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IMPLANTATION

Nerves of steel - (Source)
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SURGICAL PROCEDURE

Once cleared for a DBS operation, a patient will typically undergo two or three separate surgeries depending on a host of considerations such as the format of the therapy, the robustness of the patient, time constraints, and scheduling conflicts.  The first surgery implants the first electrode in the midbrain, the second implants the complementary electrode on the other side of the midbrain.  The third surgery implants the pulse generator below the clavicle and connects it to the electrode extensions. A patient may receive two bilateral single channel pulse generators - one under each clavicle - or a unilateral dual channel pulse generator which connects to both electrode extensions. Some centers will implant both electrodes in one session. Others, such as Scripps Memorial Hospital, will implant the electrodes and pulse generator in the same morning.  Additionally, not all pathologies require bilateral electrode placement. In those cases, only one electrode would be implanted. For clarity we will follow the course of surgery of a hypothetical patient – Jed – who is afflicted by severe, debilitating dystonia and will be receiving bilateral globus pallidus interna (GPi) DBS surgery with a unilateral dual channel pulse generator.

 

The team that will be performing Jed’s surgery has decided that he will receive his first electrode today and the second one week later.  They will implant the pulse generator one week after that if all goes well.  The team briefs Jed on the details of the day’s surgery:

 


The entire procedure will take 3-4 hours and he will need to stay awake and alert throughout the procedure to aid the team in assessing his neurological function.  His head will not be secured as in traditional neurosurgery.  He should not feel pain once the team is inside his skull; the brain lacks pain receptors.  Prior to that point, local anesthetic will take care of things.  He may feel pain with the attachment of the stereotactic apparatus, depending on the method used by the team.  When the team tests the electrode, he may hear loud buzzing and feel unusual as they probe for the optimal location. Jed acknowledges, and the surgery begins shortly.

 

A whole mounted stereotactic apparatus - (Source)
frame

 

First, the team must obtain a visual, digital reconstruction of Jed's brain. To do this, they utilize an adaptation of a stereotactic intervention. Three or more artificial reference points are drilled into Jed's skull (screw or pin sites). A stereotactic head frame is then secured to the pin sites and in conjunction with CT (computed tomography) scanning, the apparatus recreates a spatial, Cartesian, three-dimensional image of his brain. Mounting the apparatus to the skull is more comfortable for both patient and surgeon and ensures a fixed frame of reference. Additionally, CT-guided stereotaxis is preferable to MRI for its rapid speed, lower cost, and potential for greater accuracy.

 

Now, the CT images of the brain allow for the team to find Jed's intercommissural line, a point of reference used to locate the correct point of entry for a particular therapeutic target of DBS. For GPi DBS, the location is 20-22mm lateral to and 4mm below the line, and 3mm anterior to its midpoint. The journey to the midbrain begins here.

 

Next, a flap of scalp is cut back and a hole about the size of a dime is burred into the frontal bone 2-3cm lateral of the midline and anterior to the coronal suture; the dura is then opened. A stereotactic arc is secured to the frame according to coordinates determined by the CT scan, a cannula (tube) is fed through the arc and into the gyrus at which point the stylet (surgical probe) may be removed to begin microelectrode recording. Feedback is given by both Jed's behavior and the recording of audio static emanating from activity in his brain. Once the GPi is found, the electrode is tested intaoperatively and oriented such that the bottom contact touches the lower portion of the GPi; the extension coming from the lead is oriented such that it may be retrieved for the pulse generator operation. The head frame lifts off, and the day's surgery is complete.

Electrode insertion - (Source)
insert

 

For the next few days, Jed is going to feel a remarkable drop in his symptoms due to a microlesioning effect. The probing of the electrode through the cortex into the midbrain and subsequent fine-tuning of its final location ablates a small amount of the surrounding tissue; this is the effect which DBS mimics to a much greater degree without actually destroying tissue. The following week, Jed's second electrode is implanted. It's the same operation. The advantage of spacing them apart lies in a decrease in the intraoperative agitation of the brain and a decreased risk of the loss of cerebrospinal fluid. Allowing two CT-guided stereotactic scans increases precision and corrects for shifts in the brain's spatial orientation during the first implantation.

 

The week after that, Jed is ready to power up his new electrodes with a unilateral dual channel pulse generator. This time around, he's under general anesthesia. An incision is made beneath the clavicle to fashion a subcutaneous pocket for the pulse generator; another incision is made near the mastoid to retrieve the extensions coming from the leads. Everything is connected and the circuit is complete. Jed is sewn up, sent to recover and to wait for his swelling to recede so programming can begin. He can bet that he's going to feel the hurt in the morning.

 

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THERAPEUTIC EFFECTS

Once Jed has recovered from the pulse generator implant surgery, he will follow-up with his neurosurgery team to begin programming the parameters of the electrodes in his brain. Using a hand-held electronic programmer, doctors are able to adjust voltage, pulse frequency, pulse width, and electrode polarity. Finding the optimal combination for Jed's particular brain anatomy and dystonia symptoms will prove to be an adruous task - it almost always is. There are over 1,000 possible combinations of parameter settings in some devices. Finding the sweet spot is a matter of trial and error.

Home video of DBS for Parkinson's "on-off" demo in Portuguese - (Source)

Once the premium blend is attained, the electrical pulses emanating from the electrodes will quell endogenous aberrant neuronal activity in Jed's brain and his symptoms will all but disappear. He will be able to turn the deep brain stimulator on and off at whim with a hand-held wireless controller that utilizes magnetism to allow for transcutaneous operation. It's as simple as waving the controller over the pulse generator under his clavicle while simultaneously pressing a button on the controller.

The therapeutic effects of DBS are often drastic and can be found in videos circulating the Internet. Often, the DBS patient will demonstrate for the camera their symptoms with the stimulator turned off and highlight the difference by turning the stimulator back on. Shown to the left is one such video; one can even see the unilateral dual channel pulse generator protruding from under the skin beneath the man's left clavicle.

 

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COMPLICATIONS

A 2006 summary and meta-analysis of subthalamic nucleus (STN) DBS procedures offered a comprehensive review of related literature spanning a decade and 2 databases. Among the criteria analyzed were adverse events, shown below.

 

Common DBS Complications
Intraoperative Device Related Electrically Related (Stimulation)
Transient confusion (15.6%)
Electrode/Lead replacement (4.4%)
Dysarthria (9.3%)
ICH (3.9%)
Device dysfunction (3.0%)
Weight gain (8.4%)
Infection (1.7%)
Infection (1.9%)
Depression (6.8%)
Seizures (1.5%)
Migration (1.5%)
Eyelid opening apraxia (3.6%)
Pulmonary Embolism (0.3%)
Stimulation-induced dyskinesia (2.6%)
   
Manic episodes (1.9%)

 

Please note that this chart is not exhaustive; rather, it lists the most common complications related to DBS surgery. Also note that this chart draws upon data that reaches as far back in time as the mid-1990's. Therefore, one must expect complication rates to improve as technology advances and a relatively new procedure becomes more commonplace. Yet, one must also expect very similar types of complications even today as the nature of the surgery has not changed that drastically. The chart mainly serves to bestow a sense of relativity from complication to complication.

 

The most common serious intraoperative complication was intracranial hemorraghing (ICH), which fell within an acceptable range for stereotactic procedures. Other miscellaneous intraoperative events (3.3%) not listed include: "brain contusion, wound healing complications, phlebitis, hemiparesis, and CSF leak." (Kleiner-Fisman et al. 2006) The review found that two studies reported deaths directly due to the operation. The cause of death in both cases was a post-operative pulmonary embolism.

 

In 28% of patients, aberrant interventional electrical activity caused stimulation-related complications. This is largely due to imprecise electrode placement or suboptimal parameter settings. Electrode placement precision has been and will continue to improve, and parameter adjustment (such as voltage) can ameliorate placement imprecision. Please realize that this analysis reviewed only STN DBS procedures, but due to the close proximity of structures in the midbrain one can expect similar stimulation-related complications for other DBS procedures. For example, complications involving stimulation of the thalamus includes dysarthria and muscle contractions; complications related to the stimulation of the globus pallidus include flashing lights, muscle contractions, so on and so forth.

 

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