Syndrome of Inappropriate Antidiuresis and Diabetes Insipidus as Two Sides of the Same Coin in Hypothalamic Lymphoma: A Case Report

Background: Hypothalamic-pituitary region lymphoma is rare and diabetes insipidus (DI) represents one of the most common endocrine manifestations. We report the first case of hypothalamic lymphoma associated with both the syndrome of inappropriate antidiuresis (SIAD) and DI. Case Presentation: A 64-year-old woman with a history of stage IV large B-cell non-Hodgkin lymphoma, underwent atypical right lung resection for pulmonary nodules. A few days after surgery, the patient presented severe normovolemic hyponatremia and serum hypo-osmolarity, therefore, we suspected a paraneoplastic syndrome (SIAD) related to the lung neoplasm, histologically diagnosed as typical carcinoid. The brain magnetic resonance imaging (MRI) showed a 9 mm lesion in the hypothalamic region that significantly increased one month later with the onset of neurological symptoms. A trans-sphenoidal biopsy showed localization of the large B-cell lymphoma. After surgery, the patient presented with polyuria and polydipsia, so desmopressin therapy was started. In the following days, serum osmolarity and sodium fluctuated between normal and low values, then DI was excluded, and SIAD became more likely. Desmopressin therapy was discontinued and hyponatremia was treated with sodium infusion. Hypothalamic lymphoma was treated with chemotherapy and radiotherapy with substantial shrinkage. The hyponatremia persisted during anticancer treatments and improved only after radiotherapy, confirming paraneoplastic SIAD. Conclusion: Lymphomas of the hypothalamic region can cause electrolyte imbalance for various causes. The differential diagnosis between SIAD, DI and impaired thirst centers may not be straightforward and the electrolyte disorders must be evaluated step by step in all different stages of the disease.