A surgical treatment for Parkinson’s disease in which targeted areas of neurons in the pallidum, one of the brain structures that control movement, are destroyed. This process interrupts the flow of nerve signals the globus pal-lidus sends to the motor cortex, which in turn controls the muscles, reducing many of the symptoms of Parkinson’s such as tremors, bradykine-sia, dystonia, and other dyskinesia. It also can decrease or end on-off state fluctuations and decrease the amount of levodopa, the standard medical therapy, necessary to control symptoms. Because the risks of surgical treatments are greater than the risks of nonsurgical treatments and can be significant (such as infection and permanent cognitive and functional disabilities), pallidotomy generally becomes an option only when anti-parkinson’s medications can no longer control symptoms.
Radiosurgical techniques such as gamma knife provide a way to perform ablative procedures such as pallidotomy without many of the surgical risks, but because they depend upon cells dying over the ensuing months, their effectiveness or side effects often evolve over months. Many movement disorders specialists are of the opinion that the evidence for radiosurgical ablative techniques does not support its widespread use.
Although the effects of pallidotomy are permanent, they do not cure Parkinson’s disease or prevent its progression. It is necessary to continue taking anti-Parkinson’s medications, although usually the person can significantly reduce the dosages. Symptoms eventually return as dopamin-ergic neurons continue to die and brain dopamine levels correspondingly decline. However, 70 to 85 percent of people who have pallidotomy experience years of significant improvement.
The pallidotomy procedure takes several hours. The neurosurgeon first attaches to the person’s skull a stereotactic frame, a device that keeps the head motionless during the surgery. magnetic resonance imaging (MRI) is performed with the frame in place to allow surgeons to plan the surgical path to take to the globus pallidus. Computerized equipment attached to the stereo-tactic frame also assures that the electrical probes advance properly into the brain. The person remains awake although sedated during the surgery, as it is necessary for his or her responses to inform the neurosurgeon whether the probe is on target. The functional regions of the brain are not visually distinctive from one another and vary among individuals. The pallidus is very close to optic tract, the nerve structure that conveys visual signals from the optic nerve to the cerebrum. The neurosurgeon makes sure to avoid this tract by asking the person to report any visual perceptions such as “seeing” lights or stars during the surgery.
The neurosurgeon uses a local anesthetic to numb the surface tissues of the skull, which is the only location there are nerve endings capable of sensing pain, and drills a small hole through the skull through which the probe is inserted. micro-electrode RECORDING and MACROELECTRODE STIMULATION help to guide the probe into position within an interior portion of the pallidus called the globus pallidus internus (GPi). Microelectrode recording, which allows the neurosurgeon to hear the changing spontaneous firing frequencies of neurons as the probe encounters them, further refines the probe’s placement; electrical activity produces a different frequency in each type of neuron. Macro-electrode stimulation serves as a test of both the probable effectiveness and side effects of lesioning the apparent target area before any tissue is actually destroyed.
Once the probe is in place, the neurosurgeon sends a small electrical current through it to ablate or “burn” a very small number of the surrounding neurons. ablation interrupts the flow of electrical signals enough to reduce the activity of the pallidus to more normal levels, thereby quelling much of the overstimulation that causes neuromuscular symptoms such as bradykinesia. Because the person is conscious and can respond to questions and directions, the neurosurgeon knows the results immediately. The person can usually notice the change as well.
Most pallidotomies are unilateral, performed on the side of the pallidus opposite the side of the body with the more severe symptoms as the functions of the brain are contralateral (the right side of the brain controls functions on the left side of the body and the left side of the brain controls functions on the right side of the body). This technique reduces the likelihood that any side effects from the surgery would be debilitating, because they, too, would affect only one side of the body. Bilateral pallidotomies carry a significant risk of affecting speech, swallowing, problems with eyelid opening, and neuropsychological side effects such as depression, extreme passivity, and obsessive behavior. Though the probe is very thin, it must pass through brain cells on its way to its destination and can inadvertently cause damage. It also is possible for ablation to leave a larger scar than anticipated, creating some movement difficulties or vision problems. The computerized stereotactic systems that neuro-surgeons use allow the probe to target specific cells, with a precision that minimizes the risk of such problems.