A case of autoimmune encephalitis with autoimmune gastritis presenting as refractory hyponatremia

The coexistence of autoimmune encephalitis (AE) and autoimmune gastritis (AIG) is exceedingly rare, and their simultaneous presentation often manifests as refractory hyponatremia, which can be easily overlooked. This report describes a case of anti-leucine-rich glioma-inactivated 1 (LGI1) antibody-associated AE coexisting with AIG, presenting initially as refractory hyponatremia, aiming to enhance awareness of multisystem autoimmune involvement.

A 77-year-old man presented with a two-month history of poor appetite, fatigue, and drowsiness, which had worsened over the preceding week. He had an 8-year history of psoriasis. Laboratory evaluation revealed profound hyponatremia (serum sodium nadir 109 mmol/L), macrocytic anemia (hemoglobin 88 g/L, mean corpuscular volume 118 fL), and deficiencies of vitamin B12 and iron. Gastroscopy with histopathological examination confirmed the diagnosis of autoimmune gastritis. Due to poor response of hyponatremia to conventional sodium supplementation and the presence of left limb spasms, further diagnostic work-up was pursued. Cerebrospinal fluid analysis tested positive for anti-LGI1 antibodies (titer 1:64), and brain magnetic resonance imaging revealed bilateral hippocampal T2-FLAIR hyperintensity and a T2 hyperintense signal in the pituitary gland, leading to the diagnosis of anti-LGI1 antibody-associated AE complicated by central adrenal insufficiency. Following methylprednisolone pulse therapy, the patient's serum sodium normalized, left limb spasms resolved, and mental status improved. At the 8-month follow-up, the patient remained clinically stable.

This case highlights that in patients presenting with refractory hyponatremia accompanied by multisystem manifestations, the possibility of coexisting autoimmune diseases such as AE and AIG should be considered. Comprehensive autoimmune antibody screening and neuroimaging are crucial for early diagnosis and timely intervention.