Coordination and gait are usually described under a separate section because cerebellar disorders can disrupt coordination or gait while leaving other motor functions relatively intact. There is much overlap, however, between the systems being examined in this section and those examined in the earlier general motor exam section. Keep in mind that disturbances of coordination and gait can be caused by lesions in many systems other than the cerebellum.
The term ataxia is often used to describe the abnormal movements seen in coordination disorders. In ataxia there are medium- to large-amplitude involuntary movements with an irregular oscillatory quality superimposed on and interfering with the normal smooth trajectory of movement. Overshoot is also commonly seen as part of ataxic movements and is sometimes referred to as past pointing when target-oriented movements are being discussed. Another feature of coordination disorders is dysdiadochokinesia—that is, abnormal alternating movements.
Cerebellar lesions can cause different kinds of coordination problems depending on their location. One important distinction is between truncal ataxia and appendicular ataxia. Appendicular ataxia affects movements of the extremities and is usually caused by lesions of the cerebellar hemispheres and associated pathways. Truncal ataxia affects the proximal musculature, especially that involved in gait stability, and is caused by midline damage to the cerebellar vermis and associated pathways (see Neuroanatomy through Clinical Cases, Figure 15-3, and Key Clinical Concepts 15-2).