Excessive fragmentary hypnic myoclonus: Clinical and neurophysiological findings

Background: Brief involuntary sleep-related twitches occurring asymmetrically throughout the body define physiologic hypnic fragmentary myoclonus. An abnormal intensification of this entity identifies excessive fragmentary hypnic myoclonus (EFHM), a 'proposed sleep disorder' in the International Sleep Disorders Classification. Methods: We describe two patients with EFHM, one associated with a REM behaviour disorder. Results: EFHM activity was restricted to sleep prevailing during NREM sleep stages and the second part of the night. EMG was normal and EEG-EMG back-averaging did not show any cortical potentials related to the twitches. Conclusions: EFHM represents a pathological phenomenon which may or may not be associated with other sleep disorders. A brainstem generator explains its distribution throughout the body and the sleep stages.