Evaluation of defocus curve performance with two visual acuity testing systems: ETDRS and FrACT

Abstract

Purpose: An important clinical method to evaluate multifocal and accommodating intraocular lens performance is the defocus curve test. To have a clear interpretation of the defocus curve data, the test methods used and the metrics obtained from this test should be evaluated relative to other clinical measures. The aim of this study was to compare near visual acuity (VA) and defocus curve measurements with two different methods, standard letter charts (ETDRS) and a computer based technique (FrACT), in non-presbyopic and presbyopic subjects.

Methods: A total of 25 subjects were recruited (22 to 80 years; mean: 46 ± 15 years). Outcome measures included monocular manifest refraction of the right eye, distance corrected intermediate VA (DCIVA) at 66 cm and distance corrected near VA (DCNVA) at 40 cm with both the ETDRS and FrACT systems. Defocus curves (far visual acuity measured through trial lenses from +2.0 D to -4.0 D in 0.5 D steps) were measured with both systems. Near add power at 40 cm and push-down accommodative amplitude (AA) were obtained. Two depth of focus (DOF) metrics from defocus curves were evaluated: a 20/32 VA threshold (DOF-M1) and a 0.2 logMAR loss in VA from the 0.0 D defocus VA (DOF-M2). Age related trends for DCIVA, DCNVA and depth of focus measurements were evaluated through regression analyses and slopes compared.

Results: Intermediate VA (DCIVA) declined at about 0.1 logMAR per decade with both ETDRS (-0.367 + 0.009 * age; R² = 0.51) and FrACT (-0.430 + 0.011 * age; R² = 0.61). Near VA (DCNVA) declined at 0.14 logMAR per decade with both ETDRS (-0.455 + 0.014 * age; R² = 0.58) and FrACT (-0.408 + 0.014 * age; R² = 0.58). Depth of focus metrics (DOF-M1) and (DOF-M2), measured with ETDRS and FrACT, showed a significant relationship with age (p < 0.05 for all regression slopes) and had slopes between -0.6 D and -0.7 D per decade. Similarly, minimum near add declined at 0.6 D per decade (R² = 0.83). All non-presbyopes accommodated well to the ETDRS chart, but some showed reduced accommodation with the FrACT system, especially through -3.5 D and -4.0 D defocus levels.

Conclusions: The ETDRS charts and FrACT computer system measured comparable reductions in intermediate visual acuity and near visual acuity and depth of focus with increasing age. Generally, with each decade increase in age, near vision declined at 0.1 logMAR and depth of focus declined at about 0.6 D.