Isaac Syndrome with Intractable Neuropathic Pain Features: A Case Report

Isaac syndrome (IS) is a peripheral nerve hyperexcitability state associated with voltage-gated potassium channel (VGKC) complex antibodies. Major manifestations are muscle twitching, stiffness, hypertrophy, and dysautonomic features such as hyperhidrosis [Ahmed and Simmons. Muscle Nerve . 2015;52(1):5–12]. Neuropathic pain is a rare manifestation. We describe a case of IS characterized by muscle twitching and intractable neuropathic pain. Diagnostic workup included elevated VGKC complex antibodies and EMG/NC that showed neuromyotonic discharges. Neuropathic pain was initially difficult to relieve even after using multiple medications, including opiates, benzodiazepines, anticonvulsants, and intravenous immunoglobulin (IVIg). Moderate pain control was eventually achieved with long-term use of carbamazepine and subcutaneous immunoglobulin (SCIg). Common manifestations of IS are muscle twitching, stiffness hypertrophy, and dysautonomia [Ahmed and Simmons. Muscle Nerve . 2015;52(1):5–12]. Sensory manifestations such as neuropathic pain are rare, but, as illustrated by our patient, can be the most distressing symptom. In our patient, not only was neuropathic pain disabling but it also showed the least response to IVIg. The use of 200 mg of long-acting carbamazepine twice daily with weekly SCIg demonstrated the best response. This case highlights an uncommon but potentially resistant symptom of IS.