Sensory evoked potentials in clinical disorders of the nervous system

Electrophysiological Detection of the Neurotoxic Effects of Acrylamide and 2,5-Hexanedione on the Rat Sensory System

Brain stem auditory evoked potentials (BAEP) and somatosensory evoked potentials (SEP), recorded from subcutaneously placed electrodes in anesthetized rats, were used to detect neurotoxic effects of acrylamide and 2,5-hexanedione on the sensory nervous system. Both neurotoxicants were administered for 21 days by the intraperitoneal route, using dosages of 20 mg/kg/day for acrylamide and 350 mg/kg/day for 2,5-hexanedione. Recordings were made before and 1, 2, and 3 weeks after dosing was initiated. Both food-restricted and ad libitum-fed rats served as controls. Results demonstrated that SEP waveforms generated in rats were sufficiently variable that differences among the groups were not detected. However, BAEP latencies were longer than those seen in control rats after 3 weeks of acrylamide treatment and after both 2 and 3 weeks of 2,5-hexanedione treatment. The effects of 2,5-hexanedione were more pronounced than those of acrylamide, and increased with length of the dosing period.

Pathophysiological mechanisms and functional hearing consequences of auditory neuropathy

The effects of inner ear abnormality on audibility have been explored since the early 20th century when sound detection measures were first used to define and quantify 'hearing loss'. The development in the 1970s of objective measures of cochlear hair cell function (cochlear microphonics, otoacoustic emissions, summating potentials) and auditory nerve/brainstem activity (auditory brainstem responses) have made it possible to distinguish both synaptic and auditory nerve disorders from sensory receptor loss. This distinction is critically important when considering aetiology and management. In this review we address the clinical and pathophysiological features of auditory neuropathy that distinguish site(s) of dysfunction. We describe the diagnostic criteria for: (i) presynaptic disorders affecting inner hair cells and ribbon synapses; (ii) postsynaptic disorders affecting unmyelinated auditory nerve dendrites; (iii) postsynaptic disorders affecting auditory ganglion cells and their myelinated axons and dendrites; and (iv) central neural pathway disorders affecting the auditory brainstem. We review data and principles to identify treatment options for affected patients and explore their benefits as a function of site of lesion.

Auditory impairment in infants at risk for bilirubin-induced neurologic dysfunction

Classical and subtypes of kernicterus associated with bilirubin toxicity can be differentiated in part with physiological auditory measures that include auditory-evoked potentials and measures of cochlear integrity. The combination of these auditory measures suggests that bilirubin exposure results in auditory system damage initially at the level of the brainstem, progressing to the level of the VIII cranial nerve and then to greater neural centers. There is no evidence of neural damage at the level of the cochlea. Auditory neural damage from bilirubin toxicity ranges from neural timing deficits, including neural firing delays and dyssynchrony, to neural response reduction and even elimination of auditory neural responses. This condition is comprehensively described as auditory neuropathy spectrum disorder. Independent measures of cochlear function and auditory neural function up to the level of the brainstem can effectively diagnose auditory neural damage resulting from bilirubin neurotoxicity. Intervention, including cochlear implants can be effective.

Prognostic value of somatosensory-evoked potentials in the surgical management of cervical spondylotic myelopathy

STUDY DESIGN

Preoperative somatosensory-evoked potentials (SEPs) were retrospectively analyzed and classified, and compared with surgical outcome.

OBJECTIVE

To evaluate the value of the preoperative SEP waveform in predicting the clinical outcome after surgical management of cervical spondylotic myelopathy (CSM).

SUMMARY OF BACKGROUND DATA

SEPs have played an important role in spinal surgery. However, the value of SEPs in predicting the outcome of surgery for CSM remains controversial.

METHODS

This study enrolled 76 CSM patients who underwent surgical intervention. Median nerve SEPs were recorded before surgery. The Japanese Orthopedic Association (JOA) scoring system was used to evaluate the neurologic function before surgery and at postoperative follow-up at 1, 3, 6, 12, and 24 months. Patients were divided into 5 groups according to the classification of their preoperative SEP waveforms. Group I patients had normal SEPs, group IIa had normal latency and abnormal amplitude, group IIb had abnormal latency and normal amplitude, group III had abnormal latency and amplitude, and group IV had immeasurable waveforms. The myelopathic disability scores and surgical outcomes in different groups were compared by the Kruskal-Wallis test.

RESULTS

The SEP classification was found to be significantly associated with the JOA score (Pearson's chi test, chi = 53.9, P < 0.05). There were no significant differences in JOA score recovery at different follow-up times within any SEP group. At 24 months after surgery, there was no significant difference in the recovery ratio between groups I and IIa, or between groups IIb and III (Kruskal-Wallis test, P > 0.05). However, the recovery ratio was significantly higher in groups I and IIa than in all the other groups (Kruskal-Wallis test, P < 0.05), and in groups IIb and III than in group IV (Kruskal-Wallis test, P < 0.05).

CONCLUSION

SEP classification correlates well with CSM disability and postoperative recovery ratio. Median nerve SEP recordings would be a valuable and practical tool for the diagnosis and prognosis of myelopathy.

Leukoencephalopathy upon disruption of the chloride channel ClC-2

ClC-2 is a broadly expressed plasma membrane chloride channel that is modulated by voltage, cell swelling, and pH. A human mutation leading to a heterozygous loss of ClC-2 has previously been reported to be associated with epilepsy, whereas the disruption of Clcn2 in mice led to testicular and retinal degeneration. We now show that the white matter of the brain and spinal cord of ClC-2 knock-out mice developed widespread vacuolation that progressed with age. Fluid-filled spaces appeared between myelin sheaths of the central but not the peripheral nervous system. Neuronal morphology, in contrast, seemed normal. Except for the previously reported blindness, neurological deficits were mild and included a decreased conduction velocity in neurons of the central auditory pathway. The heterozygous loss of ClC-2 had no detectable functional or morphological consequences. Neither heterozygous nor homozygous ClC-2 knock-out mice had lowered seizure thresholds. Sequencing of a large collection of human DNA and electrophysiological analysis showed that several ClC-2 sequence abnormalities previously found in patients with epilepsy most likely represent innocuous polymorphisms.

Abnormal flash visual evoked potentials in malnourished infants: an evaluation using principal component analysis

OBJECTIVE

To characterize the morphology of flash visual evoked potentials (fVEPs) obtained from infants hospitalized with severe, chronic malnutrition (marasmus).

METHODS

A covariance-based principal component analysis with Promax factor rotation was applied to fVEPs obtained from malnourished infants and age-matched control subjects.

RESULTS

The analysis suggests the presence of a late positive complex in the fVEP, with at least one of its components being significantly diminished in marasmic infants. The N3 component was also diminished in marasmic infants. Following remediation, the marasmic group no longer differed with respect to these components. However, an abnormally large late, positive deflection was evident at discharge.

CONCLUSIONS

The fVEP morphology of infants hospitalized with severe malnutrition was found to be significantly different from age-matched controls. Moreover, although there was evidence of recovery following remediation, fVEPs continued to show abnormality at discharge, suggesting the possibility that nutritional rehabilitation did not fully eliminate the physiological deficit.

SIGNIFICANCE

Malnourishment during early infancy results in altered neurophysiological functioning, possibly in cortical areas responsible for higher order visual processing.

Hyperbilirubinemia and kernicterus

This article describes new findings concerning the basic science of bilirubin neurotoxicity, new considerations of the definition of clinical kernicterus, and new and useful tools to diagnose kernicterus in older children, and discusses treatments for kernicterus beyond the newborn period and why proper diagnosis is important.

Bilirubin toxicity in the developing nervous system

Bilirubin toxicity remains a significant problem despite recent advances in the care of jaundiced (hyperbilirubinemic) neonates. A recent surge in reported cases of classical kernicterus, due in part to earlier hospital discharge and relaxation of treatment criteria for hyperbilirubinemia, and new reports of hyperbilirubinemia-induced auditory dysfunction using evoked potential based infant testing and hearing screening, underscore the need to better understand how hyperbilirubinemia causes brain damage in some infants, especially because the damage is preventable. Recent progress in understanding bilirubin binding and neurotoxicity resulting from unbound or "free" unconjugated bilirubin, how bilirubin affects the central nervous system in vivo and in vitro, and the use of new clinical tools in neonates, for example magnetic resonance imaging revealing bilateral lesions in globus pallidus and subthalamus, and abnormal brainstem auditory evoked potentials with normal inner ear function, may lead to improved detection and prevention of neurologic dysfunction and damage from bilirubin. Finally, the concern is raised that partial or isolated neurologic sequelae, for example auditory neuropathy and other central auditory processing disorders, may result from excessive amount and duration of exposure to free, unconjugated bilirubin at different stages of neurodevelopment.

Impact of somatosensory evoked potential monitoring on cervical surgery

Controversy still exists about the necessity of somatosensory evoked potential (SSEP) monitoring during cervical surgery. The purpose of this prospective study is to determine the impact of SSEP monitoring on anterior cervical surgery. Intraoperative SSEP monitoring was performed in 100 patients treated by an anterior cervical approach. The patients were divided into three groups according to their preoperative clinical condition. Somatosensory evoked potential monitoring was performed during five stages of the procedure: M1, after the induction of anesthesia; M2, during positioning; M3, during distraction of the intervertebral space; M4, throughout decompression; and M5, during graft placement. Normal SSEPs were obtained during M1 from all the patients in group 2. Pathologic SSEPs were recorded at M1 in 45 patients from group 1. No SSEPs were recorded at M1 in six patients in group 3. A deterioration of the SSEPs was observed in 35 patients during M2. Deteriorated SSEPs were observed during M3 in 14 patients. No deterioration of the SSEPs was recorded during M4. Intraoperative SSEP monitoring is easy to perform and helps to increase safety during anterior cervical surgery. Critical phases of the surgical procedure were identified and the surgical strategy was modified as a result of this study.

Hemispheric lateralization of the processing of consonant-vowel syllables (formant transitions): effects of stimulus characteristics and attentional demands on evoked magnetic fields

It is still unsettled in how far temporal resolution of dynamic acoustic events (formant transitions) or phonetic/linguistic processes contribute to predominant left-hemisphere encoding of consonant-vowel syllables. To further elucidate the underlying mechanisms, evoked magnetic fields in response to consonant-vowel events (synthetic versus spoken) were recorded (oddball design: standards=binaural/ba/, deviants=dichotic/ba/-/da/; 20 right-handed subjects) under different attentional conditions (visual distraction versus stimulus identification). Spoken events yielded a left-lateralized peak phase of the mismatch field (MMF; 150-200ms post-stimulus onset) in response to right-ear deviants during distraction. By contrast, pre-attentive processing of synthetic items gave rise to a left-enhanced MMF onset (100ms), but failed to elicit later lateralization effects. In case of directed attention, synthetic deviants elicited a left-pronounced MMF peak resembling the pre-attentive response to natural syllables. These interactions of MMF asymmetry with signal structure and attentional load indicate two distinct successive left-lateralization effects: signal-related operations and representation of 'phonetic traces'. Furthermore, a right-lateralized early MMF component (100ms) emerged in response to natural syllables during pre-attentive processing and to synthetic stimuli in case of directed attention. Conceivably, these effects indicate right hemisphere operations prior to phonetic evaluation such as periodicity representation. Two distinct time windows showed correlations between dichotic listening performance and ear effects on magnetic responses reflecting early gain factors (ca. 75ms post-stimulus onset) and binaural fusion strategies (ca. 200ms), respectively. Finally, gender interacted with MMF lateralization, indicating different processing strategies in case of artificial speech signals.

Age-related slowing in face and name recognition: evidence from event-related brain potentials

Age-related slowing in recognizing famous names and faces was investigated with event-related brain potentials (ERPs). In a group of young adults, item repetition induced early (220-340 ms) and late (400-700 ms) ERP modulations, apparently signaling the access to, respectively, domain-specific representations of faces and names and domain-general semantic knowledge about the persons. These repetition effects and other ERP components were then used as process-specific time markers in middle-aged and elderly participants. For both faces and names, the elderly participants' responses were slowed, but repetition priming in reaction times was not. The ERP latencies suggested that most of the age-related slowing occurred in the access to domain-specific representations and during response decision, whereas sensory and perceptual processing was largely spared.

Somatosensory evoked potentials associated with thermal activation of type II Adelta mechanoheat nociceptive afferents

We evaluated evoked potentials (EPs) to noxious contact heat pulses delivered to hairy skin of healthy adults. Heat pulses from an adapting temperature of 34 degrees C to a target temperature of 52 degrees C, produced two scalp positive waves. The first peaked at 44 degrees to 45 degrees C (approximately 500 ms following stimulus onset), while the second peaked approximately 300 ms following the 52 degrees C heat pulse (approximately 1 s after stimulus onset). The first positive wave was absent from an adapting temperature of 39 degrees C, suggesting loss of synchronized activation of warm and/or low threshold mechanothermal afferents. The second EP was observed following stimulation from both adapting temperatures and was associated with subjective report of first pain. Latency difference of the pain EP from arm and leg were consistent with conduction in Adelta nociceptive afferents (approximately 10/ms). EPs to painful contact thermal stimuli may be of value in the evaluation of small fiber peripheral neuropathies and assessment of altered pain states.

Progressive deterioration of central components of auditory brainstem responses during postnatal development of the myelin mutant taiep rat

Auditory brainstem responses (ABRs) were evaluated during the postnatal development (P10-P180) of taiep rats, neurological mutants characterized by early abnormal myelin development and subsequent demyelination of the CNS. The disorder is produced by an autosomal recessive mutation trait that affects the oligodendrocytes but not the Schwann cells. After onset of ABRs (P12-P14), taiep rats and their nonaffected heterozygous littermates that served as controls showed a similar pattern of maturation for wave I. The central waves (In-IV) showed significantly longer latencies in the mutants. By P60-P180, the later waves (III and IV) were frequently difficult to discern. From the onset of ABRs, the interpeak latency I-IV, corresponding to the central conduction time (CCT) of the auditory pathway, showed in taiep rats significantly longer values than controls. After an initial reduction, proportional to that of control rats, the CCT value increased progressively during the second month of the mutants' lives. The electrophysiological results of the present study strongly support the hypothesis that mutation in the taiep rat impairs neuromaturation of the central auditory pathway in the brainstem by affecting the myelination process in the CNS.

Transient deafness due to temperature-sensitive auditory neuropathy

OBJECTIVE

To define mechanisms accounting for transient deafness in three children (two siblings, ages 3 and 6, and an unrelated child, age 15) when they become febrile.

DESIGN

Audiometric tests (pure-tone audiometry, speech and sentence comprehension), tympanometry, middle ear muscle reflex thresholds, otoacoustic emissions (OAEs), and electrophysiological methods (auditory brain stem responses [ABRs], sensory evoked potentials, peripheral nerve conduction velocities) were used to test the children when they were afebrile and febrile.

RESULTS

ABRs, when afebrile, were abnormal with a profound delay of the IV-V and absence of waves I-III. The ABR in one of the children, tested when febrile, showed no ABR components. Measures of cochlear receptor function using OAEs were normal in both febrile and afebrile states. Cochlear microphonic potentials were present in the three children, and a summating potential was likely present in two. When afebrile, there was a mild threshold elevation for all frequencies in the 15-yr-old and a mild elevation of thresholds for just low frequencies in the two siblings. Speech comprehension in quiet was normal but impaired in noise. One of the siblings tested when febrile had a profound elevation (>80 dB) of pure-tone thresholds and speech comprehension was absent. Acoustic reflexes subserving middle ear muscles and olivocochlear bundle were absent when febrile and when afebrile. No other peripheral or cranial nerve abnormalities were found in any of the children. Sensory nerve action potentials from median nerve in one of the children showed no abnormalities on warming of the hand to 39 degrees C.

CONCLUSION

These children have an auditory neuropathy manifested by a disorder of auditory nerve function in the presence of normal cochlear outer hair cell functions. They develop a conduction block of the auditory nerves when their core body temperature rises due, most likely, to a demyelinating disorder of the auditory nerve. The auditory neuropathy in the two affected siblings is likely to be inherited as a recessive disorder.

P300 clinical utility and control of variability

The P300 event-related brain potential (ERP) has been used to study normal aging as well as patient populations with a variety of neurologic and psychiatric disorders. The P300 has demonstrated reasonable success as a means to assess disturbances in cognitive function, and its clinical utility has been enhanced by the identification of factors that contribute to the variability of ERP measurements. In this article, the neuropsychological theory of P300 is reviewed, ways in which this brain potential can be used as a measure of cognitive efficiency are defined, and methodologic issues that must be considered for successful clinical ERP applications are outlined. This approach is then extended to specific recommendations concerning the technical and practical aspects of P300 recording, so that a well-defined normative database can be developed for evaluating individual patients. When appropriate procedures are used, the P300 can provide a highly useful means to quantify human cognitive capability.

Spinal somatosensory evoked potentials after epidural isoproterenol in awake sheep

PURPOSE

The use of 10-15 micrograms epinephrine as an epidural test-dose is controversial. Isoproterenol would be a better alternative. However, before 5 micrograms isoproterenol can be incorporated in an epidural test-dose, neurotoxicological studies have to be performed. The present study was designed to assess spinal somatosensory evoked potentials (spinal SSEP) before and after epidural isoproteronol.

METHODS

Spinal SSEPs were recorded before, 30 min after, and 72 hr after 50 micrograms isoproterenol were given epidurally (L3-4) to six chronically instrumented awake sheep. The spinal SSEPs after epidural (L3-4) administration of 15 ml lidocaine 2% were used to evaluate the model. The SSEPs were generated by transcutaneous stimulation of the sciatic nerve in the thigh. Spinal SSEPs were recorded directly from the spinal cord at vertebra T12 using a monopolar epidural electrode referenced to a subcutaneous needle electrode in the adjacent paraspinal area.

RESULTS

Thirty minutes and 72 hr after epidural injection of 50 micrograms isoproterenol the latency and the amplitude of the SSEP waves were similar to baseline values. After lidocaine, no SSEPs could be generated in three sheep while in three sheep the latency of wave 2 (W2) was prolonged and the amplitude diminished.

CONCLUSION

Administration of epidural isoproterenol did not affect spinal SSEPs in this study indicating an absence of neurotoxic side effects.

Somatosensory and brainstem auditory evoked potential in congenital craniovertebral anomaly; effect of surgical management

Clinical features and evoked potential recordings were analysed in 32 patients with congenital atlantoaxial dislocation before and after surgery. Seven patients (group 1) had atlantoaxial dislocation, while 22 patients had associated basilar invagination (group 2). In both groups, pyramidal tract signs, posterior column signs, wasting of the upper limbs, and abnormality of somatosensory evoked potentials (SSEP) were similar. Conversely, lower cranial nerve involvement and abnormal brainstem auditory evoked potentials (BAEP) were significantly more in patients with basilar invagination (p less than 0.05). All seven patients in group 1 and 17 patients in group 2 were operated upon. Clinical and electrophysiological deterioration were significant in patients with basilar invagination (group 2), following posterior fixation compared with group 1. Among the patients in group 2, who clinically deteriorated following posterior fixation, seven had transoral excision of odontoid and six of them improved both clinically and electrophysiologically. Two patients in group 2 had odontoid excision before posterior fixation, and in both the evoked potentials improved postoperatively. In group 1 the patient's BAEP remained unaffected following posterior fixation, however, in group 2, eight patients over 53% showed improvement in brainstem function following posterior fixation. This study shows the value of evoked potentials in congenital atlantoaxial dislocation, and rationalizes the surgical procedure in these patients. In patients with basilar invagination, odontoid excision is the preferred first stage procedure.

P300 in multiple sclerosis: a preliminary report

The P300 component of the event-related brain potential (ERP) elicited with auditory stimuli and pattern-shift visual evoked potentials (VEPs) was obtained from 16 patients with multiple sclerosis (MS) and 16 matched control subjects. P300 latency was significantly longer and component amplitude relatively depressed in the MS patients compared to control subjects. The P100 potential of the VEP also was delayed for both full-field and half-field stimulus conditions in the patients compared to control subjects. The findings suggest that the P300 ERP may reflect the cognitive decline associated with MS.

Association of serum testosterone levels with latencies of somatosensory evoked potentials from right and left posterior tibial nerves in right-handed young male and female subjects

The relationships between serum testosterone levels and somatosensory evoked potential (SEPs) latencies from the right and left posterior tibial nerves (PTNs) were studied in right-handed young men and women. The corrected P1 (P39) and N1 (N49) latencies from the right PTN (left hemisphere) were found to be significantly longer in females than males. The corrected P2 and N2 latencies from both PTNs were longer in females than males. Testosterone was not associated with N1 latencies. In females, there was a negative linear correlation between testosterone and latencies from the right and left PTNs. These relationships were complex in males depending upon foot and eye preferences. The following results were obtained from males: no correlation with P1, N1, and P2, but a direct relation with P2 from left PTN in total sample; in males with right eye and right foot preference, a direct correlation only with P1 from right and left PTNs; in mixed- and left-eyed males, inverse correlations with P1, N1, and P2 latencies especially from right PTN; in right-eyed males, direct correlation with P1, inverse correlation with P2 from left PTN; in right-footed males, direct correlation with P1 and N2 waves from right and left PTNs. The interpeak latencies also showed sex-related differences. The overall results suggested that the left brain would be the main target for testosterone effects in both sexes, which may be beneficial for females but mainly disadvantageous for males.

Brainstem evoked potentials in learning disabled children

Averaged brainstem auditory responses (BAER) were recorded using left and right ear stimulation with clicks of 90 dBs in two groups of primary school children (control and learning disabled). A linear multiple regression model was used in an attempt to demonstrate the effects of sex, high risk factors related to brain damage and learning disability on the evoked responses. Sex showed a strong influence in the latencies of the first five peaks, with girls having shorter latencies. Risk factors had an effect on the latency of peak V, the I-V interval and the V/I amplitude ration, but only when the left ear was stimulated. Learning disability had no significant influence according to this analysis. Multivariate test of complete homogeneity showed highly significant differences between LD and control boys when the left ear was stimulated and between control and LD girls when the right ear was stimulated. Principal component analyses revealed differences between the two groups: the BAER components of the control subjects showed a minor source of variance when the right ear was stimulated. A contrary effect was observed in LD children. Such differences might be related to ear preference and hemispheric dominance.

Endorphin mechanisms are responsible for the beneficial effects of opioid antagonists on cerebral function during relative cerebral ischaemia in rats

The present study was conducted to examine the mechanisms behind the previously reported beneficial effects of high doses of naloxone on impaired cerebral function due to hypotensive haemorrhage in spontaneously hypertensive rats (SHR). The stereospecificity of the effects of two opioid receptor antagonists, naloxone (Nal) and Win 44.441-3 (Win), was tested. The effects of thyrotropin releasing hormone (TRH) were also examined, because this peptide has been shown to have beneficial effects in neuronal ischaemia due to spinal injury. In addition, we were interested in seeing what effect the GABA antagonist, bicucculine (Bic), had on cerebral function during relative ischaemia, because Nal in high doses is suspected to antagonize GABA transmission. Mean arterial pressure (MAP), heart rate (HR) and somatosensory evoked potentials (SEP) were recorded in chloralose-anaesthetized SHR. The rats were bled and MAP was rapidly lowered to 50 mmHg. This resulted in transient bradycardia and attenuated SEP. When the first SEP component had decreased to 40-50% of control, the animals were retransfused to a MAP of 60-80 mmHg, to prevent further deterioration of SEP, and maintained at the new pressure level for the rest of the experiment. Fifteen to twenty minutes later, Nal, Win, TRH or Bic was injected i.v. Both (-)Nal (5 mg kg-1) and (-)Win (1 mg kg-1) improved SEP in a stereospecific manner. (+)Naloxone or (+)Win did not affect SEP significantly. Thyrotrophin releasing hormone (2 mg kg-1) caused further attenuation of a late SEP component.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory elicitation of the P300 event-related evoked potential in the rat

Auditory stimuli were used to elicit a P300 event-related evoked potential (ERP) in rat. Test conditions were comparable to those for eliciting ERP's in humans. A train of background tones with a randomly inserted target tone at a ten to one ratio were presented individually to ten unrestrained subjects in a baseline, a novel, and a trained condition. In the novel condition EEG's were averaged from subjects hearing both background and target stimuli for the first time. In the trained condition, subjects were previously trained using footshock in a shuttle box to discriminate the target tone. A statistical comparison of an ERP peak elicited at approximately 300 msec in both the novel and target condition compared with the baseline demonstrated the presence of the P300 in the rat.

The effects of naloxone on cerebral function in spontaneously hypertensive rats during hypotensive haemorrhage

The purpose of this study was to examine the effects of naloxone on signs of relative cerebral ischaemia induced by hypotensive haemorrhage. Mean arterial blood pressure (MAP), heart rate (HR), renal sympathetic nerve activity (rSNA) and somatosensory evoked potentials (SEP) were recorded in chloralose-anaesthetized spontaneously hypertensive rats exposed to graded bleeding. Hypotensive haemorrhage resulted, after a very brief sympathetic excitation, in marked sympathetic inhibition and bradycardia and a considerable reduction of SEP, indicating relative cerebral ischaemia. However, after 25-30 min this sympatho-inhibitory response was reversed to pronounced sympathetic excitation and tachycardia, which was accompanied by a further attenuation of SEP. A single bolus of naloxone (10 mg kg-1) caused transient sympathetic inhibition and bradycardia, which was accompanied by an improvement of SEP. A bolus injection (5-10 mg kg-1) followed by a 30 min infusion of naloxone (25-35 mg kg-1 h-1) caused a sustained SEP improvement despite the fact that MAP was kept constant during naloxone administration. We conclude that naloxone can have beneficial effects on brain function during cerebral ischaemia, effects that are probably due to blockade of opioid receptors. Our model of relative cerebral ischaemia might be useful for evaluating the mechanisms behind the naloxone effects during this condition.

Real-time monitoring of brainstem auditory evoked response (BAER) during cerebellopontine angle (CPA) surgery

The signal-to-noise ratio of brainstem auditory evoked responses (BAER) can be greatly enhanced by use of optimal digital filtering before averaging. This permits accurate assessment of auditory nerve status every 5 to 10 seconds, making real-time intraoperative monitoring possible. The major advantages yielded by real-time monitoring--in our experience thus far--have been identification of potentially adverse functional consequences of apparently uneventful surgical maneuvers, reducing postoperative dysfunction, early indication of potential for improved clinical function, and potential identification and localization of neural tissue in the face of absent surgical landmarks. Examples of these advantages will be provided from case studies, and the possibility that real-time monitoring may improve ability to preserve hearing will be discussed.

Cerebral cortical contributions to sensory evoked potentials: hydranencephaly

The contribution of the cerebral cortex to the generation of sensory evoked potentials was studied in an infant with hydranencephaly. On CT scan no tissue above the thalamus was noted. Long-latency potentials to auditory stimuli were absent whereas the short-latency or brain-stem auditory evoked potentials and some of the components of the middle latency auditory evoked potentials (No and Po) were present. To visual stimulation only the electroretinogram was detected. To somatosensory stimulation only the spinal cord potentials could be detected. The absence of long-latency components in each of the sensory modalities supports the concept that these potentials require intact cerebral hemispheres in man.

Auditory and somatosensory far-field evoked potentials in diabetes mellitus

The effects of insulin dependent diabetes mellitus (IDDM) on brainstem auditory evoked potentials and early components of somatosensory evoked potentials were evaluated in 10 young IDDM patients and 10 control subjects. The IDDM group showed significantly longer intervals between major components of both types of evoked potentials. The results indicated that IDDM patients are at risk for central as well as peripheral neuropathy.

Evoked potentials and brain stem reflexes

The recording of evoked potentials (EPs) has become one of the most useful functional diagnostic techniques in the neurosciences during recent years. In combination with the neurophysiological investigation of brain stem reflexes (BSRs) EPs provide good information concerning circumscribed and diffuse brain and brain stem lesions. In this article the role of EPs and BSRs will be pointed out with special regard to their use in neurosurgery concerning awake and comatose patients as well. Pathological findings caused by extracerebral factors or due to neurological (systemic) and otological diseases will be discussed from the aspect of differential diagnosis only. Evoked potentials described in this paper are short latency potentials which are related to more or less defined generators in the peripheral and central nervous system. EPs of long latency seem to play a role in cognitive, affective and integrative functions of the central nervous system and they will not be dealt with in this article. In recent years an increasing number of review articles and monographs dealing with several aspects of evoked potentials has appeared [16, 17, 18, 25, 69, 80, 84, 122, 123, 124, 127, 130].

Middle and long latency auditory evoked potentials in cat. I. Component definition and dependence on behavioral factors

Middle (10-50 ms) and long (50-600 ms) latency auditory evoked potentials (AEPs) were investigated in artificially respired, muscle-paralyzed cats. Similarity to human potentials of comparable latencies was examined in two ways: (1) the similarity of waveform features such as peak amplitude, polarity and latency, and (2) the effects of task-related variables on these various waveform features. Four behavioral variations of a classical pupillary conditioning paradigm were used to vary attention and arousal. Twelve peaks and troughs were identified in the AEP: P10, N13, P17, N22, P31, N41, P55, N70, N100, N140, P260 and N520. Principal components analysis (PCA) defined 7 AEP components, some of which spanned several peaks. Analysis both of peak latencies and amplitudes, and of principal component scores, revealed differential effects of the behavioral manipulations on these components: those with latencies longer than 50 ms were strongly influenced by behavioral variations, while earlier components were relatively immune to these effects. On the basis of these findings, several relationships between cat and human AEP components were suggested. Specifically, peaks P10-P41 in the cat were thought related to human middle latency components, cat P55 to human P50, cat N140 to human N300, and cat P260 to human P300. Cat N520 was comparable to several long latency components in humans. No obvious correspondences between cat AEP components and human N90 and P170 were identified.

Cortical evoked potentials in response to trigeminus nerve stimulation in humans

A standard technique for recording trigeminal somatosensory evoked potentials (TEP) in response to stimulation of the second branch of the trigeminus nerve is described. Thirty-six nerves of 18 apparently healthy subjects have been investigated. A characteristic polyphasic wave was repeatedly observed. It consisted mainly of a P20, N30, P40, N50 sequence of deflections. Intraindividual variations were minimal. The response pattern varied according to the positions of the recording and stimulating electrodes. The method is useful in detecting functional integrity of the afferent pathways of the 5th cranial nerve, and it can be used as a diagnostic tool for routine investigation of the trigeminus nerve.

Maturational sequence of the visual system: serial measurements of visual evoked potential and electroretinogram in the healthy neonatal lamb

The value of measurements of visual evoked potential (VEP) for neurological assessment of the adult is widely accepted. Its use for neonatal evaluation following birth asphyxia has been limited by our knowledge of developmental changes occurring in the newborn brain. VEPs and electroretinograms (ERGs) were simultaneously recorded from five healthy newborn lambs from birth until 30 days of life. Newborn age was then compared with amplitude (height of the signal) and latency (time from stimulus to a specific wave deflection) measurements from these neuroelectrical signals. Latencies to A wave (photoreceptor activity) and B wave (bipolar and ganglion cells) of the ERG declined in a small but significant manner, while the latency interval from A to B wave did not change with age. Latency to wave N1 of the VEP remained stable while latency to P2 declined from birth to 30 days. The ERG and VEP signals exhibited linear increases in amplitude as the lambs became older. We conclude from these observations that ganglion and bipolar cells (B wave of the ERG) within the retina and primary nerve tracts (N1 of the VEP) along the visual system are mature at birth. In contrast, photoreceptor activity within the retina (A wave of the ERG) and nerve conduction through the visual cortex (P2 of the VEP) exhibit changes which are consistent with ongoing maturation of these more specialized areas through the early newborn period.

Auditory brainstem response in neonates during the first 48 hours after birth

Auditory brainstem response (ABR) has been recorded in 20 neonates at three different time intervals during the first 48 hours after birth. The first recording was performed about 15 min after delivery, when the babies were in their first period of postnatal reactivity. During this period, we were unable to obtain reliable ABR data. During the second and third ABR test performed 2 and 48 hours after birth the babies revealed gradually better responses. Data from these recordings suggest no significant change in middle ear function during the time interval between 2 and 48 hours after birth. Observations of amplitude ratio wave V/wave I are presented. Finally, reservations regarding the interpretation of the results are discussed.

Somatosensory cortical potentials evoked by stimulation of leg nerves: analysis of normal values and variability; diagnostic significance

An attempt was undertaken to assess the diagnostic value of sensory potentials evoked by stimulation of leg nerves. Findings in normal persons were as follows. First, stimulation of the sural nerve was superior to other methods, especially stimulation of the tibial nerve. Second, variations in latency were considerable and not attributable to age, height, or skin temperature. In many normal persons latency was not stable in either short-time or long-time trials. Amplitudes varied to such an extent that they could not help in diagnosis with the possible exception of extreme side-to-side differences. Finally, both amplitudes and latencies varied in relation to stimulus intensity, repetition rate, and filtering. Since sensory potentials from the leg nerves varied considerably, normal values must extend over a range that is wide enough to avoid mistaken diagnosis of abnormality. Hence, slight disturbances of nerve conduction such as those found in certain neuropathies, root damage and many extramedullary intraspinal space-occupying lesions could not be identified, whereas extensive demyelination in all parts of the sensory neuronal chain was readily discovered.

Cortical evoked potentials following stimulation of the urinary bladder in man

Techniques are described for recording evoked potentials in humans in response to stimulation of the urinary bladder. Twenty patients with various disorders of their urinary function were investigated. Evoked cortical potentials following stimulation of the urinary bladder could be recorded from all patients except subjects with complete transverse spinal cord lesions. The response was most evident at the central site (Cz) and had its most prominent peaks at 45, 65, 82 and 102 msec. This technique may become a useful diagnostic tool in the study of neurogenic bladder dysfunctions.

Spinal cord pathways mediating somatosensory evoked potentials

Using a CO2 laser, discrete thoracic spinal cord lesions were made in cats anesthetized with ketamine and xylazine (Rompun). Differences in cortical somatosensory evoked potentials (SEP's) produced with high-intensity stimulation (20 times the motor threshold) of each posterior tibial nerve determined for nine different combinations of unilateral spinal cord lesions. The results of these studies show that nerve fibers in the ipsilateral dorsal column, the ipsilateral dorsal spinocerebellar tract, and the contralateral ventrolateral tracts with respect to the side of leg stimulation, contribute to cortical SEP's. A lesion of the dorsal spinocerebellar tract affected only the early waves (less than 30 msec) of the SEP from leg stimulation ipsilateral to the side of the lesion, whereas a solitary lesion of the ventrolateral tract caused changes primarily in the amplitude of later waves (greater than 30 msec) of the SEP produced by contralateral leg stimulation. Lesions involving one-half of the dorsal column caused changes in the amplitude of both the early and late waves produced by stimulation ipsilateral to the side of the lesion. The effects of various combinations of lesions on the cortical SEP's were not additive, which indicates significant interaction between afferent pathways. These findings suggest that high-intensity peripheral nerve stimulation, which activates both C and A fibers, could be used intraoperatively to assess spinal cord function with more accuracy than the current practice of using a stimulus strength of twice the motor threshold. The importance of using anesthetic agents that do not depress cortical activity (which may affect the later components of the SEP) is also emphasized.

Intraoperative monitoring of brain-stem auditory evoked potentials

Intraoperative brain-stem auditory evoked potentials (BAEP's) were monitored in 46 patients undergoing intracranial surgery for a variety of pathological conditions to determine whether this technique was capable of providing useful information to the operating surgeon. Intraoperative BAEP's were unchanged throughout surgery in 34 patients (74%), and these individuals had no postoperative hearing deficits. Four patients (9%) developed an abrupt ipsilateral loss of all waveform components beyond Wave I and had postoperative evidence of a pronounced hearing loss in the affected ear. An additional patient demonstrated BAEP loss contralateral to the side of surgery, and this was associated with subsequent signs of severe brain-stem dysfunction. Seven patients (15%) developed intraoperative delays of BAEP waveform latency values, but maintained recognizable waveforms beyond Wave I. Postoperatively, their hearing was either normal or mildly impaired, and there were no indications of other brain-stem abnormalities. This group represents the individuals who may have been benefited by evoked potential monitoring, since corrective surgical measures were taken when latency delays were observed. Intraoperative BAEP's can be reliably and routinely recorded in an operating room environment. They provide a good predictor of postoperative auditory status, and may have prevented permanent neurological deficits in a small segment of patients by alerting the surgeon to potentially reversible abnormalities.

Developmental profiles for the brainstem auditory evoked potential

Brainstem auditory evoked potentials (BAEP's) were recorded from 641 subjects in eleven age groups ranging from birth through adulthood, including three levels of prematurity, in order to establish an age-specific normative data base. The amplitude and latency values for waves 1-6, interwave differences for 3-1, 5-3, 5-1 and the amplitude ratios for 5/3 and 5/1 were then converted to percentile scores to permit construction of age-related BAEP profile report forms. Visual inspection of the printed output reveals the proportion of subjects falling above and below a given individual across all BAEP measures. This simple format is useful for general screening purposes as well as plotting maturational changes in healthy or high risk infants.