Increase of brain-stem high-frequency SEP subcomponents during light sleep in seizure-free epileptic patients

Three hertz spike-and-wave (SW) occurrence is caused by the abnormal functioning of the same thalamo-cortical loop generating sleep spindles. In fact, SW preferably occurs during light sleep and transitional phases of the vigilance status. Since high-frequency somatosensory evoked potentials (HF-SEPs) are powerfully modulated by sleep and arousal, we verified whether they can reveal abnormalities of arousal-related structures in two patients having showed sporadic SW discharges during light sleep.

We recorded right median nerve SEPs in two adult patients who suffered since the infancy from childhood absence epilepsy (CAE). Sleep stage-related changes of HF-SEPs were compared to those observed in five healthy volunteers.

HF-SEPs decreased during sleep in controls. By contrast, the amplitude of the subcortical component dramatically increased in CAE patients during phase II NREM sleep. Simultaneous EEG showed normally represented sleep spindles, but not SW discharges.

HF-SEP increase probably reflects the hyperactivation of brain-stem arousal-related structures. During such a hyperactivation no EEG abnormalities were observed.

We hypothesize that HF-SEP increase might reflect a protective mechanism against seizure occurrence during light sleep.