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Excerpt
Three-modality exogeneous 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 under the form of grades: Grade 0 corresponds to normal(never encountered in comatose patients), Grade 1 and Grade 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 IGCF is significantly correlated (P<0.0001) with the Glasgow Coma Score (GCS) in non-curarized patients. The IBSC is firstly quantitatively determined, and, if abnormal, qualitatively rated in terms of midbrain, pontine or medullar involvement. Proper TMEP interpretation requires consideration of all the extracerebral factors liable to interfere with the recordings: drugs, metabolic disturbances, body temperature, peripheral sensory impairment [1].
Anoxic comas are associated with prognostically relevant IGCF abnormalities while the IBSC remains intact (Pattern 1). For examinations performed between the first and the third 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 presenting with Grade 4 more than 24 h after the acute episode. This does not hold true for patients examined within the first 24 h, as three anoxic cases with Grade 4 TMEPs in the very acute stage of coma eventually recovered a good neurological function.
Head trauma is associated with both IGCF and IBSC alterations and the abnormalities can be clustered into four patterns: hemispheric damage without brain-stem involvement (Pattern 1), midbrain dysfunction (Pattern 2), transtentorial herniation (Pattern 3), and 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 a 87% and 69% rate 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 always suggest to perform magnetic resonance imaging in patients with Pattern 2. Patterns 3 and 4 were uniformly associated with death.
Thus, it appears that strongly altered exogeneous EPs are always associated with an ominous prognosis. Mildly altered exogeneous 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 does not have any prognostic value, their presence implies a very high probability of consciousness recovery. As such, cognitive EPs may very usefully complement exogeneous EPs as a prognostic tool in coma.
