Paraneoplastic Cerebellar Degeneration and Symptomless Lambert–Eaton Myasthenic Syndrome With <scp>SOX</scp>‐1 Antibodies

ABSTRACT A 62‐year‐old man was diagnosed with paraneoplastic cerebellar degeneration (PCD) at a previous hospital due to cerebellar ataxia, positive serum SOX‐1 antibodies, and small cell lung cancer. Although the tumor has shrunk by chemotherapy, the cerebellar ataxia progressed and left him unable to walk. Lambert–Eaton myasthenic syndrome (LEMS) was suspected, because nerve conduction studies showed that reduced compound muscle action potential amplitudes disproportionate to muscle weakness. PCD‐LEMS with SOX‐1 antibodies was diagnosed on the basis of waxing by high‐frequency repetitive nerve stimulation test and high P/Q‐type voltage‐gated calcium channel antibody levels. Immunoglobulin therapy and 3,4‐diaminopyridine contributed to improved activities of daily living by improving muscle weakness masked by cerebellar symptoms. In patients with SOX‐1 antibodies, investigating for tumors and LEMS is necessary, even if muscle weakness is not noticeable.