Electrophysiologic intraoperative monitoring for spine procedures

Usefulness of the H-Reflex for Intraoperative Monitoring of Thoracoabdominal Aneurysms

PURPOSE

Intraoperative neurophysiologic monitoring in thoracoabdominal aneurysms (TAAA) is essential to avoid intraoperative spinal cord injury). Motor and somatosensory evoked potentials may be considered intraoperative tools for detecting spinal cord injury. H-reflex is a well-known neurophysiologic technique to evaluate L5-S1 root. Current evidence supports the observation that H-reflex changes may occur with spinal cord damage as high as the cervical level. This study aimed to evaluate the usefulness of the H-reflex in these surgeries.

METHODS

The use of intraoperative H-reflex in TAAA monitoring was evaluated in 12 patients undergoing open or endovascular repair of TAAA for a period of four years (2016-2020) using somatosensory evoked potentials (SSEPs) and transcranial motor evoked potentials (TcMEPs) and bilateral H-reflex.

RESULTS

Six neurophysiologic alarms were recorded in five of the 12 patients. Summarizing the neurophysiologic changes of our series, we considered a peripheral change when we detected a unilateral loss of SSEPs, TcMEPs, and H-reflex. Instead, we assumed a central change when we detected a unilateral or bilateral loss of TcMEPs and H-reflex with normal SSEPs, which we considered a sign of spinal cord ischemia. Interestingly H-reflex always changed significantly in combination with TcMEPs in the same fashion.

CONCLUSIONS

According to our series, H-reflex can detect intraoperative changes with the same sensitivity as TcMEPs in TAAA surgeries. With the support of other techniques, it can be useful to localize the origin of the lesion (peripheral or central spinal cord), to help in surgical decision-making to avoid postoperative neurologic damage. Based on our results, we recommend the systematic use of H-reflex in TAAA surgeries.

Anterolateral S1 screw malposition detected with intraoperative neurophysiological monitoring during posterior lumbosacral fusion

Background:

The standard of care is to utilize intraoperative neurophysiological monitoring (IOM) of triggered electromyography (tEMG) during posterior lumbosacral instrumented-fusion surgery. IOM should theoretically signal misplacement of S1 screws into the neural L5–S1 foramen or spinal canal, utilizing screw stimulation, and recording of the lower limb muscles and the anal sphincter. Here, we evaluated when and whether anterolateral S1 screw malposition could be detected by IOM/tEMG during open posterior lumbosacral instrumented fusion surgery.

Methods:

tEMG, somatosensory-evoked potential (SSEP), and transcranial electrical motor-evoked potential (TcMEP) data were retrospectively reviewed from 2015 to 2017 during open posterior lumbosacral instrumented fusions. We utilized screw stimulation alert thresholds of <14 mA (tEMG) and recorded from the lower extremity muscles and anal sphincter. Furthermore, all patients underwent routine postoperative computed tomography (CT) scans to confirm the screw location.

Results:

There were 106 S1 screws placed in 54 patients: 52 bilateral and 2 unilateral. In 6 patients (11.1%), 7 screws (6.6%) registered at low tEMG thresholds. In 1 patient, the postoperative CT scan documented external malposition of the screw despite no intraoperative IOM/tEMG alert. When S1 misplaced screws were stimulated, the most sensitive muscle was the tibialis anterior; the sensitivity of the IOM/tEMG was 87.5%, the specificity was 97.9%, the positive predictive value was 77.8%, and the negative predictive value was 98.9%. TcMEP and SSEP did not change during any of the operations. Notably, no patient developed a new neurological deficit.

Conclusion:

Anterolateral S1 screw malposition can be detected accurately utilizing IOM/tEMG stimulation of screws. When alerts occur, they can largely be corrected by partially backing out the screw (e.g., a few turns) and/ or changing the screw trajectory.

Intraoperative Neurophysiologic Testing of the Perigastric Vagus Nerve Branches to Evaluate Viability and Signals along Nerve Pathways during Gastrectomy

Purpose

The perigastric vagus nerve may play an important role in preserving function after gastrectomy, and intraoperative neurophysiologic tests might represent a feasible method of evaluating the vagus nerve. The purpose of this study is to assess the feasibility of neurophysiologic evaluations of the function and viability of perigastric vagus nerve branches during gastrectomy.

Materials and Methods

Thirteen patients (1 open total gastrectomy, 1 laparoscopic total gastrectomy, and 11 laparoscopic distal gastrectomy) were prospectively enrolled. The hepatic and celiac branches of the vagus nerve were exposed, and grabbing type stimulation electrodes were applied as follows: 10–30 mA intensity, 4 trains, 1,000 µs/train, and 5× frequency. Visible myocontractile movement and electrical signals were monitored via needle probes before and after gastrectomy. Gastrointestinal symptoms were evaluated preoperatively and postoperatively at 3 weeks and 3 months, respectively.

Results

Responses were observed after stimulating the celiac branch in 10, 9, 10, and 6 patients in the antrum, pylorus, duodenum, and proximal jejunum, respectively. Ten patients responded to hepatic branch stimulation at the duodenum. After vagus-preserving distal gastrectomy, 2 patients lost responses to the celiac branch at the duodenum and jejunum (1 each), and 1 patient lost response to the hepatic branch at the duodenum. Significant procedure-related complications and meaningful postoperative diarrhea were not observed.

Conclusions

Intraoperative neurophysiologic testing seems to be a feasible methodology for monitoring the perigastric vagus nerves. Innervation of the duodenum via the celiac branch and postoperative preservation of the function of the vagus nerves were confirmed in most patients.

Trial Registration

Clinical Research Information Service Identifier: KCT0000823

Intraoperative Neurophysiological Monitoring : A Review of Techniques Used for Brain Tumor Surgery in Children

Intraoperative monitoring (IOM) utilizes electrophysiological techniques as a surrogate test and evaluation of nervous function while a patient is under general anesthesia. They are increasingly used for procedures, both surgical and endovascular, to avoid injury during an operation, examine neurological tissue to guide the surgery, or to test electrophysiological function to allow for more complete resection or corrections. The application of IOM during pediatric brain tumor resections encompasses a unique set of technical issues. First, obtaining stable and reliable responses in children of different ages requires detailed understanding of normal ageadjusted brain-spine development. Neurophysiology, anatomy, and anthropometry of children are different from those of adults. Second, monitoring of the brain may include risk to eloquent functions and cranial nerve functions that are difficult with the usual neurophysiological techniques. Third, interpretation of signal change requires unique sets of normative values specific for children of that age. Fourth, tumor resection involves multiple considerations including defining tumor type, size, location, pathophysiology that might require maximal removal of lesion or minimal intervention. IOM techniques can be divided into monitoring and mapping. Mapping involves identification of specific neural structures to avoid or minimize injury. Monitoring is continuous acquisition of neural signals to determine the integrity of the full longitudinal path of the neural system of interest. Motor evoked potentials and somatosensory evoked potentials are representative methodologies for monitoring. Free-running electromyography is also used to monitor irritation or damage to the motor nerves in the lower motor neuron level : cranial nerves, roots, and peripheral nerves. For the surgery of infratentorial tumors, in addition to free-running electromyography of the bulbar muscles, brainstem auditory evoked potentials or corticobulbar motor evoked potentials could be combined to prevent injury of the cranial nerves or nucleus. IOM for cerebral tumors can adopt direct cortical stimulation or direct subcortical stimulation to map the corticospinal pathways in the vicinity of lesion. IOM is a diagnostic as well as interventional tool for neurosurgery. To prove clinical evidence of it is not simple. Randomized controlled prospective studies may not be possible due to ethical reasons. However, prospective longitudinal studies confirming prognostic value of IOM are available. Furthermore, oncological outcome has also been shown to be superior in some brain tumors, with IOM. New methodologies of IOM are being developed and clinically applied. This review establishes a composite view of techniques used today, noting differences between adult and pediatric monitoring.

Adult Degenerative Lumbar Scoliosis

Adult degenerative lumbar scoliosis is a 3-dimensional deformity defined as a coronal deviation of greater than 10°. It causes significant pain and disability in the elderly. With the aging of the population, the incidence of adult degenerative lumbar scoliosis will continue to increase. During the past decade, advancements in surgical techniques and instrumentation have changed the management of adult spinal deformity and led to improved long-term outcomes. In this article, the authors provide a comprehensive review of the pathophysiology, diagnosis, and management of adult degenerative lumbar scoliosis. [Orthopedics. 2017; 40(6):e930-e939.].

Suprasegmental neurophysiological monitoring with H reflex and TcMEP in spinal surgery. Transient loss due to hypotension. A case report

OBJECTIVE

H-reflex is a well known neurophysiological test used to evaluate sensory afferent and motor efferent impulses of S1 root. Despite its simplicity and feasibility, it is not used very often in the operating room.

METHODS

We report the case of a 16-year-old male patient who undergoes a surgical correction for a severe paralytic scoliosis (160°). On account of previous deficits, intraoperative neurophysiological monitoring was achieved through TcMEP and H-reflex.

RESULTS

Intraoperative neurophysiological monitoring (IONM) showed a transient and simultaneous loss of bilateral TcMEP and H-reflex, coinciding with an abrupt hypotension during pedicle screw placement. After having dismissed mechanical injury and after increasing blood pressure, TcMEP and H-reflex were equivalent to those at baseline.

CONCLUSIONS

The H-reflex is a classic neurophysiological test not used very frequently in the operating room. It is a feasible and reliable technique that can be helpful during spine surgery IONM, especially in patients with preexisting neurological deficits. Although simultaneous TcMEP and H-reflex monitoring has been previously described, to our knowledge, this is the first recorded case of a decline in both associated with abrupt hypotension.

Significant change or loss of intraoperative monitoring data: a 25-year experience in 12,375 spinal surgeries

STUDY DESIGN

Retrospective.

OBJECTIVE

The purpose of this study was to report the spectrum of intraoperative events responsible for a loss or significant change in intraoperative monitoring (IOM) data.

SUMMARY OF BACKGROUND DATA

The efficacy of spinal cord/nerve root monitoring is demonstrated in a large, single institution series of patients, involving all levels of the spinal column (occiput to sacrum) and all spinal surgical procedures.

METHODS

Multimodality IOM included somatosensory-evoked potentials, descending neurogenic-evoked potentials, neurogenic motor-evoked potentials, and spontaneous and triggered electromyography. A total of 12,375 patients who underwent surgery for spinal pathology between January 1985 and December 2010 were reviewed. There were 59.3% female patients (7178) and 40.7% male patients (5197). Procedures by spinal level were as follows: cervical 29.7% (3671), thoracic/thoracolumbar 45.4% (5624), and lumbosacral 24.9% (3080). Age at the time of surgery was as follows: older than 18 years, 72.7% (242/8993) and younger than 18 years, 27.3% (144/3382). A total of 77.8% (9633) patients underwent primary surgical procedures and 22.2% (2742) patients underwent revision surgical procedures.

RESULTS

A total of 406 instances of IOM data change/loss occurred in 386 of 12,375 (3.1%) patients. Causes for data degradation/loss included the following: instrumentation (n = 131), positioning (n = 85), correction (n = 56), systemic (n = 49), unknown (n = 24), and focal spinal cord compression (n = 15). Data loss/change was seen in revision (6.1%/167 patients) surgical procedures more commonly than in primary procedures (2.3%/219 patients; P < 0.0001). Data improvement was demonstrated by 88.7% (n = 360) after intervention versus 11.3% (n = 46) with no improvement in IOM data. One patient with improved data after intervention versus 14 with no improvement despite intervention had a permanent neurological deficit (P < 0.0001).

CONCLUSION

IOM data identified 386 (3.1%) patients with loss/degradation of data in 12,375 spinal surgical procedures. Fortunately, in 93.3% of patients, intervention led to data recovery and no neurological deficits. Reduction from a potential (worst-case scenario) 3.1% (386) of patients with significant change/loss of IOM data to a permanent neurological deficit rate of 0.12% (15) patients was achieved (P < 0.0001), thus confirming efficacy of IOM.

A review of intraoperative monitoring for spinal surgery

BACKGROUND

Intraoperative neurophysiologic monitoring (IONM) is a technique that is helpful for assessing the nervous system during spine surgery.

METHODS

This is a review of the field describing the basic mechanisms behind the techniques of IONM. These include the most often utilized trancranial motor evoked potentials (Tc-MEPs), somatosensory evoked potentials (SSEPs), and stimulated and spontaneous EMG activity. It also describes some of the issues regarding practices and qualifications of practitioners.

RESULTS

Although the anatomic pathways responsible for the Tc-MEP and SSEP are well known and these clinical techniques have a high sensitivity and specificity, there is little published data showing that monitoring actually leads to improved patient outcomes. It is evident that IONM has high utility when the risk of injury is high, but may be only marginally helpful when the risk of injury is very low. The monitoring team must be well trained, be able to provide the surgeon feedback in real time, and coordinate activities with those of the surgical and anesthesia teams.

CONCLUSIONS

Although IONM is a valuable technique that provides sensitive and specific indications of neurologic injury, it does have limitations that must be understood. Maintaining a high quality of practice with appropriately trained personnel is critical.

Intraoperative electrophysiological monitoring in spine surgery

STUDY DESIGN

Review of the literature with analysis of pooled data.

OBJECTIVE

To assess common intraoperative neuromonitoring (IOM) changes that occur during the course of spinal surgery, potential causes of change, and determine appropriate responses. Further, there will be discussion of appropriate application of IOM, and medical legal aspects. The structured literature review will answer the following questions: What are the various IOM methods currently available for spinal surgery? What are the sensitivities and specificities of each modality for neural element injury? How are the changes in each modality best interpreted? What is the appropriate response to indicated changes? Recommendations will be made as to the interpretation and appropriate response to IOM changes.

SUMMARY OF BACKGROUND DATA

Total number of abstracts identified and reviewed was 187. Full review was performed on 18 articles.

METHODS

The MEDLINE database was queried using the search terms IOM, spinal surgery, SSEP, wake-up test, MEP, spontaneous and triggered electromyography alone and in various combinations. Abstracts were identified and reviewed. Individual case reports were excluded. Detailed information and data from appropriate articles were assessed and compiled.

RESULTS

Ability to achieve IOM baseline data varied from 70% to 98% for somatosensory-evoked potentials (SSEP) and 66% to 100% for motor-evoked potentials (MEP) in absence of neural axis abnormality. Multimodality intraoperative neuromonitoring (MIOM) provided false negatives in 0% to 0.79% of cases, whereas isolated SSEP monitoring alone provided false negative in 0.063% to 2.7% of cases. MIOM provided false positive warning in 0.6% to 1.38% of cases.

CONCLUSION

As spine surgery, and patient comorbidity, becomes increasingly complex, IOM permits more aggressive deformity correction and tumor resection. Combination of SSEP and MEP monitoring provides assessment of entire spinal cord functionality in real time. Spontaneous and triggered electromyography add assessment of nerve roots. The wake-up test can continue to serve as a supplement when needed. MIOM may prove useful in preservation of neurologic function where an alteration of approach is possible. IOM is a valuable tool for optimization of outcome in complex spinal surgery.

Upper-limb somatosensory evoked potential monitoring in lumbosacral spine surgery: a prognostic marker for position-related ulnar nerve injury

BACKGROUND CONTEXT

Somatosensory evoked potential (SSEP) is used to monitor integrity of the brain, spinal cord, and nerve roots during spinal surgery. It records the electrical potentials from the scalp after electrical stimulation of the peripheral nerves of the upper or lower limbs. The standard monitoring modality in lumbosacral spine surgery includes lower-limb SSEP and electromyography (EMG). Upper-limb SSEP monitoring has also been used to detect and prevent brachial plexopathy and peripheral nerve injury in thoracic and lumbosacral spine surgeries. We routinely monitor lower-limb SSEP and EMG in lumbosacral spine procedures at our institution. However, a few patients experienced postoperative numbness and/or pain in their ulnar distribution with uneventful lower-limb SSEP and EMG.

PURPOSE

We hypothesized that the postoperative upper extremity paresis in lumbosacral surgeries may result from compression and/or stretch of the brachial plexus and/or ulnar nerve while the patients were in prone position. Using upper-limb SSEP, we investigated whether we observe any significant change in the SSEP, and if so, whether we can prevent or reduce frequency of postoperative upper extremity deficits.

STUDY DESIGN/SETTING

In this prospective study, we monitored upper-limb SSEP, in addition to lower-limb SSEP and EMG, in 230 elective, posterior lumbosacral spinal procedures. All operations were performed by a group of four neurosurgeons.

PATIENT SAMPLE

One hundred and thirty-one female and 99 male with an age range of 28 to 86 years between January 2004 and December 2005 were studied.

OUTCOME MEASURES

Amplitude and latency of upper-limb or ulnar SSEP were continuously compared with those of the baseline. A greater than or equal to 50% decrease in SSEPs amplitude and/or a greater than or equal to 10% increase in latency were considered to be significant.

METHODS

After intubation, patients were positioned prone on Jackson or Andrews spinal table. Anesthesia was maintained with inhalant gas (desflurane or sevoflurane) and propofol infusion with and without minimal infusion of narcotics (fentanyl, sufentanyl, or remifentanil). Intraoperative neurophysiologic monitoring of upper-limb or ulnar SSEP was achieved by continuously recording cortical and subcortical responses after alternate stimulation of the ulnar nerve at the wrist. In our institutional protocol, a greater than or equal to 50% decrease in SSEPs amplitude and/or a greater than or equal to 10% increase in latency were considered to be significant to alert the operating surgeons. When significant changes occurred, the surgeon was immediately notified. Also, reevaluation of vital signs, depth of anesthesia, and patient's position, and technical troubleshootings were subsequently followed.

RESULTS

We observed a greater than or equal to 50% decrease in amplitude of ulnar SSEP in 10 patients without significant changes in lower-limb SSEP (peroneal or posterior tibial nerve SSEP) or EMG during surgery. Eight patients had changes in unilateral limbs, and two patients had changes in bilateral limbs. Two patients with significant changes in unilateral limbs showed changes twice. The mean SSEP amplitude for the 14 changes was 29.2+/-3.1% (mean+/-SEM, standard error of mean) of the baseline value at the average surgical time of 60+/-1.5 minutes. With repositioning of the arms, the amplitudes were immediately restored with the average of 70.2+/-7.1% (n=14) of the baseline value. The mean amplitude of upper-limb SSEP was 73.4+/-8.7% (n=12) of the baseline at wound closure. The average surgical time was 154+/-29.2 minutes per case for the 10 patients. There was no documented postoperative upper extremity paresis in all 230 patients.

CONCLUSIONS

The present study demonstrates that upper-limb SSEP monitoring could detect position-related ulnar neuropathy in 5.2% of the patients undergoing lumbosacral spine surgery.

Single trial somatosensory evoked potential extraction with ARX filtering for a combined spinal cord intraoperative neuromonitoring technique

Background

When spinal cord functional integrity is at risk during surgery, intraoperative neuromonitoring is recommended. Tibial Single Trial Somatosensory Evoked Potentials (SEPs) and H-reflex are here used in a combined neuromonitoring method: both signals monitor the spinal cord status, though involving different nervous pathways. However, SEPs express a trial-to-trial variability that is difficult to track because of the intrinsic low signal-to-noise ratio. For this reason single trial techniques are needed to extract SEPs from the background EEG.

Methods

The analysis is performed off line on data recorded in eight scoliosis surgery sessions during which the spinal cord was simultaneously monitored through classical SEPs and H-reflex responses elicited by the same tibial nerve electrical stimulation. The single trial extraction of SEPs from the background EEG is here performed through AutoRegressive filter with eXogenous input (ARX). The electroencephalographic recording can be modeled as the sum of the background EEG, which can be described as an autoregressive process not related to the stimulus, and the evoked potential (EP), which can be viewed as a filtered version of a reference signal related to the stimulus. The choice of the filter optimal orders is based on the Akaike Information Criterion (AIC). The reference signal used as exogenous input in the ARX model is a weighted average of the previous SEPs trials with exponential forgetting behavior.

Results

The moving average exponentially weighted, used as reference signal for the ARX model, shows a better sensibility than the standard moving average in tracking SEPs fast inter-trial changes. The ability to promptly detect changes allows highlighting relations between waveform changes and surgical maneuvers. It also allows a comparative study with H-reflex trends: in particular, the two signals show different fall and recovery dynamics following stressful conditions for the spinal cord.

Conclusion

The ARX filter showed good performances in single trial SEP extraction, enhancing the available information concerning the current spinal cord status. Moreover, the comparison between SEPs and H-reflex showed that the two signals are affected by the same surgical maneuvers, even if they monitor the spinal cord through anatomically different pathways.

Medical complications of surgical treatment of adult spinal deformity and how to avoid them

STUDY DESIGN

Review article of medical complications related to adult spinal deformity surgery.

OBJECTIVE

To identify medical complications related to surgery for adult spinal deformity and suggest ways to minimize their occurrence and to avoid them.

SUMMARY OF BACKGROUND DATA

Medical complications are a major consideration in adult spinal deformity surgery. Few studies have been done to identify the medical complication rate in relation to these procedures.

METHODS

We review the literature pertaining to medical complications regarding spinal deformity surgery.

RESULTS

Urinary tract infections are the most frequently seen complication. Additionally, pulmonary complications are the most common life-threatening complication. Medical complications are a frequent occurrence with adult deformity spinal surgery.

CONCLUSIONS

Awareness of the presentation, treatment, and prevention of medical complications of deformity surgery may allow minimization of their occurrence and optimize treatment should they occur.

Intraoperative Applications Of The H-Reflex And F-Response: A Tutorial

Traditional intraoperative monitoring of spinal cord function involves the use of three techniques: 1. Orthodromic ascending somatosensory evoked potentials (SSEPs) and 2. antIDromic descending neurogenic somatosensory evoked potentials (DNSSEPs) monitor long-tract sensory function. SSEPs and DNSSEPs do not monitor interneuronal gray matter function. 3. Transcranial motor evoked potentials (TMEPs) monitor descending long-tract motor function and measure interneuronal gray matter function by activating motor neurons. TMEPs activate from 4-5% of the motor neuron pool. When using TMEPs 95-96% of the motor spinal cord systems activating the motor neurons are not monitored. Our ability to interact with our environment involves not only intact sensation and strength, but also complex coordinated motor behavior. Complex coordinated motor behavior is controlled by groups of electrically-coupled spinal cord central pattern generators (CPGs). The components of CPGs are: descending and propriospinal systems, peripheral input, and segmental interneurons. The point-of-control is the level of excitation of interneurons, which is determined by the integrated activity of the other components. Spinal cord injury (SCI) changes segmental reflex gain by uncoupling these components. Changes in gain are detected by recordings from muscles. SSEPs, DNSSEPs and TMEPs provIDe limited information about the status of CPGs. H-reflexes measure the function of from 20-100% of the motor neuron pool. F-responses measure the function of from 1-5% of the motor neuron pool. H-reflexes and F-responses provIDe information about the degree of coupling between CPG components. Recording H-reflexes and F-responses together with SSEPs and TMEPs not only monitors spinal cord long-tract function, but also provIDes a multiple-systems approach that monitors those spinal cord systems that are responsible for the control of complex coordinated motor behavior. The objective of this paper is to describe how H-reflexes and F-responses can be used to monitor complex coordinated motor behavior.

Interventional neurophysiologic monitoring

PURPOSE OF REVIEW

Intraoperative neurophysiologic monitoring provides useful information on the functional status of the nervous system. This review focuses on recently published data concerning the impact of monitoring on patient outcome.

RECENT FINDINGS

There is level I evidence to support the use of bispectral index monitoring to prevent awareness during anesthesia in high-risk patients. A number of randomized trials have shown that monitoring-guided anesthesia using the bispectral index or other devices will expedite recovery and improve perioperative drug utilization. There are also preliminary reports suggesting that anesthesia dictated by bispectral index monitoring may alter long-term outcome and reduce mortality. In surgical procedures, however, it is less clear whether neurophysiologic monitoring will improve patient outcome. Currently, the majority of data are derived from respective case series. Nonetheless, monitoring with somatosensory evoked potential has been shown to reduce postoperative neurologic deficits after spinal surgery. There is also evidence to suggest that electromyography and motor evoked potential are essential complements to somatosensory evoked potential for monitoring of spinal cord surgery.

SUMMARY

Brain monitoring facilitates anesthetic drug administration. An increasing number of neurosurgical procedures will require some form of intraoperative neurophysiologic monitoring to achieve higher degrees of safety and accuracy. In many instances, the data derived from monitoring will guide and influence surgical decisions. In this context, neurophysiologic monitoring should be regarded as interventional.

The effects of isoflurane and propofol on intraoperative neurophysiological monitoring during spinal surgery

OBJECTIVES

To compare the effects of isoflurane and propofol on intraoperative neurophysiological monitoring (IONM) during spinal surgery.

METHODS

Thirty-five patients were randomly assigned to receive isoflurane (n = 17) or propofol (n = 18) anesthesia. Somatosensory evoked potentials (SEPs) following posterior tibial nerve stimulation were recorded before induction as baselines. Isoflurane concentrations and propofol infusions were adjusted to obtain four pre-determined BIS ranges: 65-55, 55-45, 45-35 and 35-25. For each range, a stable state was maintained for at least 10 min to perform IONM. The SEP latency P40 and amplitude P40-N50, the onset latency and amplitude of transcranial motor evoked potentials (tcMEPs), and threshold intensity of triggered electromyographic activity (EMG) following pedicle screw stimulation were statistically analyzed.

RESULTS

Compared with baseline values, P40 latency increased and P40-N50 amplitude decreased after anesthesia with isoflurane or propofol. Isoflurane caused a dose-dependent depression of SEPs, but propofol did not. TcMEPs were recordable and stable in all patients receiving propofol in each BIS range, but only recordable in 10 (58.8%) receiving isoflurane with BIS >55, and 3 (17.8%) with BIS <55. No difference was noted in triggered EMG.

CONCLUSIONS

Isoflurane inhibited IONM more than propofol. Propofol is recommended for critical spinal surgery, particularly when motor pathway function is monitored.