MON-420 Diffuse Large B-Cell Lymphoma Presenting as Hypernatremia Secondary to Adipsic Central Diabetes Insipidus

Background: Central diabetes insipidus is usually characterized by polyuria and polydipsia secondary to deficiency in anti-diuretic hormone. In some cases, patients can have hypothalamic or thalamic involvement leading to an impaired thirst mechanism. Here we present a case of a primary central nervous system (CNS) diffuse large B-cell lymphoma which presented as hypernatremia secondary to adipsic central diabetes insipidus. Clinical Case: A 53-year-old female presented to the hospital with confusion and lethargy. Family noted increased urinary frequency. She had been hospitalized at a different facility three times prior for hypernatremia. On admission, she was found to have sodium elevated to 160 mmol/L (135-145). Urine studies were notable for initial urine osmolality 360 with repeat low-normal at 144 mOsm/kg (50-1400), random urine sodium 36 mmol/L, and urine specific gravity <1.005 (1.005-1.03). Further workup included hemoglobin A1C 6.5% (4-6), serum osmolality range 282-311 mOsm/kg (280-300), and normal TSH and free T4. ADH drawn when sodium was 150 mmol/L was undetectable. ACTH stimulation test was normal at 22.6 ug/dL (>18). Free water deficit was calculated at 8 liters. MRI brain showed a 4.1 x 3.5 x 2 cm mass in the right posterior pons extending to the right cerebral peduncle and thalamus with mass effect on the third ventricle. She was hydrated with intravenous D5W then with ½ normal saline. Her urine output was 3.5-5 liters/day. She was given 1 mcg/mL DDAVP injection with urine osmolality increasing from 145 mOsm/kg to 280 mOsm/kg with a greater than 50% increase consistent with central diabetes insipidus. She was started on oral DDAVP prior to discharge with improvement in mental status. She underwent stereotactic brain biopsy with pathology demonstrating primary diffuse large B-cell lymphoma. CT chest/abdomen/pelvis was negative for primary disease and bone marrow biopsy showed no evidence of lymphoma. She was started on chemotherapy with rituximab, methotrexate, procarbazine, and vincristine with CT brain two weeks later showing rapid decrease in the size of the thalamic and midbrain mass. Conclusions: Although pituitary involvement in lymphomas is rare, there are a few cases in the literature describing posterior pituitary involvement in the setting of a primary CNS lymphoma which should be considered in new-onset, non-traumatic central diabetes insipidus. In patients with impaired thirst mechanism, evaluation of the hypothalamus and thalamus should be pursued.