Ictal and interictal EEG patterns in patients with nonconvulsive and subtle convulsive status epilepticus

Electroencephalography findings in nonconvulsive or subtle convulsive status epilepticus (NCSE and SCSE, respectively) can be heterogenous. We aimed to study the different patterns on EEG in our cohort of patients.

Our objective was to study ictal and interictal EEG patterns in patients with NCSE and SCSE.

From January 2012 to December 2013, EEGs recorded from patients admitted for altered mental status suspected of having NCSE or SCSE were reviewed retrospectively. Electroencephalography status was defined as having (a) continuous ictal discharges lasting >5 min or (b) >2 discrete bursts of ictal discharges, each lasting <5 min, without returning to previous background rhythm in between these bursts.

Among 1698 EEGs recorded for at least 30 min from hospitalized patients, 55 (3.23%) satisfied the criteria of EEG SE. The ictal onset was regional in 37 (67.2%) EEGs, multiregional independent in 8 (14.5%), and generalized in 10 (18.4%). The EEG seizure duration was >5 min in 24 (43.6%) EEGs, between 1 and 5 min in 14 (25.4%), and less than 1 min in 17 (30.8%). Twenty (36.3%) EEGs showed one continuous prolonged seizure episode of >5-minute duration, 15 (27.2%) had 10 or less discrete episodes, 20 (36.3%) had more than 10 episodes, and 11 (20%) had 2 or more ictal patterns. Thirty (54.5%) EEGs had onset ictal frequency of >8 Hz whereas the rest had <8-Hz ictal frequency. In the interictal segment, 29 patients had continuous generalized slow waves, while 12 had intermittent generalized slow waves. Eleven patients had continuous slow waves lateralized to one hemisphere, and these were ipsilateral to the ictal focus in 10 but contralateral in 1. Other interictal waves seen were PLEDS (6), sharp waves (3), suppression (5), and triphasic waves (1). The background alpha rhythm was absent in 36 patients and slow in 14, and normal background alpha was seen in the interictal period in 5 patients.

The ictal and interictal EEG patterns in NCSE and SCSE can be varied. Further study to look for etiologic and clinical correlates of each pattern could add to its clinical value. This article is part of a Special Issue entitled "Status Epilepticus".