The effect of abnormal eye movements upon visual acuity

Abstract

Purpose: Abnormal fixational eye movements such as those which occur in nystagmus can have a deleterious effect upon visual acuity measurement which is difficult to determine. The repeat letter format of the Regan Repeat Letter charts (RRL) is designed to measure visual acuity independently of abnormal eye movements, and should give higher visual acuity scores than linear test charts in subjects with fixational instability. In the present study RRL and Glasgow Acuity Cards (GAC) were used to determine visual acuity losses arising from fixational instability.Methods: Monocular visual acuity was measured in five subjects with congenital nystagmus using both GAC and RRL while eye movements were continuously recorded using an infrared limbal reflection eyetracker.Results: Mean visual acuity obtained with RRL was significantly higher (P < 0.01) than that obtained with GAC. A significant correlation (P < 0.01) was observed between the amplitude of the nystagmus and the visual acuity obtained using GAC. There was no significant correlation between the amplitude of the nystagmus and visual acuity measured using RRL.Conclusions: The results show that RRL can measure visual acuity independently of abnormal fixational eye movements. Fixational instability in nystagmus is a limiting factor in visual acuity measurements obtained using a linear test chart format.