​Transcranial magnetic stimulation: a non-invasive neurostimulation

Posted by Sam Maddox in Research News on August 02, 2016 # Research

Spinal Cord Series and CasesTranscranial magnetic stimulation (TMS) is a type of non-invasive neurostimulation recently shown to restore function in spinal cord injury (SCI), when combined with stimulation of peripheral nerves.

In a paper that came out a few days ago, “Long-term paired associative stimulation can restore voluntary control over paralyzed muscles in incomplete chronic spinal cord injury patients,” a team from Finland treated two patients with spinal cord injuries with a combination of TMS and peripheral nerve stimulation, applied together repeatedly for nearly six months. According to the research team, this was the first attempt to rehabilitate patients with chronic paralysis using the paired strategy.

During a typical TMS session, an electromagnetic coil is placed against the scalp. The magnet delivers a magnetic pulse that stimulates nerve cells in the brain, painlessly. In this study, one patient was paraplegic, the other a quad with some voluntary hand movement but no ability to grasp. Both were beyond two years post-injury; both had received conventional rehabilitation treatments.

Results: After about six months of the stimulation, the para could bend and flex both ankles, and the quad could grasp an object. The cool part about the TMS + nerve stim is that the effect was there when the stim was off. From the paper:

The newly acquired voluntary movements could be performed by the patients in the absence of stimulation and for at least 1 month after the last stimulation session.

“We observed strengthened neural connections and partial restoration of movement to muscles which the patients were previously entirely unable to use,” said investigator Dr. Anastasia Shulga, who led the study at the University of Helsinki.

TMS uses electromagnetic induction to generate electrical currents in the brain and down the spinal cord. It’s called magnetic but is indeed electrical. The main advantage of TMS over direct electrical stim is that TMS does not produce pain. It has been used as a diagnostic tool, and for monitoring nerve activity during surgery. Now it is a promising therapy for SCI as well as for epilepsy, Parkinson’s disease, stroke and more.

In the Finish study, what they think is happening is that the stim awakens plasticity of circuits in the corticospinal tract, the part of the spinal cord that controls voluntary movement in the extremities. From the abstract:

Emerging therapeutic strategies for spinal cord injury aim at sparing or restoring at least part of the corticospinal tract at the acute stage. Hence, approaches that strengthen the weak connections that are spared or restored are crucial.

Transient plastic changes in the human corticospinal tract can be induced through paired associative stimulation, a noninvasive technique in which transcranial magnetic brain stimulation is synchronized with electrical peripheral nerve stimulation. ...

In this unblinded proof-of-principle demonstration in two subjects, long-term paired associative stimulation induced persistent and clinically relevant strengthening of neural connections and restored voluntary movement in previously paralyzed muscles.

TMS isn’t a new idea; it is commonly used to treat depression and works for some types of neuropathic pain, including migraine. With further research maybe it will emerge in brain or spinal cord trauma, in combination with physical activity or rehab training. TMS already is useful as a diagnostic tool, further eroding the meaning of “complete” injury.

A research group from Canada wanted to see if they could find any preserved activity in the trunk, hip and legs muscles of 16 patients with so-called motor complete injuries. Their results came out a few days ago, Cortical and vestibular stimulation reveal preserved descending motor pathways in individuals with motor-complete spinal cord injury. From the paper:

Most individuals (14/16) with motor-complete spinal cord injury were found to have transcranial magnetic stimulation evoked, and/or voluntary evoked muscle activity in muscles innervated below the clinically classified lesion level.

... These techniques may provide clinicians with more accurate information about the state of various motor pathways, and could offer a method to more accurately target rehabilitation.

Meanwhile, there are several more studies underway in Cleveland, Boston and elsewhere to evaluate TMS therapy spinal cord injury and other nervous system disorders. I direct you to a recent column for the Neurotech Business report, written by Jennifer French (a pioneer herself in applied electrical stim for functional return after SCI). Says she:

To date, most clinical efforts [brain stimulation] in neurorehabilitation have focused on techniques to restore activities of daily living, which are heavily task- and movement-oriented. However, fueled by advances in our understanding of cortical plasticity, there has been a renewed effort to use brain stimulation to enhance neurorehabilitation.

See more of her coverage here.