Deep brain stimulation
Deep brain stimulation (DBS) delivers a constant low electrical stimulation to a small region of the brain, through implanted electrodes connected to an implanted battery. It is used to partially restore normal movements in Parkinson's disease, essential tremor, and dystonia.
Parkinson's disease is due to degeneration of a group of cells called the substantia nigra. These cells interact with other brain regions to help control movement. The normal signals from the substantia nigra inhibit these other regions, and so when it degenerates, these regions become overactive. The electrical signals from the DBS electrodes mimic the inhibitory function of the substantia nigra, helping to restore more normal movements.
The substantia nigra normally releases the chemical dopamine, which exerts its inhibitory action on the globus pallidus interna (GPi) and the subthalamic nucleus (STN). For Parkinson's disease, deep brain stimulation is performed on these two centers. The target for DBS in dystonia is the GPi as well. Treatment of essential tremor usually targets the thalamus.
Each of these brain regions has two halves, which control movement on the opposite side of the body: right controls left, and left controls right. Unilateral DBS may be used if the symptoms are much more severe on one side. Bilateral DBS is used to treat symptoms on both sides.
Parkinson's disease affects approximately one million Americans. The peak incidence is approximately age 62, but young-onset PD can occur as early as age 40. Because young-onset patients live with their disease for so many more years, they are more likely to become candidates for surgery than older-onset patients. In addition, younger patients tend to do better and have fewer adverse effects of surgery. Approximately 5% of older PD patients receive one form or another of PD surgery. Many more develop the symptoms for which surgery may be effective, but either develop them at an advanced age, making surgery inadvisable, or decide the risks of surgery are not worth the potential benefit, or do not choose surgery for some other reason.
Essential tremor is more common than Parkinson's disease, but rarely becomes severe enough to require surgery. Dystonia is a very rare condition, and the number of patients who have received DBS as of 2003 is under 100.
Deep brain stimulation relies on implanting a long thin electrode deep into the brain, through a hole in the top of the skull. In order to precisely locate the target area and to ensure the probe is precisely placed in the target, a "stereotactic frame" is used. This device is a rigid frame attached to the patient's head, providing an immobile three-dimensional coordinate system, which can be used to precisely track the location of the GPi or STN and the movement of the electrode.
For unilateral DBS, a single "burr hole" is made in the top of the skull. Bilateral DBS requires two holes. A strong topical anesthetic is used to numb the skin while this hole is drilled. Since there are no pain receptors in the brain, there is no need for deeper anesthetic. In addition, the patient must remain awake in order to report any sensory changes during the surgery. The electrode is placed very close to several important brain structures. Sensory changes during electrode placement may indicate the electrode is too close to one or more of these regions.
Once the burr hole is made, the surgeon inserts the electrode. Small electric currents from the electrode are used to more precisely locate the target. This is harmless, but may cause twitching, light flashes, or other sensations. A contrast dye may also be injected into the spinal fluid, which allows the surgeon to visualize the brain's structure using one or more imaging techniques. The patient will be asked to make various movements to assist in determining the location of the electrode.
The electrode is connected by a wire to an implanted pulse generator. This wire is placed under the scalp and skin. A small incision is made in the area of the collarbone, and the pulse generator is placed there. This portion of the procedure is performed under general anesthesia.
DBS for Parkinson's disease is considered as an option in a patient who is still responsive to levodopa (used to treat symptoms) but has developed motor complications. These include the rapid loss of benefit from a single dose (wearing off), unpredictable fluctuations in benefit (on-off), and uncontrolled abnormal movements (dyskinesias). Essential tremor patients who are candidates for surgery are those whose tremor is unsatisfactorily controlled by medications and whose tremor significantly impairs activities of daily living. Similar criteria apply for dystonia patients.
The patient who is a candidate for DBS discusses all the surgical options with his neurologist before deciding on deep brain stimulation. A full understanding of the risks and potential benefits must be understood before consenting to the surgery.
The patient will undergo a variety of medical tests, and one or more types of neuroimaging procedures, including MRI, CT scanning, angiography (imaging the brain's blood vessels) and ventriculography (imaging the brain's ventricles). On the day of the surgery, the stereotactic frame is fixed to the patient's head. A local anesthetic is used at the four sites where the frame's pins contact the head; there may nonetheless be some initial discomfort. A final MRI is done with the frame in place, to set the coordinates of the targeted area of the brain in relation to the frame.
The patient will receive a mild sedative to ease the anxiety of the procedure. Once the electrodes are positioned, the patient receives general anesthetic to implant the pulse generator.
The procedure is lengthy, and the patient will require a short hospital stay afterward to recover from the surgery. Following the procedure itself, the patient meets several times with the neurologist to adjust the stimulation. The pulse generator is programmable, and can be fine-tuned to the patient's particular needs. This can provide a higher degree of symptom relief than lesioning surgeries, but requires repeated visits to the neurologist. Pulse generator batteries must be replaced every three to five years. This is done with a small incision as an outpatient procedure. Since the generator is in the chest area, no additional brain surgery is required.
The patient's medications are adjusted after surgery, with a reduction in levodopa likely in most patients who receive DBS of the subthalamic nucleus.
Deep brain stimulation entails several risks. There are acute surgical risks, including hemorrhage and infection, and the risks of general anesthesia. The electrodes can be placed too close to other brain regions, which can lead to visual defects, speech problems, and other complications. These may be partially avoided by adjusting the stimulation settings after the procedure. Because a device is left implanted under the skin, there is the risk of breakage or malfunction, which requires surgical removal.
A patient with implanted electrodes must not receive diathermy therapy. Diathermy is the passage of radiowaves through the tissue to heat it, and is used as a physical therapy for muscle pain and other applications. Diathermy poses a risk of death in a patient with DBS electrodes.
Patients who are cognitively impaired may become more so after surgery, and cognitive impairment usually prevents a patient from undergoing surgery.
Deep brain stimulation improves the movement disorder symptoms of Parkinson's disease by 25–75%, depending on the care of the placement and the ability to find the optimum settings. These improvements are seen most while off levodopa; DBS does little to improve the best response to levodopa treatment. Levodopa dose will likely be reduced, leading to a significant reduction in dyskinesias.
Morbidity and mortality rates
The rate of complications depends highly on the skill and experience of the surgical team performing the procedure. Rates from one of the most experienced teams, in a study of over 200 patients, were as follows.
- asymptomatic intracranial bleed (10% of procedures)
- symptomatic intracranial bleed (2%)
- seizures (3%)
- headache (25%)
- infection (6%)
- lead replacements (9%)
- lead repositionings (8%)
- extension wire replacements (6%)
- implantable pulse generator replacements (17%), approximately half of which were due to malfunction
The risk of death is less than 1%.
Patients who are candidates for deep brain stimulation have usually been judged to require surgery for effective treatment of their symptoms. Other surgical alternatives for Parkinson's disease include pallidotomy and thalamotomy, which destroy brain tissue to achieve the same effect as the stimulation. Pallidotomy is rarely performed for Parkinson's disease, unless tremor is the only debilitating symptom. It is common in essential tremor. DBS for dystonia is the only really promising neurosurgical treatment for this condition. Some peripheral surgeries may be appropriate for selected patients.
Jahanshahi, M., and C. D. Marsden. Parkinson's Disease: A Self-Help Guide. New York: Demos Medical Press, 2000.
National Parkinson's Disease Foundation. Bob Hope Parkinson Research Center, 1501 N.W. 9th Avenue, Bob Hope Road, Miami, FL 33136-1494. (305) 547-6666. (800) 327-4545. Fax: (305) 243-4403. http://www.parkinson.org .
WE MOVE, Worldwide Education and Awareness for Movement Disorders. 204 West 84th Street, New York, NY 10024. (800) 437-MOV2, Fax: (212) 875-8389. http://www.wemove.org .
WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?
Deep brain stimulation is performed by a neurosurgeon in a hospital.
QUESTIONS TO ASK THE DOCTOR
- How many electrode implantations has the neurosurgeon performed?
- What is his own rate of serious complications?
- Would pallidotomy be appropriate for me?
- How will my medications change after the operation?