Abstract: FR-PO0868
Not Your Average Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Anti-LGI1 Encephalitis
Session Information
- Fluid, Electrolyte, and Acid-Base Disorders: Case Reports - 1
October 23, 2026 | Location: Exhibit Hall A, Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Fluid, Electrolytes, and Acid-Base Disorders
- 1102 Fluid, Electrolyte, and Acid-Base Disorders: Clinical
Authors
- Cuadra Aruguete, Sara Adelina, The University of Chicago, Chicago, Illinois, United States
- Ko, Benjamin S., The University of Chicago, Chicago, Illinois, United States
Introduction
This case describes a rare cause of SIADH, anti-leucine-rich-glioma-inactivated 1 (LGI1) antibody autoimmune encephalitis.
Case Description
The patient is a 65-year-old male with a history of migraine headaches, alcohol use, T2DM, and hyponatremia, presenting for a second opinion of SIADH management.
Over the past year he had 7 admissions for AMS with serum Na+ in the 120-130s with elevated urine sodium and osmolarity. He was diagnosed with SIADH and focal seizures and started on levetiracetam, 1L liter fluid restriction, NaCl 3g TID, demeclocycline 300mg BID, urea powder 15g BID, and tolvaptan 15mg QOD.
Demeclocycline and sodium tablets were discontinued. Repeat labs demonstrated a sodium 123, urine osmolality 984, urine sodium 37.
Within a few weeks he was weaned off urea and sodium remained at 130 with tolvaptan alone. However, seizures increased along with episodes of decreased responsiveness and unilateral arm raising, despite increased levetiracetam and improved sodium control. Three months after his initial visit he had increasing forgetfulness, lethargy, gait instability, incontinence, and anorexia. He became dependent on family to complete IADLs. He was admitted to the hospital for additional management.
During his admission he was found to have an unremarkable brain MRI, bilateral temporal seizures on EEG, elevated protein levels and positive LGI1 antibody on CSF analysis from lumbar puncture. He was started on valproic acid and treated with five days of IV methylprednisolone and rituximab infusion for LGI1 autoimmune encephalitis. He was continued on tolvaptan every other day on discharge.
Within two months from discharge his sodium improved and he was weaned off tolvaptan with normalization of his sodium levels.
Discussion
1. LGI1 autoimmune encephalitis is an autoantibody-mediated disorder with an insidious onset. It presents with a 2:1 ratio male to female and a median age of 64. It is associated with antegrade and retrograde amnesia, frequent focal seizures that causes posturing of the ipsilateral face and arm, and hyponatremia. Diagnosis is commonly delayed as seizures are attributed to hyponatremia.
2. The hyponatremia is associated with elevated urine osmolarity and urine sodium consistent with SIADH.The exact cause is unclear; LGI1 is expressed in mice in the hypothalamus (suggestive of ADH secretion) but also in uncharacterized renal tubules.