SOX‐1 antibodies positive Lambert–Eaton myasthenic syndrome with occult small cell lung cancer: A case report

Abstract Lambert–Eaton myasthenic syndrome (LEMS) is a rare paraneoplastic neurological syndrome of the neuromuscular transmission. The symptoms often progress slowly and can be misdiagnosed in early stage. Seropositive SOX‐1 antibodies are support for the diagnosis of LEMS and have high specificity for small cell lung cancer (SCLC). In this paper, we report a case of a 56‐year‐old man with smoking history who was admitted to hospital with progressive muscle weakness of the proximal legs. LEMS was diagnosed by repetitive nerve stimulation (RNS) testing and seropositive SOX‐1 antibodies. Primary screening with chest computed tomography (CT) and integrated PET/CT did not reveal any tumor. After continuous follow‐up, SCLC was found by chest CT and confirmed with pathological examination 10 months after the diagnosis of LEMS. Long‐term follow‐up and screening for occult SCLC in LEMS patients with positive SOX‐1 antibodies are very important.