1457: Paraneoplastic Myositis From Large B-Cell Lymphoma: A Rare and Elusive Cause of Profound Weakness

Introduction: Cancer-associated myositis is an infrequent cause of profound weakness, most commonly associated with solid tumors like pulmonary and ovarian cancers. It has rarely been linked to lymphoma. Diagnosis in the critically ill is especially challenging when confounded by multisystem disease, evolving weakness, and nondiagnostic early testing. Description: A 59-year-old man with prior melanoma treated with local excision was admitted to an outside hospital with several weeks of cough and proximal muscle weakness. Imaging revealed a right perihilar pulmonary embolism and a 3.7 cm cecal mass. Labs showed leukocytosis and elevated inflammatory markers. He was started on empiric antibiotics and transferred to our facility. Shortly after arrival, he developed worsening respiratory failure requiring intubation and ICU transfer. Infectious, autoimmune, and neurologic workups were pursued. Notable findings included elevated CK, weakly positive ANA, CSF neutrophilic pleocytosis with negative infectious studies, and cervical MRI showing linear nerve root enhancement. EMG showed motor root involvement. Broad-spectrum antimicrobials and steroids were trialed, followed by five sessions of plasma exchange. He required a feeding tube and tracheostomy. Polymyositis was considered, although anti-Jo-1 and anti-SRP were negative. Evaluation of the colonic mass included two nondiagnostic colonoscopic biopsies and a PET scan showing uptake in the cecum, stomach, and mesenteric nodes. Myomarker panel revealed elevated anti-TIF1-γ antibodies, associated with malignancy including colorectal cancer. Given ongoing concern, he underwent right hemicolectomy. Pathology revealed diffuse large B-cell lymphoma. The clinical picture was consistent with paraneoplastic myositis. He was initiated on R-CHP (no vincristine due to neuropathy). After two inpatient cycles, he was discharged to rehab with improving strength and continues to improve on follow-up. Discussion: This case highlights the diagnostic complexity of paraneoplastic syndromes. They should remain on the differential for critically ill patients with unexplained weakness and systemic findings, especially after more common diagnoses are excluded. Autoantibodies like anti-TIF-γ can support the diagnosis. Persistence and multidisciplinary coordination are key.