Imaging the Visual Pathway in Human Albinism
thesisposted on 06.07.2016, 15:28 by Sarim Ather
Albinism refers to a group of genetic abnormalities that are associated with profound defects throughout the visual pathway. These include foveal hypoplasia, optic nerve anomalies, chiasmal misrouting, visual cortex reorganisation, and nystagmus. This study utilises optical coherence tomography and magnetic resonance imaging to assess the visual pathway in a large cohort of patients. We find that in albinism, there is maldevelopment of the fovea, with a continuation of the inner retinal layers and a failure of the photoreceptor layers to specialize. The latter abnormality is the biggest contributor to reduced visual acuity seen in albinism. The optic nerve head is characterized by presence of excess glial tissue within the optic rim indicating incomplete maturation. In addition, there is reduced peripapillary retinal never fibre layer thickness consistent with previous histology reports of reduced ganglion cell numbers in albinism. We demonstrate the ability of diffusion tractography to quantify abnormal chiasmal decussation for the first time. Moreover, we find that cortical abnormalities are related to melanin levels within the retinal pigment epithelium and axonal disorganisation. Our results show that nystagmus severity is related to the degree of foveal maldevelopment. This finding adds credence to the increasing recognition of the importance of sensory abnormalities in generating nystagmus. In conclusion, we find that in albinism, normal development of the visual pathway appears to have halted prior to reaching completion. Patients with albinism show a spectrum of anomalies ranging from resembling normality to being grossly atypical. This spectrum closely resembles stages in normal visual development. Our findings represent a step forward in the scientific understanding of visual deficits associated with albinism and are likely to aid clinicians in the management of affected patients.