Factors affecting interictal spike discharges in adults with epilepsy

We investigated various factors affecting interictal spike discharges (ISDs) in standard interictal EEGs of 203 consecutive cases with seizure(s). 94 EEGs (46%) showed ISDs. Yield was maximum (68%) when recordings were done within 2 days of a seizure; beyond this period, incidence of ISD did not change with time from the last seizure. EEGs of patients having greater than 12 seizures/year were more likely to contain ISD (68%) than the records of cases with less than 12 attacks/year (37-41%; P less than 0.001). Age and neurological status at the time of EEG, etiology and anticonvulsants did not influence the frequency of ISD. Analysis of serial EEGs (n = 512) from the study group showed that if initial 3 EEGs lacked ISD, yield from further standard EEGs is small.