Clinical Validation of an Afocal Optical System in Visual Acuity Testing

To validate the feasibility of using an afocal optical system for visual acuity (VA) measurementMethods: An afocal optical system was designed and installed in an Afocal Visual Acuity Test (AVAT) device. Perceived optotype sizes from the AVAT, Snellen, and Early Treatment Diabetic Retinopathy Study (ETDRS) charts were compared at varying distances ± 15cm from recommended. Participants at the National University Hospital, Singapore, underwent VA testing with all three methods.

An afocal optical system showed lower variability in perceived optotype sizes compared to Snellen and ETDRS, with two-way ANOVA performed as a statistical analysis comparing the variability between the methods. For the logMAR 1.00 optotype, at 15cm nearer, perceived sizes increased by 1.46 ±0.43% for AVAT vs. 3.01 ±0.15% for Snellen and 4.33 ±0.31% for ETDRS. At 15cm further, perceived sizes decreased by 2.10 ±0.14% for AVAT vs. 3.95 ±0.38% for Snellen and 6.57 ±0.49% for ETDRS. The three methods were compared in a prospective study on 58 participants (mean age 54.5 ±13.2 years, 22 (36.2%) males, 49 (84.5%) Chinese). AVAT correlated strongly with ETDRS (r=0.904, P<0.001) and Snellen (r=0.906, P<0.001). Bland-Altman tests showed mean VA differences of 0.0541 ±0.122 logMAR between AVAT and ETDRS, 0.0045 ±0.126 logMAR between AVAT and Snellen. Receiver operating characteristic (ROC) analysis for AVAT, using ETDRS as gold standard (cut-off VA > logMAR 0.30), yielded an AUC of 0.975 (95% CI, 0.935 - 1.00), with a sensitivity of 100% and specificity of 94.4%).

The AVAT device, which employs an afocal optical system, correlated well with conventional VA tests. Further studies are recommended to evaluate test-retest reliability with a larger sample size.