Abnormal white matter development in children with multiple sclerosis and monophasic acquired demyelination

The use of conventional MRI as a tool in the diagnosis of multiple sclerosis (MS) is well established in both adults and children and aids in the differentiation between CNS demyelination and its mimics. Conventional MR imaging is insensitive to the tissue damage,which occurs beyond the visible MS plaques, in the so-called normal appearing white matter (NAWM). In contrast to conventional MRI, diffusion tensor imaging (DTI) allows a non-invasive and indirect assessment of axonal structure and myelin integrity by characterising the diffusion properties of water molecules in vivo in the brain. The two most commonly used diffusion indices are the mean diffusivity (MD) and the fractional anisotropy (FA). Changes in MD and FA reflect changes in tissue microstructure and organisation of white matter fibers. Although linking changes in MD and FA to pathological abnormalities is tempting, it is likely to be an approximation, considering the complexity of the tissue microstructure and underlying pathological changes. In general, pathological processes known to occur in MS, including axonal degeneration, myelin breakdown, inflammation and increased tissue water, may lead to increased MD and reduced FA in both the lesions and NAWM1. In addition, a reduction in white matter FA is thought to reflect reduced fiber coherence, which is responsible for a reduction in the directional bias of water diffusion.