Phenotypic Features of the Visual System in Albinism

Purpose: The main aims were to: 1) characterise the morphology of the iris structures and investigate the diagnostic potential of changes in albinism, 2) investigate visual field changes in albinism and 3) to analyse the relationship between refractive error/iris pigmentation and foveal hypoplasia in infantile nystagmus. Methodology: The iris was imaged in 55 individuals with albinism and 45 controls using anterior segment optical coherence tomography (OCT) and segmented using ImageJ software. Visual field assessment using Humphrey field analyser was carried out on 61 individuals with albinism and 32 individuals with idiopathic infantile nystagmus (IIN), and were compared to posterior OCT parameters. Refractive measures were compared to foveal hypoplasia in different groups with nystagmus (albinism=33, IIN=18, PAX6=9 and achromatopsia=12) and iris posterior epithelial layer (PEL) thickness in albinism. Results: The iris posterior epithelial layer (PEL) in albinism demonstrates significant thinning compared to controls, especially at the ciliary end (P<0.001). Ciliary PEL thickness demonstrated 85% sensitivity and 78% specificity in aiding the diagnosis of albinism. Visual field measurements showed that detection thresholds in albinism were significantly worse than in IIN (P<0.001). In albinism the upper nasal visual field quadrant demonstrated poorer detection thresholds compared to other quadrants (P<0.05 for all comparisons). Left eyes had lower detection thresholds than right eyes (P<0.0001). Changes in peripapillary retinal nerve fibre layer were not associated with these changes in detection thresholds. Worse foveal hypoplasia was associated with myopia in achromatopsia but with hypermetropia in albinism. Increasing hypermetropia in albinism was associated with PEL thinning (P<0.01). Conclusion: The PEL demonstrates significant thinning in albinism which may be used as a diagnostic aid. The cause of lower detection thresholds within the upper nasal visual field and left eyes in albinism is unclear. Hyperillumination appears more important than foveal hypoplasia in the development of refractive error in albinism.