Esther Berrio

Contribution of the cornea and internal surfaces to the change of ocular aberrations with age

We studied the age dependence of the relative contributions of the aberrations of the cornea and the internal ocular surfaces to the total aberrations of the eye. We measured the wave-front aberration of the eye with a Hartmann-Shack sensor and the aberrations of the anterior corneal surface from the elevation data provided by a corneal topography system. The aberrations of the internal surfaces were obtained by direct subtraction of the ocular and corneal wave-front data. Measurements were obtained for normal healthy subjects with ages ranging from 20 to 70 years. The magnitude of the RMS wave-front aberration (excluding defocus and astigmatism) of the eye increases more than threefold within the age range considered. However, the aberrations of the anterior corneal surface increase only slightly with age. In most of the younger subjects, total ocular aberrations are lower than corneal aberrations, while in the older subjects the reverse condition occurs. Astigmatism, coma, and spherical aberration of the cornea are larger than in the complete eye in younger subjects, whereas the contrary is true for the older subjects. The internal ocular surfaces compensate, at least in part, for the aberrations associated with the cornea in most younger subjects, but this compensation is not present in the older subjects. These results suggest that the degradation of the ocular optics with age can be explained largely by the loss of the balance between the aberrations of the corneal and the internal surfaces.

Compensation of corneal aberrations by the internal optics in the human eye.

The objective was to study the relative contribution of the optical aberrations of the cornea and the internal ocular optics (with the crystalline lens as the main component) to overall aberrations in the human eye. Three sets of wave-front aberration data were independently measured in the eyes of young subjects: for the anterior surface of the cornea, the complete eye, and internal ocular optics. The amount of aberration of both the cornea and internal optics was found to be larger than for the complete eye, indicating that the first surface of the cornea and internal optics partially compensate for each others aberrations and produce an improved retinal image. This result has a number of practical implications. For example, it shows the limitation of corneal topography as a guide for new refractive procedures and provides a strong endorsement of the value of ocular wave-front sensing for those applications.

Estimates of the ocular wave aberration from pairs of double-pass retinal images

We apply a computational technique to retrieve the wave aberration of the eye from the point-spread function obtained from pairs of double-pass retinal images. The method consists of an adapted pyramidal version of a nonlinear least-squares fitting procedure to a wave aberration expressed as an expansion in Zernike polynomials. Although the procedure provides accurate estimates of the wave aberration, it presents several drawbacks that are discussed in detail. In particular, since a great deal of computational time is necessary to retrieve a single wave aberration, this technique is not useful for real-time applications. We present results of wave aberrations in five normal subjects in the fovea for a 4-mm-pupil diameter. In every case there is a clear presence of comalike aberrations, while the third-order spherical aberration is usually smaller than previous estimates. The root-mean-square error in the retrieved wave aberration, when defocus and astigmatism were corrected, ranges from 0.24 to 0.5 wavelength. The particular values of the aberration coefficients present a large intersubject variability.

Optical aberrations and alignment of the eye with age

We explored the relative changes in ocular, corneal, and internal aberrations associated with normal aging with special emphasis in the role of ocular alignment and lens shape factor in the balance of aberrations. Ocular and corneal aberrations together with the angle kappa were measured for a 5-mm pupil diameter in 46 eyes with low refractive errors and ages ranging between 20 and 77 years. The root mean square (RMS) of the higher order ocular and corneal aberrations increased with age at a rate of 0.0032 μm/year and 0.0015 μm/year, respectively. While in young eyes the partial compensation of aberrations by the internal surfaces was clear, no significant difference was found between corneal and ocular RMS in the older group. The ocular spherical aberration (0.0011 μm/year) and horizontal coma (0.0017 μm/year) increased moderately with age. This is not due to changes in the optical alignment, since angle kappa did not vary significantly with age. Age-related variations in the radii of curvature of the crystalline lens modify slightly its shape factor, reducing the compensation of lateral coma. This suggests that geometrical changes in the crystalline lens with age contribute to modify its aberration structure, reducing the compensation mechanism and explaining most of the measured increment of ocular aberrations with age.

Optical aberrations and the aging eye.

We present a review of recent studies of the changes of optical aberrations in the aging eye. During normal aging, the retinal image quality gradually decreases in average. This is due to increases in both aberrations and amount of scattered light. This deterioration of retinal image quality has an impact on vision and, therefore, in the design of ophthalmic optics for the aging population. The underlying mechanisms explaining this decline of retinal image quality with age are also presented in this chapter. Normal aging largely affects different aspects of the visual system in humans. Specifically, the contrast sensitivity function (CSF) declines throughout the life span. Several reasons are responsible for this deterioration, ranging from purely optical degradation to retinal and neural losses. The relative contribution of optical and postoptical (retinal and neural) factors to the deterioration in spatial vision aroused some controversy, although it is now accepted that optical factors play the major important role in normal eyes. In the aging eye, there are a larger light absorption by the ocular media, a smaller pupil diameter (senile miosis), and a nearly complete reduction of the accommodation capability. In addition, Artal and coworkers first showed that the mean ocular modulation transfer function (MTF) in a group of older patients was lower than the average MTF for a group of younger patients. This result, although it was obtained in a rather small population, suggested that the ocular aberrations, besides intraocular scattering, increase with age. Burton and associates, by comparing CSFs obtained both with laser interferometry

Degree of polarization as an objective method of estimating scattering

A new method of determining objectively the amount of scattered light in an optical system has been developed. It is based on measuring the degree of polarization of the light in images formed after a double pass through the system. A dual apparatus composed of a modified double-pass imaging polarimeter and a wave-front sensor was used to measure polarization properties and aberrations of the system under test. We studied the accuracy of the procedure in a system that included a lanthanum-modified lead zirconate titanate (PLZT) ceramic plate able to generate variable amounts of scattered light as a function of the applied voltage. Changes in the voltage applied to the ceramics plate modified significantly the scattering contribution while hardly altering the wave-front aberration. The degree of polarization was well correlated with the level of scattering in the system as determined by direct-intensity measurements at the tails of the double-pass images. This indicates that this polarimetric parameter provides accurate relative estimates of the amount of scattering generated in a system. The technique can be used in a number of applications, for example, to determine objectively the amount of scattered light in the human eye.

Aberro-polariscope for the human eye.

We have developed an aberro-polariscope that simultaneously measures spatially resolved polarization properties and wave-front aberration in a living human eye. The setup consists of an infrared Hartmann-Shack sensor that incorporates a polariscope. A series of four Hartmann-Shack images corresponding to independent polarization states were recorded. The corresponding wave-front aberration was computed from each image. Moreover, from each set of four images spatially resolved (over the pupil plane) parameters of polarization were also determined. This instrument allows useful information on both the optical and the biomechanical properties of the eye to be obtained.

Modeling the mechanism of compensation of aberrations in the human eye for accommodation and aging

The mechanisms of compensation of aberration between cornea and lens are somehow modified during both accommodation and aging. In 15 individualized ocular models of young and unaccommodated eyes, we used morphological data of the lens to simulate the effect of accommodation and aging on these mechanisms. The predicted changes in aberrations were compared to data from the literature. In general, only the variation of the lens curvature was enough to reproduce the decrease in ocular spherical aberration (SA) during accommodation. However, the increase in SA with age could only be explained as a consequence of an increase in the conic constant of the lens and/or additional changes on the gradient index.

Corneal polarimetry after LASIK refractive surgery

Imaging polarimetry provides spatially resolved information on the polarization properties of a system. In the case of the living human eye, polarization could be related to the corneal biomechanical properties, which vary from the normal state as a result of surgery or pathologies. We have used an aberro-polariscope, which we recently developed, to determine and to compare the spatially resolved maps of polarization parameters across the pupil between normal healthy and post-LASIK eyes. The depolarization distribution is not uniform across the pupil, with post-surgery eyes presenting larger levels of depolarization. While retardation increases along the radius in normal eyes, this pattern becomes irregular after LASIK refractive surgery. The maps of slow axis also differ in normal and post-surgery eyes, with a larger disorder in post-LASIK eyes. Since these changes in polarization indicate subtle structural modifications of the cornea, this approach can be useful in a clinical environment to follow the biomechanical and optical changes of the cornea after refractive surgery or for the early diagnosis of different corneal pathologies.