A salty cause of confusion: hyponatraemia heralding an unusual case of CASPR2 autoimmune encephalitis — ASN Events
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A salty cause of confusion: hyponatraemia heralding an unusual case of CASPR2 autoimmune encephalitis (#320)

Keshini Vijayan 1 2 , Emma Hartley 1 2 , Cun Phang 1 , Christian M Girgis 1 2
  1. Endocrinology and Diabetes, Westmead Hospital, Sydney, NSW, Australia
  2. University of Sydney, Sydney, NSW, Australia

Background:

Hyponatremia is well established in cases of Leucine-rich glioma-inactivated 1 (LG-1) limbic encephalitis with the usual mechanism being postulated as syndrome of inappropriate antidiuretic hormone (SIADH) related (1). Specific cases of anti-contactin-associated-protein-like 2 (CASPR2) autoimmune encephalitis with associated hyponatremia are extremely rare with only isolated case reports (2,3). 

Case:

A 65-year-old woman presented with severe hyponatremia associated with dizziness and mild confusion following five days of poor oral intake and recurrent vomiting. Examination revealed hypovolemia, normal vital signs and neurological examination apart from a flat affect, circumstantial thought process and poor insight. Initial investigations showed: serum sodium 105 mmol/L, serum osmolality 234 mmol/kgL, urinary sodium of 38 mmol/L and urinary osmolality 310 mmol/kg. Potentially confounding medications included sacubitril-valsartan and prochlorperazine. A previously identified ovarian mass on CT raised concern for a possible paraneoplastic process.

 

She was initially gently fluid resuscitated and despite serum sodium incrementing to target, her psychiatric symptoms progressively worsened, evolving into profound non-fluctuating confusion, agitation and faciobrachial dystonic seizures confirmed on EEG. She required sedation in the intensive care unit for behavioural management and to facilitate further investigations. Hyponatraemia recurred in keeping with SIADH and she was subsequently fluid restriced: serum sodium 125mmol/L, urinary sodium 93mmol/L, urinary osmolality 501mmol/kg. MRI brain and infective screen were unremarkable. Lumbar puncture was not feasible due to significant agitation. Pelvic ultrasound characterised the known ovarian lesion as a benign simple cyst.

 

She was treated with empiric intravenous methylprednisone for presumed limbic encephalitis resulting in marked clinical improvement.  Full serum autoimmune analysis sent prior to treatment subsequently returned positive for CASPR-2 antibodies. She was discharged neurologically intact and remains well on maintenance rituximab.

 

Conclusion:

Autoimmune and paraneoplastic causes should be considered in patients presenting with unexplained hyponatremia with evolving neuropsychiatric symptoms as prompt diagnosis and early immunotherapy may significantly improve outcomes.

  1. (1) Binks SNM, Saylor D, Easton A, Thakur KT, Irani SR. Encephalitis. Lancet. 2026 May 16;407(10542):1968-1983.
  2. (2) Wang W-Y, Huang J-Y, Xue Y, Zhang A-R, Guo R-Y, Jia Z, Sun Y-F and Li B (2025) Case Report: CASPR2-associated autoimmune encephalitis with NF155 Antibody-positive autoimmune nodopathies: a rare case with hyponatremia onset. Front. Immunol. 16:1519878.
  3. (3) Wu X, Shi M, Zhang H, Fang S. Autoimmune encephalitis with CASPR2 antibody: A case series and updated literature review. Int Immunopharmacol. 2025 Sep 23.