Propriospinal myoclonus: The spectrum of clinical and neurophysiological phenotypes

Propriospinal myoclonus (PSM) is a rare type of spinal myoclonus characterized by muscle jerks that usually start in the midthoracic segments and then slowly propagate up and down into the spinal cord, resulting in repetitive and irregular jerky flexion, or extension of the trunk, neck, knees and hips. PSM can be symptomatic, but up to 80% of reported cases appear idiopathic. PSM tends to occur especially while the subject is lying down. PSM at sleep onset was first described by experts in sleep medicine. The original electrophysiological features included fixed pattern of muscle activations, slow spinal cord conduction (5-15 m/s), electromyographic burst duration less than 1000 ms, synchronous activation of agonist and antagonist muscles and no involvement of facial muscles. PSM has been reported to be a functional (psychogenic) movement disorder in a number of cohorts. The differential diagnosis between idiopathic PSM and the functional forms is not always straightforward. A consistent polymyographically documented muscle activation pattern may be supportive but by no means sufficient and additional neurophysiological investigations are required. PSM should be differentiated from other movement disorders involving the abdomen and trunk, or occurring at sleep-wake transition. This article offers a comprehensive overview of the spectrum of PSM phenotypes.