Abstract: TH-PO1158
Paraneoplastic Syndrome of Inappropriate Antidiuretic Hormone Secretion as a Harbinger of Metastatic Merkel-Cell Carcinoma: An Electrolyte Clue to Disease Progression
Session Information
- Onconephrology: Emerging Biomarkers, Preclinical Models, Clinical Challenges, and Therapeutic Strategies
October 22, 2026 | Location: Exhibit Hall A, Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Onconephrology
- 1600 Onconephrology
Authors
- Colón, Alexandra M., Universidad de Puerto Rico Recinto de Ciencias Medicas, San Juan, Puerto Rico
- Vazquez Morales, Emily, Universidad de Puerto Rico Recinto de Ciencias Medicas, San Juan, Puerto Rico
- Ocasio Melendez, Ileana E., Universidad de Puerto Rico Recinto de Ciencias Medicas, San Juan, Puerto Rico
Introduction
Hyponatremia is the most common electrolyte disorder encountered in clinical practice and is frequently associated with malignancy. The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a well-recognized paraneoplastic phenomenon, most commonly linked to small cell lung carcinoma. Its occurrence in Merkel cell carcinoma (MCC), a rare and aggressive neuroendocrine skin malignancy, is exceedingly uncommon. Early recognition of paraneoplastic SIADH may provide a critical diagnostic clue to occult disease progression or recurrence.
Case Description
An 86-year-old female with a history of MCC status post resection and radiotherapy two years prior, and remote Hodgkin lymphoma, presented with a two-day history of inability to ambulate following two months of progressive lumbar pain. Imaging revealed a metastatic recurrent of MCC with a thoracic spinal mass spanning T8-T10, causing spinal cord compression. Neurosurgical evaluation recommended initiation of intravenous corticosteroids. On admission, the patient was hemodynamically stable, euvolemic, and without focal neurological deficits. Laboratory evaluation revealed severe hypotonic hyponatremia (serum sodium 120 mmol/L) with preserved renal function. Urine studies demonstrated inappropriately elevated urine sodium (74 mmol/L) and urine osmolality (502 mOsm/kg), consistent with SIADH. Evaluation excluded alternative etiologies, including diuretic use, hypothyroidism and adrenal insufficiency. The patient was managed with fluid restriction and oral sodium supplementation, resulting in gradual correction of serum sodium. SIADH was attributed to a paraneoplastic process in the setting of metastatic MCC.
Discussion
This case underscores a rare but clinically significant association between MCC and paraneoplastic SIADH. In contrast to more commonly implicated malignancies, MCC-related SIADH is sparsely reported, which may delay recognition. The patient’s hyponatremia served as a biochemical signal of disease recurrence and systemic progression. For nephrologists and internists, distinguishing SIADH from other causes of hyponatremia is essential, as management hinges on accurate diagnosis and addressing the underlying malignancy. Moreover, malignancy-associated SIADH is linked to increased morbidity, highlighting the importance of timely identification and intervention.