The diagnostic challenge of small cell lung cancer with anti-gamma-aminobutyric-acid B receptor encephalitis: a case report

BACKGROUND: Anti-gamma-aminobutyric-acid B receptor (anti-GABA(B)R) encephalitis is a rare type of encephalitis, with an incidence of only 5% in all autoimmune encephalitis (AE). A third of patients had pathologically confirmed small-cell lung cancer (SCLC). Early identification and treatment of tumors can improve the poor prognosis. Most patients attend hospital for the treatment of neurological disorders and rarely present with any respiratory symptoms, but suffer from an underlying tumor. Thus, early proper diagnosis is very important. In this article, we report a special case of a patient with anti-GABA(B)R encephalitis with delayed SCLC pulmonary behavior, whose diagnosis prosses was tortuous and complicated. CASE DESCRIPTION: A 62-year-old man with a history of anti-GABA(B)R encephalitis presented with respiratory symptoms. His chest computed tomography (CT) scan showed significant progression of right superior lobe consolidation. A bronchoscopy was performed with no histopathological result, but the microbiological examination of the bronchial lavage fluid showed Aspergillus niger infection. With antifungal therapy, the patient’s respiratory symptoms were significantly relieved. Given the close relationship between SCLC and anti-GABA(B)R encephalitis, we insisted to clarify the pulmonary lesions. The other three biopsies were performed successively, including lung biopsy, endobronchial ultrasonography (EBUS), and transbronchial needle aspiration (TBNA). SCLC was finally proven by pathology, and the patient received systemic chemotherapy. CONCLUSIONS: SCLC is the most common pulmonary neuroendocrine tumor. It secretes onconeural antibodies and is closely associated with paraneoplastic neurologic syndromes (PNSs). Lung tumors, especially SCLC, should be a concern in patients presenting with AE or neurological symptoms, even if they have not any abnormal sign in respiratory system. The early diagnosis and intervention for underlying tumors will improve the clinical outcomes of patients significantly. Thus, the close follow-up is helpful and it is imperative to select and combine the most appropriate examinations for proper diagnosis.