Day 168: Review – Static magnetic field stimulation applied over the cervical spinal cord can decrease corticospinal excitability in finger muscle

Figure 1 of the paper showing how the intervention was applied to the spinal cord

As mentioned in yesterday’s post, this is my second critical review paper. You can read my first looking at spinal reflexes here. Today we are looking at a novel way to modulate spinal cord excitability. Overall I find the paper very interesting. Although the authors performed a limited experiment and no follow up (as of now) has been done, it still looks very promising and would provide a new way to explore the circuitry of the spinal cord. This is my second attempt at a “critical review” so you can take my opinion on the methodology and findings how you will.

Spinal cord modulation techniques typically focus on lowering the threshold for excitation, thus making it easier for the neurons in the spinal cord to fire. This is done using either invasive methods, such as epidural spinal stimulation, or more recently non-invasive transcutaneous spinal cord stimulation. However, methods for increasing excitation threshold non-invasively have been nonexistent. This study uses a low cost and more importantly noninvasive method for decreasing corticospinal excitability. Researchers used transspinal static magnetic field stimulation (tsSMS) to inhibit the reflex response stimulated by transcranial magnetic stimulation (TMS).

This study was a double-blind study (N = 25), one researcher chose a neodymium magnet or a sham cylinder of the same size and weight. The second researcher then applied the intervention to the C8 vertebra and measured the response of TMS on the first dorsal interosseous muscle of the contralateral hand. Trials were performed 5 minutes prior to intervention (either sham or magnet). Then at 5 minutes and 10 minutes during intervention. Followed by two more trials at 0-minutes and 5-minutes post intervention. Each trial consisted of 20 motor evoked potentials (MEPs) which were determined for the individual to be 120% of the resting motor threshold. Using a two-way repeated measures ANOVA, where statistical significance was considered p<0.05, the researchers found MEP amplitudes significantly smaller than the sham condition for 5 minutes (p = 0.02) and 10 minutes (p = 0.01) during intervention. Researchers found that post intervention, MEP amplitudes had minimal or no suppression. This shows that there are no long-lasting effects once the intervention was removed.

This study is unique because the researchers present a novel way to modulate corticospinal activity. During the introduction they cited several papers showing a similar effect when applying a stationary magnetic field to the human cortex, which led the researchers to try this same technique on the spinal cord. A post-experiment MRI of three of the subjects was done to determine the mean depth of the spinal cord to the surface of the neck. They found this to be 4.5 cm, comparatively the distance to the cortex from the surface of the head is about 2 cm. Because the intensity of a static magnetic field dramatically decreases with increasing distance from the magnet, they determined that the actual field applied to the spinal cord was around 50 mTesla,

There were several limitations to the study and the authors seem to be aware of some of those limitations. One of the major limitations the authors point out is that only a single stimulus level was used for the trials. However, they fail to mention that they had a very limited methodology. This means the suppressive effect they found could be very limited or non-existent under other test conditions. A simple way the researchers could have expanded the study was to apply the same intervention to the lumbar enlargement of the spinal cord instead of solely the cervical enlargement. From the text it, is unclear why they chose to limit testing to the cervical enlargement exclusively. However, should the study’s findings hold, tsSMS could become very valuable as a non-invasive tool for spinal cord modulation. Applying this technique complementary to spinal cord stimulation could help elucidate the circuitry of the spinal cord in ways that spinal cord stimulation alone could not.

Source:

Nakagawa, Kento, and Kimitaka Nakazawa. “Static magnetic field stimulation applied over the cervical spinal cord can decrease corticospinal excitability in finger muscle.” Clinical neurophysiology practice vol. 3 49-53. 23 Feb. 2018, doi: 10.1016/j.cnp.2018.02.001