New Visual Acuity and Crowding Tests for Better Detection of Amblyopia

In the UK, a national vision screening is employed to detect amblyopia and other undiagnosed visual conditions in school-aged children, so that they can be managed effectively. Better detection of amblyopia could be achieved by modifying current visual acuity tests, which measure the spatial resolution limit (or capacity for clarity and sharpness) of vision and by measuring crowding distance; previously unmeasured in a clinical setting. By strengthening crowding magnitude, and better quantifying its spatial distance in paediatric populations, it is hoped that the ability to detect amblyopia, and monitor its improvement with treatment will be significantly enhanced.

Three recent suggestions regarding modifications to current visual acuity tests may lead to improved detection of crowding-sensitive conditions, such as amblyopia. First, closer placement of surrounding letters to the target letter should increase the magnitude of crowding in the amblyopic eye in particular, leading to increased visual acuity differences between eyes. Second, contrast modulated stimuli should enhance crowding in amblyopic eyes. Third, a new thinner font will allow clinical measures of foveal crowding distance. In this project, these three modifications will be tested for the first time in the target clinical population.

Several studies reported that optotypes (letters or symbols) on a vision chart should be placed closer together than they currently are on commercially-available charts, which use 2.5 to 5 stroke-widths separation between target and neighbouring letters. Closer placement disrupts target optotype identification in normal peripheral vision and in central vision of amblyopic eyes due to crowding. In one vision test in the proposed study, a laterally-reversible target letter (e.g., H, O, T or V) will be surrounded by four other letters (U, A, L, C). This arrangement, in which letters were separated by 0.5 optotype widths (or 2.5 stroke widths) was first formally used in the Cambridge Crowding Test. In the proposed experiments, the surrounding letters will be placed 1 stroke-width away from the target letter, the optimal position recommended by recent studies.

Visual acuities for target letters will be measured for both amblyopic and non-amblyopic eyes of child participants, in a clinical environment. These results will be compared with visual acuities acquired during the clinical appointment using the Sonksen logMAR Test (SLT), which is the standard visual acuity test used in Cambridge University Hospital, Addenbrooke's Orthoptic Department

In the Sonksen logMAR Test (SLT), a single line of 4 letters separated from each other by 5 stroke-widths (or 1 optotype width), is contained within a box, separated from the letters also by 5 stroke-widths.

Recognition of target optotypes created by contrast-differences (second-order stimuli) is prone to greater crowding effects than is recognition of optotypes created by luminance-differences (first-order stimuli). This effect is present in normal vision, however even greater visual losses for contrast-defined targets, as well as stronger crowding effects, have been reported in amblyopia. By presenting a contrast-modulated (CM) target letter (H, O, T or V) surrounded by four other letters (U, A, L, C) placed 1 stroke-width away (in the modified Cambridge Crowding Test arrangement), this research aims to examine whether this second-order vision test will exaggerate differences between the eyes of amblyopic children, when compared to those measured in visually 'normal' children. The results will again be compared to those obtained with the Sonksen logMAR test (SLT), a first-order vision test, to compare their sensitivities to amblyopia detection.

Finally, crowding distance, the spatial distance over which crowding occurs, in normal foveal (central) vision is small (2-4 arcmin) and cannot be measured with current standard clinical optotypes due to their large size (5 arcmin for 0.0 logMAR acuity). To get around this difficulty, a new vision test was recently created to quantify the crowding distance or "critical spacing" of crowding . It uses a new "Pelli" font, which is much thinner horizontally than standard clinical fonts, allowing the optotypes to get closer to each other in physical space. This 'Crowding Distance Test' permits quantification of the critical spacing of crowding for the first time in a clinical population. The new "Pelli" font, each optotype appearing like tall skinny numbers, has already been trialled on 'visually normal' school-aged children, but not yet examined on children with greater sensitivity to visual crowding, i.e., amblyopes. The third vision test in the proposed research, will investigate whether or not "crowding distance" measures made in each eye, results in greater inter-ocular differences, leading to better detection of anisometropic and strabismic amblyopia than does the current clinical standard visual acuity test, the Sonksen logMAR test (SLT).

By using closer first-order target optotypes, second-order target optotypes, and quantifying crowding distance in paediatric populations, the ability to detect amblyopia and monitor amblyopia treatment, could be significantly improved for the first time since the 1960s. At that time crowding was first quantified in a clinical population of adult amblyopes and was subsequently incorporated into commercially available tests as best practice.