Brainstem Auditory Evoked Potentials Changes Research Articles

Intravenous Perflubron Emulsion Administration Improves the Recovery of Auditory Evoked Potentials after Temporary Brain Stem Ischemia in Dogs

Oxygent, a second-generation perfluorocarbon (Perflubron) emulsion (Alliance Pharmaceutical Corporation, San Diego, CA) with superior oxygen delivery characteristics and greater stability than previous perfluorocarbon emulsions, was evaluated as a cerebroprotective agent in a dog model of partial brain stem ischemia. Six dogs were exposed to 20 minutes of isolated brain stem ischemia after receiving an intravenous bolus of Oxygent at a dose of 1.5 ml/kg. Brain stem auditory evoked potentials (BAEP) and regional cerebral blood flow were measured before and during the ischemia and for 5 hours after reperfusion. Changes in BAEP in this group were compared with those in four control dogs that experienced an identical ischemic period but that did not receive Oxygent. During the ischemic period, both control and Oxygent-treated animals experienced a dramatic decline in BAEP to under 10% of the baseline value. After reperfusion, the BAEP increased in both groups to between 50 and 70% of the baseline. In the Oxygent-treated group, the BAEP continued to recover to a final sustained level of over 80% of baseline. In contrast, the control animals suffered a drop in BAEP to 23% of baseline after the brief postischemic peak. The continued improvement in the BAEP in the Oxygent-treated group compared with the control groups suggests that Oxygent may be of some value as a protective agent to the brain stem during ischemia. This effect may be the result of improved oxygen delivery to the brain stem or may be related to other effects of Oxygent, such as reduction of reperfusion injury. Results suggest that Oxygent may be useful as a cerebroprotectant during cerebrovascular surgeries that require temporarily reducing blood flow to the brain stem.

Brainstem auditory evoked potentials during hypoglycaemia in insulin-dependent diabetic children.

Brainstem auditory evoked potentials (BAEP) were studied in 10 type 1 diabetic children during normoglycaemia (5.5 +/- 0.4 mmol/l), hypoglycaemia and in the post-hypoglycaemic state. In addition, BAEP during normoglycaemia in diabetic children were compared with those of an age-, weight- and sex-matched group of healthy control children. No significant differences were observed between all latencies of the diabetic children compared with those of the healthy children during normoglycaemia. During induction of hypoglycaemia a minor (p < 0.05) prolongation of the inter-peak latency I-V at a blood glucose concentration of 4.1 +/- 0.5 mmol/l was observed. This prolongation was not aggravated at glucose nadir (1.7 +/- 0.3 mmol/l). In conclusion, and in contrast with previous findings in non-diabetic children and in adults with type 1 diabetes, no changes in BAEP were demonstrated during short-term severe hypoglycaemia in diabetic children and only minor transient changes were seen during the initial phase of a standardized induction of hypoglycaemia.

Cochlear microphonics in the jaundiced Gunn rat

Introduction: Bilirubin toxicity causes brain damage and deafness. Brainstem auditory areas are damaged, but the effects of bilirubin toxicity on the peripheral auditory system are less well defined. As a first step in the study of cochlear physiology, we performed studies of scalp-derived cochlear microphonic (CM) responses in the jaundiced (jj) Gunn rat model of acute bilirubin toxicity. Materials and Methods: CMs were obtained from scalp needle electrodes in response to acoustically delayed 500-, 1,000-, 2,000-, 4,000-, 6,000-, and 8,000-Hz tonebursts delivered by overhead speakers, and brainstem auditory-evoked potentials (BAEPs) were obtained to clicks. At 18 days of age, one nonjaundiced (Nj) and two jj Gunn rats in each of four litters were anesthetized. CMs and BAEPs were performed before and 4 hours after either (1) sulfonamide was injected into jj rats to produce acute bilirubin toxicity, (2) jj controls were injected with saline, or (3) Nj controls were given sulfonamide. In a second experiment, 16-day-old jj-sulfa and Nj-saline littermates were studied with insert speakers at 60 and 75 dB hearing level (HL) at baseline and 6 hours later, and CM amplitude was analyzed with a fast fourier transformation. Results: No statistically significant differences were found by repeated measures analysis of variance in the CMs in either experiment between groups or after sulfonamide, despite BAEP changes of decreased amplitude of waves II and III and increased latency of I–II and I–III interwave intervals in jjs given sulfa. Conclusion: Alterations of CM after acute bilirubin toxicity did not occur at a time when there was electrophysiologic evidence of brainstem dysfunction. Although more subtle effects might be detected with a larger sample studied at longer intervals after the insult, CM seems insensitive or less sensitive to acute bilirubin toxicity than brainstem auditory function. These results suggest that retrocochlear tests of central auditory function may be more sensitive to the effects of hyperbilirubinemia than tests of peripheral auditory function in humans.

An ischemic opening of the blood-brain barrier may deteriorate brain stem auditory evoked potentials following transient hindbrain ischemia in gerbils.

To clarify the effect of vasogenic brain edema on the brainstem, the relationships between waveform changes in brainstem auditory evoked potentials (BAEP) and blood-brain barrier (BBB) disturbance following transient hindbrain ischemia were investigated. Hindbrain ischemia was induced in gerbils by bilateral occlusion of the vertebral arteries. The animals were divided into three groups subjected to 0, 5, and 30 min of bilateral vertebral occlusion (BVO-0',-5', and -30' groups; n = 4 in each group). Two hours after recirculation, Evans blue (EB) solution was injected into the saphenous vein. The brains were removed after 30 min of circulation, and all areas stained macroscopically by EB were noted and recorded. During hindbrain ischemia, BAEP disappeared within 3 min. In the BVO-5' group, BAEP reappeared and returned to normal within 10 min after reperfusion, whereas in the BVO-30' group, BAEP never returned to normal and finally disappeared within 30 min after reperfusion. In the BVO-5' group, no EB staining was visible. On the other hand, in the BVO-30' group, EB staining was seen in the medial part of the tegmentum in the midbrain in two animals, and around the vestibular nucleus in the lateral parts of the pons in three. These results demonstrate the close relationship between the reversibility of ischemia-induced changes in BAEP and BBB disturbance in the brainstem.

The combined monitoring of brain stem auditory evoked potentials and intracranial pressure in coma. A study of 57 patients.

Continuous monitoring of brainstem auditory evoked potentials (BAEPs) was carried out in 57 comatose patients for periods ranging from 5 hours to 13 days. In 53 cases intracranial pressure (ICP) was also simultaneously monitored. The study of relative changes of evoked potentials over time proved more relevant to prognosis than the mere consideration of "statistical normality" of waveforms; thus progressive degradation of the BAEPs was associated with a bad outcome even if the responses remained within normal limits. Contrary to previous reports, a normal BAEP obtained during the second week of coma did not necessarily indicate a good vital outcome; it could, however, do so in cases with a low probability of secondary insults. The simultaneous study of BAEPs and ICP showed that apparently significant (greater than 40 mm Hg) acute rises in ICP were not always followed by BAEP changes. The stability of BAEP's despite "significant" ICP rises was associated in our patients with a high probability of survival, while prolongation of central latency of BAEPs in response to ICP modifications was almost invariably followed by brain death. Continuous monitoring of brainstem responses provided a useful physiological counterpart to physical parameters such as ICP. Serial recording of cortical EPs should be added to BAEP monitoring to permit the early detection of rostrocaudal deterioration.

Brainstem Auditory Evoked Potentials in Icteric Fullterm Newborns: Alterations After Exchange Transfusion

Brainstem auditory evoked potentials (BAEP) were studied in five fullterm newborns before and during an exchange transfusion (ET) for hyperbilirubinemia and 1, 24, 48 hours after, in order to evaluate the changes of BAEP following acute decreases in bilirubinemia. Hyperbilirubinemia was due to ABO incompatibility. The newborns were free of other risk factors known to alter BAEP. In comparison with our normal laboratory data, we observed significant differences in pre-ET recordings for latencies I and V (p less than 0.01), and for interpeak latency V-III (p less than 0.01). After ET, there was a tendency towards shortening of all mean latencies and interpeak latencies in correlation with the acute decrease in bilirubinemia, but post-ET BAEP alterations consisted predominantly of a wave V latency and of a wave V-I interval shortening (p less than 0.01). In four patients, during the 48 hours post-ET we observed modifications of the wave V-I interval related to variations of bilirubinemia. At 12 months, BAEP were normal. Even acceptable increase of bilirubinemia in otherwise normal newborns may have a deleterious influence on BAEP and this could have implications for the determination of a so-called threshold of bilirubin neurotoxicity. Our results suggest that bilirubin neurotoxicity is rapidly reversed by ET, but it seems important to verify BAEP in the follow-up of hyperbilirubinemic newborns.

Auditory Evoked Potentials in Vertebrobasilar Transient Ischemic Attacks

Brainstem auditory evoked potentials (BAEPs) are affected by stroke or migraine in the vertebrobasilar arterial system. Some studies have reported BAEP changes in vertebrobasilar transient ischemic attacks (TIAs), but others have shown no alterations. We recorded BAEPs in 35 patients with TIAs in the vertebrobasilar system who did not have a stroke, other neurologic disease or significant hearing loss. Thirty patients were recorded after resolution of symptoms, while five individuals still had some resolving signs or symptoms. TIA patients as a group had longer interpeak latencies, but I-III, III-V, and I-V latencies were not significantly longer than in controls. Wave V was significantly longer in latency and lower in amplitude in TIA patients, however. The patients whose TIAs had resolved at absolute and interpeak latencies were within normal limits, but three of five had interpeak latencies at or above three standard deviations beyond the normal mean in the still symptomatic group. One of these was later tested and found to be within normal limits. BAEPs after subsidence of symptoms may add little to the evaluation of vertebrobasilar ischemia, but further AEP analysis may show more definitive differences of diagnostic use. The occasional BAEP abnormality during the resolving transient ischemia supports the recently suggested continuum between ischemia and infarction in the vertebrobasilar territory.

Transtentorial Brain Herniation in the Monkey: Analysis of Brain Stem Auditory and Somatosensory Evoked Potentials

A monkey model of transtentorial brain herniation (TBH) was created to simulate the clinically encountered situation of a gradually expanding intracranial lesion. TBH was produced by extradural balloon inflation over a 4-hour period and documented by the appearance of the pupils as dilated or fixed at midposition. Intracranial pressure (ICP), brain stem auditory evoked potentials (BAEP), and short-latency somatosensory evoked potentials (SSEP) were recorded before, during, and after TBH. Statistical significance from baseline values to TBH was found for diminution of the BAEP amplitude, rise of the ICP, and diminution of the SSEP amplitude. An ICP rise to twice the baseline value and a 25% decrease in Wave V amplitude was found 1 hour before TBH. Changes in BAEP and SSEP took several minutes after deflation to return to baseline values. Analysis of Wave V of the BAEP was as sensitive as ICP in warning of TBH. Discussion centers upon previous animal studies of brain herniation and ICP elevation, and findings reported in humans deteriorating as a result of intracranial mass lesions. BAEP and SSEP monitoring may be used as noninvasive tests for brain stem compression in the setting of primate TBH, and in the future may be used to guide the effectiveness of therapy.

Intraoperative Monitoring of Brainstem Auditory Evoked Potentials

Brainstem auditory evoked potentials (BAEPs) were monitored intraoperatively during 44 posterior fossa operations. Technically satisfactory recordings were obtained from 42 patients (95%) and, in 41 of these cases (98%), the intraoperative changes in BAEPs were correlated with the postoperative neurological findings. The functional integrity of the brainstem was assessed by intraoperative BAEP monitoring in 33 patients. Transient, reversible changes were observed in six (18%), and none had postoperative brainstem dysfunction. The functional integrity of the auditory pathway was monitored in 11 cases, and nine of these patients (82%) exhibited alterations of BAEP waveforms. Transient, reversible dysfunction occurred in three, and permanent changes in six (obliteration in five). Seven patients experienced postoperative hearing loss, six of whom had intraoperative changes in BAEPs that became permanent. The seventh had only transient derangement during surgery. BAEP monitoring appeared to be a much more sensitive indicator of auditory than of brainstem function.The study suggests that the maximum delay in wave V latency in reversible dysfunction is about 1.0 msec. Therefore, surgery should be interrupted before the wave V latency is delayed by 1.0 msec, in order to avert postoperative neurological deficits. However, when BAEP changes are rapid, it is extremely difficult to halt the surgical procedure before such alterations become irreversible. The authors conclude that a more timely indicator of reversible trauma must be developed.

The use of brainstem auditory evoked potentials during posterior fossa surgery as a monitor of brainstem function.

Nine patients with unilateral ponto-cerebellar tumour, one with epidermoid, eight with acoustic neuroma had intraoperative monitoring of the brainstem auditory evoked potentials (BAEP) using bilateral scalp recording (Cz-A1 and Cz-A2) and binaural stimulation in order to monitor brainstem function during the surgery. Results were correlated with preoperative and postoperative BAEP findings. There was some intraoperative BAEP change in all the patients. Halothane anaesthesia caused bilateral wave V. prolongation in six patients. Lumbar drainage produced ipsilateral or bilateral wave V. prolongation in three of four patients in whom lumbar drainage was established. Opening the dura produced ipsilateral or bilateral wave V. depression or bilateral prolongation in four patients. Tumour dissection caused BAEP changes in all the patients: bilateral prolongation or depression in five patients, and ipsilateral prolongation and/or depression in six patients, three of them together with bilateral changes. The only patient who had no changes in wave I-IV. but in whom a big wave VI. appeared at the tumour side during removal, had epidermoid tumour. At the end of surgery there was some improvement in all patients, but in two the improvement was not striking: in one BAEP remained flat ipsilaterally, and in other, latency did not improve. All the patients had a good clinical recovery a few days later, but the control BAEP performed from the 3rd-7th postoperative day was not yet completely recovered in four patients. Intraoperative BAEP monitoring can be a useful technique to monitor brainstem function.(ABSTRACT TRUNCATED AT 250 WORDS)

Monitoring of sensory evoked potentials during surgery of skull base tumours.

Despite advances in instrumentation and the use of microsurgical techniques, neurosurgical procedures involving extensive areas of skull base or other critical areas of brain still carry significant risk for neurological injury. The use of intraoperative recording of sensory evoked potentials (SEP) has been advocated to monitor neurologic function during these major neurosurgical procedures to reduce the risk of injury to neural structures. This report summarizes our experience with intraoperative monitoring of SEP in over 200 patients, and details our findings in a group of 12 patients with skull base and posterior fossa tumours. Somatosensory evoked potentials (SSEP) were monitored in all patients, and brain stem auditory evoked potentials (BAEP) in five. While minor changes in BAEP and SSEP parameters were noted in most patients, significant changes occurred in five. Irreversible loss of BAEP in one patient was associated with complete hearing loss postoperatively. Marked, persistent alteration of both BAEP and SSEP was associated with postoperative brainstem dysfunction. No patient with stable BAEP and SSEP at the end of the procedure suffered additional neurological deficit. We conclude that intraoperative SEP monitoring may be valuable in minimizing neural injury during major neurosurgical procedures.

The Effects of Isoflurane and Isoflurane-Nitrous Oxide Anesthesia on Brainstem Auditory Evoked Potentials in Humans

Monitoring of brainstem auditory evoked potentials (BAEP) during neurological surgery can provide useful information. However, in order to interpret intraoperative BAEP changes, it is necessary to delineate the influence of anesthesia, including inhalation agents. In this study, we examined the influence of isoflurane and isoflurane-nitrous oxide anesthesia on BAEP in ten healthy volunteers during normothermic, normocapnic, and normotensive conditions. Isoflurane significantly increased the latencies of peaks III, IV, and V at all end-tidal concentrations studied (1.0%, 1.5%, and 2.0%). Addition of 50% nitrous oxide did not influence these findings. The increase in latencies with isoflurane anesthesia, however, was nonlinear and appeared to plateau after 1.5%. We suggest that during isoflurane anesthesia, an intraoperative increase in peak V latency beyond 1.0 msec is best explained by factors other than direct effects of isoflurane.

Cerebellopontine Angle Surgery and Intraoperative Monitoring of the Auditory Function

Electrocochleographic potentials from the internal auditory meatal portion of the cochlear nerve (IAM-EcoG) and brain-stem auditory evoked potentials (BAEP) from the vertex were simultaneously recorded during cerebellopontine angle (CPA) manipulations (retractions of the cerebellar hemisphere or the cochlear nerve) of dogs. The same BAEP changes as in human subjects were obtained from the animal models. IAM-EcoG showed graded deteriorations in accordance with BAEP changes. The results are as follows: 1) Prolongation of the I-V interpeak latency (IPL) of BAEP, the most common finding during CPA manipulations, is the reflection of prolongation of the I-II IPL, which is caused by conduction impairment or block of the nerve impulses between the extracranial portion of the cochlear nerve and the brain-stem. The operative manipulations act as stretch or compression injuries to the cochlear nerve in the CPA and bring about acute traumatic cranial nerve root lesion, a retrocochlear lesion. 2) The obliteration of all BAEP components including wave I cannot be caused by conduction block. This is caused by occlusion of the internal auditory artery (IAA). BAEP wave I and IAM-EcoG P1-N1 complex are important indicators for the cochlear blood flow during surgical interventions. 3) As possible causes of IAA obstructions, mechanical distortions of the relationship between the anterior inferior cerebellar artery (AICA) and IAA at their junctional portions, mechanical vasospasms of the AICA-IAA complex, and ensuing no-reflow phenomena are discussed. 4) The EcoG is expected to be a useful intraoperative, real-time monitor during CPA surgery which can detect rapid changes derived from cochlear insufficiency. This real timability can overcome the disadvantages of BAEP.

The Effects of Isoflurane and Isoflurane-Nitrous Oxide Anesthesia on Brainstem Auditory Evoked Potentials in Humans

Monitoring of brainstem auditory evoked potentials (BAEP) during neurological surgery can provide useful information. However, in order to interpret intraoperative BAEP changes, it is necessary to delineate the influence of anesthesia, including inhalation agents. In this study, we examined the influence of isoflurane and isoflurane-nitrous oxide anesthesia on BAEP in ten healthy volunteers during normothermic, normocapnic, and normotensive conditions. isoflurane significantly increased the latencies of peaks III, IV, and V at all end-tidal concentrations studied (1.0%, 1.5%, and 2.0%). Addition of 50% nitrous oxide did not influence these findings. The increase in latencies with isoflurane anesthesia, however, was nonlinear and appeared to plateau after 1.5%. We suggest that during isoflurane anesthesia, an intraoperative increase in peak V latency beyond 1.0 msec is best explained by factors other than direct effect of isoflurane.

Brain stem auditory evoked potentials in posterior circulation surgery.

The objectives of this study were to evaluate the use of brain stem auditory evoked potentials (BAEPs) in 10 adult patients with vascular disorders of the posterior circulation that were treated surgically and to compare the BAEPs with the neurological findings. The vascular lesions included basilar artery stenosis in 3 patients, vertebral artery stenosis in 1 patient, brain stem/cerebellar arteriovenous malformation in 2 patients, and basilar artery aneurysm in 4 patients. Measurement of BAEPs were carried out during operation in all cases. Eight patients had BAEPs measured before operation, and 9 patients had BAEPs measured after operation. Repeat postoperative studies were performed in patients with changes in neurological status. In general, BAEP abnormalities correlated with the neurological findings before and after operation. Six patients had normal intraoperative studies. None of them had clinical findings of pontomesencephalic dysfunction after operation. Three patients with significant BAEP abnormalities during operation had neurological findings of pontomedullary ischemia after operation. Transient BAEP changes in 1 patient were thought to be the result of brain stem retraction. The BAEPs were lost in 3 patients who died. The use of BAEP measurement did not lengthen the operative procedures. However, the technique used in this study required 4 to 8 minutes for the accumulation and interpretation of each average, thereby delaying feedback to the surgeon. The results of this study suggest a potential role for BAEP monitoring in identifying brain stem injury during posterior circulation surgery.

Evoked potentials in rats with misonidazole neurotoxicity. I. Brain stem auditory evoked potentials.

Central neurotoxicity produced in rats by daily administration of 300 mg/kg of misonidazole (MISO) 5 times/week for 4-5 weeks (total dose = 6.0 gm/kg) was evaluated weekly wit brain stem auditory evoked potentials (BAEPs). Compared to untreated control rats, all treated rats had a prolongation of the I-IV interpeak latency (p less than 0.005) at a mean of 13.2 +/- 2.7 days at a cumulative dose of approximately 4.0 gm/kg of MISO per rat. In some rats, the I-III and I-II interwave latencies were prolonged and waves III and IV were lost. Control rats did not show any significant alteration in BAEP latency or amplitude. Histopathologic examination of the brain stems of treated rats showed that necrotic lesions were present primarily in the nuclei of the tegmentum of the fourth ventricle, with scattered nuclear involvement in the cerebellar roof nuclei, inferior olive, and nucleus of the spinal tract of the trigeminal nerve. The cerebral cortex appeared to be normal in all treated rats. Changes in BAEPs caused by central neurotoxicity correlated with the histopathologic findings. We conclude that BAEPs are a sensitive method for evaluating MISO central neurotoxicity in the rat model.

Intraoperative monitoring of brain-stem auditory evoked potentials.

Brain-stem auditory evoked potentials (BAEP) were monitored during 545 neurosurgical operations in the cerebellopontine angle. The BAEP were irreversibly obliterated in five patients who required deliberate section of the auditory nerve. Technical difficulties interfered with monitoring in three cases, and three patients had deafness and absent BAEP preoperatively. Reversible alterations in BAEP were seen during 32 operations, with recovery after as long as 177 minutes of virtually complete obliteration. Changes in BAEP were associated with surgical retraction, operative manipulation, positioning of the head and neck for retromastoid craniectomy, and the combination of hypocarbia and moderate hypotension. In 19 cases, waveforms improved after specific interventions made by the surgeon or anesthesiologist because of deteriorating BAEP. In 13 other cases, BAEP recovered after maneuvers not specifically related to the electrophysiological monitoring, most often completion of operative manipulation. Whenever BAEP returned toward normal by the end of anesthesia, even after transient obliteration, hearing was preserved. Irreversible loss of BAEP occurred only when the auditory nerve was deliberately sacrificed. The authors conclude that monitoring of BAEP may help prevent injury to the auditory nerve and brain stem during operations in the cerebellopontine angle.