What is the management approach for an 11-year-old child presenting with cerebellar ataxia and nystagmus following a 10-day history of fever?

Differential Diagnosis: Cerebellar Ataxia with Nystagmus Post-Infection in an 11-Year-Old

The most likely diagnosis is post-infectious cerebellar ataxia, which accounts for approximately 50% of acute ataxia cases in children presenting to emergency departments and carries an excellent prognosis with complete recovery expected within 2-4 weeks. 1, 2

Primary Differential Diagnoses (Ranked by Likelihood)

1. Post-Infectious Cerebellar Ataxia (Most Common - 50-59%)

• This is the leading diagnosis given the 10-day fever history followed by cerebellar signs in an 11-year-old child 1, 2
• Typically follows viral infections (varicella 31%, mumps 20%, non-specific viral 15%, Mycoplasma 5%, EBV 3%) with latency of 8.8 ± 7.4 days from illness to ataxia onset 3
• Clinical features: acute-onset ataxia, nystagmus, dysmetria, and dysdiadochokinesis without extracerebellar signs 4
• Prognosis is excellent: 91% recover within 30 days, with mean recovery under 2 weeks 2, 3
• CSF pleocytosis present in 40% of cases, but brain MRI typically normal 2

2. Acute Infectious Cerebellitis (Part of 33.6% Infectious/Post-Infectious Group)

• Distinguished from post-infectious ataxia by concurrent fever, altered mental status, and more severe systemic illness 1
• MRI shows cerebellar edema in 33.3% of cases 5
• Requires more aggressive monitoring and may need acyclovir treatment 6

3. Acute Disseminated Encephalomyelitis (ADEM)

• Accounts for portion of the 33.6% infectious/post-infectious category 1
• Key distinguishing features: multifocal neurological deficits beyond cerebellar signs, altered consciousness, and characteristic MRI findings 1
• Typically presents with extracerebellar symptoms (encephalopathy, focal motor weakness, cranial nerve involvement) 1

4. Herpes Simplex Virus (HSV) or Varicella-Zoster Virus (VZV) Encephalitis

• Critical not to miss due to high mortality (25% at 12 months with treatment vs 59% without) 6
• HSV can present with ataxia and seizures, though altered mental status and temporal lobe involvement are more typical 1
• VZV-associated cerebellitis can occur days to months after rash (mean 3 months, range 1 week to 48 months) 1
• The presence of altered consciousness, bradycardia, or progressive deterioration mandates immediate empiric IV acyclovir 10 mg/kg every 8 hours 6

5. Mycoplasma pneumoniae Encephalitis with Bickerstaff Brainstem Encephalitis

• Median age 11 years (matches this patient) 1
• Characteristic triad: progressive symmetrical external ophthalmoplegia, ataxia, and areflexia 1
• Short prodrome with fever (70%), lethargy (68%), altered consciousness (58%) 1
• Respiratory symptoms present in only 44% 1

6. Miller-Fisher Syndrome (Guillain-Barré Variant)

• Classic triad: ophthalmoplegia, ataxia, and areflexia 1
• Anti-GQ1b IgG antibody positive 1
• Post-infectious timing fits the clinical scenario 1

7. Posterior Fossa Tumor

• Accounts for 11.2% of acute ataxia presentations 1
• Red flags: symptoms >3 days duration, age >3 years, extracerebellar signs (somnolence, encephalopathy, focal weakness, cranial nerve involvement) 1
• Progressive rather than acute onset more typical 1

8. Cerebellar Stroke (Rare but Critical)

• Rare (3 cases out of 364 children with acute ataxia in one series) 1
• Must be diagnosed urgently due to implications for management 1
• Sudden onset, focal neurological deficits, vascular risk factors 1

9. Drug Intoxication

• Second most common cause after post-infectious ataxia in some series 2
• Requires detailed medication/substance exposure history 1
• Typically shorter duration (<24 hours) 5

10. Influenza-Associated Acute Necrotizing Encephalopathy (ANE)

• Particularly influenza B with severe myositis 1
• MRI shows characteristic bilateral thalamic, brainstem, and white matter lesions 1
• More common in Southeast Asian populations with genetic predisposition 1

Immediate Management Algorithm

Step 1: Assess for Life-Threatening Conditions (First 30 Minutes)

• If altered mental status, bradycardia, or progressive deterioration present: Start IV acyclovir 10 mg/kg every 8 hours immediately before any diagnostic procedures 6
• Check blood glucose with glucose oxidase strip 1
• Assess for signs of increased intracranial pressure or herniation 6

Step 2: Determine Need for Urgent Neuroimaging

Brain MRI with and without contrast is superior to CT and should be performed urgently 6

• MRI detects abnormalities in 63.9% vs CT in only 29.3% of children with acute ataxia 1
• Indications for urgent imaging: 1
  • Extracerebellar neurological signs (somnolence, encephalopathy, focal weakness, cranial nerve involvement)
  • Age >3 years with symptoms >3 days
  • Progressive deterioration
  • Altered consciousness

Watchful waiting acceptable if: 1

• Isolated cerebellar signs (ataxia, nystagmus only)
• Recent viral illness history
• Age <3 years
• Symptoms <3 days
• No extracerebellar signs
• Negative urine drug screen

Step 3: Lumbar Puncture Indications

Perform LP if: 1, 6

• Clinical signs of meningism
• Complex or prolonged convulsion
• Unduly drowsy, irritable, or systemically ill
• Age <18 months (probably), almost certainly if <12 months
• Not completely recovered within 1 hour

CSF studies should include: 6

• Cell count, protein, glucose
• PCR for HSV-1, HSV-2, VZV, enteroviruses
• Bacterial culture
• Consider Mycoplasma serology/PCR

Step 4: Additional Diagnostic Testing

• EEG essential to rule out non-convulsive status epilepticus (occurs in 8% of comatose patients without clinical seizures) 6
• Anti-GQ1b antibody if Miller-Fisher syndrome suspected 1
• Mycoplasma serology if respiratory prodrome 1
• Urine drug screen 1

Common Pitfalls to Avoid

1. Do not delay acyclovir if any concern for HSV/VZV encephalitis - mortality dramatically increases with treatment delay 6

2. Do not rely on CT alone for posterior fossa evaluation - misses 34.6% of abnormalities detected by MRI 1, 6

3. Do not assume benign post-infectious ataxia if extracerebellar signs present - 86% of children with significant neuroimaging pathology had additional focal findings 1

4. Do not perform LP before brain imaging if comatose or signs of increased ICP - risk of herniation 1

5. Do not mistake post-streptococcal ataxia for other causes - timing (days to weeks post-infection) and isolated cerebellar signs are key 4

6. Do not overlook Mycoplasma in 11-year-olds - this is the median age for M. pneumoniae encephalitis, and respiratory symptoms may be absent 1

7. Do not discharge without clear recovery plan if watchful waiting chosen - must be prepared to review decision within hours and image if deterioration occurs 1

Prognosis and Expected Course

For post-infectious cerebellar ataxia (most likely diagnosis): 2, 3

• Complete recovery in 100% of cases
• 91% recover within 30 days
• Mean gait recovery <2 weeks
• Longest duration of neurologic signs: 24 days
• No neurologic sequelae expected
• Imaging studies needed only if atypical presentation or no improvement after 1-2 weeks