here, we investigated whether combined treatment of these two agents could exert a synergistic effect on paclitaxel-induced cold and mechanical allodynia, which were assessed by the acetone drop test and von Frey filament assay, respectively. Significant signs of allodynia were observed after four paclitaxel injections (a cumulative dose of 8 mg/kg, i.p.). GBT (3, 30, and 100 mg/kg, i.p.) or EA (ST36, Zusanli) alone produced dose-dependent anti-allodynic effects. The medium and highest doses of GBT (30 and 100 mg/kg) provided a strong analgesic effect, but they induced motor dysfunction in Rota-rod tests. On the contrary, the lowest dose of GBT (3 mg/kg) did not induce motor weakness, but it provided a brief analgesic effect. The combination of the lowest dose of GBT and EA resulted in a greater and longer effect, without inducing motor dysfunction. This effect on mechanical allodynia was blocked by spinal opioidergic (naloxone, 20 μg), or noradrenergic (idazoxan, 10 μg) receptor antagonist, whereas on cold allodynia, only opioidergic receptor antagonist blocked the effect. In conclusion, the combination of the lowest dose of GBT and EA has a robust and enduring analgesic action against paclitaxel-induced neuropathic pain, and it should be considered as an alternative treatment method.

Gabapentin, opioids, and the risk of opioid-related death: A population-based nested case-control study

BACKGROUND

Prescription opioid use is highly associated with risk of opioid-related death, with 1 of every 550 chronic opioid users dying within approximately 2.5 years of their first opioid prescription. Although gabapentin is widely perceived as safe, drug-induced respiratory depression has been described when gabapentin is used alone or in combination with other medications. Because gabapentin and opioids are both commonly prescribed for pain, the likelihood of co-prescription is high. However, no published studies have examined whether concomitant gabapentin therapy is associated with an increased risk of accidental opioid-related death in patients receiving opioids. The objective of this study was to investigate whether co-prescription of opioids and gabapentin is associated with an increased risk of accidental opioid-related mortality.

METHODS AND FINDINGS

We conducted a population-based nested case-control study among opioid users who were residents of Ontario, Canada, between August 1, 1997, and December 31, 2013, using administrative databases. Cases, defined as opioid users who died of an opioid-related cause, were matched with up to 4 controls who also used opioids on age, sex, year of index date, history of chronic kidney disease, and a disease risk index. After matching, we included 1,256 cases and 4,619 controls. The primary exposure was concomitant gabapentin use in the 120 days preceding the index date. A secondary analysis characterized gabapentin dose as low (<900 mg daily), moderate (900 to 1,799 mg daily), or high (≥1,800 mg daily). A sensitivity analysis examined the effect of concomitant nonsteroidal anti-inflammatory drug (NSAID) use in the preceding 120 days. Overall, 12.3% of cases (155 of 1,256) and 6.8% of controls (313 of 4,619) were prescribed gabapentin in the prior 120 days. After multivariable adjustment, co-prescription of opioids and gabapentin was associated with a significantly increased odds of opioid-related death (odds ratio [OR] 1.99, 95% CI 1.61 to 2.47, p < 0.001; adjusted OR [aOR] 1.49, 95% CI 1.18 to 1.88, p < 0.001) compared to opioid prescription alone. In the dose-response analysis, moderate-dose (OR 2.05, 95% CI 1.46 to 2.87, p < 0.001; aOR 1.56, 95% CI 1.06 to 2.28, p = 0.024) and high-dose (OR 2.20, 95% CI 1.58 to 3.08, p < 0.001; aOR 1.58, 95% CI 1.09 to 2.27, p = 0.015) gabapentin use was associated with a nearly 60% increase in the odds of opioid-related death relative to no concomitant gabapentin use. As expected, we found no significant association between co-prescription of opioids and NSAIDs and opioid-related death (OR 1.11, 95% CI 0.98 to 1.27, p = 0.113; aOR 1.14, 95% CI 0.98 to 1.32, p = 0.083). In an exploratory analysis of patients at risk of combined opioid and gabapentin use, we found that 46.0% (45,173 of 98,288) of gabapentin users in calendar year 2013 received at least 1 concomitant prescription for an opioid. This study was limited to individuals eligible for public drug coverage in Ontario, we were only able to identify prescriptions reimbursed by the government and dispensed from retail pharmacies, and information on indication for gabapentin use was not available. Furthermore, as with all observational studies, confounding due to unmeasured variables is a potential source of bias.

CONCLUSIONS

In this study we found that among patients receiving prescription opioids, concomitant treatment with gabapentin was associated with a substantial increase in the risk of opioid-related death. Clinicians should consider carefully whether to continue prescribing this combination of products and, when the combination is deemed necessary, should closely monitor their patients and adjust opioid dose accordingly. Future research should investigate whether a similar interaction exists between pregabalin and opioids.

Opioid analgesics-related pharmacokinetic drug interactions: from the perspectives of evidence based on randomized controlled trials and clinical risk management

BACKGROUND

Multimorbidity results in complex polypharmacy which may bear a risk of drug interactions. A better understanding of opioid analgesics combination therapy used for pain management could help warrant medication safety, efficacy, and economic relevance. Until now there has been no review summarizing the opioid analgesics-related pharmacokinetic drug interactions from the perspective of evidence based on randomized controlled trials (RCTs).

METHOD

A literature search was performed using PubMed, MEDLINE, and the Cochrane Library, using a PRISMA flowchart.

RESULTS

Fifty-two RCTs were included for data interpretation. Forty-two RCTs (80.8%) were conducted in healthy volunteers, whereas 10 RCTs (19.2%) enrolled true patients. None of the opioid-drug/herb pairs was listed as contraindications of opioids involved in this review. Circumstances in which opioid is comedicated as a precipitant drug include morphine-P2Y12 inhibitors, morphine-gabapentin, and methadone-zidovudine. Circumstances in which opioid is comedicated as an object drug include rifampin-opioids (morphine, tramadol, oxycodone, methadone), quinidine-opioids (morphine, fentanyl, oxycodone, codeine, dihydrocodeine, methadone), antimycotics-opioids (buprenorphine, fentanyl, morphine, oxycodone, methadone, tilidine, tramadol), protease inhibitors-opioids (ritonavir, ritonavir/lopinavir-oxycodone, ritonavir-fentanyl, ritonavir-tilidine), grapefruit juice-opioids (oxycodone, fentanyl, methadone), antidepressants-opioids (paroxetine-tramadol, paroxetine-hydrocodone, paroxetine-oxycodone, escitalopram-tramadol), metoclopramide-morphine, amantadine-morphine, sumatriptan-butorphanol nasal sprays, ticlopidine-tramadol, St John's wort-oxycodone, macrolides/ketolides-oxycodone, and levomepromazine-codeine. RCTs investigating the same combination, almost unanimously, drew consistent conclusions, except two RCTs on amantadine-intravenous morphine combination where a different amantadine dose was used and two RCTs on morphine-ticagrelor combination where healthy volunteers and true patients were enrolled, respectively. RCTs investigating in true patients may reflect a realistic clinical scenario and overcome the limitation of RCTs performed in healthy volunteers under standardized conditions. Further research opportunities are also presented in this review.

CONCLUSION

Effective and safe combination therapy of opioids can be achieved by promoting the awareness of potential changes in therapeutic efficacy and toxicities, prescribing alternatives or changing administration strategy, tailoring dose, reviewing the appropriateness of orders, and paying attention to medication monitoring.

A case-controlled comparison of postoperative analgesic dosing between girls with Rett syndrome and girls with and without developmental disability undergoing spinal fusion surgery

BACKGROUND

Rett syndrome is associated with severe motor and communicative impairment making optimal postoperative pain management a challenge. There are case reports documenting reduced postoperative analgesic requirement in Rett syndrome.

AIM

The goal of this preliminary investigation was to compare postoperative analgesic management among a sample of girls with Rett syndrome compared to girls with and without developmental disability undergoing spinal fusion surgery.

METHOD

The medical records of eight girls with Rett syndrome (mean age = 13.2 years, sd = 1.9), eight girls with developmental disability (cerebral palsy; mean age = 13.1 years, sd = 2.0), and eight girls without developmental disability (adolescent idiopathic scoliosis; mean age = 13.4, sd = 1.8) were reviewed. Data related to demographics, medications, and route of drug administration were recorded.

RESULTS

Girls with Rett syndrome received significantly fewer morphine equivalent opioids postoperatively (M = 0.26 mg·kg^-1 ·day^-1 , sd = 0.10) compared to girls with adolescent idiopathic scoliosis (M = 0.47mg·kg^-1 ·day^-1 , sd = 0.13; 95% CI -0.34 to -0.08; P = 0.001) and girls with CP (M = 0.40 mg·kg^-1 per day, sd = 0.14; 95% CI -0.27 to -0.02; P = 0.01). Girls with Rett syndrome received significantly fewer opioid patient-controlled analgesic (PCA) bolus doses (given by proxy; M = 42.63, sd = 17.84) compared to girls with adolescent idiopathic scoliosis (M = 98.25, sd = 52.77; 95% CI -96.42 to -14.83; P = 0.01). There was also some evidence indicating girls with Rett syndrome received fewer bolus doses compared to girls with CP (M = 80.88, sd = 38.93; 95% CI -79.05 to 2.55; P = 0.06). On average, girls with Rett syndrome also received smaller total doses of acetaminophen, diazepam, and hydroxyzine.

CONCLUSION

This study highlights possible discrepancies in postoperative pain management specific to girls with Rett syndrome and suggests further investigation is warranted to determine best practice for postoperative analgesic management for this vulnerable patient population.

Review about gabapentin misuse, interactions, contraindications and side effects

The current work is targeted to review the risks of gabapentin misuse, its potential interactions with other drugs, side effects and use contraindications. This review consists of a total of 99 biographical references (from the year 1983 to 2016). A publication search of PubMed was performed from January 1983 to December 2016. It included animal studies, clinical studies, case studies and reviews related to gabapentin misuse, potential interactions, side effects and use contraindications. The search terms were gabapentin, anticonvulsant and antiepileptic. In general, it seems that gabapentin has risks of being misused based on the increased level of prescriptions, related fatalities, recreational misuse and higher doses of self-administration. The main reasons for gabapentin misuse are as follows: getting high, alleviating opioid withdrawal symptoms and potentiating methadone effects. Some of the main substances that interact with gabapentin are morphine, caffeine, losartan, ethacrynic acid, phenytoin, mefloquine and magnesium oxide. Some of the side effects caused by gabapentin are teratogenicity, hypoventilation, respiratory failure and myopathy. Finally, reports in general contraindicate the use of gabapentin in conditions such as myasthenia gravis and myoclonus.

The research on long-term clinical effects and patients' satisfaction of gabapentin combined with oxycontin in treatment of severe cancer pain

Gabapentin has been used as an adjuvant for treatment of cancer pain. Previous studies showed that opioids combined with gabapentin for management of cancer pain reduced the dosage of opioids.The objective of this study was to explore the clinical effect and patients' satisfaction of oxycontin combined with gabapentin in treatment of severe cancer pain. After titration of morphine, 60 severe cancer patients with visual analog score (VAS) more than 7 were randomly divided into trial group (n = 30) and control group (n = 30). The control group was administered oxycontin and placebo, and the trial group was given oxycontin and gabapentin. VAS score was recorded pre- and post-treatment; while daily dose of oxycontin, daily cost of pain relief and life quality score were observed 1 week, 1 month, 2 months, 3 months, and 6 months post-treatment. We found that daily dose of oxycontin 1 month post was comparable between the 2 groups (P > 0.05). Three months post, compared with control group (58.0 ± 15.2 mg), average daily dose of oxycontin was significant lower in trial group (33.4 ± 11 mg) (P < 0.001). Average daily cost of pain relief in trail group (34.5 ± 10.2 RMB) was less than the control group (52.4 ± 13.7 RMB) (P < 0.001). Life quality score increased in all of the patients in both group post-treatment (P < 0.05); while life quality score in trail group was greater than in control group 3 months (46.8 ± 4.5 vs 43.5 ± 4.6, P = 0.007) and 6 months (46.5 ± 4.8 vs 41.4 ± 4.3, P < 0.001) post-treatment. The incidence of drowsiness and dizziness was comparable between the 2 groups (P > 0.05), while the incidence of nausea and vomiting (P = 0.038), and constipation (P < 0.001) was higher in the control group.We concluded that oxycontin combined with gabapentin used in severe cancer pain management can control pain effectively, decreased the dose of oxycontin and the cost of cancer pain relief, and reduced the incidence of nausea and vomiting, and constipation, increased the life quality.

Population Pharmacokinetic Modelling of Morphine, Gabapentin and their Combination in the Rat

PURPOSE

The combination of morphine and gabapentin seems promising for the treatment of postoperative and neuropathic pain. Despite the well characterised pharmacodynamic interaction, little is known about possible pharmacokinetic interactions. The aim of this study was to evaluate whether co-administration of the two drugs leads to modifications of their pharmacokinetic profiles.

METHODS

The pharmacokinetics of morphine, morphine-3-glucuronide and gabapentin were characterised in rats following subcutaneous injections of morphine, gabapentin or their combination. Non-linear mixed effects modelling was applied to describe the pharmacokinetics of the compounds and possible interactions.

RESULTS

The plasma-concentration-time profiles of morphine and gabapentin were best described using a three- and a one-compartment disposition model respectively. Dose dependencies were found for morphine absorption rate and gabapentin bioavailability. Enterohepatic circulation of morphine-3-glucuronide was modelled using an oscillatory model. The combination did not lead to pharmacokinetic interactions for morphine or gabapentin but resulted in an estimated ~33% diminished morphine-3-glucuronide formation.

CONCLUSIONS

The finding of a lack of pharmacokinetic interaction strengthens the notion that the combination of the two drugs leads to better efficacy in pain treatment due to interaction at the pharmacodynamic level. The interaction found between gabapentin and morphine-3-glucuronide, the latter being inactive, might not have any clinical relevance.

Opioid analgesics increase incidence of somnolence and dizziness as adverse effects of pregabalin: a retrospective study

BACKGROUND

Pregabalin, a gabapentinoid, is an adjuvant analgesic for treatment of neuropathic pain, but it has serious adverse effects such as somnolence and dizziness, particularly in elderly patients. Although decreased renal function is considered to the contributing factor for high frequency of these adverse effects in elder patients, only a few systematic clinical investigations, especially for hospitalized patients, have been performed on factors that might affect the incidence of its adverse effects. In this study, we performed a retrospective study on the effect of concomitant drugs on induction of somnolence and dizziness as adverse effects of pregabalin in hospitalized patients.

METHODS

The subjects were all pregabalin-administered patients in Shiga University of Medical Science Hospital from September 2010 to September 2012, and the subject number was 195. Multivariate logistic regression analysis was performed to determine predictors of the adverse effects, creatinine clearance, duration of pregabalin therapy, initial and maintenance doses of pregabalin, and concomitant drugs, including hypoglycemic drugs, anti-hypertensive ones, non-steroidal anti-inflammatory ones, opioids and central nervous system depressants, being used as independent variables.

RESULTS

The median initial doses of pregabalin in each renal function group were the same with the case of the defined dose. Although renal function is a well-known factor for prediction of development of adverse effects of pregabalin, we did not detect significant contribution of it. Alternatively, it was demonstrated that concomitant administration of opioids was the significant factor of the incidence of somnolence and dizziness. The first onset date of the adverse effects was frequently detected in the early days of the pregabalin therapy.

CONCLUSIONS

The fine tuning of pregabalin dosage schedule based on the renal function appeared to be critical for prevention of development of its adverse effects. Adverse effects tended to develop in the initial phase of pregabalin therapy. Concomitant administration of opioids with pregabalin has the potential to increase the incidence of adverse effects, and thus much more careful attention has to be paid especially in those situations.

A Survey on the Effect of Oral Gabapentin on Hemodynamic Changes During Direct Laryngoscopy and Tracheal Intubation and Intraoperative Bleeding in Patients Undergoing Septorhinoplasty

BACKGROUND

In patients undergoing septorhinoplasty, control of bleeding and hemodynamic variables is of great importance and laryngoscopy and tracheal intubation could be followed by a specific and transient increase in systemic blood pressure and heart rate.

OBJECTIVES

This study aimed to examine the effects of oral gabapentin on hemodynamic changes, during direct laryngoscopy, and the amount of bleeding in the patients undergoing septorhinoplasty.

PATIENTS AND METHODS

In a double-blind clinical trial, 103 patients (American society of anesthesiologists class I, II) aged 18 - 45 years old, who were septorhinoplasty candidates, were randomly assigned into two groups, a 900 mg gabapentin group and placebo, in Amir-Al-Momenin academic hospital. The drug was prescribed to the patients orally, 2 hours before the operation. Anesthetic technique was similar for all the patients. Heart rate (HR), systolic (SBP) and diastolic blood pressures (DBP), mean arterial blood pressure (MAP), oxygen saturation percentage of arterial blood (SaO2), before induction of anesthesia, 3, 5, 10, and 15 minutes after the intubation and tracheal extubation, and the amount of bleeding during operation were measured. Statistical analysis was performed with the SPSS (v. 16) software.

RESULTS

Variations in the HR, DBP and SaO2, in the specified time intervals, did not show any statistically significant difference, although variations in SBP were statistically significant (higher in gabapentin group). Regarding the average amount of bleeding volume, although there was a lower amount of bleeding in the gabapentin group, the difference was not statistically significant. Also, regarding the pain, there was no significant difference between the two groups in terms of visual analog scale (VAS) average and the received analgesic.

CONCLUSIONS

The present study showed that premedication with 900 mg gabapentin did not affect the hemodynamic changes induced by laryngoscopy and the amount of bleeding. However, the decreased amount of bleeding was observed in the gabapentin group.

A retrospective study to identify risk factors for somnolence and dizziness in patients treated with pregabalin

Background

Pregabalin is a therapeutic drug for neuropathic pain that is associated with somnolence and dizziness. These adverse events are often experienced shortly after initiating pregabalin, and may lead to treatment discontinuation. The purpose of this study was to explore factors that influence the incidence of somnolence and dizziness induced by pregabalin, and to identify patients at higher risk of adverse events.

Methods

A retrospective analysis was conducted of patient characteristics (age, gender, renal function, initial daily dose of pregabalin, co-administration of strong opioids and hypnotics) and the incidence of somnolence and dizziness during the first week of pregabalin treatment. An electronic chart was used to collect data from 204 inpatients prescribed pregabalin at Nagoya University Hospital from June 2011 to November 2012.

Results

Among 36 patients who regularly received strong opioids, 18 (50.0 %) reported somnolence or dizziness during the first week of pregabalin treatment. The remaining 168 patients did not regularly receive strong opioids, and 25 (14.9 %) had an adverse event. In multivariate analysis, age (≧65 years, adjusted odds ratio: 2.507, 95 % CI: 1.164-5.397, p = 0.019) and regular co-administration of strong opioids (adjusted odds ratio: 5.507, 95 % CI: 2.460-12.328, p < 0.001) correlated with somnolence or dizziness.

Conclusions

These data suggest that age (≧65 years) and co-administration of strong opioids are risk factors for somnolence or dizziness during pregabalin treatment for neuropathic pain. More careful dose titration is recommended for elderly patients and those receiving concomitant strong opioids.

Attenuation of the haemodynamic responses to tracheal intubation with gabapentin, fentanyl and a combination of both: A randomised controlled trial

Background and Aims:

We conducted a prospective, randomised, double-blind, controlled clinical trial to examine (1) whether a single preoperative dose of 800 mg gabapentin would be as effective as 2 μg/kg of intravenous (IV) fentanyl in blunting the haemodynamic response to tracheal intubation and (2) whether a combination of both would be more effective in this regard.

Methods:

Seventy-five patients (American Society of Anaesthesiologists physical status I), aged 20–50 years were allocated into one of three groups: 2 μg/kg IV fentanyl, 800 mg oral gabapentin or a combination of both. Gabapentin was administered 2 h and fentanyl 5 min before induction of anaesthesia, which was achieved with 5 mg/kg thiopentone, and tracheal intubation facilitated with 0.1 mg/kg vecuronium. Laryngoscopy lasting a maximum of 30 s was attempted 3 min after administration of the induction agents. Serial values of mean arterial pressure (MAP) and heart rate (HR) were compared among the three groups and with the respective preinduction measurements.

Results:

Patients receiving gabapentin 800 mg alone showed remarkable increases in HR and MAP in response to tracheal intubation (P < 0.05). The increases were similar for the other two regimens. These haemodynamic changes were lesser in patients receiving fentanyl and the combination of gabapentin and fentanyl.

Conclusion:

Oral gabapentin does not produce significant reduction in laryngoscopy and tracheal intubation induced sympathetic responses as compared to IV fentanyl or the combination of gabapentin and fentanyl.

Randomized, placebo-controlled pilot trial of gabapentin during an outpatient, buprenorphine-assisted detoxification procedure

This pilot study examined the efficacy of the N-type calcium channel blocker gabapentin to improve outcomes during a brief detoxification protocol with buprenorphine. Treatment-seeking opioid-dependent individuals were enrolled in a 5-week, double-blind, placebo-controlled trial examining the effects of gabapentin during a 10-day outpatient detoxification from buprenorphine. Participants were inducted onto buprenorphine sublingual tablets during Week 1, were randomized and inducted onto gabapentin or placebo during Week 2, underwent a 10-day buprenorphine taper during Weeks 3 and 4, and then were tapered off gabapentin/placebo during Week 5. Assessments included thrice-weekly opioid withdrawal scales, vitals, and urine drug screens. Twenty-four individuals (13 male; 17 Caucasian, 3 African American, 4 Latino; mean age 29.7 years) participated in the detoxification portion of the study (gabapentin, n = 11; placebo, n = 13). Baseline characteristics did not differ significantly between groups. Self-reported and observer-rated opioid withdrawal ratings were relatively low and did not differ between groups during the buprenorphine taper. Urine results showed a Drug × Time interaction, such that the probability of opioid-positive urines significantly decreased over time in the gabapentin versus placebo groups during Weeks 3 and 4 (OR = 0.73, p = .004). These results suggest that gabapentin reduces opioid use during a 10-day buprenorphine detoxification procedure.

Use of a soluble epoxide hydrolase inhibitor as an adjunctive analgesic in a horse with laminitis

HISTORY

A 4-year old, 500 kg Thoroughbred female horse diagnosed with bilateral forelimb laminitis and cellulitis on the left forelimb became severely painful and refractory to non-steroidal anti-inflammatory therapy (flunixin meglumine on days 1, 2, 3 and 4; and phenylbutazone on days 5, 6 and 7) alone or in combination with gabapentin (days 6 and 7).

PHYSICAL EXAMINATION

Pain scores assessed independently by three individuals with a visual analog scale (VAS; 0 = no pain and 10 = worst possible pain) were 8.5 on day 6, and it increased to 9.5 on day 7. Non-invasive blood pressure monitoring revealed severe hypertension.

MANAGEMENT

As euthanasia was being considered for humane reasons, a decision was made to add an experimental new drug, trans-4-{4-[3-(4-Trifluoromethoxy-phenyl)-ureido]-cyclohexyloxy}-benzoic acid (t-TUCB), which is a soluble epoxide hydrolase (sEH) inhibitor, to the treatment protocol. Dose and frequency of administration were selected based on the drug potency against equine sEH to produce plasma concentrations within the range of 30 nmol L(-1) and 2.5 μmol L(-1) . Pain scores decreased sharply and remarkably following t-TUCB administration and blood pressure progressively decreased to physiologic normal values. Plasma concentrations of t-TUCB, measured daily, were within the expected range, whereas phenylbutazone and gabapentin plasma levels were below the suggested efficacious concentrations.

FOLLOW UP

No adverse effects were detected on clinical and laboratory examinations during and after t-TUCB administration. No new episodes of laminitis have been noted up to the time of writing (120 days following treatment).

CONCLUSIONS

Inhibition of sEH with t-TUCB was associated with a significant improvement in pain scores in one horse with laminitis whose pain was refractory to the standard of care therapy. No adverse effects were noticed. Future studies evaluating the analgesic and protective effects of these compounds in painful inflammatory diseases in animals are warranted.

Effects of gabapentin on pain and opioid consumption after abdominal hysterectomy

BACKGROUND

Postoperative pain is an important factor affecting anesthesia and surgery.

OBJECTIVES

The present study assessed the effects of 1200 mg gabapentin, an anticonvulsant drug that acts through voltage-dependent calcium channels, for the control of postoperative pain in patients undergoing abdominal hysterectomy.

METHODS

Fifty patients undergoing hysterectomy were enrolled in the present study. Subjects received either 1200 mg gabapentin or placebo 2 h before surgery. The amount of morphine consumption and level of postoperative pain at 2 h, 6 h, 12 h and 24 h after surgery were measured.

RESULTS

There were no significant differences in age, duration of surgery and anesthesia, or body mass index between the two groups. The mean intensity of pain in the gabapentin group was significantly lower than in the placebo group. The mean amount of morphine used in the placebo group (5.2 ± 2.8 mg) was significantly higher than in gabapentin group (1.2 ± 0.29 mg; P=0.001). Nausea and vomiting in the placebo group was more common than in the gabapentin group (P=0.001). The time interval for initial ambulation after surgery was significantly shorter in the gabapentin group (12.24 ± 2.18 h) compared with the placebo group (15 ± 3.61 h; P=0.002).

CONCLUSION

1200 mg gabapentin reduced postoperative pain and the need for opioids, and enabled earlier ambulation of the patient. Significant side effects were not observed.

Once-daily gastroretentive gabapentin for the management of postherpetic neuralgia: an update for clinicians

Gabapentin was originally developed as an add-on anticonvulsant drug, but has been widely used in the USA for the management of postherpetic neuralgia since its approval for this indication in 2002. Gabapentin has a short elimination half life and limited absorption due to a saturable L-amino acid transport system, which is expressed predominantly in the proximal small intestine. Hence, the original immediate-release gabapentin formulation (gabapentin TID) must usually be taken three times a day for optimal efficacy. Gabapentin TID is also associated with a high incidence of dizziness and somnolence and some patients are unable to tolerate the doses required for maximum pain relief. A once-daily, gastroretentive formulation of gabapentin was recently approved by the US Food and Drug Administration (FDA) for the management of postherpetic neuralgia. This formulation provides gradual release of gabapentin to the optimal site of absorption in the proximal small intestine and reduces the chance of saturating intestinal uptake, thus enabling once-daily dosing of gabapentin. This gradual release and absorption have been demonstrated in pharmacokinetic studies in healthy subjects. The efficacy of once-daily gastroretentive gabapentin for the management of postherpetic neuralgia has been demonstrated in placebo-controlled clinical studies. In addition, data from these studies suggest that the incidence of dizziness and somnolence may be reduced compared with similar studies using gabapentin TID. This article provides an overview of the pharmacokinetics, efficacy, and safety of once-daily gastroretentive gabapentin for the management of postherpetic neuralgia.

Human experimental pain models for assessing the therapeutic efficacy of analgesic drugs

Pain models in animals have shown low predictivity for analgesic efficacy in humans, and clinical studies are often very confounded, blurring the evaluation. Human experimental pain models may therefore help to evaluate mechanisms and effect of analgesics and bridge findings from basic studies to the clinic. The present review outlines the concept and limitations of human experimental pain models and addresses analgesic efficacy in healthy volunteers and patients. Experimental models to evoke pain and hyperalgesia are available for most tissues. In healthy volunteers, the effect of acetaminophen is difficult to detect unless neurophysiological methods are used, whereas the effect of nonsteroidal anti-inflammatory drugs could be detected in most models. Anticonvulsants and antidepressants are sensitive in several models, particularly in models inducing hyperalgesia. For opioids, tonic pain with high intensity is attenuated more than short-lasting pain and nonpainful sensations. Fewer studies were performed in patients. In general, the sensitivity to analgesics is better in patients than in healthy volunteers, but the lower number of studies may bias the results. Experimental models have variable reliability, and validity shall be interpreted with caution. Models including deep, tonic pain and hyperalgesia are better to predict the effects of analgesics. Assessment with neurophysiologic methods and imaging is valuable as a supplement to psychophysical methods and can increase sensitivity. The models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. Knowledge obtained from this review can help design experimental pain studies for new compounds entering phase I and II clinical trials.

Effect of gabapentin on morphine consumption and pain after surgical debridement of burn wounds: a double-blind randomized clinical trial study

Background:

Burn pain is recognized as being maximal during therapeutic procedures, and wound debridement can be more painful than the burn injury itself. Uncontrolled acute burn pain increases the stress response and the incidence of chronic pain and associated depression. Although opiates are excellent analgesics, they do not effectively prevent central sensitization to pain. The anticonvulsant gabapentin has been proven effective for treating neuropathic pain in large placebo-controlled clinical trials. Experimental and clinical studies have demonstrated antihyperalgesic effects in models with central neuronal sensitization. It has been suggested that central neuronal sensitization may play an important role in postoperative pain.

Objectives:

The aim of this study was to investigate the effect of gabapentin on morphine consumption and postoperative pain in burn patients undergoing resection of burn wounds.

Patients and Methods:

In a randomized, double-blind, placebo-controlled study, 50 burn patients received a single oral dose of gabapentin (1200mg) or placebo 2h before surgery. Anesthesia was induced with propofol and fentanyl and maintained by infusingpropofol, remifentanil, and 50% N2O in O2. All patients received patient-controlled analgesia with morphine at doses of 2.5 mg bolus and a lock-out time of 10 min for 24h before the operation. Pain was assessed on a visual analog scale (VAS) at rest and during movement at 1,4,8,12,16,20, and 24 h before the operation. Heart rate, oxygen saturation, mean arterial blood pressure, respiratory rate, sedation score, and morphine consumption were studied.

Results:

All the enrolled patients were able to complete the study; therefore, data from 50 patients wereanalyzed. The VAS scores at rest andduring movement at 1,4,8,12,16,20, and 24 h after the operation were significantly lower in the gabapentin group than in the placebo group (P < 0.05). Morphine consumption was significantly lessr in the gabapentin group than in the placebo group (P < 0.05). Sedation scores were similar in the 2 groups at all measured times. There were no differences in adverse effects between the groups.

Conclusions:

A single oral dose of 1200mg gabapentin resulted in a substantial reduction in postoperative morphine consumption and pain scores after surgical debridement in burn patients.

Role of gabapentin in preventing fentanyl- and morphine-withdrawal-induced hyperalgesia in rats

PURPOSE

This study was undertaken to examine the effect of gabapentin for preventing hyperalgesia induced by morphine and fentanyl withdrawal in rats.

METHODS

To induce hyperalgesia, Sprague Dawley (SD) rats were subcutaneously injected with fentanyl four times at 15-min intervals (60 μg/kg per injection), resulting in total dose of 240 μg/kg over 1 h, and morphine 10 mg/kg twice daily for 7 days. The effect of gabapentin was detected with behavioral tail-flick and paw-withdrawal tests.

RESULTS

Drug termination produced significant decrease in antinociception thresholds (P < 0.05 vs. saline group), indicating that the rats became sensitive to thermal stimuli. In rats that received combined treatment with fentanyl/morphine and gabapentin (25/50 mg/kg), results demonstrated that there were no significant decreases in antinociception thresholds (vs. saline group) after opioid withdrawal. Gabapentin (50 mg/kg) could also prevent morphine tolerance. The 50% effective dose (ED50) value was 12.5 mg/kg in tail-flick and 13.6 mg/kg in paw-withdrawal tests.

CONCLUSIONS

The study showed that gabapentin can significantly prevented opioid-induced hyperalgesia (OIH) induced caused by fentanyl and morphine, suggesting a role for the addition of gabapentin in the perioperative period and during chronic pain treatment as an effective drug to prevent OIH.

Morphine use in cancer surgery

Morphine is the core of perioperative pain management. However, when it comes to cancer surgery the possibility that this drug might affect tumor recurrence and metastasis has raised concerns. The results of two recent retrospective clinical trials indicated that regional anesthesia/analgesia might be beneficial in prostate and breast cancer surgery. It was proposed that morphine could be responsible for the higher recurrence and mortality rate observed in the general anesthesia/opioid analgesia groups. Nevertheless, the results of several other retrospective studies and one randomized prospective trial failed to confirm any advantage for regional anesthesia/analgesia over general anesthesia and opioid analgesia. Moreover laboratory data on the effect of morphine on cancer are contradictory, ranging from tumor-promoting to anti-tumor effects. Considering that surgical stress and pain promote the recurrence and spread of cancer, choosing a proper analgesic strategy is of high significance. Although the question of whether morphine causes any harm to cancer patients remains unanswered, alternative analgesic regimens could be used concomitant to or instead of morphine to limit its potential adverse effects.

Study of the effect of oral gabapentin used as preemptive analgesia to attenuate post-operative pain in patients undergoing abdominal surgery under general anesthesia

Aims:

To study the effect of oral gabapentin used as preemptive analgesia to attenuate post operative pain in patients undergoing abdominal surgery under general anesthesia.

Materials and Methods:

In a randomized double blind study, 60 patients were divided into two groups. Group A received 600mg gabapentin and group B oral received placebo 1 h prior to surgery. Anesthesia was induced with Propofol 2 mg/kg and Vecuronium 0.1mg/kg and maintained with 60% N₂O in O₂ and Vecuronium 0.02 mg/kg. All cases were given Fentanyl 2µg/kg as pre medication and a repeat dose 1µg/kg at the end of the first hour. Assessment of post-operative pain was made with the visual analog score (VAS) at extubation (0 h), 2, 4, 6, 12, and 24 h post-operatively. Post-operative analgesia was provided with intravenous Tramadol. The first dose was given in the Post Anesthesia Care Unit as 2mg/kg, and repeated at 8 and 16 h. Rescue analgesia was given with Diclofenac 1.5mg/kg, slow intravenous. The number of doses of rescue analgesia in both the groups was noted.

Results:

The VAS scores at 0, 2, 4, 6, 12, and 24 h were 1.9 vs. 2.4 (P=0.002), 2.3 vs. 3.0 (P=0.000), 3.2 vs. 3.7 (P=0.006), 2.9 vs. 4.4 (P=0.000), 3.6 vs. 4.6 (P=0.000), and 3.7 vs.4.6 (P=0.000), respectively. Numbers of patients requiring rescue analgesia with Diclofenac were 3 vs. 14 (P=0.004).

Conclusion:

A single oral dose of gabapentin given pre-operatively enhanced the analgesic effect of Tramadol as it also reduced the requirement of rescue analgesia with Diclofenac.

Optimal dose of pre-incision/post-incision gabapentin for pain relief following lumbar laminectomy: a randomized study

BACKGROUND

Gabapentin has been introduced as an effective agent for post-operative pain control. This study aimed to test the effects of pre- and post-incision administration of different doses of gabapentin on post-operative morphine requirement and pain following lumbar laminectomy.

METHODS

In this randomized clinical trial, 175 patients were allocated into seven groups of 25 patients each to receive placebo or gabapentin 600, 900 or 1200 mg pre- or post-incision. Total patient-controlled intravenous morphine consumption during the first 24 post-operative hours, and the time to the first demand for morphine were recorded. Pain score at rest (visual analogue scale) was recorded every 30 min in the first 4 h and then every 2 h until 24 h post-operatively. Side-effects were observed.

RESULTS

In the first 12 h, morphine consumption was less, pain scores were lower and the time to the first demand for analgesia was longer in groups receiving gabapentin 900 or 1200 mg either pre- or post-incision, compared with placebo and gabapentin 600 mg (P<0.001). There was no difference between gabapentin 900 and 1200 mg. Pain score, morphine consumption and time to the first demand for analgesia in equal pre- or post-incision doses of gabapentin were not significantly different. No differences in the side-effects were observed between groups.

CONCLUSION

Gabapentin 900 or 1200 mg, administered either pre- or post-incision, was found to be effective in pain management following lumbar laminectomy. Similar doses of gabapentin provide the same post-operative analgesia whether administered pre- or post-incision.

Effects of gabapentin on brain hyperactivity related to pain and sleep disturbance under a neuropathic pain-like state using fMRI and brain wave analysis

Neuropathic pain is the most difficult pain to manage in the pain clinic, and sleep problems are common among patients with chronic pain including neuropathic pain. In the present study, we tried to visualize the intensity of pain by assessing neuronal activity and investigated sleep disturbance under a neuropathic pain-like state in mice using functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG)/electromyogram (EMG), respectively. Furthermore, we investigated the effect of gabapentin (GBP) on these phenomena. In a model of neuropathic pain, sciatic nerve ligation caused a marked decrease in the latency of paw withdrawal in response to a thermal stimulus only on the ipsilateral side. Under this condition, fMRI showed that sciatic nerve ligation produced a significant increase in the blood oxygenation level-dependent (BOLD) signal intensity in the pain matrix, which was significantly decreased 2 h after the i.p. injection of GBP. Based on the results of an EEG/EMG analysis, sciatic nerve-ligated animals showed a statistically significant increase in wakefulness and a decrease in non-rapid eye movement (NREM) sleep during the light phase, and the sleep disturbance was almost completely alleviated by a higher dose of GBP in nerve-ligated mice. These findings suggest that neuropathic pain associated with sleep disturbance can be objectively assessed by fMRI and EEG/EMG analysis in animal models. Furthermore, GBP may improve the quality of sleep as well as control pain in patients with neuropathic pain.

Combination drug therapy for chronic pain: a call for more clinical studies

Chronic pain is a debilitating clinical condition associated with a variety of disease entities including diabetic neuropathy, postherpetic neuralgia, low back pathology, fibromyalgia, and neurological disorders. For many general practitioners and specialists, managing chronic pain has become a daunting challenge. As a modality of multidisciplinary chronic pain management, medications are often prescribed in combinations, an approach referred to as combination drug therapy (CDT). However, many medications for pain therapy, including antidepressants and opioid analgesics, have significant side effects that can compound when used in combination and impact the effectiveness of CDT. To date, clinical practice of CDT for chronic pain has been based largely on clinical experiences. In this article, we will focus on (1) the scientific basis and rationales for CDT, (2) current clinical data on CDT, and (3) the need for more clinical studies to establish a framework for the use of CDT.

PERSPECTIVE

More preclinical, clinical, and translational studies are needed to improve the efficacy of combination drug therapy that is an integral part of a comprehensive approach to the management of chronic pain.

A comparison of the pharmacokinetics and pharmacodynamics of pregabalin and gabapentin

Pregabalin and gabapentin share a similar mechanism of action, inhibiting calcium influx and subsequent release of excitatory neurotransmitters; however, the compounds differ in their pharmacokinetic and pharmacodynamic characteristics. Gabapentin is absorbed slowly after oral administration, with maximum plasma concentrations attained within 3-4 hours. Orally administered gabapentin exhibits saturable absorption--a nonlinear (zero-order) process--making its pharmacokinetics less predictable. Plasma concentrations of gabapentin do not increase proportionally with increasing dose. In contrast, orally administered pregabalin is absorbed more rapidly, with maximum plasma concentrations attained within 1 hour. Absorption is linear (first order), with plasma concentrations increasing proportionately with increasing dose. The absolute bioavailability of gabapentin drops from 60% to 33% as the dosage increases from 900 to 3600 mg/day, while the absolute bioavailability of pregabalin remains at > or = 90% irrespective of the dosage. Both drugs can be given without regard to meals. Neither drug binds to plasma proteins. Neither drug is metabolized by nor inhibits hepatic enzymes that are responsible for the metabolism of other drugs. Both drugs are excreted renally, with elimination half-lives of approximately 6 hours. Pregabalin and gabapentin both show dose-response relationships in the treatment of postherpetic neuralgia and partial seizures. For neuropathic pain, a pregabalin dosage of 450 mg/day appears to reduce pain comparably to the predicted maximum effect of gabapentin. As an antiepileptic, pregabalin may be more effective than gabapentin, on the basis of the magnitude of the reduction in the seizure frequency. In conclusion, pregabalin appears to have some distinct pharmacokinetic advantages over gabapentin that may translate into an improved pharmacodynamic effect.

The effects of gabapentin in human experimental pain models

Background

The antidepressant drugs imipramine and venlafaxine relieve clinical neuropathic pain and have been shown to increase pain thresholds in healthy volunteers during repetitive electrical sural nerve stimulation causing temporal pain summation, whereas pain during the cold pressor test is unaltered by these drugs. If this pattern of effect in experimental pain models reflects potential efficacy in clinical neuropathic pain, the pain summation model may potentially be used to identify new drugs for such pain conditions. Gabapentinoids are evidence-based treatments of clinical neuropathic pain and could contribute with additional knowledge of the usefulness of the pain summation model.

The aim of this study

To test the analgesic effect of the gabapentinoid gabapentin in a sural nerve stimulation pain model including temporal pain summation and the cold pressor test.

Method

18 healthy volunteers completed a randomized, double-blind, cross-over trial with medication of 600 mg gabapentin orally dosed 3 times over 24 h against placebo. Pain tests were performed before and 24 h after medication including pain detection and tolerance to single sural nerve stimulation and pain summation threshold to repetitive stimulation (3 Hz). Peak pain intensity and discomfort were rated during a cold pressor test.

Results

Compared to placebo, gabapentin had a highly significant effect on the threshold of pain summation to repetitive electrical sural nerve stimulation (P = 0.009). Gabapentin significantly increased the pain tolerance threshold to single electrical sural nerve stimulation (P = 0.04), whereas the pain detection threshold to single electrical sural nerve stimulation tended to be increased (P = 0.06). No significant differences were found on pain ratings during the cold pressor test.

Conclusion

Gabapentin had a selective hypoalgesic effect in a human experimental pain model of temporal pain summation and the results lend further support to the usefulness of the pain summation model to identify drugs for neuropathic pain.

Assessing efficacy of non-opioid analgesics in experimental pain models in healthy volunteers: an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of analgesics and target the clinical indications for their use. This review, the second in a series of two, addresses how the efficacy of non-opioid analgesics have been assessed in human volunteers using experimental pain models.

METHODS

A literature search was completed for randomized controlled studies that included human experimental pain models, healthy volunteers and non-opioid analgesics.

RESULTS

Nonsteroidal anti-inflammatory drugs worked against various types of acute pain as well as in hyperalgesia. Analgesia from paracetamol was difficult to detect in experimental pain and the pain needed to be assessed with very sensitive methods like evoked brain potentials. The N-methyl-D-aspartate antagonists exemplified by ketamine generally needed strong, long-lasting or repeated pain in the skin for detectable analgesia, whereas pain in muscle and viscera generally was more easily attenuated. Gabapentin worked well in several models, particularly those inducing hyperalgesia, whereas lamotrigine was weak in modulation of experimental pain. Imipramine attenuated pain in most experimental models, whereas amitriptyline had weaker effects. Delta-9-tetrahydrocannabinol attenuated pain in only a few models.

CONCLUSIONS

Pain induction and assessment are very important for the sensitivity of the pain models. Generally, experimental pain models need to be designed with careful consideration of the pharmacological mechanisms and pharmacokinetics of analgesics. The drawback with the different study designs is also discussed. This knowledge can aid the decisions that need to be taken when designing experimental pain studies for compounds entering Phase I and II trials.

Treatment considerations for patients with neuropathic pain and other medical comorbidities

The efficacy of drugs for neuropathic pain has been established in randomized controlled trials that have excluded patients with comorbid conditions and those taking complex medications. However, patients with neuropathic pain frequently present with complex histories, making direct application of this evidence problematic. Treatment of neuropathic pain needs to be individualized according to the cause of the pain, concomitant diseases, medications, and other individual factors. Tricyclic antidepressants (TCAs), gabapentinoids, selective noradrenergic reuptake inhibitors, and topical lidocaine are the first-line choices; if needed, combination therapy may be used. When a new drug is added, screening for potential drug interactions is recommended. The TCAs have anticholinergic adverse effects and may cause orthostatic hypotension. They should be avoided or used cautiously in patients with cardiac conduction disturbances or arrhythmias. Patients who lack cytochrome P450 2D6 isoenzyme activity are prone to adverse effects of TCAs and venlafaxine and have a weaker analgesic response to tramadol. A combination of several serotoninergic drugs may lead to serotonin syndrome. Risk of gastrointestinal tract bleeding is increased in patients taking selective serotonin reuptake inhibitors or venlafaxine, especially when combined with nonsteroidal anti-inflammatory drugs. Dose adjustment may be needed in patients with renal or hepatic impairment. Depending on the drug, the dose is reduced or the dosage interval lengthened. Slow titration and careful follow-up are needed. No drug is absolutely safe during pregnancy and lactation. Particular care must be exercised during the first trimester when drug dose should be as low as possible. Individual weighing of benefits and risks should guide therapeutic decisions.

Assessing analgesic actions of opioids by experimental pain models in healthy volunteers - an updated review

AIM

Experimental pain models may help to evaluate the mechanisms of action of analgesics and target the clinical indications for their use. This review addresses how the efficacy of opioids can be assessed in human volunteers using experimental pain models. The drawback with the different study designs is also discussed.

METHOD

A literature search was completed for randomized controlled studies which included human experimental pain models, healthy volunteers and opioids.

RESULTS

Opioids with a strong affinity for the micro-opioid receptor decreased the sensation in a variety of experimental pain modalities, but strong tonic pain was attenuated more than short lasting pain and non-painful sensations. The effects of opioids with weaker affinity for the micro-opioid receptor were detected by a more narrow range of pain models, and the assessment methods needed to be more sensitive.

CONCLUSION

The way the pain is induced, assessed and summarized is very important for the sensitivity of the pain models. This review gives an overview of how different opioids perform in experimental pain models. Generally experimental pain models need to be designed with careful consideration of pharmacological mechanisms and pharmacokinetics of analgesics. This knowledge can aid the decisions needed to be taken when designing experimental pain studies for compounds entering phase 1 clinical trials.

An update on the pharmacological management of post-herpetic neuralgia and painful diabetic neuropathy

Neuropathic pain is a persistent pain condition that develops secondary to nerve injury. The two most common types of peripheral neuropathic pain are post-herpetic neuralgia (PHN) and painful diabetic neuropathy (PDN). Amitriptyline, nortriptyline, desipramine and imipramine are TCAs that have been shown to be effective for the symptomatic relief of PHN and PDN. Serotonin noradrenaline reuptake inhibitors (SNRIs) such as venlafaxine and duloxetine have been shown to be very promising for the treatment of PDN with fewer adverse effects than TCAs. Selective serotonin reuptake inhibitors (SSRIs) were shown in a number of studies to have some efficacy in relieving PDN-related pain, yet other studies of the SSRIs have demonstrated conflicting outcomes. Most of the older antiepileptic studies were performed in patients with PDN; consequently, little is known about the efficacy of these drugs in patients with PHN. Carbamazepine, phenytoin and valproic acid were shown to be effective in ameliorating PDN-related pain. Other antiepileptic agents, including lamotrigine, oxcarbazepine and topiramate, have demonstrated some beneficial effects for the treatment of PDN, although they were also found to be ineffective in some PDN studies. alpha2delta Ligands such as gabapentin and pregabalin have been proven to be effective for the treatment of PHN and PDN in a number of large placebo-controlled trials. These drugs are useful not only in relieving pain but also in improving quality of life. Although the use of opioids for the treatment of neuropathic pain is controversial, a number of studies support the efficacy and safety of opioids in the treatment of neuropathic pain. Of these, oxycodone and tramadol have been shown to be superior to placebo for the treatment of PHN and PDN. A number of small studies have shown that dextromethorphan was effective in patients with PDN but not in patients with PHN. Topical agents such as lidocaine 5% patches and topical capsaicin are useful in ameliorating pain in patients with PHN but these agents are unsatisfactory for use as a sole agent. Although a number of drug treatments are available for the symptomatic relief of neuropathic pain symptoms, these agents do not provide satisfactory relief in all patients. For these patients, other treatment alternatives such as combination drug therapy that produces pain relief via distinctly different mechanisms may be successful. The purpose of this review is to compare the efficacy and limitations of currently available pharmacological treatments for the symptomatic relief of PHN and PDN, and to discuss the potential of combination therapy in PHN and PDN.

The interaction of gabapentin and N6-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA) on mechanical allodynia in rats with a spinal nerve ligation

We examined the antiallodynic interaction between gabapentin and adenosine A1 receptor agonist, N(6)-(2-phenylisopropyl)-adenosine R-(-)isomer (R-PIA), in a rat model of nerve ligation injury. Rats were prepared with ligation of left L5-6 spinal nerves and intrathecal catheter implantation for drug administration. Mechanical allodynia was measured by applying von Frey filaments. Gabapentin and R-PIA were administered to obtain the dose-response curve and the 50% effective dose (ED(50)). Fractions of ED(50)s were administered concurrently to establish the ED(50) of the drug combination. The drug interaction between gabapentin and R-PIA was analyzed using the isobolographic method. Adenosine A1 receptor antagonist was administered intrathecally to examine the reversal of the antiallodynic effect. Locomotor function changes were evaluated by rotarod testing. Intrathecal gabapentin and R-PIA and their combination produced a dose-dependent antagonism against mechanical allodynia without severe side effects. Intrathecal gabapentin synergistically enhanced the antiallodynic effect of R-PIA when coadministered. There were no significant changes in rotarod performance time, except gabapentin 300 microg. In the combination group, the maximal antiallodynic effect was reversed by A1 adenosine receptor antagonist. These results suggest that activation of adenosine A1 receptors at the spinal level is required for the synergistic interaction on the mechanical allodynia.

Effects of gabapentin on early postoperative pain, nausea and vomiting in laparoscopic surgery for assisted reproductive technologies

Prevention and treatment of postoperative pain, nausea and vomiting continues to be a major challenge in postoperative care. This study was designed to compare the effects of small dose of oral gabapentin with placebo as premedication on early postoperative pain, nausea and vomiting in patients undergoing ambulatory laparoscopic surgery for Assisted Reproductive Technologies (ART). Seventy women undergoing ambulatory laparoscopic surgery were randomly assigned to receive oral gabapentin 300 mg or placebo as premedication 1 h before surgery. Patients were anesthetized with the same anesthetic techniques. Duration of anesthesia, severity of postoperative pain and presence of Post Operative Nausea and Vomiting (PONV) were compared between the study groups. Demographic data and the duration of anesthesia were not statistically different between the study groups. There were significant differences in median VAS scores (25th-75th) measurements at all time points in the study groups (p < 0.05). Ten patients (28%) in control and one patient (0.02%) in gabapentin group required additional IV analgesic that was statistically significant (p = 0.012). Two patients in gabapentin and nine patients in placebo group had nausea (p = 0.022). None of patients in gabapentin but four patients in placebo group had vomiting (p = 0.114). Administration of oral gabapentin 300 mg before ambulatory laparoscopic surgeries, decreased postoperative pain, analgesic requirement and nausea.

Myoclonus induced by the use of gabapentin

Myoclonus is a rare side effect of gabapentin (GBP) and has been reported in patients with preexisting myoclonus, mental retardation, chronic static encephalopathy, diffuse brain damage, impaired renal function, or end stage renal disease. We report a case of myoclonus in a patient with normal renal function and no previous disorders. A 69-year-old female underwent diskectomy and foraminotomy at the left L4-L5 level. Postoperatively, she complained of paresthesia in her left leg, which was thought to be due to root manipulation during surgery. To relieve the paresthesia, she was given tramadol, an oral opioid agonist, and GBP. One week after GBP was increased to 900 mg per day, myoclonus developed, which severely impaired her normal activity. Her symptoms resolved 2 days after discontinuation of GBP. The coadministration of tramadol and GBP may mutually enhance the myoclonic potential of each drug. The causal relationship between GBP and myoclonus was suggested by cessation of myoclonus after GBP discontinuation despite continued therapy with tramadol.

Gabapentin evoked changes in functional activity in nociceptive regions in the brain of the anaesthetized rat: an fMRI study

BACKGROUND AND PURPOSE

Gabapentin (GBP; 1-(aminomethyl)cyclohexane acetic acid) is used clinically in the treatment of pain. Nevertheless, the sites and mechanisms of action of GBP are poorly defined. Herein, the effects of GBP on brain activation have been studied.

EXPERIMENTAL APPROACH

Changes in blood oxygen level dependent (BOLD) haemodynamic signal following intravenous infusion of GBP (equivalent to 30 mg kg(-1) p.o., followed by 100 mg kg(-1) p.o.), compared to saline control, were studied in isofluorane anaesthetized rats (n=8 per group). Effects of GBP on mean arterial blood pressure (MAP) were also recorded.

RESULTS

Random effect analysis revealed that the lower dose of GBP produced significant (P<0.001) increases in BOLD signal intensity in several brain regions, including the thalamus and periaqueductal grey (PAG), compared to basal. This dose of GBP also produced significant (P<0.001) decreases in BOLD signal intensity in the amygdala and the entorhinal cortex. Increasing the dose of GBP (100 mg kg(-1)) produced significantly greater changes in BOLD signal intensity in several brain regions including the thalamus and PAG. MAP was not significantly altered by GBP, compared to saline.

CONCLUSIONS AND IMPLICATIONS

GBP had marked positive and negative effects on BOLD signal intensity in a number of brain regions in naïve rats. The activation of key areas involved in nociceptive processing indicate a supraspinal site of action of GBP and this may contribute to its well-described analgesic effects in animal models of pain and clinical studies.

Use of gabapentin for perioperative pain control -- a meta-analysis

BACKGROUND

Gabapentin, an anticonvulsant, has recently been suggested as an effective postoperative 'analgesic' agent. The objective of the present study was to examine the analgesic effectiveness, opioid-sparing effects and side effects associated with the use of gabapentin in a perioperative setting.

METHODS

Following the Quality of Reporting of Meta-analyses recommendations, nine electronic databases until February 2006 were searched, without language restriction, for randomized controlled trials comparing gabapentin with control for postoperative pain control. Outcome measures, namely, 24 h cumulative opioid consumption, visual analogue scale pain scores and adverse effects, were expressed as odds ratios, ratio of means or weighted mean differences (as appropriate), which were aggregated under the fixed or random effects models.

RESULTS

Gabapentin caused a 35% reduction in total opioid consumption over the first 24 h following surgery (ratio of means 0.65, 95% CI 0.59 to 0.72), a significant reduction in postoperative pain at rest (in the first 24 h) and with movement (at 2 h, 4 h and 12 h), regardless of whether treatment effects were expressed as ratios of means or weighted mean differences, and a reduction of vomiting (relative risk [RR] 0.73, 95% CI 0.56 to 0.95) and pruritus (RR 0.30, 95% CI 0.13 to 0.70). It was associated with a significant increase in dizziness (RR 1.40, 95% CI 1.06 to 1.84) and an increase in sedation of borderline significance (RR 1.65, 95% CI 1.00 to 2.74).

CONCLUSION

Gabapentin improves the analgesic efficacy of opioids both at rest and with movement, reduces analgesic consumption and opioid-related adverse effects, but is associated with an increased incidence of sedation and dizziness.

Chronic pain after surgery: What can we do to prevent it?

The development of chronic pain after surgery is not an uncommon event. Despite increased attention devoted to this topic in the recent medical literature, little is known about the underlying mechanisms, natural history, and response to therapy of each syndrome. Central nervous system plasticity that occurs in response to tissue injury may contribute to the development of persistent postsurgical pain. As evidence continues to accumulate concerning the role of central sensitization in the prolongation of postoperative pain, many researchers have focused on methods to prevent central neuroplastic changes from occurring through the use of preemptive or preventative analgesic techniques. Effective preventative analgesic techniques may be useful in reducing not only acute pain but also chronic postsurgical pain and disability. This review examines the efficacy of using a variety of analgesic techniques aimed at preventing or reducing chronic pain after surgery. Specific chronic postsurgical pain syndromes evaluated include complex regional pain syndrome, phantom limb pain, chronic donor site pain, post-thoracotomy pain syndrome, and postmastectomy pain syndrome.