Evoked potentials and brain stem reflexes

A Comprehensive Review and Practical Guide of the Applications of Evoked Potentials in Neuroprognostication After Cardiac Arrest

Cardiorespiratory arrest is a very common cause of morbidity and mortality nowadays, and many therapeutic strategies, such as induced coma or targeted temperature management, are used to reduce patient sequelae. However, these procedures can alter a patient's neurological status, making it difficult to obtain useful clinical information for the reliable estimation of neurological prognosis. Therefore, complementary investigations are conducted in the early stages after a cardiac arrest to clarify functional prognosis in comatose cardiac arrest survivors in the first few hours or days. Current practice relies on a multimodal approach, which shows its greatest potential in predicting poor functional prognosis, whereas the data and tools to identify patients with good functional prognosis remain relatively limited in comparison. Therefore, there is considerable interest in investigating alternative biological parameters and advanced imaging technique studies. Among these, somatosensory evoked potentials (SSEPs) remain one of the simplest and most reliable tools. In this article, we discuss the technical principles, advantages, limitations, and prognostic implications of SSEPs in detail. We will also review other types of evoked potentials that can provide useful information but are less commonly used in clinical practice (e.g., visual evoked potentials; short-, medium-, and long-latency auditory evoked potentials; and event-related evoked potentials, such as mismatch negativity or P300).

Visual P300 latency predicts treatment response to modafinil in patients with narcolepsy

OBJECTIVE

To evaluate the hypothesis that visual P300 latency (VL) predicts treatment response to modafinil (a new wake-promoting agent) in patients with narcolepsy.

METHODS

DESIGN

Comparison of responders and non-responders in a double-blind randomized placebo-controlled trial.

SETTING

Private practice referral sleep disorders center.

PATIENTS

Twenty one patients with narcolepsy (ages 17-65 years).

INTERVENTIONS

Auditory and visual P300 testing using 31 evenly spaced scalp electrodes, and baseline polysomnograms and objective and subjective tests of daytime sleepiness, followed by modafinil treatment for 9 weeks. Polysomnograms and tests of sleepiness were then repeated.

MAIN OUTCOME MEASURE

The Maintenance of Wakefulness Test (MWT). Response defined as a final MWT > 7.3min (normative sample mean - 3 SD), plus an increase > 1SD based on normative sample (3.6 min) over baseline MWT.

RESULTS

Non-responders had longer age-adjusted 31-electrode mean VL (448.4 ms vs. 410.8 ms, P = 0.024), and larger auditory P300 amplitude, with no topographical P300 differences. Non-responders and responders did not differ on any other baseline clinical variable. Using a cut-off of 0.5 SE from normal regression constant, shorter age-adjusted VL predicted modafinil response, with specificity of 0.71 and sensitivity of 0.86.

CONCLUSIONS

VL predicts treatment response to modafinil in patients with narcolepsy.

Longer auditory and visual P300 latencies in patients with narcolepsy

To compare auditory and visual P300 amplitude and latency magnitudes and topographies in patients with narcolepsy and normal subjects, 20 patients with polysomnographically-confirmed narcolepsy and 40 normal subjects were administered auditory and visual P300 testing using 31 evenly spaced scalp electrodes. Patients with narcolepsy were then administered baseline polysomnograms and objective (MSLT, Maintenance of Wakefulness Test or MWT) and subjective tests (Epworth Sleepiness Scale, Clinical Global Impression) of daytime sleepiness. Patients had longer 31-electrode mean age-adjusted auditory P300 latencies (406.0 +/- 27.8 vs. 385.7 +/- 28.9 ms, p = 0.012) and visual P300 latencies (427.3 +/- 29.0 vs. 411.4 +/- 27.7 ms., p = 0.044) than 40 normal subjects in the same age range. Age-adjusted auditory P300 latency was correlated with MWT (r = -0.49, p = 0.028), but not with any other clinical variable or measure of sleepiness. Age-adjusted visual P300 latency was not correlated with any clinical variable or measure of sleepiness. Patients with narcolepsy had longer auditory and visual P300 latencies than normal subjects.

Obstructive sleep apnea and abnormal P300 latency topography

This study was conducted to evaluate cognitive abnormalities in obstructive sleep apnea (OSA) using cognitive evoked potentials (P300), and to clarify if such cognitive dysfunction is related to the OSA itself or to the hypersomnolence in OSA. Subjects were administered a polysomnogram, auditory and visual P300 testing using 31 scalp electrodes, and the multiple sleep latency test. There were 40 normal subjects ages 26 to 75. Of 143 consecutive OSA patients ages 26 to 75, 56 had severe OSA (Respiratory Disturbance Index or RDI 40-80/h sleep) with objective somnolence (Mean Sleep Latency < 5 min). Thirty-three had severe OSA without objective somnolence. Fifty-four had profound OSA (RDI > 80/h sleep) with or without objective somnolence. The normals and the three OSA groups did not differ in age. Patients with profound OSA or with severe OSA without somnolence had longer visual P300 latency than normals. The groups also differed in visual P300 latency topography. OSA patients had significantly longer latencies frontally than normals. Thus, OSA, even in the absence of hypersomnolence, is associated with abnormalities in cognitive evoked potentials. Visual P300 latency at frontal electrodes seems to be a neurophysiological index of dysfunction in OSA that is independent of tests of sleepiness.

Abnormal auditory P300 topography in attention deficit disorder predicts poor response to pemoline

P300 is a cognitive evoked potential that evaluates attention and information processing. This study uses auditory and visual P300 topography to develop a classification of attention deficit disorder (ADD), and to find predictors of treatment response to the stimulant pemoline. Forty-five ADD children ages 6 to 15 were administered auditory and visual P300 using 31 scalp electrodes. They were compared with 39 normals. Patients were treated with pemoline, and good and poor responders compared. There were no P300 differences between normals and ADD patients. Good and poor responders to pemoline were clinically identical. Poor pemoline responders had smaller right fronto-central auditory P300 amplitudes than good responders. The ratio of right fronto-central to parietal auditory P300 amplitude, had a sensitivity of 0.70 and specificity of 0.76, as a test for good pemoline response. A ratio greater than 0.5 predicted good response to pemoline, while a ratio less than 0.5 predicted poor response. Treatment with pemoline produced no P300 changes. We conclude that P300 topography classifies ADD into group 1 with normal P300 topography and good response to pemoline, and group 2 with small right fronto-central auditory P300 amplitudes and poor response to pemoline.

P300 latency: abnormal in sleep apnea with somnolence and idiopathic hypersomnia, but normal in narcolepsy

To evaluate cognitive abnormalities in excessive daytime sleepiness (EDS) using cognitive evoked potentials (P300), and to evaluate if P300 measures differentiate among disorders of EDS, a series of EDS subjects were administered a polysomnogram, auditory and visual P300 testing using 31 scalp electrodes, and a multiple sleep latency test. P300 variables were compared with those of normal subjects. Forty normal subjects ages 16 to 65 years, and 69 EDS patients ages 16 to 65 years were used. Of these, 39 had profound obstructive sleep apnea (OSA, Respiratory Disturbance Index or RDI > 80/h sleep) with severe somnolence (Mean Sleep Latency < 5 min). Twenty-two had idiopathic hypersomnia (IH). Eight had narcolepsy. The normals and the three EDS groups did not differ in age. IH and profound OSA patients had longer visual P300 latency than normals or narcolepsy patients (p < 0.05). (p < 0.05). IH and profound OSA patients had longer auditory P300 latency than normals. They had smaller auditory P300 amplitude than narcolepsy patients. There were visual P300 latency topographic differences between normals and profound OSA patients. In conclusion, IH and profound OSA patients show cognitive evoked potential evidence of cognitive dysfunction. Narcolepsy patients do not show such evidence. Visual P300 latency differentiates among disorders of EDS.

A comparative study of brain-stem auditory evoked potentials and blink reflexes in posterior fossa tumor patients

Brain-stem auditory evoked potentials (BAEP's) and blink reflexes (BR's) were recorded from 40 patients with clinical and radiological evidence of posterior fossa tumors. They were examined in three groups according to the anatomical location of the lesion: Group A included 15 patients with midline tumors; Group B included 14 patients with cerebellar hemispheric tumors; and Group C included 11 patients with cerebellopontine angle (CPA) tumors. More of the 40 patients had BAEP abnormalities (32) than BR abnormalities (24). All of the 11 patients with CPA tumors had some kind of BAEP and BR abnormalities. The 14 patients with cerebellar tumors showed the next most frequent abnormalities: 12 related to the BAEP's and seven to the BR's. The 15 patients with midline tumors showed the least number of abnormalities: nine related to BAEP's and six to the BR's. In the analysis of BAEP wave latencies and interpeak latencies, a wave III latency delay occurred in all groups; latencies of waves IV and V were more significantly delayed in patients with CPA and cerebellar hemispheric tumors, and the interpeak latencies of waves III-V and I-V were markedly prolonged only in patients with CPA tumors (p less than 0.01). In all tumor groups, early response (R1) of BR's was significantly delayed in comparison to a healthy volunteer control group (p less than 0.01), but R1 was more pronounced in cases of CPA tumors when compared with the other tumor groups. Although significant delays in direct and consensual late reflex components (R2) of BR's were noted in comparison to the control group, this delay could not differentiate one tumor group from another. In can be concluded that, while these tests reflect the functions of different cranial nerves and brain-stem tracts, BAEP monitoring is more sensitive than BR testing for the detection of brain-stem involvement in posterior fossa tumors. Cerebellopontine angle tumors almost always cause severe abnormalities in both tests. Cerebellar hemispheric tumors and midline tumors cause fewer changes in both BAEP's and BR's.

Electrically elicited blink reflex and early acoustic evoked potentials in circumscribed and diffuse brain stem lesions

In the present paper, the function of the brain stem in patients with brain stem lesions of various aetiology is investigated with electrophysiological methods. The clinical observations are supplemented by experimental investigations on cats, in which the blink reflex and the early acoustic evoked potentials were registered during the acute elevation of intracranial pressure. The findings in patients with circumscribed space-occupying lesions in the posterior fossa document that the registration of the BR and the BAEP have a functional diagnostic significance above and beyond the neurological and radiological investigation. In the case of the cerebellar space occupations, specific alterations could not be observed. On the contrary, the alterations of BR and BAEP indicate a general disturbance of brain stem function, possibly as a result of a general increase of intracranial pressure. In cerebellopontine angle tumours, both BR and BAEP showed specific alterations which were usually asymmetrical. The BR changes ipsilateral to the tumour are of major topodiagnostic significance, whereas the alterations of the contralateral potential are especially informative in the registration of BAEP. The alterations of BR and BAEP also allow an appraisal of the localization and extent of the lesion in primary space occupations in the brain stem: A pathological R1 indicates a pontine lesion, whereas pathological R2 responses are found in medullary and in oral pontine and mesencephalic lesions. In contrast to cerebellopontine angle tumours, the BAEP tends to show symmetrical alterations in primary brain stem lesions. The prolongations of interpeak latencies correspond to the brain stem segment concerned, and the same also applies to pathological amplitude reduction and deformations of individual potentials. In patients with localized brain stem damage, the reflex pathway of R2 is discussed on the basis of the BR findings. In contrast to the view held up to now that only structures situated caudal of the facial nucleus area are responsible for the genesis of the R2 response, it is assumed on the basis of our own observations that pontomesencephalic structures rostral to the facial nuclei are also important for the genesis of R2. Registration of BR and BAEP in patients with acute diffuse brain stem damage shows that both methods have a high diagnostic and prognostic value. Isolated damage and combined brain stem lesion can be demonstrated and the course can be followed up. Normalization of pathological findings reflects clinical recovery, and conversely a secondary deterioration indicates the presence of complications.(ABSTRACT TRUNCATED AT 400 WORDS)

The future role of neurosurgery in the case of vascular diseases of the central nervous system

On the basis of 2542 cerebro-spinal vascular diseases (933 aneurysms, 689 cerebro-spinal angiomas, 410 spontaneous intracerebral hematomas, 361 vascular bypass operations and 149 endarterectomies of neck vessels) the present situation and problems, as well as the future prospects of cerebral vascular neurosurgery are reviewed. It is expected that the main development will take place in the field of pathophysiology and pathochemistry of vascular diseases through the acquisition of data obtained not in experimental studies but in patients. This will refine the diagnosis and indications. It is not expected that the operative technique will undergo very substantial change. New methods in the application of laser and photosensitization techniques will be probably included in the technical armamentarium. Endovascular methods of treatment will be further developed and will lead to the limitation of the indications for direct operation. It is assumed that mortality and morbidity can be further reduced by improving the diagnosis and establishing the optimal lines of therapy.

Microsurgery of gliomas

The author describes his microsurgical operative technique used since 1980 for gliomatous tumours. Instead of extensive resection and lobectomy, a pergyral or intergyral persulcal approach with partial gyrectomy, interhemispheric, transsylvian and transventricular exposure of the tumour surface were used. The resection of the tumour begins from its centre. In the first phase 1980-1982 bipolar coagulation, micro-sucker and pincer were used, since 1983 tumour resections have been performed with the CO2 and Nd-Yag laser and CUSA. Tumours located in functionally important regions such as the speech area, thalamus, brain stem, etc. could be removed without additional morbidity and there was a rapid improvement in neurological deficits. The early prognosis of patients harbouring these tumours is improved thanks to minimized operative trauma. The quality of life during the recurrence free period is improved and surgery of recurrence is indicated more frequently than in the past. There is no evidence that these techniques influence the length of the total survival. The use of CT and MRI improved the early diagnosis of small tumours and intraparenchymal lesions. This requires exact intraoperative localization and identification of the lesion. The technical aspects of these procedures are described. Thanks to the improvement in operative technique some limitations of surgery such as location, nature of the tumour and the age of the patient have lost much of their importance.