MON-014 Impact of Chiari 1 Malformation Decompression on Arginine Vasopressin Deficiency and Desmopressin Dosing: A Case Report

Abstract Disclosure: D. Kaul: None. M. Paravastu: None. M. Shah: None. T. Yasmeen: None. Introduction: Chiari 1 Malformation (CM 1) is characterized by the downward herniation of the cerebellar tonsils into the spinal canal through an opening called the foramen magnum. The prevalence of CM1 is approximately 1% and is associated with a range of neurological symptoms, including headaches, visual disturbances, paresthesias, and numbness. In 2007, Guo et al. reported two arginine vasopressin (AVP) deficiency cases among 128 Chiari Malformation cases analyzed. More recently, in 2022, Roohollahi et al. highlighted AVP deficiency as a rare but possible presentation of CM1, noting that it can resolve following decompression surgery. Clinical Case: A 32-year-old female with a past medical history of arginine vasopressin deficiency (diagnosed at age 3) and a CM 1 (diagnosed in childhood but never further evaluated) presented to the neurosurgery clinic for evaluation of progressively worsening headaches for the past 4 months. She also reported intermittent left-sided facial tingling, nausea, and lightheadedness. An MRI of the brain revealed a 12-mm cerebellar tonsillar herniation with mild crowding at the foramen magnum without evidence of a syrinx in the upper cervical cord. These findings were consistent with CM 1. The patient was admitted for surgical management and underwent Chiari decompression with duraplasty. Before hospitalization, she was on desmopressin (DDAVP) nasal spray (0.01%), using one spray in the morning and afternoon and two sprays in the evening. Postoperatively, the medical team restarted the patient on her pre-admission DDAVP regimen. However, on post-op day 2, she developed moderate to severe symptomatic hyponatremia, with a serum sodium of 118 mmol/L (reference range 135-145 mmol/L). Her urine output was 1950 mL, with a serum osmolality of 281 mOsm/kg (reference range 275-300 mOsm/kg) and urine osmolality of 566 mOsm/kg (reference range 50-1,200 mOsm/kg), suggesting excessive DDAVP administration. Subsequently, the medical team suspended her DDAVP and placed her on fluid restriction. Over the next 48 hours, her serum sodium gradually improved. Notably, she did not exhibit any clinical or laboratory signs of AVP deficiency until three days postoperatively. At that time, DDAVP was reintroduced at a lower dose, with doses intermittently withheld due to recurrent hyponatremia. The patient was ultimately discharged on a reduced DDAVP dose of 25 mcg twice daily, significantly lower than her pre-admission dose. Conclusion: This case underscores the significance of recognizing the potential impact of Chiari type 1 malformation decompression surgery on arginine vasopressin deficiency. AVP deficiency, though rare, can be an atypical manifestation of CM1. Following decompression surgery, the patient's need for desmopressin may decrease or even resolve, highlighting the importance of reassessing endocrine treatment requirements post-surgery. Presentation: Monday, July 14, 2025