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Interhemispheric Plasticity in Response to Peripheral Nerve Injury
Thomas Hansson, MD, PhD; Lotta Fornander, MD; Torbjörn Nyman, MD
Linköping University, Linköping, Sweden

Objective: Recovery from peripheral nerve injury in adults results in profound abnormalities in sensory perception despite improved microsurgical technique. It is of great importance to increase the understanding of cortical changes in response to peripheral nerve injury to improve the rehabilitation of these patients. We have previously described a transient increase in the contralateral primary somatosensory cortical activation area (fMRI) following nerve damage and repair (Hansson,Brismar 2003). In addition, there is growing evidence of interhemispheric plasticity. Studies of rats with a denervated paw show a transfer of activity from the contralateral to the ipsilateral hemisphere. Our objective was to study the ipsilateral cortical activation pattern in patients with median nerve injury.

Methods: Eleven healthy volunteers (mean age 24) and four patients (mean age 39) with median nerve injury at the wrist, repaired with epineural suture at least two years prior to the examination, were included in the study. 3T Functional MRI (fMRI) was used to measure brain activity while the median- and ulnar nerve innervated fingers of both hands respectively were given tactile stimulation by an air-driven brush. The static two point discrimination (2pd) test was used to evaluate sensory function. Data analysis was performed using SPM12 and the laterality index (LI) was calculated to evaluate redistribution of hemispheric dominance.

Results: All patients showed abnormal 2pd in the injured median nerve innervation area (2pd 8->15mm). The patients had normal 2pd (2pd <5mm) in the healthy hand and in the ulnar fingers of the injured hand. All healthy volunteers had normal 2pd. fMRI showed a higher degree of ipsilateral activity in the parietal lobe of the patients than in the group of healthy volunteers and there was a significant difference in LI between the two groups (mean patients 0.21, mean healthy volunteers 0.60, p <0.05).

Conclusions: Our study showed that patients with median nerve injury have a lower LI, meaning a more bilateral activation pattern in the somatosensory cortex compared to a group of healthy individuals. Healthy individuals display a greater degree of contralateral dominance to unilateral median nerve stimulation. The relative increase in the ipsilateral contribution may compensate for the disorganised contralateral somatosensory cortex i.e. it may be a part of the interhemispheric plasticity in response to peripheral nerve injury.

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