The practice of lesioning parts of the brain in dystonia patients was very common in the 1950s and 1960s, since at that time it was essentially the only available treatment for severe cases. These procedures, as practiced over 50 years ago, had mixed results. By the 1980s, brain surgery for dystonia had widely fallen out of favor and was not widely practiced. However, the increased understanding of the basis of movement disorders such as Parkinson’s disease and the success in treating it with surgical approaches, plus the development of brain imaging technology, led to a re-evaluation of surgery as an option for patients with dystonia.
The procedure that involves creating a therapeutic lesion in the globus pallidus is called pallidotomy, and the procedure that involves creating a lesion in the thalamus is called thalamotomy. A permanent lesion is made in the brain tissue by heating the tip of an electrode and coagulating the intended tissue.
When lesioning surgery is chosen, pallidotomy is now preferred over thalamotomy and provides a reasonable alternative to pallidal DBS for patients who are averse to the cosmetic appearance of the implanted pulse generator or do not want to be burdened by repeated battery replacements. Bilateral pallidotomy has shown an average of 67-80% improvement in the Burke-Fahn-Marsden dystonia rating scale in patients with generalized dystonia. Primary generalized patients may respond better than focal or secondary dystonias. In Parkinson's patients, bilateral pallidotomies are avoided because they cause hypophonia, a quieting of speech. This has not been observed in dystonia patients, and many dystonia patients have had bilateral pallidotomies without significant worsening of speech.
Although thalamotomy was once the most common brain surgery performed for dystonia, it is now used almost exclusively in cases of stable hemidystonia, and a very specific site in the thalamus is targeted. The procedure is performed unilaterally. Bilateral brain surgeries increase the risk of complications, and bilateral thalamotomies in particular are known to often cause speech impairment.
The primary factor that distinguishes modern lesioning procedures from those of 50-plus years ago is that surgeons are able to locate the lesioning target more accurately. The following factors make it much easier for the surgical team to locate the target within the brain, which is crucial to reducing the risk of complications:
• Stereotactics—Surgeons are able to target the precise area of the brain with a computerized, 3-dimensional scale using MRI and CAT scans.
• Microelectrode recording and brain mapping—The surgical team has the ability to listen to the sounds of brain cells firing messages to one another. Cells in different parts of the brain fire at very specific rates and in characteristic patterns, and by listening to these cells the surgeon knows exactly where the electrode is within the brain. Several recording tracts may be necessary to identify the precise target.
Once a physician has recommended brain surgery, and pre-operative screening tests and preparations are complete, the basic plan of operation for pallidotomy and thalamotomy are the same. The individual is fitted with a head frame under general or local anesthesia. The brain is mapped with imaging technology to create a blueprint for planning and measuring the placement of the electrode. Under local anesthesia, the electrode is inserted through a small hole in the skull into the brain. The brain itself does not feel pain, and the patient is awake during most of the procedure. The surgical team interacts with the patient throughout the procedure, and the patient provides feedback about symptoms and how he/she feels. Microelectrode recording is used to confirm the target. The mapping procedure alone may take up to several hours. Once the target is defined, the surgeon inserts the thermal electrode and creates a lesion. The thermal electrode is removed and the procedure is complete. A bilateral procedure may be done in a single surgery or in two separate surgeries. If a second target is to be lesioned, the mapping procedure is repeated for that specific target. Most patients are in the hospital for two or three days. Medications may be temporarily resumed, and after a short time the patient returns to the neurologist for a follow-up exam.
There is a small but real risk of complications associated with lesioning. The most serious risk is a 1-2% incidence of stroke or hemorrhage during the mapping phase of the surgery. Also, the target of the pallidotomy, the internal segment of the globus pallidus, is located right above the optic tract which may be damaged if the electrode is not targeted precisely. There also exists the risk that the pallidotomy will not improve the symptoms. However, the procedure has been shown to dramatically improve dystonia in some patients.