Three-modality exogenous evoked potentials (TMEPs) have been used since several years as a prognostic tool in acute anoxic or traumatic coma. The whole information provided by TMEPs can be summarized by means of two indices : the index of global cortical function (IGCF), derived from flash visual and cortical somatosensory EPs, and the index of brain-stem conduction (IBSC), derived from subcortical somatosensory and brainstem auditory EPs. The IGCF is expressed by grades : Grade 0 corresponds to normal (never encountered in comatose patients), Grade 1 and 2 to the variable preservation of associative cortical activities, Grade 3 to the sole preservation of primary cortical activities, and Grade 4 to the loss of all cortical EPs with preservation of brain-stem components. The IBSC is firstly quantitatively determined, and, if abnormal, qualitatively rated in terms of midbrain, pontine or medullar involvement.
Anoxic comas are associated with prognostically relevant IGCF abnormalities while the IBSC remains intact (Pattern 1). For examinations performed between the 1st and the 3d day after the acute episode, Grade 1 and Grade 2 were associated in our series with a 64% and 38% rate of good outcome, respectively, while we never observed any recovery in patients with Grade 4 more than 24 hours after the acute episode.
Head trauma is associated with both IGCF and IBSC alterations and the abnormalities can be clustered into 4 patterns : hemispheric damage without brain-stem involvement (Pattern 1), midbrain dysfunction (Pattern 2), transtentorial herniation (Pattern 3), brain death (Pattern 4). In our series of traumatic patients with Pattern 1, IGCF Grade 1 and Grade 2 observed within the first 3 days were associated with 89% and 78% rates of good outcome, respectively. The outcome of patients with Pattern 2 depended on the extent of hemispheric diffuse axonal lesions (HDAL) associated with the midbrain lesion (67% of good outcome in the absence of HDAL, 20% of good outcome in the presence of HDAL). Therefore, we suggest performing MRI in patients with Pattern 2. Patterns 3 and 4 were always associated with death.
Thus, it appears that strongly altered exogenous EPs are always associated with an ominous prognosis. Mildly altered exogenous EPs were associated with a better prognosis in our series, although 36% of anoxic and 13% of traumatic patients with mildly altered EPs presented a poor evolution. We examined whether the presence of cognitive EPs (oddball paradigm) recorded in passive conditions could be associated with a better prognosis. These were recorded in more than 150 anoxic and traumatic comatose patients (GCS £ 8). Although the P3b component was almost never obtainable, the mismatch negativity (MMN) and the P3a component were recordable in more than 20% of patients, and their persistence was associated in more than 90% of cases with some consciousness recovery. Moreover, the latencies of both the MMN and the P3a were significantly correlated with the GCS (p < 0.006). Thus, while the absence of cognitive EPs in comatose patients doesn't have any prognostic value, their presence implies a very high probability of consciousness recovery. As such, cognitive EPs may very usefully complement exogenous EPs as a prognostic tool in coma.

Selected references:
Guerit JM, Fischer C, Facco E, Tinuper P, Murri L, Ronne-Engstrom E, Nuwer M. Standards of clinical practice of EEG and EPs in comatose and other unresponsive states. The International Federation of Clinical Neurophysiology. Electroencephalogr Clin Neurophysiol Suppl. 1999;52:117-31
Guerit JM. Medical technology assessment EEG and evoked potentials in the intensive care
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Guerit JM, Verougstraete D, de Tourtchaninoff M, Debatisse D, Witdoeckt C. ERPs obtained with the auditory oddball paradigm in coma and altered states of consciousness: clinical relationships, prognostic value, and origin of components. Clin Neurophysiol. 1999 Jul;110(7):1260-9