Role of Neurophysiologic Testing in Prognostication of Hypoxic-Ischemic Encephalopathy

Electroencephalography (EEG) and somatosensory evoked potentials (SSEP) are important components of multimodal prognostication in comatose patients after cardiac arrest (CA). The EEG changes considerably during the first days after CA. The prognostic value of some EEG patterns is time-dependent, whereas that of others is not. Persistence of burst-suppression with heterogeneous (variable) bursts and generalized suppression (<10 µV) ≥24 hours after CA strongly predicts poor outcome. Burst-suppression with identical or highly epileptiform bursts and periodic discharges on a suppressed background strongly predict poor outcome, also within the first 24 hours during ongoing sedation and temperature management. These synchronous patterns are best assessed with early continuous EEG-monitoring (cEEG), because their prevalence gradually decreases. Early (<12 hours) return of a continuous normal-voltage background activity on cEEG is a strong predictor of good outcome. A reactive and continuous background on a late routine-EEG also indicates a good prognosis. Full-montage or simplified cEEG provides early prognostic information and improves sensitivity for both poor and good outcome prediction compared with a single late routine-EEG. Bilateral absence of cortical potentials (N20) on median nerve SSEP is one of the most reliable tools for predicting poor outcome, including during ongoing sedation. Sensitivity is limited, but specificity is near 100% if recordings are of sufficient quality, with clearly discernible peripheral potentials and low noise levels. Currently not included in guidelines, very low cortical SSEP amplitudes (approximately 0.5 µV or less) are strongly predictive of poor outcome. However, further research is needed to establish consensus criteria.