Autoimmune Encephalitis

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Disclaimer

This reference is intended for medical professionals and serves as a comprehensive educational and clinical guidance tool. It is not a substitute for clinical judgment, direct patient evaluation, or consultation with specialists. Treatment decisions must be individualized based on the patient's specific condition, comorbidities, and current clinical guidelines.

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Overview

Definition

Autoimmune Encephalitis (AE) refers to a heterogeneous group of inflammatory diseases affecting the brain, characterized by the presence of autoantibodies targeting neuronal surface antigens (e.g., NMDA-R, LGI1, CASPR2) or intracellular antigens, leading to acute or subacute onset of neurocognitive, psychiatric, movement, and seizure disorders.

Epidemiology

• Incidence: AE, particularly N-methyl-D-aspartate receptor (NMDA-R) encephalitis, is increasingly recognized as a major cause of non-infectious encephalopathy, potentially more common than viral etiologies (e.g., Herpes Simplex Encephalitis [HSE]) in some centers. Estimated incidence ranges from 0.8 to 1.5 per 100,000 person-years.
• Demographics: AE can affect all ages, but specific types have peaks:
  • Anti-NMDA-R AE: Most common in children, adolescents, and young adults (often associated with ovarian teratomas in women).
  • LGI1/CASPR2 AE (Limbic Encephalitis): More common in older adults, often presenting with seizures and memory deficits.
• Sex: Anti-NMDA-R AE is significantly more common in females (ratio up to 4:1) in association with tumors. Tumor-negative cases show a more even distribution.

Classification

AE is broadly classified based on the target autoantibody:

Classification Type	Target Antigen	Typical Presentation	Paraneoplastic Association
I. Autoantibodies to Neuronal Surface Antigens
Anti-NMDA-R	NMDA Receptor	Psychosis, movement disorders (dyskinesia), central hypoventilation, seizures	Ovarian teratoma (up to 50%)
Anti-LGI1	Leucine-rich glioma-inactivated 1	Limbic features, faciobrachial dystonic seizures (FBDS)	Thymoma, small cell lung cancer (SCLC) (rare)
Anti-CASPR2	Contactin-associated protein-like 2	Morvan syndrome (insomnia, hyperthermia, dysautonomia), neuromyotonia	Thymoma, SCLC
Anti-GABA-A/GABA-B	Gamma-aminobutyric acid receptor	Severe refractory seizures, status epilepticus	SCLC
II. Autoantibodies to Intracellular Antigens (Paraneoplastic)
Anti-Hu (ANNA1)	Hu protein	Primarily brainstem/limbic (Paraneoplastic Limbic Encephalitis)	SCLC
Anti-Ma2 (Ta)	Ma2 protein	Hypothalamic/brainstem dysfunction, testicular cancer	Testicular tumor

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Pathophysiology

Mechanism

• Surface Antigens (e.g., NMDA-R, LGI1): Primarily involves antibody-mediated receptor modulation and internalization, leading to a functional deficit rather than direct cell death. Example: Anti-NMDA-R antibodies reduce NMDA receptor density on the neuronal surface, causing hypofunction, consistent with psychiatric symptoms. These are generally better responsive to immunotherapy.
• Intracellular Antigens (e.g., Hu, Ma2): These antibodies are thought to be markers of a severe T-cell mediated cytotoxic response against neurons expressing the antigen. They are highly associated with underlying cancer and often result in greater irreversible neuronal damage, leading to poorer prognosis.

Risk Factors

• Underlying Cancer: High risk for AE related to intracellular antibodies (e.g., SCLC, breast, ovarian, testicular cancer). Common tumors include teratoma (NMDA-R), SCLC (Hu, Ma2, CRMP5), and Thymoma (LGI1, CASPR2).
• Infection: A preceding viral infection (e.g., HSV, Mycoplasma pneumoniae) may trigger an autoimmune response via molecular mimicry, leading to cross-reactivity (particularly noted in pediatric anti-NMDA-R cases).
• Genetic Predisposition: Associations with specific HLA haplotypes (e.g., HLA-DRB1*07:01, HLA-DQB1*02:02 for LGI1 AE).

Protective Factors

• No definitive protective factors are known, as AE is an idiopathic autoimmune condition. Early and aggressive immunotherapy is the most critical factor influencing functional outcome.

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Clinical Presentation

Signs and Symptoms

Feature	Anti-NMDA-R AE	Anti-LGI1/CASPR2 AE	Anti-GABA-B AE
Cardinal Features	Psychosis, Seizures, Dyskinesia, Cognitive decline	Amnesia (anterograde), Seizures (FBDS), Hyponatremia	Refractory Epilepsy, Status Epilepticus
Early Signs	Flu-like prodrome, Headache, Insomnia, Rapid onset of psychiatric symptoms (paranoia, agitation)	Subacute onset of short-term memory loss (Limbic Encephalitis)	Onset with focal or generalized seizures
Advanced Signs	Catatonia, Central hypoventilation (ICU required), Autonomic instability, Refractory status epilepticus (RSE)	Profound amnesia, Coma (rare), Morvan syndrome (CASPR2)	Oromotor dyskinesia
Atypical Presentations	Isolated seizures, Minimal psychiatric features	Isolated neuromyotonia (CASPR2), Pure limbic presentation	Primary psychiatric presentation

Faciobrachial Dystonic Seizures (FBDS): Highly specific for anti-LGI1 AE (up to 70%). These are ultrashort (seconds-long), asymmetrical dystonic posturing of the face and arm/leg, often rapidly recurring (many per day) and frequently misdiagnosed as focal motor seizures.

Physical Examination

• Inspection:
  • Mental Status: Severe agitation, catatonia (especially NMDA-R), reduced level of consciousness.
  • Movement: Orofacial dyskinesia, choreoathetosis, stereotypic movements, generalized rigidity/dystonia.
  • Vitals: Significant autonomic instability (tachycardia, labile blood pressure, hyperthermia/hypothermia).
• Palpation: General evaluation for peripheral masses (e.g., lymphadenopathy, testicular/ovarian masses).
• Special Tests: Evaluation for ocular movement abnormalities (e.g., vertical gaze palsy in anti-Ma2 AE), signs of peripheral nerve hyperexcitability (neuromyotonia, fasciculations, associated with CASPR2).

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Diagnostic Approach

AE is a diagnosis of exclusion that requires a high index of suspicion, often necessitating treatment initiation before confirmatory antibody results are available.

History Taking

• Key Questions:
  • Onset and trajectory of symptoms (acute vs. subacute, usually < 12 weeks).
  • Presence of psychiatric symptoms (new onset psychosis, behavioral changes).
  • Detailed seizure description (focal, generalized, FBDS).
  • Autonomic instability (unexplained fever, cardiac arrhythmias, blood pressure lability).
  • History of cancer (personal or family), or recent viral prodrome.
• Red Flags (Require Urgent Immunotherapy):
  • Rapidly progressive encephalopathy.
  • Refractory status epilepticus (RSE).
  • Severe autonomic instability or central hypoventilation requiring ICU support.
  • Catatonia.

Laboratory Tests

Test Category	First-Line (Empiric/Exclusion)	Confirmatory (Diagnostic)	Monitoring (Follow-up)
General Labs	CBC, CMP, LFTs, VDRL/RPR, TSH. Electrolytes (Hyponatremia is suggestive of LGI1/CASPR2 AE).		General health/Drug toxicity.
Infectious Panel	Blood cultures, CSF analysis (Cell count, protein, glucose), CSF/Serum PCR for HSV, VZV, EBV, Enterovirus, HIV.
CSF Analysis	Mild pleocytosis (usually < 100 cells/µL, lymphocytic predominance), Elevated protein. Oligoclonal bands (OCBs) present in ~50-80% of cases.		IgG index/OCBs to assess disease activity (not standardized).
Autoimmunity	ANA, ESR, CRP, Complement levels.	CSF and Serum Antibody Panels (Cell-Based Assays are gold standard) for: NMDA-R, LGI1, CASPR2, GABA-B, AMPA-R, GAD65, Hu, Ma2, CRMP5, Amphiphysin.	Repeat antibody titres (NMDA-R titers correlate with relapse/response).
Tumor Screen	Age-appropriate tumor screening (e.g., LDH, AFP, Beta-HCG, CA-125, mammogram, testicular exam/ultrasound).		Long-term surveillance as indicated by underlying tumor type.

Critical Note: Antibodies should be tested in both serum and CSF. CSF testing is more specific and often remains positive after serum clearance, especially for NMDA-R antibodies.

Imaging

• Recommended (Standard):
  • Brain MRI with Contrast: Essential to rule out infectious, vascular, or structural etiologies.
    • Classic Findings: T2/FLAIR hyperintensities, primarily involving the medial temporal lobes, often bilateral (Limbic Encephalitis pattern, typical of LGI1, GABA-B, and intracellular antibodies).
    • NMDA-R AE: Often normal (50%), or shows non-specific changes in the cerebellum, cortex, or basal ganglia.
• Advanced (Specialized):
  • Whole-Body PET/CT: Highly recommended in all AE subtypes, especially those with high paraneoplastic risk, and mandatory if intracellular antibodies are found. Used to identify underlying occult tumors (teratoma, SCLC).
  • FDG-PET (Brain): Often shows abnormal metabolism.
    • NMDA-R AE: Frontotemporal and basal ganglia hypermetabolism during the acute phase, followed by global hypometabolism during recovery.
    • Limbic AE: Hypermetabolism in the medial temporal lobes.
  • Pelvic/Abdominal Ultrasound: Targeted search for ovarian teratomas (especially in young women with NMDA-R AE).

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Management

Acute Management

AE is a medical emergency requiring hospitalization, often in an Intensive Care Unit (ICU) for stabilization and monitoring.

Immediate (First-hour interventions)

1. Airway/Breathing: Assess for central hypoventilation (common in advanced NMDA-R AE) and prepare for intubation/mechanical ventilation.
2. Seizure Control: Treat seizures aggressively with standard anticonvulsants (e.g., benzodiazepines, levetiracetam). If refractory, proceed to RSE protocols.
3. Vitals: Manage autonomic instability (labile BP, arrhythmias) with short-acting agents in ICU setting.

Stabilization (Early Management Priorities)

1. Infectious Exclusion: Start empiric broad-spectrum antibiotics and Acyclovir (10 mg/kg IV q8h) for presumed viral encephalitis (e.g., HSV) until infectious etiology is ruled out by CSF PCR.
2. Initiate Immunotherapy: Once high suspicion for AE exists and life-threatening etiologies are ruled out, initiate first-line immunotherapy immediately.
3. Tumor Search: Start expedited tumor screening (body PET/CT, pelvic US/MRI).

Emergency (Crisis Management)

• Refractory Status Epilepticus (RSE): Use continuous infusions (e.g., midazolam, propofol) and consider third-line agents (ketamine, magnesium, deep anesthesia/coma induction).
• Malignant Catatonia: Treat with high-dose benzodiazepines (lorazepam challenge), aggressive immunotherapy, and consider Electroconvulsive Therapy (ECT) if unresponsive to lorazepam.

Chronic Management

• Anticonvulsants: Patients typically require ongoing seizure or FBDS prophylaxis (e.g., LGI1 AE) for at least 6-12 months post-recovery.
• Psychiatric Support: Aggressive management of residual behavioral and psychiatric symptoms (antipsychotics, mood stabilizers).

Treatment Algorithms

The treatment approach is stratified into three main phases: First-line, Second-line (Rescue), and Refractory.

First-Line Treatment

Initiated immediately upon strong clinical suspicion (often before positive antibody results).

1. High-Dose Glucocorticoids:
   • Dosage: Methylprednisolone 1000 mg IV daily for 3-5 days.
   • Followed by: Oral prednisone taper over 4-6 weeks.
2. Intravenous Immunoglobulin (IVIg):
   • Dosage: 0.4 g/kg/day for 5 days (Total cumulative dose of 2 g/kg).
3. Plasma Exchange (PLEX/PE):
   • Protocol: 5-7 exchanges over 7-10 days, exchanging 1-1.5 plasma volumes per session.

Note: PLEX and IVIg are generally considered interchangeable and provide equivalent efficacy. Some clinicians prefer PLEX for rapid removal of antibodies in severely ill patients.

Second-Line Treatment (Rescue Therapy)

Used if improvement is absent or minimal after 10-14 days of first-line therapy, or in severe/paraneoplastic cases.

• Rituximab:
  • Mechanism: Targets CD20 B-cells, reducing antibody production.
  • Dosage: 375 mg/m² weekly for 4 doses OR 1000 mg on Day 1 and Day 15 (fixed dose).
• Cyclophosphamide:
  • Dosage: 750 mg/m² monthly for 6 months, often combined with Rituximab (used primarily for refractory disease or paraneoplastic AE).

Refractory Cases

(Lack of improvement or relapse after First- and Second-line treatments.)

• Combination Therapy: Rituximab + Cyclophosphamide.
• Deeper Immunosuppression: Mycophenolate mofetil (MMF) or Azathioprine (AZA) for maintenance, or Alemtuzumab (rarely used, high risk).

Tumor Management

• Removal of Tumor: In paraneoplastic AE (especially NMDA-R with teratoma), tumor removal is crucial for symptom resolution and prevention of relapse. Surgery should be performed promptly once the patient is stable enough.

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Complications

Common

• Relapse (10-30% in NMDA-R AE, lower in LGI1, higher if tumor untreated).
• Residual cognitive impairment (memory deficits).
• Residual psychiatric symptoms (mood disorders, anxiety).

Serious

• Refractory Status Epilepticus (leading to permanent neuronal injury).
• Central hypoventilation and autonomic collapse (requiring prolonged ICU stay).
• Rhabdomyolysis due to severe catatonia or dyskinesias.
• Immunosuppression-related complications (infections—PCP prophylaxis often needed with combined therapy).

Long-term

• Epilepsy (high risk, especially in GABA-B AE).
• Residual neurocognitive deficits affecting return to work/school.
• Need for chronic psychotropic medication.

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Prognosis

Factors Affecting Outcome

Favorable Factors	Poor Prognostic Factors
Early initiation of immunotherapy (< 4 weeks).	Delayed diagnosis (> 3 months).
Antibodies to neuronal surface antigens (e.g., NMDA-R, LGI1).	Antibodies to intracellular antigens (e.g., Hu, Ma2, CRMP5).
Tumor removal (if applicable).	Need for ICU admission or prolonged mechanical ventilation.
Lack of severe cerebral atrophy on follow-up MRI.	Refractory status epilepticus.

Survival

• Overall prognosis for most surface-antibody AE is good to excellent, with 70–80% achieving good functional recovery (Modified Rankin Scale [mRS] 0-2).
• AE associated with intracellular antibodies (paraneoplastic) carries a significantly worse prognosis, often with permanent functional deficits and lower survival rates linked to underlying cancer progression.

Quality of Life

Full cognitive and psychosocial recovery may take months to years. Children often recover faster and more completely than adults. Long-term quality of life is highly dependent on residual cognitive deficits, memory issues, and psychiatric sequelae.

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Prevention

Primary

There is no known method for primary prevention of idiopathic AE.

Secondary

Early detection and Prompt Intervention: High awareness among neurologists and psychiatrists is key. Prompt initiation of empiric immunotherapy in seronegative but clinically suggestive encephalopathy improves outcomes.

Tertiary

• Relapse Prevention: Monitoring antibody titers and scheduling maintenance therapy (e.g., Rituximab every 6 months for 2 years) may be necessary, particularly after a first relapse.
• Cancer Surveillance: Regular screening (every 6 months for 3-5 years) for tumor recurrence or initially missed tumors is vital in seropositive AE.

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Special Populations

Pediatric

• Diagnosis: Often presents initially with behavioral problems or sleep disorders, followed by seizures or movement disorders. Psychiatric symptoms are often less severe than in adults.
• Treatment: Dosing is weight-based. Full recovery is often seen, but long-term neurodevelopmental follow-up is critical.

Geriatric

• Presentation: More frequently presents as simple limbic encephalitis (memory loss, seizures). Paraneoplastic association is higher. FBDS are highly characteristic of LGI1 AE in this group.
• Comorbidities: Greater risk for complications from immunosuppressive therapy (infection, cardiac events).

Pregnancy

AE during pregnancy is rare but documented. Treatment must balance maternal benefit and fetal risk: * Immunotherapy: PLEX is generally considered safer than IVIg or steroids in the first trimester. Rituximab is generally avoided unless the disease is life-threatening. * Delivery: Premature delivery may be required if maternal condition rapidly deteriorates.

Comorbidities

• Pre-existing Psychiatric Disease: Symptom overlap can delay diagnosis. A rapid change or worsening inconsistent with the baseline illness should prompt AE workup.
• Chronic Infections: Increased risk of infection; prophylactic antibiotics (e.g., trimethoprim-sulfamethoxazole for PCP) are recommended if long-term, high-dose corticosteroids or cytotoxic agents are used.

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Clinical Pearls

Diagnostic

• "Triple R Rule": If a patient presents with Rapid progression, Refractory seizures, and Requires ICU care (especially with central hypoventilation), think Anti-NMDA-R AE.
• Hyponatremia and FBDS: Consider anti-LGI1 AE strongly if these two features are present, even with subtle inflammation on CSF/MRI.

Treatment

• Treat Empirically: Do not wait weeks for antibody results if the clinical picture is compelling. Delaying immunotherapy is the greatest predictor of poor outcome.
• Tumor vs. Antibodies: In paraneoplastic AE, tumor removal is a primary therapy, not just an adjunct.
• Psychiatry Management: Avoid typical high-potency antipsychotics, which may worsen movement disorders or lower the seizure threshold. Low-dose quetiapine or benzodiazepines are often preferred for agitation/psychosis.

Pitfalls

• Misdiagnosis: Often misdiagnosed as viral encephalitis (due to mild CSF pleocytosis), primary psychosis, or non-epileptic seizures.
• FBDS Misinterpretation: Misinterpreting FBDS as simple focal motor seizures or psychiatric movements leads to poor seizure control with standard AEDs. FBDS usually respond better to immunotherapy.

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Recent Updates

Guidelines

• The International Autoimmune Encephalitis Study Group (IAESG) Diagnostic Criteria: Emphasize the importance of combining clinical findings (defined syndrome), results of core labs (MRI, CSF), and antibody findings to establish definite, probable, or possible AE. Seronegative AE diagnosis is now explicitly recognized but requires rigorous exclusion of alternative causes.

Evidence

• Maintenance Therapy: Increasing evidence supports the use of maintenance immunotherapy (e.g., Rituximab) for at least 1-2 years in patients with NMDA-R AE who have experienced a severe presentation or relapse, although standardized protocols are still evolving.

Controversies

• Optimal Timing and Sequence of PLEX vs. IVIg: While both are frontline, debate continues on whether one is superior for specific syndromes, though most evidence suggests equivalent efficacy.
• Role of Antibody Titers: While serum anti-NMDA-R antibody titers typically correlate with disease activity and relapse risk, the utility of routine CSF titer monitoring for guiding treatment de-escalation remains controversial.

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Key References

• Graus, F., et al. (2016). Classification and diagnostic criteria for autoimmune encephalitis. Lancet Neurology.
• Dalmau, J., et al. (2017). Anti-NMDA-receptor encephalitis: clinical spectrum and long-term care. Nature Reviews Neurology.
• Irani, S. R. (2018). Autoimmune encephalitis—therapeutic challenges and future directions. Clinical Neuropharmacology.
• Lancaster, E. (2016). The diagnosis and treatment of autoimmune encephalitis. Journal of Clinical Neurology.