Autoimmune Encephalitis
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Direct answer
Autoimmune encephalitis is inflammation of the brain driven by antibodies against neuronal surface or synaptic proteins, most commonly the NMDA receptor. It causes subacute psychiatric change, seizures, memory loss and movement disorders, and it is often linked to a tumour. Antibody testing of CSF and serum guides the tumour search and the choice of immunotherapy, which is often effective in the treatable forms.
What you must remember
- Anti-NMDA receptor encephalitis: Typically in young women, with a prodrome, psychiatric change, reduced speech, orofacial dyskinesias, seizures and autonomic instability.
- Ovarian teratoma: Strongly associated with anti-NMDA receptor encephalitis in young women, and tumour removal improves recovery.
- LGI1 antibody: Older men with faciobrachial dystonic seizures, memory loss and often hyponatraemia.
- GABA-B receptor antibody: Limbic encephalitis with seizures, commonly linked to small cell lung cancer.
- CSF findings: Lymphocytic pleocytosis is common, and CSF antibody testing is more sensitive than serum for NMDA receptor antibodies.
- First-line immunotherapy: Corticosteroids, intravenous immunoglobulin or plasma exchange, started early.
- Mimic to exclude: HSV encephalitis, which needs intravenous aciclovir while CSF PCR and antibody results are awaited.
Antibody types, tumour links and clinical clues
Each antibody has a characteristic clinical picture and tumour association, which guides the search. The table summarises the most examined forms.
| Antibody | Typical picture | Tumour link |
|---|---|---|
| NMDA receptor | Psychiatric change, mutism, dyskinesia, seizures | Ovarian teratoma in young women |
| LGI1 | Faciobrachial dystonic seizures, memory loss, hyponatraemia | Rarely tumour-associated |
| GABA-B receptor | Limbic encephalitis with seizures | Small cell lung cancer |
| AMPA receptor | Limbic encephalitis with confusion | Lung, breast or thymic tumours |
The antibody result therefore tells the clinician where to look for a tumour, such as the ovary in young women or the lung in small cell disease. A negative antibody panel does not exclude the diagnosis, since some antigens remain unidentified and the clinical picture still guides treatment.
Tumour-linked forms, such as NMDA receptor disease with teratoma, need tumour removal alongside immunosuppression. Forms without a tumour, such as many LGI1 cases, often respond to immunotherapy alone, so the tumour search is never skipped.
Where the exam tries to catch you
Examiners reward recognising the antibody picture early and excluding the look-alike infection.
- Trap: Calling a young woman with psychosis, seizures and dyskinesia a primary psychiatric disorder Correct: new psychosis with seizures and dyskinesia should prompt NMDA receptor antibody testing and a pelvic tumour search.
- Trap: Ruling out autoimmune encephalitis on a negative serum antibody Correct: CSF testing is more sensitive for NMDA receptor antibodies, so both serum and CSF should be sent.
- Trap: Withholding aciclovir because the picture looks autoimmune Correct: HSV encephalitis is treatable and fatal if missed, so aciclovir starts empirically while PCR and antibodies are awaited.
Frequently asked questions
Which antibody is most commonly associated with autoimmune encephalitis?
Antibodies against the NMDA receptor are the most common cause of antibody-mediated autoimmune encephalitis in young people. They produce a characteristic evolution from psychiatric symptoms to seizures, dyskinesia and reduced consciousness. CSF antibody positivity is an important diagnostic clue.
Why must ovarian tumours be searched for in anti-NMDA receptor encephalitis?
Ovarian teratoma is strongly associated with anti-NMDA receptor encephalitis, particularly in young women, and it may trigger the immune response. Pelvic ultrasound or MRI is used to search for the tumour, and surgical removal is recommended when one is found. Tumour removal is linked to a better long-term outcome.
Which immunotherapy is used first for autoimmune encephalitis?
First-line immunotherapy for autoimmune encephalitis uses corticosteroids, intravenous immunoglobulin or plasma exchange, usually started early. Rituximab or cyclophosphamide is added as second-line therapy if the response is inadequate. The choice depends on the antibody, the severity and the response to the first line.
How does LGI1 encephalitis differ from anti-NMDA receptor encephalitis?
LGI1 encephalitis typically affects older men with memory loss and faciobrachial dystonic seizures, often with hyponatraemia. Anti-NMDA receptor encephalitis more often affects young women with psychiatric change, dyskinesia and autonomic instability. Antibody testing separates the two when the clinical picture overlaps.