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Deep Brain Stimulation

H L Low and A Misbahuddin

The functional neurosurgical unit at Queen’s Hospital, Romford is the newest in the United Kingdom and became operative in August 2007. The catchment area for the unit is the whole of Essex and parts of Northeast London. The service presently offers surgical procedures for movement disorders such tremor, Parkinson’s disease and dystonia but expects to include operations for the control of pain in the near future. In this article we give an overview of how deep brain stimulation is performed at Queen’s Hospital.

What is deep brain stimulation?

Deep brain stimulation (DBS) involves the insertion of an electrode into a specific brain target. The electrode has a diameter of 1.3mm and has four platinum-iridium contacts at its end. The brain electrode is attached to a neurostimulator which generates an electrical current which is passed into the brain. The current creates an electrical field between the contacts. The shape and size of this field can be altered to modulate the activity of surrounding brain tissue.

How is it performed?

There are minor variations as to how the operation is done at various centres in the UK and the world. At our unit, DBS surgery consists of three stages: application of stereotactic frame to the head, microelectrode recording and brain electrode insertion, and connection of brain electrodes to a neurostimulator. The first two stages are performed under local anaesthetic with the patient awake. The last stage is done with the patient asleep. The entire operation is completed in a single day and patients are discharged the day after surgery.

A) Before surgery: Patient assessment and selection

All patients referred to the functional neurosurgery clinic at Queen’s Hospital are seen by both the neurosurgeon and the movement disorder neurologist. The principal aim of this consultation is to confirm the diagnosis, exclude any neurological conditions which may mimic essential tremor and to determine whether surgery is indicated. Nearly half of the patients seen at the clinic do not require surgery. Many will experience symptomatic improvement as a result of a change in the dosage or combination of their medications. The remaining patients could possibly benefit from DBS. Arrangements are made for patients in this group to have high resolution MRI scans of the brain and to undergo neuropsychological testing. In a small number of cases, neuropsychiatric assessment is also required. Patients with Parkinson’s disease are administered a levodopa- challenge test.

The functional movement disorder multidisciplinary team meets to discuss each case once the results of all assessments become available. Deep brain stimulation is offered if the patient fulfils certain criteria. The time taken from the first consultation to the offer of surgery is usually 8 weeks.

B) Day before surgery

All patients are admitted to the neurosurgical unit the day before surgery and relevant medications are stopped at midnight.

C) Day of surgery:

i. Application of stereotactic frame

A titanium stereotactic frame is first attached to the head under local anaesthetic. This is followed by an MRI scan of the brain. DBS involves the insertion of an electrode into a specific area deep inside the brain. The stereotactic frame allows the surgeon to calculate the coordinates of the target seen on the MRI scan. The choice of target depends on the primary problem causing the tremor. It may be the thalamus, zona incerta, globus pallidus interna or the subthalamic nucleus.

Whilst the surgeon calculates the target coordinates, the surgeon’s assistant positions the patient and shaves a thin strip of hair from the middle of the patient’s scalp. We do not do a full head shave. A small incision is made after infiltrating the scalp with local anaesthetic. This is followed by the creation of a small burr hole the size of a 5 pence coin. A microelectrode is then passed into the brain. The patient is awake throughout this part of the operation.

ii. Microelectrode recording and insertion of brain electrode

The microelectrode is a very thin wire used to measure brain activity. Different parts of the brain give rise to different patterns of brain activity. These brain patterns act as ‘road signs’ that guide the surgeon to the target. We find that microelectrode recording increases the accuracy of the operation and, in our hands, has not been associated with any increased risk of complications.

A small electrical current is passed into the brain once the electrode recordings confirm that the target has been reached. Tremor on the side of the body opposite the operation side will instantly stop. The current is then increased slowly. The patient is asked to repeat certain words and perform several simple tasks. As the stimulation amplitude is increased, some current may spread into surrounding structures causing temporary pins and needles, slurring of speech or double vision. This means that the electrode is too close to another critical brain structure. The electrode is re-positioned and the process repeated if this occurs. If the electrodes are optimally placed, not only will the patient not have a tremor but also experience no side effects at stimulation amplitudes up to 6 volts. The final position of the electrode is critically dependent on what the patient tells us hence this part of the operation is performed with the patient awake.

The microelectrode is exchanged for the permanent brain electrode which is secured to the skull via a special plastic ring and the scalp wound is closed. All patients have another MRI scan of the brain once the permanent brain electrodes are inserted to confirm optimal electrode positioning.

iii. Insertion of neurostimulator

This part of the operation is done with the patient asleep. The stereotactic frame is removed and the patient is given a general anaesthetic. Whilst they are anaesthetised, a small pocket is made under the skin just below the left collarbone to house the neurostimulator. Similarly, wires connecting the stimulator to the brain electrodes are passed just under the skin. All wounds are closed with dissolvable sutures.

Patients are allowed to fully mobilise later that day and are well enough to go home by lunchtime the day after surgery. The neurostimulator is only switched on 4-5 weeks after surgery to allow the brain swelling around the electrodes to subside.

D) After surgery: Programming the neurostimulator

Most of our patients experience some improvement immediately after surgery despite the neurostimulator not being switched on. This improvement is usually temporary and will wear off as brain swelling subsides. The neurostimulator is turned on 4 to 5 weeks after surgery and various stimulation settings are tried. It usually takes between 4 to 8 programming sessions to get a setting that controls the symptoms and is not associated with undesirable side effects. More sessions are required in some cases. The patients’ medication is altered or reduced at the same time. Our patients are taught how to alter their stimulation parameters using their own personal programmers which are the size of mobile phones. All our patients have direct access to members of the functional neurosurgery team.

What are the risks of surgery?

The risks can be divided into those resulting from the actual operation, those resulting from the spread of the electrical current into surrounding structures and general anaesthetic risks. The main complications resulting from the operation include a small risk of brain haemorrhage causing a stroke and its consequences, infection and equipment failure. Problems resulting from current spread include problems with speech, visual disturbances, sensory and motor abnormalities. We have not found this to be a significant long term problem as we actively test for these symptoms intraoperatively. This is why the first part of the operation is done with the patient awake. Rarely, the patient may experience psychological problems after surgery as a result of current spread. This can be expeditiously corrected by changing the stimulation parameters.

What are the chances of success?

The outcome of surgery depends on numerous patient factors and needs to be individualised. In general, more than 80% of our patients experience an improvement following surgery.

Conclusion

DBS has been performed at centres throughout the world for more than ten years and there is good clinical evidence that it works. Despite this, relatively few patients in the UK are offered DBS for tremor. The aim of this article is to make patients and their medical referrers aware that DBS is a valid treatment option in patients who have poorly controlled tremor. In appropriately selected patients, the procedure is a relatively safe and has the potential to significantly improve the quality of life of the afflicted individual.