Noxious radiant heat evokes bi-component nociceptive withdrawal reﬂexes in spinal cord injured humans—A clinical tool to study neuroplastic changes of spinal neural circuits

Front. Hum. Neurosci., 2023 · DOI: 10.3389/fnhum.2023.1141690 · Published: May 2, 2023

Spinal Cord Injury Neurology Pain Management

Simple Explanation

This study investigates how people with spinal cord injuries (SCI) respond to painful heat stimuli, using lasers to trigger reflexes and measuring muscle activity. The goal was to see if these reflexes could indicate hyperexcitability in the spinal cord after SCI. The study found that individuals with SCI showed stronger and more frequent reflex responses to heat compared to those without SCI, suggesting increased excitability in their spinal cord circuits. These reflexes also seem to be linked to spasticity, a common issue after SCI. Interestingly, the study did not find a direct link between these reflexes and neuropathic pain, another common problem after SCI. This might mean that different mechanisms are at play for spasticity and neuropathic pain after a spinal cord injury.

Study Duration

Not specified

Participants

27 (15 SCI, 12 non-disabled controls)

Evidence Level

Level III, Cross-sectional observational study

Key Findings

1
Individuals with SCI showed a significantly higher percentage of responses, response rates, and reflex magnitude to noxious heat compared to non-disabled controls, indicating hyperresponsiveness.
2
SCI-related reflexes clustered in two time-windows, suggesting the involvement of both A-delta and C-fibers in mediating nociceptive withdrawal reflexes.
3
Spasticity was associated with facilitated reflexes in SCI, while neuropathic pain was inversely associated with the occurrence/severity of spasticity, but not directly related to reflex behavior.

Research Summary

This study explored nociceptive withdrawal reflexes in individuals with SCI using radiant heat laser stimuli, revealing hyperreflexia and the involvement of both A-delta and C-fibers. The findings indicated a positive association between hyperreflexia and spasticity, suggesting a common underlying mechanism of spinal hyperexcitability. However, no direct association was found between the reflex behavior and neuropathic pain, suggesting that different maladaptive plasticity mechanisms may contribute to these distinct complications of SCI.

Practical Implications

Clinical Assessment Tool

Laser-evoked withdrawal reflexes may serve as a valuable outcome parameter to explore maladaptive spinal circuitries in SCI and assess the effectiveness of targeted treatments.

Targeted Therapies

Understanding the distinct mechanisms underlying spasticity and neuropathic pain may lead to the development of more targeted and effective therapeutic strategies for SCI-related complications.

Further Research

Future research should focus on further elucidating the specific roles of A-delta and C-fibers in nociceptive processing after SCI and investigating the relationship between structural changes and reflex circuits.

Study Limitations

1
The study was not primarily designed to examine specific differences in reflex behavior with respect to stimulation or recording sites.
2
The reflex receptive fields (RRFs) of the study participants were not investigated, limiting the assessment of central sensitization.
3
The relatively small sample size and the use of medications like anticonvulsants or baclofen by some participants may have biased the results.

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