Pyramidal tract lesions in comatose patients

Summary In 197 comatose patients transcranial magnetic evoked potentials were registered to investigate the integrity of the pyramidal tract. Findings were compared to somatosensory, visual and auditory evoked potentials.Preservation, abnormalities and absence of evoked potentials were related to survival and non-survival. Transcranial magnetic and somatosensory evoked potentials proved to be of high prognostic value. While the accuracy of prediction of a fatal outcome based on transcranial magnetic evoked potentials was close to 90%, the prediction of survival was less dependable.Transcranial magnetic evoked potentials are concluded to be a valuable adjunct to the neurophysiological assessment of the comatose patient.     

Evoked potentials following diazepam or fentanyl

The effects of fentanyl or diazepam on somatosensory, visual and brainstem auditory evoked potentials were studied in 13 healthy patients scheduled for elective surgery. Following control recordings of evoked potentials, either diazepam 20 mg or fentanyl 200 micrograms was administered intravenously. Evoked potentials were then recorded twice in the subsequent hour. No significant changes occurred in the latency or amplitude of somatosensory, visual or brainstem auditory evoked potentials. Although dose-related changes in evoked potential latencies and amplitudes have been demonstrated with both the inhalational and intravenous anaesthetics, these changes did not occur with diazepam or fentanyl used alone. An anaesthetic technique based on these two drugs would be suitable when intra-operative evoked potential monitoring is required to assess ischaemia and preservation of evoked responses.     

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.     

EFFECT OF NITROUS OXIDE ON VISUAL, AUDITORY AND SOMATOSENSORY EVOKED POTENTIALS

The effects of 10%, 30% and 50% nitrous oxide on visual, auditoryand somatosensory evoked potentials were studied in seven healthyvolunteers. The evoked potentials were averaged from the electroencephalogramfollowing repeated peripheral sensory stimulation of the appropriatemodality. Latencies and amplitudes of the resulting potentialswere measured and compared with control values. In five subjects,increasing concentrations of nitrous oxide were associated witha graded reduction in amplitude of the visual (P < 0.02)and somatosensory (P < 0.02) evoked potentials. The latencyof the first major negative potential of the visual evoked potentialwas significantly increased (P< 0.02). Latencies of brainstemauditory evoked potentials did not alter. In the other two subjectsthe amplitudes of the visual and somatosensory evoked potentialsshowed graded increase with decreasing concentrations of nitrousoxide, confirming that the changes are dose related. As nitrousoxide is used almost universally during anaesthesia, these changesmust be taken into account when assessing variations observedduring operation in anaesthetic-related evoked potential studies.     

Evaluation of post-traumatic coma using evoked potentials

In 130 cases of post-traumatic coma a study of multimodality evoked potentials (MEP) was carried out shortly after the traumatism (within 72 h). According to the morphology of the records they were classified in grades (I-IV) in agreement with Greenberg et al. The worst grade of EP in the different types was related to the outcome at 6 months, distinguishing between focal lesions and diffuse lesions. Only a group (22.9%) of the 109 patients in which the auditory brain-stem evoked potentials (ABEP) could be studied, showed severe disorders (grade III-IV). There was a significant correlation between the ABEP grades and the outcome in the focal lesions (p less than 0.001). In the diffuse lesions the ABEP grades did not show significant differences with the outcome. A 22.4% of the patients with diffuse lesions had some visual evoked potentials (VEP) in grades III-IV against almost the double (41.6%) in the focal lesions. The grades of the VEP in the focal lesions did not have differences with outcome and in the diffuse lesions they did (p less than 0.05). Approximately half of the cases with focal lesions and one third of the diffuse lesions showed severe disorders (grades III-IV) of the somatosensory evoked potentials (SEP). In the two types of lesions there were significant differences with the outcome (p less than 0.001) and (p less than 0.01). The multimodality evoked potentials (MEP) are useful for predicting the outcome, especially in the patients where the neurological examination is impossible. Among the different modalities, the SEP were shown to be more predictive.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reappraisal of the role of motor and somatosensory evoked potentials during open distal aortic repair

ObjectiveIntraoperative motor and somatosensory evoked potentials have been applied to monitor spinal cord ischemia during repair. However, their predictive values remain controversial. The purpose of this study was to evaluate the impact of motor evoked potentials and somatosensory evoked potentials on spinal cord ischemia during open distal aortic repair.MethodsOur group began routine use of both somatosensory evoked potentials and motor evoked potentials at the end of 2004. This study used a historical cohort design, using risk factor and outcome data from our department's prospective registry. Univariate and multivariable statistics for risk-adjusted effects of motor evoked potentials and somatosensory evoked potentials on neurologic outcome and model discrimination were assessed with receiver operating characteristic curves.ResultsBoth somatosensory evoked potentials and motor evoked potentials were measured in 822 patients undergoing open distal aortic repair between December 2004 and December 2019. Both motor evoked potentials and somatosensory evoked potentials were intact for the duration of surgery in 348 patients (42%). Isolated motor evoked potential loss was observed in 283 patients (34%), isolated somatosensory evoked potential loss was observed in 18 patients (3%), and both motor evoked potential and somatosensory evoked potential loss were observed in 173 patients (21%). No spinal cord ischemia occurred in the 18 cases with isolated somatosensory evoked potential loss. When both signals were lost, signal loss happened in the order of motor evoked potentials and then somatosensory evoked potentials. Immediate spinal cord ischemia occurred in none of those without signal loss, 4 of 283 (1%) with isolated motor evoked potential loss, and 15 of 173 (9%) with motor evoked potential plus somatosensory evoked potential loss. Delayed spinal cord ischemia occurred in 12 of 348 patients (3%) with intact evoked potentials, 24 of 283 patients (8%) with isolated motor evoked potentials loss, and 27 of 173 patients (15%) with motor evoked potentials + somatosensory evoked potentials loss (P < .001). Motor evoked potentials and somatosensory evoked potentials loss were each independently associated with spinal cord ischemia. For immediate spinal cord ischemia, no return of motor evoked potential signals at the conclusion of the surgery had the highest odds ratio of 15.87, with a receiver operating characteristic area under the curve of 0.936, whereas motor evoked potential loss had the highest odds ratio of 3.72 with an area under the curve of 0.638 for delayed spinal cord ischemia.ConclusionsSomatosensory evoked potentials and motor evoked potentials are both important monitoring measures to predict and prevent spinal cord ischemia during and after open distal aortic repairs. Intraoperative motor evoked potential loss is a risk for immediate and delayed spinal cord ischemia after open distal aortic repair, and somatosensory evoked potential loss further adds predictive value to the motor evoked potential.     

Evoked potentials in the follow-up and prognosis of patients with craniocerebral trauma

The aims of this study were to find a reliable way of establishing the prognosis for the final outcome in the first week after head injury, to show the correlation between abnormalities in evoked potentials (EP) and clinical coma score, and finally, to document EP results in patients with the clinical diagnosis of brain death. We examined 46 patients, 23 in different states of coma and 23 with bulbar syndrome (complete absence of cortical and brain stem function). In the group of comatose patients brain stem auditory EP (BAEP) and somatosensory EP (SEP) were recorded in the first 48 h, 3-5 days, 1 week and 4 weeks after the head injury. The depth of coma was scaled with a scoring system devised by the authors and with the Innsbruck coma scale. Outcome was evaluated with the Glasgow outcome scale after 3, 6, and 9 months. BAEP were recorded bilaterally after stimulation with clicks; SEP were recorded from the neck (C2) and the contralateral cortex (C3', C4') after electrical stimulation of the median nerve. Evoked potentials were scored according to a four-point scale from grade 1 (normal) to grade 4 (only component I present in BAEP or absence of cortical responses on both sides in SEP). We found a significant correlation between the mean SEP score of the first week and the Glasgow outcome of the 3rd month, but no significant correlation between the BAEP score of the first week and the Glasgow outcome. There was a significant correlation between SEP (BAEP) scores and the corresponding clinical score.(ABSTRACT TRUNCATED AT 250 WORDS)     

Topographical relationships between the brainstem auditory and somatosensory evoked potentials and the location of lesions in posterior fossa stroke

The topographical relationships between the location of brainstem lesions detected by magnetic resonance imaging and abnormality of brainstem auditory evoked potentials (BAEPs) and short-latency somatosensory evoked potentials (SSEPs) were studied in 57 patients with stroke in the posterior fossa. Abnormal BAEPs or SSEPs were associated with lesions involving the pontine tegmentum, and abnormal BAEPs also with lesions at the cerebellar peduncle. Absence of the V wave in BAEPs and N20 in SSEPs was associated with a localized overlapping area in the pontine tegmentum contralateral to stimulation. The overlapping area associated with loss of N20 coincided with the location of the medial lemniscus. Lesions widely involving the pontine tegmentum caused the disappearance of multiple waves in the BAEPs and SSEPs. Patients who entered prolonged coma or died had total loss of the III, IV, and V waves, bilateral absence to the contralateral response in BAEPs, or loss of N18 in SSEPs. The loss of N18 in SSEPs had a statistically significant correlation with bad outcome, which suggests the superiority of SSEPs for predicting the outcome of stroke and indicates the involvement of some system excluding the medial lemniscus in the generation of N18.     

EVOKED POTENTIALS DURING ISOFLURANE ANAESTHESIA

Somatosensory, visual and brainstem auditory evoked potentialswere recorded in 10 unpre-medicated patients anaesthetized withisoflurane in oxyen. Recordings were made at 0. 5%, 1. 1 % and1. 65% (six patients) end-tidal isoflurane concentration. Therewere statistically significant increases in the latencies ofthe somatosensory (N20), visual and brainstem auditory potentials(waves III and V) with increasing concentrations of isoflurane.The central conduction time was prolonged. Amplitudes of thesomatosensory and visual potentials were reduced with increasingconcentrations of isoflurane. The effects of isoflurane on evokedpotentials are similar to those of halothane and enflurane.It is possible that changes in evoked potential measurementssmay be useful as a neurophysiological indicator of anaestheticdepth. Department of Anesthesiology, Duke University Medical Center,Durham, North Carolina 27710, U. S.A. ^*Department of Clinical Neurophysiology, The London Hospital,Whitechapel, London EI IBB.     

Cortical evoked potentials by stimulation of the vesicourethral junction: clinical value and neurophysiological considerations

In 21 healthy volunteers and 42 patients with either neurogenic bladder dysfunction (24), partial peripheral denervation of the bladder (12) or nonneurogenic bladder dysfunction (6) scalp-derived evoked potentials after stimulation of the vesicourethral junction (cortical evoked potentials) were recorded. In addition, evoked potentials from the posterior tibial nerve (tibial somatosensory evoked potentials) and from the pudendal nerve (pudendal somatosensory evoked potentials) were evaluated. The results obtained in normal subjects were reproducible and comparable to those reported in previous studies. Cortical evoked potentials of vesicourethral junction consisted of a prominent negativity with a mean latency of 95 msec. Tibial and pudendal somatosensory evoked potentials were similar and showed a typical W-shaped complex. In normal subjects stimulation of the vesicourethral junction was described as a stimulus-synchronous pulsation combined with a continuous burning feeling and sometimes with a desire to void. In 4 normal subjects no cortical evoked potentials of the vesicourethral junction could be obtained because of a decreased pain threshold. In regard to clinical value, the results demonstrate that in patients with lesions of the central nervous system (in the group with cauda equina and conus medullaris lesions, and in the group with suprasacral spinal cord lesions) the results of cortical evoked potentials of the vesicourethral junction and pudendal somatosensory evoked potentials widely correlate due to similar afferent nervous pathways within the central nervous system. However, in patients with partial peripheral denervation of the bladder with suspected additional secondary local detrusor damage the results of cortical evoked potentials obtained by stimulation of the vesicourethral junction differ mostly from the results of somatosensory evoked potentials obtained by stimulation of the pudendal nerve. The pattern obtained (increased sensory and pain threshold, normal cortical evoked potentials of the vesicourethral junction with normal latencies and normal or increased amplitude) is indicative of local detrusor damage. In 21 patients the ability to detect cortical evoked potentials of the vesicourethral junction was combined with the sensation of stimulus-synchronous pulsation, whereas in the other 21 patients the absence of this sensation during stimulation was combined with the absence of cortical evoked potentials. On the other hand, no correlation was found between the ability of obtaining cortical evoked potentials of the vesicourethral junction and the stimulus-induced sensation of pain and/or desire to void.(ABSTRACT TRUNCATED AT 400 WORDS)     

Human auditory evoked potentials in the assessment of brain function during major cardiovascular surgery

Focal neurologic and intellectual deficits or memory problems are relatively frequent after cardiac surgery. These complications have been associated with cerebral hypoperfusion, embolization, and inflammation that occur during or after surgery. Auditory evoked potentials, a neurophysiologic technique that evaluates the function of neural structures from the auditory nerve to the cortex, provide useful information about the functional status of the brain during major cardiovascular procedures. Skepticism regarding the presence of artifacts or difficulty in their interpretation has outweighed considerations of its potential utility and noninvasiveness. This paper reviews the evidence of their potential applications in several aspects of the management of cardiac surgery patients. The sensitivity of auditory evoked potentials to the effects of changes in brain temperature makes them useful for monitoring cerebral hypothermia and rewarming during cardiopulmonary bypass. The close relationship between evoked potential waveforms and specific anatomic structures facilitates the assessment of the functional integrity of the central nervous system in cardiac surgery patients. This feature may also be relevant in the management of critical patients under sedation and coma or in the evaluation of their prognosis during critical care. Their objectivity, reproducibility, and relative insensitivity to learning effects make auditory evoked potentials attractive for the cognitive assessment of cardiac surgery patients. From a clinical perspective, auditory evoked potentials represent an additional window for the study of underlying cerebral processes in healthy and diseased patients. From a research standpoint, this technology offers opportunities for a better understanding of the particular cerebral deficits associated with patients who are undergoing major cardiovascular procedures.     

Detection of ischemia in endovascular therapy of cerebral aneurysms: a perspective in the era of neurophysiological monitoring

A prospective study was performed to evaluate the efficacy of neurophysiological monitoring (NPM) techniques in the detection of ischemic changes that may be seen during endovascular treatment of cerebral aneurysms. Sixty three patients underwent NPM during first-stage endovascular treatment of cerebral aneurysms. The endovascular procedures included coil embolization (26 patients), balloon-remodeling coiling (16 patients), stent-assisted coiling (10 patients), balloon-stent-assisted coiling (9 patients), and balloon test occlusion (2 patients). NPM included electroencephalography, somatosensory evoked potentials, and brain stem auditory evoked potentials, depending on the location of the aneurysm and its associated vascular territory. NPM changes were seen in 3 (4.8%) patients and the procedures were altered immediately. No neurological changes were found postendovascularly. Ten patients demonstrated abnormal angiographic findings without concurrent NPM changes, of which 5 patients developed visual disturbance or hemiparesis. It is concluded that NPM is a valuable monitoring tool for endovascular treatment of cerebral aneurysms.     

Detection of ischemia in endovascular therapy of cerebral aneurysms: a perspective in the era of neurophysiological monitoring

A prospective study was performed to evaluate the efficacy of neurophysiological monitoring (NPM) techniques in the detection of ischemic changes that may be seen during endovascular treatment of cerebral aneurysms. Sixty three patients underwent NPM during first-stage endovascular treatment of cerebral aneurysms. The endovascular procedures included coil embolization (26 patients), balloon-remodeling coiling (16 patients), stent-assisted coiling (ten patients), balloon-stent-assisted coiling (nine patients), and balloon test occlusion (two patients). NPM included electroencephalography, somatosensory evoked potentials, and brain stem auditory evoked potentials, depending on the location of the aneurysm and its associated vascular territory. NPM changes were seen in three patients (4.8%), and the procedures were altered immediately. No neurological changes were found postendovascularly. Ten patients demonstrated abnormal angiographic findings without concurrent NPM changes, of which five patients developed visual disturbance or hemiparesis. It is concluded that NPM is a valuable monitoring tool for endovascular treatment of cerebral aneurysms.     

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.     

Monitoring of sensory evoked potentials during neurosurgical operations: methods and applications

Monitoring of sensory evoked potentials (SEPs) may help minimize the risk of neurological injury during neurosurgical operations. The author describes the current state of the art, summarizing basic principles and reviewing current clinical applications. Experience with intraoperative monitoring of auditory, somatosensory, and visual evoked potentials is presented. The pitfalls and limitations of presently available methods are discussed, with some speculation regarding future developments. Given adequate quality control in the acquisition, processing, and interpretation of electrophysiological signals, monitoring of SEPs can be a valuable adjunct to the intraoperative care of selected neurosurgical patients.     

Loss of SSEP during sitting craniotomy

A 54-year-old woman with a past medical history of asthma and depression presented with right side hearing loss and ataxia. She was scheduled for a sitting craniotomy for cerebellopontine angle tumor resection. Somatosensory evoked potential, brainstem auditory evoked response, and facial nerve EMG were monitored intraoperatively. Approximately 30 minutes into the case, there was an episode of air embolism, which resolved after the source was identified and treated. Near the conclusion of the case, there was an abrupt loss of the right cortical somatosensory evoked potential signal, which never returned to baseline. A postoperative CT scan showed a substantial amount of subarachnoid air and intraventricular air in the frontal and temporal regions. The patient awakened in the ICU with no new neurologic deficit besides preoperative hearing loss on the right side. Despite the high specificity of somatosensory evoked potential change associated with postoperative neurodeficit when the change never returns to the baseline, there was no postoperative neurologic deficit in this patient. This case indicates the false-positive somatosensory evoked potentials caused by pneumocephalus in the sitting position.     

Failure of motor evoked potentials to predict neurologic outcome in experimental thoracic aortic occlusion

Motor evoked potential monitoring was tested as an alternative to somatosensory evoked potential monitoring in evaluating spinal cord function during thoracic aortic occlusion in dogs. Twenty-seven animals underwent 60 minutes of cross-clamping of the proximal descending thoracic aorta with (n = 18) or without (n = 9) cerebrospinal fluid drainage. Spinal cord blood flow was measured with microspheres, and neurologic outcome was evaluated at 24 hours with Tarlov's scoring system. Cerebrospinal fluid drainage improved neurologic outcome (p less than 0.05). Motor evoked potentials recorded over the lumbar spinal cord were lost in 9 of 20 dogs with ischemic cord injury and were not lost in any of the 7 dogs that were neurologically normal. Somatosensory evoked potential were lost in 19 of 20 paraplegic/paraparetic dogs and lost in 3 of 7 normal dogs (p less than 0.01). After reperfusion, motor evoked potentials returned in all nine neurologically injured dogs that lost the potentials and were still present at 24 hours. Changes in amplitude, latency, or time until loss or return of motor evoked potentials or somatosensory evoked potentials did not predict neurologic injury. Loss of somatosensory evoked potentials had a high sensitivity (95%) but had low specificity (67%) because of peripheral nerve ischemia. Loss of motor evoked potentials recorded from the spinal cord had high specificity (100%) but a low sensitivity (46%) and was therefore not a reliable predictor of neurologic injury. Return of motor evoked potentials during reperfusion did not correlate with functional recovery. Motor evoked potentials stimulated in the cortex and recorded from the spinal cord had low overall accuracy (59%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Monitoring of somatosensory evoked potentials during surgical procedures on the thoracoabdominal aorta. II. Use of somatosensory evoked potentials to assess adequacy of distal aortic bypass and perfusion after thoracic aortic cross-clamping

Pulsatile left atrial-femoral artery bypass was instituted after aortic cross-clamping distal to the left subclavian artery in a canine experimental model to determine the relationship of distal aortic perfusion pressure with spinal cord blood flow and somatosensory evoked potentials. In six animals (Group I) distal aortic perfusion pressure was maintained at 100 mm Hg throughout a 1 hour interval of aortic cross-clamping. During this period, somatosensory evoked potentials and spinal cord blood flow (radioactive microspheres) showed no significant change from baseline. In six other dogs (Group II) distal aortic perfusion pressure was initially maintained at 100 mm Hg after aortic cross-clamping and then progressively decreased to 70, 40, and 25 mm Hg. Somatosensory evoked potentials and spinal cord blood flow were preserved at baseline levels for all distal perfusion pressures greater than 70 mm Hg. At 40 mm Hg, abnormalities in amplitude of the somatosensory evoked potentials were noted in all animals with progression to complete loss of evoked potential activity at lower perfusion pressures. Maintenance of adequate somatosensory spinal cord conduction after thoracic aortic cross-clamping is dependent on a critical level of distal aortic perfusion that can be accomplished by use of an adjunct such as pulsatile left atrial-femoral artery bypass. The critical level of distal aortic perfusion pressure to maintain normal somatosensory evoked potentials and spinal cord blood flow in this canine experimental study was 70 mm Hg or greater. Because inadequate distal aortic perfusion can be easily detected by monitoring of somatosensory evoked potentials, these techniques should prove helpful in evaluating the effectiveness of distal perfusion techniques during clinical aortic cross-clamping for procedures on the thoracoabdominal aorta.     

The changes in EEG, evoked potentials and neurological recovery after global brain ischemia in dogs

Electroencephalography (EEG), evoked potentials and neurological recovery score were compared between 10 min and 15 min transient global brain ischemia in 18 dogs. The transient global brain ischemia was induced by occluding aorta, superior and inferior caval veins. The grade of EEG (1: normal approximately 5: flat) 2 hrs after ischemia was significantly lower with the 10 min ischemic group (n = 9) than with the 15 min group (n = 9) (3.7 +/- 0.5 vs 4.1 +/- 0.3, P less than 0.05). The rate of reappearance in evoked potential waves 2 hrs after ischemia was higher with the 10 min ischemic group than with the 15 min group (auditory brainstem response 5 wave: 100% vs 33%, middle latency response Pa wave: 80% vs 0%, somatosensory evoked potential N2 wave: 83% vs 78%, N3 wave: 67% vs 33%). The neurological recovery score (0: death approximately 100: normal) 7 days after ischemia was significantly higher with the 10 min group than with the 15 min group (58 +/- 34 vs 27 +/- 23, P less than 0.05). In both groups, there was a significant correlation (r = +0.85, P less than 0.01) between the total score of EEG and evoked potential waves (0: no wave appeared approximately 6: all waves appeared) 2 hours after ischemia and the neurological recovery score 7 days after ischemia. These results suggest that the neurological recovery after transient global brain ischemia would be estimated by EEG and evoked potential waves.     

Effects of isoflurane and halothane on the auditory and somatosensory evoked potentials

The effects of isoflurane and halothane anaesthesia on brainstem auditory (BEAR) and somatosensory evoked potentials (SEP) were recorded in 15 patients; eight received isoflurane and seven halothane. Atropine alone was given as premedication. After induction of anaesthesia with thiopentone, it was maintained with 50% nitrous oxide in oxygen. Ventilation was controlled. Isoflurane and halothane concentrations were gradually increased to 3%. Recordings were made at 1%, 2.5% and 3%. There were statistically significant differences in the latencies of SEP and BEAR with increasing concentrations of anaesthetic. The BEAR waves significantly affected were wave V and the latency between peaks I to V (p less than 0.001); for the SEP, it was wave N20. SEP central conduction time was prolonged with both anaesthetics (p less than 0.001). The effects of isoflurane and halothane were similar. These results could not be explained by changes in central body temperature or end-tidal carbon dioxide concentration. The study showed a dose-related direct effect of both anaesthetic agents on the brainstem auditory and somatosensory evoked responses. It may be that the measurement of changes in evoked potentials could be a useful indicator of anaesthetic depth.