A population based electroencephalographic study of absence epilepsy in 97 children is reported from the Departments of Neurophysiology and Pediatrics, Goteborg University, Sweden. All patients had regular bilaterally synchronous and symmetrical 2-4 Hz spike-and-slow wave discharges and absences with or without generalized tonic-clonic seizures (GTCS). Patients with absence seizures not complicated by GTCS had long episodes of 2-4 Hz spike-and-slow wave discharges (more than 10 seconds). Posterior delta rhythms occurred only in patients with pure uncomplicated absence whereas focal abnormalities were predictive of GTCS complication. There was no correlation between poly spike and slow wave and the development of GTCS. Brief episodes of 2-4 Hz spike-and-slow wave (less than 10 seconds) were predictive of a two-fold increased risk of GTCS. GTCS appeared in spite of a normalized EEG in seven of 14 patients. Favorable seizure control was correlated with the normalization of EEG but a normal EEG was no guarantee that GTCS would not develop. 
COMMENT. This study demonstrates the predictive value of the EEG in prognosis of absence epilepsy. Posterior delta rhythm and long episodes of spike-and-wave with clinical correlates favor a good prognosis whereas brief spike-and-wave discharges without clinical correlates increase the risk of future generalized tonic-clonic seizures.