The usefulness of EEG, exogenous evoked potentials, and cognitive evoked potentials in the acute stage of post-anoxic and post-traumatic coma

Three-modality evoked potentials (TMEPs) have been used for several years in association with the EEG as a diagnostic and prognostic tool in acute anoxic or traumatic coma. Cognitive EPs have been recently introduced. EEG and cognitive EPs provide functional assessment of the cerebral cortex. TMEP parameters can be described by two indices: the index of global cortical function (IGCF) and the index of brainstem conduction (IBSC). Although it remains a unique tool for epilepsy assessment, the value of EEG is largely limited by its high sensitivity to the electrical environmental noise, its dependence on sedative drugs. and its inability to test the brainstem. Major TMEP alterations (absence of cortical activities more than 24 hours after the onset of post-anoxic coma, major pontine involvement in head trauma) are associated in all cases with an ominous prognosis (death or vegetative state). However even if mild TMEP changes are associated with a good prognosis in 65% (post-anoxic coma) to 90% (head trauma) of cases, some patients never recover despite exogenous TMEPs that are only mildly altered in the acute stage. Thus, cognitive EPs can usefully complement exogenous EPs as a prognostic tool in coma. Indeed, even if the absence of cognitive EPs in comatose patients does not have any prognostic value, their presence implies a very high (more than 90%) probability of consciousness recovery. The major technical challenge for the future will be the development of reliable tools for continuous EEG and TMEP monitoring

Neurophysiological patterns of vegetative and minimally conscious states.

This paper reviews the possible usefulness of electroencephalogram (EEG) and evoked potential (EP) recording in vegetative and poorly-responsive patients. There is a marked inter-individual EEG and EP variability, which reflects the state heterogeneity. Four clinical applications are described: (1) the identification of primary midbrain dysfunction--and, therefore, a possible reversibility--in post-traumatic states; (2) the identification of the permeability of sensory channels; (3) quantitative follow-up; and (4) individual assessment of cognitive functions and/or consciousness. Regarding this last issue, the loss of primary cortical EPs, although rarely observed, constitutes one major argument against consciousness. Conversely, cognitive EPs definitely proved the persistence of cognitive functions in several vegetative patients. Whether these cognitive functions are conscious or not remains a matter of debate

Le neuromonitorage en salle d’opération et en unité de soins intensifs: une technique neurophysiologique pour non-neurophysiologistes?

The successfulness of neuromonitoring in the operating room and intensive care unit relies on an adequate choice of the neurophysiological tool (electroencephalogram or evoked potentials), which should correctly target the neural structure at risk, be sensitive to the pathophysiological process feared, and correctly disentangle pathological and non-pathological factors. The neurophysiological information should be coded into a message readily interpretable by non-neurologists and continuous neuromonitoring should be provided. (C) 1998 Elsevier, Paris

EEG and evoked potentials in the intensive care unit

We review the principal aspects of EEG and evoked potential (EP) neuromonitoring in the intensive care unit. The electrophysiological methods allow functional assessment of comatose patients and can be used (a) as a help to diagnose the origin of coma, (b) as a means to predict outcome, and (c) for monitoring purposes. The combination of the EEG and long-, middle-, and short-latency EPs allows widespread assessment of the cerebral cortex, the brain-stem, and the spinal cord. The EEG and the EP interpretation first requires taking into account non-neurological factors that may interfere with the recorded activities (sensory pathologies, toxic or metabolic problems, body temperature). The sensitivity and the specificity of any neurophysiological technique depend on the etiology of coma. Anoxic comas are associated with a predominantly cortical involvement, while the cortical and brain-stem functions are to be taken into account to interpret the EEG and the EPs in head trauma. The EEG and the EPs can be used to differentiate the comas due to structural lesions from those of metabolic origin, to confirm brain death and help to diagnose psychogenic unresponsiveness or a de-efferented state. While the prognostic value of the EEG is markedly hampered by the widespread use of sedative drugs, it has been possible to design efficient systems based on early- and middle-latency multimodality evoked potentials in anoxic and traumatic comas and, more generally, in all comas associated with an increase of the intracranial pressure. Continuous neuromonitoring techniques are currently under development. They have already been proven useful for the early detection and for the prevention of subclinical seizures, transtentorial herniation, vasospasm, and other causes of brain or spinal-cord ischemia. (C) 1999 Editions scientifiques et medicales Elsevier SAS

Unexpected Myogenic Contaminants Observed in the Somatosensory Evoked-potentials Recorded in One Brain-dead Patient

Visual, brain-stem auditory and somatosensory evoked potentials (EPs) have been recorded in a 53-year-old woman who fulfilled all criteria of brain death. We observed the persistance of 48 msec latency somatosensory activities. Their muscular, and not neural, origin was, however, suggested by their disappearance after curarization. The practical consequences of this observation are two-fold. On the one hand, more precautions must be taken before reaching the conclusion that central neural activities can be obtained in coma dépassé patients. On the other, it suggests a possible test to enhance the sensitivity of EPs in diagnosing brain death

The concept of brain death

The first description of “coma dépassé” dates from 1959.1 Ten years later, it was followed by the introduction of the “Brain Death” (“BD”) concept. BD was considered as equivalent to individual human death.2 Since then, this concept gave rise to three partially overlapping, though qualitatively distinct, debates, which were well summarized by Bernat:3 a philosophical debate on the definition of death, a medico-philosophical debate on the identification of a measurable criterion of death, and a medical debate on the definition of which tests should be used to demonstrate that this criterion has been fulfilled. Actually, these debates are closely related to each other: the choice of medical tests depends on the criterion, and the choice of the criterion depends on which definition of death has been accepted

Evoked potentials: a safe brain-death confirmatory tool?

The diagnosis of brain death (BD) relies primarily on prerequisites (clear knowledge of the cause of coma, all remedial procedures proven unsuccessful) and clinical arguments (areactive coma, loss of brainstem reflexes, apnea). Confirmatory tests should be applied whenever any misleading factor (CNS depressant drugs, hypothermia, metabolic disturbances) can interfere with the clinical diagnosis. This paper reviews the different available confirmatory methods (EEG, four-vessel angiography, radioisotopic techniques, intracranial Doppler, evoked potentials). Both the author's own experience and the data from the literature indicate that evoked potentials are actually a safe and rapid BD confirmatory tool that can be performed at the patient's bedside. It is suggested that they be used in association with the clinical examination for all BD-suspected patients, except for children younger than 6 months of age in whom the guidelines of the Task Force for Brain Death in children (1987) are still recommended