Deep brain stimulation

The use of DBS in patients with movement disorders is growing, explains Keyoumars Ashkan.

Plain lateral skull X-ray showing deep brain stimulation electrodes (Photograph: Author image)
Plain lateral skull X-ray showing deep brain stimulation electrodes (Photograph: Author image)

The use of electricity to treat brain disorders is not new. For example, there are records of torpedo fish being used to treat headaches in Roman times. More recently, since the early 20th century, electroconvulsive therapy has been widely used to treat psychiatric disease.

Deep brain stimulation (DBS) as a target-specific means of electrical neuromodulation emerged in the 1980s as a direct result of developments in technology. This allowed many advances, such as the availability of miniaturised hardware for chronic stimulation in neurophysiology, which enabled improved understanding of brain circuits to modulate.

In neuroimaging, it permitted visualisation of surgical targets and facilitated safer surgery in neurosurgery.

The procedure
The technique of DBS involves implanting tiny electrodes, around 1.3mm in diameter, into specific targets within the brain.

The electrodes are inserted through burr holes made in the skull using stereotaxy, a neurosurgical method reliant on high-quality imaging of the brain to localise the DBS targets with millimetre accuracy.

Depending on whether the patient has unilateral or bilateral symptoms, one or two electrodes may be inserted.

Although still debated, most neurosurgeons agree that, where possible, awake surgery to implant the electrodes under local anaesthetic achieves the best outcome. This is because it allows on-table clinical examination of the patient to determine the most physiologically effective and safe location within the radiologically defined target volume, thus correcting for the expected anatomical variations.

The electrodes are then connected via cables to a pulse generator battery, which is placed in a subcutaneous pocket below the clavicle.

The latter part of the surgery is performed under general anaesthetic. Following the operation, the battery is turned on and the voltage is gradually increased, usually over several weeks on an outpatient basis, until optimal clinical effects are obtained.1

Unlike traditional forms of surgery which involve altering structure, DBS is used to alter the function. The particular advantages of the technique are that it is non-destructive, largely reversible and adjustable. The latter two are possible since the electric current delivered by the electrodes can be turned off, decreased or increased using an external controller held against the battery.

Multidisciplinary approach
At the present time, DBS is used most commonly in patients with movement disorders. The procedure has NICE approval for Parkinson's disease (PD), tremor and dystonia. NICE further emphasises the importance of a multidisciplinary team in the selection of patients for DBS and their subsequent management.

A subspecialist movement disorder neurologist and DBS neurosurgeon are the obvious members of such a team. Other crucial members include the neuropsychologist and neuropsychiatrist to identify and manage cognitive impairment and ongoing mood disturbance, which otherwise at best could confound the benefit of DBS and at worse might deteriorate after surgery.

A clinical nurse specialist is also vital to support patients after DBS and act as a point of contact for the patient, carers and other clinicians.

Which patients benefit?
Medically refractory disease or development of drug-related side-effects are the most common indications for DBS. In patients with PD, drug-induced side-effects, such as dyskinesias and also on/off fluctuations, in which patients unpredictably and suddenly become rigid and immobile, can develop as early as five years after diagnosis.

In such patients DBS can be effective in improving symptoms, such as tremor, rigidity and bradykinesia, by 60-70 per cent, ironing out the motor fluctuations and reducing the drug-related side-effects by enabling medications to be reduced. The reduction in the medication after DBS accounts for the long-term cost-effectiveness of the procedure.

For DBS to be beneficial in PD, the patient must still have some response to medication and therefore all patients should undergo a dopa challenge test as part of the pre-DBS assessment to ensure at least a 30 per cent response.

Hence DBS should not be thought as a last-resort treatment for end-stage PD but as an effective alternative treatment when quality of life can no longer be maintained by medication.

In dystonia, patients with primary generalised disease respond best to DBS. A good response has also been seen in other forms of dystonia, such as torticollis or tardive dystonia. Given the nature of the condition, children represent a significant proportion of patients undergoing DBS for dystonia. In this group, early surgery is associated with the best outcome before fixed deformities are established.

Outcomes
In terms of long-term outcome, several studies with more than five years' follow-up have confirmed continued improvement of patients following DBS.2

With respect to the complications of surgery, risk of major morbidity, such as haemorrhage, is usually quoted as 1 per cent. Other risks include hardware infection, migration or breakage, further emphasising the importance of good surgical techniques and an experienced team when managing these patients.

It should be noted that the manufacturers essentially prohibit an MRI in patients with DBS. Unipolar diathermy and a strong magnetic field, such as that produced by an airport security gate, are also contraindicated after DBS.

With the growing use of DBS in patients with movement disorders as well as for emerging indications, GPs will play an increasingly important role, not only as a referrer but also in the long-term management of these patients.

  • Mr Ashkan is consultant neurosurgeon and senior lecturer at King's College Hospital, London

References
1. Ashkan K, Wallace B, Bell B A, et al. Deep brain stimulation of the subthalamic nucleus in Parkinson's disease 1993-2003: where are we 10 years on? Br J Neurosurg. 2004; 18(1): 19-34.

2. Krack P, Batir A, Van Blercom N, et al. Five-year follow-up of bilateral stimulation of the subthalamic nucleus in advanced Parkinson's disease. N Engl J Med. 2003; 349(20): 1,925-34.

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