Ataxia is fundamentally a disturbance of muscle incoordination, which is not the result of muscle weakness or loss of muscle tone, or the intrusion of abnormal muscle movements. Implicit in the definition of ataxia is the presumption that muscle strength is not impaired, or not sufficiently impaired, to cause muscle incoordination. Those with ataxia have problems with the accuracy and organization of voluntary muscle actions, resulting in uncoordinated movements involving the trunk and/or limbs. The incoordination consists of irregularities in the rhythm, rate, and amplitude of voluntary movements causing voluntary movements to become jerky and erratic.
Plaques or scarring in the cerebellum, brain stem, basal ganglia or in other regions of the brain under the cerebral cortex (subcortical regions) are cause for multiple sclerosis (MS) movement disorders.
Cerebellar gait disturbances involve disturbances in stance and gait. Ataxia may primarily involve the trunk (truncal ataxia) and the person may not be able to sit or stand unsupported (astasia); truncal ataxia is usually due to midline cerebellar disease, and associated limb ataxia due to lateral cerebellar hemisphere disease may not be present. Incoordination of walking (gait ataxia) that is so severe that the person can't walk is called abasia, and the term astasia-abasia is used if the persons balance is so impaired that they can't maintain their balance when either sitting or walking. Persistent incoordination of the truncal axial musculature may produce body tilts, pelvic tilts or head tilts.