How Much GAD65 Do You Have? High Levels of GAD65 Antibodies in Autoimmune Encephalitis

Abstract

Neurologic Syndromes Related to Anti-GAD65 Clinical and Serologic Response to Treatment Muñoz-Lopetegi A, de Bruijn MAAM, Boukhrissi S, Bastiaansen AEM, Nagtzaam MMP, Hulsenboom ESP, Boon AJW, Neuteboom RF, de Vries JM, Sillevis Smitt PAE, Schreurs MWJ, Titulaer MJ. Neurol Neuroimmunol Neuroinflamm. 2020;7(3):e696. doi:10.1212/NXI.0000000000000696 Objective: Antibodies against glutamic acid decarboxylase 65 (anti-GAD65) are associated with a number of neurologic syndromes. However, their pathogenic role is controversial. Our objective was to describe clinical and paraclinical characteristics of anti-GAD65 patients and analyze their response to immunotherapy. Methods: Retrospectively, we studied patients (n = 56) with positive anti-GAD65 and any neurologic symptom. We tested serum and cerebrospinal fluid with enzyme-linked immunosorbent assay (ELISA), immunohistochemistry, and cell-based assay. Accordingly, we set a cutoff value of 10 000 IU/mL in serum by ELISA to group patients into high-concentration (n = 36) and low-concentration (n = 20) groups. We compared clinical and immunologic features and analyzed response to immunotherapy. Results: Classical anti–GAD65-associated syndromes were seen in 34 of 36 patients with high concentration (94%): stiff-person syndrome (7), cerebellar ataxia (3), chronic epilepsy (9), limbic encephalitis (9), or an overlap of 2 or more of the former (6). Patients with low concentrations had a broad, heterogeneous symptom spectrum. Immunotherapy was effective in 19 of 27 treated patients (70%), although none of them completely recovered. Antibody concentration reduction occurred in 15 of 17 patients with available pre- and posttreatment samples (median reduction 69%; range 27%-99%), of which 14 improved clinically. The 2 patients with unchanged concentrations showed no clinical improvement. No differences in treatment responses were observed between specific syndromes. Conclusion: Most patients with high anti-GAD65 concentrations (>10 000 IU/mL) showed some improvement after immunotherapy, unfortunately without complete recovery. Serum antibody concentrations’ course might be useful to monitor response. In patients with low anti-GAD65 concentrations, especially in those without typical clinical phenotypes, diagnostic alternatives are more likely.