Interictal quantitative EEG in epilepsy

The interictal EEG is often normal in epilepsy patients, particularly with partial seizures of extratemporal origin. Quantitative techniques of EEG analysis may increase the yield of diagnostic abnormality in such patients. Thirty patients with partial seizures of frontal or temporal origin had EEG recorded from left frontal (F7-C3), right frontal (F8-C4), left posterior (T5-O1), and right posterior (T6-O2) derivations. Four-second epochs were used to compute power in the delta (0.25-4.0 Hz), theta (4.25-8.0 Hz), alpha (8.25-13 Hz), and beta (13.25-30 Hz) bands. The ratio of high (8.25-30 Hz) to low (0.25-8 Hz) power on the left and the right was measured, as was the ratio between the left and the right hemisphere total power. The mean frequency deviation in the alpha band between the left and the right hemispheres was also measured, and spectral mobility was determined in the right and the left frontal regions. These values were also calculated in normal subjects and tension headache patients with normal EEGs. Seizure patients with abnormal interictal EEGs had decreased ratios of high to low power, greater asymmetry of total power and alpha frequency, and reduced spectral mobility on the side of their EEG foci. Epileptics with normal interictal EEGs had lower ratios of high to low power, greater alpha frequency asymmetry, and lower spectral mobility than did headache patients or normal controls. Power and frequency measurements, and determination of spectral measures such as mobility, can be done with commercially available digital EEG equipment. They may demonstrate otherwise obscure asymmetries in the interictal EEG and thereby aid in epilepsy diagnosis and classification.