SUN-002 - A Challenging Case of Sodium Fluctuations in an Elderly Patient with Arginine Vasopressin Deficiency Secondary to Eosinophilic Granuloma, A Rare Subtype of Langerhans Cell Histiocytosis

Introduction
Arginine vasopressin deficiency (AVD) is characterized by reduced antidiuretic hormone (ADH) production in the posterior pituitary gland. AVD can result from Langerhans cell histiocytosis (LCH) by infiltrating and damaging the posterior pituitary–hypothalamic axis. Eosinophilic granuloma (EG), a subtype of LCH, causing AVD is uncommon. Our case is unique as it describes a patient with a known history of the EG and AVD, who faced significant unexpected sodium (Na) fluctuations during hospitalization.

Clinical case
A 90-year-old female with a history of LCH with EG subtype involving hip bones, AVD, central hypothyroidism, osteoporosis, atrial fibrillation, emphysema, and pulmonary hypertension requiring home oxygen presented to the hospital after a mechanical fall and dyspnea. At home, she was taking oral desmopressin 100 mcg once or twice a day every 48 hours, depending on symptoms, and levothyroxine 75mcg six days a week. Serum Na was 143 mmol/L on admission. On day three, Na increased to 150 mmol/L, with urine output of 250 cc/hour, urine osmolality (UO) of 100 mos/kg, and urine specific gravity of 1.004. She received one dose of intravenous desmopressin 0.5 mcg. Within a day, Na decreased to 143 mmol/L, UO increased to 629 mos/kg, and urine output dropped to less than 100 cc/hour. She was advised to avoid excessive water intake. The next day, Na continued to decrease to 132 mmol/L, and UO was 604 mos/kg. She was fluid restricted to 800 mL/day. Adrenocorticotropic hormone and 8AM cortisol levels were 8.2 pg/mL and 19.2 ug/dL, respectively. Free thyroxine was 1 ng/dL. After several days of fluid restriction, Na increased to 142 mmol/L, with UO of 226 mos/kg, prompting discontinuation of fluid restriction. Her hospital course was complicated by severe dyspnea and was ultimately discharged to long-term comfort care with half a tablet of oral desmopressin 0.1mg as needed. Na was 142 mmol/L at discharge.

Discussion
Our patient presented with complete AVD and central hypothyroidism, both from pituitary involvement by EG. While LCH subtypes are known to cause AVD, there are very few cases in literature describing EG infiltrating the pituitary gland leading to AVD. Despite clear evidence of complete AVD, our patient didn’t require daily desmopressin at home. She experienced significant Na changes after being given desmopressin once while hospitalized. We hypothesize that the patient has clinical AVD from EG with only partial pituitary or stalk infiltration resulting in preserved but insufficient endogenous vasopressin secretion. This likely complicated Na management following exogenous desmopressin administration. Na and urine osmolality were vigilantly tracked to avoid dangerous hyponatremia. Ultimately, fluid restriction improved Na levels after several days. Clinicians should be cognizant of sudden, unpredictable changes in Na in patients with LCH affecting the pituitary gland.

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