Postpartum-Onset Dermatomyositis Presenting With Malignant Pleural Effusion: A Case Report.

Dermatomyositis (DM) is a rare autoimmune inflammatory myopathy with cutaneous and muscular manifestations. Malignancy is associated with up to one-third of cases in women; gynecological cancer is among the most frequent associations. Postpartum-onset DM is exceptionally rare, with only a few cases in the literature. We report a case of postpartum-onset DM complicated by malignant pleural effusion. A 40-year-old Pakistani woman, 15 days postpartum following a cesarean section, presented with progressive facial swelling, heliotrope rash, photosensitivity, and proximal muscle weakness. She also reported dysphagia, alopecia, and weight loss. A strong family history of cancer was noted. Electromyography and MRI confirmed an inflammatory myopathy. Subsequently, she developed dyspnea and cough. Imaging revealed a right-sided pleural effusion with pleural nodules. Pleural fluid cytology and biopsy confirmed metastatic adenocarcinoma. Immunohistochemistry was positive for PAX8 and cytokeratin-7, suggesting a gynecological origin, although no primary gynecological tumor was identified on imaging. Positron emission tomography (PET)-computed tomography (CT) and laparoscopic staging were not feasible due to financial and clinical constraints. The patient was started on prednisolone and azathioprine, which significantly improved her DM symptoms. For the malignancy, she began systemic chemotherapy. The pleural effusion was managed with chest tube drainage, pleurodesis, and thoracotomy. Despite treatment, her prognosis remained poor due to advanced metastatic disease. Postpartum-onset DM presenting with malignant pleural effusion is rare and may signal an occult malignancy. This case highlights the importance of early cancer screening in similar presentations, as well as the need for a coordinated, multidisciplinary approach to optimize patient management and outcomes.