Scalp-recorded ictal patterns in focal epilepsy.

Scalp-recorded focal EEG seizure patterns are usually expressed as rhythmic metamorphic evolving patterns (with or without epileptiform morphology) that progress through two or more ictal phases into a postictal change. Such patterns are almost invariably seen in temporal complex partial seizures but less often detected in frontal complex partial seizures and least of all in simple partial seizures. The failure of scalp recordings to detect activity from a focal seizure can usually be explained by the seizure's distant location, limited extent, or disadvantageous orientation with respect to scalp electrodes. The elimination of these disadvantages with properly implanted electrodes explains why these recordings are able to detect seizure discharges missed by scalp electrodes. Although the lateralization of a scalp-recorded seizure can be misleading, it usually accurately identifies the focus when it remains well-lateralized throughout its various ictal phases and into the postictal state.