Unaided Visual Acuities Correlated with Refractive Errors: A Study

The dynamic optotype (DYOP) visual acuity (VA) test is based on motion detection rather than element resolution and has been proposed for routine clinical assessment. This investigation examined the validity, inter- and intra-session repeatability and subjective preference for the DYOP versus a static letter chart and examined its utility in detecting astigmatic defocus.

VA of 103 participants was measured three times with the letter and DYOP charts and repeated within two weeks in 75 participants who also rated their subjective experience. The VA of 29 participants was measured using DYOP, letter, Landolt C, and Tumbling E charts, with habitual correction and astigmatism induced with +1.00, +2.00 or +3.00 cylinders at 45, 60, 90 and 180°.

The charts differed by a mean of 0.02 logMAR, with 81% of the measurements within one line of acuity. Inter-session, intraclass correlation coefficients, within-subject SD and repeatability were 0.03 logMAR, 0.95, 0.11 and 0.30 versus 0.01 logMAR, 0.92, 0.15 and 0.42 for the DYOP and letter charts, respectively. The DYOP was significantly more frustrating (1.79 vs.1.36), with 59% preferring the letter chart. The DYOP was least affected by induced astigmatism.

The DYOP and letter charts differed significantly in their mean values with wide limits of agreement. DYOP had better within-subject SD and narrower limits of agreement between sessions, though clinically insignificant, and performed significantly worse for the detection of uncorrected astigmatism. Thus, it is difficult to recommend this test for the clinical determination of refractive error.

We investigated the limits at which blur due to defocus, crossed-cylinder astigmatism, and trefoil became noticeable, troublesome or objectionable. Black letter targets (0.1, 0.35 and 0.6 logMAR) were presented on white backgrounds. Subjects were cyclopleged and had effectively 5 mm pupils. Blur was induced with a deformable, adaptive-optics mirror operating under open-loop conditions. Mean defocus blur limits of six subjects with uncorrected intrinsic higher-order ocular aberrations ranged from 0.18 ± 0.08 D (noticeable blur criterion, 0.1 logMAR) to 1.01 ± 0.27 D (objectionable blur criterion, 0.6 logMAR). Crossed-cylinder astigmatic blur limits were approximately 90% of those for defocus, but with considerable meridional influences. In two of the subjects, the intrinsic aberrations of the eye were subsequently corrected before the defocus and astigmatic blur were added. This resulted in only minor reductions in their blur limits. When assessed with trefoil blur and corrected intrinsic ocular aberrations, the ratio of objectionable to noticeable blur limits in these two subjects was much higher for trefoil (3.5) than for defocus (2.5) and astigmatism (2.2).

The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center, self-controlled clinical trial of a standardized technique of radial keratotomy in 435 patients who had physiologic myopia with a preoperative refraction between −2.00 and −8.00 diopters. The surgical technique consisted of eight incisions using a diamond micrometer knife with blade length determined by intraoperative ultrasonic pachymetry and the diameter of central clear zone determined by preoperative refraction. At one year after surgery, myopia was reduced in all eyes; 60% were within ±1.00 diopter of emmetropia; 30% were undercorrected and 10% were overcorrected by more than 1.00 diopter (range of refraction, −4.25 to +3.38 D). Uncorrected visual acuity was 20/40 or better in 78% of eyes. The operation was most effective in eyes with a refraction between −2.00 and −4.25 diopters.

Thirteen percent of patients lost one or two Snellen lines of best corrected visual acuity. However, all but three eyes could be corrected to 20/20. Ten percent of patients increased astigmatism more than 1.00 diopter. Disabling glare was not detected with a clinical glare tester, but three patients reduced their driving at night because of glare. Between six months and one year, the refraction changed by >0.50 diopters in 19% of eyes.