Autoimmune encephalopathy represents a complex category of disease with diverse immunologic associations, clinical manifestations, and therapeutic outcomes. Three main subgroups include autoimmune encephalopathies without cancer but with neural nonspecific serologic evidence of autoimmunity (encompassing "Hashimoto's encephalopathy") that is the main focus of this review, paraneoplastic encephalopathies, and central nervous system (CNS) vasculitis (primary or secondary). Diagnosis of autoimmune encephalopathy can be suspected based on the clinical course, serologic evidence of autoimmunity, severe but nonspecific slowing on electroencephalography, and evidence of intrathecal inflammation in the cerebrospinal fluid. Rarely, there will be evidence of meningeal enhancement or increased fluid attenuated inversion-recovery signal in symptomatic regions on magnetic resonance imaging, but diagnosis may require brain biopsy demonstration of perivascular lymphocytic infiltrates. Nonspecific autoimmune encephalopathies are generally much more therapeutically responsive than paraneoplastic and vasculitic encephalopathies, so that high-dose corticosteroids may produce dramatic improvement within as little as a few days, although exceptional patients can require months of therapy. Paraneoplastic syndromes typically require tumor removal and often prove fatal. CNS vasculitides may respond to steroid therapy, but often require a second agent such as cyclophosphamide.
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