Autoimmune encephalitis with GABAB antibodies, thymoma, and GABAB receptor thymic expression

Antibody-mediated autoimmune encephalopathies comprise a group of severe conditions with a varying degree of motor and cognitive symptoms that respond to immunotherapies. The associated antibodies are directed against intracellular targets, such as the classic paraneoplastic autoantigens Hu, Yo, Ri, CV2, Ma2/Ta, or the enzyme GAD, or against cell surface antigens such as receptors and ion channels. A newly described pathogenic antibody in autoimmune encephalopathies is directed against the g-aminobutyric acid receptor B (GABAB), a G-protein–coupled receptor. In 2 large series of more than 35 GABAB-seropositive patients, the most common symptoms were limbic encephalitis, seizures, ataxia, and opsoclonus myoclonus; only a single case presented with brainstem encephalitis. The GABAB-associated syndrome can be also paraneoplastic up to 35% of the cases, most often associated with small-cell lung cancer and quite often in conjunction with another paraneoplastic antibody. We present a case of GABAB-associated autoimmune encephalopathy with brainstem manifestations, thymoma, and co-occurrence of anti-Hu and anti-CV2 antibodies. The main novelty of the case is GABAB expression in the thymus, implicating reactivity against the same antigen between thymus and brain.