Acute Cerebellar Ataxia

1. Definition

• The most common cause of acute childhood ataxia (accounting for ~30–50% of cases).
• It is a self-limiting, autoimmune, post-infectious demyelinating disorder affecting the cerebellum.
• Key Concept: It is a diagnosis of exclusion.

2. Etiology

• Post-Infectious: Typically follows a viral illness by 1–3 weeks.
  • Varicella Zoster (Chickenpox): Classic association (approx. 1 in 4000 cases).
  • Others: Mumps, Epstein-Barr Virus (EBV), Echovirus, Coxsackie, Influenza.
• Post-Vaccination: Rarely reported after Varicella or MMR vaccination.

3. Clinical Features

• Age: Toddlers and school-age children (2–6 years).
• Onset: Sudden (over hours to days).
• The "Happy Ataxic": The hallmark is severe truncal/gait ataxia with preserved sensorium. The child is alert and interactive, unlike in meningitis or toxic ingestion.
• Cerebellar Signs:
  • Gait: Wide-based, reeling, "drunken" gait. Inability to walk or sit without support.
  • Tremor: Intention tremor (finger-nose test), titubation of head/trunk.
  • Speech: Scanning speech/dysarthria (in severe cases).
  • Eyes: Horizontal nystagmus (approx. 50%).
• Absence of Red Flags: No fever (at onset of ataxia), no nuchal rigidity, no signs of raised ICP.

4. Differential Diagnosis (Exclusion is Vital)

• Drug Intoxication: Phenytoin, Carbamazepine, Benzodiazepines, Alcohol. (Check history/access).
• Acute Labyrinthitis: Associated with vertigo/tinnitus.
• Guillain-Barré Syndrome (Miller-Fisher Variant): Ataxia + Areflexia + Ophthalmoplegia.
• Posterior Fossa Tumor: Medulloblastoma (usually subacute, but can present acutely with bleed/hydrocephalus).
• Opsoclonus-Myoclonus Syndrome: "Dancing eyes," irritability (Neuroblastoma).

5. Investigations

• Neuroimaging (MRI Brain):
  • Usually Normal.
  • May show mild cerebellar swelling or hyperintensity.
  • Indication: Mandatory if there is asymmetry, altered sensorium, or raised ICP.
• CSF Analysis: Usually normal or mild lymphocytic pleocytosis with slightly elevated protein.
• Toxicology Screen: If history is ambiguous.

6. Management and Prognosis

• Treatment:
  • Supportive: Prevention of falls (gait assistance).
  • Pharmacotherapy: No specific treatment. Steroids and IVIG are not routinely indicated as the course is benign.
  • Acyclovir: Only if active Varicella is present (controversial utility for ataxia).
• Prognosis: Excellent.
  • Gait improves within 1–2 weeks.
  • Complete resolution in 2–3 months.
  • Permanent sequelae (learning disability/mild incoordination) are rare.