Functional brain reorganization for hand movement in patients with multiple sclerosis: defining distinct effects of injury and disability
Reddy H, Narayanan S, Woolrich M, Mitsumori T, Lapierre Y, Arnold DL and Matthews PM;
Commented by , 28 Jan 2003
Background
Even though axonal loss as well as demyelination are early pathological features of multiple sclerosis (MS), functional recovery from relapses is often good, which classically has been ascribed reparatory processes such as remyelination.
Recent studies have shown that neural plasticity account for considerable functional redundancy in the CNS, consisting of both compensatory motor strategies as well as true synaptic reorganization of activation. In MS these functional changes may account in part for the limited relationship between brain pathology and disability, an issue which the current study seeks to enlighten.
Aim
To test if brain injury and disability in MS lead to distinguishable patterns of activation with hand movement and to determine if the observed changes are caused by compensation or reflect true functional reorganization.
Methods
Patients with relapsing-remitting MS were selected for three study groups: (1) Low diffuse central brain injury (DCBI) (n=6), (2) greater DCBI and normal hand function (n=4), (3) greater DCBI and impaired hand function (n=4).
In order to determine the degree of brain injury, a structural MRI was done to estimate brain and cord parenchymal volume and MR spectroscopy was done to measure relative N-acetylaspartate concentration, which is considered to be a marker of axon dysfunction or loss.
Functional MRI (fMRI) was used to map brain activation. The motor paradigm was a four-finger flexion-extension movement task as well as a one finger active and passive task. Eight healthy controls were used for comparison.
Results
The authors found activation in ipsi- and contralateral sensorimotor and premotor cortex and in the contralateral supplementary motor area (SMA). Similar results were found for the one finger passive and active movement task.
There was good correlation between the extents of active and passive activation in the ipsilateral motor cortex (r=0,82; p=0,001) and in the contralateral motor cortex (r=0,67; p=0,007).
A contrast between patients from group 3 and 2 (matched DCBI but different hand disability) showed greater bilateral primary and secondary somatosensory cortex activation. A contrast between group 2 and 1 (matched hand disability but different DCBI) showed activation in ipsilateral premotor cortex and bilateral SMA. A full report of the study results will not be contained within the limits of this article.
Discussion
It is well characterized that brain activation is reorganized after brain injury and the current study show comparable results. Furthermore, the study shows that brain activation in MS changes both with increasing DCBI and hand disability, and that the changes related to disability may reflect responses to altered patterns of use.
The most interesting conclusion relies on the assumption that active and passive movement shows similar patterns of activation if it reflects a true functional neural organization. In the study, the motor activation pattern was distinctly changed, but there was a strong correlation between active and passive activation.
This indicates a neural reorganization of the motor cortex related to MS pathology. The results together suggest that both functional impairment and brain injury burden contribute to the altered brain activation in MS.