Repeatability of Aberrometry-Based Automated Subjective Refraction in Healthy and Keratoconus Subjects

Non-orthogonal spectacle correction for irregular astigmatism

PURPOSE

To investigate the potential improvement in visual acuity and subjective perception of image quality in patients with keratoconus using non-orthogonal correction (NOC) cylinder trial lenses where the steep and flat power meridians are set at angles less or greater than 90°.

METHODS

A set of NOC plano/cylindrical trial lenses, where the axes between the power meridians were set at a range of non-orthogonal angles, were used to refract 18 participants with keratoconus in whom 23 eyes were used for testing. Corneal elevation data were processed by bespoke MATLAB code from Pentacam Scheimpflug tomographer scans. Each participant first underwent subjective refraction using standard orthogonal cylinder trial lenses, and the monocular best-corrected visual acuity (BCVA, logMAR) was recorded for each eye. They then underwent a second subjective refraction using NOC cylinder trial lenses created for the study and completed a questionnaire to elicit their subjective appraisal of letter clarity and ghosting.

RESULTS

Fourteen (61%) eyes demonstrated an increase in objective BCVA with the NOC versus the orthogonal correction; seven (30%) eyes showed no change and in two (9%) eyes, the BCVA was slightly worse. Further, 87% and 79% experienced an increase in letter clarity and a reduction in ghosting, respectively, independent of changes in BCVA. The majority of non-orthogonal angles were in the range of 80°-85°, and it was possible to refine the cylinder and axis of the NOC further compared with the orthogonal correction. All but one of the participants said they would be interested in trying non-orthogonal spectacles if the opportunity arose.

CONCLUSIONS

Correcting irregular astigmatism in keratoconic individuals with non-orthogonal spectacle correction may provide benefit in terms of increased visual acuity, improvements in letter clarity and a reduction of ghosting effects. This type of correction has the potential to improve the overall quality of life for patients with keratoconus.

Accuracy and precision of automated subjective refraction in young hyperopes under cycloplegia

PURPOSE

To assess the agreement between the Eye Refract, an instrument to perform subjective automated refraction, and the traditional subjective refraction, as the gold standard, in young hyperopes under noncycloplegic and cycloplegic conditions.

METHODS

A cross-section and randomized study was carried out, involving 42 participants (18.2 ± 7.7 years, range 6 to 31 years). Only one eye was chosen for the analysis, randomly. An optometrist conducted the refraction with the Eye Refract, while another different optometrist conducted the traditional subjective refraction. Spherical equivalent (M), cylindrical components (J0 and J45), and corrected distance visual acuity (CDVA) were compared between both refraction methods under noncycloplegic and cycloplegic conditions. A Bland-Altman analysis was performed to assess the agreement (accuracy and precision) between both refraction methods.

RESULTS

Without cycloplegia, the Eye Refract showed significantly lower values of hyperopia than the traditional subjective refraction (p < 0.009), the mean difference (accuracy) and its 95% limits of agreement (precision) being -0.31 (+0.85, -1.47) D. Conversely, there were no statistical differences between both refraction methods under cycloplegic conditions (p ≥ 0.05). Regarding J0 and J45, both refraction methods manifested no significant differences between them under noncycloplegic and cycloplegic conditions (p ≥ 0.05). Finally, the Eye Refract significantly improved CDVA (0.04 ± 0.01 logMAR) compared with the traditional subjective refraction without cycloplegia (p = 0.01).

CONCLUSIONS

The Eye Refract is presented as a useful instrument to determine the refractive error in young hyperopes, the use of cycloplegia being necessary to obtain accurate and precise spherical refraction.

Measuring corneal astigmatism using OCT in keratoconus

PURPOSE

To measure net corneal astigmatism using optical coherence tomography (OCT) (Avanti) in individuals with keratoconus and compare the repeatability and accuracy with those obtained using Scheimpflug imaging (Pentacam HR).

SETTING

Casey Eye Institute, Portland, Oregon.

DESIGN

Prospective cross-sectional observational study.

METHODS

Net corneal astigmatism was calculated in keratoconic and normal eyes using OCT and Scheimpflug imaging with 4 settings-pupil or vertex centration settings with a 3 or 4 mm circular analytical zone. Corneal elevation maps were obtained from OCT images and fitted with the Zernike polynomials to obtain net corneal astigmatism. Manifest refraction astigmatism was used to evaluate the accuracy of net corneal astigmatism measurements. The coefficient of repeatability from 2 repeated measures was calculated.

RESULTS

46 eyes with manifest or subclinical keratoconus and 52 normal control eyes were analyzed. For OCT measurements in keratoconus, better accuracy was achieved with pupil centration and 3 mm analytical zone; however, better repeatability was achieved with vertex centration and 4 mm analytical zone (coefficient of repeatability = 0.53 diopters, the Fligner-Killeen test with Bonferroni adjustment P &lt; .0017). Agreement with manifest refraction was significantly better with OCT compared with that using Pentacam HR (generalized mixed-effect model with Bonferroni adjustment P &lt; .00625). No statistically significant difference was found between instruments or settings in control eyes.

CONCLUSIONS

OCT was able to measure net corneal astigmatism with better accuracy and precision in keratoconic eyes than the Pentacam HR. Measurements may be more accurate using pupil centration and a smaller analytical zone in patients with keratoconus.

Dioptric power and refractive behaviour: a review of methods and applications

Myopia is a global healthcare concern and effective analyses of dioptric power are important in evaluating potential treatments involving surgery, orthokeratology, drugs such as low-dose (0.05%) atropine and gene therapy. This paper considers issues of concern when analysing refractive state such as data normality, transformations, outliers and anisometropia. A brief review of methods for analysing and representing dioptric power is included but the emphasis is on the optimal approach to understanding refractive state (and its variation) in addressing pertinent clinical and research questions.

Although there have been significant improvements in the analysis of refractive state, areas for critical consideration remain and the use of power matrices as opposed to power vectors is one such area. Another is effective identification of outliers in refractive data. The type of multivariate distribution present with samples of dioptric power is often not considered. Similarly, transformations of samples (of dioptric power) towards normality and the effects of such transformations are not thoroughly explored. These areas (outliers, normality and transformations) need further investigation for greater efficacy and proper inferences regarding refractive error. Although power vectors are better known, power matrices are accentuated herein due to potential advantages for statistical analyses of dioptric power such as greater simplicity, completeness, and improved facility for quantitative and graphical representation of refractive state.