Electroencephalographic (EEG) changes are generally prominent in patients with encephalitis. In most of acute encephalitis, the EEG is characterized by non-specific diffuse slow activity. However several diseases such as herpes simplex encephalitis, slow virus infection or prion disease often show periodic EEG abnormalities, such as periodic synchronous discharges (PSD) and periodic lateralized epileptiform discharges (PLED). These findings are sometimes diagnostic, although the abnormality is not disease specific. EEG is often helpful in following up the clinical course of illness and in evaluating therapeutic effects.