by G. Rees Cosgrove, M.D., F.R.C.S. (C.)

Neurosurgical Service

Massachusetts General Hospital

Parkinson's Disease (PD) is a progressive neurological disorder caused by a loss of nerve cells in the substantia nigra, a small area deep within the brainstem. In most cases, the cause of PD is unknown although Parkinson's-like conditions can be seen after stroke, encephalitis, carbon monoxide or manganese poisoning and head trauma. The onset is usually insidious and occurs in most patients in their 50's and 60's. The major manifestations of the disease consist of resting tremor, rigidity, bradykinesia [slowness of movements] and involuntary movements. Gait disturbance is also a prominent symptom of Parkinson's Disease.

In the early 1900s, before the advent of modern anti-Parkinsonian drugs, surgical treatment of Parkinson's Disease was common. A variety of operations aimed at destroying certain areas of the brain were carried out in an attempt to relieve severe tremor and rigidity. In 1947, special stereotactic techniques were introduced which allowed for safer, more precise surgical treatment and many deep brain structures within the basal ganglia were targeted with varied degrees of success. Pallidotomy was introduced in 1952 by Dr. Lars Leksell and was successful in relieving many Parkinsonian symptoms in patients. At the same time, many surgeons were performing surgery on the thalamus and for a variety of reasons, thalamotomy became widely accepted, replacing pallidotomy as the surgical treatment of choice for Parkinson's Disease. Thalamotomy, which has an excellent effect on the tremor, was not quite as effective at reducing rigidity. In addition, bradykinesia was often aggravated by the procedure.

In 1985, Dr. Lauri Laitinen, who had worked with Leksell, re-introduced the pallidotomy, as a treatment for patients who had previously undergone thalamotomy but remained symptomatic. Many of his patients suffered from severe bradykinesia, rigidity, tremor and other unusual involuntary movements. These patients had long standing, severe PD that had been treated with medications for many years and exhibited what is known as drug-induced dyskinesias. He reported his first pallidotomy series of 38 patients in January of 1992 and found that 80-90% of patients had a long lasting relief of symptoms. This encouraging experience prompted other specialists to re-examine the role of pallidotomy in PD and currently several centers in the Unitied States carry out the procedure.

Stereotactic pallidotomy is not without certain risks although major morbidity and mortality is less than 1%. One side effect of pallidotomy has been a contra-lateral visual field defect seen in approximately 7-10% of patients. This visual field defect or scotoma creates a blind spot in the lower visual field and if this occurs on the left side it is generally well tolerated, but on the right side it may disturb reading. The incidence of this side effect and other potential side effects are minimized by intraoperative physiologic testing during the procedure.

Stereotactic pallidotomy or thalamotomy is only mildly painful. The surgical target within the pallidum is defined by a CT and/or MRI scan carried out with a special stereotactic frame attached to the head. Once the appropriate target coordinates have been selected on a computer work station, the patient is taken back to the operating room for the surgical procedure itself. A small patch of hair is shaved in the frontal region and the surgery is then carried out under intravenous sedation. A 3 cm skin incision is made in the scalp after infiltration with local anesthesia and a burr hole is drilled through the skull. A 1.8 mm insulated stimulating electrode is then introduced under impedance monitoring into the postero-ventro-lateral globus pallidus. The target area is stimulated with very small electrical impulses which may give rise to a variety of different reactions. The purpose of the stimulation is to make sure that the probe lies in the correct area of the pallidum. With electrical stimulation, tremor and rigidity can be reduced almost immediately in the operating room and this confirms accurate placement of the electrode tip. Electrical stimulation may also give rise to visual, motor, sensory or other untoward symptoms and this would indicate that the probe may need repositioning. If symptoms occur even after repositioning, there is a risk that the surgery cannot be performed safely and the probe would be removed without actual creation of the lesion.

When the intraoperative stimulation indicates that the tip of the electrode lies in the optimal location, a temporary (nonpermanent) lesion is first made. This allows for detailed testing of the patient intraoperatively to insure that no neurologic deficit will be incurred with creation of a permanent lesion. It also will allow for assessment of beneficial effect on tremor, rigidity and bradykinesia. If all of these conditions are met, then a permanent lesion is created at the target site. During the lesioning, the patient will be given a variety of motor, visual and psychological tests to check that no adverse effects develop. If unexpected reactions are observed, further lesioning is stopped immediately. It should be noted that none of the stimulation or lesioning is at all painful.

Post-operatively the patient is observed in the recovery room for approximately one hour and then returned to his hospital room. He may eat and drink immediately after the surgery and is often able to leave the hospital in a few days. The hypokinesia, rigidity and dyskinesia generally improves immediately. Sometimes the tremor does not disappear immediately but gradually diminishes over several days to weeks. If the surgery is successful without side effects, no special post-operative care or training is required. Stitches can be removed one week after surgery. Headache in the post-operative phase is minimal and can generally be controlled with Tylenol.