GFAP and optic papillitis: Glial fibrillary acidic protein (GFAP) immunoglobulin G (IgG) is one of the most common neural autoantibodies in patients with central nervous system (CNS) autoimmunity and a biomarker of a steroid‐responsive subacute meningoencephalomyelitis with or without papillitis.1, 2, 3