When should autoimmune encephalitis be suspected?

When to Suspect Autoimmune Encephalitis

Suspect autoimmune encephalitis in any patient presenting with subacute onset (days to weeks) of altered mental status, new psychiatric symptoms, cognitive decline, seizures, or movement disorders—particularly when accompanied by focal or multifocal brain abnormalities on MRI/EEG or CSF inflammation. 1, 2

Key Clinical Red Flags

Core Presenting Features

The threshold for suspicion should be lower than formal diagnostic criteria to avoid missing cases 3. Consider AE when patients present with:

• Altered mental status or cognitive dysfunction developing over days to weeks
• New-onset psychiatric symptoms (behavioral changes, psychosis, agitation, personality changes) 2, 4
• Seizures, especially:
  • New-onset seizures in adults >50 years (8% prevalence of AE in this population) 5
  • Seizures without clear metabolic or structural cause
  • Refractory seizures or new-onset refractory status epilepticus (NORSE) 1
• Movement disorders, particularly faciobrachial dystonic seizures (highly suggestive of LGI1-antibody encephalitis) 1
• Autonomic instability (dysautonomia, hypoventilation) 1, 4
• Memory deficits, especially mesial temporal dysfunction 5

High-Risk Clinical Scenarios

Immediately suspect AE in these presentations:

• NORSE (new-onset refractory status epilepticus) 1
• Severe dysautonomia requiring ICU admission 1, 4
• Rapid cognitive decline with psychiatric features in young adults
• Encephalitis picture with negative infectious workup 6
• Late-onset seizures (≥50 years) with mesial temporal neuropsychological deficits 5

Supportive Diagnostic Clues

Laboratory and Imaging Findings That Increase Suspicion

CSF abnormalities 1, 5:

• Pleocytosis (≥5 WBC/mm³, though absence doesn't exclude AE)
• Oligoclonal bands present
• Elevated IgG index or synthesis rate
• Note: CSF may be normal in immunocompromised patients or early disease 6

MRI findings 1, 2:

• Focal or multifocal brain abnormalities
• Mesial temporal lobe involvement
• Cortical hyperintensity on diffusion-weighted imaging 7
• Caveat: Normal MRI does not exclude AE

EEG abnormalities 1, 6:

• Nonspecific slowing to distinctive patterns
• Epileptiform discharges
• Subclinical seizure activity

Associated Risk Factors

• Active or known malignancy (significantly increases likelihood) 5
• Recent immune checkpoint inhibitor therapy 1
• Known autoimmune thyroid disease 1
• Young women with ovarian teratoma (NMDAR encephalitis) 4

Critical Pitfalls to Avoid

1. Don't wait for antibody results to initiate treatment 2, 7. A substantial subset remains seronegative, and diagnosis relies on clinical criteria 3, 7, 8

2. Don't dismiss cases with normal paraclinical studies 8. Some patients are "paraclinically silent" and require diagnosis based purely on clinical presentation and exclusion of alternatives

3. Don't confuse post-ictal state with encephalitis 6. The diagnostic criteria require ≥24 hours of altered mentation to exclude simple febrile seizures

4. Don't overlook mimics 3. The proportion of AE mimics is high—actively search for:

   • Infectious encephalitis (HSV, VZV)
   • Primary CNS lymphoma
   • Neurosarcoidosis
   • CNS vasculitis
   • Metabolic encephalopathies

Practical Diagnostic Algorithm

When clinical suspicion is raised:

1. Perform brain MRI and EEG immediately to look for focal/multifocal abnormalities 1

2. Lumbar puncture (once safe) to:

   • Rule out infection (HSV/VZV PCR, cultures)
   • Document inflammation (cell count, protein, oligoclonal bands, IgG index)
   • Send CSF for neuronal autoantibodies 1

3. Send serum for neuronal autoantibodies and rule out alternative causes 1

4. Consider FDG-PET when clinical suspicion is high but MRI/EEG/CSF are uninformative 1

5. Cancer screening (CT chest/abdomen/pelvis) in relevant cases, especially with antibodies against intracellular antigens 1, 9

Once infection is reasonably excluded based on initial CSF results, start empiric immunotherapy without waiting for antibody confirmation 1, 2, 7. Early treatment initiation is critical for favorable outcomes 4, 7.

The diagnosis of "probable" or "possible" AE can be made through logical differential diagnosis even without antibody confirmation, allowing prompt treatment that significantly impacts morbidity and mortality 2, 7.