Morvan fibrillary chorea, or Morvan syndrome, is a rare autoimmune syndrome characterized by neuromyotonia (peripheral nerve hyperexcitability resulting in continuous muscle fiber activity) and central nervous system symptoms.

Morvan syndrome is seen almost exclusively in men, with a ratio of 19:1. Patients usually present in their 50s-60s.

Patients typically present with a combination of peripheral and central nervous system symptoms, including neuromyotonia (muscle twitching and cramping, and stiffness) and encephalopathy, with marked insomnia and hallucinations. In addition, patients often have neuropathic pain and autonomic disturbance (tachycardia, hyperhidrosis, salivation, lacrimation, ileus, and urinary symptoms). Some patients also exhibit weight loss, seizures, delusions, agitation, and itching.

The syndrome is caused by autoantibodies to voltage-gated potassium channel (VGKC) complexes. Approximately 40% of patients with Morvan syndrome will have a thymoma.