Segmental myoclonus in a patient affected by syringomyelia

We describe a patient who has been complaining of brief jerk-like, rhythmic , involuntary movements involving the second digit of the left hand for the last three months. These involuntary jerks produced an adduction movement of the second digit and were unaffected by peripheral sensory stimuli. In a ddition, the patient experienced loss of dexterity in the left hand. On exa mination the patient showed hypotrophy of the first dorsal interosseous (FD I) muscle of the left hand and a dissociated sensory loss involving the C8- T1 dermatomes. Magnetic resonance imaging of the brain and spinal cord reve aled a tonsilar herniation characteristic of the Chiari I malformation asso ciated with a syrinx extending from C4 to D5 that did not communicate with the fourth ventricle. The electrophysiological evaluation indicated the pre sence of a focal myoclonus of spinal origin that is likely to be caused by the syrinx.