Cerebellar syndromes: clinical observations leading to the recognition of the three types.

Cerebellar syndrome is traditionally categorized into three primary types: cerebellar motor syndrome (CMS), vestibulocerebellar syndrome (VCS), and cerebellar cognitive affective syndrome (CCAS) or Schmahmann syndrome (SS). The first type is subdivided into five elemental features: dysmetria, kinetic tremor, asynergia, adiadochokinesis and dyschronometria. The second is characterized by dysmetria of saccades and jerky pursuit, as well as downbeat nystagmus and gaze-evoked nystagmus. And the third type is associated with a broader spectrum of cognitive and affective symptoms, including impairments in executive function, spatial cognition, language processing and emotional regulation. In its extreme form, cerebellar mutism can also develop during childhood following cerebellar vermis surgery. Recent physiological studies have shed light on the underlying neural mechanisms of these syndromes by identifying a common link of dysfunction within the cerebellum's internal forward model. This is essential to the prediction of the outcomes of motor and cognitive actions and underlines dysmetria as the core common element. Despite the diversity in clinical presentation, cerebellar syndromes can be understood as disruptions of a unified neural mechanism, providing a new framework for better understanding of cerebellar deficits.