What is the most likely diagnosis and appropriate management for a 16-year-old female with a one-week history of gait instability (falling to the left), recent vertigo, a viral prodrome one month ago, normal limb cerebellar testing, inability to perform tandem gait, negative Romberg test, and normal laboratory studies?

Post-Infectious Cerebellar Ataxia (Acute Cerebellitis)

The most likely diagnosis is post-infectious cerebellar ataxia, a benign self-limited condition that typically resolves within 2-4 weeks without specific treatment, though MRI brain with contrast should be obtained to exclude alternative diagnoses and acyclovir should be initiated if there is any concern for viral encephalitis. 1, 2

Clinical Reasoning

This 16-year-old presents with the classic triad supporting post-infectious cerebellar ataxia:

• Viral prodrome one month prior followed by delayed neurological symptoms 2, 3
• Isolated truncal/gait ataxia (inability to perform tandem gait, falling to left) with preserved limb coordination (normal finger-to-nose, heel-to-shin) 4, 1
• Negative Romberg test confirming cerebellar rather than sensory ataxia 4

The pattern of truncal ataxia with preserved limb coordination specifically localizes to the cerebellar vermis, which is the characteristic distribution in post-infectious cerebellitis 4. The one-week duration and recent vertigo are consistent with the typical presentation 2, 3.

Diagnostic Workup

Neuroimaging (Essential)

MRI brain with and without IV contrast is the preferred initial imaging modality and should be obtained urgently 5, 1:

• Detects abnormalities in 63.9% of children with acute ataxia 1
• Superior to CT, which misses 34.6% of posterior fossa abnormalities 1
• Contrast enhancement improves detection of inflammatory, infectious, or demyelinating processes 5
• In post-infectious cerebellar ataxia specifically, MRI is typically normal or shows cerebellar edema 2, 6

Additional Testing

• CSF analysis if MRI shows abnormalities or clinical deterioration occurs—pleocytosis is present in 40% of post-infectious cases 2
• EEG should be considered to exclude non-convulsive status epilepticus, which can present with encephalopathy 5, 1
• Serology for recent viral infections (VZV, EBV, Mycoplasma) may support the diagnosis but does not change acute management 7, 3

Management Algorithm

Immediate Assessment (Rule Out Life-Threatening Causes)

Start IV acyclovir 10 mg/kg every 8 hours immediately if ANY of the following are present 1:

• Altered mental status or progressive drowsiness
• Fever or signs of acute encephalitis
• Bradycardia or other autonomic instability
• Clinical deterioration

Do not delay acyclovir while awaiting MRI if encephalitis is suspected—mortality increases dramatically with treatment delay 1.

If Stable Without Encephalitis Features

For isolated cerebellar ataxia with normal mental status (as in this case):

• Supportive care with close observation is the primary management 2, 3
• Fall precautions and assistance with ambulation 4
• Serial neurological examinations to monitor for progression 1, 3

Escalation Criteria

Consider IV immunoglobulin (IVIG) if 7:

• No improvement after 1-2 weeks
• Progressive worsening despite supportive care
• Failure to respond to corticosteroids (if initiated)

High-dose corticosteroids (dexamethasone) may be considered in severe cases, though evidence is limited 7.

Expected Clinical Course

• 91% of patients recover within 30 days 2
• Mean recovery time is less than 2 weeks for gait, with longest duration typically 24 days 3
• Complete recovery without neurological sequelae is the rule 3
• Imaging studies are only needed if atypical presentation or no spontaneous improvement after 1-2 weeks 3

Critical Pitfalls to Avoid

Do not miss alternative diagnoses that require urgent intervention:

• Acute infectious cerebellitis (concurrent fever, altered mental status, more severe systemic illness) requires immediate acyclovir 1
• Posterior circulation stroke (though rare in adolescents, can present with isolated vertigo and ataxia in 11% of cases) 5
• Miller Fisher syndrome (look for areflexia and ophthalmoplegia in addition to ataxia) 4, 1
• Opsoclonus-myoclonus ataxia syndrome (chaotic eye movements, myoclonic jerks) 2

Do not rely on CT imaging alone—it has poor sensitivity for posterior fossa pathology 1, 8.

Do not assume drug intoxication is excluded without a comprehensive drug screen, especially in adolescents where a second peak of ingestions occurs 8.

Age-Specific Considerations

While post-infectious cerebellar ataxia is most common in children 1-6 years old (85% of cases), it does occur in adolescents 2. At age 16, the differential broadens slightly to include:

• Demyelinating disease (first presentation of multiple sclerosis)—contrast-enhanced MRI is essential 5
• Paraneoplastic syndromes—though typically with more systemic symptoms 1
• Episodic ataxia syndromes—though these are typically recurrent 2

The one-month latency from viral illness to symptom onset is longer than the typical 8.8 ± 7.4 days but still within the reported range (1 week to 48 months for VZV-associated complications) 5, 3.