Neurophysiological findings in the northern epilepsy syndrome.
We describe the neurophysiological findings in the northern epilepsy syndrome (NES), an autosomal recessively inherited childhood onset epilepsy with associated mental deterioration. Sixty-five EEGs of 18 patients (10 females and 8 males) from the age of 5 years to 52 years were analyzed. EEG showed relatively slight changes at the outbreak of epilepsy at the mean age of 6.6 years (range 5-10 years). Slowing of the background activity to 6-7 Hz theta activity, abundant diffuse or intermittent theta and delta activity and disappearance of sleep-specific activity characterized the EEGs at puberty. The amount of diffuse delta and theta activity diminished in adulthood. Epileptiform findings were scanty. Spikes and sharp waves occurred in 43% of the recordings with varying localization, form and extent. Intermittent 2-4 Hz sharp and slow wave rhythm was seen in 32% of the recordings. Of the three ictal recordings, one showed a primarily generalized discharge pattern, while two were clearly asymmetric. Clonazepam was the most effective antiepileptic drug, and it also normalized the EEG when started in childhood or at the onset of puberty. Visual evoked potentials were abnormal in 44% and brainstem auditory evoked potentials in 35%. The neurophysiological findings suggest an extensive, probably multifocal degenerative brain process, which reaches its peak at puberty. Although the abnormal features of EEG often decreased in adulthood, the clinical course of NES showed slow progression throughout the lifetime.