Clinical meaning
Autoimmune encephalitis (AE) encompasses a group of inflammatory brain disorders caused by antibodies directed against neuronal cell-surface or synaptic proteins, resulting in encephalopathy with cognitive impairment, psychiatric symptoms, seizures, and movement disorders. Unlike paraneoplastic encephalitides that involve T-cell-mediated neuronal destruction (targeting intracellular antigens), antibody-mediated AE targets extracellular epitopes on neuronal surface proteins, making the neuronal injury potentially reversible with immunotherapy and antibody removal.
Anti-NMDA receptor encephalitis is the most common and best-characterized form of AE, affecting predominantly young women and children. The N-methyl-D-aspartate receptor (NMDAR) is a glutamate-gated ion channel critical for synaptic plasticity, learning, memory, and excitatory neurotransmission. The NMDAR is a heterotetrameric receptor composed of two GluN1 (NR1) obligatory subunits and two GluN2 (NR2A-D) regulatory subunits that form a cation channel permeable to calcium, sodium, and potassium. In anti-NMDA receptor encephalitis, IgG antibodies target the GluN1 subunit, causing receptor internalization through cross-linking and antibody-mediated endocytosis. This dramatically reduces NMDAR density on the synaptic surface without destroying the neurons themselves.
The reduction in NMDAR function produces a predictable clinical syndrome that mirrors the effects of NMDAR antagonists such as ketamine and phencyclidine (PCP). The pathogenesis proceeds through distinct clinical phases. The prodromal phase (1 to 2 weeks) presents with viral-like symptoms: headache, fever, malaise, and upper respiratory or gastrointestinal symptoms. The psychiatric phase follows with behavioral changes, psychosis (paranoia, delusions, hallucinations), agitation, anxiety, and cognitive dysfunction (memory deficits, language difficulties, disorientation). Many patients are initially misdiagnosed with a primary psychiatric disorder and admitted to psychiatric facilities. The neurological phase then emerges with seizures (often refractory), movement disorders (orofacial dyskinesias, choreoathetosis, dystonia, opisthotonus), speech deterioration (echolalia, mutism), decreased level of consciousness, and autonomic instability. Autonomic dysfunction includes central hypoventilation (requiring mechanical ventilation in 75% of severe cases), cardiac arrhythmias, labile blood pressure, hyperthermia, and sialorrhea. In the most severe cases, patients become catatonic and unresponsive.