A Second Look at Refractory Edema: Delayed Diagnosis of Paraneoplastic Cushing's Syndrome in Small Cell Lung Cancer

Paraneoplastic Cushing syndrome (PCS) is a rare manifestation of ectopic adrenocorticotropic hormone (ACTH) production, mostly associated with bronchial carcinoid and small cell lung cancer (SCLC). Its clinical manifestations: refractory hypertension, profound hypokalemia, metabolic alkalosis, worsening hyperglycemia, and edema, can easily be misattributed to more common conditions, especially in older adults with multiple comorbidities, leading to diagnostic errors. We present a case of an 84-year-old man with a history of stage IA non-SCLC treated one year earlier, who developed progressive dyspnea, orthopnea, bilateral extremity edema, severe hypokalemia, metabolic alkalosis, and new-onset hypertension. His symptoms were initially managed as volume overload and diuretic-resistant heart failure in the outpatient setting. During hospitalization, persistent metabolic alkalosis, worsening hyperglycemia, resistant hypertension, and refractory hypokalemia prompted further evaluation. Laboratory studies demonstrated markedly elevated early morning cortisol (102.7 µg/dL) and ACTH (293 pg/mL). Computed tomography (CT) imaging revealed a new right infrahilar mass, extensive mediastinal adenopathy, and bilateral adrenal metastases. Endobronchial ultrasound-guided biopsy confirmed SCLC. The patient was diagnosed with paraneoplastic ACTH-dependent CS and initiated on systemic chemotherapy. This case highlights several diagnostic vulnerabilities, including anchoring bias, confirmation bias, premature closure, and failure to integrate multiple abnormal findings into a unifying diagnosis. Earlier recognition of the characteristic cluster of hypercortisolism signs-refractory hypokalemia, metabolic alkalosis, resistant hypertension, and hyperglycemia- may have accelerated diagnosis and treatment. Clinicians should maintain a high index of suspicion for PCS in older adults with a history of lung cancer who present with unexplained electrolyte disturbances and rapidly worsening cardiometabolic parameters. Early diagnosis is critical given the high morbidity and mortality associated with untreated paraneoplastic Cushing's syndrome.