The Interest of Multimodality Evoked Potentials in the Evaluation of Chronic Coma

The clinical usefulness of three-modality evoked potentials (TMEPs) has been widely demonstrated in acute coma, but is still a matter of debate in the persistent vegetative state. We observed that TMEPs in vegetative patients could be situated along a continuum, one extremity of which corresponds to absent cortical, but preserved brain-stem activities, and the other extremity to major abnormalities of somatosensory conductions at the midbrain level, but variable involvement of auditory pontine and visual cortical pathways. The former pattern is most often associated with ischemic-anoxic encephalopathy, while the latter is most often consecutive to head trauma. However, all intermediate states may be observed. TMEPs can usefully complement the clinical evaluation in answering two questions. The first one concerns the chances of subsequent improvement that are lower in the absence of quantitative TMEP evolution, but are likely to be higher in the presence of non-fixed mesencephalic dysfunction associated with a relative preservation of cortical function. The second question concerns the evaluation of remaining cognitive abilities that are more likely to be present if only mild abnormalities of cortical activities are found and if some cognitive EPs are still obtained.