Geometrical technique to determine the influence of monochromatic aberrations on retinoscopy

IMI - Clinical Myopia Control Trials and Instrumentation Report

The evidence-basis based on existing myopia control trials along with the supporting academic literature were reviewed; this informed recommendations on the outcomes suggested from clinical trials aimed at slowing myopia progression to show the effectiveness of treatments and the impact on patients. These outcomes were classified as primary (refractive error and/or axial length), secondary (patient reported outcomes and treatment compliance), and exploratory (peripheral refraction, accommodative changes, ocular alignment, pupil size, outdoor activity/lighting levels, anterior and posterior segment imaging, and tissue biomechanics). The currently available instrumentation, which the literature has shown to best achieve the primary and secondary outcomes, was reviewed and critiqued. Issues relating to study design and patient selection were also identified. These findings and consensus from the International Myopia Institute members led to final recommendations to inform future instrumentation development and to guide clinical trial protocols.

Effect of phenylephrine on static and dynamic accommodation

PURPOSE

We tested the hypothesis that changes in accommodation after instillation of Phenylephrine Hydrochloride (PHCl) observed in some studies could be caused by changes in optics.

METHODS

We performed two experiments to test the effects of PHCl on static and on dynamic accommodation in 8 and 6 subjects, respectively. Objective wavefront measurements were recorded of the static accommodation response to a stimulus at different distances or dynamic accommodation response to a sinusoidally moving stimulus (between 1 and 3 D of accommodative demand at 0.2Hz). The responses were characterized using two methods: one that takes into account the mydriatic optical effects on the accommodation produced by higher-order aberrations of the eye and another that takes into account only power changes paraxially due to the action of the ciliary muscle and regardless of the pupil size.

RESULTS

When mydriatic optical effects were taken into account, differences in responses before and after PHCl instillation were 0.51±0.53 D, and 0.12±0.15, for static and dynamic accommodation, respectively, and were statistically significant (p<0.039). When mydriatic optical effects were not taken into account, the differences in responses before and after PHCl instillation were -0.20±0.51 D, and -0.05±0.14, for static and dynamic accommodation, respectively, and were not statistically significant (p>0.313).

CONCLUSIONS

The mydriatic effect of the PHCl causes optical changes in the eye that can reduce the objective and subjective measurement of accommodation.

Potential Effect on the Retinoscopic Reflex of Scleral Expansion Surgery for Presbyopia

PURPOSE

In this initial study, we model the impact of crystalline lens movement and tilt, which we postulate are a potential consequence of scleral expansion surgery (SES). We demonstrate the possibility that these lead to an improvement in near vision with no concurrent restoration of lens accommodative function. With the implantation of scleral expansion bands (SEB), Schachar predicts an increase in the amplitude of accommodation not observed objectively. We postulate that the SEB implants cause the crystalline lens to be shifted forward and become misaligned. Using mathematical models of the optics of the human eye, we assess the effects of our postulate on the power of the eye, the depth of field, optical aberrations, and the appearance of the horizontal retinoscopic reflex.

METHODS

Using anatomically based models of the human eye, the postulated effects of SES are theoretically modeled and optically analyzed in Code V for unintentional lens anterior movement, tilts, and decentrations of up to 1 mm, +/- 3 degrees , and +/- 0.3mm, respectively. The transverse aberrations are calculated before and after SES. Because it has been reported that the appearance of the retinoscopic reflex is consistent with the presence of excess aberrations, we also predict the appearance of the one-dimensional retinoscopic reflex for our models. The change in refractive error is also determined. RESULTS.: Unintentional lens shift, tilt, and decentration, as might occur as a result of SES surgery, would result in an increase in the total power of the eye along with an increase in asymmetric aberrations and little change in symmetric aberrations. The calculated appearances of the retinoscopic reflexes pre- and postsurgery are consistent with observations in the literature.

CONCLUSIONS

Scleral expansion surgery could produce an improvement in near vision through an unintentional anterior displacement of the crystalline lens in combination with excess tilts and decentrations rather than as a result of a restoration of lens accommodation. There is a need for measurements and further analysis of the optical and visual properties of SES patients postsurgery.

Sources of normal and anomalous motion in retinoscopy

PURPOSE

Besides the classic "with," "against," and "neutral" absence of motion, retinoscopic reflexes can display anomalous "with" motion in myopia. A model is presented that explains the source of this anomalous motion, as well as quantifies the appearance of retinoscopic motion in myopia and hyperopia.

METHODS

Various 2 x 2 matrices were created to describe schematic eyes for a +20 D trial lens, a Gullstrand #1 schematic eye, and an infant schematic eye. Rays from the retinoscope were traced paraxially through these matrices over a full transit of the retinoscope beam across the pupil. Retinal position of the edge of the reflex visible to the observer was plotted as a function of pupil sizes from 2 mm to 16 mm for -5.00 D and +2.00 D refractive errors for the +20 D trial lens.

RESULTS

The edge of the retinoscopic reflex could be formed by one of two sources: the edge of the retinoscope beam itself, or the shadow cast by the beam against the edge of the pupil. Anomalous "with" motion arose in myopia when the edge of the reflex was formed by the edge of the beam. The edge of the beam was also visible in hyperopia but did not create anomalous motion. The retinoscope peephole was not involved in the formation of the edge of the reflex. The degree of anomalous motion increased with greater myopia and pupil size. Measured pupil sizes needed to completely eliminate anomalous motion agreed well with those predicted by the model, except at the largest pupil sizes. The limit for anomalous motion depended only on pupil size, refractive error, and working distance but not on whether the system matrix represented the trial lens, a Gullstrand #1 eye, or an infant eye.

CONCLUSIONS

Seeing the edge of the beam at large pupil sizes during retinoscopy creates anomalous "with" motion in myopia but may make the reflex easier to see in hyperopia. Anomalous "with" motion in myopia can be managed by adjustment of pupil size, working distance, or net corrected refractive error. Aside from possible effects of aberrations, retinoscopic motion appears to be consistent across various paraxial optical systems for a given refractive error and pupil size.

The streak retinoscopy pupil reflex in the presence of astigmatism

Computer simulation is being increasingly used as a teaching tool. Having developed a computer-generated virtual focimeter, we are now in the process of developing a computer-generated virtual streak retinoscope to teach the principles of retinoscopy and the effect of residual refractive error and mirror movement on the pupil reflex. One of the important requirements was to provide as accurate a simulation as possible for the completely general case of an astigmatic patient and a streak in any orientation being moved also in any orientation. This required a thorough understanding of the optical theory of the retinoscope and equations that describe the behaviour of the pupil reflex. We have taken this opportunity to review the optics of the streak retinoscope and derive equations for the behaviour of the pupil streak reflex.

A method to predict refractive errors from wave aberration data

We explored the impact of the eye's higher-order aberrations on subjective refraction comparing two classes of methods for estimating refractive state, one based directly on the wave aberration defined in the pupil plane and another based on the retinal image plane. The method defined in the pupil plane chose the sphere and cylinder that either minimized the wave aberration root mean square or minimized the sum of all the spherical and cylindrical components in the wave aberration. The method defined in the image plane chose the sphere and cylinder that optimized an image-quality metric such as the Strehl intensity ratio, the entropy and the intensity variance of the point-spread function, the volume under the modulation transfer function, or the volume under the contrast-sensitivity function. All these methods were compared in a population of six eyes for which we measured both the wave aberration with a Shack-Hartmann wavefront sensor and the subjective refraction under identical conditions. Pupil plane methods predicted subjective refraction poorly. The mean absolute error of the prediction, in spherical equivalent, was about 0.5 D (range, 0.1 to 0.8 D) and increased with increases in higher-order aberrations. However, for all the retinal image plane methods, the mean error between predicted and subjective refraction was about 0.1 D (range, 0 to 0.25 D). The reliability of the method based on the image-quality optimization was further confirmed in a large population of 146 eyes. In conclusion, higher-order aberrations influence the amount of sphere and cylinder required to correct vision. The results indicate that subjective refraction can be predicted from the eye's optics alone by optimizing computed retinal image quality.