An Evaluation of Visual Acuity Measures as a Means of Screening for Refractive Error
Authors:   , Oduro Bright Anyimah 
Published: May 2024

Due to the complexity and hierarchical organisation of the visual system, a battery of tests is required to thoroughly quantify visual performance. Visual function can be assessed with various techniques that each measure particular aspects. Visual Acuity (VA) is a commonly used measure of visual function in clinical practice. The measurement of VA in children is important in the detection of visual anomalies, including uncorrected refractive error. The aim of this thesis was to investigate both the measurement of VA and the use of VA measures as a means of screening for refractive error. The initial experimental chapters investigated the assessment of VA using a novel set of picture optotypes, which were developed for refractive error screening in learning-disabled adults. VA measures for the novel picture optotypes were poorer when compared to those from a standard logMAR letter test. Between the novel picture optotypes, discrimination thresholds showed significant variability and VA thresholds were better when optotypes were presented in an uncrowded compared to a crowded format. Variability amongst the novel picture optotypes arose from identification cues such as shape, varying stroke width, distribution of visual information, orientation and confusion. A subset of 4 optotypes (dog, face, fish and tree) is proposed to be used. In a further experiment, the effects of optical and non-optical defocus on VA measures obtained with the novel picture optotypes was investigated. VA measures for the novel optotypes under both types of blur were considerably better than those obtained with a standard logMAR letter chart. The novel picture optotypes are not sensitive to blur and therefore may not be suitable for use in screening for refractive error. In a subsequent study, a novel approach using “scalar blur values” (SBV) to quantify refractive error was used to investigate the diagnostic accuracy of measures of unaided VA in the detection of refractive error. The results of the study highlight that the sensitivity and specificity of screening tests based on measures of VA are highly dependent on both the SBV criteria for defining significant refractive error, as well as the test pass/fail VA criterion. Further experiments showed that the use of additional plus and minus lenses in screening tests does not improve the diagnostic accuracy for the detection of hyperopia in young adults. Following on from this, a final study investigated the diagnostic accuracy of measures of unaided VA in the detection of refractive error in school age children. It was found that VA measures could reliably detect uncorrected myopia with a cut-off of ≥ -1.25D, but was only reliable for the detection of hyperopia when a cut off of ≥ +3.50D is used. Assessing the difference in unaided VA compared to VA measures obtained with an additional +2.50D lens allowed hyperopia of +2.25D to be reliably detected in this cohort. The work presented in the final two experimental chapters highlights important considerations for those involved in the design of vision screening programmes. Careful thought is required in relation to both the level of refractive error that is deemed significant and what level of sensitivity and specificity are hoped to be achieved through the screening programme, in order to be able to determine the pass/fail criterion that should be used within screening tests.