A cross-sectional cohort survey in 2100 patients with painful diabetic neuropathy and postherpetic neuralgia: Differences in demographic data and sensory symptoms

Sensory profiles of patients with neuropathic pain based on the neuropathic pain symptoms and signs

This manuscript aimed to characterize the clinical profile of various neuropathic pain (NeP) disorders and to identify whether patterns of sensory symptoms/signs exist, based on baseline responses on the Neuropathic Pain Symptom Inventory (NPSI) questionnaire and the quantitative sensory testing (QST). These post hoc analyses were based on data from 4 randomized, double-blind, placebo-controlled clinical studies of pregabalin (150-600mg/day) in patients with NeP syndromes: central poststroke pain, posttraumatic peripheral pain, painful HIV neuropathy, and painful diabetic peripheral neuropathy. The NPSI questionnaire includes 10 different pain symptom descriptors. QST was used to detect sensory thresholds of accurately calibrated sensory stimuli and to quantify the intensity of evoked sensation. To identify symptoms/signs clusters and select the number of clusters, a principal component analysis (PCA) and hierarchical clustering methods with clinical input were used. Analysis of the NPSI pain qualities and individual QST measures at baseline indicated no clear association between particular symptoms/signs profiles and etiologies. Based on NPSI symptoms, PCA identified 3 pain dimensions: provoked, deep, and pinpoint. A hierarchical cluster analysis identified 3 clusters with distinct pain characteristics profiles independent of NeP syndrome. Based on QST signs, PCA identified 2 pain dimensions: evoked by cold and evoked by touch. A hierarchical cluster analysis identified 4 clusters with distinct pain characteristics profiles. These "trans-etiological" profiles may reflect distinct pathophysiological mechanisms and therefore, potential differential responses to treatment.

Reappraising neuropathic pain in humans--how symptoms help disclose mechanisms

Neuropathic pain--that is, pain arising directly from a lesion or disease that affects the somatosensory system--is a common clinical problem, and typically causes patients intense distress. Patients with neuropathic pain have sensory abnormalities on clinical examination and experience pain of diverse types, some spontaneous and others provoked. Spontaneous pain typically manifests as ongoing burning pain or paroxysmal electric shock-like sensations. Provoked pain includes pain induced by various stimuli or even gentle brushing (dynamic mechanical allodynia). Recent clinical and neurophysiological studies suggest that the various pain types arise through distinct pathophysiological mechanisms. Ongoing burning pain primarily reflects spontaneous hyperactivity in nociceptive-fibre pathways, originating from 'irritable' nociceptors, regenerating nerve sprouts or denervated central neurons. Paroxysmal sensations can be caused by several mechanisms; for example, electric shock-like sensations probably arise from high-frequency bursts generated in demyelinated non-nociceptive Aβ fibres. Most human and animal findings suggest that brush-evoked allodynia originates from Aβ fibres projecting onto previously sensitized nociceptive neurons in the dorsal horn, with additional contributions from plastic changes in the brainstem and thalamus. Here, we propose that the emerging mechanism-based approach to the study of neuropathic pain might aid the tailoring of therapy to the individual patient, and could be useful for drug development.

Painful and painless diabetic neuropathy: one disease or two?

Painful diabetic polyneuropathy (PDPN) is generally considered a variant of diabetic polyneuropathy (DPN) but the identification of distinctive aspects that characterize painful compared with painless DPN has however been addressed in many studies, mainly with the purpose of better understanding the mechanisms of neuropathic pain in the scenario of peripheral nerve damage of DPN, of determining risk markers for pain development, and also of recognizing who might respond to treatments. This review is aimed at examining available literature dealing with the issue of similarities and differences between painful and painless DPN in an attempt to respond to the question of whether painful and painless DPN are the same disease or not and to address the conundrum of why some people develop the insensate variety of DPN whilst others experience distressing pain. Thus, from the perspective of comparing painful with painless forms of DPN, this review considers the clinical correlates of PDPN, its distinctive framework of symptoms, signs, and nerve functional and structural abnormalities, the question of large and small fiber involvement, the peripheral pain mechanisms, the central processing of pain and some new insights into the pathogenesis of pain in peripheral polyneuropathies and PDPN.

Pain qualities and satisfaction with therapy: a survey of subjects with neuropathic pain

OBJECTIVES

Pain qualities may reflect neurobiological mechanisms and guide therapy. The objective was to assess whether pain qualities were associated with satisfaction with pain relief in subjects with neuropathic pain.

METHODS

Subjects responded to a web survey that included current pain qualities, type of medications, and satisfaction with pain relief.

RESULTS

A total of 1,502 subjects, primarily with diabetic neuropathy, completed the survey and were grouped into six clusters based on their pain qualities. Subjects in the Broadest spectrum endorsed all pain types (paroxysmal, throbbing, paresthesias, evoked pain, and numbness). Subjects in the Broad spectrum endorsed all types of pain, but to a lesser degree. Subjects in the Sharp paroxysmal and paresthesias endorsed mainly stabbing-like pain, subjects in the throbbing dull pain endorsed throbbing and pressure, subjects in the numbness endorsed mainly numbness, and subjects in the least affected endorsed few symptoms. The degree of satisfaction in each cluster varied with the type of medication. Two clusters were dissatisfied with antidepressants: subjects with paroxysmal pain and paresthesias (odds ratio [OR] = 0.38; 95% confidence interval [CI], 0.19-0.74) and subjects with the broadest spectrum of symptoms (OR = 0.63; 95% CI, 0.41-0.97). Two clusters were dissatisfied with opioids: subjects with throbbing and dull pain (OR = 0.44; 95% CI, 0.26-0.75) and subjects with numbness (OR = 0.48; 95% CI, 0.23-0.99).

DISCUSSION

Pain qualities may help guide pain therapy and permit individualization of therapy.

Personalized medicine and opioid analgesic prescribing for chronic pain: opportunities and challenges

Use of opioid analgesics for pain management has increased dramatically over the past decade, with corresponding increases in negative sequelae including overdose and death. There is currently no well-validated objective means of accurately identifying patients likely to experience good analgesia with low side effects and abuse risk prior to initiating opioid therapy. This paper discusses the concept of data-based personalized prescribing of opioid analgesics as a means to achieve this goal. Strengths, weaknesses, and potential synergism of traditional randomized placebo-controlled trial (RCT) and practice-based evidence (PBE) methodologies as means to acquire the clinical data necessary to develop validated personalized analgesic-prescribing algorithms are overviewed. Several predictive factors that might be incorporated into such algorithms are briefly discussed, including genetic factors, differences in brain structure and function, differences in neurotransmitter pathways, and patient phenotypic variables such as negative affect, sex, and pain sensitivity. Currently available research is insufficient to inform development of quantitative analgesic-prescribing algorithms. However, responder subtype analyses made practical by the large numbers of chronic pain patients in proposed collaborative PBE pain registries, in conjunction with follow-up validation RCTs, may eventually permit development of clinically useful analgesic-prescribing algorithms.

PERSPECTIVE

Current research is insufficient to base opioid analgesic prescribing on patient characteristics. Collaborative PBE studies in large, diverse pain patient samples in conjunction with follow-up RCTs may permit development of quantitative analgesic-prescribing algorithms that could optimize opioid analgesic effectiveness and mitigate risks of opioid-related abuse and mortality.

Axial low back pain: one painful area--many perceptions and mechanisms

Axial low back pain can be considered as a syndrome with both nociceptive and neuropathic pain components (mixed-pain). Especially neuropathic pain comprises a therapeutic challenge in practical experience and may explain why pharmacotherapy in back pain is often disappointing for both the patient and the therapist. This survey uses epidemiological and clinical data on the symptomatology of 1083 patients with axial low back pain from a cross sectional survey (painDETECT). Objectives were (1) to estimate whether neuropathic pain contributes to axial low back pain and if so to what extent. (2) To detect subgroups of patients with typical sensory symptom profiles and to analyse their demographic data and co-morbidities. (3) To compare patients with and without prior intervertebral disc surgery (IVD). Neuropathic pain components could be detected in 12% of the entire cohort. Cluster analyses of these patients revealed five distinct subgroups of patients showing a characteristic sensory profile, i.e. a typical constellation and combination of symptoms. All subgroups occurred in relevant numbers and some showed distinct neuropathic characteristics while others showed nociceptive features. Post-IVD-surgery patients showed a tendency to score more “neuropathic” than patients without surgery (not statistically significant). Axial low back pain has a high prevalence of co-morbidities with implication on therapeutic aspects. From these data it can be concluded that sensory profiles based on descriptor severity may serve as a better predictor for therapy assessment than pain intensity or sole diagnosis alone. Standardized phenotyping of pain symptoms with easy tools may help to develop an individualized therapy leading to a higher success rate in pharmacotherapy of axial low back pain.

Investigation of depression in Greek patients with diabetic peripheral neuropathy

Background:

Considerable studies directly connect the complications in diabetic patients, and especially peripheral neuropathy, with the emergence of depression. Neuropathetic pain may deteriorate the general health status of the diabetic patient and glycaemic regulation.

Purpose:

The purpose of this study was to investigate the appearance and degree of diabetic peripheral neuropathy and its correlation with depression, with other parameters of the disease and also duration.

Methods:

57 diabetic patients participated with diagnosed diabetic peripheral neuropathy (male n=27, female n= 30, mean of age 72.7±6.35 years). The first part of Michigan Neuropathy Screening Instrument and the Zung Depression Rating Scale were used as tools for our study. Data was analysed with the SPSS 18.0 statistic program.

Results:

57.9% of the patients were overweight, 35.1% were obese and only 7% were within normal weight range. The BMI findings between the two genders indicate that male participants are more often obese than females. Women surpassed men in the category of overweight patients (p<0.05). The score based on MNSI was high and between 3 to 12 (mean average of 8.19±2.60 with 8 as intermediate rate). Almost 60% of patients had severe neuropathy, only 2 were found with mild symptoms and the rest had moderate neuropathtic symptoms, based on the score summary from the questionnaire. Investigating in detail the relation of diabetic neuropathy and depression, it derives that a high degree of diabetic neuropathy is related with high score of depression [F(3.160)=9.821, p=0.001]. Moderate and severe neuropathy was found with almost the same levels of depression.

Conclusions:

The correlation between diabetic neuropathy and depression is confirmed, while a very high depression rate was found in patients with severe neuropathy. The issue needs further study by using common instruments to obtain comparative results from the scientific community.

Mechanism- and experience-based strategies to optimize treatment response to the capsaicin 8% cutaneous patch in patients with localized neuropathic pain

The capsaicin 8% cutaneous patch is an emergent new treatment option for patients with peripheral neuropathic pain. In randomized controlled clinical studies relevant pain relief for 12 weeks was achieved in about one third of patients following a single application. The first part of this paper is a review of the pathophysiology, pharmacology, and published clinical trials with the capsaicin 8% cutaneous patch. The second part reports on outcomes of an interdisciplinary expert workshop, where new treatment results of three major German pain centers were presented and reviewed with the objectives of obtaining responder rates for different pain syndromes, assessing maintenance of effect under real-life conditions, and giving recommendations for practical care. The 12 week responder rates with pain relief of ≥ 30% were comparable in patients with mononeuropathies (37.9%) and postherpetic neuralgia (38.8%). Similar responder rates were seen in a subgroup of patients with cervical spine radiculopathy and back pain (46.7%). In HIV-associated neuropathy the responder rates were high (47.8%) but lower in patients with other polyneuropathies (17.6%). Response rates were nearly identical after 1 week (46.6%) and 4 weeks (43.3) and dropped only slightly at 12 weeks (37.4%). In a subgroup of 54 patients who underwent a second treatment, efficacy was maintained. Response rates in patients with or without lidocaine pretreatment were comparable. Treatment with the capsaicin 8% cutaneous patch was generally safe and well tolerated. The workshop panel recommended further investigation of opportunities to improve the application procedure and to perform studies on the skin penetration and distribution of capsaicin. A modified quantitative sensory testing (QST) should be developed for clinical practice in order to better understand the correlation of sensory profiles and response to capsaicin treatment.

Neuropathic pain and use of PainDETECT in patients with fibromyalgia: a cohort study

Backround

Fibromyalgia has a plethorae of symptoms, which can be confusing and even misleading. Accurate evaluation is necessary when patients with fibromyalgia are treated. Different types of instruments are available for the clinicians to supplement evaluation. Our objective was to study the applicability of the PainDETECT instrument to screen neuropathic pain in patients with fibromyalgia.

Methods

158 patients with primary fibromyalgia underwent a neurological examination including bedside sensory testing. They also fulfilled four questionnaires: PainDETECT, Beck depression inventory IA (BDI IA), Fibromyalgia Impact Questionnaire (FIQ) and a self-made questionnaire regarding present pain and pain relieving methods of the patients. The results of the clinical evaluation and questionnaires were then compared.

Results

Clinically verified neuropathic pain was diagnosed in 53/158 [34% (95% Cl: 26 to 41)] patients. The ROC curve achieved a maximum Youden´s index at score of 17 when sensitivity was 0.79 (95% Cl: 0.66 to 0.89) and specificity 0.53 (95% Cl: 0.43 to 0.63). The PainDETECT total score (OR: 1.14 95% Cl: 1.06 to 1.22), FM as the worst current pain (OR: 0.31; 95% 0.16 to 0.62), body mass index (BMI) (OR: 1.05; 95% Cl: 1.00 to 1.11) and the intensity of current pain (OR: 1.20; 95% Cl: 1.01 to 1.41) were significantly associated with the presence of neuropathic pain in univariate analyses.

Conclusion

This study highlights the importance of thorough clinical examination. The Neuropathic pain screening tool PainDETECT is not as useful in patients with fibromyalgia as in patients with uncompromised central pain control.

A mechanism-based classification of pain in multiple sclerosis

Pharmacological treatment of pain in multiple sclerosis (MS) is challenging due to the many underlying pathophysiological mechanisms. Few controlled trials show adequate pain control in this population. Emerging evidence suggests that pain might be more effectively classified and treated according to symptoms and underlying mechanisms. The new mechanism-based classification we propose here distinguishes nine types of MS-related pain: trigeminal neuralgia and Lhermitte's phenomenon (paroxysmal neuropathic pain due to ectopic impulse generation along primary afferents), ongoing extremity pain (deafferentation pain secondary to lesion in the spino-thalamo-cortical pathways), painful tonic spasms and spasticity pain (mixed pains secondary to lesions in the central motor pathways but mediated by muscle nociceptors), pain associated with optic neuritis (nerve trunk pain originating from nervi nervorum), musculoskeletal pains (nociceptive pain arising from postural abnormalities secondary to motor disorders), migraine (nociceptive pain favored by predisposing factors or secondary to midbrain lesions), and treatment-induced pains. Identification of various types of MS-related pain will allow appropriate targeted pharmacological treatment and improve clinical practice.

Drug design for neuropathic pain regulation from traditional Chinese medicine

FAAH-like anandamide transporter (FLAT) regulates anandamide transport for hydrolysis and may be an attractive drug target for pain regulation. We aimed to discover potential FLAT antagonists from traditional Chinese medicine (TCM) using virtual screening, ligand-based drug design and molecular dynamics simulation (MD). Guineensine and Retrofractamide A exhibited high Dock Scores in FLAT. Consensus from multiple linear regression (MLR; R² = 08973) and support vector machine (SVM; R² = 0.7988) showed similar bioactivities for Guineensine and the FAAH-1 inhibitor (9Z)-1-(5-pyridin-2-yl-1,3,4-oxadiazol-2-yl)octadec-9-en-1-one. Contour of Guineensine to CoMFA and CoMSIA features also imply bioactivity. MD revealed shake or vibration in the secondary structure of FLAT complexed with Guineensine and (9Z)-1-(5-pyridin-2-yl-1,3,4-oxadiazol-2-yl)octadec-9-en-1-one. Ligand movement might contribute to protein changes leading to vibration patterns. Violent vibrations leading to an overall decrease in FLAT function could be the underlying mechanism for Guineensine. Here we suggest Guineensine as a drug-like compound with potential application in relieving neuropathic pain by inhibiting FLAT.

Burden of illness associated with painful diabetic peripheral neuropathy among adults seeking treatment in the US: results from a retrospective chart review and cross-sectional survey

BACKGROUND

The purpose of this study was to characterize the burden of illness among adult subjects with painful diabetic peripheral neuropathy (pDPN) seeking treatment in the US.

METHODS

This observational study recruited 112 subjects with pDPN during routine visits from general practitioner and specialist sites. Subjects completed a one-time questionnaire, which included demographics, symptom duration, health care resource use, out-of-pocket costs, employment status, and validated measures that assessed pain, functioning, sleep, anxiety and depression, health status, and productivity. Investigators completed a case report form based on a 6-month retrospective chart review to capture clinical information, pDPN-related treatments, and other pDPN-related health care resource use over the past 6 months. Annualized costs were extrapolated based on reported 6-month health care resource use.

RESULTS

The mean age of the subjects was 61.1 years, 52.7% were female, and 17.9% were in paid employment. The most common comorbid conditions were sleep disturbance/insomnia (43.8%), depressive symptoms (41.1%), and anxiety (35.7%). The mean pain severity score was 5.2 (0-10 scale), and 79.5% reported moderate or severe pain. The mean pain interference with function score was 5.0 (0-10 scale) overall, with 2.0 among mild, 5.1 among moderate, and 7.0 among severe. The mean Medical Outcomes Study sleep problems index score was 48.5 (0-100 scale). The mean health state utility score was 0.61. Among subjects employed for pay, mean overall work impairment was 43.6%. Across all subjects, mean overall activity impairment was 52.3%. In total, 81.3% were prescribed at least one medication for their pDPN; 50.9% reported taking at least one nonprescription medication. Adjusted mean annualized total direct and indirect costs per subject were $4841 and $9730, respectively. Outcomes related to pain interference with function, sleep, health status, activity impairment, prescription medication use, and direct and indirect costs were significantly worse among subjects with more severe pain (P < 0.0020).

CONCLUSION

Subjects with pDPN exhibited high pain levels, which were associated with poor sleep, function, and productivity. Health care resource utilization in pDPN was prevalent and costs increased with greater pain severity. The burden of pDPN was greater among subjects with greater pain severity.

The burden associated with neuropathic pain in Western Europe

BACKGROUND

The purpose of this study is to assess the burden of neuropathic pain (NeP) on health-related quality-of-life (HRQoL), health status, employment status, absenteeism and presenteeism, and direct medical costs in Western Europe.

METHODS

Data are from the 2010 National Health and Wellness Survey (NHWS) for five countries in western Europe: the UK, France, Spain, Germany, and Italy. Among subjects who reported experiencing pain in the past month, those who attributed their pain to NeP were compared with those who attributed their pain to another chronic pain condition other than NeP (the latter was the reference group). These two groups were compared on demographic and both pain and non-pain related comorbidities. Generalized linear models were used to estimate the independent contribution of the presence of NeP on: (a) HRQoL (using the SF-12v2); (b) self-reported health status (the first item of the SF-12v2); (c) employment status; (d) absenteeism and presenteeism (using the WPAI questionnaire); and (e) direct medical costs (estimated from self-reported healthcare resource use and unit costs from the literature).

RESULTS

Relative to the chronic pain reference group, subjects with NeP reported a higher prevalence of severe daily pain (38.12% vs 12.67%, p < 0.05), lower labor force participation (39.68% vs 55.56%; p < 0.05), higher prevalence of sleep difficulties (59.14% vs 46.73%; p < 0.05), insomnia (45.61% vs 29.78%; p < 0.05) anxiety (42.42% vs 31.99%; p < 0.05), and depression (35.25% vs 24.03%; p < 0.05). NeP subjects reported higher rates of absenteeism (39.78% vs 21.47%; p < 0.05) and presenteeism (86.48% vs. 66.70%; p < 0.050). Direct medical costs were approximately twice as high compared to non-NeP controls. In addition, >80% of NeP patients reported having other pain conditions. Regression results amplified these findings by indicating the independent contribution of confounding factors on the presence of NeP.

LIMITATIONS

The NHWS is an Internet-based survey and may not be representative of the respective country populations if Internet access is limited. Second, respondents are asked to report their experience of pain. Although respondents are asked if their pain condition has been diagnosed by a physician there is no separate clinical confirmation of the presence of pain, pain conditions reported, and the presence of comorbidities.

CONCLUSIONS

The presence of NeP is associated with an increased disease burden in the chronic pain population. This is seen in terms of HRQoL, health status, employment experience, and direct medical costs.

Subgrouping of patients with neuropathic pain according to pain-related sensory abnormalities: a first step to a stratified treatment approach

BACKGROUND

Patients with neuropathic pain present with various pain-related sensory abnormalities. These sensory features form different patterns or mosaics-the sensory profile-in individual patients. One hypothesis for the development of sensory profiles is that distinct pathophysiological mechanisms of pain generation produce specific sensory abnormalities. Several controlled trials of promising new drugs have produced negative results, but these findings could have been a result of heterogeneity in the patient population. Subgrouping patients on the basis of individual sensory profiles could reduce this heterogeneity and improve trial design.

RECENT DEVELOPMENTS

A statistical categorisation of patients with neuropathic pain showed that subgroups of patients with distinct sensory profiles who perceive their pain differently do exist across a range of neuropathic disorders, although some distinct disorder-specific profiles were also detected. Results of the first clinical trials to use the subgroup approach at baseline could show a superior effect of the study drugs in specific subgroups, rather than in the entire cohort of patients. WHERE NEXT?: A new classification of neuropathic pain should take into account subgroups of patients with different sensory profiles. Sensory phenotyping has the potential to improve clinical trial design by enriching the study population with potential treatment responders, and might lead to a stratified treatment approach and ultimately to personalised treatment.

Sensory functions in the foot soles in victims of generalized torture, in victims also beaten under the feet (falanga) and in healthy controls - A blinded study using quantitative sensory testing

Background

Falanga torture (beatings on the foot soles) produces local chronic pain and severe walking difficulties. We have previously reported signs of neuropathic pain in the feet of falanga victims. The objective here was to clarify underlying pain mechanisms by quantifying sensory impairments in the feet of torture victims who had experienced both generalized torture and those who had been exposed to falanga in addition. An ethnically matched control group was available.

Methods

We employed quantitative sensory testing (QST) by investigators blinded to whether the patients, 32 male torture victims from the Middle East, had (n=15), or had not (n=17) been exposed to falanga. Pain intensity, area and stimulus dependence were used to characterize the pain as were interview data on sensory symptoms. QST included thresholds for touch, cold, warmth, cold-pain, heat-pain, deep pressure pain and wind-up to cutaneous noxious stimuli in the foot soles. Clinical data on anxiety and depression were retrieved.

Results

Almost all falanga victims had moderate or strong pain in their feet and in twice as large an area of their foot soles as other torture victims. One-third of the latter had no pain in their feet and many reported slight pain; in spite of this, there were no differences in foot sole QST data between the tortured groups. A comparison with normal data indicated that both tortured groups had hypoesthesia for all cutaneous sensory fibre groups except those transmitting cold and heat pain, in addition to deep mechano-nociceptive hyperalgesia.

Conclusion

A comparison of the QST data between victims having been exposed to generalized torture and victims who in addition had been exposed to falanga, showed no differences on the group level. The sensory disturbances in relation to our control group are compatible with central sensitization and de-sensitization, pointing to a core role of central mechanisms. A further analysis to create individual sensory profiles from our measurements is in progress.

0.025% capsaicin gel for the treatment of painful diabetic neuropathy: a randomized, double-blind, crossover, placebo-controlled trial

BACKGROUND

Topical therapy may provide additional benefit in patients with painful diabetic neuropathy (PDN). This study was conducted to study the safety and efficacy of 0.025% capsaicin gel in this condition.

METHODS

A 20-week, double-blind, crossover, randomized, single-center study enrolled subjects with PDN. They received 0.025% capsaicin gel or placebo for 8 weeks, with a washout period of 4 weeks between the two treatments. Primary efficacy end point was percent change in visual analog scale (0-100 mm) of pain severity. Secondary outcomes were score change in Neuropathic Pain Scale (NPS), short-form McGill Pain Questionnaires (SF-MPQ), proportion of patients who had pain score reductions of 30% and 50%, and adverse event.

RESULTS

Of the 35 subjects screened, 33 were enrolled and 33 completed at least an 8-week treatment period. Intention-to-treat analysis showed no significant improvement in pain with capsaicin gel, compared with placebo with visual analog scale (VAS) score 28.8 mm vs. 34.6 mm (P = 0.53). No significant difference between the groups was found in SF-MPQ (7.4 vs. 7.71, P = 0.95), NPS (29.4 vs. 31.3, P = 0.81), and proportion of patients who had 30% or 50% pain relief. Capsaicin gel was well tolerated with minor skin reaction.

CONCLUSIONS

0.025% capsaicin gel is safe and well tolerated, but does not provide significant pain relief in patients with PDN.

Diagnosing and managing postherpetic neuralgia

Postherpetic neuralgia (PHN) represents a potentially debilitating and often undertreated form of neuropathic pain that disproportionately affects vulnerable populations, including the elderly and the immunocompromised. Varicella zoster infection is almost universally prevalent, making prevention of acute herpes zoster (AHZ) infection and prompt diagnosis and aggressive management of PHN of critical importance. Despite the recent development of a herpes zoster vaccine, prevention of AHZ is not yet widespread or discussed in PHN treatment guidelines. Diagnosis of PHN requires consideration of recognized PHN signs and known risk factors, including advanced age, severe prodromal pain, severe rash, and AHZ location on the trigeminal dermatomes or brachial plexus. PHN pain is typically localized, unilateral and chronic, but may be constant, intermittent, spontaneous and/or evoked. PHN is likely to interfere with sleep and daily activities. First-line therapies for PHN include tricyclic antidepressants, gabapentin and pregabalin, and the lidocaine 5 % patch. Second-line therapies include strong and weak opioids and topical capsaicin cream or 8 % patch. Tricyclic antidepressants, gabapentinoids and strong opioids are effective but are also associated with systemic adverse events that may limit their use in many patients, most notably those with significant medical comorbidities or advanced age. Of the topical therapies, the topical lidocaine 5 % patch has proven more effective than capsaicin cream or 8 % patch and has a more rapid onset of action than the other first-line therapies or capsaicin. Given the low systemic drug exposure, adverse events with topical therapies are generally limited to application-site reactions, which are typically mild and transient with lidocaine 5 % patch, but may involve treatment-limiting discomfort with capsaicin cream or 8 % patch. Based on available clinical data, clinicians should consider administering the herpes zoster vaccine to all patients aged 60 years and older. Clinicians treating patients with PHN may consider a trial of lidocaine 5 % patch monotherapy before resorting to a systemic therapy, or alternatively, may consider administering the lidocaine 5 % patch in combination with a tricyclic antidepressant or a gabapentinoid to provide more rapid analgesic response and lower the dose requirement of systemic therapies.

Systems biology approaches to finding novel pain mediators

Chronic pain represents a major health burden; this maladaptive pain state occurs as a consequence of hypersensitivity within the peripheral and central components of the somatosensory system. High throughput technologies (genomics, transciptomics, lipidomics, and proteomics) are now being applied to tissue derived from pain patients as well as experimental pain models to discover novel pain mediators. The use of clustering, meta-analysis and other techniques can help refine potential candidates. Of particular importance are systems biology methods, such as co-expression network generating algorithms, which infer potential associations/interactions between molecules and build networks based on these interactions. Protein-protein interaction networks allow the lists of potential targets generated by these different platforms to be analyzed in their biological context. Outputs from these different methods must also be related to the clinical pain phenotype. The improved and standardized phenotyping of pain symptoms and sensory signs enables much better subject stratification. Our hope is that, in the future, the use of computational approaches to integrate datasets including sensory phenotype as well as the outputs of high throughput technologies will help define novel pain mediators and provide insights into the pathogenesis of chronic pain.

Peripheral nociceptor sensitization mediates allodynia in patients with distal symmetric polyneuropathy

Patients with painful neuropathy frequently complain of pain in response to normally non-painful brushing, namely dynamic mechanical allodynia. Despite many animal studies suggesting that allodynia arises when the spontaneous firing in damaged nociceptive afferents sensitise second-order nociceptive neurons to Aβ-fibre input, no studies have sought to confirm this mechanism by investigating Aβ-fibre sparing in human patients with allodynia. In this study we compared data from Aβ-fibre-mediated nerve conduction studies and nociceptive-fibre-mediated laser-evoked potentials (LEPs) in 200 patients with distal symmetric polyneuropathy (114 with neuropathic pain, 86 without). Of the 114 patients with painful neuropathy studied, 44 suffered from allodynia. Whereas no statistical difference was found in nerve conduction study data between patients with and without allodynia, LEP amplitudes were larger in patients with allodynia than in those without (P < 0.01 by Mann-Whitney U test). The lack of difference in NCS data between patients with and without allodynia suggest that this type of pain, rather than arising through second-order nociceptive neuron sensitization to Aβ-fibre input, might reflect a reduced mechanical threshold in sensitised intraepidermal nociceptive nerve terminals.

Construct validity of an instrument to measure neuropathic pain in women with bladder pain syndrome

AIMS

To determine the construct validity of an instrument to measure neuropathic pain in women with bladder pain syndrome (BPS). Our hypothesis is that neuropathic, bladder, and bowel pain represent different constructs in women with BPS.

METHODS

Secondary planned analysis of a prospective cross-sectional study of 150 women with BPS. The relationship between neuropathic pain, urinary, and bowel symptoms was assessed.

RESULTS

The correlation of the total neuropathic pain score with total urinary and bowel symptom scores was low to moderate (r = 0.28-0.49). The correlation of specific neuropathic pain items with bladder and bowel pain was also low to moderate (r = 0.12-0.36). Women with neuropathic pain had significantly higher scores for urinary urgency, bladder pain, abdominal pain, diarrhea, and constipation than women with non-neuropathic pain (all P < 0.0001).

CONCLUSION

Somatosensory neuropathic pain and "visceral" bladder and bowel pain represent separate but related constructs in women with BPS.

Epidemiology of neuropathic pain and its impact on quality of life

Epidemiology is an important clinical tool in designing and evaluating management and prevention strategies, and is particularly relevant to neuropathic pain. However, there is a relative lack of accurate information available. In one sense, neuropathic pain describes a symptom or a mechanism, rather than a specific disease; on the other hand, there are sufficient similarities in the effects and response to treatment between different causes to make it worthwhile to consider neuropathic pain as a distinct condition. However, there are important specific disease-based factors that need to be considered separately. Estimates of prevalence that are based on specific causes of neuropathic pain tend to be lower (1-2%) than those that are based on reports of the classic symptoms (6-8%), and further methodological research is needed. All neuropathic pain is associated with poor general health, comparable with other severe chronic diseases. The importance of newly proposed risk factors, including genetic factors, still needs to be assessed at a population level.

HIV-associated sensory neuropathy: still a problem in the post-stavudine era?

Sensory neuropathy (SN) is a common and difficult to manage cause of chronic pain in HIV. Recent recommendations for earlier HIV treatment and avoidance of neurotoxic antiretroviral drugs (such as stavudine) have led to optimism that HIV-SN rates may decline. We present several reasons as to why HIV-SN is likely to remain prevalent, despite improvements in HIV management, together with clinical evidence confirming high HIV-SN rates in cohorts never exposed to neurotoxic medications. A combination of epidemiologic studies, laboratory work and clinical trials are needed to understand the problem of HIV-SN in the post-stavudine era. Improved HIV-SN prevention and management strategies are needed if the morbidity associated with HIV infection is to improve along with life expectancy.

Neuropathic pain: is quantitative sensory testing helpful?

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory system and is characterised by a combination of positive and negative sensory symptoms. Quantitative sensory testing (QST) examines the sensory perception after application of different mechanical and thermal stimuli of controlled intensity and the function of both large (A-beta) and small (A-delta and C) nerve fibres, including the corresponding central pathways. QST can be used to determine detection, pain thresholds and stimulus-response curves and can thus detect both negative and positive sensory signs, the second ones not being assessed by other methods. Similarly to all other psychophysical tests QST requires standardised examination, instructions and data evaluation to receive valid and reliable results. Since normative data are available, QST can contribute also to the individual diagnosis of neuropathy, especially in the case of isolated small-fibre neuropathy, in contrast to the conventional electrophysiology which assesses only large myelinated fibres. For example, detection of early stages of subclinical neuropathy in symptomatic or asymptomatic patients with diabetes mellitus can be helpful to optimise treatment and identify diabetic foot at risk of ulceration. QST assessed the individual's sensory profile and thus can be valuable to evaluate the underlying pain mechanisms which occur in different frequencies even in the same neuropathic pain syndromes. Furthermore, assessing the exact sensory phenotype by QST might be useful in the future to identify responders to certain treatments in accordance to the underlying pain mechanisms.

Understanding chronic inflammatory and neuropathic pain

This meeting report highlights the main topics presented at the conference "Chronic Inflammatory and Neuropathic Pain," convened jointly by the New York Academy of Sciences, MedImmune, and Grünenthal GmbH, on June 2-3, 2011, with the goal of providing a conducive environment for lively, informed, and synergistic conversation among participants from academia, industry, clinical practice, and government to explore new frontiers in our understanding and treatment of chronic and neuropathic pain. The program included leading and emerging investigators studying the pathophysiological mechanisms underlying neuropathic and chronic pain, and experts in the clinical development of pain therapies. Discussion included novel issues, current challenges, and future directions of basic research in pain and preclinical and clinical development of new therapies for chronic pain.

Anti-inflammatory mesenchymal stem cells (MSC2) attenuate symptoms of painful diabetic peripheral neuropathy

Mesenchymal stem cells (MSCs) are very attractive candidates in cell-based strategies that target inflammatory diseases. Preclinical animal studies and many clinical trials have demonstrated that human MSCs can be safely administered and that they modify the inflammatory process in the targeted injured tissue. Our laboratory developed a novel method that optimizes the anti-inflammatory effects of MSCs. We termed the cells prepared by this method MSC2. In this study, we determined the effects of MSC2-based therapies on an inflammation-linked painful diabetic peripheral neuropathy (pDPN) mouse model. Streptozotocin-induced diabetic mice were treated with conventionally prepared MSCs, MSC2, or vehicle at three specific time points. Prior to each treatment, responses to radiant heat (Hargreaves) and mechanical stimuli (von Frey) were measured. Blood serum from each animal was collected at the end of the study to compare levels of inflammatory markers between the treatment groups. We observed that MSC2-treated mice had significant improvement in behavioral assays compared with the vehicle and MSC groups, and moreover these responses did not differ from the observations seen in the healthy wild-type control group. Mice treated with conventional MSCs showed significant improvement in the radiant heat assay, but not in the von Frey test. Additionally, mice treated with MSC2 had decreased serum levels in many proinflammatory cytokines compared with the values measured in the MSC- or vehicle-treated groups. These findings indicate that MSC2-based therapy is a new anti-inflammatory treatment to consider in the management of pDPN.

5% lidocaine medicated plaster in elderly patients with postherpetic neuralgia: results of a compassionate use programme in France

BACKGROUND

Postherpetic neuralgia (PHN) is a common, debilitating complication of herpes zoster that has a major impact on patients' quality of life. PHN prevalence increases with advancing age. One treatment option is the topical analgesic 5% lidocaine (lignocaine) medicated plaster (Versatis®), which has been proven to be efficacious and well tolerated in a number of randomized clinical studies.

OBJECTIVE

The aim of this analysis was to assess the use of the lidocaine medicated plaster under clinical practice conditions in a patient population whose previous PHN treatment with antidepressant and/or antiepileptic agents was inadequate or was not tolerated, or for whom such treatment was contraindicated or not recommended.

METHODS

This was a prospective, multicentre, non-interventional observation conducted in private and public health centres in France under a compassionate use programme (CUP). To obtain this new - and, at the time, unauthorized - PHN treatment alternative, physicians (in accordance with French guidelines) had to complete standardized case report forms for each patient before his/her inclusion in the CUP. As it was a CUP and therefore a non-interventional observation, returning documented information on follow-up visits to the medication provider was voluntary, and only a limited number of physicians returned completed forms. Documentation was, however, mandatory for adverse events (AEs) occurrence. Depending on the size of the painful skin area, up to three lidocaine plasters daily were applied for a maximum of 12 hours with plaster-free intervals of at least 12 hours. The study assessed changes in the prescription of concomitant PHN medication from the start of lidocaine plaster treatment to the last follow-up visit, both in terms of the sum of all concomitant PHN treatments and stratified by type of treatment: antiepileptic drugs, tricyclic antidepressants (TCAs), serotonin reuptake inhibitors (SRIs), classical analgesics (classified as step 1, 2 or 3 according to the WHO cancer pain ladder), transcutaneous electrical nerve stimulation, and others (mainly NSAIDs). AEs were monitored for safety.

RESULTS

A total of 625 patients were included in the CUP and permitted to receive lidocaine plaster treatment. Physicians returned 273 documented follow-up visit report forms. The mean ± SD CUP duration (i.e. duration of lidocaine plaster treatment) was 2.4 ± 2.5 months (median 1 month). Efficacy was assessed in the group of patients with documented follow-up visits (n = 273; mean ± SD age 73.6 ± 11.2 years), of whom 184 were aged ≥70 years (elderly efficacy population). The safety analysis included 625 patients (mean ± SD age 73.2 ± 11.9 years). Lidocaine plaster treatment resulted in a significant mean reduction of one concomitant PHN treatment per patient in the overall efficacy population analysed at the end of the observation (p < 0.001). In both populations (overall efficacy and elderly efficacy population), significantly fewer patients received TCAs (p = 0.003 and p = 0.001, respectively), step 3 analgesics (p = 0.001 and p = 0.005, respectively), and other miscellaneous treatments (p < 0.001 for both populations); there was also a significant reduction in the proportion of patients who took step 2 analgesics (p = 0.009) in the overall efficacy group. AEs (mainly related to local plaster application) were documented for 2.6% of the patients in the safety population; none were considered serious.

CONCLUSIONS

In day-to-day clinical practice management of PHN, treatment with the 5% lidocaine medicated plaster permitted a significant quantitative reduction in concomitant treatments for neuropathic pain in the overall efficacy population. In the subgroup aged ≥70 years, the quantitative reduction was non-significant. However, in both populations, 5% lidocaine medicated plaster reduced use of TCAs and step 3 analgesics. An improved polymedication status and good tolerability in this likely multimorbid age group indicate that the plaster is a new therapeutic alternative for patients suffering from PHN in France.

Skin innervation at different depths correlates with small fibre function but not with pain in neuropathic pain patients

BACKGROUND

Neuropathy can lead not only to impaired function but also to sensory sensitization. We aimed to link reduced skin nerve fibre density in different levels to layer-specific functional impairment in neuropathic pain patients and tried to identify pain-specific functional and structural markers.

METHODS

In 12 healthy controls and 36 patients with neuropathic pain, we assessed clinical characteristics, thermal thresholds (quantitative sensory testing) and electrically induced pain and axon reflex erythema. At the most painful sites and at intra-individual control sites, skin biopsies were taken and innervation densities in the different skin layers were assessed. Moreover, neuronal calcitonin gene-related peptide staining was quantified.

RESULTS

Perception of warm, cold and heat pain and nerve fibre density were reduced in the painful areas compared with the control sites and with healthy controls. Warm and cold detection thresholds correlated best with epidermal innervation density, whereas heat and cold pain thresholds and axon reflex flare correlated best with dermal innervation density. Clinical pain ratings correlated only with epidermal nerve fibre density (r = 0.38, p < 0.05) and better preserved cold detection thresholds (r = 0.39, p < 0.05), but not with other assessed functional and structural parameters.

CONCLUSIONS

Thermal thresholds, axon reflex measurements and assessment of skin innervation density are valuable tools to characterize and quantify peripheral neuropathy and link neuronal function to different layers of the skin. The severity of small fibre neuropathy, however, did not correspond to clinical pain intensity and a specific parameter or pattern that would predict pain intensity in peripheral neuropathy could not be identified.

Considerations for improving assay sensitivity in chronic pain clinical trials: IMMPACT recommendations

A number of pharmacologic treatments examined in recent randomized clinical trials (RCTs) have failed to show statistically significant superiority to placebo in conditions in which their efficacy had previously been demonstrated. Assuming the validity of previous evidence of efficacy and the comparability of the patients and outcome measures in these studies, such results may be a consequence of limitations in the ability of these RCTs to demonstrate the benefits of efficacious analgesic treatments vs placebo ("assay sensitivity"). Efforts to improve the assay sensitivity of analgesic trials could reduce the rate of falsely negative trials of efficacious medications and improve the efficiency of analgesic drug development. Therefore, an Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials consensus meeting was convened in which the assay sensitivity of chronic pain trials was reviewed and discussed. On the basis of this meeting and subsequent discussions, the authors recommend consideration of a number of patient, study design, study site, and outcome measurement factors that have the potential to affect the assay sensitivity of RCTs of chronic pain treatments. Increased attention to and research on methodological aspects of clinical trials and their relationships with assay sensitivity have the potential to provide the foundation for an evidence-based approach to the design of analgesic clinical trials and expedite the identification of analgesic treatments with improved efficacy and safety.

Pharmacodynamic analysis of the analgesic effect of capsaicin 8% patch (Qutenza™) in diabetic neuropathic pain patients: detection of distinct response groups

Treatment of chronic pain is associated with high variability in the response to pharmacological interventions. A mathematical pharmacodynamic model was developed to quantify the magnitude and onset/offset times of effect of a single capsaicin 8% patch application in the treatment of painful diabetic peripheral neuropathy in 91 patients. In addition, a mixture model was applied to objectively match patterns in pain-associated behavior. The model identified four distinct subgroups that responded differently to treatment: 3.3% of patients (subgroup 1) showed worsening of pain; 31% (subgroup 2) showed no change; 32% (subgroup 3) showed a quick reduction in pain that reached a nadir in week 3, followed by a slow return towards baseline (16% ± 6% pain reduction in week 12); 34% (subgroup 4) showed a quick reduction in pain that persisted (70% ± 5% reduction in week 12). The estimate of the response-onset rate constant, obtained for subgroups 1, 3, and 4, was 0.76 ± 0.12 week⁻¹ (median ± SE), indicating that every 0.91 weeks the pain score reduces or increases by 50% relative to the score of the previous week (= t½). The response-offset rate constant could be determined for subgroup 3 only and was 0.09 ± 0.04 week⁻¹ (t½ 7.8 weeks). The analysis allowed separation of a heterogeneous neuropathic pain population into four homogenous subgroups with distinct behaviors in response to treatment with capsaicin. It is argued that this model-based approach may have added value in analyzing longitudinal chronic pain data and allows optimization of treatment algorithms for patients suffering from chronic pain conditions.

P2X3-mediated peripheral sensitization of neuropathic pain in resiniferatoxin-induced neuropathy

Patients suffering from sensory neuropathy due to skin denervation frequently have paradoxical manifestations of reduced nociception and neuropathic pain. However, there is a lack of satisfactory animal models to investigate these phenomena and underlying mechanisms. We developed a mouse system of neuropathy induced by resiniferatoxin (RTX), a capsaicin analog, and examined the functional significance of P2X3 receptor in neuropathic pain. From day 7 of RTX neuropathy, mice displayed mechanical allodynia (p<0.0001) and thermal hypoalgesia (p<0.0001). After RTX treatment, dorsal root ganglion (DRG) neurons of the peripherin type were depleted (p=0.012), while neurofilament (+) DRG neurons were not affected (p=0.62). In addition, RTX caused a shift in neuronal profiles of DRG: (1) increased in P2X3 receptor (p=0.0002) and ATF3 (p=0.0006) but (2) reduced TRPV1 (p=0.036) and CGRP (p=0.015). The number of P2X3(+)/ATF3(+) neurons was linearly correlated with mechanical thresholds (p=0.0017). The peripheral expression of P2X3 receptor in dermal nerves was accordingly increased (p=0.016), and an intraplantar injection of the P2X3 antagonists, A-317491 and TNP-ATP, relieved mechanical allodynia in a dose-dependent manner. In conclusion, RTX-induced sensory neuropathy with upregulation of P2X3 receptor for peripheral sensitization of mechanical allodynia, which provides a new therapeutic target for neuropathic pain after skin denervation.

Deconstructing the neuropathic pain phenotype to reveal neural mechanisms

After nerve injury maladaptive changes can occur in injured sensory neurons and along the entire nociceptive pathway within the CNS, which may lead to spontaneous pain or pain hypersensitivity. The resulting neuropathic pain syndromes present as a complex combination of negative and positive symptoms, which vary enormously from individual to individual. This variation depends on a diversity of underlying pathophysiological changes resulting from the convergence of etiological, genotypic, and environmental factors. The pain phenotype can serve therefore, as a window on underlying pathophysiological neural mechanisms and as a guide for developing personalized pain medicine.

Association of neuropathic pain with bladder, bowel and catastrophizing symptoms in women with bladder pain syndrome

PURPOSE

In this study we determined if there is an association of neuropathic pain with urinary, bowel and catastrophizing symptoms in women with bladder pain syndrome.

MATERIALS AND METHODS

Female patients with a diagnosis of bladder pain syndrome completed validated questionnaires to assess neuropathic pain, urinary and bowel symptoms, quality of life and pain catastrophizing. Women were dichotomized into neuropathic pain and nonneuropathic pain groups. Urinary and bowel symptoms, pain catastrophizing and quality of life scores were compared between the 2 groups using parametric and nonparametric tests.

RESULTS

Of 150 women with bladder pain syndrome 40 (27%) had features of neuropathic pain while 110 (73%) did not. Women with features of neuropathic pain had significantly worse urinary urgency (mean ± SD 3.1 ± 3.1 vs 2.1 ± 1.7, p <0.001), bladder pain (3.0 ± 1.1 vs 2.0 ± 1.3, p <0.001), bowel pain (8.8 ± 4.0 vs 5.3 ± 3.6, p <0.001), diarrhea (7.8 ± 6.1 vs 4.1 ± 4.3, p <0.001), quality of life (12.2 ± 5.5 vs 9.8 ± 3.8, p <0.001) and higher pain catastrophizing (32.2 ± 12.4 vs 23.1 ± 14.3, p <0.001) scores than those without neuropathic pain.

CONCLUSIONS

In women with bladder pain syndrome the presence of neuropathic pain is significantly associated with the severity of bladder and bowel pain, urinary urgency and diarrhea. Women with features of neuropathic pain also have worse pain catastrophizing and quality of life than those without features of neuropathic pain.

Relationship of axonal voltage-gated sodium channel 1.8 (NaV1.8) mRNA accumulation to sciatic nerve injury-induced painful neuropathy in rats

Painful peripheral neuropathy is a significant clinical problem; however, its pathological mechanism and effective treatments remain elusive. Increased peripheral expression of tetrodotoxin-resistant voltage-gated sodium channel 1.8 (NaV1.8) has been shown to associate with chronic pain symptoms in humans and experimental animals. Sciatic nerve entrapment (SNE) injury was used to develop neuropathic pain symptoms in rats, resulting in increased NaV1.8 mRNA in the injured nerve but not in dorsal root ganglia (DRG). To study the role of NaV1.8 mRNA in the pathogenesis of SNE-induced painful neuropathy, NaV1.8 shRNA vector was delivered by subcutaneous injection of cationized gelatin/plasmid DNA polyplex into the rat hindpaw and its subsequent retrograde transport via sciatic nerve to DRG. This in vivo NaV1.8 shRNA treatment reversibly and repeatedly attenuated the SNE-induced pain symptoms, an effect that became apparent following a distinct lag period of 3-4 days and lasted for 4-6 days before returning to pretreatment levels. Surprisingly, apparent knockdown of NaV1.8 mRNA occurred only in the injured nerve, not in the DRG, during the pain alleviation period. Levels of heteronuclear NaV1.8 RNA were unaffected by SNE or shRNA treatments, suggesting that transcription of the Scn10a gene encoding NaV1.8 was unchanged. Based on these data, we postulate that increased axonal mRNA transport results in accumulation of functional NaV1.8 protein in the injured nerve and the development of painful neuropathy symptoms. Thus, targeted delivery of agents that interfere with axonal NaV1.8 mRNA may represent effective neuropathic pain treatments.

Phenotypic changes in diabetic neuropathy induced by a high-fat diet in diabetic C57BL/6 mice

Emerging evidence suggests that dyslipidemia is an independent risk factor for diabetic neuropathy (DN) (reviewed by Vincent et al. 2009). To experimentally determine how dyslipidemia alters DN, we quantified neuropathic symptoms in diabetic mice fed a high-fat diet. Streptozotocin-induced diabetic C57BL/6 mice fed a high-fat diet developed dyslipidemia and a painful neuropathy (mechanical allodynia) instead of the insensate neuropathy (mechanical insensitivity) that normally develops in this strain. Nondiabetic mice fed a high-fat diet also developed dyslipidemia and mechanical allodynia. Thermal sensitivity was significantly reduced in diabetic compared to nondiabetic mice, but was not worsened by the high-fat diet. Moreover, diabetic mice fed a high-fat diet had significantly slower sensory and motor nerve conduction velocities compared to nondiabetic mice. Overall, dyslipidemia resulting from a high-fat diet may modify DN phenotypes and/or increase risk for developing DN. These results provide new insight as to how dyslipidemia may alter the development and phenotype of diabetic neuropathy.

Review of current guidelines on the care of postherpetic neuralgia

An unfortunate minority of patients with acute herpes zoster (AHZ) experience pain beyond the typical 4-week duration, and roughly 10% develop the distressing complication of postherpetic neuralgia (PHN), often defined as pain persisting for > 4 months after the onset of the rash. Elderly patients are at increased risk of PHN. The pathophysiology of PHN is complex, likely involving both peripheral and central processes. This complexity may create opportunities for pharmacologic interventions with multiple differing mechanisms of action. Consequently, complementary combinations of pharmacologic agents are frequently more effective than any monotherapy. Current US and international guidelines on the care of patients with PHN are reviewed and interpreted here to facilitate their effective incorporation into the practice of primary care physicians, acknowledging the contrasts that often exist between the clinical trial populations analyzed to craft so-called evidence-based medicine and the individual patients seen in daily practice, many of whom may not have been candidates for those clinical trials. First-line treatments for PHN include tricyclic antidepressants, gabapentin and pregabalin, and the topical lidocaine 5% patch. Opioids, tramadol, capsaicin cream, and the capsaicin 8% patch are recommended as either second- or third-line therapies in different guidelines. Therapies that have demonstrated effectiveness for other types of neuropathic pain are discussed, such as serotonin-norepinephrine reuptake inhibitors, the anticonvulsants carbamazepine and valproic acid, and botulinum toxin. Invasive procedures such as sympathetic blockade, intrathecal steroids, and implantable spinal cord stimulators have been studied for relief of PHN, mainly in patients refractory to noninvasive pharmacologic interventions. The main guidelines considered here are those issued by the American Academy of Neurology for the treatment of postherpetic neuralgia (2004) and general guidelines for the treatment of neuropathic pain issued by the Special Interest Group on Neuropathic Pain of the International Association for the Study of Pain (2007) and the European Federation of Neurological Societies (2010).

Microneurographic identification of spontaneous activity in C-nociceptors in neuropathic pain states in humans and rats

C-nociceptors do not normally fire action potentials unless challenged by adequate noxious stimuli. However, in pathological states nociceptors may become hyperexcitable and may generate spontaneous ectopic discharges. The aim of this study was to compare rat neuropathic pain models and to assess their suitability to model the spontaneous C-nociceptor activity found in neuropathic pain patients. Studies were performed in normal rats (n=40), healthy human subjects (n=15), peripheral neuropathic pain patients (n=20), and in five rat neuropathic pain models: nerve crush (n=24), suture (n=14), chronic constriction injury (n=12), STZ-induced diabetic neuropathy (n=56), and ddC-induced neuropathy (n=15). Microneurographic recordings were combined with electrical stimulation to monitor activity in multiple C fibers. Stimulation at 0.25 Hz allowed spontaneous impulses to be identified by fluctuations in baseline latency. Abnormal latency fluctuations could be produced by several mechanisms, and spontaneous activity was most reliably identified by the presence of unexplained latency increases corresponding to two or more additional action potentials. Spontaneous activity was present in a proportion of mechano-insensitive C-nociceptors in the patients and all rat models. The three focal traumatic nerve injury models provided the highest proportion (59.5%), whereas the two polyneuropathy models had fewer (18.6%), and the patients had an intermediate proportion (33.3%). Spontaneously active mechano-sensitive C-nociceptors were not recorded. Microneurographic recordings of spontaneous activity in diseased C-nociceptors may be useful for both short- and long-term drug studies, both in animals and in humans.

Pathophysiological mechanisms of neuropathic pain

Neuropathic pain is a common problem in clinical practice and one that adversely affects patients’ quality of life. Converging evidence from animal and human studies demonstrates that neuropathic pain arises from a lesion in the somatosensory system. Injured peripheral nerve fibers give rise to an intense and prolonged ectopic input to the CNS and, in some cases, also to secondary changes in dorsal horn neuronal excitability. Convincing evidence now suggests that classifying neuropathic pain according to a mechanism-based rather than an etiology-based approach might help in targeting therapy to the individual patient and would be useful in testing new drugs. This article summarizes our current understanding of the peripheral and central pathophysiological mechanisms underlying neuropathic pain and focuses on how symptoms translate into mechanisms.