Constipation in a Patient With Thymoma: A Diagnostic Mimic of Myasthenia Gravis

Constipation is a common complaint, but may present atypically in patients with underlying malignancies. We present a 54-year-old man with metastatic thymoma who was admitted with a one-week history of constipation, abdominal discomfort, and nausea. Imaging showed proximal colonic fecal loading with sparing of the descending colon, raising suspicion of dysmotility possibly secondary to a paraneoplastic syndrome. Myasthenia gravis (MG), a known thymoma-associated disorder, was suspected. However, acetylcholine receptor and muscle-specific tyrosine kinase (MuSK) antibodies were negative, and no neuromuscular symptoms were present. The patient responded well to conservative management and was discharged. Two months later, he was readmitted with rapidly progressive bulbar and generalized weakness, severe dysphagia, diplopia, and ptosis, raising renewed concern for MG. Despite persistently negative antibodies, he improved with pyridostigmine and corticosteroids. This case illustrates the evolving nature of paraneoplastic MG, the need to consider both common and rare causes of gastrointestinal symptoms in thymoma patients, and the importance of avoiding premature diagnostic anchoring.