Daily variation in an intracranial EEG feature in humans detected by a responsive neurostimulator system.

PURPOSE

Based on the observation that epileptic seizures can occur at specific times of the day, we looked for daily variation in an intracranial electrographic feature used by a responsive neurostimulator system to detect seizures.

METHODS

A computationally efficient measure of intracranial EEG energy or complexity, the line length baseline, was calculated and reported by an external responsive neurostimulator during a clinical trial of device safety. Data were obtained from 24 consecutive patients with medically intractable epilepsy undergoing intracranial monitoring over 2 to 54 days to localize the seizure onset zone. Measurements from individual subjects made at different times of day over many days were displayed on a single 24-h cycle and fit with a cosine function to characterize the time of the maximum value. The timing of epileptic seizures was also noted.

RESULTS

The time of the maximum line length baseline value had a bimodal distribution with relative peaks at 05:30 and 15:00 hours. The time of the maximum value did not associate with specific brain regions, except that a nocturnal peak was not measured from temporal neocortex. The temporal distribution of maximum values was similar to the timing of epileptic seizures.

CONCLUSION

The line length baseline feature of the intracranial EEG shows daily variation with location specific characteristics within individual subjects.