About 20 years ago the group of diseases currently known as ‘autoimmune encephalitis’ or ‘antibody-mediated encephalitis’ was unknown and the entire field of ‘autoimmune neurology’ non-existent. Since then, 18 autoimmune encephalitis and the corresponding syndromes have been described, including 16 in which the antigens are expressed on the cell surface of neurons and two on the surface of glial cells. The characterization of these autoimmune encephalitis was facilitated by the cumulative knowledge provided by research on autoimmune disorders of the neuromuscular junction (myasthenia gravis and Lambert–Eaton myasthenic syndrome) and the paraneoplastic neurological syndromes. Up to 12.6 per 100,000 persons are affected by encephalitis annually. Of these, it has been estimated that 20–30% are caused by autoimmune mechanisms. In children the most frequent types of autoimmune encephalitis are acute disseminated encephalomyelitis (ADEM), anti-MOG, and anti-NMDAR encephalitis. In young adults, particularly women, anti-NMDAR encephalitis, and in late adulthood, anti-LGI1 encephalitis, are the most prevalent autoimmune encephalitis. The most frequently used classifications combine information related to three features: mechanisms of disease (cytotoxic T cell or antibody-mediated mechanisms), type of antigen (intracellular vs cell surface), and presence or absence of a tumour. The detection of a neoplasm frequently serves to categorize the autoimmune encephalitis as paraneoplastic.

Anti-NMDAR encephalitis is the most frequent autoimmune encephalitis. It predominantly occurs in children and young females. Up to 80% of patients present with severe insomnia and psychiatric and behavioural symptoms that resemble those of psychotic episodes caused by primary psychiatric diseases. In addition to the psychiatric manifestations, patients develop neurological symptoms including seizures, abnormal movements, reduced verbal output, and dysautonomic features. Up to 50% of young females have an underlying ovarian teratoma that contains nervous tissue and NMDAR, which probably trigger the immune response. Less frequently, the encephalitis is triggered by an episode of herpes simplex encephalitis probably through the release of antigens by neurons damaged by the virus. The diagnosis of anti-NMDAR encephalitis requires the demonstration of the antibodies in CSF. Up to 14% of patients do not have detectable antibodies in serum. A positive result in serum but negative in CSF must be taken with caution as these patients do not present clinical features of encephalitis and many represent false positive results. Between 80% and 90% of patients respond to treatment which includes immunotherapy and removal of the tumour when it applies.