Effects of cerebellar retraction on brainstem auditory evoked potentials in an experimental animal model of cerebellopontine angle tumor

The effects of cerebellar retraction on brainstem auditory evoked potentials (BAEPs) were studied in 15 New Zealand rabbits. In the first series, a Fogarty catheter was placed in the cerebellopontine angle of 5 rabbits. When a balloon volume of 0.2 mL was produced, only the latencies of waves III-V of the ipsilateral side increased. With a volume of 0.4 mL the ipsilateral BAEPs were irreversibly lost and the contralateral one reversibly changed. A volume of 0.6 mL caused loss of the BAEP on both sides and death of the animal. In another series, the cerebellum of 10 rabbits was retracted by a self-retaining retractor laterally to medially, so an approach to the cerebellopontine angle was simulated. With a retraction of up to 36 power units (p) the ipsilateral BAEPs were reversibly changed. A retraction of 54 p caused irreversible loss of wave V on both sides. With 84 p ipsilateral and contralateral waves III-V were immediately lost. The animals died within 7 minutes. Our experimental investigation shows that not only a defined volume in the cerebellopontine angle can cause irreversible impairment of brainstem function, but also uncontrolled retraction of the cerebellum.     

Auditory evoked potentials

Auditory evoked potentials (AEPs) are an electrical manifestation of the brain response to an auditory stimulus. The waveform represents the passage of electrical activity provoked by auditory stimuli from the cochlea to cortex. The waves represented by I-VII are generated mainly in the brainstem. These waves are called the brain stem auditory evoked potentials (BAEPs) or the auditory brain stem response (ABR). The middle latency AEPs (MLAEP) are generated from the medial geniculate and primary auditory cortex. The long latency AEPs (LLAEP) are generated from the frontal cortex and association areas. The BAEPs appear to be an exquisitely sensitive monitor for pathological events during surgery. Anesthetics and mild hypothermia have minimum effect, if any, on the BAEPs. The BAEPs are useful during the microvascular decompression of the fifth or seventh cranial nerve, resection of acoustic neuroma and posterior fossa operations. Because the auditory pathway occupies a small area in the brainstem, combined use of other evoked potentials such as short latency sensory evoked potentials is recommended. The MLAEPs are most promising evoked responses for monitoring awareness or depth of anesthesia. When the concentration of anesthetics is increased, the amplitudes of the MLAEP's peaks are decreased and their latencies are elongated. Commercially developed A-line AEP monitor or aepEX can extract the AEPs waveform in a short period and automatically analyze the changes in the MLAEPs. These AEP based monitors may be superior to bispectral index (BIS) in detecting the transition from unconsciousness to consciousness.     

Diagnostic value of short latency somatosensory evoked potentials for various intracranial lesions

Diagnostic value of short latency somatosensory evoked potentials (SSEP) was studied in 124 patients with various intracranial lesions. Abnormal SSEPs were recorded in 58 of 124 patients (46.8%) and were classified into three types. Type I (6 cases) showed abnormality of late components with N18 being preserved. All patients with type I abnormality had cortical or subcortical lesions in the parietal lobe. Type II (20 cases) was characterized by abnormality of N18 and late components with N16 being preserved, and mainly seen in patients with a lesion involving thalamus and internal capsule. Type III (31 cases) showed abnormality of N16 and N18 which was elicited by unilateral stimulation (IIIa: 11 cases), or bilateral stimulation (IIIb: 21 cases), and this indicated brainstem impairment. The incidence of SSEP abnormality was as high as 93.1% in patients with sensory disturbance, and 23.1% in patients without such disturbance, and it was suggested that SSEP is useful to detect subclinical dysfunction in the somatosensory pathway. The SSEP grades defined by Anderson et al were found to be well correlated with the outcome of patients with severe head injury, and the SSEP was more reliable for predicting the outcome of patients than the auditory brainstem evoked responses. The SSEP grades were also fairly well correlated with the outcome of patients with cerebrovascular accidents, although the outcome was not consistent in patients with moderately abnormal SSEP.     

Combined measurement of brainstem auditory and somatosensory evoked potentials in a surgically treated brainstem hematoma

Both brainstem auditory evoked potentials and short latency somatosensory evoked potentials were studied in a surgically successfully treated case of brainstem hematoma. The brainstem lesion, evaluated neurologically, radiologically, and surgically, was restricted to the right side of the pons. Comparison between the preoperative and postoperative evoked potentials indicated that wave III is dependent on an intact auditory pathway in the pons of the same side. Wave V appears to have a projection mainly, but not exclusively, from the generator of wave III on the same side. The neural generator of the P15, recorded from the scalp with an ear reference, appears to be in the medial lemniscus above the level of the pons.     

Improvement in brainstem auditory evoked potentials after suboccipital decompression in patients with chiari I malformations

OBJECT: The optimal treatment for patients with symptoms related to Chiari I malformation remains controversial. Although a suboccipital decompression with duraplasty is most commonly performed, there may be a subset of patients who improve in response to bone decompression alone. In an initial attempt to identify such patients, we performed a continuous study of intraoperative brainstem auditory evoked potentials (BAEPs) in patients undergoing a standard decompression with duraplasty and compared conduction times at three different time points: 1) baseline while the patient is supine (before positioning); 2) immediately after opening of the bone and release of the atlantooccipital membrane (that is, the dural band); and 3) after opening of the dura mater. METHODS: Eleven children and young adults (mean age 9.8 years) with symptoms related to Chiari I malformation underwent suboccipital decompression and duraplasty with intraoperative monitoring of BAEPs and somatosensory evoked potentials (SSEPs). Six patients (55%) had associated syringomyelia. At baseline, the I to V interpeak latency (IPL) for both sides (total 21 BAEPs) was 4.19 +/- 0.22 msec (mean +/- standard deviation). After complete bone decompression and before the dura mater was opened, the I to V IPL decreased to 4.03 +/- 0.25 msec (p = 0.0005). When the dura was opened, however, no further decrease in the I to V IPL was detected (4.03 +/- 0.25 msec; p = 0.6). The SSEPs remained stable throughout the procedure. CONCLUSIONS: In children and young adults undergoing suboccipital decompression with duraplasty for Chiari I malformation, the vast majority of improvement in conduction through the brainstem occurs after bone decompressionand division of the atlantooccipital membrane, rather than after opening of the dura. Additional studies are needed to establish whether the improvement seen with BAEP monitoring during bone decompression will predict long-term clinical improvement in these patients.     

Evoked potentials and post-traumatic evolution

Visual, somatosensory, and brainstem auditory evoked potentials provide functional quantitative assessment of the cerebral cortex and brainstem. Their contribution at the acute stage of coma concerns diagnosis, prognosis, and follow-up. Four patterns are observed in traumatic coma: pattern 1=dysfunction of the cerebral cortex, brainstem integrity: good prognosis in more than 80% of cases; pattern 2=midbrain dysfunction: prognosis depends on both the reversibility of midbrain dysfunction and the extent of associated axonal lesions in the hemispheric white matter; pattern 3=pontine dysfunction due to transtentorial herniation: ominous prognosis, this pattern must be early detected by continuous monitoring; pattern 4=brain death: we currently use evoked potentials at the only brain-death confirmatory test, even in sedated patients. The contribution of evoked potentials in vegetative or minimally responsive states concerns the identification of these patients whose state is determined by midbrain dysfunction and the evaluation of persisting cognitive abilities in individual cases.     

Early prognosis in severe cranio-cerebral trauma using the Glasgow Coma Score and evoked potentials

During 72 h following severe head injury, 103 patients in acute posttraumatic coma were assessed by clinical examinations (documented by Glasgow Coma Score) and brain stem auditory evoked potentials (BAEP) as well as short-latency somatosensory evoked potentials (SEP) following median-nerve stimulation. Patient outcomes were classified at 6 months or more according to the following categories: good recovery, severely disabled or vegetative, and brain dead. Patients who had died of systemic complications (pneumonia, septicemia, renal failure, etc.) were excluded from the study. The Glasgow Coma Score was reliable in forecasting a favorable outcome; all patients with a Score over 9 points had a good recovery. The Glasgow Coma Score was not reliable in predicting an unfavorable outcome, however; some patients with the lowest possible Glasgow Coma Score (3 points) at the early clinical examination survived with good recovery. The BAEPs were reliable predictors of an unfavorable outcome; the outcome was unfavorable when a missing wave V or more missing waves pointed toward a secondary brainstem lesion. Normal BAEPs were not reliable, however, in predicting a favorable outcome. SEP data served as a prognostic indicator of unfavorable as well as favorable outcomes. In summary, evoked potentials add valuable information to the clinical examination in assessing a patient's outcome after severe head injury.     

"Kissing" brainstem cavernous angiomas removed using a trans-fourth ventricular floor approach--case report

A 39-year-old woman presented with a rare case of "kissing" brainstem cavernomas formed by separate lesions enlarging with simultaneous recurrent hemorrhages, which was successfully treated by staged resection using a trans-fourth ventricular floor approach. She had a familial history of cerebral cavernous angioma, and presented with a history of four episodes of sudden neurological deterioration. Magnetic resonance (MR) imaging obtained at each neurological event demonstrated two distinct brainstem cavernomas located in the pontine tegmentum and ventral part of the lower pons, both of which enlarged stepwise caused by simultaneous recurrent hemorrhages. Both cavernomas contacted and formed "kissing" lesions. She underwent midline suboccipital craniotomy in the prone position. The cavernoma in the pontine tegmentum was resected through a trans-fourth ventricular floor approach. Although "kissing" formation appeared on preoperative MR imaging, parenchyma was identified at the bottom of the removal cavity of the dorsal lesion, and resection was terminated. MR imaging following the first surgery revealed complete resection of the pontine tegmentum cavernoma and the ventral pontine cavernoma, which was located adjacent to the bottom of the removal cavity and aligned in same direction along the fourth ventricular floor approach. At 10 days after first surgery, she underwent the same procedure with the aid of neuronavigation to resect the ventral pontine cavernoma through the former removal cavity. This approach through the previous removal route, particularly for resection of "kissing" lesions which are difficult to access in the brainstem, is a technically feasible microsurgical procedure.     

Somatosensory evoked potential monitoring during carotid surgery

Controversy exists over the value of intraoperative monitoring and shunting in patients undergoing carotid endarterectomy. Although it is widely believed that contralateral carotid occlusion and previous stroke mandate intraoperative shunting, the susceptibility of these two groups of patients to cerebral ischemia during carotid artery endarterectomy is not well defined. Somatosensory evoked potentials (SSEPs) were monitored in 113 carotid artery endarterectomy patients. Of these, 32 (28.3%) had a previous stroke, 24 (21.2%) had a contralateral carotid occlusion and 33 (29.2%) were diabetic. There were no deaths and only one perioperative stroke (0.9%). Cerebral ischemia occurred in 14 patients (12.4%). Six of these patients had a contralateral carotid occlusion. Some 29 patients (25.7%) were shunted, including 10 with contralateral carotid occlusions that did not have major SSEP changes. In the latter half of the study, 14 patients with contralateral carotid occlusions were selectively shunted (six shunted, eight not shunted) with no neurological complications. Thirty-two patients with prior strokes were selectively shunted (nine shunted, 23 not shunted); of these, one shunted patient undergoing combined carotid artery endarterectomy and coronary artery bypass grafting had a perioperative stroke. Intraoperative monitoring with SSEPs accurately identifies cerebral ischemia secondary to carotid clamping as well as patients requiring shunts. With the use of intraoperative SSEP monitoring, selective shunting may be safely performed in patients with a contralateral carotid occlusion or a previous stroke.     

Characteristics and distribution of somatosensory evoked potentials in the subthalamic region

OBJECT: The aim of the present study is to evaluate the topographical distribution of somatosensory evoked potentials (SSEPs) in the subthalamic area, including the zona incerta (ZI). Determination of this distribution may help in the correct placement of deep brain stimulation (DBS) leads. METHODS: Intraoperative SSEPs were recorded from contacts of DBS electrodes at 221 sites in 41 patients: three patients with essential tremor and 38 with Parkinson disease who underwent implantation of DBS electrodes for the relief of severe tremor or parkinsonism. RESULTS: Two distinct SSEPs were recorded in the subthalamic area. One was a monophasic positive wave with a mean latency of 15.8 +/- 0.9 msec, which the authors designated subthalamic P16. Using both cephalic and noncephalic references, subthalamic P16 was only recorded in the ventral part of the ZI (mean 6.6 +/- 1.3 mm posterior to the midcommissure point, 4.8 +/- 1.2 mm inferior to the anterior commissure-posterior commissure line, and 9.7 +/- 0.6 mm lateral to the midline). When bipolar recordings were made, the traces showed a phase reversal at the caudal part of the ZI. The second potential is a positive-negative SSEP recorded throughout the entire subthalamic area. The mean latencies of the initial positive peak and the major negative peak were 13.6 +/- 1.1 msec and 16.4 +/- 1.1 msec, respectively. Several small notches were superimposed on the peaks, and their amplitudes were largest at the contact close to the medial lemniscus. CONCLUSIONS: The results indicate that intraoperative SSEPs from DBS electrodes are helpful in refining stereotactic targets in the thalamus and subthalamic areas.     

Influence of blood supply,thermal and mechanical traumata on hearing function in an animal model

Summary Even with modern neurosurgical techniques preservation of functional hearing in acoustic neurinoma surgery is still impossible in a large number of cases. Due to the necessity of averaging the brainstem auditory evoked potentials (BAEP) this monitoring is not a real time measurement. Therefore the surgeon cannot be sure which manipulation during the tumour dissection has caused the loss of the BAEPs. The direct monitoring of the cochlear nerve (CNAP) may warn the surgeon earlier. But it is not able to explain, which manipulation has caused the worsening of the potentials. A loss of the waves after coagulation of a vessel next to the cochlear nerve may be the result of the heat or of the disturbance of the blood supply. Potentially harmful to cochlear nerve function may be the interruption of inner ear blood supply, thermal or mechanical traumta. Experimental studies are rare to nonexistent. We therefore tested selectively each trauma for its influence on the BAEPs in an animal model. In New Zealand rabbits a lateral craniectomy of the posterior fossa was performed. Care was taken not to retract the cerebellum or to open the inner ear system, because both factors might disturb the BAEPs. Each step of the operation was followed by BAEP recording. After reaching the internal auditory canal, the cerebellopontine angle of 6 animals was exposed to heated water with definitive increasing temperature. The BAEPs did not react significantly until 71 °C was reached and protein coagulation started. In the second group, the internal auditory artery of 6 rabbits was compressed with a microdissector for 3 minutes. Subsequently the BAEPs disappeared in all animals. In the last group a constant pressure of 10 g was applied to 6 cochlear nerves for 1 minute consistently causing the loss of the BAEPs. The results are statistically significant (p=0.03). We therefore concluded that the blood supply of the inner ear is of the upmost importance for cochlear nerve function. Mechanical manipulation should be minimized whereas thermal traumatization of the nerve is only critical when the nerve itself is coagulated.     

Research on the effectiveness of intermittent cervical traction therapy, using short-latency somatosensory evoked potentials

Short-latency somatosensory evoked potentials (SSEPs) were measured before and after intermittent cervical traction therapy to serve as objective indicators of therapy effectiveness. The subjects were 29 patients with myelopathy, 23 with cervical radiculopathy, 28 with cervical sprain, and 26 healthy individuals. SSEPs were recorded by stimulating the median nerve, and the negative potentials elicited from the brachial plexus (N9), neck (N11, lcN13, ucN13), and somatosensory area (N18) were measured to determine interpeak latencies and then corrected latency. As to the changes in SSEPs following traction, the N11-lcN13 and lcN13-ucN13 interpeak latencies for patients with type I and II myelopathy decreased, and the severity of myelopathy was inversely related to the degree of decrease. The ucN13-N18 interpeak latency for some patients with severe myelopathy increased. The N9-N11 and N11-lcN13 interpeak latencies for patients with cervical radiculopathy decreased, and the ucN13-N18 for patients with cervical sprain accompanied by autonomic nervous symptoms also decreased. Traction therapy might improve conduction disturbance primarily by increasing the amount of blood flow from the nerve roots to the spinal parenchyma. Received: April 16, 2001 / Accepted: November 12, 2001     

Prolonged somatosensory evoked potential depression following a brief exposure to low concentrations of inhalation anaesthetic in a 3-year-old child

A 3-year-old child was brought to the operating room for removal of a brainstem juvenile pilocytic astrocytoma. Following inhalation induction and intubation, he was maintained on 0.5% isoflurane. Somatosensory evoked potentials (SSEPs) were recorded but unobtainable initially and up to 90 min after all inhalation agents were discontinued. The operation was cancelled and the patient was transported to the paediatric intensive care unit (PICU). Subsequent PICU testing revealed a depression of amplitude with propofol and absence of potentials with 0.5% isoflurane. He returned to the operating room, was induced with propofol, and maintained with a propofol: nitrous oxide:fentanyl technique. This anaesthetic technique allowed adequate tumour resection with appropriate monitoring of SSEPs. These findings suggest that a total intravenous anaesthetic technique may be preferable for resection of spinal cord tumours where SSEPs are monitored.     

Ascending limb of the N20 component of somatosensory evoked potential--an analysis on direct recording from the human midbrain

Somatosensory evoked potential elicited by median nerve stimulation at the wrist was recorded from five loci on the trajectory of stereotactic rostral mensencephalic reticulotomy. Four distinct positive waves followed by one negative wave, the peak latency being 16.3 msec, were recorded from the rostral midbrain reticular formation near the medial lemniscus. The four positive waves were named as I, II, III, and IV respectively. Peak latency of these positive waves was 12.6, 13.7, 14.7 and 15.8 msec respectively. The first two positive waves (I, II) corresponded to P13 and P15 recorded over the scalp. The other two positive waves (III, IV) changed their polarity to negative at the level of the ventral thalamus and formed the ascending limb of N20 recorded over the scalp. N16 was most prominent at the level of nucleus ventrocaudalis externus. These findings suggest that the ascending limb of N20 is composed of at least three components, wave III, IV, and N16. The present report is compatible with the investigations by Abbruzzese et al.2) and Eisen et al.11) that there are several distinct dipoles between P15 and N20 of somatosensory evoked potential in man.     

Neurophysiologic monitoring in posterior fossa surgery

Summary In 135 cases of posterior fossa surgery almost exclusively in the cerebellopontine angle (CPA) intraoperative monitoring of brainstem acoustic evoked potentials (BAEP) and partly somatosensory evoked potentials (SEP) was performed. The series consisted of 20 microvascular decompressions, 63 acoustic neurinomas, 7 vascular lesions and 45 other space occupying lesions, mostly in the CPA. BAEP monitoring alone was employed in 76 cases, combined BAEP und SEP monitoring less frequently. The technique of anaesthesia and intraoperative monitoring is presented in detail including an analysis of technical problems (17 in 135=13% of cases) and technical failures (11 of 135=8%). The results of monitoring brainstem pathways contralateral to the lesion are detailed. It is concluded that the technical principles of evoked potential monitoring in posterior fossa surgery are well established. The applications and limits of this technique including its modifications are described.     

Monitoring of motor evoked potentials compared with somatosensory evoked potentials and microvascular Doppler ultrasonography in cerebral aneurysm surgery

OBJECT: The aims of this study were to compare the efficiency of motor evoked potentials (MEPs), somatosensory evoked potentials (SSEPs), and microvascular Doppler ultrasonography (MDU) in the detection of impending motor impairment from subcortical ischemia in aneurysm surgery; to determine their sensitivity for specific intraoperative events; and to compare their impact on the surgical strategy used. METHODS: Motor evoked potentials, SSEPs, and MDU were monitored during 100 operations for 129 aneurysms in 95 patients. Intraoperative events, monitoring results, and clinical outcome were correlated in a prospective observational design. Motor evoked potentials indicated inadequate temporary clipping, inadvertent occlusion, inadequate retraction, vasospasm, or compromise to perforating vessels in 21 of 33 instances and deteriorated despite stable SSEPs in 18 cases. Microvascular Doppler ultrasonography revealed inadvertent vessel occlusion in eight of 10 cases and insufficient clipping in four of four cases. Stable evoked potentials (EPs) allowed safe, permanent vessel occlusion or narrowing despite reduced flow on MDU in five cases. Two patients sustained permanent and 10 showed transient new weakness, which had been detected by SSEPs in two of 12 patients and MEPs in 10 of 11 monitored cases. The surgical strategy was directly altered in 33 instances: by MEPs in 16, SSEPs in four, and MDU in 13. CONCLUSIONS: Monitoring of MEPs is superior to SSEP monitoring and MDU in detecting motor impairment, particularly that from subcortical ischemia. Microvascular Doppler ultrasonography is superior to EP monitoring in detecting inadvertent vessel occlusion, but cannot assess remote collateral flow. Motor evoked potentials are most sensitive to all other intraoperative conditions and have a direct influence on the course of surgery in the majority of events. A controlled study design is required to confirm the positive effect of monitoring on clinical outcome in aneurysm surgery.     

The usefulness of the somatosensory evoked potentials of the pudendal nerve in diagnosis of probable multiple sclerosis.

STUDY DESIGN: The aim of the study was to evaluate the sensitivity of pSEP in patients affected by probable MS. OBJECTIVES: Bladder dysfunction is the presenting symptom in 2% of patients affected by multiple sclerosis (MS) and may be present in up to 78% of them. Abnormalities of somatosensory evoked potentials of the pudendal nerve (pSEP) have been found by many authors in patients affected by clinically defined MS, but little is known of diagnostic reliability of pSEP in early stage of MS. METHODS: Sixteen patients, eleven females and five males, aged between 18 and 45 years old (mean age 28.9), affected by clinically probable MS, were studied. Six of them reported retention or urge incontinence. pSEP with P1 (P40) scalp wave was analyzed. All patients also underwent visual evoked potentials (VEP), SEP of median and tibial nerves (mSEP, tSEP), brainstem acoustic evoked potentials (BAEPs), MRI of the brain and cerebrospinal fluid (CSF) evaluation. Urodynamic study with simultaneous measurement of intravesical, intraurethral and abdominal pressures with external sphincter electromyography was performed. RESULTS: Abnormalities of the evoked potentials were found in all patients. Abnormalities of the pSEP were observed in all the symptomatic cases and in eight of the remaining ten patients; ten showed no responses from the scalp and four showed P1 increased latency. Urodynamic abnormalities were found in 12 patients and MRI showed demyelinating lesions in 13 patients and oligoclonal bands were found in eight of them. CONCLUSION: pSEP can be worthwhile as part of the initial diagnostic evaluation in patients affected by MS. It provides information of diagnostic relevance and plays a role in screening patients for urodynamic testing, which, however, is more specific for detecting urethrovesical dysfunctions and preventing urological complications.     

The effect of increased intracranial pressure during the appearance of pressure waves on the brainstem.

The effects of increased intracranial pressure (ICP) during the appearance of plateau waves or B-waves to the brainstem functions were investigated by evaluation of evoked potentials. The ICP and systemic blood pressure were continuously recorded in nine cases of intracranial hypertension. In the four cases demonstrating plateau waves in ICP recording, the latencies of the auditory brainstem evoked potentials (ABEP) measured during the appearance of plateau waves showed no significant differences compared to those measured during the interval phase between two plateau waves. In the five cases demonstrating B-waves in ICP recording, four cases showed significant (p less than 0.05) prolongation of the V wave of ABEP and three showed significant (p less than 0.05) prolongation of the P15 of somatosensory evoked potentials during the phase of increased ICP level with B-waves compared to those measured during the phase of decreased ICP level without B-waves, produced by external drainage of the cerebrospinal fluid or mannitol administration. The results indicated that during the appearance of plateau waves there were no significant changes of electrical activity in the brainstem, even though there was a significant reduction in cerebral perfusion pressure. It is suggested that in cases with the appearance of B-waves, an increase in ICP frequently affects the electrical activity in the brainstem.     

Experimental study of evoked potentials in predicting the reversibility of brain function following tentorial herniation

Brainstem auditory evoked potentials (BAEP), somatosensory evoked potentials (SEP) and EEG were recorded sequentially in cat with mass-induced intracranial hypertension in correlation with mass volume, intracranial pressure (ICP), systemic blood pressure (BP) and size of the pupils. 1) As the intracranial pressure was raised by expansion of a supratentorial epidural balloon, suppression of cortical SEP (CSEP) and pupillary abnormality appeared first, EEG, waves IV and III of BAEP and wave III of short latency SEP (SSEP) were suppressed in this order. 2) As far as wave IV of BAEP remained and decompression was started within 30 minutes after peaks of CSEP completely suppressed, changes in P1 and N1 of CSEP were reversible. 3) Further raising of ICP was followed by loss of waves IV and III of BAEP and wave III of SSEP in this order. Simultaneously with loss of wave III in SSEP, systemic blood pressure dropped rapidly. By immediate balloon deflation after disappearance of wave III of SSEP, animals recovered from hypotension, but hardly from suppression in EEG, CSEP and BAEP. 4) Preservation of wave IV of BAEP indicated good improvement of pupillary abnormality even after removal of compression. These results suggest that for the patient with disturbed consciousness caused by supratentorial mass lesion, decompressive procedure should be started before wave V of BAEP and brainstem components of SEP disappear. EP monitor seems to be useful clinical method for preventing irreversible change of the brain in patients with coma caused by supratentorial mass lesions.