Hippocampal sleep spindles revisited: physiologic or epileptic activity?

Objective: Few reports have described sleep spindles in intracranial electrode recordings from human hippocampus. Controversy exists regarding whether hippocampal spindles represent a physiologic or epileptic phenomenon. Methods: We reviewed hippocampal recordings in 8 subjects to characterize events resembling sleep spindles. Results: In 6 subjects, events occurred exclusively during non-rapid eye movement (NREM) sleep, were similar in morphology to surface spindles, occurred simultaneously or independently of surface spindles, and did not show a consistent relationship to the epileptic region. In an additional subject, a proportion of the hippocampal activity recorded differed slightly in morphology from surface spindles, was present during both NREM and rapid eye movement (REM) sleep, occurred in the same channels as isolated interictal epileptiform discharges, attenuated just prior to seizures, and occurred postictally as repetitive discharges. This activity occurred simultaneously or independently of surface spindles, but differed from surface spindles by both visual and signal analysis measures. Conclusions: Most examples of hippocampal activity resembling spindles are probably physiologic, originating within the hippocampus or propagated from neighboring regions. However, in one subject, spindle activity and epileptiform discharges may have coincided, supporting experimental evidence that neurophysiological processes associated with spindle generation and NREM sleep contribute to the activation of epileptiform discharges.