Clinical presentation, pathophysiology and neurophysiology of choreatic disorders

Choreatic hyperkinesias are found in a variety of neurodegenerative disorde rs. In addition, choreatic symptoms may also accompany many generalised imm unological, infectious and metabolic diseases or may be sig ns of side effe cts of prescribed or illicit drugs. A number of clinical scares have been d eveloped enabling the clinician to quantify the degree of the choreatic sym ptoms. Choreatic hyperkinesias are caused by functional or structural distu rbances of the basal ganglia. Based on neuropathological and experimental e vidence models of basal ganglia connections have been developed which attri bute chorea to a disinhibition of thalamocortical pathways via a loss of in hibitory input from the main basal ganglia output structures. In Huntington 's disease the loss of GABAergic fibres from the caudate nucleus to the ext ernal pallidal segment is thought to be responsible for the disinhibition o f the basal ganglia motor loop. Recently, new findings concerning the effic acy of stereotactic pallidotomy in treating hyperkinetic syndromes have she d some doubt on the validity of current basal ganglia models. It is conceiv able that current models will be revised in the near future. Abnormalities in a variety of neurophysiological methods have been reported in patients w ith choreatic syndromes. Patients with Huntington's disease were found to h ave reduced amplitudes of early somatosensory evoked potentials, absent cor tically mediated long-latency stretch reflexes in small hand muscles, delay ed R2 components of the blink reflex as well as a prolonged "silent period" following cortical magnetic stimulation. These abnormalities were not only detected in patients with the classical hyperkinetic form of the disease b ut also in patients with the rigid-akinetic Westphal variant as well as in still asymptomatic carriers of the Huntington's disease gene. The character istic pattern of neurophysiological abnormalities seen in Huntington's dise ase patients could not be replicated in patients with choreatic syndromes o f other etiologies. Clinical neurophysiology in hyperkinetic syndromes is, therefore, not a mere reflection of the patient's symptomatology but gives clues to the underlying pathophysiological process.