Disorders of the cerebellum: ataxia, dysmetria of thought, and the cerebellar cognitive affective syndrome.

Many diseases involve the cerebellum and produce ataxia, which is characterized by incoordination of balance, gait, extremity and eye movements, and dysarthria. Cerebellar lesions do not always manifest with ataxic motor syndromes, however. The cerebellar cognitive affective syndrome (CCAS) includes impairments in executive, visual-spatial, and linguistic abilities, with affective disturbance ranging from emotional blunting and depression, to disinhibition and psychotic features. The cognitive and psychiatric components of the CCAS, together with the ataxic motor disability of cerebellar disorders, are conceptualized within the dysmetria of thought hypothesis. This concept holds that a universal cerebellar transform facilitates automatic modulation of behavior around a homeostatic baseline, and the behavior being modulated is determined by the specificity of anatomic subcircuits, or loops, within the cerebrocerebellar system. Damage to the cerebellar component of the distributed neural circuit subserving sensorimotor, cognitive, and emotional processing disrupts the universal cerebellar transform, leading to the universal cerebellar impairment affecting the lesioned domain. The universal cerebellar impairment manifests as ataxia when the sensorimotor cerebellum is involved and as the CCAS when pathology is in the lateral hemisphere of the posterior cerebellum (involved in cognitive processing) or in the vermis (limbic cerebellum). Cognitive and emotional disorders may accompany cerebellar diseases or be their principal clinical presentation, and this has significance for the diagnosis and management of patients with cerebellar dysfunction.