1. bilateral ulnar nerve
2. bilateral median nerve
3. bilateral posterior tibial nerve
4. bilateral Erb’s point
5. bilateral popliteal fossa
6. FPz
7. CPz
8. CP3
9. CP4
10. Cs2
11. Ground
12. M1 tcMEP
13. M2 tcMEP
14. bilateral trapezius
15. bilateral deltoid
16. bilateral bicep
17. bilateral triceps
18. bilateral first dorsal interosseous
19. bilateral tibialis anterior
20. bilateral abductor hallucis brevis
21. d-wave reference electrode

If the cervical tumor is located below C6, I might elect to get rid of the trapezius (control) and add another lower extremity muscle for redundancy (extensor digitorum brevis). The deltoid would then be my control for tcMEP.

All I need to do is remember to bring a concentric bipolar probe and 2 spinal recording electrodes for d-waves.

The spinal recording electrode (on left) is meant to be placed on the spinal cord for d-wave recording. The surgeon can place it either epidural or subdural. They should use 2 electrodes for the procedure. The first should be placed rostral to the surgical area, with the tip going up towards the head. The second one should be placed caudal to the surgical area with the tip going towards the feet. The surgeon can then secure both with a loop tie.

You’ll notice some other items in the box. There is a needle and tubing for fluid if you were going to place the electrode percutaneously. I personally have never done percutaneous d-wave monitoring. I would image it to be pretty difficult to get something reproducible.

Don’t let the concentric bipolar probe fool you. It looks like a monopolar probe at first. But it is, in fact, a bipolar probe. You will plug the anode and cathode into your stimulator box and do not have to worry which goes where. No reference needed, like when using a monopolar probe.

If you were using a bipolar probe, then you would need to make sure the cathode is position correctly, depending on the dorsal column mapping technique you’re planning on using. You also need to monitor the surgeon to make sure they have the probe position correctly. The concentric probe eliminates this need and provides a very localized stimulation. Both are very useful when stimulating over such a small area.

Writing The D-Wave And Dorsal Column Mapping Program

Since the D-wave recording electrodes have 3 channels, it gives you 3 opportunities to record a response. In the rostral electrode, you will want to reference 1-2, 2-3, and 1-3. When recording the traces, you can elect to use the best one or 2 to monitor. Since the caudal electrode is facing the other direction, you’ll want to reverse your active and reference electrodes so that you get the same negative response to monitor off of. That would be 2-1, 3-2 and 3-1.

I would also recommend having a reference electrode placed beforehand in case of an emergency. Say you’re unable to get a bipolar response for some reason. It only cost you 1 electrode to test out a referential response, say 1-ref, 2-ref or 3-ref. If you’re using something less user-friendly, like the eclipse, try to have it programmed ahead of time. If you’re using a program where changes on the fly aren’t a big deal, like the Cadwell, you may elect to not have it preprogrammed if you’re running short on channels.

Stimulating a D-wave should be setup exactly like a tcMEP with 1 major exception. Because we are only recording the direct cortical activity from the lateral cortical spinal tract we do not have to worry about synapses. This means that having multiple train impulses is unnecessary. This allows us to stimulate without any patient movement and allows for continuous monitoring. But you’ll be able to use the same M1/M2 electrodes you’re using for tcMEP.

The amplitude is somewhere around 5-50uV. With an amplitude that large, it should have a pretty good signal-to-noise ratio. You’ll still want to set up some sweeps (5-20) to make sure that the amplitude you’re measuring is really representative of the conduction along the lateral cortical spinal tracts.

When setting up the filters, a low-cut of 100, a high-cut of 5,000 and a gain 200 of have worked for me.

As far as stimulation intensities, it is going to be less than what is used to elicit a compound muscle action potential in the limbs when using a multi-pulsed tcMEP. What you’re looking to achieve is the largest D-wave amplitude without causing the patient to move. That way you know that you’re monitoring most of the fast conducting fibers along the lateral cortical spinal tract.

As you move up in intensity, you’ll notice the onset latency of the response will decrease. This is most likely due to the fact that the higher intensity will cause deeper stimulation, which will shorten the length the potential will have to travel.

Dorsal column mapping is relatively easy to do from a programming standpoint. You are going to set up a stimulator through a probe for an evoked potential, just like you would for an SSEP. Since the stimulation is directly over the spinal cord, you can use much less power in order to evoke a response. At 3.11 Hz, a pulse width of 200us and starting at 0.2mA, you should be able to get a response below 2mA. In fact, you want the intensity low enough to get a reliable response, but not high enough to cause current spread.

The cortical channels you’re going to use is CPz-CP4, CP3-CP4, CPz-FPz, CPz-CP3, and CP4-CP3.

When stimulating over the left dorsal column (just like when stimulating over the left posterior tibial nerve) you’ll see a P40 response (which will actually be much shorter than 40ms) on CP3-CP4 and CPz-CP4, with phase reversal at CP4-CP3 and CPz-CP3. The opposite pattern will be seen when the surgeon stimulates over the right side of the cord.

CPz-FPz will always show a P40 response unless the surgeon is directly over the dorsal median sulcus. If this is the case, you should see an isoelectric response from all cortical channels. That is the anatomical area the surgeon will want to enter. You can see the phase reversal below in the CP3-CP4 channel from #4 (left dorsal column), #5 (dorsal median sulcus) and #6 (right dorsal column).

Is TIVA Needed For D-Wave Monitoring?

Here we really need to discuss what is best for the patient. We need to be able to really rely on tcMEP as they match up with D-wave recordings in order to best assess this patient throughout the case. This is one of those cases where you really need to work hard to get TIVA with full 4/4 during the time of laminotomy. While you don’t need TIVA to get D-waves, you need TIVA in order to best compare your motor evoked potentials to D-wave recordings. You’re going to want to make sure that you have as reliable of tcMEP at that time as you possibly can. While the aesthetics will not really affect the D-wave monitoring, and only affect the mapping as much as it would SSEPs, we need to make sure we have optimal tcMEPs. (We will get into why later when we talk about alarm criteria).

For cervical tumors, these patients will often times have a positive Lhermitte’s sign, weakness, loss of sensation and other spinal cord injury signs and symptoms. You’ll want to do prepositional baselines for SSEP and tcMEP. You’ll want to see if the anesthesiologist is willing or able to intubate without muscle relaxation. If not, then they need to use very little or something short acting.

The concentric bipolar goes into our stimulator box and the 2 spinal recording electrodes go to the pods or amp. You showed me the phase reversal in dorsal column mapping, but can you go over the alarm criteria for D-waves now?

D-Wave Alarm criteria

The best way to use reduction of D-wave amplitude is to compare it not only to the baseline measure but also against tcMEP responses.

• If D-wave <30-50% + mMEP stable = No deficit
• If D-wave <30-50% + mMEP lost = Transient deficit
• If D-wave > 50% + mMEP lost = Permanent deficit
• Any decrease in D-wave should be reported

This is important information for the surgeon to know. If the tumor is one that has great outcomes with complete removal, then the surgeon might elect to continue with tumor resection with lost tcMEP and D-waves with minimal reduction. If they remove the tumor and have a transient weakness, that’s a victory.

But if the tumor isn’t in this class, then the surgeon might elect to stop tumor removal in order to preserve function. This might happen in cases where the life expectancy is not too long. The patient might choose to have motor and sensory function in their last days over getting all the tumor.

Troubleshooting Dorsal Column Mapping

• The surgeon needs to make sure they are absolutely still while placing the probe on the spinal cord.
• The stimulation intensity needs to be low enough to collect a response and not cause shunting
• CSF needs to be suctioned out and the surface dried off
• Take your time with the initial baselines to get optimal responses. This might mean testing out CP1 and CP2 also.
• Be able to try out other ways to perform dorsal column mapping

Troubleshooting D-wave Monitoring

• Check impedances. Poor contact with the spinal cord can be a reoccurring problem. You may never get great impedances.
• Surface electrodes get noisy. You might have to play with your filters to get rid of noise and/or stim artifact. Do not let the changes in latency fool you. It will happen.
• If you stimulate too high, muscle artifact from the paraspinal muscles can be seen. This should be later than the typical response latency. You might get it regardless in the cervical spine.
• Latency is around 2-­3 ms in the cervical spine and 9-10 ms in the thoracic spine. The lower you go, the longer the onset latency. The higher you stimulate, the shorter the onset latency.
• Do not place your ground in the head. Not a good move whenever stimulating transcortical.
• Use a referential montage if you can’t get a bipolar response.