SAT-062 Anti-rabphilin-3A Antibodies as Diagnostic Clues in Idiopathic Arginine vasopressin deficiency: Differentiating Germinoma from Lymphocytic Infundibuloneurohypophysitis in Children and Young Adults

Abstract Disclosure: Y. Sugimura: None. H. Fujisawa: None. T. Takeuchi: None. N. Iwata: None. S. Nakayama: None. A. Ishii: None. J. Muto: None. H. Kamasaki: None. A. Suzuki: None. [Background] Arginine vasopressin (AVP) deficiency (AVP-D), also known as central diabetes insipidus (CDI), is a rare condition caused by various underlying diseases, including neoplasms, autoimmune disorders, and infiltrative conditions. Differentiating between the etiologies of AVP-D can be challenging. Cases initially classified as "idiopathic" AVP-D may involve pathogenic mechanisms that are not immediately apparent at diagnosis. Notably, even when idiopathic AVP-D is diagnosed at onset, tumors such as germinoma may emerge during follow-up. Importantly, delayed diagnosis of germinoma is associated with worse prognosis. Recently, the detection of anti-rabphilin-3A antibodies (RPH3Aab) has been identified as a highly sensitive and specific marker for lymphocytic infundibuloneurohypophysitis (LINH), an autoimmune-mediated form of AVP-D.[Case presentation] We present three cases: a 13-year-old boy (patient 1), a 19-year-old man (patient 2), and a 20-year-old man (patient 3), all diagnosed with idiopathic AVP-D.In patients 1 and 2, panhypopituitarism developed. Magnetic resonance imaging (MRI) revealed pituitary stalk thickening and pituitary swelling approximately 1.5 years after the onset of AVP-D. Serum RPH3Aab were negative in both patients. They were subsequently diagnosed with germinoma based on pathological examination and underwent chemotherapy followed by radiation therapy. Notably, in patient 2, testosterone and insulin-like growth factor-1 levels normalized, and libido and beard growth recovered after chemoradiotherapy.Patient 3 worked at a family-run ironworks, where habitual fluid intake was encouraged, leading to a daily water intake of 16 liters. MRI revealed no mass lesions or pituitary enlargement but showed a slightly reduced high signal in the posterior pituitary on T1-weighted imaging. A hypertonic saline infusion test demonstrated a relative decline in AVP secretion. The vasopressin challenge test confirmed AVP-D. Following desmopressin treatment and guidance on fluid intake, both urine output and daily fluid intake decreased. RPH3Aab were positive. After five years of follow-up, MRI findings remained stable without tumor development, and the case was ultimately diagnosed as idiopathic AVP-D.[Conclusion] Our findings suggest that when AVP-D is diagnosed in children or young adults, the presence or absence of RPH3Aab provides important diagnostic clues. Specifically, the absence of RPH3Aab may increase the likelihood of non-lymphocytic lesions, including germinoma, while the presence of these antibodies strongly suggests the possibility of LINH. These findings highlight the importance of measuring RPH3Aab in the initial evaluation of idiopathic AVP-D to guide appropriate monitoring and management strategies. Presentation: Saturday, July 12, 2025