Ataxia

Ataxia is a disturbance of the coordination of movement that is not due to weakness, producing clumsy, imprecise, and poorly timed actions. It most often reflects dysfunction of the cerebellum or its connecting pathways, and manifests as unsteady gait, incoordination of the limbs, abnormal eye movements, and impaired speech. Ataxia can be acquired or inherited, and the hereditary forms include a large and growing group of genetic syndromes.

Definition

Ataxia is impaired coordination of voluntary movement, in the absence of significant weakness, typically arising from dysfunction of the cerebellum or its afferent or efferent pathways and expressed as incoordination of gait, limbs, eye movements, and speech.

Scope

This entry covers the definition of ataxia as a coordination disorder, the cerebellar signs by which it is recognised, the distinction between its acquired and hereditary causes, and the inherited cerebellar ataxias as an important genetic subgroup. It is a reference description of the syndrome and its signs and does not provide diagnostic criteria or management guidance for any individual.

Core questions

• Is the incoordination cerebellar in origin, or does it arise from sensory or vestibular pathways?
• Which domains are affected — gait, limb movements, eye movements, speech?
• Is the ataxia acquired or inherited?
• If inherited, what is the pattern of inheritance and the likely genetic group?

Key concepts

• Coordination of movement
• Cerebellar syndrome
• Dysmetria
• Dysdiadochokinesia
• Intention tremor
• Gait ataxia
• Hereditary versus acquired ataxia

Key theories

Cerebellar control of coordination and timing

The cerebellum coordinates the timing, amplitude, and smoothness of movement across muscle groups; its dysfunction produces the characteristic constellation of dysmetria, dysdiadochokinesia, intention tremor, gait ataxia, abnormal eye movements, and dysarthria that defines the cerebellar syndrome.

Mechanisms

Ataxia most often reflects disruption of cerebellar circuits or of the sensory and vestibular pathways that feed coordination. The cerebellum regulates the timing and scaling of movement, so its dysfunction yields dysmetria (inaccurate reaching), dysdiadochokinesia (impaired rapid alternating movement), intention tremor, abnormal eye movements, and scanning dysarthria — the cluster catalogued in the consensus revisiting of cerebellar signs by Bodranghien and colleagues. In the hereditary ataxias, a large group of genetic mechanisms, including the polyglutamine repeat expansions described by Durr, damage cerebellar and related neurons and produce progressive incoordination.

Clinical relevance

Identifying ataxia and localising it to the cerebellum or its pathways, then distinguishing acquired from inherited causes, structures the description of an individual's condition and the search for an underlying cause. This entry describes how ataxia is defined and recognised for reference and educational purposes and does not offer diagnostic thresholds or treatment advice for any person.

Epidemiology

Ataxia is a final common expression of many disorders, ranging from acquired causes to a large family of hereditary cerebellar ataxias. As Durr (2010) describes, the autosomal dominant cerebellar ataxias include numerous genetically distinct subtypes, several caused by polyglutamine repeat expansions; prevalence is therefore reported for specific aetiologies and genetic subtypes rather than for ataxia as a single entity.

Evidence & guidelines

The reference description of cerebellar signs in this topic follows the international consensus paper revisiting the symptoms and signs of cerebellar syndrome (Bodranghien et al., 2016), and the account of the inherited cerebellar ataxias follows Durr's (2010) review. Together these provide the standard descriptive framework for the syndrome and its hereditary forms.

History

Cerebellar incoordination was characterised by classical neurologists who linked specific signs to cerebellar lesions. Twentieth- and twenty-first-century work catalogued the full cerebellar syndrome and, with the advent of molecular genetics, defined an expanding set of hereditary ataxias; the polyglutamine repeat expansions detailed by Durr exemplify how genetic discovery reshaped the classification of these inherited disorders.

Debates

How should the hereditary ataxias be classified as new genes are discovered?

The number of recognised genetic ataxia subtypes has grown rapidly, and classification has shifted from purely clinical descriptions toward genetic and mechanistic groupings such as the polyglutamine expansion ataxias, leaving the optimal organising scheme an evolving question.

Key figures

• Mario Manto
• Jeremy Schmahmann
• Alexandra Durr
• Amy Bastian
• Dagmar Timmann

Seminal works

• bodranghien-2016
• durr-2010

Frequently asked questions

Does ataxia always come from the cerebellum?

Cerebellar dysfunction is the most common source of ataxia, but incoordination can also arise from disruption of the sensory (proprioceptive) or vestibular pathways that feed movement coordination. Distinguishing these sources is part of characterising the ataxia.

Are all ataxias inherited?

No. Ataxia can be acquired or inherited. The hereditary ataxias form a large and genetically diverse group — including the autosomal dominant cerebellar ataxias — but many cases of ataxia have acquired causes instead.