Promoting Gait Recovery and Limiting Neuropathic Pain After Spinal Cord Injury

The predictive value of cortical activity during motor imagery for subacute spinal cord injury-induced neuropathic pain

OBJECTIVE

The aim of this study is to explore whether cortical activation and its lateralization during motor imagery (MI) in subacute spinal cord injury (SCI) are indicative of existing or upcoming central neuropathic pain (CNP).

METHODS

Multichannel electroencephalogram was recorded during MI of both hands in four groups of participants: able-bodied (N = 10), SCI and CNP (N = 11), SCI who developed CNP within 6 months of EEG recording (N = 10), and SCI who remained CNP-free (N = 10). Source activations and its lateralization were derived in four frequency bands in 20 regions spanning sensorimotor cortex and pain matrix.

RESULTS

Statistically significant differences in lateralization were found in the theta band in premotor cortex (upcoming vs existing CNP, p = 0.036), in the alpha band at the insula (healthy vs upcoming CNP, p = 0.012), and in the higher beta band at the somatosensory association cortex (no CNP vs upcoming CNP, p = 0.042). People with upcoming CNP had stronger activation compared to those with no CNP in the higher beta band for MI of both hands.

CONCLUSIONS

Activation intensity and lateralization during MI in pain-related areas might hold a predictive value for CNP.

SIGNIFICANCE

The study increases understanding of the mechanisms underlying transition from asymptomatic to symptomatic early CNP in SCI.

Effective Connectivity in Spinal Cord Injury-Induced Neuropathic Pain

AIM

The aim of this study was to differentiate the effects of spinal cord injury (SCI) and central neuropathic pain (CNP) on effective connectivity during motor imagery of legs, where CNP is typically experienced.

METHODS

Multichannel EEG was recorded during motor imagery of the legs in 3 groups of people: able-bodied (N = 10), SCI with existing CNP (N = 10), and SCI with no CNP (N = 20). The last group was followed up for 6 months to check for the onset of CNP. Source reconstruction was performed to obtain cortical activity in 17 areas spanning sensorimotor regions and pain matrix. Effective connectivity was calculated using the directed transfer function in 4 frequency bands and compared between groups.

RESULTS

A total of 50% of the SCI group with no CNP developed CNP later. Statistically significant differences in effective connectivity were found between all groups. The differences between groups were not dependent on the frequency band. Outflows from the supplementary motor area were greater for the able-bodied group while the outflows from the secondary somatosensory cortex were greater for the SCI groups. The group with existing CNP showed the least differences from the able-bodied group, appearing to reverse the effects of SCI. The connectivities involving the pain matrix were different between able-bodied and SCI groups irrespective of CNP status, indicating their involvement in motor networks generally.

SIGNIFICANCE

The study findings might help guide therapeutic interventions targeted at the brain for CNP alleviation as well as motor recovery post SCI.

Posteroanterior Cervical Transcutaneous Spinal Cord Stimulation: Interactions with Cortical and Peripheral Nerve Stimulation

Transcutaneous spinal cord stimulation (TSCS) has demonstrated potential to beneficially modulate spinal cord motor and autonomic circuitry. We are interested in pairing cervical TSCS with other forms of nervous system stimulation to enhance synaptic plasticity in circuits serving hand function. We use a novel configuration for cervical TSCS in which the anode is placed anteriorly over ~C4–C5 and the cathode posteriorly over ~T2–T4. We measured the effects of single pulses of TSCS paired with single pulses of motor cortex or median nerve stimulation timed to arrive at the cervical spinal cord at varying intervals. In 13 participants with and 15 participants without chronic cervical spinal cord injury, we observed that subthreshold TSCS facilitates hand muscle responses to motor cortex stimulation, with a tendency toward greater facilitation when TSCS is timed to arrive at cervical synapses simultaneously or up to 10 milliseconds after cortical stimulus arrival. Single pulses of subthreshold TSCS had no effect on the amplitudes of median H-reflex responses or F-wave responses. These findings support a model in which TSCS paired with appropriately timed cortical stimulation has the potential to facilitate convergent transmission between descending motor circuits, segmental afferents, and spinal motor neurons serving the hand. Studies with larger numbers of participants and repetitively paired cortical and spinal stimulation are needed.

Impact of Experimental Tonic Pain on Corrective Motor Responses to Mechanical Perturbations

Movement is altered by pain, but the underlying mechanisms remain unclear. Assessing corrective muscle responses following mechanical perturbations can help clarify these underlying mechanisms, as these responses involve spinal (short-latency response, 20-50 ms), transcortical (long-latency response, 50-100 ms), and cortical (early voluntary response, 100-150 ms) mechanisms. Pairing mechanical (proprioceptive) perturbations with different conditions of visual feedback can also offer insight into how pain impacts on sensorimotor integration. The general aim of this study was to examine the impact of experimental tonic pain on corrective muscle responses evoked by mechanical and/or visual perturbations in healthy adults. Two sessions (Pain (induced with capsaicin) and No pain) were performed using a robotic exoskeleton combined with a 2D virtual environment. Participants were instructed to maintain their index in a target despite the application of perturbations under four conditions of sensory feedback: (1) proprioceptive only, (2) visuoproprioceptive congruent, (3) visuoproprioceptive incongruent, and (4) visual only. Perturbations were induced in either flexion or extension, with an amplitude of 2 or 3 Nm. Surface electromyography was recorded from Biceps and Triceps muscles. Results demonstrated no significant effect of the type of sensory feedback on corrective muscle responses, no matter whether pain was present or not. When looking at the effect of pain on corrective responses across muscles, a significant interaction was found, but for the early voluntary responses only. These results suggest that the effect of cutaneous tonic pain on motor control arises mainly at the cortical (rather than spinal) level and that proprioception dominates vision for responses to perturbations, even in the presence of pain. The observation of a muscle-specific modulation using a cutaneous pain model highlights the fact that the impacts of pain on the motor system are not only driven by the need to unload structures from which the nociceptive signal is arising.

Ankle proprioception during gait in individuals with incomplete spinal cord injury

INTRODUCTION

Proprioception is known to be affected after a spinal cord injury (SCI). However, it is currently assessed during simple tasks that do not reflect activities of daily living. To better understand how proprioception affects movement, assessing it during a functional sensorimotor task such as walking is therefore of primary importance. Therefore, the objectives of this study were as follows: (a) measure the protocol reliability of a new robotic test in nondisabled controls; (b) evaluate the effect nonlesion-related factors such as sex, age, pain, and gait speed on ankle proprioception; and (c) assess ankle proprioception during walking in individuals with SCI.

METHODS

In the current study, ankle proprioception was assessed during gait in individuals with an incomplete spinal cord injury (iSCI; n = 15) using an electrohydraulic robotized ankle-foot orthosis (rAFO). Ankle proprioceptive threshold was quantified as the participants' ability to detect torque perturbations of varied amplitude applied during swing by the rAFO. In addition, test-retest reliability and the potential effect of nonlesion-related factors (sex, age, pain, and gait speed) were evaluated in nondisabled (ND; n = 65) participants.

RESULTS

During gait, individuals with iSCI had a 53% poorer proprioceptive threshold than ND controls (p < .05). Test-retest reliability was good (ICC = 0.78), and only gait speed affected proprioceptive threshold (p = .018).

CONCLUSION

This study is the first to show that ankle proprioception assessed during gait is impaired in individuals with an iSCI. The developed test can now be used to better characterize proprioception in population with other neurological conditions and has potential to maximize functional recovery during gait training in those populations.

Impaired Motor Learning Following a Pain Episode in Intact Rats

Motor learning and pain are important factors influencing rehabilitation. Despite being mostly studied independently from each other, important interactions exist between them in the context of spinal cord injury, whether to the spinal cord or the body. Ongoing or recent past episodes of nociceptive activity can prevent motor learning in spinalized rats. In intact animals, it has been proposed that supraspinal activity could counter the repressive effect of nociception on motor system plasticity, but this has not yet been verified in behavioral conditions. The aim of this study was to test whether a recent episode of nociception affects subsequent motor learning in intact animals. We trained rodents to walk on a custom-made horizontal ladder. After initial training, the rats underwent a week-long rest, during which they were randomly assigned to a control group, or one out of two pain conditions. Nociceptive stimuli of different durations were induced through capsaicin or Complete Freund's Adjuvant injections and timed so that the mechanical hypersensitivity had entirely subsided by the end of the resting period. Training then resumed on a modified version of the horizontal ladder. We evaluated the animals' ability to adapt to the modified task by measuring their transit time and paw misplacements over 4 days. Our results show that prior pain episodes do affect motor learning in neurologically intact rats. Motor learning deficits also seem to be influenced by the duration of the pain episode. Rats receiving a subcutaneous injection of capsaicin displayed immediate signs of mechanical hypersensitivity, which subsided rapidly. Nonetheless, they still showed learning deficits 24 h after injection. Rats who received a Complete Freund's Adjuvant injection displayed mechanical hypersensitivity for up to 7 days during the resting period. When trained on the modified ladder task upon returning to normal sensitivity levels, these rats exhibited more prolonged motor learning deficits, extending over 3 days. Our results suggest that prior pain episodes can negatively influence motor learning, and that the duration of the impairment relates to the duration of the pain episode. Our results highlight the importance of addressing pain together with motor training after injury.

The Spinal Cord, Not to Be Forgotten: the Final Common Path for Development, Training and Recovery of Motor Function

Research on learning, memory, and neural plasticity has long focused on the brain. However, the spinal cord also exhibits these phenomena to a remarkable degree. Following a spinal cord injury, the isolated spinal cord in vivo can adapt to the environment and benefit from training. The amount of plasticity or recovery of function following a spinal injury often depends on the age at which the injury occurs. In this overview, we discuss learning in the spinal cord, including associative conditioning, neural mechanisms, development, and applications to clinical populations. We take an integrated approach to the spinal cord, one that combines basic and experimental information about experience-dependent learning in animal models to clinical treatment of spinal cord injuries in humans. From such an approach, an important goal is to better inform therapeutic treatments for individuals with spinal cord injuries, as well as develop a more accurate and complete account of spinal cord and behavioral functioning.

Assessments of sensory plasticity after spinal cord injury across species

Spinal cord injury (SCI) is a multifaceted phenomenon associated with alterations in both motor function and sensory function. A majority of patients with SCI report sensory disturbances, including not only loss of sensation, but in many cases enhanced abnormal sensation, dysesthesia and pain. Development of therapeutics to treat these abnormal sensory changes require common measurement tools that can enable cross-species translation from animal models to human patients. We review the current literature on translational nociception/pain measurement in SCI and discuss areas for further development. Although a number of tools exist for measuring both segmental and affective sensory changes, we conclude that there is a pressing need for better, integrative measurement of nociception/pain outcomes across species to enhance precise therapeutic innovation for sensory dysfunction in SCI.