How can NMDA receptor encephalitis be suspected?

How to Suspect NMDA Receptor Encephalitis

Suspect NMDA receptor encephalitis in any patient with acute or subacute onset (<12 weeks) of psychiatric symptoms combined with seizures, movement disorders, or altered consciousness, particularly when accompanied by CSF inflammation. 1

Core Clinical Criteria for Suspicion

The diagnosis should be considered when all three of the following are present: 1

• Acute or subacute onset (<12 weeks) of neuropsychiatric symptoms
• Evidence of CNS inflammation (CSF pleocytosis, oligoclonal bands, elevated IgG index, or MRI abnormalities such as mediotemporal FLAIR/T2 hyperintensities)
• Exclusion of infectious, toxic, metabolic, or other structural causes

High-Yield Clinical Presentations

Characteristic Symptom Progression

The disease typically unfolds in a recognizable temporal sequence, though individual variability is high: 2

• Psychiatric symptoms (agitation, paranoid thoughts, hallucinations, behavioral changes) often appear first and may dominate the initial presentation 3, 2
• Seizures occur in approximately 72.5% of cases, frequently early in the course 2
• Movement disorders (dyskinesias, dystonia, orofacial dyskinesias) develop in 77.8% and are highly characteristic 2
• Speech dysfunction (mutism, reduced verbal output) appears in 40.7% 2
• Autonomic instability (dysautonomia) emerges later, particularly in adults 4
• Reduced consciousness progresses in severe cases 2

Age-Specific Red Flags

In pediatric patients (<18 years): 4

• Speech disturbances are significantly more common (56.5% vs. 34.6% in adults, p=0.02)
• Movement disorders predominate (56.5% vs. 34.6%, p=0.06)
• EEG abnormalities are more frequent (69.6% vs. 42.3%, p=0.01)
• Male predominance when seizures are the initial symptom (78.57% of cases) 5

In adult patients (≥18 years): 4

• Confusion is more prominent (57.7% vs. 13%, p=0.003)
• Mood disturbances are more frequent (61.5% vs. 26.1%, p=0.03)
• Cognitive impairment is more severe (76.9% vs. 36.1%, p=0.001)
• Dysautonomia occurs almost exclusively in adults (19.2% vs. 0%, p=0.04)
• Female predominance is more pronounced (61.5% vs. 30.4%, p=0.03)

Critical Warning Signs

Immediately suspect NMDA receptor encephalitis when: 2

• New behavioral symptoms arise after a viral prodrome (fever, headache, or flu-like illness precedes neuropsychiatric symptoms in 31.1% of cases)
• Psychiatric symptoms are accompanied by dyskinesias, seizures, or insomnia (insomnia occurs in 39.5%)
• Psychiatric features are atypical (e.g., non-verbal auditory hallucinations, catatonia-like states)
• Young male patients present with seizures as the initial symptom, especially with concurrent headache or fever 5

Diagnostic Workup to Confirm Suspicion

CSF Analysis

CSF findings strongly support the diagnosis: 1

• Mild to moderate lymphocytic pleocytosis (commonly 20-200 cells, but can reach 900 cells)
• Elevated protein (more common in early stages and correlates with abnormal brain MRI) 5
• Positive oligoclonal bands (found in 60% of pediatric cases) 6
• Normal CSF does not exclude the diagnosis (CSF was normal in 12/13 children in one series) 6
• CSF pleocytosis independently predicts ICU admission (OR=11, p=0.01) 4

Antibody Testing Strategy

Test both CSF and serum, as CSF is more sensitive for NMDA receptor antibodies: 1

• CSF antibody detection is superior to serum for NMDAR antibodies 1, 5
• Do not delay immunotherapy while awaiting antibody results—clinical suspicion alone justifies immediate treatment 7
• Send samples for autoimmune encephalopathy panel including NMDAR, VGKC complex proteins, and GAD antibodies 1

Neuroimaging and Electrophysiology

Brain MRI is often normal but may show characteristic findings: 6

• Normal MRI in 100% of one pediatric series does not exclude diagnosis 6
• Mediotemporal FLAIR/T2 hyperintensities when present are supportive 1
• Functional imaging (SPECT) is more sensitive—abnormal in 100% (6/6) of tested children despite normal structural MRI 6

EEG abnormalities are common, especially in children: 6, 4

• EEG changes occur in 84.6% of pediatric cases (11/13 children) 6
• More frequent in pediatric patients (69.6%) than adults (42.3%, p=0.01) 4

Tumor Screening at Initial Suspicion

Cancer screening should begin immediately, as the antibody result is often unknown at presentation: 1

Initial Screening

• CT chest, abdomen, and pelvis with contrast for all patients 1, 7
• Pelvic ultrasound (transvaginal or transabdominal) for young and middle-aged adults to detect ovarian teratoma 1, 7
• Testicular ultrasound for males with typical NMDAR encephalitis presentation 1, 7

Additional Screening When Indicated

• Mammogram for females not up-to-date with screening or with strong family history 7
• Breast MRI if mammogram negative but suspicion remains high 7
• Pelvic MRI if ultrasound is equivocal for teratoma 7
• Whole-body FDG-PET when initial CT negative but malignancy suspicion persists (smoking history, advanced age) 7

Common Pitfalls to Avoid

Do not dismiss the diagnosis because: 6, 2

• CSF is normal (occurs in >90% of some pediatric cohorts)
• Brain MRI is normal (occurs in 100% of some series)
• The patient is male (though less common, males represent 30-40% of pediatric cases)
• Psychiatric symptoms dominate without obvious neurological signs initially

Do not delay treatment: 7

• Begin immunotherapy immediately once infection is excluded, without waiting for antibody confirmation
• Time to treatment directly impacts outcomes—implementation of clinical practice guidelines reduced time to treatment from 7.6 to 3.9 days (p=0.018) and time to diagnostic evaluation from 5.4 to 1.5 days (p=0.0082) 8

Supportive Features That Strengthen Suspicion

The following increase diagnostic probability: 1

• History of other antibody-mediated disorders (e.g., myasthenia gravis) or organ-specific autoimmunity
• Preceding viral prodrome (febrile illness or viral disease-like symptoms)
• Rapid progression from psychiatric symptoms to neurological deterioration over days to weeks