Pallidotomy is the destruction of a small region of the brain, the globus pallidus internus, in order to treat some of the symptoms of Parkinson's disease.
The symptoms of Parkinson's disease (PD) include rigidity, slowed movements, and tremor, along with postural instability and a variety of non-motor symptoms (i.e., symptoms not involving movement). These symptoms are due to degeneration of a small portion of the brain called the substantia nigra, the cells of which secrete the chemical dopamine that influences cells in another brain region called the globus pallidus internus (GPi). Together with other brain regions, these two structures take part in complex control loops that govern certain aspects of movement and, when substantia nigra cells degenerate, these loops are disrupted and movements become unregulated, producing the symptoms of Parkinson's disease.
The effects of dopamine on the brain can be mimicked by the drug levodopa: levodopa therapy is the mainstay of PD treatment in its early stages. Unfortunately, levodopa becomes less effective over time, and also produces unwanted and uncontrolled movements called dyskinesias. This may occur after five to 10 years or more of successful levodopa treatment. Once a patient can no longer be treated effectively with levodopa, surgery is considered as a management option. Pallidotomy is one of the main surgical options for treatment of advanced PD.
The effect of dopamine on the cells of the GPi is to suppress them by preventing them from firing. Pallidotomy mimics this action by permanently destroying the GPi cells. It may seem odd that the treatment for degeneration of one brain area is to destroy another, but in the absence of dopamine, the GPi cells are overactive, and therefore, eliminating them is an appropriate treatment.
The GPi has two halves that control movements on opposites sides: right controls left, left controls right. Unilateral (one-sided) pallidotomy may be used if symptoms are markedly worse on one side or the other, or if the risks from bilateral (two-sided) pallidotomy are judged to be too great.
Parkinson's disease affects approximately one million Americans. The peak incidence is approximately at age 62, but young-onset PD can occur as early as age 40. Because young-onset patients live with their disease for so many more years, they are more likely to become candidates for surgery than older-onset patients. In addition, younger patients tend to do better with surgery and suffer fewer adverse effects from the surgery. Approximately 5% of older PD patients receive one form or another of PD surgery; many more develop the symptoms for which surgery may be effective, but either develop them at an advanced age, making surgery inadvisable, or decide the risks of surgery are not worth the potential benefit, or do not choose surgery for some other reason.
Pallidotomy requires the insertion of a long needle-like probe deep into the brain through a hole in the top of the skull. In order to precisely locate the GPi target, and to ensure the probe is precisely placed in the target, a "stereotactic frame" is used. This device is a rigid frame attached to the patient's head, providing an immobile three-dimensional coordinate system, which can be used to precisely track the location of the GPi and the movement of the probe.
For unilateral pallidotomy, a single "burr hole" is made in the top of the skull; bilateral pallidotomy requires two holes. A strong topical anesthetic is used to numb the shaved area before this hole is drilled. Since there are no pain receptors in the brain, there is no need for deeper anesthetic. In addition, the patient must remain awake in order to report any sensory changes during the surgery. The lesion made in the GPi is very close to the optic tract that carries visual information from the eyes to the rear of the brain. Visual changes may indicate the probe is too close to this region.
Once the burr hole is made, the surgeon inserts a microelectrode probe, which is used to more precisely locate the GPi. Electrical stimulation of the brain through the electrode can help determine exactly which structure is being stimulated. This is harmless, but may cause twitching, light flashes, or other sensations. A contrast dye may also be injected into the spinal fluid, which allows the surgeon to visualize the brain's structure using one or more imaging techniques. During the procedure, the patient will be asked to make various movements to assist in determining the location of the electrode.
When the proper target is located, the electrode tip is briefly heated, carefully destroying the surrounding tissue to about the size of a pearl. If bilateral pallidotomy is being performed, the localizing and lesioning will be repeated on the other side.
Pallidotomy is performed in patients with Parkinson's disease who are still responsive to levodopa, but who have developed disabling drug treatment complications known as motor fluctuations, including rapid wearing off of drug effect, unpredictable "off states" (times of low levodopa levels in the blood), and disabling dyskinesias. Those who are very elderly, demented, or with other significant medical conditions that would be compromised by surgery are usually not candidates for pallidotomy.
The surgical candidate should discuss all the surgical options with the neurologist before deciding on pallidotomy. A full understanding of the risks and potential benefits must be understood before consenting to the surgery.
The patient will undergo a variety of medical tests, and one or more types of neuroimaging procedures, including magnetic resonance imaging (MRI), computed tomagraphy (CT) scanning, angiography (imaging the brain's blood vessels), and ventriculography (imaging the brain's ventricles). On the day of the surgery, the stereotactic frame will be fixed to the patient's head. First, a local anesthetic is applied at the four sites where the frame's pins contact the head; there may nonetheless be some initial discomfort. A final MRI is done with the frame in place to help set the coordinates of the GPi in relation to the frame.
The patient will receive a mild sedative to ease the anxiety of the procedure.
The procedure requires several hours. Some centers perform pallidotomy as an outpatient procedure, sending the patient home the same day. Most centers keep the patient overnight or longer for observation. Patients will feel improved movement immediately. Medications may be adjusted somewhat to accommodate the changes in symptoms.
The key to successful outcome in pallidotomy is extremely precise placement of the electrode. While there are several controversies in the field of PD surgery, all experts agree that risks are reduced in procedures performed by the most experienced neurosurgeons.
Hemorrhage in the brain is a possible complication, as is infection. There are small but significant risks of damage to the optic tract, which can cause visual deficits. Speech impairments may also occur, including difficulty retrieving words and slurred speech. Some cognitively fragile patients may become even more impaired after surgery.
Pallidotomy improves the motor ability of patients, especially during "off" periods. Studies show the procedure generally improves tremor, rigidity, and slowed movements by 25–60%. Dyskinesias typically improve by 75% or more. Improvements from unilateral pallidotomy are primarily on the side opposite the surgery. Balance does not improve, nor do non-motor symptoms such as drooling, constipation, and orthostatic hypotension (lightheadness on standing).
Among the best surgeons, the risk of serious morbidity or mortality is 1–2%. Hemorrhage may occur in 2–6%, visual field deficits in 0–6%, and weakness in 2–8%. Most patients gain weight after surgery.
Patients whose symptoms are well managed by drugs are not recommended for surgery, and significant effort will usually be made to adjust medications to control symptoms before surgery is considered.
Thalamotomy, surgery to the thalamus, was recommended in the past to control tremor. It is rarely performed today, and few centers would consider thalamotomy for any patient unless tremor was the only troubling and uncontrolled symptom.
Deep-brain stimulation (DBS) of the GPi is an alternative treatment in widespread use, as is DBS of another brain region, the subthalamic nucleus. Both procedures use permanemtly implanted, programmable electrodes to deliver a very small, continuous electric current to the target region. This has the same effect as a lesion, but is adjustable. DBS of the subthalamic nucleus typically produces better symtomatic results that either DBS to the GPi or pallidotomy. However, both forms of DBS carry the risk of long-term complications from the implanted hardware, as well as other risks.
See also Deep brain stimulation .
Jahanshahi, M., and C. D. Marsden. Parkinson's Disease: A Self-Help Guide. New York: Demos Medical Press, 2000.
Pallidotomy is performed in the hospital by a neurosurgeon, in coordination with the patient's neurologist.