Dystonia is a general term used to describe failure of regulation of muscle tone, usually a sustained increase in muscle tone without hyper-reflexia (Fig. 431-6). When opposing muscles of an extremity have increased tone, there is limitation of voluntary activity. When the increased tone of the opposing muscles is not symmetrical, the extremity or trunk may be drawn into a distorted posture that cannot be overcome voluntarily. The most common form of dystonia, and the one of greatest interest to the neurosurgeon, is dystonia musculorum deformans. In the usual history, the first symptoms begin just prior to puberty, but they may occur between the ages of 3 and 15 or not until early adulthood. When the disease begins early, it usually involves the lower extremities first, and gait disturbance may be the only symp­tom for some time. When the disease begins later in life, it is more likely that the first manifestation is trunk or neck involvement. Some patients, usually adults, begin with what appears to be clas­sic spasmodic torticollis but then develop dystonia of the shoulders and the remainder of the trunk, in distinction to spasmodic torticol­lis, which always involves only the neck. Since it appears that patients with dystonia and spasmodic torti­collis do not respond the same to stereotactic surgery, and since the diagnosis is not always apparent at the time the adult patient is first seen by the neurosurgeon, some confusion has developed in the literature about the effectiveness of stereotactic surgery in both dystonia and spasmodic torticollis. The rate of progression of symptoms is variable, but appears to be more rapid in those patients whose symptoms appear early. Although the dystonia may arrest at any level, it may progress to the point of almost complete immobility and death from secondary complications, usually pneumonia.48 On the other hand, it appears that there may be only partial expression in such conditions as writer's cramp. Dystonia musculorum deformans is usually inherited as a dom­inant trait, although cases with no apparent family history are not uncommon. There is a particularly high incidence following an autosomal recessive pattern of inheritance among Ashkenazi Jews. Families that carry the gene for dystonia have a high incidence of familial tremor. The pathophysiology of dystonia musculorum deformans, as is true of most of the dystonias, is unknown. It seems reasonable to assume that the malfunction lies in the basal ganglia, which are significantly involved in the regulation of muscle tone, since the interruption of extrapyramidal pathways may affect the dystonia. However, neuropathologic study fails to reveal any consistent ab­normality. Since there is no known medical therapy,6 stereotactic surgery is indicated when the dystonia becomes disabling (Fig. 431-6 ).21.56 By that time, the trunk and extremities on both sides are usually involved, and bilateral surgery is generally necessary. The procedure of choice involves producing bilateral stereotac­tic lesions in the posterior half of the ventrolateral nucleus 10; as an alternative, a lesion in Forel's field on one side may be combined with a thalamic lesion on the other.74 Improvement is not apparent for several days or weeks after the lesion is made, but may prog­ress for some weeks thereafter. Consequently, the recommended program involves making the initial lesion in Forel's field or the ventrolateral nucleus on the side opposite the greater involvement. If necessary, lesions can be made on the second side in the ventro­lateral nucleus 3 months later. The lesions may be repeated or enlarged at 3-month intervals as long as the patient requires, pro­viding progress is being made. If involvement is symmetric, the first lesion may be placed in the dominant side, anticipating that a second lesion would be nec­essary later. Operating on the nondominant side at that later date with knowledge of the patient's response to the initial lesion al­lows the surgeon to be more generous in the size of the lesion. The results of surgery are variable and unpredictable, but some improvement can be anticipated in 50 to 70 percent of patients. 10 Some patients have almost total alleviation of symptoms following the first or second procedure, whereas others continue their pro­gressive course unabated despite large bilateral lesions. Some pa­tients may have initial improvement, but the progressive nature of the disease catches up later as other parts of the body become involved. The most distressing complication of surgery is dysphonia, which may occur in 15 percent 10 to 33 percent 11.56 of patients.