WHAT IS DEEP BRAIN STIMULATION?
Deep Brain Stimulation (DBS), also 'a pacemaker for the brain', is a surgical procedure used to treat movement disorders such as Parkinson’s disease, Alzheimer’s disease, cluster headaches, chronic pain, dystonia, epilepsy, and stroke. It has also been used on patients suffering from certain behavior disorders, most specifically Tourette’s syndrome, obsessive-compulsive disorder, and depression. In DBS, electrodes are surgically implanted into the brain and deliver electrical impulses that can be externally adjusted based on the patient’s needs.
DBS modulates brain activity through the direct electrical stimulation of different regions of the brain. The exact mechanism of action is ultimately unclear; however, the electrode that is implanted emits pulses of energy that block abnormal brain activity, which is responsible for many movement disorders. The efficacy of the surgery is dependent on isolating and stimulating the areas of the brain that are responsible each specific disorder.
DBS, though a relatively new therapy, has been studied extensively and has been FDA approved for certain neuromuscular disorders. The therapeutic format of DBS is especially novel because it can directly affect brain activity in a controlled manner and its effects are reversible, unlike those of lesioning.
Deep brain stimulation surgery varies among doctors and clinics. At the Cleveland Clinic, one of the foremost centers for DBS surgery, the surgical procedure entails implanting a very thin lead containing four electrodes into the target location of the brain. The lead is threaded through a small opening in the patient’s skull and is connected to an extension wire, which is attached to a pulse generator/pacemaker/neurostimulator which is implanted under the skin over the chest.
The stimulator(s) can either be implanted at the same time as electrode placement or at a later date, when the patient is placed under general anesthesia. The surgeon utilizes computerized brain-mapping technology to locate the source of nerve signals that initiate various symptoms. The physical structure and function of the brain are continuously mapped with the use of imaging and recording devices.
The patient is kept awake during the procedure to enable the surgical team to assess the patient’s brain function. When the electrode is threaded through the brain the patient experiences no pain, as the human brain does not generate pain signals. However, a local anesthetic is used when the surgeon drills a hole in the skull. Patients generally remain in the hospital for approximately three days post-operatively. For a more detailed explanation of the implantation procedure, click here.
Mechanism of action
DBS of the normal and diseased brain depends on the physiological properties of the brain tissue – which can be affected by disease state – the parameters of stimulation, and the geometric configuration of the electrode and the surrounding tissue. The effect of DBS on different neural elements depends on the inverse relationship between the stimulus duration and the amplitude that is necessary to stimulate the element. Within normal DBS parameters, the postsynaptic response results from the activity of efferent axons.
The exact parameters of DBS such as stimulus amplitude, duration, and the frequency band vary based on the therapy and the targeted area of the brain. With most commercially available stimulators, these factors can be altered with an external programmer; however, the technology only permits an open-loop continuous stimulation, which cannot take into account the continuous neural feedback of the patient. Currently physicians use the patient’s behavioral state to adjust the therapy, and such ambiguity lends itself to stimulation-induced side-effects. The efficacy of DBS is also dependent on the configuration of the neural elements in relation to the electrode; proximity of the axons and cell bodies to the electrode influences the responsiveness of the neural elements.
Although the mechanism of action is unknown, there is an emerging view that DBS has both excitatory and inhibitory effects. According to the theory of synaptic inhibition, DBS controls neural output by the activation of axon terminals that make synaptic connections with neurons near the electrode. The theory of depolarization blockade suggests that stimulation-induced changes occur in the activation of voltage-gated currents, which block neural output near the electrodes. The aforementioned theories can be supported by recordings of local somatic activity in the stimulated nucleus.
One hypothesis about the functioning of DBS is that stimulation from an electrode blocks the somatic activity of other neural elements that are close by and causes sub-threshold activity in distal neural elements, which allows the myelinated axons of intermediate elements to receive the effects of stimulation and pass them onto other connected brains structures.