Diffusion tensor imaging of normal-appearing white matter in neuromyelitis optica

Neuromyelitis optica (NMO) is an inflammatory disease of the central nervous system characterized by severe attacks of optic neuritis and myelitis. Brain was classically, unlike in multiple sclerosis (MS), spared. Nevertheless recent studies showed that brain lesions can be seen with MRI. We studied the diffusion characteristics of normal-appearing white matter (NAWM) and abnormal white matter in NMO patients compared with NAWM in healthy subjects.

Diffusion tensor imaging (DTI) scans of the brain and spinal cord were obtained from 25 patients with NMO and 20 age- and gender-matched healthy subjects. Region of interest (ROI) analysis of the apparent diffusivity coefficient (ADC) and fractional anisotropy (FA) was performed in brain NAWM (optic radiations, corpus callosum [CC] and anterior and posterior limbs of the internal capsule [IC]) and in spinal cord NAWM and in lesions.

ADC was increased and FA decreased in NMO patients in the posterior limb of the IC in the optic radiations and in spinal cord NAWM. FA was lower in spinal cord lesions. In contrast, there was no difference between the two groups in the anterior limb of the IC nor in the CC.

These results suggest that DTI abnormalities are very severe in NMO spinal cord lesions. In our study, DTI abnormalities in NAWM were restricted to optic radiations and cortico-spinal tracts, suggesting secondary Wallerian degeneration. In contrast, NAWM outside these tracts (CC and anterior IC) remained normal suggesting that, unlike what is observed in MS, there is no infra-lesional abnormality in NMO.