A case report of Isaacs' syndrome with treatment-resistant pain responsive to rituximab.

Isaacs' syndrome is an immune-mediated primary peripheral nerve hyperexcitability syndrome characterized by muscle twitches, cramps, and muscle stiffness. In 30-50% of cases, antibodies against voltage-gated potassium channel (VGKC) complex proteins are present. Symptomatic treatment, intravenous immunoglobulin (IVIg), plasma exchange (PE), and intravenous methylprednisolone (IVMP) are generally effective in controlling symptoms. However, some cases remain treatment-refractory. We present a 52-year-old female patient with walking difficulty, fasciculations, and severe back and hip pain. The patient was positive for anti-VGKC antibodies with a history of thymoma. Symptomatic treatment, highdose IVMP, IVIg and PE treatments improved the patient's gait partially but failed to control the pain and fasciculations. Eventually, we treated the patient with rituximab (RTX), because of its effect on antibody-mediated autoimmunity, resulting in complete symptomatic relief without side effects. RTX is a safe and effective option for controlling symptoms in treatment-refractory cases with Isaacs' syndrome.