A case of primary spinal myoclonus: clinical presentation and possible mechanisms involved.

Spinal myoclonus is a rare movement disorder characterized by myoclonic involvement of a group of muscles supplied by a few contiguous segments of the spinal cord. Structural lesions are usually the cause, but in primary spinal myoclonus the etiology remains unknown. We present the case of a 26-year-old woman with cervical spinal myoclonus in which both clinical and electromyographic findings pointed to the segment C1-C3 as the origin of the myoclonus. Laboratorial examinations were normal and no structural lesion was found in magnetic resonance imaging (MRI). Botulinum toxin type A was injected in infrahyoid muscles and cervical paraspinal musculature. The patient remained free of symptoms for almost five months. The pathophysiology of spinal myoclonus remains speculative, but there is evidence that various possible mechanisms can be involved: loss of inhibitory function of local dorsal horn interneurons, abnormal hyperactivity of local anterior horn neurons, aberrant local axons re-excitations and loss of inhibition from suprasegmentar descending pathways.