[Case of Isaacs' syndrome associated with Hashimoto disease].

We report a case of Isaacs' syndrome associated with Hashimoto disease. A 26-year-old woman, who had a past history of Hashimoto disease, complained of involuntary movements and muscle cramp in lower extremities. On examination, myokymia was seen in lower extremities. Myokymia was observed even during sleep, and worsened by exercise or bathing. The antibody against voltage-gated potassium channel (VGKC) was positive. Myokymic discharges were recorded with needle EMG in lower extremities. The patient was diagnosed as having Isaacs' syndrome. Isaacs' syndrome tends to be associated with some other autoimmune diseases. We discussed the correlation between Isaacs' syndrome and autoimmune disease. About 23% of Isaacs' syndrome cases are associated with some other autoimmune diseases and myasthenia gravis was most common. This is the first case report of Isaacs' syndrome associated with Hashimoto disease in Japan.