Paraneoplastic Encephalitis Unmasking Small-Cell Lung Cancer With Impending Superior Vena Cava Obstruction: A Case Report.

Small-cell lung cancer (SCLC) is an aggressive neuroendocrine carcinoma that accounts for approximately 15% of all lung cancers and is strongly associated with paraneoplastic syndromes. Among these, paraneoplastic encephalitis (PE) is a rare, immune-mediated inflammation that leads to neuropsychiatric symptoms, seizures, and memory impairment. It may precede the diagnosis of malignancy by weeks to months. Superior vena cava obstruction (SVCO), in contrast, is a more common oncological emergency. Their concurrent presentation as the initial manifestations of SCLC is an uncommon occurrence. We report the case of a 57-year-old woman with a strong history of smoking, who presented with acute behavioral changes and seizures. Despite negative onconeural antibody testing, her presentation was strongly suggestive of autoimmune encephalitis. Immunotherapy with corticosteroids, plasma exchange, and intravenous immunoglobulin was trialed, but her neurological symptoms persisted. Subsequently, she developed neck swelling and dyspnea - imaging revealed mediastinal lymphadenopathy with venous compression concerning for impending SVCO. Endobronchial ultrasound-guided biopsy confirmed small-cell lung carcinoma. She was commenced on carboplatin-etoposide chemotherapy, which led to a clinical improvement in both neurological and obstructive symptoms. This case highlights the diagnostic complexity of paraneoplastic encephalitis, which can mimic psychiatric or infectious causes. Awareness of this rare presentation is crucial, as timely identification and prompt oncological intervention can significantly improve outcomes in patients presenting with unexplained neuropsychiatric symptoms or new-onset seizures.