Unusual EEG patterns.

Some of the unusual patterns that can be encountered on the EEG are described briefly. The patterns are grouped according to the predominant frequencies involved and/or by distinctive morphology or distribution. Those involving predominantly the alpha frequency range are alpha squeak, retained alpha, alpha-delta sleep, unilateral decrease in reactivity of alpha activity, and extreme spindles. The patterns involving the beta frequency range are the fast alpha variant, posterior temporal fast activity in children, the fast spiky spindle variant, central fast activity, and diffuse paroxysmal or continuous fast activity. The patterns of predominant theta frequencies are the slow alpha variant, frontal arousal rhythm, rhythmic temporal theta activity of drowsiness, midline theta rhythms, and focal parietal theta activity. Activity in the delta range includes the transient rhythmic slowing occurring after eye closure and the more continuous posterior rhythmic slowing. Patterns with a distinct morphology or distribution include the breach rhythm, wicket spikes, zeta waves, periodic frontal sharp complexes, subclinical rhythmic electrographic discharge of adults, and the EEG pattern of holoprosencephaly.