Propriospinal Myoclonus.

Propriospinal myoclonus (PSM) consists of paroxysmal and sudden jerks involving axial flexion trunk and hip muscles, conditioning sudden myoclonias of the trunk and arms/limbs, both spontaneous and triggered by sensory stimulations, emerging in relaxed wakefulness typically during the transition between wake and sleep. Generally, PSM originates from a thoracic myelomere and spreads caudally and rostrally, provoking flexion and/or extension movements, leading to jumps or trunk jerks. They appear triggered by the lying-down position and disappear when the subject stands up. The main consequences are the difficulties in sleep start and the reappearance during the period of wakefulness after sleep onset.