DBS Targets
 
 

Use of DBS for treatment of movement disorders has evolved out of older surgical treatments which used various techniques to destroy small areas of brain tissue.  DBS seems to produce effects which are similar to the old lesion techniques, but without destroying tissue.  On this page we will elaborate on the three target areas and why they are used. 


The operation to implant DBS electrodes is pretty much the same for any of these targets.  The holes in the skull are even in pretty much the same places.  The only thing that changes is the path of the electrode to the target, and of course, what happens when the DBS is turned on. 

There are three targets approved for DBS

Subthalamic Nucleus (STN)

About half of our DBS cases are being treated for Parkinson’s Disease, and until very recently most PD patients have had DBS electrodes targeted to the STN, so the STN is by far the most common DBS site.  The STN is in the brainstem, and is in fact just below the thalamus, as you might guess from the name.  Here is a screen shot from a computer showing a target point in the STN.   (There is not really a hole in the top of this person’s head- the computer trimmed off the image a bit.) 


The red dot is the target point, at about the level of the deepest contact on the DBS electrode.  The three images are “slices” taken in the three dimensions which intersect that point.  The three planes (slices) are called axial (lower left) sagittal (top right) and coronal (upper left).  The STN can be seen as a darker “smear” in the T2 MR images, right at the red dot.








DBS of the STN provides very effective therapy for some of the motor symptoms of Parkinson’s disease, most especially bradykinesia, stiffness, and tremor.  This is why it is such a popular target for treating PD.  


The STN, however, is in somewhat of a dangerous neighborhood.  The STN is just behind the corticospinal and corticobulbar tracts, which are the nerve fibers that control the muscles of the body.  It is fairly common for DBS to affect some of these fibers, which can produce problems with speech and swallowing or even cause some weakness or cramping in the face or hand. 


Some patients with STN DBS will report double vision with some settings.  This is due to recruitment of the oculomotor nerve (cranial nerve III), which runs just below the STN.  The oculomotor nerve controls most of the muscles that move the eye. 


Most, if not all patients will experience some “tingling,” or paresthesias, when certain contacts are stimulated.  These sensations are not painful, and usually go away within a few seconds.  They may be produced by stimulation of sensory fibers which pass behind the STN. 


Apart from side effects caused by direct stimulation of neighboring structures, DBS of STN will commonly interact with DBS medication.  It acts like an extra dose of carbidopa/levodopa (Sinemet), a dose that never goes away the way a real dose would.  This is probably because the DBS is doing the same thing that the drug does, and it is actually a very good sign.  The problem for some patients is that they will get the wiggly, squirmy movements of dyskinesia (see “DBS and Medication”).  In such cases we need to turn down the DBS, slowly reduce the medication and then slowly go up on the DBS so that the patient does well without strong dyskinesias.


Programming the DBS properly involves finding settings which will reduce or eliminate these side effects while providing optimal relief of the PD symptoms.  It can sometimes be a challenge!

Globus Pallidus pars interna (GPi)

Patients with dystonia will almost certainly opt for DBS targets in the GPi.  Stimulation of this target can provide very dramatic remission of the painful cramping and posturing typical of this disease.  GPi stimulation is also very effective for motor symptoms of Parkinson’s disease.  There was a large, multi-center study run at major Veterans Administration facilities which went on for many years to determine whether there is a difference in efficacy between stimulation of GPi and STN for treatment of PD symptoms.  The study was recently published, and the main finding seems to be that DBS of the STN and GPi are equally effective.  GPi stim may require more current, while STN stim may have more risk of side effects and certainly more interaction with PD medication.


The GPi is part of the basal ganglia, and in the screen shot below you can see two trajectories aimed at the GPi.  The trajectories were drawn along the actual trajectory of the electrodes in patient as seen in an MR taken the day after the implant.  The trajectories were then overlaid onto the MR taken before the operation.  This screenshot has an overlay of a brain atlas showing outlines of some of the nuclei in the area.  The red dot on the tip of the green trajectory represents the approximate location of the deepest contact on the right DBS electrode.

Ventral Intermediate Nucleus of the Thalamus (Vim)

The Vim is the target of choice for tremor, and is the first target approved by the FDA for DBS.  All patients with essential tremor who opt for DBS treatment will have electrodes targeted to the Vim. 

A few patients with “tremor dominant” Parkinson’s may also opt for DBS of Vim, even though the thalamic stimulation will not help with any of the other symptoms of PD.  This is because Vim stimulation is thought to provide the best possible tremor control among the DBS targets.  We have even seen two patients with implants in BOTH Vim and STN on the same side!  


Tremor control with DBS of the Vim occurs almost instantly- within a few seconds.  Patients with essential tremor typically do not have a problem sleeping with DBS off, and they are encouraged to turn the device off at night.  This will conserve battery life by about 30% (depending on how long they sleep).


Common side effects of DBS of Vim include a “pins & needles” sensation caused by stimulation of the sensory nucleus (VPL) which is just posterior to the Vim.  This is usually transient, going away within 10 to 20 seconds.  If it is not too strong it simply confirms that the DBS was indeed just turned on.  Less commonly, bilateral DBS of Vim can affect speech, swallowing, and fine motor control, such as writing or knitting.  In a few cases there can be a difficult trade-off between tremor control and side effects, usually slurred speech.  The patient controller (see “DBS Devices”) will enable the patient to adjust stim voltage to find what feels like the best balance, and some people facing this trade-off opt to change the DBS voltage depending on the task at hand.