Objective: This is the first report of a case of anti-gamma-aminobutyric acid B receptor(GABA_BR)antibody-associated encephalitis with refractory hyponatremia and small-cell lung cancer, in order to further understand the clinical features, possible pathogenesis diagnosis and treatment, and prognosis of such patients

Methods: A case of anti-GABA_BR-associated encephalitis with refractory hyponatremia and small-cell lung cancer was reported. The clinical manifestations, assistant examinations diagnosis and treatment and prognosis were reviewed and analyzed. Combined with the review of relevant literature, the possible mechanism was explored.

Results:The patient was an elderly woman who went to the hospital because of "episodic unconsciousness with limb twitching",and soon progressed to status epilepticus accompanied by mental abnormalities and cognitive impairment, The anti-GABA_BR antibodies were detected in blood and cerebrospinal fluid. The refractory hyponatremia was observed. The cerebral MRl showed no obvious abnormality. The PET-CT image showed nodules and masses in the left hilar and lower lobe of the left lung with abnormal increase of ¹⁸F-fuorodeoxyglucose (¹⁸F-FDG metabolism). The small-cell lung cancer with mediastinal lymph node metastasis was diagnosed by percutaneous lung biopsy. After glucocorticoid impact therapy, intravenous infusion of gamma globulin, chemotherapy and radiotherapy for small-cell lung cancer, and symptomatic treatment for seizures, mental symptoms and hyponatremia, the seizures, mental symptoms and cognitive impairment were alleviated, but the hyponatremia still could not be corrected, and the lung cancer lesion gradually increased.In follow-up, it was found that the patient may have brain metastasis from small-cell lung cancer.

Conclusion: Anti-GABA_BR-associated encephalitis with refractory hyponatremia may be related to the abnormal secretion of antidiuretic hormone caused by autoimmunity or small-cell lung cancer, but its mechanism needs to be further explored.