Up the Voltage

Last Thursday was the event I've been waiting for all summer.  In previous years, I would have reserved those words some something like a fishing trip to the Missouri River in Montana.  This time, it was my first follow-up appointment with the Deep Brain Stimulation (DBS) team at the Mayo Clinic in Rochester, MN.  I quit taking my Parkinson's Disease medication about 12 hours before the appointment, which made for a terrible night's sleep.   Morning arrived, and I hobbled through the shower and got dressed, trying not to fall down and wake anyone up prematurely.

Check-in time for my appointment was 7:45 AM.  We arrived at the complex early so we could get a good parking spot and find breakfast in the "subway level," the sprawling complex of tunnels beneath the streets of Rochester, akin to the "Skyway" system of Minneapolis.  When it's winter in Minnesota, most folks would prefer not to walk outside.  The tunnels link several Mayo buildings, hotels, and other businesses.  They also happen to be home to plenty of food options, such as bagels, Cinnabon, and Caribou coffee.  Mmm, morning mocha....



We were called back into an exam room shortly after checking in.  We were introduced to a new nurse, who would be doing the testing and programming.  I was happy to see her, because the nurse I'd worked with previously... well, let's just say she and I didn't get along.  Some medical professionals don't take kindly to have their work questioned...

I reported that my DBS experience had been a yoyo ride, where I felt great for a week or so after the system was activated, then the beneficial effects faded, followed be a similar up-down cycle when I started adjusting the voltage upward and my initial limits.  I asked if there was a plan or strategy for the visit and whether there would be another round of threshold test.  There was indeed a comprehensive plan!  It turns out that the DBS team has planning meetings for each DBS patient prior to their appointment.   The plan for me was simple, and I couldn't have been happier about it:

Figure 1- Each lead implanted in my brain has 4 electrodes,
which can be controlled individually

1. Ignore the right brain/left body for now.  My PD symptoms are still primarily on my body's right side.
2. For left brain/right side, they planned to introduce 3 new program "groups."  For my Medtronic neurostimulator device, a program group is a collection of settings. In this case, each group would use the same pulse frequency (130 cycles/sec) and pulse width (60 microseconds), but each group would utilize a different electrode on the leads implanted in my brain.

You may recall that the wires they pushed down into my brain each have four electrodes, spaced about a millimeter apart.  The spacing provides the ability to stimulate different areas (actually, volumes) of tissue.   The image below shows a configuration very similar to my own.

Figure 2
Source: http://www.lead-dbs.org/?p=2301

The subthalamic nucleus (STN) is a tiny region of the brain (though not the only one!) where research as shown DBS to effective in treating Parkinson's Disease symptoms.  As you can see, only one or two of the leads have much chance of stimulating neurons in the STN, so placement is critical to success.  All other factors held equal, each electrode has a different Volume of Tissue Activated (VTA) with each pulse.  So, it's important to find which electrode (or combination of electrodes) provides the best benefit with the least side effects.

Figure 3
Another visualization of the subthalamic nucleus (STN)
and the DBS electrodes.
Source: http://jn.physiology.org/content/115/1/19

As shown in Figure 2, I had been using the third electrode from the bottom, called "electrode 2," but it isn't labeled "2" because it's second from the top.  They actually start at the bottom with "electrode 0" and go up from there.   It's digital technology, and we computer science-types like to start counting at zero. 

The plan was to create a program group for each electrode and let me try those for about 4 weeks at a time.  Within each program group, the frequency and pulse width would remain constant, and I would be allowed to change the voltage using my "patient programmer" device.  Alas, no plan survives contact with the enemy...

Figure 4
Approximate location of the subthalamic nucleus (STN)

Source: http://www.extremetech.com/extreme/
160203-deep-brain-stimulation-the-bleeding-edge-of-neurohacking-and-transhumanism

For the safety of the patient, it's important to test each program AT LEAST up to limits that the patient is allowed to adjust.  As I learned during surgery and initial programming, its possible for even low amplitude DBS signals to incapacitate a person.  When the nurse and another neurologist started testing electrodes 0 and 1, there were adverse effects such as shaking, trouble thinking and speaking clearly, vertigo, and numbness or electrical sensations in my hand and arm.  These symptoms did not occur using electrodes 2 and 3.

The decision was quickly made to remove the bottom two electrodes (0 and 1) from the plan based on their adverse effects.  It's likely, though not impossible, that neither of those electrodes will be of any future benefit. So, what to do?

Well, it turns out there's a lot more than can be done with electrode configurations.  So far, I've only talked about configurations where a single electrode is active, configured as the negative pole in the circuit.  This a called a monopolar configuration, even though the case of the neurostimulator in my chest is configured as the positive pole.  As shown in Figure 2 above, this results in a electrical field (and VTA) that's relatively spherical.

The neurostimulator can also be programmed to activate each electrode with either a negative or positive polarity.  Using two electrodes in the brain, one negative and one positive creates an electrical field with a different shape, intensity, and vastly different properties. 

Source: Montgomery, E.B., Deep brain stimulation programming. Principles and practice.
Oxford University Press, USA, 2010.

From here, the physics and math start to get really complicated.   Even the orientation of the electrical field (reversing negative and positive) can have a huge impact because of how neurons work and the way electrons flow.

Because I did a lot of research before surgery, I was already familiar with all of this.  And I was very pleased when they decided to add a pair of bipolar program groups to my configuration using the electrodes 2 and 3.  In one program group, electrode 2 is negative and 3 is positive, then it's reversed in the other program group.

Then they let me decide which group I wanted to have active when I left the clinic.  When I arrived, I was using electrode 2 (2nd from the top) at 2.1 volts.  Using electrode 3 (top) at 2.0 volts, I was feeling and walking much better.   And the bipolar configuration was providing similar relief.  Ultimately, the nerd in me won the argument: stick with the monopolar configuration using the top electrode, and leave the bipolar configuration for future comparison testing.

When the supervising neurologist stopped in at the conclusion of the appointment to review and sign off on everything, one of the things he said was, "Well, it sounds like we didn't give you enough to keep you busy last time."  Very true.  I've got more possibilities to work with this time.  After 4 days, I'm up to 2.3 volts on the top electrode.  I'm walking smoothly and my right arm swings a little.  I didn't have a big struggle getting out of bed this morning and I wasn't stumbling around waiting for my morning meds to kick in.  The right arm is still stiff and occasionally painful, so I'll be staying here for a while to see if that improves at all.  When it's time to try something else, I have 3 other program groups and up to 3.0 volts to try.