Several movement disorders are of particular interest to the neurosurgeon. Most are diagnosed by the history of onset and progression and by the characteristics of the involuntary movements or abnormality in muscle tone. The etiologies and pathophysiologies of many movement disorders are not known, but since most movement disorders involve the extrapyramidal system, surgical management has, for the most part, focused on neurological structures constituting that system. As a rule, surgical therapy should not be considered until medical management has been exhausted. With a single exception, however, medical treatment of movement disorders is nonspecific and generally inadequate, and severely disabled patients may be referred early to the neurosurgeon. A knowledge of the indications and shortcomings of surgical management is necessary to advise those patients appropriately. Since there are many movement disorders, most of which do not lend themselves to surgical intervention, a summary is in order. Before proceeding, a review of the anatomy of the extrapyramidal system is in order. The pathways of greatest interest to the stereotactic neurosurgeon are two interlocking circuits within the thalamus. One pathway involves the basal ganglia. The neurons of the corpus striatum, that is, the caudate and putamen, project to the globus pallidus, from which there are two major pathways to the ventral anterior nucleus of the thalamus. One runs by way of the lenticular fasciculus above the subthalamic nucleus, and the other runs via the ansa lenticularis below that nucleus. They join together in Forel's field H, where the pallidothalamic fibers are compacted most densely. The axons ascend together as the thalamic fasciculus HI to the ventral anterior nucleus. From the thalamus, fibers project to the supplementary motor cortex. Cortical neurons then lead to the caudate nucleus, which projects to the putamen to complete a circuit: putamen - globus pallidus - thalamus (ventral anterior) - cortex - caudate - putamen. The second circuit links the cerebellum to the motor portion of the thalamus. The main outflow from the cerebellar cortex is via the dentate nucleus, which sends a projection to the ventrolateral nucleus of the thalamus, with some fibers directly connected and others synapsing in the red nucleus, the so-called dentatorubrothalamic pathway. The area of the ventrolateral nucleus receiving these fibers lies adjacent to the area receiving fibers from the globus pallidus. The ventrolateral nucleus projects to the primary and secondary motor cortex. Neurons from these cortical areas descend through the internal capsule to the pontine nuclei, which in turn project mainly to the cortex of the cerebellum in somatotopic distribution to complete the motor control circuit summarized as cerebellum - dentate nucleus - red nucleus - thalamus (ventrolateral) - cortex - pontine nuclei - cerebellum.
Although the early stereotactic literature was concerned with production of lesions in the globus pallidus (pallidotomy) or ansa lenticularis (ansotomy), present-day procedures stress interrupting these pathways in the ventrolateral nucleus of the thalamus (thalamotomy) or Forel's field (campotomy, from campus Foreli), with occasional lesions made in the dentate nucleus for cerebral palsy (dentotomy or dentatotomy).
Note that all targets for surgical interruption are based on physiologic considerations. Although a stereotactic atlas of the brain can approximate the target for stereotactic intervention, the final localization must be done on the basis of physiologic parameters wherever possible. It usually is necessary to insert an electrode into the area of interest in order to verify localization by stimulation and/or recording, or to temporarily suppress the function of the target by cooling. Although there has recently been a movement toward using the Gamma Knife or other stereotactic radiosurgery to produce lesions for functional neurosurgery, such procedures do not provide the opportunity for important physiologic localization, and should be condemned.
Stereotactic surgery can be a valuable treatment in well-selected patients with several specific types of movement disorders. Although the therapeutic response may be less than optimal in many cases, it must be recognized that in those cases there is often no other available treatment. With care and a compulsive approach to surgery, potential risks can be minimized. Stereotactic surgery has had an increasing role in the management of movement disorders during the past several years. At the present time, the usual approach to movement disorders is ablative, not reparative. However, techniques of transplanting tissue containing specific neurotransmitters deficient elsewhere may allow the correction of the deficit with cure of the disorder, rather than merely a masking of symptoms. We can anticipate that neurochemists, taking the lead from the success in Parkinson's disease, will identify a deficiency in neurotransmitters in other motor disorders, so that opportunities for tissue transplantation or transplantation of sources of neurotransmitters will arise.
There has been considerable activity in the use of transplanted tissue for the management of Parkinson's disease. This has been based on the demonstration that there is a deficiency in the neurotransmitter dopamine in the basal ganglia. The original reports concerned insertion of the patient's own adrenal medulla tissue, which produces dopamine, into the caudate nucleus under direct vision. Although many patients obtained moderate and sometimes dramatic improvement in their symptoms, this technique was fraught with complications, such as prolonged coma, which outweighed the potential benefit, and improvement was generally not prolonged. Injection of a slurry of adrenal tissue through a stereotactically inserted cannula into the head of the caudate nucleus has been done consistently without complication, but, again, the benefit was not permanent enough to justify the surgery that involved removal of an adrenal gland.
Presently under investigation are techniques to transplant fetal tissue, especially embryonic substantia nigra, which can produce dopamine. Available reports suggest that the results are no better than with adrenal transplantation, and the practical considerations of obtaining and processing the fetal tissue make this procedure not practical for general use. The future holds promise, however, for transplanting cells that have been cultured and genetically modified to produce dopamine for Parkinson's disease or other neurotransmitters in other diseases. Techniques are used to apply chronic stimulation to subcortical areas with stereotactically inserted electrodes for the treatment of pain. As we learn more about the central regulation of movement and can identify the pathways involved in specific motor disorders, controlling these pathways with chronic stimulation may bring the motor system back into adjustment. Stimulation can interfere with physiologic transmission and block messages that might produce involuntary movements. Recent reports describe the use of implanted deep brain stimulators in lieu of lesion production to block pathways to treat the tremor of Parkinson's disease. This opens the door to more sophisticated techniques of controlled or intermittent stimulation for management of various neurological conditions. Regarding movement disorders, computer techniques are already available to regulate the stimulation by implanted monitors of muscle function. However, the engineers must wait until the physiologists learn what structure to stimulate and with which parameters.