Brain stem or cerebellar tumors that grow rapidly, acutely hemorrhage, or shift can cause acute decompensation of the brain stem, leading to acute ataxia.

Brain stem encephalitis can be caused by a variety of agents. Common etiologies include Epstein-Barr virus, varicella-zoster virus, enterocytopathogenic human orphan virus, Coxsackievirus, Mycoplasma, and borreliosis (Lyme disease). Ataxia occurs, usually in association with other cranial nerve abnormalities, in children
with diffuse encephalitis; symptoms typically include altered awareness and seizures. Many children recover completely, although some are left with significant neurologic impairment.

Acute cerebellar ataxia is characterized by the sudden onset of ataxia 2 to 3 weeks after infection. Varicella and Epstein-Barr virus are the most commonly identified triggers, but most often no agent is identified. Children between 1 and 5 years of age are most often affected. Diagnosis often relies on clinical signs because confirmatory laboratory studies are not always present. Cerebrospinal fluid (CSF) may show a mild pleocytosis (in about half of cases), and magnetic resonance imaging (MRI) may show mildly abnormal signal in the cerebellum (in a minority of patients). Generally, improvement begins within days of presentation, and there is a good outcome within weeks. Corticosteroids are of unproven benefit in the treatment of acute cerebellar ataxia.

Guillain-Barré syndrome (GBS) is a postinfectious demyelinating disease of the peripheral nerves, which can occur in children at any age. It is a postinfectious disorder that usually occurs in otherwise healthy individuals after a nonspecific infection, although certain infections, such as Campylobacter, are more often associated. Most affected children will present with progressive, ascending weakness, and areflexia. Up to 15% of patients develop a sensory ataxia before other signs of peripheral demyelination are apparent, and some others, though rare, are dominated by autonomic dysfunction. While GBS is still a potentially serious disorder, even life-threatening, intravenous immunoglobulin has dramatically improved the outcome.