Diffusion tensor imaging of the optic tracts in multiple sclerosis: association with retinal thinning and visual disability

Abstract

Background and purpose: Visual disability is common in multiple sclerosis, but its relationship to abnormalities of the optic tracts remains unknown. Because they are only rarely affected by lesions, the optic tracts may represent a good model for assessing the imaging properties of normal-appearing white matter in multiple sclerosis.

Methods: Whole-brain diffusion tensor imaging was performed on 34 individuals with multiple sclerosis and 26 healthy volunteers. The optic tracts were reconstructed by tractography, and tract-specific diffusion indices were quantified. In the multiple-sclerosis group, peripapillary retinal nerve-fiber-layer thickness and total macular volume were measured by optical coherence tomography, and visual acuity at 100%, 2.5%, and 1.25% contrast was examined.

Results: After adjusting for age and sex, optic-tract mean and perpendicular diffusivity were higher (P=.002) in multiple sclerosis. Lower optic-tract fractional anisotropy was correlated with retinal nerve-fiber-layer thinning (r=.51, P=.003) and total-macular-volume reduction (r=.59, P=.002). However, optic-tract diffusion indices were not specifically correlated with visual acuity or with their counterparts in the optic radiation.

Conclusions: Optic-tract diffusion abnormalities are associated with retinal damage, suggesting that both may be related to optic-nerve injury, but do not appear to contribute strongly to visual disability in multiple sclerosis.

Figure 1

Regions of interest used for diffusion tensor imaging-based tractography of the optic tract and three-dimensional reconstruction of the optic tract. Regions of interest were drawn in the coronal plane (generated by reformatting the axially acquired data) anteriorly around the optic chaism (panel A) and posteriorly at the level of, or just anterior to, the lateral geniculate nucleus (panel B). Fibers that ran through both regions of interest were selected for analysis. The reconstructed optic tracts conformed well to the known anatomy and are shown superimposed on axial (panel C) and coronal (panel D) diffusion-tensor-imaging color maps.

Figure 2

Spatially normalized tract profiles of the optic tracts along the anatomic course from the optic chiasm to the lateral geniculate nucleus (LGN). Data are averaged across 34 participants with multiple sclerosis (solid lines) and 26 healthy volunteers (dashed lines) and are adjusted to the average age of our cohort (39.6 years). Error bars denote standard errors of the mean. (A) MD, mean diffusivity; (B) FA, fractional anisotropy; (C) λ_∥, parallel diffusivity; (D) λ_⊥, perpendicular diffusivity.