Spinal cord injuries are one very prevalent type of traumatic injury that occurs in the United States daily. According to recent data, there are over 17,000 new spinal cord injury (SCI) cases per year in the United States alone (University of Alabama at Birmingham, 2020). Although incomplete tetraplegia, where there is only partial damage to the spinal cord, is more common compared to complete paraplegia, which is usually accompanied by a total loss in motion and/or sensory function, the data recorded also found that less than 1% of SCI patients have complete recovery by the time of discharge and this injury can also decrease life expectancy significantly (University of Alabama at Birmingham, 2020). Due to the poor prognosis often associated with SCIs, there is a drastic need for refined treatments in order to help resolve symptoms of these patients, as well as striving for recovery. Two recent studies conducted in 2020 have examined how stimulation can be used to treat SCI patients. Jo and Perez examine how pairing corticospinal-motor neuronal stimulation and exercise in incomplete SCI patients can cause some recovery in voluntary movement that lasts beyond the time of active stimulation. On the other hand, Isabela Peña Pino and colleagues examine how an epidural spinal cord stimulation alone can lead to some recovery in voluntary movement that lasts beyond the time of active stimulation in complete SCI patients.
Previous literature has shown that exercise training paired with neural stimulation can increase recovery effects in SCI patients (Harkema et al., 2012; Behrman et al., 2017). Due to this, researchers Jin Jo and Perez hypothesized that pairing exercise with paired cortico-motor stimulation (PCMS) of spinal synapses would promote functional recovery in incomplete SCI patients (Jo & Perez, 2020). They believed that this was due to enhancing transmission in the surviving corticospinal pathways. In this study, 25 individuals with incomplete spinal cord injuries between the C2 and L3 vertebrae were randomly separated into either the PCMS group, where they received stimulation, or the sham-PCMS group, where they did not actually receive any stimulation but believed they did. PCMS, or paired corticospinal-motor neuronal stimulation, uses TMS to evoke corticospinal volleys in the primary motor cortex (Bunday & Perez, 2012; Urbin et al., 2017; Bunday et al., 2018). It is believed that this type of stimulation can lead to recovery of SCI patients. Participants completed 10 sessions of exercise combined with PCMS/sham-PCMS. A further group of 13 individuals was tested with PCMS alone.
This study found many different results. Motor evoked potentials (MEPs) increased from the beginning to the end of the study in groups that received PCMS, regardless of if they received exercise training (Jo & Perez, 2020). On the other hand, these increases were not seen in the sham-PCMS group. These trends in data were also seen for the level of maximal voluntary contraction (MVC) from before to after the study. PCMS groups increased MVC throughout the study, but the sham-PCMS group did not. An interesting aspect of these results were discovered six months after the initial study. Participants were invited back to the lab for a follow-up study to test MEPs and MVCs. Both MEP and MVC amplitude remained at an increased level from baseline in the PCMS with exercise group, but not the sham-PCMS group (Jo & Perez, 2020). These results suggest that there may be some plasticity in the surviving corticospinal tracts in the injured spinal cord. Plasticity may be facilitated by PCMS, which leads to the functional recovery of incomplete SCI patients that may last up to 6 months after stimulation is ceased. Although this may be a potential treatment for incomplete SCI patients, there is still the question if this type of stimulation can also help complete SCI patients.
To address this question, Pino and colleagues explore how epidural spinal cord stimulation (eSCS) can lead to sustained voluntary movement abilities in complete SCI patients (Pino et al., 2020). The researchers in this study did not change the exercise or rehabilitation regimes of any participants, so the effects observed should be due to the stimulation alone. Seven participants with chronic complete SCIs in the thoracic spinal cord. These participants had complete loss of motor function below the level of injury, but they retained full arm and hand strength. After completing a baseline MRI and questionnaire, participants were implanted with an epidural stimulator into the spinal cord (Tripole and Proclaim Elite, Abbot, Plano, TX, United States). Participants were provided with a programmer in which they had the ability to change stimulation settings depending on which goal they are attempting to achieve in the moment, such as voluntary movement or control of spasms. After implantation, participants completed 13 follow-up assessments where they completed a questionnaire and participated in a Brain Motor Control Assessment (BMCA). The BMCA assesses voluntary motor function by using surface electromyography (sEMG) and observation by researchers (Sherwood et al., 1996). Functional movement was also assessed using a Muvi 300 stationary bike developed by MOTOmed (Pino et al., 2020). This cycle has a motor-assisted setting so participants can train with minimal muscle strength.
By the time the study was concluded, four participants developed voluntary movements even when the stimulation was turned off (Pino et al., 2020). These participants were then named the SVM group, or spontaneous volitional movement group, and some participants in this group showed sustained voluntary movement as early as 3 months post-implantation.. The non-SVM group also demonstrated voluntary movement, but only in the presence of active stimulation. Overall, voluntary movement was recorded much more significantly when stimulation was active. When viewing the baseline scores, researchers noted that baseline spasticity scores were significantly higher in the SVM group compared to the non-SVM group (Pino et al., 2020). This suggests that high levels of spasticity may indicate some surviving corticospinal tracts that can be trained to induce volitional movement. This indicates that these tracts may be plastic and can potentially be restored to a degree. Furthermore, since this study indicates plasticity in the spinal cord axons, and the SVM group had a higher rate of spasticity at baseline, high rates of spasticity may indicate how likely one is to be able to have a level of recovery from stimulation. Therefore, these ‘complete’ spinal cord injuries may not be as complete as one believed. Although they had complete motor loss at the beginning of the study, plastic changes were still induced leading to a recovery in volitional movement.
Overall, both studies described present a promising route for recovery in both incomplete and complete SCI patients. The second study indicates that complete SCIs may not be as complete as once believed, as plastic changes can still be induced in these participants. Although more research is needed to determine how long these changes can last, and if they can become permanent, using stimulation to treat SCIs is a significant first step in restoring voluntary movement to these patients.
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Urbin MA, Ozdemir RA, Tazoe T, Perez MA. Spike-timing-dependentplasticity in lower-limb motoneurons after human spinal cord injury.J Neurophysiol 2017; 118: 2171–80.
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