Propriospinal Myoclonus (psm) Is Character- Ized by Flexion or Extension Rhythmic- Arrhythmic Jerks Arising in Axial Muscles and Spreading to More Caudal and Rostral

PROPRIOSPINAL MYOCLONUS (PSM) IS CHARACTERIZED BY FLEXION OR EXTENSION RHYTHMICARRHYTHMIC JERKS ARISING IN AXIAL MUSCLES AND SPREADING TO MORE CAUDAL AND ROSTRAL SEGMENTS. In PSM a spinal generator is believed to recruit axial and limb muscles via slowly conducting propriospinal pathways to produce the myoclonic jerks.1 Several cases of PSM have been reported and multiple sclerosis, cervical trauma, thoracic herpes zoster, HIV infection, excision of cervical hemangioblastoma, thoracic arachnoid cyst, syringomyelia, and ischemic myelopathy have occasionally been described as associated lesions.1-10 In many cases, however, PSM remains idiopathic. PSM may be spontaneous and stimulus-sensitive and may be worse with the patient leaning back or lying in bed, as an effect of posture.1,11 In three previous reported cases PSM showed a striking relationship with vigilance level, since it occurred in a semirrhythmic fashion only during the wakefulness period preceding sleep onset. In these patients jerks recurred every 10—20 seconds and were of such intensity as to cause severe sleep-onset insomnia. The jerks were absent during sleep proper. It was speculated that changes in supraspinal control peculiar to the pre-dormitum stage set the spinal generator responsible for PSM into motion.8 This peculiar relationship of PSM with relaxed wakefulness and the ensuing disturbance of sleep allowed the proposition that PSM may represent a disorder of the sleep-wake transition period in some patients.