Characterization of Optic Tract Degeneration in Patients with Damage to the Visual Pathway

Abstract

Trans-synaptic retrograde degeneration (TRG) in the optic tract (OT) has recently been demonstrated in patients with damage to the visual cortex. However, the time-course of this degeneration is unknown, as is the role of lesion extent on the severity of TRG. The current study used evidence from MRI structural images from patients with substantial visual field deficits to investigate the effect of lesion size and time since damage on OT degeneration. Fourteen patients with visual field deficits stemming from damage to the visual pathway were scanned, together with ten age-matched controls. OT size was measured from T1-weighted scans, by drawing a mask of each tract on a series of coronal images, and then computing OT volume at different voxel intensity thresholds. A laterality index (LI) was then computed for each subject to quantify the amount of degeneration present in the ipsilesional tract. Patients with occipital lobe resections showed the largest laterality indices, followed by those with damage to the tract itself. Patients with cortical lesions showed a highly significant correlation between LI and lesion extent. The LI in two patients with recent lesions did not differ significantly from controls. Degeneration over time was evident in four patients with recent damage who were each rescanned a year or more after the initial scan. Optic tract degeneration was present in all patients with long standing lesions, and severity of cortical loss was correlated significantly with the extent of atrophy. Repeated tests in patients with recent damage show increasing degeneration over time. This study shows conclusively that MRI can be used for monitoring retrograde degeneration in patients. Modulation of TRG may have a role in minimising collateral effects of brain damage and an understanding of the time course of the process is essential in the design of such a strategy.

Meeting abstract presented at VSS 2012