Optic radiation changes after optic neuritis detected by tractography-based group mapping

Abstract

Postmortem data suggest that trans-synaptic degeneration occurs in the lateral geniculate nucleus after optic nerve injury. This study investigated in vivo the optic radiations in patients affected by optic neuritis using fast marching tractography (FMT), a diffusion magnetic resonance imaging (MRI) fiber tracking method, and group mapping techniques, which allow statistical comparisons between subjects. Seven patients, 1 year after isolated unilateral optic neuritis, and ten age and gender-matched controls underwent whole-brain diffusion tensor MR imaging. The FMT algorithm was used to generate voxel-scale connectivity (VSC) maps in the optic radiations in each subject in native space. Group maps of the left and right optic radiations were created in the patient and control group in a standardized reference frame using statistical parametric mapping (SPM99). The reconstructed optic radiations in the patient group were localized more laterally in the posterior part of the tracts and more inferiorly than in the control group. Patients showed reduced VSC values in both tracts compared with controls. These findings suggest that the group mapping techniques might be used to assess changes in the optic radiations in patients after an episode of optic neuritis. The changes we have observed may be secondary to the optic nerve damage.

Figure 1

VSC maps of the left and right optic radiations of four patients who showed incidental optic radiation lesions (i.e., the first four patients in Table I) obtained in native space and overlaid on FA maps. Lesions are outlined on the PD images. The color scale indicates the VSC values. Optic radiations show posterior connection to the visual cortex and medial connections to the LGN. There are also false positive tracts that project anteriorly, which represent fronto‐occipital fibers that run close to the optic radiations for a short distance. The reconstructed optic radiations pass through all the lesions, and in two cases (b and d), the VSC of the voxels in the lesions seemed lower than that of voxels outside the lesion (see white arrows in b and d). VSC values of the tract downstream of lesions remained high regardless of local pathology.

Figure 2

A: Difference in the degree of spatial variability between patients and controls overlaid onto a structural template (MNI coordinates x, y, z [in mm] = −30.2, −70.1, and 0.4). The hot color scale indicates the difference in overlapping between the two groups, with yellow representing a bias toward the control group and red representing a bias toward the patient group. Reconstructed optic radiations do not show only posterior connections to the visual areas and medial connections to the LGN, but also some false positive connections, as anterior and superior projections from the LGN, which may reflect inferior fronto‐occipital fibers, and are known to accompany optic radiations for a short distance. B: Left optic radiation (a). Patients show reduced voxel‐scale connectivity (VSC) compared to that in controls with the peak of the difference posterior and adjacent to the trigone (corrected P = 0.04; MNI coordinates x, y, z [in mm] = −28, −62, and 4). A second area localized more distally shows a trend for reduced VSC in patients (corrected P = 0.06; MNI coordinates x, y, z [in mm] = −24, −88, and 6). Right optic radiation (b). Patients show reduced VSC compared to that in controls with the peak localized in the distal part of the tract (corrected P = 0.01; MNI coordinates x, y, z [in mm] = 18, −88, and 4). A second area posterior and adjacent to the trigone also shows reduced VSC in patients (corrected P = 0.04; MNI coordinates x, y, z [in mm] = 28, −66, and 2). At the termination of the right optic radiation, a third area shows a trend for reduced VSC in patients (corrected P = 0.07; MNI coordinates x, y, z [in mm] = 10, −96, and 6). The color scales indicate the t scores.