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Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a well-recognized form of autoimmune encephalitis, manifesting with a diverse range of neurological symptoms. In this case, we highlight a unique presentation of relapse NMDA encephalitis accompanied by cerebral salt wasting (CSW), characterized by hyponatremia, hypovolemia, and natriuresis. Notably, the association between cerebral salt wasting and autoimmune encephalitis has not been previously studied.

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A 39-year-old man, with a history of anti-NMDAR encephalitis in 2016, presented with late onset relapse marked by refractory status epilepticus. MRI brain w/wo consistent with left temporal lobe encephalitis. Treatment included multiple antiepileptic medications, IVIG, steroids, and rituximab infusion. During second week of hospitalization, he developed polyuria, initially urine output reaching 11L/daily and averaging 4-7L for up to 2 weeks. His labs showed sodium level 130 mEq/L, urine specific gravity 1.011, urine sodium 193 mm/l, and urine osmolality 455 MOSM/Kg. Management involved hypertonic saline (1.8%), 1:1 replacement of urine with normal saline, and continuous sodium monitoring. As NMDA encephalitis responded to treatment and seizure control improved, polyuria gradually resolved.

This seems to be a first case report of NMDA encephalitis leading to cerebral salt wasting (CSW), accentuating atypical manifestations in autoimmune encephalitis management. The previously unexplored connection emphasizes the need for heightened awareness. Further research is urged to comprehend the association of salt wasting with autoimmune encephalitis, an underexplored area.