Residual descending motor pathways influence spasticity after spinal cord injury

Abstract

Objective: Spasticity is one of the most common symptoms manifested in humans with spinal cord injury (SCI). The neural mechanisms contributing to its development are not yet understood. Using neurophysiological and imaging techniques, we examined the influence of residual descending motor pathways on spasticity in humans with SCI. Methods: We measured spasticity in 33 individuals with motor complete SCI (determined by clinical examination) without preservation of voluntary motor output in the quadriceps femoris muscle. To examine residual descending motor pathways, we used magnetic and electrical stimulation over the leg motor cortex to elicit motor evoked potentials (MEPs) in the quadriceps femoris muscle and structural magnetic resonance imaging to measure spinal cord atrophy. Results: We found that 60% of participants showed symptoms of spasticity, whereas the other 40% showed no spasticity, demonstrating the presence of 2 clear subgroups of humans with motor complete SCI. MEPs were only present in individuals who had spasticity, and MEP size correlated with the severity of spasticity. Spinal cord atrophy was greater in nonspastic compared with spastic subjects. Notably, the degree of spared tissue in the lateral regions of the spinal cord was positively correlated with the severity of spasticity, indicating preservation of white matter related to motor tracts when spasticity was present. Interpretation: These results support the hypothesis that preservation of descending motor pathways influences spasticity in humans with motor complete SCI; this knowledge might help the rehabilitation and assessment of people with SCI.

Imbalanced Corticospinal and Reticulospinal Contributions to Spasticity in Humans with Spinal Cord Injury

“…Participants with a MAS score of 1 and ϩ1 were grouped together as MAS 1. Recently, we observed no differences in outcomes related to spasticity between individuals with MAS score of 0 and 1 ( post hoc analysis not reported in the paper) (Sangari et al, 2019). The same result was observed here.…”

“…medication to reduce spasticity are unable to modulate corticospinal responses, as is found in individuals that do take antispastic medication (Barry et al, 2013;Bunday et al, 2014), which is consistent with the view that corticospinal responses are affected when spasticity is present. This also agrees with recent results in humans with motor complete SCI showing that MEPs elicited by TMS were only present in spastic but not in nonspastic participants (Sangari et al, 2019). Second, we found that the magnitude of maximal voluntary output was decreased in spastic compared with non-spastic and control subjects.…”

“…For example, self-reported questionnaires and clinical examinations indicate that most people with incomplete SCI or residual descending voluntary drive have a high prevalence of spasticity (Little et al, 1989;Maynard et al, 1990;Sköld et al, 1999;Holtz et al, 2017). Recent electrophysiological data showed the presence of motorevoked potentials (MEPs), likely involving the corticospinal tract, in humans with a diagnosis of clinically motor complete SCI who have spasticity but not in those individuals without spasticity (Sangari et al, 2019), suggesting that spasticity involved the presence of residual descending connections. Which descending motor pathways influence spasticity in humans with SCI, and to which extent, however, remains poorly understood.…”

Crossed Corticospinal Facilitation Between Arm and Trunk Muscles Correlates With Trunk Control After Spinal Cord Injury

“…For instance, it has been demonstrated that in people with SCI amplitudes of MEPs are reduced, latencies are prolonged, and motor thresholds are increased compared to uninjured controls (Davey et al, 1998;Ellaway et al, 2007). Alterations in corticospinal function have been shown to relate to the functional recovery of limbs after SCI (Belci et al, 2004;Wirth et al, 2008;Sangari et al, 2019). Previous work on the trunk has shown that the MEP latencies in paraspinal muscles were prolonged in SCI patients with severe motor impairment compared with those with mild motor impairment (Ogura et al, 2003).…”

Effects of a Contusive Spinal Cord Injury on Spinal Motor Neuron Activity and Conduction Time in Rats

“…Targeted therapy of an intact reticulospinal tract in rats with severe spinal cord contusion has been shown to result in the reorganization of the cortico-reticulospinal circuit and improvement of motor function (Asboth et al, 2018). Stimulation therapies targeting specific descending motor pathways using time-dependent stimulation techniques have shown substantial recovery of motor function after injury in humans (Bunday et al, 2018;Sangari et al, 2019).…”