The background EEG is usually normal, although minor background slowing may occur. One third of patients have temporal intermittent rhythmic delta activity (TIRDA), seen on the side of the hippocampal sclerosis.
The interictal EEG may be normal or may show focal sharp waves, spikes or spike-and-wave, maximally over the anterior temporal electrodes on the side of the hippocampal sclerosis. The EEG may also show bitemporal abnormality.
EEG abnormality is enhanced by sleep deprivation and in sleep. Focal slowing of the background may be accentuated during hyperventilation.
Ictal onset in the hippocampus is typically associated with clinical seizure onset before scalp EEG change appears. This EEG change, when it occurs, may be preceded by diffuse or lateralized suppression. The ictal pattern is typically a gradual build up of rhythmic alpha or theta activity, with or without superimposed spikes or sharp waves, maximal in anterior temporal electrodes on the side of the hippocampal sclerosis. Post-ictal slow activity is seen in the same electrodes.