Decreased Electroencephalographic Alpha Power During Anesthesia Induction Is Associated With EEG Discontinuity in Human Infants.

BACKGROUND: Electroencephalogram (EEG) discontinuity can occur at high concentrations of anesthetic drugs, reflecting suppression of electrocortical activity. This EEG pattern has been reported in children and reflects a deep state of anesthesia. Isoelectric events on the EEG, a more extreme degree of voltage suppression, have been shown to be associated with worse long-term neurologic outcomes in neonates undergoing cardiac surgery. However, the clinical significance of EEG discontinuities during pediatric anesthesia for noncardiac surgery is not yet known and merits further research. In this study, we assessed the incidence of EEG discontinuity during anesthesia induction in neurologically normal infants and the clinical factors associated with its development. We hypothesized that EEG discontinuity would be associated with sevoflurane-induced alpha (8-12 Hz) power during the period of anesthesia induction in infants. METHODS: We prospectively recorded 26 channels of EEG during anesthesia induction in an observational cohort of 54 infants (median age, 7.6 months; interquartile range [IQR] [4.9-9.8 months]). We identified EEG discontinuity, defined as voltage amplitude <25 microvolts for >2 seconds, and assessed its association with sevoflurane-induced alpha power using spectral analysis and multivariable logistic regression adjusting for clinically important variables. RESULTS: EEG discontinuity was observed in 20 of 54 subjects (37%), with a total of 25 discrete events. Sevoflurane-induced alpha power in the posterior regions of the head (eg, parietal or occipital regions) was significantly lower in the EEG discontinuity group (midline parietal channel on the electroencephalogram, International 10-20 System [Pz]; 8.3 vs 11.2 decibels [dBs]; P = .004), and this association remained after multivariable adjustment (adjusted odds ratio [aOR] = 0.51 per dB increase in alpha power [95% CI, 0.30-0.89]; P = .02). There were no differences in the baseline (unanesthetized) EEG between groups in alpha power or power in any other frequency band. CONCLUSIONS: We demonstrate that EEG discontinuity is common during anesthesia induction and is related to the level of sevoflurane-induced posterior alpha power, a putative marker of cortical-thalamic circuit development in the first year of life. This association persisted even after adjusting for age and propofol coadministration. The fact that this difference was only observed during anesthesia and not in the baseline EEG suggests that otherwise hidden brain circuit properties are unmasked by general anesthesia. These neurophysiologic markers observed during anesthesia may be useful in identifying patients who may have a greater chance of developing discontinuity.

Seizure control, stress, and access to care during the COVID-19 pandemic in New York City: The patient perspective.

OBJECTIVE: Our epilepsy population recently experienced the acute effects of the COVID-19 pandemic in New York City. Herein, we aimed to determine patient-perceived seizure control during the surge, specific variables associated with worsened seizures, the prevalence of specific barriers to care, and patient-perceived efficacy of epilepsy care delivered via telephone and live video visits during the pandemic. METHODS: We performed a cross-sectional questionnaire study of adult epilepsy patients who had a scheduled appointment at a single urban Comprehensive Epilepsy Center (Montefiore Medical Center) between March 1, 2020 and May 31, 2020 during the peak of the COVID-19 pandemic in the Bronx. Subjects able to answer the questionnaire themselves in English or Spanish were eligible to complete a one-time survey via telephone or secure online platform (REDCap). RESULTS: Of 1212 subjects screened, 675 were eligible, and 177 adequately completed the questionnaire. During the COVID-19 pandemic, 75.1% of patients reported no change in seizure control, whereas 17.5% reported that their seizure control had worsened, and 7.3% reported improvement. Subjects who reported worsened seizure control had more frequent seizures at baseline, were more likely to identify stress and headaches/migraines as their typical seizure precipitants, and were significantly more likely to report increased stress related to the pandemic. Subjects with confirmed or suspected COVID-19 did not report worsened seizure control. Nearly 17% of subjects reported poorer epilepsy care, and 9.6% had difficulty obtaining their antiseizure medications; these subjects were significantly more likely to report worse seizure control. SIGNIFICANCE: Of the nearly 20% of subjects who reported worsened seizure control during the COVID-19 pandemic, stress and barriers to care appear to have posed the greatest challenge. This unprecedented pandemic exacerbated existing and created new barriers to epilepsy care, which must be addressed.

Electroencephalographic assessment of infant spinal anesthesia: A pilot prospective observational study.

INTRODUCTION: Spinal anesthesia is utilized as an alternative to general anesthesia in infants for some surgeries. After spinal anesthesia, infants often become less conscious without administration of sedative medications. The aim of this study was to assess electroencephalographic (EEG) correlates after spinal anesthesia in a cohort of infants. PATIENTS AND METHODS: This pilot study included 12 infants who underwent spinal anesthesia. Unprocessed electroencephalography was recorded. The electroencephalogram was interpreted by four neurologists. Processed analyses compared electroencephalogram changes 30 min after spinal anesthesia to baseline. RESULTS: Following spinal anesthesia, all 12 infants became sedated. Electroencephalography in all 12 demonstrated Stage 2 sleep with the appearance of sleep spindles (12-14 Hz) in the frontal and central leads in 8/12 (67%) of subjects. The median time to onset of sleep spindles was 24.7 interquartile range (21.2, 29.9) min. The duration of sleep spindles was 25.1 interquartile range (5.8, 99.8) min. Voltage attenuation and background slowing were the most common initial changes. Compared to baseline, the electroencephalogram 30 min after spinal anesthesia showed significantly increased absolute delta power (p = 0.02) and gamma power (p < 0.0001); decreases in beta (p = 0.0006) and higher beta (p < 0.0001) were also observed. The Fast Fourier Transform power ratio difference for delta/beta was increased (p = 0.03). Increased coherence was noted in the delta (p = 0.02) and theta (p = 0.04) bandwidths. DISCUSSION: Spinal anesthesia in infants is associated with increased electroencephalographic slow wave activity and decreased beta activity compared to the awake state, with appearance of sleep spindles suggestive of normal sleep. The etiology and significance of the observed voltage attenuation and background slowing remains unclear. CONCLUSIONS: The EEG signature of infant spinal anesthesia is distinct from that seen with general anesthesia and is consistent with normal sleep. Further investigation is required to better understand the etiology of these findings. Our preliminary findings contribute to the understanding of the brain effects of spinal anesthesia in early development.

Electroencephalographic Findings and Clinical Behavior During Induction of Anesthesia With Sevoflurane in Human Infants: A Prospective Observational Study.

Seizures and interictal epileptiform discharges (IEDs) have been described during sevoflurane. We prospectively estimated their incidence in 54 otherwise neurologically healthy infants by obtaining the full-head video electroencephalogram (EEG). No infants had clinical seizures, but 1 had an electrographic seizure; 3 others had focal IEDs (7.4%; 95% confidence interval [CI], 2.1%-17.9%). We detected no differences in demographic or clinical characteristics between normal and abnormal EEG groups. Diffuse slowing was the most common initial EEG change followed by fast (α, ß) activity in all head leads. Larger studies with more statistical power are needed to further investigate the hypotheses generated with this research.

Termination of seizure clusters is related to the duration of focal seizures.

OBJECTIVE: Clustered seizures are characterized by shorter than usual interseizure intervals and pose increased morbidity risk. This study examines the characteristics of seizures that cluster, with special attention to the final seizure in a cluster. METHODS: This is a retrospective analysis of long-term inpatient monitoring data from the EPILEPSIAE project. Patients underwent presurgical evaluation from 2002 to 2009. Seizure clusters were defined by the occurrence of at least two consecutive seizures with interseizure intervals of <4 h. Other definitions of seizure clustering were examined in a sensitivity analysis. Seizures were classified into three contextually defined groups: isolated seizures (not meeting clustering criteria), terminal seizure (last seizure in a cluster), and intracluster seizures (any other seizures within a cluster). Seizure characteristics were compared among the three groups in terms of duration, type (focal seizures remaining restricted to one hemisphere vs. evolving bilaterally), seizure origin, and localization concordance among pairs of consecutive seizures. RESULTS: Among 92 subjects, 77 (83%) had at least one seizure cluster. The intracluster seizures were significantly shorter than the last seizure in a cluster (p = 0.011), whereas the last seizure in a cluster resembled the isolated seizures in terms of duration. Although focal only (unilateral), seizures were shorter than seizures that evolved bilaterally and there was no correlation between the seizure type and the seizure position in relation to a cluster (p = 0.762). Frontal and temporal lobe seizures were more likely to cluster compared with other localizations (p = 0.009). Seizure pairs that are part of a cluster were more likely to have a concordant origin than were isolated seizures. Results were similar for the 2 h definition of clustering, but not for the 8 h definition of clustering. SIGNIFICANCE: We demonstrated that intracluster seizures are short relative to isolated seizures and terminal seizures. Frontal and temporal lobe seizures are more likely to cluster.

Monitoring surgery around the cranial nerves.

Intraoperative neurophysiologic monitoring (IONM) of cranial nerve (CN) function is an essential component in multimodality monitoring of surgical procedures where CNs are at risk for injury. In most cases, IONM consists of localizing and mapping CNs and their pathways, and monitoring of CN motor function during surgery. However, CN VIII, which has no motor function, and is at risk for injury in many surgical procedures, can be easily and accurately monitored using brainstem auditory evoked potentials. For motor CNs, the literature is clear that function can be safely and adequately performed using basic electromyographic (EMG) techniques, such as recording of continuous EMG activity and electrically evoked compound muscle actions potentials. Newer techniques, such as corticobulbar motor evoked potentials and reflex studies, show good potential for a greater degree of functional assessment but require further study to determine their clinical utility. EMG remains the basic clinical neurophysiologic technique with the greatest clinical research supporting its utility in IONM of motor CN function and should be used as part of a comprehensive multimodality IONM protocol. Understanding the physiologic basis of EMG and the changes associated with altered motor function will allow the practitioner to alter surgical course to prevent injury and improve patient safety.

Factors associated with patients not proceeding with proposed resective epilepsy surgery.

BACKGROUND: This study evaluated the association between eligible patients not proceeding with resective epilepsy surgery and various demographic, disease-specific, and epilepsy-evaluation variables. METHODS: This retrospective case-control study included patients identified as candidates for resective epilepsy surgery at the Montefiore Medical Center between January 1, 2009 and June 30, 2017. Chi-squared, two-tailed, independent sample t-test, Mann-Whitney U test and logistic regression were utilized to identify variables associated with patients not proceeding with surgery. RESULTS: Among the 159 potential surgical candidates reviewed over the 8.5-year study period, only 53 ultimately proceeded with surgery (33%). Eighty-seven (55%) out of these 159 patients were identified as appropriate for resective epilepsy surgery during the study period. Thirty-four (39%) of these 87 patients did not proceed with surgery. Variables independently correlated (either positively or negatively) with the patient not proceeding with surgery were: being employed [Odds Ratio (OR) 4.2, 95% confidence interval (CI) 1.12-15.73], temporal lobe lesion on MRI (OR 0.35, 95% CI 0.14-0.84), temporal lobe EEG ictal onsets (OR 0.21, 95% CI 0.07-0.62), and temporal lobe epileptogenic zone (OR 0.19, 95% CI 0.07-0.55). CONCLUSION: The novel finding in this study is the association between employment status and whether the patient had epilepsy surgery: employed patients were 4.2 times more likely to not proceed with surgery compared to unemployed patients. In addition, patients with a temporal lobe lesion on MRI, temporal lobe EEG ictal onsets, and/or a temporal epileptogenic zone were more likely to proceed with surgery. Future work will be needed to evaluate these findings prospectively, determine if they generalize to other patient populations, explore the decision whether or not to proceed with epilepsy surgery from a patient-centered perspective, and suggest strategies to reduce barriers to this underutilized treatment.

Use of an extracorporeal membrane oxygenation circuit as a bridge to salvage a major upper-extremity replant in a critically ill patient.

Major replantation of the upper extremity is defined as replantation at or above the level of the wrist. Selection of appropriate candidates is complex and requires consideration of many patient- and injury-associated factors including patient age, associated injuries, patient desire, mechanism of injury, ischemia time, wound condition, and presence of multiple-level injury. With respect to age, younger patients, especially children, are deemed to have a distinct advantage over more elderly patients due to improved nerve regeneration, and many advocate making every effort to replant this population. The risks of major upper-extremity replantation are significant and include bleeding, depletion of coagulation factors, secondary infection, and sepsis. As a result, major systemic illness and significant associated injuries are accepted as contraindications to limb salvage in this patient population. Herein we describe the use of an extracorporeal membrane oxygenation (ECMO) circuit as a potential bridge for short-term preservation of the extremity in a young patient with an acute, concomitant systemic illness. In the authors' opinion, use of ECMO perfusion is a viable means of maintaining extremity perfusion over hours or even days and may lead to broadened replant criteria in patients with associated injuries.

EEG findings in acutely ill patients investigated for SARS-CoV-2/COVID-19: A small case series preliminary report.

OBJECTIVE: Acute encephalopathy may occur in COVID-19-infected patients. We investigated whether medically indicated EEGs performed in acutely ill patients under investigation (PUIs) for COVID-19 report epileptiform abnormalities and whether these are more prevalent in COVID-19 positive than negative patients. METHODS: In this retrospective case series, adult COVID-19 inpatient PUIs underwent EEGs for acute encephalopathy and/or seizure-like events. PUIs had 8-channel headband EEGs (Ceribell; 20 COVID-19 positive, 6 COVID-19 negative); 2 more COVID-19 patients had routine EEGs. Overall, 26 Ceribell EEGs, 4 routine and 7 continuous EEG studies were reviewed. EEGs were interpreted by board-certified clinical neurophysiologists (n = 16). EEG findings were correlated with demographic data, clinical presentation and history, and medication usage. Fisher's exact test was used. RESULTS: We included 28 COVID-19 PUIs (30-83 years old), of whom 22 tested positive (63.6% males) and 6 tested negative (33.3% male). The most common indications for EEG, among COVID-19-positive vs COVID-19-negative patients, respectively, were new onset encephalopathy (68.2% vs 33.3%) and seizure-like events (14/22, 63.6%; 2/6, 33.3%), even among patients without prior history of seizures (11/17, 64.7%; 2/6, 33.3%). Sporadic epileptiform discharges (EDs) were present in 40.9% of COVID-19-positive and 16.7% of COVID-19-negative patients; frontal sharp waves were reported in 8/9 (88.9%) of COVID-19-positive patients with EDs and in 1/1 of COVID-19-negative patient with EDs. No electrographic seizures were captured, but 19/22 COVID-19-positive and 6/6 COVID-19-negative patients were given antiseizure medications and/or sedatives before the EEG. SIGNIFICANCE: This is the first preliminary report of EDs in the EEG of acutely ill COVID-19-positive patients with encephalopathy or suspected clinical seizures. EDs are relatively common in this cohort and typically appear as frontal sharp waves. Further studies are needed to confirm these findings and evaluate the potential direct or indirect effects of COVID-19 on activating epileptic activity.

Electrophysiology of Cranial Nerve Testing: Auditory Nerve.

The electrocochleogram and brainstem auditory evoked potentials (BAEPs) are electrophysiologic signals used to assess the auditory nerve. The electrocohleogram includes the cochlear microphonic, the cochlear summating potential, and the eighth nerve compound action potential. It is used predominantly for hearing assessment and for diagnosis of Ménière disease and auditory neuropathy. Brainstem auditory evoked potentials are used for hearing assessment, diagnosis of dysfunction within the cochlea, the auditory nerve, and the brainstem auditory pathways up to the level of the mesencephalon, and intraoperative monitoring of these structures. The earliest BAEP component, wave I, and the eighth nerve compound action potential reflect the same process-the initial depolarization in the distal auditory nerve. Brainstem auditory evoked potential wave II receives contributions from the region of the cochlear nucleus and from the second depolarization in the distal auditory nerve. Wave III and later components are entirely generated rostral to the auditory nerve. Interpretation of BAEP studies is based on waves I, III, and V; auditory nerve dysfunction is manifested as prolongation of the I-III interpeak interval or absence of waves III and V. Eighth nerve tumors can cause a variety of BAEP abnormalities depending on which structures they affect. Adverse intraoperative BAEP changes can have many etiologies, including direct mechanical or thermal injury of tissue, ischemia (including cochlear ischemia or infarction due to compromise of the internal auditory artery), eighth nerve stretch, systemic or localized hypothermia, and artifactual BAEP changes due to technical factors.

Burst Suppression Pattern on Electroencephalogram Secondary to Valproic Acid-Induced Hyperammonemic Encephalopathy.

BACKGROUND: Valproic acid may induce hyperammonemic encephalopathy. Various electroencephalogram (EEG) abnormalities have been documented in association with this condition, but not burst suppression, an abnormal EEG pattern that is associated with severe encephalopathy. METHODS: Serial EEGs, clinical observations, and laboratory findings were analyzed. PATIENT DESCRIPTION: This 13-year-old girl with autism and intractable epilepsy experienced increased seizures; her valproic acid dose was increased and other antiepileptic drugs were administered. She became lethargic, and her EEG showed a burst suppression pattern. Her ammonia concentration was increased to 101 µmol/L and her valproic acid level was increased to 269.9 mg/L. Valproic acid was discontinued and carnitine was administered. Subsequently she became more alert, her ammonia concentration decreased, and her EEG changed from a burst suppression pattern to a continuous pattern. Within three days, she was back to her baseline level of functioning. CONCLUSIONS: Valproic acid-induced hyperammonemic encephalopathy can produce a burst suppression EEG patternin the patient's.

Clinical outcomes in patients with generalized periodic discharges.

PURPOSE: Generalized periodic discharges (GPDs) are frequently identified in the EEGs of hospitalized patients but their prognostic significance remains unclear. We retrospectively reviewed clinical data in patients with GPDs to elucidate factors associated with in-hospital mortality. METHOD: We reviewed data from inpatients at three different hospitals affiliated with our institution in whom GPDs were reported on routine EEGs by fellowship-trained electroencephalographers during the years 2010-2012. Cox regression was used to determine statistical association between in-hospital death and demographics, medical comorbidities, neurological and neuroimaging abnormalities and antiepileptic drug use. RESULTS: We identified 113 patients with GPDs. The mean age was 70.4 years and 70 (61.9%) were women. There were 60 inpatient deaths (53.1%). The variables significantly associated with in-hospital mortality were dementia, poor mental status at the time of the EEG, chronic focal abnormalities on neuroimaging, cardiac arrest and chronic obstructive pulmonary disease (COPD). CONCLUSION: Dementia, poor mental status during EEG, chronic focal abnormalities on neuroimaging, cardiac arrest and COPD are independently associated with increased in-hospital mortality in patients with GPDs (P<0.05).

Intramedullary spindle cell hemangioma: case report.

The authors describe the case of a 48-year-old man found to have the first reported intramedullary spinal cord spindle cell hemangioma. Previous research indicates that spindle cell hemangiomas are rarely found in the spine. Only 3 previous cases exist, all in the intradural, extramedullary space. In the present case, gross-total resection of the tumor was possible with no loss of function from baseline. This report presents the successful resection of the first reported intramedullary spindle cell hemangioma and reports 4-month follow-up, demonstrating the biological behavior of this rare tumor.

Somatosensory evoked potential monitoring detection of carotid compression during ACDF surgery in a patient with a vascularly isolated hemisphere.

The authors report herein a case of anterior cervical discectomy and fusion (ACDF) surgery in which findings on somatosensory evoked potential (SSEP) monitoring led to the correction of carotid artery compression in a patient with a vascularly isolated hemisphere (no significant collateral blood vessels to the carotid artery territory). The amplitude of the cortical SSEP component to left ulnar nerve stimulation progressively decreased in multiple runs, but there were no changes in the cervicomedullary SSEP component to the same stimulus. When the lateral (right-sided) retractor was removed, the cortical SSEP component returned to baseline. The retraction was then intermittently relaxed during the rest of the operation, and the patient suffered no neurological morbidity. Magnetic resonance angiography demonstrated a vascularly isolated right hemisphere. During anterior cervical spine surgery, carotid artery compression by the retractor can cause hemispheric ischemia and infarction in patients with inadequate collateral circulation. The primary purpose of SSEP monitoring during ACDF surgery is to detect compromise of the dorsal column somatosensory pathways within the cervical spinal cord, but intraoperative SSEP monitoring can also detect hemispheric ischemia. Concurrent recording of cervicomedullary SSEPs can help differentiate cortical SSEP changes due to hemispheric ischemia from those due to compromise of the dorsal column pathways. If there are adverse changes in the cortical SSEPs but no changes in the cervicomedullary SSEPs, the possibility of hemispheric ischemia due to carotid artery compression by the retractor should be considered.

Triggered EMG Potentials in Determining Neuroanatomical Safe Zone for Transpsoas Lumbar Approach: Are They Reliable?

STUDY DESIGN: In vivo analysis in swine model. OBJECTIVE: The purpose of this study was to determine the accuracy of triggered EMG (t-EMG) and its reliability in lateral lumbar interbody fusions surgery. We also aim to document changes in psoas muscle produced during the approach. SUMMARY OF BACKGROUND DATA: Lateral lumbar interbody fusions is preferred over direct anterior approach because of lower complications, blood loss, and shorter recovery time. Threshold-EMGs are utilized for real-time feedback about nerve location; however, neurological deficits are widely reported, and are unique to this approach. Multiple factors have been hypothesized including neuropraxia from retractors and compression from psoas hematoma/edema. The variable reports of neurological complication even with t-EMGs indicate the need to study them further. METHODS: Eight swines underwent left-sided retroperitoneal approach. The nerve on the surface of the psoas was identified and threshold-EMGs were obtained utilizing a ball-tip, and needle probe. First EMG and threshold responses required to elicit 20-µV responses were recorded for 2 mm incremental distances up to 10 mm. In the second part, a K-wire was inserted into the mid-lumbar disc space, and a tubular retractor docked and dilated adequately. Postmortem CT scans were carried out to evaluate changes in psoas muscle. RESULTS: A t-EMG stimulus threshold of <5 mA indicates a higher probability that the probe is close to or on the nerve, but this was not proportional to the distance suggesting limitations for nerve mapping. Negative predictive value of t-EMGs is 76.5% with the ball-tipped probe and 80% with the needle probe for t-EMG ≥10 mA and indicates that even with higher thresholds, the nerve may be much closer than anticipated. Postoperative hematoma was not seen on CT scans. CONCLUSION: Threshold measurements are unreliable in estimating distance from the nerve in an individual subject and higher values do not always correspond to a 'safe zone." LEVEL OF EVIDENCE: 5.

Electrophysiologic auditory tests.

Auditory stimulation elicits a complex series of electric signals in the ear and nervous system which can be used for hearing assessment, audiologic and neurologic diagnosis, intraoperative monitoring, and neurophysiologic research. The earliest components comprise the electrocochleogram. The cochlear microphonic arises from receptor potentials from cochlear hair cells, and is used to assess hair cell function in patients with auditory neuropathy. The summating potential becomes larger in Ménière's disease. The eighth-nerve compound action potential is useful for objective audiometry. Brainstem auditory evoked potentials, a series of components generated in the eighth nerve and the brainstem auditory pathways, can be used for diagnostic assessment and intraoperative monitoring of the ears and of the auditory pathways up through the mesencephalon. They are relatively easy to record, highly consistent in normal subjects, and little unaffected by surgical anesthesia. Middle-latency and long-latency auditory evoked potentials are generated in multiple areas of cerebral cortex that are activated by auditory stimulation. Anesthetic effects limit their utility for intraoperative monitoring, and substantial intersubject variability limits their utility as a diagnostic test in individual patients, but they are important research tools for the study of memory processes and the way in which the brain analyzes auditory stimuli.

Mechanisms of intraoperative brainstem auditory evoked potential changes.

Brainstem auditory evoked potential (BAEP) changes during intraoperative monitoring may reflect damage to or potentially reversible dysfunction of the ear, the eighth nerve, or the brainstem auditory pathways up to the level of the mesencephalon. They may also be caused by other physiologic mechanisms such as anesthesia, hypothermia, and acoustic masking from drilling noise, or they may result from technical factors that prevent proper stimulus delivery or recording of an evoked potential that is actually present. Cochlear ischemia or infarction resulting from compromise of the internal auditory artery and inner ear damage during temporal bone drilling will affect all BAEP components, including wave I. Direct mechanical or thermal trauma to the eighth nerve will delay, attenuate, and possibly eliminate waves III and V, but wave I, which is generated at the cochlear end of the eighth nerve, may be preserved. During scraping of tumor off the eighth nerve, force applied in an ear-toward-brainstem direction can avulse the fragile fibers of the distal eighth nerve at the area cribrosa. Prolonging the I-to-III interpeak interval during retraction of the cerebellum and brainstem reflects stretching of the eighth nerve, and is often reversible. Vasospasm within the eighth nerve can cause similar, potentially reversible BAEP changes. Damage to the brainstem auditory pathways at or below the level of the mesencephalon will delay and attenuate or eliminate wave V. Wave III is affected similarly if the damage is at or caudal to the region of the superior olivary complex. These BAEP changes may reflect direct mechanical or thermal damage to the brainstem, brainstem compression, or ischemia or infarction resulting from vascular compromise. During BAEP monitoring, examination of the pattern of BAEP changes, analysis of their correlation with surgical maneuvers, and investigation for possible contributory technical factors can help to determine the cause of the BAEP changes and provide the appropriate information to the rest of the surgical team.