Is EEG useful in assessing patients with acute encephalitis treated with acyclovir?

EEG has been used widely in diagnosing encephalitis, as it demonstrates rather typical abnormalities, especially in herpes simplex virus encephalitis (HSVE). We analysed 204 EEG recordings from 98 consecutive acyclovir-treated patients with acute encephalitis between 1984 and 1994. Periodic complexes (PC) in the acute phase predicted poor outcome (Kendall tau 0.40, P<0.001). However, unlike in many other diseases, e.g. stroke and intracerebral haemorrhage, the diffuse slowing of the background activity at acute phase did not predict outcome (Kendall tau −0.6, P=0.35). At follow-up, the emergence of diffuse slow background activity was significantly associated with a less favourable outcome (Kendall tau 0.33, P=0.0016). Among clinical variables, only epileptic seizures early during the course of the disease correlated significantly with outcome. EEG does have value as a prognostic indicator in acute encephalitides, but it seems that diffuse slowing of background activity or irritative features acutely are not as important as previously thought, based on the experiences of the pre-acyclovir era.