Teaching Video NeuroImages: myokymia and nerve hyperexcitability as components of Morvan syndrome due to malignant thymoma.

A 50-year-old woman with a history of metastatic malignant thymoma presented with diffuse neuropathic pain involving the extremities and torso prior to chemotherapy and radiation. She also developed episodic diarrhea, diaphoresis, fevers, insomnia, and encephalopathy. Examination revealed rippling muscles (video on the Neurology® Web site at www.neurology.org). Prolonged afterdischarges were noted in motor nerve studies, suggestive for nerve hyperexcitability (figures 1 and 2). Electromyography (approximately 8 years after mediastinal radiation) revealed fasciculations, doublets, triplets, and myokymic discharges. Elevated serum antibodies for voltage-gated potassium channel (0.30 nmol/L, normal <0.02) and striational muscle (1:30,720, normal <1:60) were suggestive of paraneoplastic Morvan syndrome, which includes all of the features noted in our case.(1,2) The CNS features of Morvan syndrome differentiate it from Isaac syndrome, which is most often due to an autoimmune etiology. A substantial proportion of Morvan syndrome cases are paraneoplastic, the majority of which are due to thymomas.(1).