Patient’s Right Leg Before Treatment

This video illustrates muscle twitching (myokymia) in the calf of a 42-year-old man with psoriasis due to Isaacs syndrome, a syndrome of neuromyotonia (peripheral nerve hyperexcitability) that can be genetic, due to autoimmune disease, or be the manifestations of a paraneoplastic syndrome. This patient had voltage-gated potassium channel (VGKC)–complex antibodies against CASPR2 and LGI1, previously described in association with psoriasis, so the etiology was presumed to be autoimmune. The patient responded to mycophenolate mofetil and prednisone.