Evidence for sustained neural plasticity after chronic spinal cord injury – generated by vibration administration

During the first year after spinal cord injury, an electrophysiological index – the so-called Hoffmann reflex depression (H) – gradually disappears. This index is used to study the function of neuronal pathways and, in the case of spinal cord injury, reflects the reorganisation of its interneurons after the loss of descending inhibition from the cortex. This reorganisation influences the development of spasticity. Researchers from the US University of Iowa observed on a group of five chronically injured spinal cord patients the restoration of H-reflex depression after vibration training, suggesting that even after long-term damage to the spinal cord, neurons of its segments associated with reflex pathways still maintain plasticity.

The aim of the study presented here was to test whether long-term vibration training could promote improvement in H-reflex parameters impaired by chronic spinal cord injury.

• In limbs not subjected to vibration, H-reflex depression was within the range observed in previous studies in patients with chronic spinal cord injury.
• Vibration had the effect of restoring H-reflex depression values to those previously observed in acute injury or in the absence of spinal cord injury.
• The mean difference in H-reflex depression between the limb subjected and not subjected to vibration was 35%.

Study population

Men with complete spinal cord injury (n = 5).

Test procedure

Vibration was administered to one of the two lower limbs in a seated position in a wheelchair. The contralateral limb served as a control. Pulses generating H-reflexes were obtained before, during and after the vibration session.

Use of vibration in the study

Vibration was administered twice a week (30 Hz, 0.6 g).

Results

H-reflex depression values in the limb not subjected to vibration were comparable to those observed in previous studies in patients with chronic spinal cord injury. H-reflex depression values in the vibration-treated limb oscillated in all subjects within the limits previously observed in acute injury or in the absence of spinal cord injury, and the mean difference in H-reflex depression between vibration-treated and non-vibration-treated limbs was 34.98% (p = 0.016).

Comment

The recovery of H-reflex depression after vibration training indicates that, even after long-term spinal cord injury, spinal reflex pathway neurons still maintain plasticity. Similarly, therefore, spinal cord neurons involved in the clinical symptoms of spasticity may still maintain adaptive plasticity.

The results described in the present report suggest that vibration training may be extremely relevant in anti-spasticity-targeted rehabilitation.

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