Spinal Tumors That Need Dorsal Column Mapping

Intraoperative neurophysiological mapping of dorsal columns isn’t really one of the neuromonitoring community’s “bread and butter” cases. Not that there isn’t anything of benefit we offer the surgeon, but because intramedullary spinal cord tumors a pretty rare. The 2 most common tumors that need dorsal column mapping are astrocytomas (seen more in kids) and ependymomas (seen more in adults).

Image from http://bit.ly/192u02F

Another reason neuromonitoring companies aren’t marketing our services for these surgeries is due to the expensive electrode used in these cases. There are a couple of versions of the electrode out there, and the one I used was custom-made.

Custom-made = big $$$.

The electrodes have a shelf life, so it’s not unthinkable to order a couple of these electrodes and never use them. (UPDATE: Adtech no longer makes these electrodes for dorsal column mapping. I have no real proof, but it is probably because it was not a money make for them or neuromonitoring companies. There are different techniques available to do dorsal column topography that are much, much more cost-effective).

But none-the-less, mapping of the dorsal column will help the surgeon find the midline of the posterior cord to perform the myelotomy. The reason this is of help is because the space occupying lesion will rotate and distort the spinal cord.

The surgeon uses landmarks, like splitting the elevated dorsal column, to find the midline dorsal median sulcus. The surgeon uses the dorsal median vein as an option to enter the midline raphe and help localize the midline cord.

But edema, scar formation and increased vascularization feeding the tumor ruin their landmarks. And that is we come in.

Dorsal Column Mapping: Setting It Up

Much like lower extremity somatosensory evoked potentials, we stimulate the posterior tibial nerve at the ankle. The surgeon places the sterile recording electrode (like the one at the top of this post) intraoperatively over the exposed spinal cord. The reference is a sterile needle in a nearby muscle. Placing the electrode perpendicular to the posterior columns allows us to determine where midline is located by assessing the largest amplitude to the ipsilateral SSEP stimulation. The largest amplitude number is recorded and compared to the numbered recording wires running parallel to the dorsal column. Once completed on both the right and left leg, the surgeon knows that midline is between the largest waveform on each side.

Image from http://bit.ly/12TWg2X

In the picture above, the surgeon looks between the 2nd and 3rd numbered wired on the spinal electrode, since those 2 numbers had the largest amplitude. Why are they larger? Because those recording electrodes are more near field to the very midline posterior columns carrying the sensory information.

After Mapping of the Dorsal Column Is Complete: MEP, SSEP, D-Wave

Now that the surgeon has located the midline, it’s time to perform the myelotomy and then remove the tumor or cyst. But you’re not done reducing morbidity rates just yet. Now it’s time to go from mapping to monitoring for prevention of injury. Using 50% reduction in SSEP, 30-50% reduction in D-wave and significant reduction to a loss of motor evoked potentials makes for excellent multimodality monitoring of spinal cord during the spinal cord cases.

And that’s a quick synopsis of mapping the dorsal columns. If this is a new surgery type for you, watch this video of a surgeon I used to work with performing a spinal cord tumor removal with a laser (although he wasn’t using dorsal column mapping in this procedure)