Types of symptoms and patients who are likely to benefit from Movement Disorder Surgery (MDS)
Parkinson's Disease
Surgical therapies are reserved for patients who have disabling symptoms of Parkinson's Disease (PD). When tremor, slowness, rigidity, balance and dyskinesia (wiggly, writhing movements) are no longer adequately controlled by medications, the patient may be evaluated to determine if they are a candidate for MDS.
 The operation usually recommended in this situation is bilateral or unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) (figure 1). This procedure offers approximately 50% improvement in control of PD symptoms. There is often 50% reduction in antiparkinsonian medication dosage once the stimulator is activated, which is usually done 2-4 weeks after the device is implanted. DBS of the STN is extremely effective at reducing dyskinesia. There is usually a dramatic reduction of wiggling, writhing movements that are called "choreifrom dyskinesia" that typically occur at the peak dose of levodopa response. In large part, this relief from dyskinesia is related to the dramatic reduction in antiparkinsonian medication dose. The painful foot cramping and twisting, called "dystonia" that often occurs at the end of a dose cycle of levodopa can often be remedied by this form of treatment. It is now appreciated that stimulation of the STN is more effective for control of PD tremor than is the thalamus, which was the previous choice target for this problem. The other symptoms of PD that include muscle rigidity and slowness likewise improve with DBS.
One of the unexpected benefits of DBS of the STN has been the significant improvement in sleep. It is well recognized that patients who have more hours of sleep and have better quality of sleep have better control of their PD symptoms the following day.
In counseling patients about this procedure, we inform them that the DBS device will help all of the symptoms that are currently helped by levodopa. For example, if a patient has severe slowness and inability to walk when their levodopa is not working (in the "off" state), and all of these problems are remedied when levodopa is working (in the "on"state), then we would expect DBS of the STN to help the patient with mobility. If a patient has severe painful leg spasms and curling under of the toes every time their levodopa wears off, and this problem disappears when they take another dose of levodopa, then we would expect this symptom to improve with DBS.
On the contrary, certain symptoms do not necessarily improve with levodopa in an individual patient (speech, walking, writing) and it is important to recognize that one would not expect these to improve with DBS.
In general, we expect the patient who has DBS of the STN to have more hours of good periods ("on" periods) during the day. The magnitude of the "on" state is not expected to increase-that is-the DBS apparatus will only be as helpful as is levodopa. It will lessen the number of hours of bad PD control ("off" time).
Some patients who have strongly lateralized PD symptoms and who do not want to have the responsibility of maintaining an implanted electrical device, may be candidates for pallidotomy if their symptoms are not well-controlled with medication. Again this procedure is quite effective in improving symptoms of dyskinesia, bradykinesia, freezing, rigidity, and tremor. It does not however allow the medication reduction described above with DBS and for safety reasons can only be done on one side.
How the decision is made to recommend surgery for a patient
The type of patient who stands the best chance of improving from DBS of the STN has the following characteristics:
- Patient must have Parkinson's disease, and not any of the "parkinsonisms" such as progressive supranuclear palsy, multiple system atrophy, cortical-basal degeneration, among others
- Patient must have a robust and persistent response to levodopa
- Patient must not have thinking or memory problems -- those patients who have even mild dementia do not do well with this procedure
- Patient should be less than 70 years of age. There may be some flexibility in this regard however, the patients in the literature who benefited the most from this surgery were in their late 50's
- Patient must not be currently depressed
- Patient must be compliant with medications, be able to provide adequate feedback for programming, and be prepared to comply with complex instructions about the safety guidelines critical for all patients harboring this electrical device
- Patient must have a realistic expectation of the benefits and risks of the DBS form of therapy
Essential Tremor (ET)
Patients with ET who are refractory to medical management are also candidates for DBS therapy. The target, in this case however is the VIM nucleus of the thalamus (figure 1). If the tremor involves only one side of the body, then the DBS is usually done unilaterally. Bilateral symptoms, of course, require bilateral implantation. Initially, it was felt that DBS did not help "axial" symptoms such as titubation (head tremor) or vocal tremor, but more recent results including some of our own patients suggest that bilateral thalamic DBS may, in fact be very effective for these complaints.
Dystonia
Recent experience has demonstrated that bilateral DBS implantation into Globus Pallidus internus (GPi) (figure 1) is quite effective for treatment of some forms of dystonia.
Surgical Procedures
Surgery for movement disorders is a type of stereotactic surgery, which refers to a procedure in which a probe is passed through a "silent area" of the brain guided by a metal frame attached to the patient's head to access a "target" which is usually deep to the surface of the brain. Generally, the target is one of three structures - the subthalamic nucleus (STN), the internal part of the globus pallidus (GPi) or the VIM nucleus of the thalamus (figure 1). Selection of the target is governed primarily by the type of symptoms that one is trying to relieve. Once the target has been decided upon, there are two types of procedure which can be done. Recent results suggest that for most patients, best results are achieved with the implantation of a Deep Brain Stimulator (DBS). Prior to the late 1990's, surgeons tended to favor "lesioning procedures" in which a small patch of overactive cells was destroyed after being localized. These operations had names like "pallidotomy" and "thalamotomy" and are still useful for a select group of patients. DBS surgery and lesion surgery are identical with regard to electrophysiologic localization of the target. The surgery for various targets varies only in the setting of the stereotactic frame. Therefore, in what follows, an operation for DBS implantation into the STN will be described for convenience with the understanding that other operations would vary in only minor details (figure 2).
Patients typically report to the hospital on the morning of surgery after being off their anti-Parkinson medications since 8 PM the prior evening. People are often apprehensive about this latter requirement, but since beginning the surgery in 1995, we have never had a patient who has been unable to manage it. There is a hotel across the street from the hospital with wheelchair access which can be useful for some patients.
After the patient checks in to the hospital, they meet with the anesthesiologist and have an IV started. This allows for administration for intravenous sedative medication prior to beginning the actual surgery. This sedative can be continued throughout the case if necessary. The first step of the actual operation is to apply the stereotactic frame. This is the a metal device (figure 3) which subsequently guides the placement of all the probes that are passed through the brain to the target area. Obviously, it must be firmly attached to the head in such a way that it does not move relative to the head. This is accomplished using four pins - two in front and two in back. The pins pass through the skin and anchor into the outer part of the skull. They do not pass through the skull. Local anesthesia in used to numb the skin, so there is no sharp pain as the pins pass through. The most uncomfortable part of the frame placement is typically the needle sticks required to put in the local anesthetic. The needle is small but it still feels sharp when it is inserted through the skin. Also, the anesthetic burns for a few seconds initially before the scalp becomes numb. As the pins are placed on the skull and are tightened, the patients do experience a sensation they describe as "pressure" or "like a tight hat". This abates within about 10 minutes even though the frame stays in place and it is very well tolerated after that.
Once the frame is on, an MRI scan is obtained with markers attached to the frame. This typically takes about 15 - 20 minutes and medication can be used to control tremor or claustrophobia Utilizing the MRI computer, these markers (which can be seen on MRI), and some fairly simple trigonometric calculations, one can then plot a path (technically called a "trajectory") between any point on the surface of the head and the target within the brain. These trajectories are selected so that they pass through "silent" parts of the brain and so that they avoid identifiable blood vessels.
Patients are then transported to the operating room and made as comfortable as possible on the operating table with the stereotactic frame also attached to the table. The frame is attached to the table using a special bracket to prevent head movement. The anesthesiologist starts an IV and a catheter will be placed into the bladder. The hair is then removed, the scalp is scrubbed and sterile drapes will be placed to prevent infection being introduced during the surgery. These drapes shield the patient's vision from the actual mechanics of the surgery but they are able to see out on the other side of the drapes into the operating room. The skin on one side of the front of the head is injected with local anesthesia and an incision measuring about 2 ½ inches is made. A power device is then used to place a dime-sized opening in the skull. Patients feel no pain during this but they can hear the drill similarly to being at the dentist. This takes about a minute. From that point on, there are no further sensations related to the surgery since the brain itself is not pain sensitive (figure 4). Patients are awake throughout this procedure to allow accurate monitoring of their neurological status and to permit testing of the stimulator electrodes when they are implanted. They typically do not experience any significant pain from the surgery itself. However, the need to remain relatively stationary on the operating table for the several hours needed to complete the procedure does sometimes induce discomfort. The table is padded and every effort is made to provide pillows and help the patient move in ways to maximize their comfort but this remains the primary complaint of patients relative to the surgery. It is possible for the anesthesiologist (who is constantly present throughout the case) to give pain medications and sedatives intravenously but many of these drugs suppress the electrical function of the neurons (see below) so they must be used judiciously.
Once the opening has been placed in the skull, intra-operative electrophysiology (mapping) is carried out. This utilizes small recording probes (microelectrodes) (figure 5) guided by the frame which are passed into the target area. These probes record the firing patterns of individual (or small groups) of neurons (nerve cells) and these patterns generate both an audio and a visual display. The firing patterns of the individual targets - STN in this case are quite distinctive and allow us to "fine-tune" the target localization selected originally from the MRI scan. This mapping is a crucial part of the procedure and typically takes 2 - 3 hours.
When the ideal location for electrode implantation has been identified, the stimulating electrode (figure 6) itself is passed into the area and test stimulation is carried out. Patients are asked to report if they feel any tingling or other unpleasant sensations. Also the effect of the stimulation on their symptoms is evaluated. Often, however, it is difficult to assess the effect of stimulation on symptoms like gait disturbance or dyskinesia which may not be present in the operating room. This is one of the reasons that the electrophysiologic mapping is so important. If the results of the test stimulation are satisfactory, the outside portion of the electrode is buried beneath the scalp and the skin incision is closed. A portable x-ray (fluoroscopy) unit is used to confirm the position of the electrode with respect to the frame (figure 7).
The procedure beginning with the skin incision is then repeated on the second side. Usually, since the target is approximately symmetrical with that on the original side the second placement is much faster. The advantages of being able to use this symmetry and of only having to put the stereotactic frame on once generally outweigh the additional operating room time required to do both sides at one sitting. Altogether, it takes about eight hours to implant both sides.
On the following day, the patients have another MRI scan to check electrode placement and then on Friday they are returned to the operating room for a short (2 - 3 hours) procedure under general anesthesia to implant the generators (figure 6). One generator is implanted on each side. An incision about three inches long on the front of the chest, below the collarbone is used to make a pocket to bury the generator which is about the size of a small metal tape-measure. The brain electrode is removed from its pocket beneath the scalp and attached to a "lead-extender" wire which is tunneled under the skin to attach to the generator on the front of the chest. An additional small (about ¾ inch) incision is made behind the ear to facilitate connecting the electrode to the lead-extender wire. Patients are typically discharged on Saturday.
Risks of Surgery
The most serious risk of stereotactic surgery is hemorrhage (bleeding) that can occur if one of the probes injures a blood vessel in the brain. Fortunately, this is quite uncommon, occurring in about 1% of patients who have stereotactic surgery. Most of the blood vessels in the brain are on the surface and we can see these through the skull opening and cauterize them before passing the probe. Also the larger vessels will show up on the MRI scan and so as noted above, we can plan the trajectory of the probe to avoid these areas. Unfortunately, it is not possible to reduce the incidence of hemorrhage to zero. This is an important reason that the brain electrode implantation part of the surgery is done with the patients awake. If bleeding does occur, we can detect a change in neurological status if the patient is awake much more quickly than if we have to wait until they awaken from general anesthesia. If there is a neurological change, the procedure is stopped and an emergency CT scan is done. If necessary then the patient is brought back to the operating room and an emergency surgery is done to remove the clot. There is good evidence that doing so quickly after the bleeding occurs can minimize the effects of the hemorrhage.
Like other surgical implants of man-made devices (total joints, heart valves, pacemakers, shunts, etc.) it is possible for the system to become infected either from contamination at the time of surgery (it is impossible to completely sterilize the skin) or if the patient has a bacteremia (blood stream infection) at some time after surgery. This infection usually presents as redness, swelling or drainage of the skin over the implanted components. This does not generally present a life-threatening problem but often does require removal of the stimulator on the involved side. It can then be replaced when the infection is cleared. Also the electrodes can fracture and if this occurs, of course, they must be replaced. This can be relatively easy or hard (requiring replacement of the stereotactic frame) depending upon exactly where the break is located.
Finally, there is at least one article in the literature suggesting that bilateral STN DBS stimulation causes a slight decrease in cognitive (thinking) function. In people who have normal cognitive reserve, this is not usually noticeable. If their thinking processes are already marginal, however, it could produce symptoms and this why all patients undergo neuropsychological screening prior to being approved for surgery (see above).
|
