Pallidotomy is the destruction of a small portion of the brain within the globus pallidus internus, or GPi. The GPi helps control voluntary movements.
Pallidotomy is performed to treat the symptoms of Parkinson's disease (PD), which results from the death of cells in a part of the brain that controls movement, called the substantia nigra. Part of the normal function of the substantia nigra is to inhibit overactivity of the GPi, which itself communicates with other portions of the brain in
Early on in PD, symptoms can be effectively treated with medication, especially levodopa and the dopamine agonists (drugs that act like levodopa). As the disease progresses, increasing amounts of drugs are needed to control symptoms, and the patient's response to the drugs declines. Typically, within 10 years of starting treatment, the patient will develop uncontrolled movements, called dyskinesias, in response to drug treatment. At this point, surgery is considered an option.
The GPi has two halves, which control movements on opposites sides of the body: 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.
Pallidotomy is major surgery on the brain. It may cause excessive bleeding, and care must be taken in patients susceptible to uncontrolled bleeding or who are on anticoagulant therapy.
To destroy tissue in the GPi, a long needle-like probe is inserted deep into the brain, through a hole in the top of the skull. To make sure the probe reaches its target exactly, a rigid "stereotactic frame" is attached to the patient's head. This provides an immobile three-dimensional coordinate system, which can be used both to determine the precise position of the GPi and to track the probe on its way to the target.
A single "burr hole" is made in the top of the skull for a unilateral pallidotomy; two holes are made for a bilateral procedure. General anesthesia is not used for two reasons: first, the brain does not feel any pain; second, the patient must be awake and responsive in order to respond to the neurosurgical team as they monitor the placement of the probe. The GPi is close to the nerve that carries visual information from the eyes to the rear of the brain. Visual abnormalities during probe placement may indicate that it is too close to this region, and thus needs repositioning.
Other procedures may be used to ensure precise placement of the probe, including electrical recording and injection of a contrast dye into the spinal fluid. The electrical recording can cause some minor odd sensations, but is harmless.
When the probe is in the correct position, its tip is heated briefly. This destroys the surrounding tissue in an area about the size of a pearl. If bilateral pallidotomy is being performed, the localizing and lesioning will be repeated on the other side.
A variety of medical tests are needed to properly locate the GPi and fit the frame. These may include computed tomography (CT) scans, magnetic resonance imaging (MRI), and injection of dyes into the spinal fluid or ventricles (fluid-filled cavities) of the brain. The frame is attached to the head on the day of surgery, which may be somewhat painful, although the pain is lessened by local anesthetic. A mild sedative is given to ease anxiety.
Pallidotomy takes several hours to perform. In some medical centers, pallidotomy is performed as an outpatient procedure, and patients are sent home the same day. Most centers provide an overnight stay or longer for observation and recuperation. Movement usually improves immediately, and typically requires the reduction of medication to accommodate the improvement.
Pallidotomy carries significant risks, especially in patients who are in poor health or who are cognitively impaired. Brain hemorrhage is a possible complication, as is infection. Damage to the optic tract, which carries visual messages from the eye to the brain, is a small but significant risk, and is more significant in bilateral pallidotomy. Speech impairments may also occur, including difficulty retrieving words, and slurred speech.
All PD experts agree that risks are lowest when the surgery is performed by neurosurgeons with the most experience in the procedure. Among the best surgeons, the risk of serious morbidity or mortality (i.e., serious consequences or death) is 1–2%. Hemorrhage may occur in 2–6%, visual deficits in 0–6%, and weakness in 2–8%.
Pallidotomy improves the patient's ability to move, especially between levodopa doses (so-called "off" periods). Studies show the surgery 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).
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