Cortical plasticity in response to median nerve trauma

Abstract

Median nerve injuries in adults, repaired with nerve suture, lead to incomplete functional recovery despite improved surgical technique. This results in a reduction in quality of life, poorer working ability and a considerable expense for society. Misrouting of axons at the suture site connects regenerating axons to the wrong distal end organs. When distorted signals are conveyed to the dorsal root ganglia, spinal cord, thalamus and the somatosensory cortex, somatotopic maps at all levels become reorganised in a disorderly fashion. Children often regain full sensory function after median nerve injury and repair despite impaired conduction across the injured segment. There is growing evidence that cortical plasticity is the main mechanism behind the superior recovery seen in young patients, but the exact pattern of reorganisation and its impact on functional recovery are not fully understood.

The general aim of this thesis was to investigate various aspects of cortical plasticity, in particular the response to median nerve injury. To this end we used two non-invasive brain imaging techniques, functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG). In Paper I we investigated the concept of audio-tactile interaction in a healthy population. We found an increased overlap between cortical activation areas (fMRI) in patients trained with coupled tactile and auditory stimuli indicating modulation of cortical plasticity induced by cross-modal training.

In Paper II we studied ageand time-dependent effects on cortical activity patterns in patients with median nerve injury by correlating age at the time of injury and time passed since injury to sensory function, and cortical activation. We found a time-dependent decline in the size of the cortical activation area during stimulation of both the median and the ulnar nerve (fMRI). Furthermore, there was greater ipsilateral activation in the patient group than in a control group from a previous study. However, the results were not conclusive on this point because the stimulation paradigms differed between the two studies (event-related in the present and block paradigm in the previous study).

Paper III was performed using MEG in order to further study cortical plasticity in patients with median nerve injury. We found decreased N1 and P1 amplitudes during stimulation of the injured median nerve, and an increase in these amplitudes during ulnar nerve stimulation. Paper IV was designed to reveal any possible differences in lateralisation of cortical activation after median nerve injury and to see if this was influenced by the stimulus paradigm used. By means of a laterality index (LI) the extent of contra- and ipsilateral activation was calculated. LI is decreased (more ipsilateral activation) in patients with a median nerve injury compared to controls. This means that median nerve injury causes a shift of activity from the contralateral to the ipsilateral SI. The type of stimulus paradigm (event-related or block) did not affect LI. Our findings add to the evolving knowledge of the cortical plasticity following median nerve injury.