The influence of regional spinal cord hypothermia on transcranial myogenic motor-evoked potential monitoring and the efficacy of spinal cord ischemia detection

A Case of Decreased Amplitude in Motor Evoked Potentials Under Remimazolam Anesthesia

Remimazolam is a newly developed benzodiazepine derivative. Although one case report on the use of remimazolam for motor evoked potential (MEP) monitoring has been reported, there has been no report of changes in the MEP response under remimazolam anesthesia, which is associated with impairment of the corticospinal motor track. This is a case of a 54-year-old woman who was diagnosed with an extradural extramedullary tumor. The patient reported being allergic to chicken eggs. We used remimazolam instead of propofol for anesthesia management. During tumor resection, the amplitudes of MEP responses at the left quadriceps femoris, left tibialis anterior, and left abductor hallucis muscle decreased. The surgery was scaled down and the tumor was removed in a reduced size. The patient had muscle weakness immediately after surgery but eventually recovered. In this case, we could detect changes in MEP response under remimazolam anesthesia, which suggested impairment of the motor tracts during surgery.

Effects of Changes in Body Temperature on Perioperative Bleeding in Adolescent Idiopathic Scoliosis Surgery

Perioperative bleeding is a critical challenge in adolescent idiopathic scoliosis (AIS) surgery. Preventing hypothermia is associated with decreased development of coagulopathy, blood transfusion rate in various surgery groups. We hypothesized that blood loss would be reduced in patients who were kept normothermic by implementation of aggressive warming methods in AIS. This randomized-controlled study included patients aged 12-18 years who were scheduled to undergo elective scoliosis deformity correction surgery. The patients were divided into two groups: the study group (Group S) was heated aggressively with three different heaters including compressed-air blower heater, intravenous fluid heating, and a heating bed, while the control group (Group C) received only heating with a standard compressed-air blower heater. Tympanic, esophageal, and axillary body temperatures were measured, and hemoglobin and arterial blood gas analyses were repeated during the anesthesia period. Daily bleeding-coagulation parameters were recorded on postoperative days 0, 1, and 2. Forty-eight patients were randomized, and 39 patients were included into the final analysis. The total amount of intraoperative bleeding (p = 0.027) was significantly lower, and duration of surgery (p = 0.025) and length of hospital stay (p = 0.002) were significantly shorter in Group S. Significant linear relationships were found between the core body temperature and the amount of bleeding (β = 0.0001; p = 0.009), operation time (β = 0.003; p = 0.015), and length of hospital stay (β = 0.027; p = 0.044) with linear logistic regression analysis. We reported that normothermia was preserved in the multiheated group, which diminished blood loss, operation time, and the length of hospital stay. Consequently, we suggested that active heating should be applied in AIS surgeries. ClinicalTrials.gov (NCT04686214).

Comparison of Intraoperative Neuromonitoring Outcome in Treating Thoracic Ossification of the Ligamentum Flavum Through En Bloc Versus Piecemeal Laminectomy

STUDY DESIGN

A retrospective cohort analysis.

OBJECTIVE

The aim of this study was to investigate the impact of piecemeal versus en bloc laminectomies on spinal cord in thoracic ossification of ligamentum flavum (TOLF) through intraoperative changes of motor-evoked potentials (MEPs) and somatosensory-evoked potentials (SEPs).

SUMMARY OF BACKGROUND DATA

Surgical treatment is indicated for symptomatic TOLF, and both piecemeal and en bloc laminectomies are commonly used methods. However, few studies compared both intraoperative interference and prognostic impact of these two laminectomies on spinal cord in TOLF patients.

METHODS

MEPs were recorded from abductor hallucis (AH) and tibialis anterior, and SEPs were performed on tibial nerve in 55 TOLF patients (piecemeal vs. en bloc: 23 vs. 32). Patients were categorized based on MEP/SEP improvement, deterioration, and no change, and MEP/SEP improvement rates were measured in the improvement group. Additionally, all patients were assessed by American Spinal Injury Association (ASIA) scores, Ashworth scores, and modified Japanese Orthopedic association (mJOA) scores before and after operation.

RESULTS

The incidences of both MEP/SEP improvement and deterioration were similar between the two laminectomy groups (P > 0.05), and no significant difference is noted in both MEP and SEP amplitudes between the baseline and different critical manipulations in both laminectomy groups (P > 0.05). In the improvement group, patients receiving en bloc laminectomy exhibited increased improvement rates of both MEPs in bilateral AH and left-side SEPs compared to piecemeal laminectomy (P < 0.05). Clinically, all functional scales clearly improved in both laminectomy groups after operation (P < 0.05), and postoperative 1-year mJOA improvement rates were highly correlated with MEP improvement rates (P < 0.05).

CONCLUSION

Intraoperative changes of MEPs and SEPs potentially provide a valid method for quantitatively evaluating the safety of different intraoperative manipulations and their prognostic impacts on spinal cord. Both laminectomies are safe and effective methods to treat TOLF, and en bloc laminectomy may cause relatively better spinal cord functional recovery.Level of Evidence: 3.

A practical guide for anesthetic management during intraoperative motor evoked potential monitoring

Postoperative motor dysfunction can develop after spinal surgery, neurosurgery and aortic surgery, in which there is a risk of injury of motor pathway. In order to prevent such devastating complication, intraoperative monitoring of motor evoked potentials (MEP) has been conducted. However, to prevent postoperative motor dysfunction, proper understanding of MEP monitoring and proper anesthetic managements are required. Especially, a variety of anesthetics and neuromuscular blocking agent are known to attenuate MEP responses. In addition to the selection of anesthetic regime to record the baseline and control MEP, the measures to keep the level of hypnosis and muscular relaxation at constant are crucial to detect the changes of MEP responses after the surgical manipulation. Once the changes of MEP are observed based on the institutional alarm criteria, multidisciplinary team members should share the results of MEP monitoring and respond to check the status of monitoring and recover the possible motor nerve injury. Prevention of MEP-related adverse effects is also important to be considered. The Working Group of Japanese Society of Anesthesiologists (JSA) developed this practical guide aimed to help ensure safe and successful surgery through appropriate anesthetic management during intraoperative MEP monitoring.

Association Between Motor-Evoked Potentials and Spinal Cord Damage Diagnosed With Magnetic Resonance Imaging After Thoracoabdominal and Descending Aortic Aneurysm Repair

OBJECTIVES

The authors investigated the association between intraoperative motor-evoked potential (MEP) changes and the severity of spinal cord infarction diagnosed with magnetic resonance imaging (MRI) to clarify the discrepancy between them, which was observed in patients with postoperative motor deficits after thoracic and thoracoabdominal aortic surgery.

DESIGN

A multicenter retrospective study.

SETTING

Motor-evoked potential <25% of control values was deemed positive for spinal cord ischemia. The severity of spinal cord infarction was categorized into grades A to D based on previous studies using the most severe axial MRI slices. The associations between MRI grade, MEP changes, and motor deficits were examined using logistic regression.

PARTICIPANTS

Twenty-three of 1,245 patients (from 1999 to 2013, at 12 hospitals in Japan) were extracted from medical records of patients who underwent thoracic and thoracoabdominal aortic repair, with intraoperative MEP examinations and postoperative spinal MRI.

INTERVENTIONS

No intervention (observational study).

MEASUREMENTS AND MAIN RESULTS

Motor-evoked potential <25% of control value was associated significantly with motor deficits at discharge (adjusted odds ratio [OR], 130.0; p = 0.041), but not with severity of spinal cord infarction (adjusted OR, 0.917; p = 0.931). Motor deficit at discharge was associated with severe spinal cord infarction (adjusted OR, 4.83; p = 0.043), MEP <25% (adjusted OR, 13.95; p = 0.031), and combined deficits (motor and sensory, motor and bowel or bladder, or sensory and bowel or bladder deficits; adjusted OR, 31.03; p = 0.072) in stepwise logistic regression analysis.

CONCLUSION

Motor-evoked potential <25% was associated significantly with motor deficits at discharge, but not with the severity of spinal cord infarction.

Intravenous hydrogen sulfide does not induce neuroprotection after aortic balloon occlusion-induced spinal cord ischemia/reperfusion injury in a human-like porcine model of ubiquitous arteriosclerosis

Objective

In rodents, intravenous sulfide protected against spinal cord ischemia/reperfusion (I/R) injury during aortic balloon occlusion. We investigated the effect of intravenous sulfide on aortic occlusion-induced porcine spinal cord I/R injury.

Methods

Anesthetized and mechanically ventilated “familial hypercholesterolemia Bretoncelles Meishan” (FBM) pigs with high-fat-diet-induced hypercholesterolemia and atherosclerosis were randomized to receive either intravenous sodium sulfide 2 h (initial bolus, 0.2 mg kg body weight (bw)⁻¹; infusion, 2 mg kg bw⁻¹ h⁻¹; n = 4) or vehicle (sodium chloride, n = 4) prior to 45 min of thoracic aortic balloon occlusion and for 8 h during reperfusion (infusion, 1 mg kg bw⁻¹ h⁻¹). During reperfusion, noradrenaline was titrated to maintain blood pressure at above 80% of the baseline level. Spinal cord function was assessed by motor evoked potentials (MEPs) and lower limb reflexes using a modified Tarlov score. Spinal cord tissue damage was evaluated in tissue collected at the end of experiment using hematoxylin and eosin and Nissl staining.

Results

A balloon occlusion time of 45 min resulted in marked ischemic neuron damage (mean of 16% damaged motoneurons in the anterior horn of all thoracic motor neurons) in the spinal cord. In the vehicle group, only one animal recovered partial neuronal function with regain of MEPs and link motions at each time point after deflating. All other animals completely lost neuronal functions. The intravenous application of sodium sulfide did not prevent neuronal cell injury and did not confer to functional recovery.

Conclusion

In a porcine model of I/R injury of the spinal cord, treatment with intravenous sodium sulfide had no protective effect in animals with a pre-existing arteriosclerosis.

Multipulse transcranial electrical stimulation (TES): normative data for motor evoked potentials in healthy horses

Background

There are indications that transcranial electrical stimulation (TES) assesses the motor function of the spinal cord in horses in a more sensitive and reproducible fashion than transcranial magnetic stimulation (TMS). However, no normative data of TES evoked motor potentials (MEP) is available.

Results

In this prospective study normative data of TES induced MEP wave characteristics (motor latency times (MLT); amplitude and waveform) was obtained from the extensor carpi radialis (ECR) and tibial cranialis (TC) muscles in a group of healthy horses to create a reference frame for functional diagnostic purposes. For the 12 horses involved in the study 95% confidence intervals for MLTs were 16.1–22.6 ms and 31.9–41.1 ms for ECR and TC muscles respectively. Intra-individual coefficients of variation (CV) and mean of MLTs were: ECR: 2.2–8,2% and 4.5% and TC: 1.4–6.3% and 3.5% respectively. Inter-individual CVs for MLTs were higher, though below 10% on all occasions.

The mean ± sd of MEP amplitudes was respectively 3.61 ± 2.55 mV (ECR muscle left) and 4.53 ± 3.1 mV (right) and 2.66 ± 2.22 mV (TC muscle left) and 2.55 ± 1.85 mV (right). MLTs showed no significant left versus right differences.

All MLTs showed significant (p < 0.05) voltage dependent decreases with slope coefficients of linear regression for ECR: − 0.049; − 0.061 ms/V and TC: − 0.082; − 0.089 ms/V (left; right). There was a positive correlation found between height at withers and MLTs in all 4 muscle groups. Finally, reliable assessment of MEP characteristics was for all muscle groups restricted to a transcranial time window of approximately 15–19 ms.

Conclusions

TES is a novel and sensitive technique to assess spinal motor function in horses. It is easy applicable and highly reproducible. This study provides normative data in healthy horses on TES induced MEPs in the extensor carpi radialis and tibialis cranialis muscles bilaterally. No significant differences between MLTs of the left and right side could be demonstrated. A significant effect of stimulation voltage on MLTs was found. No significant effect of height at the withers could be found based upon the results of the current study. A study in which both TMS and TES are applied on the same group of horses is needed.

Neuromonitoring Using Motor and Somatosensory Evoked Potentials in Aortic Surgery

BACKGROUND

Motor evoked potentials (MEP) and somatosensory evoked potentials (SSEP) are established methods of neuromonitoring aimed at preventing paraplegia after descending or thoracoabdominal aortic repair. However, their predictive impact remains controversial. The aim of this study was to evaluate our single-center experience using this monitoring technique.

METHODS

Between 2009 and 2014, 78 patients (mean age 66 ± 12, 53% male) underwent either descending or thoracoabdominal aortic repairs. Of these, 60% had an aortic aneurysm, 30% dissection, and 10% other etiologies. Intraoperatively, MEPs and SSEPs were monitored and, if necessary, clinical parameters (blood pressure, hematocrit, oxygenation) were adjusted in response to neuromonitoring signals. This analysis is focused on the neurological outcome (paraplegia, stroke) after the use of intraoperative neuromonitoring.

RESULTS

Thirty-day mortality was 10 (12.8%). All patients with continuously stable signals or signals that returned after signal loss developed no spinal cord injury, whereas two out of six of the evaluable patients with signal loss (without return) during the procedure suffered from postoperative paraplegia (one transient and one permanent). Sensitivity and specificity of use of MEP and SSEP were 100% and 94.20% regarding paraplegia, respectively.

CONCLUSIONS

(1) Preservation of signals or return of signals is an excellent prognostic indicator for spinal cord function. (2) Intraoperative modifications in direct response to the signal change may have averted permanent paralysis in the patients with signal loss without neurologic injury. We have found MEP and SSEP neuromonitoring to be instrumental in the prevention of paraplegia. doi: 10.1111/jocs.12739 (J Card Surg 2016;31:383-389).

Role of induced hypothermia in thoracoabdominal aortic aneurysm surgery

For more than 50 years, hypothermia has been used in aortic surgery as a tool for neuroprotection. Hypothermia has been introduced into thoracoabdominal aortic aneurysm (TAAA) surgery by many cardiovascular centers to protect the body's organs, including the spinal cord. Numerous publications have shown that hypothermia can prevent immediate and delayed motor dysfunction after aortic cross-clamping. Here, we reviewed the historical application of hypothermia in aortic surgery, role of hypothermia in preclinical studies, cellular and molecular mechanisms by which hypothermia confers neuroprotection, and the role of systemic and regional hypothermia in clinical protocols to reduce and/or eliminate the devastating consequences of ischemic spinal cord injury after TAAA repair.

Changes of motor evoked potentials during descending thoracic and thoracoabdominal aortic surgery with deep hypothermic circulatory arrest

BACKGROUND

Paraplegia is a serious complication of descending and thoracoabdominal aortic aneurysms (dTAAs and TAAAs) surgery. Motor evoked potentials (MEPs) enable monitoring the functional integrity of motor pathways during dTAA and TAAA surgery. Although MEPs are sensitive to temperature changes, there are few human data on changes of MEPs during mild and deep hypothermia. Therefore, we investigated changes of MEPs in deep hypothermic circulatory arrest (DHCA) in dTAA and TAAA surgery.

METHODS

Fifteen consecutive patients undergoing dTAA and TAAA surgery using DHCA were enrolled. MEPs were elicited and recorded during each degree Celsius change in nasopharyngeal temperature during both the cooling and rewarming phases. Hand and leg skin temperature were also recorded simultaneously.

RESULTS

In the cooling phase MEP amplitude decreased lineally in both the hand and leg. The MEP disappeared at ~16°C in both the hand and leg in 10 of 15 patients, but was still elicited in 5 patients. In the rewarming phase MEP in the hand recovered before the temperature reached 20°C for eight patients and 25°C for the other seven patients. In contrast, MEP in the leg recovered below 20°C for two patients and 30°C for three patients. For the other eight patients MEP waves did not recover during the rewarming phase.

CONCLUSION

In the cooling phase of DHCA, MEP disappeared at ~16°C in some patients but was still elicited in others. MEP recovered below 25°C in the hand. Recovery of MEP in the leg was, however, extremely variable.

Comparison of carbamylated erythropoietin-FC fusion protein and recombinant human erythropoietin during porcine aortic balloon occlusion-induced spinal cord ischemia/reperfusion injury

PURPOSE

Recombinant human erythropoietin (rhEPO) attenuated ischemia/reperfusion (I/R) injury-induced spinal cord damage. Since carbamylated EPO derivatives are stated to be devoid of rhEPO side effects, we tested the hypothesis that a newly developed carbamylated EPO-FC fusion protein (cEPO-FC) would compare favorably with rhEPO.

METHODS

Anesthetized and mechanically ventilated pigs randomly received cEPO-FC (50 μg kg(-1)), rhEPO (5,000 IU kg(-1)) or vehicle (n = 9 per group) 30 min prior to 30 min of aortic occlusion and over the 4 h of reperfusion. During aortic occlusion, mean arterial pressure (MAP) was maintained at 80-120% of baseline values by esmolol, nitroglycerin, and adenosine-5'-triphosphate (ATP). During reperfusion, noradrenaline was titrated to keep MAP at pre-ischemic levels. Spinal cord function was assessed by motor evoked potentials (MEP) and lower limb reflexes. Tissue damage was evaluated using hematoxylin and eosin, Nissl, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) staining. Plasma levels of interleukin-6, tumor necrosis factor-α, and 8-isoprostanes were measured as markers of systemic inflammation and oxidative stress.

RESULTS

While only cEPO-FC restored MEP amplitude to values close to pre-occlusion levels, both cEPO-FC and rhEPO comparably restored lower limb reflexes and reduced the percentage of damaged neurons. Infiltration of mononuclear inflammatory cells was moderate without intergroup difference; positive TUNEL staining was barely detectable in any group. I/R injury increased blood cytokine levels without intergroup difference, whereas both cEPO-FC and rhEPO significantly lowered 8-isoprostane levels.

CONCLUSIONS

In a porcine model of aortic balloon occlusion-induced spinal cord I/R injury, cEPO-FC and rhEPO comparably protected against ischemic spinal cord dysfunction and neuronal damage. This effect coincided with attenuated oxidative stress.

Erythropoietin during porcine aortic balloon occlusion-induced ischemia/reperfusion injury

BACKGROUND

Aortic occlusion causes ischemia/reperfusion injury, kidney and spinal cord being the most vulnerable organs. Erythropoietin improved ischemia/reperfusion injury in rodents, which, however, better tolerate ischemia/reperfusion than larger species. Therefore, we investigated whether erythropoietin attenuates porcine aortic occlusion ischemia/reperfusion injury.

MATERIALS AND METHODS

Before occluding the aorta for 45 mins by inflating intravascular balloons, we randomly infused either erythropoietin (n = 8; 300 IU/kg each over 30 mins before and during the first 4 hrs of reperfusion) or vehicle (n = 6). During aortic occlusion, mean arterial pressure was maintained at 80% to 120% of baseline by esmolol, nitroglycerine, and adenosine 5'-triphosphate. During reperfusion, noradrenaline was titrated to keep mean arterial pressure >80% of baseline. Kidney perfusion and function were assessed by fractional Na-excretion, p-aminohippuric acid and creatinine clearance, spinal cord function by lower extremity reflexes and motor evoked potentials. Blood isoprostane levels as well as blood and tissue catalase and superoxide dismutase activities allowed evaluation of oxidative stress. After 8 hrs of reperfusion, kidney and spinal cord specimens were taken for histology (hematoxylin-eosin, Nissl staining) and immunohistochemistry (TUNEL assay for apoptosis).

RESULTS

Parameters of oxidative stress and antioxidative activity were comparable. Erythropoietin reduced the noradrenaline requirements to achieve the hemodynamic targets and may improve kidney function despite similar organ blood flow, histology, and TUNEL staining. Neuronal damage and apoptosis was attenuated in the thoracic spinal cord segments without improvement of its function.

CONCLUSION

During porcine aortic occlusion-induced ischemia/reperfusion erythropoietin improved kidney function and spinal cord integrity. The lacking effect on spinal cord function was most likely the result of the pronounced neuronal damage associated with the longlasting ischemia.

Intraoperative neurophysiological monitoring of the spinal cord during spinal cord and spine surgery: a review focus on the corticospinal tracts

Recent advances in technology and the refinement of neurophysiological methodologies are significantly changing intraoperative neurophysiological monitoring (IOM) of the spinal cord. This review will summarize the latest achievements in the monitoring of the spinal cord during spine and spinal cord surgeries. This overview is based on an extensive review of the literature and the authors' personal experience. Landmark articles and neurophysiological techniques have been briefly reported to contextualize the development of new techniques. This background is extended to describe the methodological approach to intraoperatively elicit and record spinal D wave and muscle motor evoked potentials (muscle MEPs). The clinical application of spinal D wave and muscle MEP recordings is critically reviewed (especially in the field of Neurosurgery) and new developments such as mapping of the dorsal columns and the corticospinal tracts are presented. In the past decade, motor evoked potential recording following transcranial electrical stimulation has emerged as a reliable technique to intraoperatively assess the functional integrity of the motor pathways. Criteria based on the absence/presence of potentials, their morphology and threshold-related parameters have been proposed for muscle MEPs. While the debate remains open, it appears that different criteria may be applied for different procedures according to the expected surgery-related morbidity and the ultimate goal of the surgeon (e.g. total tumor removal versus complete absence of transitory or permanent neurological deficits). On the other hand, D wave changes--when recordable--have proven to be the strongest predictors of maintained corticospinal tract integrity (and therefore, of motor function/recovery). Combining the use of muscle MEPs with D wave recordings provides the most comprehensive approach for assessing the functional integrity of the spinal cord motor tracts during surgery for intramedullary spinal cord tumors. However, muscle MEPs may suffice to assess motor pathways during other spinal procedures and in cases where the pathophysiology of spinal cord injury is purely ischemic. Finally, while MEPs are now considered the gold standard for monitoring the motor pathways, SEPs continue to retain value as they provide specificity for assessing the integrity of the dorsal column. However, we believe SEPs should not be used exclusively--or as an alternative to motor evoked potentials--during spine surgery, but rather as a complementary method in combination with MEPs. For intramedullary spinal tumor resection, SEPs should not be used exclusively without MEPs.

Prolonged loss of leg myogenic motor evoked potentials during thoracoabdominal aortic aneurysm repair, without postoperative paraplegia

No postoperative paraplegia occurred in a patient whose leg myogenic motor evoked potentials (mMEPs) disappeared during thoracoabdominal aortic aneurysm repair. A 69-year-old man underwent resection and repair of a type III (Crawford classification) thoracoabdominal aneurysm. An epidural catheter was placed into the epidural space for epidural cooling, and a Swan-Ganz catheter was placed into the subarachnoid space for cerebrospinal fluid (CSF) drainage. Continuous CSF pressure and temperature measurement was carried out the day before surgery. The mMEPs gradually disappeared 10 min after proximal double aortic clamping and complete aortic transection. Selective perfusion of intercostal arteries was started about 20 min after the loss of the mMEPs, but the mMEPs were not restored. Possibly, spinal cord hyperemia, induced by selective perfusion of the intercostal vessels, narrowed the subarachnoid space so that CSF could not be satisfactorily drained during surgery. The spinal cord hyperemia may have decreased spinal function and suppressed the leg mMEPs. The persistence of the loss of mMEPs was undeniably due to the influence of the anesthetic agent or a perfusion disorder in the lower-extremity muscles. Of note, moderate spinal cord hypothermia and postoperative CSF drainage probably resulted in improved lower-limb motor function.

Effects of systemic deep hypothermia and subarachnoid block on the longitudinally conducting evoked spinal cord potentials in man

The present study reports the effects of systemic deep hypothermia (SDH) and subarachnoid block (SAB) on the longitudinally conducting evoked spinal cord potential (conducting ESCP) in man. Before induction of anesthesia, a pair of bipolar catheter electrodes was introduced to the epidural space: one at the level of the cervical enlargement and the other at the lumbosacral enlargement. The conducting ESCP was produced by electrical stimulation through the upper electrode and recorded through the lower electrode, and vice versa. SDH Study: Subjects were 6 patients who underwent replacement surgery of an aortic aneurysm with deep hypothermia anesthesia. The peak latency of the ESCP was gradually prolonged and the duration was widened with cooling via extracorporeal circulation. The amplitude of ESCP showed a biphasic change over the course of cooling with a turning point of around 30 degrees C in esophageal temperature. The ESCP was well observed until blood temperatures as low as near 10 degrees C. The result shows that ESCP is available as an intra-operative monitoring parameter of the spinal function even under SDH. SAB Study: Subjects were 7 patients, 6 of whom had SAB and the remaining 1 intravenous application of a local anesthetic. The conducting ESCP was markedly depressed or disappeared completely even after SAB with clinical doses of various local anesthetics, while it was hardly affected by the intravenous application. The result implies that SAB causes, at least partially, the conduction block within the spinal cord.

An alternative for descending thoracic aortic aneurysm repair

A simple technique termed "clamp test and spinal cord-plegia" for descending thoracic aortic aneurysm surgery is described. The clamp test is a passive method of creating ischemic conditions, whereas spinal cord-plegia is an active method of decreasing metabolism. This technique is a practical test that double-checks for critical feeding arteries and seems to have an excellent spinal cord-preserving effect.

Anesthetic Management for Pediatric Spinal Fusion: Implications of Advances in Spinal Cord Monitoring

Currently, the detection of emerging injury through intraoperative neurologic monitoring is the best way to prevent neurologic injury. This requires a team approach that includes the anesthesiologist, neurophysiologist, and surgeon. The monitoring modalities available for the patient must be considered in planning the anesthetic management. In addition, intraoperative care for the patient requires an ongoing attention to how the anesthetic drugs affect spinal cord monitoring.

Influence of electrode impedance on threshold voltage for transcranial electrical stimulation in motor evoked potential monitoring

Motor potentials evoked by transcranial electrical stimulation (TES) are used for monitoring the motor pathways, with emphasis on the spinal cord and brainstem. The stimulus voltage threshold is the voltage below which no motor response can be elicited. It has frequently been used as a monitoring parameter. However, its value can be limited, because it is affected by the impedance of the stimulus electrode. For example, the voltage threshold can change owing to formation of oedema of the scalp. The relationship between the TES voltage threshold and the electrode impedance of different electrode types was studied and discussed in the context of neuromonitoring: 323 impedance and voltage threshold pairs were studied, and TES was performed with disc cup EEG electrodes (six), corkscrew electrodes (type I: seven, type II: eight), multiple EEG needle electrodes (16) and a large needle electrode Cz' (anode) together with a ground strip over the forehead (cathode) (286). The study found the voltage threshold to be strongly dependent on electrode impedance when the impedance was higher than 460 omega (correlation: R2=0.87; p < 0.001). Below 460 omega, which included 91% of the category with the largest electrode surfaces, 25% of the multiple EEG electrodes and 75% of type II corkscrew electrodes, no significant correlation (R2=0.0064; p=0.15) was found. It was concluded that the correlation between the TES voltage threshold and electrode impedance can be markedly reduced by using TES electrodes with large contact surfaces, resulting in limit values for these parameters. This also may improve the reliability of TES motor evoked potential monitoring.

Effects of Anesthetic Agents and Physiologic Changes on Intraoperative Motor Evoked Potentials

Motor evoked potentials (MEPs) have shown promise as a valuable tool for monitoring intraoperative motor tract function and reducing postoperative plegia. MEP monitoring has been reported to contribute to deficit prevention during resection of tumors adjacent to motor structures in the cerebral cortex and spine, and in detecting spinal ischemia during thoracic aortic reconstruction. Many commonly used anesthetic agents have long been known to depress MEP responses and reduce MEP specificity for motor injury detection. Although new stimulation techniques have broadened the spectrum of anesthetics that can be used during MEP monitoring, certain agents continue to have dose-dependent effects on MEP reliability. Understanding the effects of anesthetic agents and physiologic alterations on MEPs is imperative to increasing the acceptance and application of this technique in the prevention of intraoperative motor tract injury. This review is intended as an overview of the effects of anesthetics and physiology on the reproducibility of intraoperative myogenic MEP responses, rather than an analysis of the sensitivity and specificity of this monitoring method in the prevention of motor injury.

The effect of hypothermia on myogenic motor-evoked potentials to electrical stimulation with a single pulse and a train of pulses under propofol/ketamine/fentanyl anesthesia in rabbits

In the present study, we investigated the effect of hypothermia on myogenic motor-evoked potentials (MEPs) in rabbits. The influence of stimulation paradigms to induce MEPs was evaluated. Twelve rabbits anesthetized with ketamine, fentanyl, and propofol were used for the study. Myogenic MEPs in response to electrical stimulation of the motor cortex with a single pulse and a train of three and five pulses were recorded from the soleus muscle. After the control recording of MEPs at 38 degrees C of esophageal temperature, the rabbits were cooled by surface cooling. Esophageal temperature was maintained at 35 degrees C, 32 degrees C, 30 degrees C, and 28 degrees C, and MEPs were recorded at each point. MEP amplitude to single- pulse stimulation was significantly reduced with a re-duction of core temperature to 28 degrees C compared with the control value at 38 degrees C (0.8 +/- 0.4 mV versus 2.3 +/- 0.3 mV; P < 0.05), whereas MEP amplitude to train-pulse stimulation did not change significantly during the cooling. MEP latency was increased linearly with a reduction of core temperature regardless of stimulation paradigms. In conclusion, these results indicate that a reduction of core temperature to 28 degrees C did not influence MEP amplitudes as long as a train of pulses, but not a single pulse, was used for stimulation in rabbits under propofol/ketamine/fentanyl anesthesia.

IMPLICATIONS

Intraoperative monitoring of myogenic motor-evoked potentials (MEPs) may be required under hypothermic conditions because of its neuroprotective efficacy. However, data on the influence of hypothermia on myogenic MEPs are limited. The results indicate that multipulse stimulation may be better than single-pulse stimulation when monitoring MEPs during hypothermia.

Effect of nitrous oxide on myogenic motor evoked potentials during hypothermia in rabbits anaesthetized with ketamine/fentanyl/propofol

BACKGROUND

A number of authors have reported that anaesthetics suppress myogenic motor evoked potentials (MEPs). However, the influence of hypothermia on these effects is unknown. Therefore we investigated the effects of hypothermia on nitrous oxide-induced suppression of myogenic MEPs.

METHODS

Twenty-two rabbits anaesthetized with ketamine, fentanyl and propofol were randomly allocated to one of three groups, with oesophageal temperatures of 40 degrees C (n = 8), 35 degrees C (n = 7) and 30 degrees C (n = 7). Myogenic MEPs in response to electrical stimulation of the motor cortex with a train of five pulses were recorded from the soleus muscle. Following the control recording, nitrous oxide was administered at concentrations of 30%, 50%, and 70% in random order, and MEPs were recorded. Control MEP amplitudes and percentage of control MEP amplitudes (%MEP amplitude) during the administration of nitrous oxide were compared between the three groups.

RESULTS

Control MEP amplitudes were similar between the three groups. Nitrous oxide suppressed MEPs in a dose-dependent manner in all groups. During the administration of nitrous oxide, % MEP amplitudes at 35 degrees C and 30 degrees C (hypothermia) were significantly lower than those at 40 degrees C (normothermia).

CONCLUSION

These results suggest that nitrous oxide-induced suppression of MEPs may be augmented during hypothermia.

Spinal cord monitoring: somatosensory- and motor-evoked potentials

Monitoring myogenic motor EPs after transcranial electrical stimulation is effective in detecting spinal cord ischemia. During thoracoabdominal aortic aneurysm surgery, this technique is sufficiently rapid to allow timely interventions aimed at correcting ischemic conditions and preserving spinal cord blood flow. If strategies are applied to protect the spinal cord during thoracoabdominal aortic aneurysm repair (e.g., distal bypass, cerebrospinal fluid drainage, reattachment of segmental arteries), motor EP monitoring should be included in this protocol to improve neurologic outcome further. Although SSEPs provide information regarding the adequacy of spinal cord blood flow, monitoring SSEPs during thoracoabdominal aortic aneurysm repair has serious limitations. The response time is too slow to be of practical use. SSEPs also do not provide information regarding anterior horn motor function and supply, whereas the motor neurons in the anterior horn are most likely to sustain ischemic injury.