Acute cerebellar ataxia, acute cerebellitis, and opsoclonus-myoclonus syndrome.

Acute cerebellar ataxia and acute cerebellitis represent a process characterized by parainfectious, postinfectious, or postvaccination cerebellar inflammation. There is considerable overlap between these entities. The mildest cases of acute cerebellar ataxia represent a benign condition that is characterized by acute truncal and gait ataxia, variably with appendicular ataxia, nystagmus, dysarthria, and hypotonia. It occurs mostly in young children, presents abruptly, and recovers over weeks. Neuroimaging is normal. Severe cases of cerebellitis represent the other end of the spectrum, presenting with acute cerebellar signs often overshadowed by alteration of consciousness, focal neurological deficits, raised intracranial pressure, hydrocephalus, and even herniation. Neuroimaging is abnormal and the prognosis is less favorable than in acute cerebellar ataxia. Acute disseminated encephalomyelitis may be confused with acute cerebellitis when the clinical findings are predominantly cerebellar, but lesions on neuroimaging are usually widespread. Paraneoplastic opsoclonus-myoclonus syndrome is often initially misdiagnosed as acute cerebellar ataxia, but has very specific features, course, and etiopathogensis.