Higher order aberrations in keratoconus‏

INTRODUCTION

Keratoconus is a progressive disorder of the cornea that causes thinning (Sedaghat et al. in Sci Rep 11(1):11971, 2021), ectasia, and irregular astigmatism, resulting in poor visual acuity that cannot be corrected with standard sphero-cylindrical spectacle lenses. One feature distinguishing keratoconic corneas is ocular aberrations, manifesting up to five or six times the amount of higher-order aberrations than a normal, healthy eye. These aberrations can cause visual disturbances even at the very early stages of the disease.

METHODS

In the past, a diagnosis was derived from clinical symptoms, but technological advances have revealed multiple pre-clinical features, allowing for the differentiation between keratoconic and normal eyes at a much earlier stage. These include anterior and posterior corneal surface elevations, the corneal pachymetry profile, corneal epithelial patterns, wavefront aberration metrics, and corneal biomechanics (Sedaghat et al. in Sci Rep 11(1):11971, 2021).This review discusses the aberrations associated with keratoconus, how to measure them, and treatment methods to minimize their negative influence.

CONCLUSIONS

Early diagnosis can lead to early treatment and may allow for arresting progression, thereby improving the long-term prognosis. With the acceleration of refractive surgery, it is important to identify patients with keratoconus, as they are usually contraindicated for refractive surgery.

Influence of perfluorohexyloctane (Evotears®) on higher order aberrations

PURPOSE

To prospectively assess the effect of regular application of perfluorohexyloctane (F6H8; Evotears®) on the tear film lipid layer, higher order aberrations (HOA) and the repeatability of measurements in healthy eyes.

METHODS

This prospective clinical study included 104 eyes treated with F6H8 four times daily for four weeks (group A) and 101 eyes that served as controls (group B). Measurements were performed with the WASCA aberrometer (Carl Zeiss Meditec GmbH, Jena, Germany). Main outcome measurement in addition to subjective refraction were the root mean square values of HOA measured before and after the intervention.

RESULTS

Regular use of F6H8 over a period of four weeks significantly increases HOA in healthy eyes (p < 0.05). In addition, the repeatability of measurement increases after the application of F6H8.

CONCLUSION

F6H8 may be a suitable treatment option to improve the accuracy of refractive assessment, although it increases HOA. Further studies are needed to confirm the effect on HOA and the repeatability of measurement.

A review of imaging modalities for detecting early keratoconus

OBJECTIVES

Early identification of keratoconus is imperative for preventing iatrogenic corneal ectasia and allowing for early corneal collagen cross-linking treatments to potentially halt progression and decrease transplant burden. However, early diagnosis of keratoconus is currently a diagnostic challenge as there is no uniform screening criteria. We performed a review of the current literature to assess imaging modalities that can be used to help identify subclinical keratoconus.

METHODS

A Pubmed database search was conducted. We included primary and empirical studies for evaluating different modalities of screening for subclinical keratoconus.

RESULTS

A combination of multiple imaging tools, including corneal topography, tomography, Scheimpflug imaging, anterior segment optical coherence tomography, and in vivo confocal microscopy will allow for enhanced determination of subclinical keratoconus. In patients who are diagnostically borderline using a single screening criteria, use of additional imaging techniques can assist in diagnosis. Modalities that show promise but need further research include polarization-sensitive optical coherence tomography, Brillouin microscopy, and atomic force microscopy.

CONCLUSIONS

Recognition of early keratoconus can reduce risk of post-refractive ectasia and reduce transplantation burden. Though there are no current uniform screening criterion, multiple imaging modalities have shown promise in assisting with the early detection of keratoconus.

Wavefront aberration reconstruction from tangential refractive powers measured with spatial dynamic skiascopy

The aim of this work was to study, using numerical simulations, the attainable level of accuracy to reconstruct the wavefront aberrations from tangential refractive power data measured with dynamic skiascopy. Two mathematical methods have been implemented. The first one is based on curve integration of the curvature data, previously interpolated with cubic splines. The second one reconstructs the three-dimensional wavefront surface, represented by a Zernike polynomial expansion, using a two-step least-squares method. The different factors affecting the reconstruction--noise, sampling, and wavefront patterns--were quantified. The results provide useful information to design more efficient experimental setups based on spatial dynamic skiascopy.

Precision of higher-order aberration measurements with a new Placido-disk topographer and Hartmann-Shack wavefront sensor

PURPOSE

To assess the intrasession and intersession precision of ocular, corneal, and internal higher-order aberrations (HOAs) measured using an integrated topographer and Hartmann-Shack wavefront sensor (Topcon KR-1W) in refractive surgery candidates.

SETTING

IOBA-Eye Institute, Valladolid, Spain.

DESIGN

Evaluation of diagnostic technology.

METHODS

To analyze intrasession repeatability, 1 experienced examiner measured eyes 9 times successively. To study intersession reproducibility, the same clinician obtained measurements from another set of eyes in 2 consecutive sessions 1 week apart. Ocular, corneal, and internal HOAs were obtained. Coma and spherical aberrations, 3rd- and 4th-order aberrations, and total HOAs were calculated for a 6.0 mm pupil diameter.

RESULTS

For intrasession repeatability (75 eyes), excellent intraclass correlation coefficients (ICCs) were obtained (ICC >0.87), except for internal primary coma (ICC = 0.75) and 3rd-order (ICC = 0.72) HOAs. Repeatability precision (1.96 × S(w)) values ranged from 0.03 μm (corneal primary spherical) to 0.08 μm (ocular primary coma). For intersession reproducibility (50 eyes), ICCs were good (>0.8) for ocular primary spherical, 3rd-order, and total higher-order aberrations; reproducibility precision values ranged from 0.06 μm (corneal primary spherical) to 0.21 μm (internal 3rd order), with internal HOAs having the lowest precision (≥0.12 μm). No systematic bias was found between examinations on different days.

CONCLUSIONS

The intrasession repeatability was high; therefore, the device's ability to measure HOAs in a reliable way was excellent. Under intersession reproducibility conditions, dependable corneal primary spherical aberrations were provided.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Design and validity of a miniaturized open-field aberrometer

PURPOSE

To design and validate a new miniaturized open-field wavefront device that can be attached to an ophthalmic surgical microscope or slitlamp.

SETTING

Solihull Hospital and Aston University, Birmingham, United Kingdom.

DESIGN

Comparative noninterventional study.

METHODS

The dynamic range of the Aston aberrometer was assessed using a calibrated model eye. The validity was compared with that of a conventional desk-mounted Hartmann-Shack aberrometer (Topcon KR1W) in dilated eyes. The instruments were used in random order, with measurements repeated 5 times to assess intrasession repeatability.

RESULTS

The open-field aberrometer had a large dynamic range of at least +21.0 diopters (D) to -25.0 D. It gave similar measurements to the conventional aberrometer for mean spherical equivalent (SE) (mean difference 0.02 D ± 0.49 [95% confidence interval]; correlation r = 0.995; P<.001), astigmatic components (J0: 0.02 ± 0.15 D; r = 0.977, P<.001; J45: 0.03 ± 0.28, r = 0.666, P<.001), and higher-order aberration (HOA) root mean square (RMS) (0.02 ± 0.20 D, r = 0.620, P<.001). Intraclass correlation coefficient assessments of intrasession repeatability were excellent (SE = 1.000, P<.001; J0 = 0.998, P<.001; J45 = 0.980, P<.01; HOA RMS = 0.961, P<.001).

CONCLUSIONS

The new aberrometer gave valid, repeatable measurements of refractive error and HOAs over a large range. It can measure continuously, thus providing direct feedback on the optical status of the visual system to surgeons during intraocular lens implantation and corneal surgery.

Optical quality in eyes with aspheric intraocular lenses and in younger and older adult phakic eyes: comparative study

PURPOSE

To compare ocular higher-order aberrations (HOAs) in eyes with aspheric intraocular lenses (IOL) and in phakic eyes.

SETTING

Department of Ophthalmology, Military Health Service Institute, Warsaw, Poland.

METHODS

Higher-order aberrations with a 5.0 mm pupil and best spectacle-corrected visual acuity (BSCVA) were analyzed in a young phakic group (age 20 to 40 years), older phakic group (age 40 to 60 years), and a pseudophakic group (age 62 to 84 years; SN60WF aspheric IOL).

RESULTS

The mean coma was 0.13 microm+/-0.07 (SD) in the younger phakic group, 0.18+/-0.08 microm in the older phakic group, and 0.16+/-0.08 microm in the pseudophakic group; the difference between the 2 phakic groups was statistically significant (P< .005). The mean spherical aberration was 0.07+/-0.05 microm, 0.11+/-0.05 microm, and 0.06+/-0.04 microm, respectively; the only statistically significant difference was between the 2 phakic groups (P< .005). The mean HOA value was 0.54+/-0.18 microm, 0.76+/-0.12 microm, and 0.92+/-0.31 microm, respectively; the difference was statistically significant between the 2 phakic groups or between the younger phakic group and the pseudophakic group (both P< .005). There were no statistically significant differences in BSCVA between the 3 groups.

CONCLUSIONS

Spherical and coma aberrations were similar in eyes with an aspheric IOL and younger phakic eyes, although HOAs were higher in pseudophakic eyes. There were no differences between pseudophakic eyes and older phakic eyes in coma and HOA, although pseudophakic eyes had lower spherical aberration.

Relationship between contrast sensitivity and spherical aberration: comparison of 7 contrast sensitivity tests with natural and artificial pupils in healthy eyes

PURPOSE

To find a contrast sensitivity test that can be used clinically to evaluate interventions aimed at minimizing spherical aberration and determine the circumstances under which these tests should be performed.

SETTING

Laboratory of Experimental Ophthalmology, University of Groningen, Groningen, The Netherlands.

METHODS

Contrast sensitivity tests were performed using 2 experimental designs. Design 1 was with a natural pupil under mesopic and photopic conditions. Design 2 was with a 5.0 mm artificial pupil after cycloplegia under photopic conditions only. Two computerized tests (vertical sine-modulated gratings [VSG] and Holladay circular sine-modulated patterns [HACSS]) and 5 chart tests (Pelli-Robson, acuity-measuring letter charts at low contrast [2.5% and 10%], VectorVision, and edge contrast sensitivity) were used. Spherical aberration was assessed with a Hartmann-Shack wavefront analyzer.

RESULTS

Forty-nine healthy subjects aged 20 to 35 years (n = 24) and 55 to 70 years (n = 25) participated. Design 2 showed a significant relationship between contrast sensitivity and spherical aberration with the HACSS at 3 cycles per degree (cpd) (P = .03) and 6 cpd (P = .01) and with the VSG at 6 cpd (P = .01). Design 1 yielded no significant relationships.

CONCLUSIONS

Using an artificial pupil, a relationship between contrast sensitivity and spherical aberration was established with the VSG and HACSS tests but not with the chart tests. No test showed a relationship using natural pupils under either lighting condition. Chart tests are unsuitable for uncovering contrast sensitivity differences related to differences in spherical aberration, as typically found in healthy phakic eyes.