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Day #268: Review – Magnetospinography visualizes electrophysiological activity in the cervical spinal cord

Reconstructed currents of MSG from the cervical spondylotic myelopathy subject. There are two different sets of plots, the furthest left shows the electrophysiological recording taken from the epidural space and the other set shows the reconstructed currents using MSG. They agree fairly well.

This is the results of the cervical spondylotic myelopathy subject. The left graphs are the ascending spinal cord evoked potentials (this was electrophysiological recordings taken from the epidural space) by stimulation of the lower thoracic cord showing conduction block at the C4/5 disc level. The right graphs are the reconstructed currents at the midline of the cervical spinal canal (red) and 2 cm lateral (blue). The leading component (the first waveform in red) attenuated and disappeared through C4–6, and the trailing component (the second waveform in red) disappeared at C5/6. The perpendicular inflow components greatly attenuated at C4/5 (the second waveform in blue).

Another two weeks, another critical review. This time I was more critical than review, unlike the last one where I was blown away at the possibilities. Why was I more critical with this one? Well in my opinion, the authors took a baby step when they should’ve taken a leap. All that aside, it is an interesting study and one I hope has several follow up experiments. This one is open access as well, so have a read for yourself if you’re interested.

Each spinal cord injury is unique in that it disrupts or eliminates communication between the brain and the body differently between individuals. Therefore, when a spinal cord injury occurs the ideal treatment would be personalized. Currently, in order to assess the damage, MRI or CT are used to determine the affected area. While this information is valuable to the diagnosis and determining the extent and repairability of the damage, it does not provide information on the change in communication of spinal circuitry. This study uses magnetospinography (MSG) to determine how accurately it can measure and localize signals in the spinal cord.

The researchers worked with three different right-handed healthy human subjects and a single subject with cervical spondylotic myelopathy (CSM) for the first part and ten subjects for the second portion. For the first portion of the experiment an epidural cathode was inserted at T-11 and used to stimulate the spinal cord. Recording was done using a 120-channel superconducting quantum interference device (SQUID) and was placed at the cervical region of the spinal cord. For the second portion the researchers stimulated the median nerve at the elbow and recorded the cervical region of the spinal cord.

For the first experiment they averaged 4,000 individual responses and for the second portion, 1,500 recordings were averaged. Using a recording cathode located in the epidural space of the CSM patient, they compared their findings to the reconstructed values using MSG. Overall, they found that this technique could visualize the propagating signal with high temporal and spatial resolution for both conditions. Using this technique with the CSM subject, they were able to visualize the conduction block at the site of spinal stenosis. They found that the “reconstructed current map showed that the leading component was blocked around C4/5.”

What I find fascinating about this study is the inclusion of the CSM subject. This was an important way to specifically characterize how well this tool performs as a method for evaluation after a spinal cord injury. Of course, there are quite a few limitations with this study. First is with the method of recording. MSG is stationary so the application is limited to specialized magnetically shielded rooms. Admittedly, SQUID technology has progressively gotten better, so a mobile platform could be feasible in the future. Another limitation that is inherent to magnetoneurography is that it can only register components that are tangential to the sensor. Any information in a radial component is lost. While it can be assumed that the flow of current in the spinal cord would primarily be tangential to the sensor, the activation of grey mater circuits in the cord itself should cause radial components that the sensors would be blind to and thus important information about how the spinal cord processes information locally would be lost.

One thing that would have been interesting would have been looking at changes in the spinal cord signaling during volitional movement. By using evoked potentials, the researchers are creating an artificial electrical signal that is significantly larger in magnitude than the signals created naturally. While this tells you a lot about how the signal propagates across the cord, it does not give you information about how the spinal cord communicates in its natural state. Nor does it tell you how that natural state changes after injury. Because we do not know how intraspinal cord communication changes when a spinal cord injury occurs, a follow up study showing that a small and simple volitional movement task causes repeatable and recordable changes in the cord would further strengthen the case that MSG could be used for informing treatment and would open the door to understanding intraspinal cord communication.

Source:

Sumiya, S., Kawabata, S., Hoshino, Y. et al. Magnetospinography visualizes electrophysiological activity in the cervical spinal cord. Sci Rep 7, 2192 (2017). DOI: 10.1038/s41598-017-02406-8

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