What EEG pattern is most characteristic of ongoing seizure activity?

The most characteristic EEG pattern of ongoing seizure activity is the periodic lateralized epileptiform discharges (PLEDs). PLEDs are specific patterns that appear in the EEG, typically representing focal seizure activity. They generally consist of repetitive spikes or sharp waves that occur in a lateralized manner (i.e., more prominent on one side of the brain) and are associated with a variety of pathological states, including ongoing seizures or a postictal state following a seizure.

This pattern is significant because it helps clinicians identify the localization and nature of the seizure activity, informing diagnosis and potential treatment options. PLEDs can indicate a focus of epileptic activity, often seen in conditions such as brain lesions, strokes, or recent trauma, thus providing crucial information for medical management.

In contrast, other EEG patterns such as alpha rhythm, interictal spikes, and fast activity do not reflect ongoing seizure activity. Alpha rhythm is a normal background activity seen in an awake and relaxed state, interictal spikes occur between seizures and do not signify immediate seizure activity, and fast activity may indicate various states but is nonspecific regarding seizures.