Direct Motor Evoked Potentials

Continuing on from the post about intraoperative monitoring for gliomas (and trying my best not to ramble on too much in this post), I’d like to discuss some pros and cons to using a surface electrode strip as an anode directly on the motor cortex with a cathode at F1/F2 or Fz for direct motor evoked potentials.

Performing dcMEP with a surface electrode has been used with great success in reducing postoperative deficits. It’s been successfully used in various brain tumor, cyst and arteriovenous malformations to help reduce morbidity rates.

Most surgical applications of this technique follows the original work of Taniguchi (1993), where a strip electrode is placed over the motor strip and uses a high frequency anodal square wave pulse. This electrical impulse stimulates the primary motor strip, including Betz pyramidal cells, to transmit motor activity along the corticospinal tract.

Image from http://bit.ly/128FvhT

There are a number of studies that describe the pros and cons of using an electrode strip for direct cortical MEPs. Here’s a summary:

Pros

1. Can produce reliable CMAPs at low intensity (<25mA) and no patient movement. Set it up on a timer and let it go.
2. Can monitor semi-continuously throughout the craniotomy, possibly giving early warning for surgical intervention
3. Even though shunting/spread is a concern due to monopolar stimulation (could stimulate the corticospinal tracts directly, so deficits in the cortex could go undetected), the false-negative rates are low. This has been attributed to the lower intensity stimulation.

Cons

1. Electrode is bulky and can be in the way
2. Monopolar stimulation does not allow for the most reliable mapping of the motor strip
3. Requires TIVA and no muscle relaxant
4. Cannot be used in kids under the age of 2 (incomplete myelinization of pyramidal tract)
5. Cannot be used on people with less than 3/5 muscle strength
6. Inadvertent change in electrode position can affect CMAP
7. Fluctuations have been observed and attributed to complexity of motor system

Final note: I’ve done them by manually switching the electrode in the pod, as well as using a switch box.  Both have worked great. Just make sure to find the best response at the lowest stimulation level possible.