Modified logMAR chart : consideration for greater crowding in amblyopia detection

Abstract

Early diagnosis of amblyopia is an important factor in saving child’s vision. Thus, precise assessment of visual acuity is pivotal. Existing charts such as Snellen and logMAR charts that are widely used in clinical settings differ in their features, being questioned on their standard and too complicated to be used in paediatric and special needs population. The crowding effects are important clinically in detecting amblyopia. To overcome these weaknesses, this study aims to develop a new portable modified logMAR chart with 0.5 letter width separation between the letters, which potentially produces greater crowding and also makes it children friendly. As such, it is suggesting a sensitive test in diagnosing amblyopia or any other visual abnormalities. The modified logMAR chart was designed and created using Microsoft PowerPoint 2016, version 1709 (Build 8528.2139) for 64bit Windows 10 operating system (Copyright ® Microsoft Corp. The US). The chart can be presented via a portable electronic device. Each letter size was calculated at 3meters viewing distance which subtended visual angle of 0.16º to 0.01º (range from 0.0 logMAR (6/6) to 1.0 logMAR (6/60)). Each presentation of the chart consists of 44 slides that are presented in random order of acuity level. Each slide has 3 letters with 0.5 letter width separation to produce greater crowding as compared to conventional charts (conventional logMAR: one letter width separation). The visual acuity of 17 normal participants for reliability (mean age: 24.30 ±1.50), 17 normal participants for repeatability (mean age 24.70 ±2.0), 10 normal participants for mimicking anisometropic amblyopia (mean age 19.6 ±1.4), 10 normal participants for mimicking strabismic amblyopia (mean age 21.20 ±2.00) and 17 amblyope participants (mean age 12.20 ± 1.50) participants were measured using the modified logMAR and conventional logMAR charts. Then, the crowding magnitude was calculated. The modified logMAR chart was proven to be reliable and repeatable compared to conventional logMAR chart (Bland-Altman: Mean difference -0.0006 ±0.05 with 95% CI ±0.01, RM ANOVA: p>0.05). One-Way ANOVA analyses showed no statistically significant differences in visual acuities (p>0.05) and crowding magnitude (p>0.05) in mimic amblyope participants between charts, while in amblyope participants paired t test analyses also showed a non-significant difference (p>0.05) in crowding magnitude between charts except in strabismic amblyopes (p<0.05). Modified logMAR chart consistently produced greater crowding than conventional logMAR chart. This study conclude that newly developed portable modified logMAR chart is suitable to be used in clinical settings and as a visual screening tool for detecting amblyopia or any visual abnormalities.