In a 2‑year‑old child presenting with fever and gait instability, with normal ear, throat, and lung examinations and no cough, vomiting, or diarrhea, what is the most likely diagnosis?

Acute Post-Infectious Cerebellar Ataxia

The most likely diagnosis in this 2-year-old with fever and gait instability (unsteady walking) with an otherwise normal examination is acute post-infectious cerebellar ataxia, though serious bacterial infections—particularly meningitis and urinary tract infection—must be urgently excluded given the high morbidity and mortality risk in this age group.

Critical Immediate Considerations

The combination of fever and ataxia (gait instability) in a toddler is concerning and requires systematic evaluation to rule out life-threatening conditions before attributing symptoms to a benign viral syndrome.

High-Risk Diagnoses to Exclude First

Bacterial Meningitis is the most critical diagnosis to exclude, as only 58% of infants with bacteremia or bacterial meningitis appear clinically ill 1. In children beyond the neonatal period, bacterial meningitis commonly presents with fever (92-93%), but classic signs like neck stiffness are frequently absent in younger children 2. Ataxia can be a neurological manifestation of meningitis, and the absence of neck stiffness, altered mental status, or other classic signs does not rule out this diagnosis 2.

Urinary Tract Infection (UTI) is another serious bacterial infection that must be considered. In well-appearing febrile children aged 2 months to 2 years without an identified source, UTI prevalence is 3-7%, with girls having rates of 8.1% at ages 1-2 years 3. UTIs in young children present with nonspecific symptoms including fever, vomiting, and irritability—not localizing urinary symptoms 3.

Diagnostic Approach

Immediate workup should include:

• Urinalysis and urine culture (via catheterization or suprapubic aspiration, NOT bag collection due to high false-positive rates of 12-83%) 3
• Lumbar puncture with CSF analysis to definitively exclude bacterial meningitis, given the ataxia and the fact that clinical examination alone cannot rule out meningitis in this age group 2
• Complete blood count and inflammatory markers (though these are adjunctive)
• Blood culture given the risk of occult bacteremia, though this has declined significantly in the post-pneumococcal vaccine era 1

The presence of ataxia elevates concern because neurological signs in febrile children can indicate:

• Central nervous system infection (meningitis, encephalitis)
• Post-infectious cerebellitis/cerebellar ataxia
• Acute disseminated encephalomyelitis (ADEM)
• Intoxication or metabolic derangement

Most Likely Diagnosis: Acute Post-Infectious Cerebellar Ataxia

If serious bacterial infections are excluded, the most probable diagnosis is acute post-infectious cerebellar ataxia, which:

• Typically occurs in children aged 1-3 years
• Follows a viral illness (often varicella, but also other viral infections including respiratory viruses)
• Presents with acute onset of truncal and gait ataxia
• Usually has fever that may be resolving or resolved
• Has a benign, self-limited course with full recovery in most cases over days to weeks
• Does not require specific treatment beyond supportive care

Clinical Reasoning

The normal ear, throat, and lung examinations make otitis media, pharyngitis, and pneumonia unlikely. The absence of respiratory symptoms, vomiting, or diarrhea narrows the differential but does not exclude serious infections, as young children with serious bacterial infections often present with nonspecific symptoms 1.

Common Pitfall: Assuming a well-appearing child with fever cannot have a serious bacterial infection. Guidelines emphasize that appearance alone is insufficient—only 58% of infants with bacteremia or meningitis appear ill 1. The presence of ataxia makes this child NOT entirely "well-appearing" and mandates thorough evaluation.

Risk Stratification

According to ACEP guidelines 1:

• Children under 2 years with fever without source require systematic evaluation for UTI
• The presence of neurological signs (ataxia) necessitates CSF evaluation to exclude meningitis
• Even in the post-vaccine era, serious bacterial infections occur, though at lower rates

Management Algorithm

1. Stabilize and assess: Vital signs, hydration status, level of consciousness
2. Obtain cultures BEFORE antibiotics: Urine (catheterized), blood, CSF
3. Initiate empiric antibiotics if any concern for bacterial infection while awaiting culture results
4. If all cultures negative and child stable: Diagnosis of post-infectious cerebellar ataxia is supported
5. Observe for clinical deterioration: Worsening ataxia, altered consciousness, seizures would require neuroimaging and broader infectious/inflammatory workup

Do not discharge this child without excluding serious bacterial infection through appropriate testing. The mortality of serious bacterial infections in young children can be as high as 10% in neonates and remains significant in older infants if untreated 4.