What is the most likely diagnosis in a 9-year-old girl who developed fever followed by loss of consciousness, with MRI showing bilateral patchy basal ganglia lesions, mild lymphocytic CSF pleocytosis, and who improved with intravenous immunoglobulin (IVIG) and intravenous methylprednisolone (MP)?

Acute Disseminated Encephalomyelitis (ADEM)

This 9-year-old girl most likely suffered from Acute Disseminated Encephalomyelitis (ADEM), a postinfectious immune-mediated encephalomyelitis that characteristically presents with fever followed by neurological symptoms, bilateral basal ganglia involvement on MRI, mild lymphocytic CSF pleocytosis, and dramatic response to immunotherapy with corticosteroids and IVIG.

Clinical Reasoning

The clinical presentation fits the classic pattern of ADEM:

• Age and demographics: ADEM is more commonly seen in children and adolescents 1
• Temporal sequence: Fever followed by acute neurological deterioration (unconsciousness) is characteristic of postinfectious encephalomyelitis
• MRI findings: Bilateral patchy basal ganglia lesions are consistent with ADEM, which typically shows multifocal, usually bilateral but asymmetric hyperintense lesions on T2/FLAIR involving subcortical and deep gray matter 1
• CSF profile: Mild lymphocytic pleocytosis (20 lymphocytes) is typical—ADEM generally shows <1000 WBC/mm³ with lymphocyte predominance 2
• Treatment response: Dramatic improvement with IVIG and methylprednisolone is pathognomonic for immune-mediated rather than infectious encephalitis

Distinguishing ADEM from Infectious Encephalitis

The key distinction is that ADEM is presumed to be mediated by an immunologic response to an antecedent antigenic stimulus (the initial fever/infection) rather than direct CNS infection 1. This distinction is critical because management approaches differ fundamentally:

• Infectious encephalitis requires antimicrobial therapy (e.g., acyclovir for HSV)
• ADEM requires immunosuppression and does NOT benefit from antimicrobials

The bilateral symmetric basal ganglia involvement strongly suggests an immune-mediated process rather than HSV encephalitis, which typically shows temporal lobe predilection 3.

Recommended Treatment Approach

According to IDSA guidelines, high-dose corticosteroids are recommended for ADEM (B-III evidence) 1. The treatment hierarchy is:

1. First-line: High-dose intravenous methylprednisolone (typically 20-30 mg/kg/day for 3-5 days) 4
2. Second-line alternatives (if no response to steroids):
   • Plasma exchange (B-III) 1
   • Intravenous immunoglobulin (C-III) 1

In this case, the patient received both IVIG and methylprednisolone, which is appropriate for severe presentations 4. Recent guidelines suggest combination therapy from the beginning for severe cases with altered consciousness 4.

Alternative Diagnoses to Consider

While ADEM is most likely, other possibilities in the differential include:

Basal Ganglia Autoimmune Encephalitis

Recent literature describes a distinct syndrome of "basal ganglia encephalitis" frequently associated with Mycoplasma pneumoniae infection 5. This presents with:

• Symmetric basal ganglia T2/FLAIR hyperintensities
• Mild CSF pleocytosis
• Positive oligoclonal bands in ~42% of cases
• Excellent response to immunotherapy (85.7% complete recovery) 5

However, this is likely a subtype or variant of ADEM rather than a separate entity, as the pathophysiology and treatment are identical.

MOG-Antibody Associated Disease (MOGAD)

MOG-antibody encephalomyelitis should be considered, particularly with:

• Basal ganglia involvement 6
• Good response to steroids and IVIG
• Potential for relapse (occurs in ~25% of pediatric cases) 3

Testing for MOG-IgG antibodies should be performed 6, though treatment should not be delayed awaiting results 7.

Common Pitfalls to Avoid

1. Do not delay immunotherapy while awaiting antibody results: MOG-IgG and other autoantibody tests can take 5-10 days to result 7. Clinical and radiological suspicion should prompt immediate treatment.

2. Do not assume infectious etiology based on fever alone: Fever is common in ADEM as part of the initial triggering infection 2. The bilateral symmetric basal ganglia pattern argues against HSV or other direct CNS infections.

3. Do not use corticosteroids in suspected infectious encephalitis: If HSV cannot be excluded, start acyclovir empirically while awaiting CSF PCR results 1. However, this patient's bilateral basal ganglia pattern makes HSV unlikely.

4. Monitor for relapse: ADEM is typically monophasic 1, but if symptoms recur, consider MOGAD or other relapsing autoimmune conditions requiring long-term immunosuppression 3.

Follow-up Recommendations

• Clinical monitoring: Most children with ADEM achieve complete recovery (85.7% in recent series) 5
• Repeat MRI: Follow-up imaging at 3-6 months typically shows significant lesion resolution 5
• Antibody testing: Send MOG-IgG, AQP4-IgG to identify relapsing phenotypes requiring different management 6
• Long-term surveillance: If MOG-positive, monitor for relapses which occur in ~25% of pediatric cases 3