Repeatability (test-retest variability) of refractive error measurement in clinical settings

Real world data on digital remote refraction in a healthy population of 14,680 eyes

Refractive errors are the leading cause of preventable visual impairment, to which web-based remote refraction could contribute. We report real-world 2021-2022 data of the underlying algorithm and validated these to conventional prescriptions among healthy individuals (high visual acuity and satisfactied current refraction). Participants were 18-45 years with a spherical (S) error between -3.50 + 2.00S to -2.00 Diopter Cylinder (DC), reported as Spherical Equivalent (SEQ) in mean differences and 95% Limits of agreement. Consecutive measurements (n = 14,680) were assessed of which n = 6386 selected for validation. The mean difference was 0.01D(SD 0.69) and -0.73D(SD 0.92) for myopes and hyperopes respectively. This algorithm shows variation, nonetheless, 67% and 82% of myopes were within ±0.5 and ±0.75D. The test underestimates hyperopes (34% and 50% within ±0.5D and ±0.75D) and had inconsistencies distinguishing hyperopia. This proof-of-concept shows home testing has the potency to increase accessibility to care by delivering a valuable alternative for uncomplicated refractive assessments.

Fluctuations of Anterior Chamber Depth and Astigmatism in Pseudophakic Eyes

Purpose

To explore the significance of changes in anterior chamber depth (ΔACD) and astigmatism between 4 and 8 weeks following uneventful phacoemulsification.

Patients and Methods

Anterior chamber depth (ACD, mm), autorefractometry and subjective refraction were monitored in pseudophakic eyes implanted with non-toric IOLs (group 1, SA60AT, n=36; group 2, SN60WF, n=34; group 3, ICBOO, n=16) and phakic control group (n=30, group 4a, for ACD and autorefractometry) over four weeks. Changes in subjective refractions were compared with repeatability in normal phakic eyes (n=30, group 4b).

Results

Reporting key results (p<0.01), mean (±sd 95% CI) ΔACD values (ACD at start minus ACD at four weeks) were +0.02 (±0.37, -0.16 to 0.08), +0.22 (±0.51,0.05 to 0.39), -0.33 (±0.51, -0.58 to -0.08), -0.02 (±0.07, -0.04 to 0.01) in groups 1-4a respectively. Differences were significant (1-way ANOVA, F=7.02). Pooling data from the pseudophakic eyes (n=86) induced astigmatism (IA) by autorefractometry was significantly greater in comparison with group 4a [-0.78D (±0.67, -0.92 to -0.64) and -0.19D (±0.16, -0.25 to - 0.13)]. IA power correlated with the initial power of refractive astigmatism at 4 weeks (A), [IA = 0.36A-0.30 (r2=0.207) and IA = 0.39A-0.29 (r2=0.232) by autorefractometry and subjective refraction, respectively]. In groups 1-3, vector analysis revealed i) the change in refraction in over 55% of eyes was beyond the 95% confidence interval limits observed in groups 4a and 4b, ii) some significant associations between changes in vectors describing astigmatism with ΔACD and IOL labelled power.

Conclusion

Changes in ACD and refraction still occur four weeks after unremarkable phacoemulsification and the inter-relationship depends on IOL design/type. Other factors, such as tilt or dislocation of the IOL along the X-Y axes parallel to Listing's plane, accompanying changes in ACD are expected to affect the postop astigmatism.

Ametropia detection using a novel, compact wavefront autorefractor

INTRODUCTION

Despite the well-known reproducibility issues of subjective refraction, most studies evaluating autorefractors compared differences between the device and subjective refraction. This work evaluated the performance of a novel handheld Hartmann-Shack-based autorefractor using an alternative protocol, which considered the inherent variability of subjective refraction.

METHODS

Participants underwent an initial measurement with a desktop autorefractor, two subjective refractions (SR1 and SR2) and a final measurement with the QuickSee Free (QSFree) portable autorefractor. Autorefractor performance was evaluated by comparing the differences between the QSFree and each of the subjective refractions with the difference between the subjective refractions (SR1 vs. SR2) using Bland-Altman analysis and percentage of agreement.

RESULTS

A total of 75 subjects (53 ± 14 years) were enrolled in the study. The average difference in the absolute spherical equivalent (M) between the QSFree and the SR1 and SR2 was ±0.24 and ±0.02 D, respectively, that is, very similar or smaller than the SR1 versus SR2 difference (±0.26 D). Average differences in astigmatic components were found to be negligible. The results demonstrate that differences between QSFree and both subjective refractions in J0 and J45 were within ±0.50 D for at least 96% of the measurements. The limits of agreement (LOAs) of the differences between QSFree and SR1, as well as QSFree and SR2, were higher than those observed between SR1 and SR2 for M, J0 and J45 .

CONCLUSIONS

A protocol was designed and validated for the evaluation of a refractive device to account for the variability of subjective refraction. This protocol was used to evaluate a novel portable autorefractor and observed a smaller difference between the device and subjective refractions than the difference between the two subjective refraction measurements in terms of mean bias error, although the standard deviation was higher.

Comprehensive Primary Eye Care: A Comparison Between an In-Person Eye Exam and a Tele-Eye Care Exam

Introduction

Proper access to primary eye care is essential in addressing vision impairment, and tele-eye care examinations are a promising solution that could facilitate this access in many rural or remote areas. Even though remote eye exams are becoming increasingly frequent, comprehensive tele-eye care exams are still limited by the lack of published data. The aim of this study is to compare a comprehensive tele-eye care exam with a gold standard in-person primary eye care exam with an emphasis on refractive measurements, ocular health assessment, confidence level of the eye care providers and patient satisfaction.

Methods

Sixty-six participants underwent two comprehensive eye exams performed by two eye care providers. One was a gold standard in-person exam, while the other was a remote exam performed by an eye care provider through videoconference. An overall patient satisfaction survey and a questionnaire for visual comfort with a trial frame from each modality were completed and the eye care providers scored their confidence level for each test. Exam results and diagnoses were compared between both modalities.

Results

Tele-refraction has a good to excellent agreement with in-person subjective refraction in terms of sphero-cylindrical power and best corrected visual acuity. There was no statistically significant difference for visual comfort between both modalities. The agreement between in-person and remote exams for ocular health assessment ranged from fair to almost perfect, but there was a low prevalence of ocular pathologies within the study sample. The confidence level of the eye care providers and patient satisfaction were statistically higher in-person.

Conclusion

Tele-eye care appears to be statistically and clinically non-inferior to in-person eye exams, especially for refraction, but the low prevalence of ocular pathologies somewhat limits the comparison of its efficacy for ocular health assessment. More studies on comprehensive tele-eye care exams are needed.

Multivariate analysis of repeatability for the Near Eye Tool for Refractive Assessment (NETRA)

OBJECTIVE

To investigate repeatability of refractive state using a smartphone-based assessment tool, the Near Eye Tool for Refractive Assessment (NETRA).

METHODS AND ANALYSIS

This study included 279 participants, predominantly female (66.7%) of African descent (49.1%). The age range was 9-63 years with mean age (s) 22.6 (8.9) years. Two consecutive measurements per eye with the NETRA were measured for both eyes of all participants. However, analyses for the right eyes only are included here. Multivariate statistical analysis included stereo-pair comets and scatterplots with 95% surfaces of constant probability density. Correlation coefficients for repeated samples were determined. Repeatability and agreement for NETRA were assessed with Bland-Altman plots, coefficients of repeatability ([Formula: see text] ; [Formula: see text] is the SD of differences) and intraclass correlation coefficients (ICCs).

RESULTS

Bland-Altman plots, within-subject SD (sw ), coefficients of repeatability and ICC indicated that repeated measurements were similar for many but not all eyes and there was good agreement (ICC=0.96) for the spherical coefficient (F I=M) but less so for antistigmatic coefficients (F J=J 0 and F K=J 45) of power. Although mean differences for repeated samples were almost zero, 95% limits of agreement widths were larger for the stigmatic coefficients. Without cycloplegia, repeatability (2.77sw ) was 1.63 D, 0.58 D and 0.56 D for the stigmatic and antistigmatic coefficients, respectively.

CONCLUSION

NETRA is a potentially useful and inexpensive portable method in clinical and primary health settings, and especially in less-developed regions of the world. The subjective nature of the self-refraction task can be challenging for younger individuals, and cycloplegia is recommended for NETRA with such patients.

Subjective refraction using power vectors by updating a conventional phoropter with a Stokes lens for continuous astigmatic power generation

PURPOSE

To implement a pure power vector method for monocular subjective refraction using a regular phoropter with the only modification being the inclusion of a Stokes lens. The proposed methodology was tested with three different Stokes lenses, and the results were compared with conventional clinical refraction procedures.

METHODS

Power vector subjective refraction was performed by attaching a Stokes lens to the Risley prism holder. Stokes lenses allow for pure astigmatic compensation in the form of the J0 , J45 components while the spherical lenses in the phoropter allow determination of the spherical component in the form of M (spherical equivalent). The proposed routine is presented step-by-step using three Stokes lenses having different astigmatic powers.

RESULTS

Monocular subjective refraction was performed on 26 healthy subjects with a mean age of 44 ± 16 years, mean spherical equivalent of -0.56 D (range -5.50 to +2.38 D) and refractive astigmatism ≤1.50 D. No differences were found between the results obtained with the conventional technique versus the vector-based procedure for the spherical equivalent (p = 0.28) or astigmatic components (p = 0.34). In addition, visual acuity (VA) was equivalent through the refractions measured with the conventional and vector procedures (p = 0.12). Repeatability coefficients for J0 and J45 with the new vector methodology were <0.38 D.

CONCLUSIONS

The proposed routine could be helpful for cases where it is difficult to get a valid starting point for conventional refraction (e.g., irregular corneas and media opacities), for testing facilities with limited resources/equipment and/or for motivated clinicians who wish to know about alternative methods of refractive error determination.

Descemet Membrane Endothelial Keratoplasty-Induced Refractive Shift and Descemet Membrane Endothelial Keratoplasty-Induced Intraocular Lens Calculation Error

PURPOSE

The aim of this study was to determine the mechanisms leading to the refractive shift and intraocular lens calculation error induced by Descemet membrane endothelial keratoplasty (DMEK), using ocular biometry and corneal elevation tomography data.

METHODS

This is a retrospective, monocentric cohort study. Eyes which underwent uncomplicated DMEK surgery with available pre-DMEK and post-DMEK Scheimpflug rotating camera data (Pentacam, Oculus, Wetzlar, Germany) were considered for inclusion with an age-matched control group of healthy corneas. Cataract surgery data were collected for triple-DMEK cases. DMEK-induced refractive shift (DIRS) and intraocular lens calculation error (DICE) were calculated. Pearson r correlation coefficient was calculated between each corneal parameter variation and both DIRS and DICE.

RESULTS

DIRS was calculable for 49 eyes from 43 patients. It was 30.61% neutral, 53.06% hyperopic (36.73% > 1D), and 16.32% myopic (6.12% > 1 D). DICE was calculable for 30 eyes of 26 patients: It was 46.67% neutral, 40.00% hyperopic (10.00% > 1D), and 13.33% myopic (3.33% > 1D). DIRS and DICE were mainly associated with variations in PRC/ARC ratio, anterior average radii of curvature (ARC), posterior average radii of curvature (PRC), and posterior Q.

CONCLUSIONS

Our results suggest that ARC variations, PRC/ARC ratio variations, PRC variations, and posterior Q variations are the most influential parameters for both DIRS and DICE. We suggest that a distinction between those different phenomenons, both currently described as "hyperopic shift" in the literature, should be made by researchers and clinicians.

Refraction and defocus curves in eyes with monofocal and multifocal intraocular lenses

Several clinical techniques have been described to evaluate visual performance and optical quality with intraocular lenses (IOL). However, subjective refraction remains one of the most important methods for assessing post-surgery results, taking decisions about retreatments, advanced spectacle prescription and the refinement of the constant for the formula used in the IOL power calculation. Beyond clinical refraction, defocus curve measurement has been described as a complementary tool for assessing visual performance and taking clinical decisions. However, to date, there are no clinical guidelines or evidence-based protocols published in the scientific literature recommended for pseudophakic patients implanted with either monofocal or multifocal IOLs. This narrative review highlights the importance of clinical refraction in pseudophakic eyes, its utility in the decision of different types of IOL implantation, and describes a clinical refraction protocol for eyes implanted with monofocal and multifocal IOLs.

Interexaminer reproducibility for subjective refractions for an ametropic participant

Objective

To investigate interexaminer reproducibility of non-cycloplegic subjective refractions. Subjective refractions are frequently determined, and it is important to know whether differences in refractive state over time constitute meaningful, non-random change.

Methods and analysis

Fifty registered and experienced (≥5 years) optometrists from a single geographic region performed non-cycloplegic subjective refractions for a participant with moderate left eye(OS) to severe right eye (OD) ametropia. Subjective refractions were transformed to power matrices for analysis with stereopairs, distribution ellipsoids and polar profiles of variance of dioptric power. Absolute 95% limits of reproducibility (1.96(2)(SD)) for excesses of subjective refractions for the right and left eyes separately from mean subjective refractions were determined.

Results

Mean subjective refractions were −7.68–4.50×10 and −4.59–1.85×178 for the right and left eyes, respectively. The 95% absolute reproducibility limits for the stigmatic coefficients (spherical equivalents) were ≤1.71 D and ≤0.75 D for the right and left eyes, but corresponding limits for astigmatic coefficients were smaller (≤0.69 D).

Conclusion

Removal of possible outliers for OD and OS, respectively, reduces the absolute 95% reproducibility limits for the stigmatic and astigmatic coefficients to ≤0.97 D and ≤0.49 D, thus improving interexaminer reproducibility. However, these results suggest caution with analysis of refractive data where subjective rather than objective methods are applied for longitudinal and epidemiological studies.

Influence of the invariant refraction assumption in studies of formulas for monofocal and multifocal intraocular lens power calculation

PURPOSE

To assess the influence in paired design studies of formulae comparison for intraocular lens (IOL) power calculation of using a single formula for deciding the implanted power with monofocal (mIOL) and multifocal (MIOL) lenses.

DESIGN

Retrospective observational.

METHODS

Ninety-six right eyes were retrospectively analyzed. Eyes were assigned in two independent groups, SG and HG, depending on the formula used for deciding the implanted power, SRK-T (n = 54) and Haigis (n = 42), respectively. Median absolute prediction error (MedAE) was evaluated between independent samples (SRK-T in SG vs Haigis in HG) and between paired samples (SRK-T vs Haigis in both SG and HG). Percentages of eyes within a specific range of prediction error (PE) were also calculated for both, the standard steps and the clinically relevant steps.

RESULTS

MedAE difference was lower than 0.09 D between both formulas for the comparison of independent samples in the mIOL (p = 0.62) and MIOL (p = 0.83) groups. However, paired samples resulted in better MedAE for SRK-T in the SG (0.14 D lower, p = 0.003) and for Haigis in the HG (0.07 D lower, p = 0.015), but only in the mIOL group. These small differences were also manifested, but not reaching statistical significance (p > 0.05), in the percentage of eyes achieving a specific range of PE, especially in the mIOL group.

CONCLUSIONS

A small superiority for the formula used for selecting the final implanted IOL power can appear in studies following current standards. These studies should clearly specify which formula was used for selecting the implanted power.

A critical evaluation of longitudinal changes of astigmatism following implantation of toric implantable collamer lens (TICL): a comparison between treated and untreated cases over 4 years

PURPOSE

To evaluate residual astigmatism following correction with toric implantable collamer lens (TICL) (group I) over a period of 4 years and compare with the change of astigmatism in spectacle wearers (group II).

METHODS

Groups I (86 cases implanted with TICL [EVO + Visian ICL, Staar Surgical, Nidau, Switzeland], preop refractive error [sphere and cylinder] - 22.25 DS to - 1.00 DS and - 5.50 DC to - 1.00 DC) and II (80 cases initial refractive error [sphere and cylinder] - 18.00 DS to 0.00 DS and - 7.00 DC to - 1.00 DC) were reviewed annually. Refractive and tomography data were subjected to vector analysis to determine surgically induced astigmatism (SIA), angle of error [Δθ° = angle of target-induced astigmatism (TIA) - angle of SIA], and ΔC [TIA-SIA powers] and total corneal astigmatism (TCA) in group I and induced change in astigmatism (ICA) in group II.

RESULTS

In group I, on all occasions, SIA correlated with TIA (p < 0.05); differences between SIA and TIA means were insignificant and changes in TCA were not correlated with ΔC. Mean (± sd, 95% CI) residual astigmatic powers (RA) in attending group I cases (1-4 years) were - 0.40 DC (0.58, - 0.52 to - 0.28), - 0.40 DC (0.59, - 0.52 to - 0.27), - 0.41 DC (0.58, - 0.54 to - 0.28), and - 0.61 DC (0.74, - 0.82 to - 0.40). In group II, the corresponding ICA powers were - 0.47 DC (0.53, - 0.61 to - 0.32), - 0.49 DC (0.48, - 0.69 to - 0.29), - 0.60 DC (0.40, - 0.76 to - 0.44), and - 0.86 DC (0.71, - 1.19 to - 0.52). Differences between RA and ICA were not significant. Of the group I cases presenting at 1-4 years postop, 23, 18, 16, and 28 had RA powers ≤  - 0.75 DC. Of these 12, 10, 6, and 16 were associated with Δθ° > 5° (ΔC - 0.50 to 0 DC) and 5, 1, 4, and 4 were related to ΔC (Δθ° < 5°).

CONCLUSION

The development of astigmatism after TICL implantation is on par with the natural change in astigmatism in untreated cases. In about 50% of TICL cases presenting with astigmatism ≤  - 0.75 DC, the residual astigmatism could be neutralized by realigning the TICL.

What are patients' beliefs about, and experiences of, adaptation to glasses and how does this affect their wearing habits?

PURPOSE

It is well known that some patients experience difficulties adapting to new glasses. However, little is known about what patients themselves understand of the adaptation process, and how this influences their attitudes and the decisions they make when adapting to a new pair of glasses. Nor is it understood whether these factors affect their wearing habits.

METHODS

We conducted four focus groups. Participants were 22 glasses wearers (mean ± SD age 43 ± 14 years, range 21-71 years) who reported they: (1) wore spectacle correction for distance vision (single vision, bifocal or progressive lenses); (2) had struggled to get used to a new pair of glasses and (3) sometimes chose not to wear their distance correction. Focus groups were audio recorded, transcribed verbatim and analysed thematically.

RESULTS

We identified three themes. Trust is about how participants' trust in their optometrist and themselves influences the likelihood of them adapting successfully to new glasses. Conflict describes how the advice patients have received about adapting to glasses can conflict with what they have experienced and how this conflict influences their expectations. Part of Me explores how participants' experiences and feelings about their glasses are important to adaptation and this includes physical, visual, emotional and behavioural aspects.

CONCLUSIONS

The traditional optometric perspective of adaptation to glasses is much narrower than that held by patients, and significantly underestimates the physical, behavioural and emotional adaptation that patients must go through in order to feel fully comfortable wearing their glasses. Patients should receive significantly more information about adaptation, including symptoms that may be experienced and why these happen, practical tips to aid adaptation, and when and how to raise concerns. Patients should also receive information about the day-to-day effects of blur adaptation to avoid them not wearing their glasses, including for vision-critical tasks such as driving.

Bearing fixing: A new computer algorithm method for subjective determination of astigmatism

PURPOSE

This article aims to illustrate the principles of bearing fixing, a new computer-controlled procedure for subjective determination of astigmatism, and to compare it with conventional clinician-controlled Jackson crossed-cylinder refraction.

METHODS

The principles and method for bearing fixing are described. Astigmatism was measured using bearing fixing and Jackson cross-cylinder twice on 20 visually normal participants aged between 18 and 34 years. After final sphere adjustment, acuity measurements were made with each refraction estimate.

RESULTS

Bearing fixing results could be obtained for all participants. Mean bearing fixing cylinder magnitude was slightly higher than Jackson cross-cylinder measures of cylinder magnitude, by 0.05D. Using vector analysis to take into account cylinder power and axis, mean bearing fixing astigmatism was not significantly different from cross-cylinder astigmatism, but did have higher test-retest variability (p < 0.05). Acuity with bearing fixing and cross-cylinder corrections did not significantly differ in average value or repeatability.

CONCLUSIONS

Bearing fixing is a feasible method for subjective determination of astigmatism, and there are a number of potential improvements to the method which could make it faster and more precise.

Agreement and variability of subjective refraction, autorefraction, and wavefront aberrometry in pseudophakic patients

PURPOSE

To evaluate the comparability and variability of subjective refraction (SR), autorefraction (AR), and wavefront aberrometry (WA) in pseudophakic patients.

SETTING

Hanusch Hospital, Vienna, Austria.

DESIGN

Prospective study.

METHODS

Subjective refraction was performed by 2 independent examiners at 2 study visits. Furthermore, 5 measurements of AR and WA were performed in each patient at both visits. Agreement between the 3 refraction methods for spherical equivalent (M) and cylindrical vectors (J0, J45) was analyzed using Friedman multiple comparison and Bland-Altman plots. Predictability of spherical equivalent determined by SR from AR and WA measurements was tested with partial least squares regression and random forest regression.

RESULTS

Ninety eyes of 90 pseudophakic patients were measured. The mean absolute error and arithmetic mean difference of SR measurements of M, J0, and J45 were comparable between both examiners. A small mean difference was found for SR between both visits. Spherical equivalent was more negative when measured by AR [-0.87 diopters (D)] and WA (-0.90 D) compared with SR (-0.60 D), whereas astigmatic vectors agreed well. Good test-retest reliability was found between all 3 refraction methods for M, J0, and J45. Partial least squares regression and random forest regression showed moderate predictive power for M measured by objective refraction and SR.

CONCLUSIONS

Reproducibility and reliability of SR measurements in pseudophakic patients showed good agreement. AR and WA measured the spherical equivalent more myopic than SR, whereas astigmatic vectors were comparable between the 3 methods after uneventful cataract surgery.

Are changes in visual acuity and astigmatism after corneal cross-linking (CXL) in keratoconus predictable?

PURPOSE

To evaluate changes in corrected distance visual acuity (CDVA), ratio of anterior and posterior corneal radii over the thinnest region of the cornea (ARC/PRC), and astigmatism after cross-linking (CXL) in keratoconus.

METHODS

Subjective refraction and ARC/PRC (using Pentacam™) were monitored over 1 year in (I) keratoconus treated with routine CXL (n = 53), (II) relatively stable keratoconus (n = 23), and (III) age/gender matched controls (n = 24).

RESULTS

CDVA (median, mode, interquartile range) improved significantly in group I, compared with groups II and III (p < 0.05), from 0.45 (0.60, 0.20-0.63) to 0.80 (0.95, 0.60-0.95); change in CDVA was associated with preop CDVA (p < 0.01 at all times postop). ARC/PRC (mean ± sd, 95% CI) changed from 1.362 (± 0.048, 1.347-1.377) to 1.425 (± 0.073, 1.401-1.449). CDVA and ARC/PRC remained stable in II and III. Significant relationships were revealed between logCDVA and ARC/PRC in I and II (at 12 months, I r_s =  - 0.464, II r_s - 0.449) and logCDVA at postop(y), log CDVA at preop(x₁), and ARC/PRC at preop(x₂) in I (at 12 months, y = 0.356x₁ - 1.312x₂ + 1.806, r²₁ = 0.494, r²₂ = 0.203). Astigmatic power (mean ± sd, 95% CI) improved from - 3.10DC (± 1.52, - 3.55 to - 2.66) to - 2.53DC (± 1.24, - 2.90 to - 2.17) in I, and worsened from - 1.27DC (± 1.32, - 1.81 to - 0.73) to - 1.61DC (± 1.28, - 2.13 to - 1.09) in II. Vector analysis revealed in group I (a) the power of the surgically induced astigmatism (SIA) was linked to astigmatic power at preop and (b) the difference between the axis of astigmatism at preop(ø) and the axis of the SIA was linked to ø.

CONCLUSION

CXL improved CDVA, increased the ARC/PRC ratio, and modified the association between CDVA and ARC/PRC. The change in CDVA was linked to preop CDVA and ARC/PRC values. The association between SIA and preop astigmatism implies there is not a simple cause and effect relationship with CXL.

Spectacle non-tolerance in clinical practice - a systematic review with meta-analysis

PURPOSE

Spectacle non-tolerance or adverse events to spectacle wear are serious concerns for both patients and practitioners. Non-tolerance may contribute to a negative impact on the practitioner's ability and practice. Therefore, a detailed understanding of frequency and causes of spectacle non-tolerance in clinical ophthalmic practice is essential. This review aimed to determine the prevalence and causes of non-tolerance to spectacles prescribed and dispensed in clinical practice.

METHOD

The current systematic review included quantitative studies published in the English language that reported spectacle non-tolerance in clinical practice. A comprehensive search was conducted in PubMed, Scopus and the Web of Science database for studies published until 13 July 2020. An adapted version of the Newcastle-Ottawa Scale (NOS) modified for cross-sectional studies was used to assess the quality of each included study. Five investigations with 205,478 study participants were included in the review. The prevalence of spectacle non-tolerance from individual studies was pooled using MetaXL software. The pooled prevalence of spectacle non-tolerance was 2.1% (95% CI: 1.6-2.7) ranging from 1.6% to 3.0%. The papers were also reviewed to identify the potential causes of non-tolerances. Nearly half reported that non-tolerance (47.4%) was due to an error in refraction. Other causes identified were errors related to communication (16.3%), dispensing (13.5%), non-adaptation (9.7%), data entry (8.7%), binocular vision (7.4%) and ocular pathology (6.4%).

SUMMARY

This review improves our understanding of spectacle non-tolerance in clinical practice. This is important because non-tolerance may lead to spectacle wear discontinuation, which may deprive patients of optimal vision. Increased non-tolerance in clinical practice may affect a clinician's reputation and incur additional costs associated with reassessments and replacements. Spectacle non-tolerance occurred due to a multitude of factors related to optical dispensing and wearer adaptation. Therefore, there is a need for vigilance while prescribing spectacles. The limited evidence highlights the need for more studies, especially in limited-resource settings, to improve the quality of refractive error services.

A Comparison Between Refraction From an Adaptive Optics Visual Simulator and Clinical Refractions

Purpose

The Visual Adaptive Optics (VAO) is an adaptive optics visual simulator with an embedded Hartmann–Shack aberrometer that can give objective and subjective refraction measures. The aim of the present study was to compare the findings of the objective and subjective refractions from the VAO with a commercial autorefractometer (Topcon Corp., Tokyo, Japan) and a subjective refraction by an optometrist. The influence of age, refractive error type, and presence of ocular diseases was ascertained.

Methods

The refractive error was obtained in 469 participants using the four techniques mentioned. Data were analyzed with power vectors mean spherical equivalent, the vertical Jackson-Cross-Cylinder, and the oblique Jackson-Cross-Cylinder. Age, refractive error type (myopia, emmetropia, hyperopia) and presence of ocular diseases (yes, no) were included as covariates. Agreement was assessed using the 95% interval of agreement.

Results

The median spherical equivalent difference and the interval of agreement for all the participants with the VAO subjective, VAO objective, and autorefraction with the clinical subjective refraction were (+0.13, 1.80 diopters [D]), (+0.38, 1.80 D), and (−0.38, 2.10 D), respectively. When considering only healthy participants, the results were (+0.06, 1.70 D), (+0.38, 1.60 D) and (−0.25, 1.80 D), respectively. When considering only those participants with any ocular condition, the results with VAO subjective, VAO objective and autorefraction were (+0.13, 2.50 D), (+0.31, 2.70 D), and (−0.50, 4.80 D), respectively.

Conclusions

The VAO subjective refraction is more accurate than VAO objective refraction and autorefraction, regardless of refractive error, age, or the presence of ocular conditions. The presence of ocular conditions significantly deteriorates the accuracy of all refraction methods.

Translational Relevance

Reported clinical comparisons between different types of standard refraction methods and a new adaptive optics refraction instrument (VAO) are in good agreement and support the further development of this method to increase refraction accuracy and to refract quicker than standard procedures.

Using the Lens Paradox to Optimize an In Vivo MRI-Based Optical Model of the Aging Human Crystalline Lens

Purpose

To optimize our in vivo magnetic resonance imaging (MRI)-based optical model of the human crystalline lens, developed with a small group of young adults, for a larger cohort spanning a wider age range.

Methods

Subjective refraction and ocular biometry were measured in 57 healthy adults ages 18 to 86 years who were then scanned using 3T clinical magnetic resonance imaging (MRI) to obtain lens gradient of refractive index (GRIN) and geometry measurements. These parameters were combined with ocular biometric measurements to construct individualized Zemax eye models from which ocular refractive errors and lens powers were determined. Models were optimized by adding an age-dependent factor to the transverse relaxation time (T₂)-refractive index (n) calibration to match model-calculated refractive errors with subjective refractions.

Results

In our subject cohort, subjective refraction shifted toward hyperopia by 0.029 diopter/year as the lens grew larger and developed flatter GRINs with advancing age. Without model optimization, lens powers did not reproduce this clinically observed decrease, the so-called lens paradox, instead increasing by 0.055 diopter/year. However, modifying the T₂-n calibration by including an age-dependent factor reproduced the decrease in lens power associated with the lens paradox.

Conclusions

After accounting for age-related changes in lens physiology in the T₂-n calibration, our model was capable of accurately measuring in vivo lens power across a wide age range. This study highlights the need for a better understanding of how age-dependent changes to the GRIN impact the refractive properties of the lens.

Translational Relevance

MRI is applied clinically to calculate the effect of age-related refractive index changes in the lens paradox.

The Impact of Changes in Corneal Back Surface Astigmatism on the Residual Astigmatic Refractive Error following Routine Uncomplicated Phacoemulsification

Purpose

To determine the significance of any association between intersessional changes in ocular residual astigmatism (RA) and astigmatism at corneal front (FSA) and back (BSA) surfaces following uneventful routine phacoemulsification.

Methods

Astigmatism was evaluated by autorefractometry and subjective refraction and at both the corneal surfaces with Orbscan II™ (Bausch & Lomb) over central 3 mm and 5 mm optical zones at 1, 2, and 3 months after routine phacoemulsification in 103 patients implanted with monofocal nontoric intraocular lenses (IOLs, one eye/patient). Data were subjected to vector analysis to determine the actual change (Δ) in astigmatism (power and axis) for the refractive and Orbscan II findings.

Results

The number of cases that attended where ΔRA was ≥0.50 DC between 1 and 2 months was 52 by autorefractometry and 36 by subjective refraction and between 2 and 3 months was 24 by autorefractometry and 19 by subjective refraction. Vector analysis revealed significant correlations between ΔFSA and ΔRA for data obtained by autorefractometry but not by subjective refraction. At all times, ΔBSA was greater than ΔFSA (p < 0.01). Key findings for ΔBSA values over the central 3 mm zone were between (A) the sine of the axis of ΔRA (y) and sine of the axis of ΔBSA (x) for the data obtained by autorefractometry (between 1 and 2 months, y = 0.749 - 0.303x, r = 0.299, n = 52, p=0.031) and subjective refraction (between 2 and 3 months, y = 0.6614 - 0.4755x, r = 0.474, n = 19, p=0.040) and (B) ΔRA (y) and ΔBSA (x) powers between 2 and 3 months postoperatively for the data obtained by autorefractometry (ΔRA = 0.118 ΔBSA + 0.681 r = 0.467, n = 24, p=0.021) and subjective refraction (ΔRA = 0.072 ΔBSA + 0.545 r = 0.510, n = 19, p=0.026).

Conclusion

Changes in the ocular residual refractive astigmatic error after implanting a monofocal nontoric IOL are associated with changes in astigmatism at the back surface of the cornea within the central optical zone.

Incorporating Refraction Measurement Error Into Surgery Planning for Corneal Ablation Procedures, Comparing the Flattening and Correction Indices

PURPOSE

To assess the impact of measurement error on cylinder treatment planning in corneal refractive surgery and to compare the reliability of two indices of cylinder change: the Correction Index, based on the surgically induced astigmatism, and the Flattening Index, based on the flattening effect.

METHODS

Preoperative refractions and surgical outcomes were evaluated using a Monte Carlo simulation. Cylinder change was calculated using vector analysis. The initial distribution of cylinder magnitudes was matched to a population sample of 1,000 eyes prior to LASIK. Refraction measurement error was modeled both preoperatively and postoperatively, with a non-linear optimization solver optimizing treatment effect for each amount of preoperative cylinder. Model assumptions were subsequently tested against LASIK outcomes data.

RESULTS

The mean Flattening Index was approximately 1 for all levels of preoperative cylinder, whereas the Correction Index was significantly higher than 1 for cylinder treatments of 1.00 diopters (D) or less when preoperative and postoperative measurement errors were taken into consideration. Targeting undercorrection minimized postoperative cylinder compared with targeting full correction, with the optimal target dependent on the amount of random error in refraction measurement. It was optimal to partially treat 0.25 D of cylinder, even if the presumed level of measurement error was relatively high.

CONCLUSIONS

The Flattening Index outperforms the Correction Index when accounting for errors in preoperative and postoperative cylinder measurement, particularly when pre-operative cylinder is 1.00 D or less. Treatment nomograms should be adjusted to use the Flattening Index to assess cylinder change, and reporting guidelines should be updated to include the Flattening Index. [J Refract Surg. 2020;36(3):169-174.].

The impact of exposure on the magnitude of astigmatism formed within the precorneal tear film over the central optical zone of the cornea in ocular surface disease

PURPOSE

To estimate the astigmatic power and axis of the tear film over the central optical zone of the cornea by vector analysis of topographic data, in ocular surface disease (OSD) and controls, during blink suppression.

METHODS

Video-keratoscopic images were captured on opening the eyes after a single blink 5, 10 & 15 s later during blink suppression in OSD patients (mixed aetiology, group 1 age 20 - 50 years, n = 12, group 2 > 50 years, n = 38) and controls (group 3, n = 19). The SimK and axis values were used to calculate the astigmatism (power and axis) that formed in the precorneal tear film during each period. Data were aggregated into 3 periods; T0-T5 (between 0 & 5 s after the blink), T5-T10 (5 & 10 s later, T10-T15 (between 10 & 15 s later).

RESULTS

Mean (± SD, 95%CI) astigmatic power (DC) formed in the tear film over each period was respectively : Group 1, -0.81 DC (0.99, -1.44 to -0.17), -2.65 DC(1.36, -3.52 to -1.79), -1.37 DC (2.15, -2.73 to -0.01). Group 2, -0.33 DC (0.38, -0.45 to -0.20), -0.57 DC (0.97, -0.91 to -0.24) -0.96 DC (2.10, -1.68 to -0.24), Group 3, -0.57 DC (0.55 -0.76 to -0.38), -0.56 DC (0.57, -0.76 to -0.37), -0.31 DC (0.44, -0.46 to -0.16). Changes were significant in groups 1 (p = 0.013) & 3 (p = 0.033) but not in 2 (p = 0.078). Intergroup differences were significant at all periods (p < 0.05). Significant correlations were detected following vector analysis, e.g Group2 between the astigmatism formed during T5-T10 (y) and ocular surface astigmatism at 5 s was y = 0.281x - 0.834 (r = 0.328, n = 38, p < 0.05). In all three groups apparent changes in the axes of astigmatism were not significant (p > 0.05).

CONCLUSIONS

Changes in the precorneal tear film after blinking are predominately astigmatic indicating that changes in the central region of the tear film following the natural blink are quasi-orthogonal.

Assessment of subjective refraction with a clinical adaptive optics visual simulator

PURPOSE

To clinically validate an adaptive optics visual simulator (VAO) that measures subjective refraction and visual acuity.

SETTING

Optics Laboratory, University of Murcia, Murcia, Spain.

DESIGN

Prospective case series.

METHODS

Using the adaptive optics visual simulator, 2 examiners measured the subjective refraction and visual acuity in healthy eyes of volunteers; 1 examiner also used a trial frame as a gold standard. The interexaminer reproducibility and agreement with the gold standard were estimated using the following statistical parameters: limits of agreement from Bland-Altman analysis, significance between differences (P value), and intraclass correlation coefficient (ICC).

RESULTS

Seventy-six eyes of 38 volunteers were measured. Interexaminer reproducibility for subjective refraction was excellent (ICC ≥0.96; P > .05), with low 95% confidence interval (CI) values for the power vectors M (spherical equivalent of the given refractive error), J0 (Jackson cross-cylinder, axes at 180 degrees and 90 degrees), and J45 (Jackson cross-cylinder, axes at 45 degrees and 135 degrees) (±0.51 diopter [D], ±0.14 D, and ±0.14 D, respectively). No significant differences in subjective refraction and visual acuity were found between the visual simulator and gold standard (P > .05), with 95% CIs for M, J0, and J45 (subjective refraction) of ±0.67 D, ±0.14 D, and ±0.16 D, respectively, and a ±0.10 logarithm of the minimum angle of resolution (visual acuity).

CONCLUSION

Subjective refraction results using the adaptive optics visual simulator agreed with those of the gold standard and can be used as the baseline for visual simulation of any optical corneal profile or intraocular lens design for refractive surgery patients.

Accuracy of noncycloplegic refraction performed at school screening camps

Purpose:

The aim of this study was to compare noncycloplegic refraction performed in school camp with that performed in eye clinic in children aged 6–16 years.

Methods:

A prospective study of children with unaided vision <0.2 LogMAR who underwent noncycloplegic retinoscopy (NCR) and subjective refraction (SR) in camp and subsequently in eye clinic between February and March 2017 was performed. A masked optometrist performed refractions in both settings. The agreement between refraction values obtained at both settings was compared using the Bland–Altman analysis.

Results:

A total of 217 eyes were included in this study. Between the school camp and eye clinic, the mean absolute error ± standard deviation in spherical equivalent (SE) of NCR was 0.33 ± 0.4D and that of SR was 0.26 ± 0.5D. The limits of agreement for NCR were +0.91D to − 1.09D and for SR was +1.15D to -1.06D. The mean absolute error in SE was ≤0.5D in 92.62% eyes (95% confidence interval 88%–95%).

Conclusion:

A certain degree of variability exists between noncycloplegic refraction done in school camps and eye clinic. It was found to be accurate within 0.5D of SE in 92.62% eyes for refractive errors up to 4.5D of myopia, 3D of cylinder, and 1.5D of hyperopia.

Determination of Personalized IOL-Constants for the Haigis Formula under Consideration of Measurement Precision

The capabilities of a weighted least squares approach for the optimization of the intraocular lens (IOL) constants for the Haigis formula are studied in comparison to an ordinary least squares approach. The weights are set to the inverse variances of the effective optical anterior chamber depth. The effect of random measurement noise is simulated 100000 times using data from N = 69 cataract patients and the measurement uncertainty of two different biometers. A second, independent data set (N = 33) is used to show the differences that can be expected between both methods. The weighted least squares formalism reduces the effect of measurement error on the final constants. In more than 64% it will result in a better approximation, if the measurement errors are estimated correctly. The IOL constants can be calculated with higher precision using the weighted least squares method.

Evaluations of refraction competencies of ophthalmic technicians in Mozambique

PURPOSE

Ophthalmic technicians (OT) work at health facilities in Mozambique and are trained to provide primary and secondary eye care services including basic refraction. This study was designed to assess OT competence and confidence in refraction, and investigate whether an upskilling programme is effective in developing their competence and confidence at refraction.

METHODS

Thirty-one trainee OTs and 16 qualified OTs were recruited to the study. A background questionnaire was administered to determine the demographic profile of the OTs. A confidence levels questionnaire explored their self-reported skills. Clinical competencies were assessed in relation to knowledge (theory exam) and clinical skills (patient exams). 11 OTs were upskilled and the clinical evaluations carried out post training.

RESULTS

Initial evaluations demonstrated that confidence and competence levels varied depending on the OTs training (location and duration), and their location of work (clinical load, availability of equipment and other eye care personnel). The qualified OTs were more competent than trainee OTs in most of the evaluations. Post upskilling results demonstrated significant positive impact on confidence and competence levels.

CONCLUSION

These evaluations identified factors affecting the refraction competencies of the OTs and demonstrated that upskilling is effective in improving confidence and competence levels for refraction. They demonstrate the need for a refraction competency framework. The overarching aim of this research was to inform the development of a nationwide programme of OT mentoring, upskilling and leading to the establishment of clinical competency standards for the new OT curricula, relevant to the professional demands.

Factors affecting the academic performance of optometry students in Mozambique

PURPOSE

The Mozambique Eyecare Project is a higher education partnership for the development, implementation, and evaluation of a model of optometry training at UniLúrio in Mozambique. There are many composite elements to the development of sustainable eye health structures, and appropriate education for eye health workers remains a key determinant of successful eye care development. However, from the first intake of 16 students, only 9 students graduated from the program, whereas only 6 graduated from the second intake of 24 students. This low graduation rate is attributable to a combination of substandard academic performance and student dropout. The aim of this article was to identify factors affecting the academic performance of optometry students in Mozambique.

METHODS

Nine lecturers (the entire faculty) and 15 students (9 from the first intake and 6 from the second) were recruited to the study. Clinical competency assessments were carried out on the students, semistructured individual interviews were conducted with the course lecturers, and a course evaluation questionnaire was completed by students. The results were combined to understand the complexities surrounding the optometry student training and performance.

RESULTS

One student out of nine from the first intake and three students out of six from the second were graded as competent in all the elements of the refraction clinical competency examination. Analysis of data from the interviews and questionnaire yielded four dominant themes that were viewed as important determinants of student refraction competencies: student learning context, teaching context, clinic conditions and assessment, and the existing operating health care context.

CONCLUSIONS

The evaluations have helped the university and course partners to better structure the teaching and adapt the learning environments by recommending a preparatory year and a review of the curriculum and clinic structure, implementing more transparent entry requirements, increasing awareness of the program, and improving Internet infrastructure.

Prediction of residual astigmatism after cataract surgery using swept source fourier domain optical coherence tomography

PURPOSE

To compare corneal measurements obtained by a swept source fourier domain OCT (CASIA SS-1000), an autokeratometer (Haag-Streit Lenstar), a hybrid topographer (Tomey TMS-5), a Placido topographer (Tomey TMS-5 in Placido mode) and a Scheimpflug tomographer (Oculus Pentacam) to manifest subjective refraction.

METHODS

One hundred and four pseudophacic patients with non-toric IOLs were measured at least 6 months after surgery. Corneal astigmatism as measured on the anterior corneal surface as well as total corneal astigmatism including posterior surface data was compared to manifest refractive cylinder (cross-cylinder strategy) by computing difference vectors and correlation analysis of power vectors.

RESULTS

The OCT (0.43 ± 0.25 D) and the hybrid topographer (0.44 ± 0.25 D) yielded the smallest difference vector to subjective cylinder and by far the lowest percentage of outliers >0.75 D (≈10%). The rotating Scheimpflug camera showed the largest (0.70 ± 0.41 D) difference vector. The best predictive precision (0.37 ± 0.22) could be achieved by vector averaging Lenstar keratometry and OCT.

CONCLUSIONS

Autokeratometry yielded the least measuring noise but OCT as well as hybrid topography had better predictive precision due to posterior curvature data. Scheimpflug tomography suffered from high measuring noise. Combination of keratometry and OCT data yielded the best precision for planning of toric IOL implantation. To get a reliable target cylinder for TIOL calculation, accuracy of the measuring device is crucial. Keratometry and Placido topography lack the information of the posterior corneal curvature while Scheimpflug devices suffer from higher measuring noise. In this paper, a combination of ssOCT with autokeratometry yielded the best predictive quality.

Prevalence of habitual refractive errors and anisometropia among Dutch schoolchildren and hospital employees

PURPOSE

Refractive error (RE) is suggested to cause not only visual impairment, but also functional problems such as aspecific health complaints and lower levels of school achievement. During the last few decades the prevalence of myopia has increased worldwide, especially in Asia. We investigated the prevalence of habitual RE and anisometropia in a Dutch population of children and employees.

METHODS

In a cross-sectional study, RE in both eyes of 520 children (aged 11-13 years) and 444 hospital employees (aged 17-60 years) were measured using an autorefractometer. The measurements were performed without using a cycloplegium. Pearson's correlation coefficient (r) was used to analyse correlations between the right and left eyes. Chi-square tests were used to test the differences between subgroups according to gender and age.

RESULTS

In schoolchildren 28% of right eyes were myopic (> 0.50 D) and 8% hyperopic (> 0.50 D). Pearson's r between right and left eyes for spherical equivalent power (SEP) was 0.93. The mean cylinder deviation in right eyes was 0.26 D (range 0.00-4.50 D). Anisometropia > 1.00 D was present in 4.6% of children; 22% of children were not optimally (> 0.50 D) corrected. In hospital employees, 30% of right eyes were myopic (> 0.50 D) and 10% hyperopic (> 0.50 D). Pearson's r between right and left eyes for SEP was 0.53. The mean cylinder deviation in right eyes was 0.35 D (range 0.00-5.75 D). Anisometropia > 1.00 D was present in 25% of employees. Anisometropia was more frequently present in employees aged 40-60 years, than in those aged 17-39 years (30% versus 18%; p = 0.02, Cramer's V = 0.15).

CONCLUSIONS

Refractive errors are common in children aged 11-13 years and in working adults aged 17-60 years. Distributions of sphere and cylinder deviations are similar for Dutch schoolchildren and hospital employees. Surprisingly, anisometropia proved to be more prevalent with age. In children many eyes are not optimally corrected. Increased attention should be paid to uncorrected and miscorrected REs.

Refractive predictability of partial coherence interferometry and factors that can affect it

PURPOSE

To evaluate the refractive predictability of a partial coherence interferometry (PCI) biometry device (IOL Master) for cataract surgery and to investigate factors that may affect it.

METHODS

Retrospective review of 209 eyes from 151 patients that had undergone preoperative PCI biometry and an uneventful phacoemulsification cataract surgery with posterior chamber intraocular lens (IOL) implantation was conducted. Prediction error defined as the intended refraction minus the postoperative refraction in spherical equivalent (SE) and the absolute error were analyzed according to IOL calculation formulas, patient characteristics, preoperative visual acuity (VA) and refraction, posterior subcapsular cataract (PSC), signal-to-noise ratio (SNR), and axial length (AL).

RESULTS

The overall refractive predictability of the PCI device was good. Generally, the SRK/T formula performed better than the SRK-II formula. Refractive predictability was slightly worse in eyes with >or=+2.0 diopters (D) of preoperative SE (with both SRK-II and SRK/T) and in eyes with an AL<or=23.0 mm (only with SRK-II. No other factors significantly affected the refractive predictability of the PCI, although poor VA, dense PSC, and poor SNR were closely interrelated.

CONCLUSIONS

The SRK/T formula performed significantly better than the SRK-II formula. Eyes with an AL<or=23.0 mm were associated with significantly greater hyperopic shifts in postoperative refraction with the SRK-II formula, but not with the SRK/T formula. A preoperative SE>or=+2.0D was related to a significantly greater hyperopic shift in postoperative refraction. With proper verification of measured data and a suitable IOL calculation formula, good refractive predictability is expected from PCI biometry regardless of patient characteristics, preoperative VA, SNR, PSC, and AL.

Measuring the refractive properties of the diabetic eye during blurred vision and hyperglycaemia using aberrometry and Scheimpflug imaging

PURPOSE

This study aimed to measure the refraction and geometry in the diabetic eye during the presence and absence of hyperglycaemia and blurred vision, using aberrometry and Scheimpflug imaging.

METHODS

Aberrometry and Scheimpflug imaging were used to examine ocular refraction and higher-order aberrations, as well as the shape of the cornea and the lens, in 25 patients with diabetes mellitus. From these parameters, the equivalent refractive index of the lens was calculated. Using paired t-tests, comparisons were made between a first series of measurements (Visit 1) taken in the presence of blurred vision and hyperglycaemia (> 10.0 micromol/l), and a second series of measurements (Visit 2) taken under normal conditions.

RESULTS

The mean difference in blood glucose between Visits 1 and 2 was 5.9 mmol/l (standard deviation [SD] 3.1) (p < 0.0001). Both small hyperopic and myopic shifts of equivalent refractive error (ERE) were found in nine patients (mean absolute difference ERE: 0.38 D [SD 0.12]; p = 0.02). Furthermore, higher-order aberrations (root mean square [RMS] error) were slightly increased in four patients (mean difference RMS error: 0.07 microm [SD 0.02]; p = 0.04) at Visit 1, compared to Visit 2. No significant changes were observed in the shape of the cornea or lens in any of the patients. No significant correlations were found between changes in blood glucose levels and the measured parameters in diabetic eyes.

CONCLUSIONS

The present study suggests that subjective symptoms of blurred vision during hyperglycaemia are not necessarily caused by changes in the refractive properties of the diabetic eye.

Assessment and statistics of surgically induced astigmatism

The aim of the thesis was to develop methods for assessment of surgically induced astigmatism (SIA) in individual eyes, and in groups of eyes. The thesis is based on 12 peer-reviewed publications, published over a period of 16 years. In these publications older and contemporary literature was reviewed(1). A new method (the polar system) for analysis of SIA was developed. Multivariate statistical analysis of refractive data was described(2-4). Clinical validation studies were performed. The description of a cylinder surface with polar values and differential geometry was compared. The main results were: refractive data in the form of sphere, cylinder and axis may define an individual patient or data set, but are unsuited for mathematical and statistical analyses(1). The polar value system converts net astigmatisms to orthonormal components in dioptric space. A polar value is the difference in meridional power between two orthogonal meridians(5,6). Any pair of polar values, separated by an arch of 45 degrees, characterizes a net astigmatism completely(7). The two polar values represent the net curvital and net torsional power over the chosen meridian(8). The spherical component is described by the spherical equivalent power. Several clinical studies demonstrated the efficiency of multivariate statistical analysis of refractive data(4,9-11). Polar values and formal differential geometry describe astigmatic surfaces with similar concepts and mathematical functions(8). Other contemporary methods, such as Long's power matrix, Holladay's and Alpins' methods, Zernike(12) and Fourier analyses(8), are correlated to the polar value system. In conclusion, analysis of SIA should be performed with polar values or other contemporary component systems. The study was supported by Statens Sundhedsvidenskabeligt Forskningsråd, Cykelhandler P. Th. Rasmussen og Hustrus Mindelegat, Hotelejer Carl Larsen og Hustru Nicoline Larsens Mindelegat, Landsforeningen til Vaern om Synet, Forskningsinitiativet for Arhus Amt, Alcon Denmark, and Desirée and Niels Ydes Fond.