Various kinds of abnormal movements are seen in neurological disorders involving the basal nuclei (basal ganglia), the subthalamic nucleus and the cerebellum. These may take the form of involuntary shaking (tremor) of the hands, head or other parts of the body, because of rapid alternating contraction of opposing groups of muscles. In some instances the tremor comes on when the patient tries to perform voluntary movement (intention tremor). Another type of abnormal movement consists of a slow twisting of a limb, or of the face or neck (athetosis). Sudden, jerky, shock-like movements involving any part of the body (myoclonus) may also occur. These can cause objects held in the hand to be thrown away (hemiballism). Sometimes different, complex, involuntary movements occur in succession, particularly in the distal parts of the limbs (chorea).
It has not been possible to precisely correlate specific abnormal movements with disease in specific regions of the basal nuclei and neighbouring structures. The movements are not a result of inactivity of the diseased centres but are, on the contrary, to be regarded as release phenomena, due to abolition of inhibitory influences. Hemiballism is known to be produced by lesions in the subthalamic nucleus. Intention tremor is seen in disorders of the cerebellum.
In many cases, abnormal movements are accompanied by rigidity of muscles, because of increased muscle tone. The increased tone is also a release phenomenon. One common syndrome characterised by rigidity and abnormal movements is called Parkinsonism orparalysis agitans (shaking palsy). It is characterised by marked rigidity which leads to a stooped posture, a slow shuffling gait, difficulty in speech, and a mask-like face. Characteristic involuntary ‘pill-rolling’ movements of the hands are seen. The condition is believed to be due to degenerative changes in the striatum and the substantia nigra.
In patients of Parkinsonism positron emission tomography reveals deficit of dopamine in the striatum. Grafting of embryonic ventral mesencephalic tissue, which is rich in dopamine producing neurons, has been tried as a treatment of Parkinsonism with limited success. Further refinements of the technique may make this a clinically valuable treatment. It has also been found that in some disorders of the striatum (e.g., progressive supranuclear gaze palsy) adequate amounts of dopamine reach the striatum from the substantia nigra, but receptors for dopamine are deficient in striatal neurons.