Hidden in the Mediastinum: A Grade 1 Neuroendocrine Tumor Revealed by Refractory Hypokalemia and Ectopic Adrenocorticotropic Hormone (ACTH)-Dependent Cushing's Syndrome
Abstract
Ectopic adrenocorticotropic hormone (ACTH)-dependent Cushing's syndrome (CS) is a rare, fulminant variant of endogenous hypercortisolism that demands rapid localization of the ACTH source to achieve a cure. A 51-year-old woman presented with progressive proximal muscle weakness, centripetal obesity, new-onset hypertension with a blood pressure of 150/90 mmHg at arrival, and persistent hypokalemia (2.2 mmol/L). Biochemical testing confirmed severe hypercortisolism that failed to suppress after an overnight 1-mg dexamethasone test. Plasma ACTH was markedly elevated (798 pg/mL), indicating ACTH-dependent disease. Pituitary MRI showed no adenoma, so an ectopic source was suspected. Contrast-enhanced thoracoabdominal CT revealed a 24 × 14-mm anterior mediastinal node, which was excised via video-assisted thoracoscopic surgery (VATS). Histopathology confirmed a grade 1 neuroendocrine tumor with diffuse positivity for synaptophysin, chromogranin A, and neuron-specific enolase. Twenty-four hours after surgery, serum cortisol decreased to 11.1 µg/dL and ACTH to 2.0 pg/mL; serum potassium normalized without supplementation. Octreotide receptor scintigraphy showed no residual or metastatic disease, and the patient remained normotensive and normokalemic at follow-up. Rapidly progressive hypercortisolism accompanied by refractory hypokalemia should prompt consideration of ectopic ACTH secretion. A stepwise diagnostic approach, including biochemical confirmation, exclusion of pituitary…
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