What is the appropriate management for autoimmune encephalitis presenting with bilateral hippocampal hyperintensity on FLAIR MRI and cerebrospinal fluid pleocytosis?

Management of Autoimmune Encephalitis with Bilateral Hippocampal FLAIR Hyperintensity and CSF Pleocytosis

Once infection is ruled out based on CSF analysis, immediately initiate high-dose intravenous methylprednisolone (IVMP) as first-line immunotherapy, or alternatively use IVIG or plasma exchange if steroids are contraindicated. 1

Immediate Diagnostic Priorities

Before starting immunotherapy, you must:

• Rule out infection using basic CSF parameters (cell count, glucose, protein, Gram stain, cultures)
• Exclude CNS lymphoma or neurosarcoidosis if biopsy is being considered
• Send CSF and serum for neuronal autoantibodies (NMDAR, LGI1, CASPR2, AMPA, GABA-B, etc.), oligoclonal bands, IgG index, and IgG synthesis rate 1
• Obtain EEG to assess for epileptiform activity
• Perform cancer screening with CT chest/abdomen/pelvis with contrast, as this presentation can be paraneoplastic 1

Critical pitfall: Do not delay immunotherapy while waiting for antibody results—commercial panels often take weeks, and antibody-negative cases exist. The clinical presentation with bilateral hippocampal FLAIR hyperintensity and CSF pleocytosis is sufficient to justify empiric treatment. 1

First-Line Immunotherapy Algorithm

Standard Presentation (Non-Severe)

Start with IVMP alone (typically 1 gram daily for 5 days), chosen by 84% of experts for general autoimmune encephalitis presentations 1

Alternative first-line options if steroids contraindicated:

• IVIG (0.4 g/kg/day for 5 days) - preferred if patient is agitated or has bleeding disorders 1
• Plasma exchange (5-10 sessions every other day) - preferred if severe hyponatremia, high thromboembolic risk, or associated demyelination 1

Severe Presentation

Use combination therapy from the start (IVMP + IVIG or IVMP + PLEX) if the patient presents with: 1

• Severe NMDAR-antibody picture (if suspected based on age, psychiatric features, movement disorder)
• New-onset refractory status epilepticus (NORSE)
• Severe dysautonomia
• Rapidly progressive deterioration

Sequential Treatment Strategy

If No Improvement After First-Line Therapy

Add a second first-line agent (IVIG or PLEX) if no clinical, radiological, or electrophysiological improvement by end of initial treatment cycle 1

• Add IVIG first if: agitated patient, bleeding disorder
• Add PLEX first if: severe hyponatremia, high thromboembolic/cancer risk, associated demyelination 1

If No Improvement After Combined First-Line Therapy

Initiate second-line immunotherapy 2-4 weeks after completing combined acute therapy if no clinical or radiological improvement and clinical suspicion remains high 1

Choice of second-line agent:

• Rituximab - for known or highly suspected antibody-mediated autoimmunity (e.g., NMDAR, LGI1, CASPR2) - chosen by 80% of experts 1
• Cyclophosphamide - for known or suspected cell-mediated autoimmunity (classical paraneoplastic syndromes with intracellular antibodies) 1

Special Considerations for Bilateral Hippocampal Involvement

The bilateral hippocampal FLAIR hyperintensity pattern suggests:

• LGI1-antibody encephalitis is highly likely - this typically presents with limbic encephalitis, faciobrachial dystonic seizures, and hyponatremia 1, 2
• NMDAR-antibody encephalitis is possible, though less commonly shows prominent hippocampal changes 2
• Seronegative autoimmune encephalitis remains possible even with negative antibody panels 3, 4

Important prognostic finding: Acute hippocampal swelling on MRI in anti-LGI1 encephalitis is associated with worse 12-month functional outcomes and development of mesial temporal sclerosis 5. This reinforces the need for aggressive early immunotherapy.

Bridging/Maintenance Therapy

After acute treatment response, start bridging therapy with: 1

• Gradual oral prednisone taper, OR
• Monthly IVIG, OR
• Monthly intravenous methylprednisolone

Serial antibody titers (if initially positive) can guide duration of maintenance therapy 2

Critical Pitfalls to Avoid

1. Waiting for antibody results before treating - Start empiric immunotherapy once infection is excluded; seronegative cases are well-documented 3, 4, 6

2. Undertreating based on initial antibody negativity - Approximately 20-40% of autoimmune encephalitis cases are seronegative; clinical and radiological features justify treatment 3, 4

3. Missing underlying malignancy - Perform comprehensive cancer screening, especially in patients >40 years or with risk factors 1

4. Stopping treatment prematurely - Improvement may take weeks; complete the full treatment course before declaring failure 1

5. Ignoring hyponatremia - Common in LGI1 encephalitis; requires careful fluid restriction and slow correction to avoid central pontine myelinolysis 1