Focal demyelinative damage and neighboring white matter integrity: An optic neuritis study

Background: Neuronal loss following damage is often greater than expected from the severity of injury to the nerve itself. The visual pathways, which comprise a well-defined system, and optic neuritis (ON), which is usually a discrete event, make a fine model to study this phenomenon. Objective: Understand the effect of focal optic nerve demyelination on neighboring white matter. Methods: Diffusion tensor imaging and probabilistic tractography were used to identify and characterize the optic tracts and radiations of 17 ON and matched controls. Data were correlated with retinal nerve fiber layer (RNFL) thickness. Results: Patients' optic tracts exhibited reduced axial diffusivity, which correlated with RNFL thickness values. Patients' optic radiations demonstrated intact axial diffusivity but reduced fractional anisotropy and elevated radial diffusivity, which could be explained by intra-bundle lesions. No correlations were found between diffusivity measurements in patients' optic tracts and radiations; or between RNFL thickness and optic radiations' diffusivity. Conclusions: Following ON, chronic axonal loss develops distally in the optic tracts, demonstrating Wallerian degeneration. Degeneration did not proceed to the optic radiations, opposing anterograde transneuronal changes. DTI in ON provides fine in-vivo human model for studying histological abnormalities in normal appearing white matter, localized in close proximity to damaged bundle.