Aberrant visual pathway development in albinism: From retina to cortex

Albinism refers to a group of genetic abnormalities in melanogenesis that are associated neuronal misrouting through the optic chiasm. We perform quantitative assessment of visual pathway structure and function in 23 persons with albinism (PWA) and 20 matched controls using optical coherence tomography (OCT), volumetric magnetic resonance imaging (MRI), diffusion tensor imaging and visual evoked potentials (VEP). PWA had a higher streamline decussation index (percentage of total tractography streamlines decussating at the chiasm) compared with controls (Z=-2.24, p=.025), and streamline decussation index correlated weakly with inter-hemispheric asymmetry measured using VEP (r=.484, p=.042). For PWA, a significant correlation was found between foveal development index and total number of streamlines (r=.662, p<.001). Significant positive correlations were found between peri-papillary retinal nerve fibre layer thickness and optic nerve (r=.642, p<.001) and tract (r=.663, p<.001) width. Occipital pole cortical thickness was 6.88% higher (Z=-4.10, p<.001) in PWA and was related to anterior visual pathway structures including foveal retinal pigment epithelium complex thickness (r=-.579, p=.005), optic disc (r=.478, p=.021) and rim areas (r=.597, p=.003). We were unable to demonstrate a significant relationship between OCT-derived foveal or optic nerve measures and MRI-derived chiasm size or streamline decussation index. Our novel tractographic demonstration of altered chiasmatic decussation in PWA corresponds to VEP measured cortical asymmetry and is consistent with chiasmatic misrouting in albinism. We also demonstrate a significant relationship between retinal pigment epithelium and visual cortex thickness indicating that retinal pigmentation defects in albinism lead to downstream structural reorganisation of the visual cortex.