Changes in crystalline lens radii of curvature and lens tilt and decentration during dynamic accommodation in rhesus monkeys

The effect of near work on the anterior eye and associations with myopia: a narrative review

The global prevalence of myopia has increased significantly in recent decades, and it is anticipated that half the population of the world will be myopic by 2050. The dramatic increase in myopia cannot be explained solely by genetic factors; hence, environmental factors such as near work may play an important role in myopia development. Near work activities involve accommodation, convergence, and pupil constriction, which lead to various mechanical changes that alter the structural and optical properties of the anterior eye. Mechanical changes associated with near work activities include ciliary body contraction, medial rectus contraction, lateral rectus relaxation, changes in the eyelid-cornea interaction, pupil size, and crystalline lens shape and position. These structural variations lead to optical changes including a change in spherical refractive power, astigmatism, accommodative convergence, higher order aberrations, and retinal image quality. Several differences in near work-related optical and structural changes have been observed between myopes and non-myopes. These differences elucidate mechanisms that potentially underpin near work-associated axial elongation and myopia development. This narrative review explores anterior segment structural and optical changes during near work and their potential mechanistic contribution to myopia development, while highlighting literature gaps that require further research.

Effects of cycloplegia on crystalline lens morphology and location in acute acquired concomitant esotropia

PURPOSE

The study aims to compare morphology and location of crystalline lens between acute acquired concomitant esotropia (AACE) patients and control subjects, both before and after cycloplegia.

METHODS

This is a prospective and observational clinical study. Morphological and locational parameters of the crystalline lens in 53 AACE patients and 32 control subjects were assessed before and after cycloplegia using CASIA2 system, which represents the latest swept-source anterior segment optical coherence tomography. Cycloplegic refraction was recorded by administering 1% atropine in patients younger than 12 years and 1% cyclopentolate in those > 12 years old. Morphological parameters included anterior radius of curvature (ARC), posterior radius of curvature (PRC), lens thickness (LTH), and equivalent diameter of lens (LED). Locational parameters comprised lens decentration (LD) and lens tilt (LT). Comparison of these parameters before and after cycloplegia were conducted between AACE and controls. Additionally, the study analyzed and compared the changes in these parameter post-cycloplegia.

RESULTS

Our findings suggest no significant difference in morphological parameters including ARC, PRC, LTH and LED between AACE patients and controls before or after cycloplegia. However, 2D-modeling data in the 0° meridian revealed that variation post-cycloplegia of LD (lens shift) in right eyes was different in AACE patients, measuring - 0.03(0.08) [median(interquartile range)] which was significantly distinct from the control group, exhibiting a measurement of 0.01(0.06) (z =  - 2.373, p = 0.018). In left eyes, a similar trend was observed with lens shift in the 0° meridian being 0.02(0.06) in AACE, significantly differing from control group's measurement of - 0.02(0.08) (z =  - 2.809, p = 0.005). Further, correlation analysis revealed that larger temporal shift of lens was associated with greater changes in ARC (r = 0.294, p = 0.006) and LTH (r =  - 0.230, p = 0.031).

CONCLUSIONS

The morphological features of the crystalline lens were similar in AACE patients and controls; however, the change of lens location by cycloplegia was observed only in AACE patients, suggesting an association with excessive accommodation.

Autostereoscopic 3D viewing can change the dimensions of the crystalline lens in myopes

PURPOSE

Autostereoscopic displays have become increasingly common, but their impact on ocular dimensions remains unknown. We sought to identify changes in the crystalline lens dimensions induced by autostereoscopic three-dimensional (3D) viewing.

METHODS

Forty young adults (age: 22.6 ± 2.0 years, male/female: 15/25) were consecutively enrolled and randomly divided into two groups (3D and two-dimensional [2D] viewing groups) to watch a 30-min movie clip displayed in 3D or 2D mode on a tablet computer. The lens thickness (LT), diameter, curvature, decentration and tilt were measured with anterior segment optical coherence tomography under both non-accommodating (static) and accommodating conditions.

RESULTS

In the static condition, the LT decreased by 0.03 ± 0.03 mm (p < 0.001) and the anterior radius of curvature (ARC) increased by 0.49 ± 0.59 mm (p = 0.001) post-3D viewing. In contrast, following 2D viewing, the ARC decreased by 0.23 ± 0.25 mm (p = 0.001). Additionally, the increase in the steep ARC post-3D viewing was greater in high-myopic eyes than low to moderate myopic eyes (p = 0.04). When comparing the accommodative with the static (non-accommodative) condition, for 3D viewing the lens decentration decreased (-0.03 ± 0.05 mm, p = 0.02); while for 2D viewing, the posterior curvature radius (-0.14 ± 0.20 mm, p = 0.006) and diameter (-0.13 ± 0.20 mm, p = 0.01) decreased.

CONCLUSIONS

Viewing with the autostereoscopic 3D tablet could temporally decrease the thickness and curvature of the lens under non-accommodating conditions. However, its long-term effect requires further exploration.

BCLA CLEAR presbyopia: Mechanism and optics

With over a billion adults worldwide currently affected, presbyopia remains a ubiquitous, global problem. Despite over a century of study, the precise mechanism of ocular accommodation and presbyopia progression remains a topic of debate. Accordingly, this narrative review outlines the lenticular and extralenticular components of accommodation together with the impact of age on the accommodative apparatus, neural control of accommodation, models of accommodation, the impact of presbyopia on retinal image quality, and both historic and contemporary theories of presbyopia.

[Current capabilities of anterior segment optical coherence tomography]

Optical coherence tomography of the anterior segment of the eye (AS-OCT) is a non-invasive method based on the principles of optical reflectometry (measurement of the degree of backscattering of light passing through transparent or translucent media). Limitations of the first devices of this type were associated with insufficient image quality of the details of the anterior chamber angle and the posterior parts of the lens, primarily due to the "working" level of the scanning wavelength (within 800 μm). Fundamentally new possibilities in the structural and functional assessment of the anterior segment of the eye are associated with the introduction into clinical practice of swept-source AS-OCT device - the CASIA2 anterior optical coherence tomograph (Tomey Corporation, Japan). Its high scanning speed (50 000 A-scans per second) with a wavelength of 1310 μm allows high-quality visualization by building a scan at a depth of 13 mm. The previous model (CASIA SS-1000, Tomey Corporation, Japan) supported scan depth of only 6 mm. This review summarizes the results of research on the clinical use of CASIA2 tomograph.

The evolution of mechanism of accommodation and a novel hypothesis

Myopia and presbyopia are two major optometry problems facing the whole society. The mechanism of accommodation is strongly related to the treatments of myopia and presbyopia. However, the key mechanism of accommodation has puzzled us for over 400 years and is still not clear at present, leading to the stagnation of prevention and treatment of myopia and presbyopia. With the continued development of experimental technologies and equipment, the approaches to elucidate accommodation's intricacies have become more methodological and sophisticated. Fortunately, some significant progress has been made. This article is to review the evolution of the mechanism of accommodation. Helmholtz proposed a classical theory of "zonules relax during accommodation." In contrast, Schachar put forward a theory of "zonules taut during accommodation." Those hypotheses are relatively complete, but either do not fully explain everything about the accommodation mechanism or lack sufficient experimental and clinical evidence to support them. Then, some contentious issues are discussed in detail to find the truth. Finally, we proposed our hypothesis about accommodation based on the anatomy of the accommodative apparatus.

Distributions of crystalline lens tilt and decentration and associated factors in age-related cataract

PURPOSE

To investigate the characteristics and factors associated with crystalline lens tilt and decentration measured by CASIA2 anterior segment optical coherence tomography.

SETTING

Zhongshan Ophthalmic Center, Sun Yat-sen University, Guangzhou, China.

DESIGN

Cross-sectional study.

METHODS

1097 eyes of 1097 patients who planned to undergo cataract surgery were enrolled. All patients underwent a general ophthalmologic examination. Lens thickness (LT), front curvature radius (FCR), back curvature radius, lens equator diameter (LED), tilt, and decentration of preoperative crystalline lenses were measured by CASIA2. Univariate and multivariate regression analyses were performed to evaluate the relationships between the tilt and decentration of crystalline lens with related factors.

RESULTS

The natural crystalline lenses showed a mean tilt of 5.16 degrees toward the inferotemporal direction and a mean decentration of 0.22 mm toward the temporal direction. Of the total 1097 eyes, 119 eyes (10.85%) had a tilt greater than 7 degrees, and 89 eyes (8.11%) had a decentration more than 0.4 mm. Multivariate regression analysis showed that larger decentration, thicker LT, shorter axial length (AL), and FCR were associated with greater lens tilt (P < .001, P = .007, P = .006, and P = .003, respectively). In addition, greater tilt, older and thinner LT were correlated with larger decentration (all P < .001).

CONCLUSIONS

Preoperative crystalline lens had a certain degree of tilt and decentration in age-related cataract. The greater tilt of the crystalline lens was, the larger decentration of it was. In addition, AL, FCR, LT, and age also correlated with tilt and decentration.

Measuring mental workload: ocular astigmatism aberration as a novel objective index

This study assessed the effect of two perceptually matched mental tasks with different levels of mental demand on ocular aberrations in a group of young adults. We measured ocular aberration with a wavefront sensor, and total, internal and corneal RMS (root mean square) aberrations were calculated from Zernike coefficients, considering natural and scaled pupils (5, 4.5, and 4 mm). We found that total, internal and corneal astigmatism RMS showed significant differences between mental tasks with natural pupils (p < .05), and this effect was maintained with 5 mm scaled pupils (total RMS astigmatism, p < .05). Consistently, pupil size, intraocular pressure, perceived mental load and cognitive performance were influenced by the level of mental complexity (p < .05 for all). The findings suggest that ocular astigmatism aberration, mediated by intraocular pressure, could be an objective, valid reliable index to evaluate the impact of cognitive processing in conjunction with others physiological markers in real world contexts. Practitioner Summary: The search continues for a valid, reliable, convenient method of measuring mental workload. In this study we found ocular astigmatism aberration is sensitive to the cumulative effect of mental effort. It shows promise of being a novel ocular index which may help to assess mental workload in real situations.

Protein Disulfide Levels and Lens Elasticity Modulation: Applications for Presbyopia

PURPOSE

The purpose of the experiments described here was to determine the effects of lipoic acid (LA)-dependent disulfide reduction on mouse lens elasticity, to synthesize the choline ester of LA (LACE), and to characterize the effects of topical ocular doses of LACE on mouse lens elasticity.

METHODS

Eight-month-old mouse lenses (C57BL/6J) were incubated for 12 hours in medium supplemented with selected levels (0-500 μM) of LA. Lens elasticity was measured using the coverslip method. After the elasticity measurements, P-SH and PSSP levels were determined in homogenates by differential alkylation before and after alkylation. Choline ester of LA was synthesized and characterized by mass spectrometry and HPLC. Eight-month-old C57BL/6J mice were treated with 2.5 μL of a formulation of 5% LACE three times per day at 8-hour intervals in the right eye (OD) for 5 weeks. After the final treatment, lenses were removed and placed in a cuvette containing buffer. Elasticity was determined with a computer-controlled instrument that provided Z-stage upward movements in 1-μm increments with concomitant force measurements with a Harvard Apparatus F10 isometric force transducer. The elasticity of lenses from 8-week-old C57BL/6J mice was determined for comparison.

RESULTS

Lipoic acid treatment led to a concentration-dependent decrease in lens protein disulfides concurrent with an increase in lens elasticity. The structure and purity of newly synthesized LACE was confirmed. Aqueous humor concentrations of LA were higher in eyes of mice following topical ocular treatment with LACE than in mice following topical ocular treatment with LA. The lenses of the treated eyes of the old mice were more elastic than the lenses of untreated eyes (i.e., the relative force required for similar Z displacements was higher in the lenses of untreated eyes). In most instances, the lenses of the treated eyes were even more elastic than the lenses of the 8-week-old mice.

CONCLUSIONS

As the elasticity of the human lens decreases with age, humans lose the ability to accommodate. The results, briefly described in this abstract, suggest a topical ocular treatment to increase lens elasticity through reduction of disulfides to restore accommodative amplitude.

Three-dimensional OCT based guinea pig eye model: relating morphology and optics

Custom Spectral Optical Coherence Tomography (SOCT) provided with automatic quantification and distortion correction algorithms was used to measure the 3-D morphology in guinea pig eyes (n = 8, 30 days; n = 5, 40 days). Animals were measured awake in vivo under cyclopegia. Measurements showed low intraocular variability (<4% in corneal and anterior lens radii and <8% in the posterior lens radii, <1% interocular distances). The repeatability of the surface elevation was less than 2 µm. Surface astigmatism was the individual dominant term in all surfaces. Higher-order RMS surface elevation was largest in the posterior lens. Individual surface elevation Zernike terms correlated significantly across corneal and anterior lens surfaces. Higher-order-aberrations (except spherical aberration) were comparable with those predicted by OCT-based eye models.

OCT-based full crystalline lens shape change during accommodation in vivo

The full shape of the accommodating crystalline lens was estimated using custom three-dimensional (3-D) spectral OCT and image processing algorithms. Automatic segmentation and distortion correction were used to construct 3-D models of the lens region visible through the pupil. The lens peripheral region was estimated with a trained and validated parametric model. Nineteen young eyes were measured at 0-6 D accommodative demands in 1.5 D steps. Lens volume, surface area, diameter, and equatorial plane position were automatically quantified. Lens diameter & surface area correlated negatively and equatorial plane position positively with accommodation response. Lens volume remained constant and surface area decreased with accommodation, indicating that the lens material is incompressible and the capsular bag elastic.

Full 3-D OCT-based pseudophakic custom computer eye model

We compared measured wave aberrations in pseudophakic eyes implanted with aspheric intraocular lenses (IOLs) with simulated aberrations from numerical ray tracing on customized computer eye models, built using quantitative 3-D OCT-based patient-specific ocular geometry. Experimental and simulated aberrations show high correlation (R = 0.93; p<0.0001) and similarity (RMS for high order aberrations discrepancies within 23.58%). This study shows that full OCT-based pseudophakic custom computer eye models allow understanding the relative contribution of optical geometrical and surgically-related factors to image quality, and are an excellent tool for characterizing and improving cataract surgery.

OCT-based crystalline lens topography in accommodating eyes

Custom Spectral Domain Optical Coherence Tomography (SD-OCT) provided with automatic quantification and distortion correction algorithms was used to measure anterior and posterior crystalline lens surface elevation in accommodating eyes and to evaluate relationships between anterior segment surfaces. Nine young eyes were measured at different accommodative demands. Anterior and posterior lens radii of curvature decreased at a rate of 0.78 ± 0.18 and 0.13 ± 0.07 mm/D, anterior chamber depth decreased at 0.04 ± 0.01 mm/D and lens thickness increased at 0.04 ± 0.01 mm/D with accommodation. Three-dimensional surface elevations were estimated by subtracting best fitting spheres. In the relaxed state, the spherical term accounted for most of the surface irregularity in the anterior lens (47%) and astigmatism (70%) in the posterior lens. However, in accommodated lenses astigmatism was the predominant surface irregularity (90%) in the anterior lens. The RMS of high-order irregularities of the posterior lens surface was statistically significantly higher than that of the anterior lens surface (x2.02, p<0.0001). There was significant negative correlation in vertical coma (Z3 (-1)) and oblique trefoil (Z3 (-3)) between lens surfaces. The astigmatic angle showed high degree of alignment between corneal surfaces, moderate between corneal and anterior lens surface (~27 deg), but differed by ~80 deg between the anterior and posterior lens surfaces (including relative anterior/posterior lens astigmatic angle shifts (10-20 deg).

Stretch-dependent changes in surface profiles of the human crystalline lens during accommodation: a finite element study

BACKGROUND

A non-linear isotropic finite element (FE) model of a 29-year-old human crystalline lens was constructed to study the effects of various geometrical parameters on lens accommodation.

METHODS

The model simulates dis-accommodation by stretching of the lens and predicts the change in surface profiles of the lens capsule, cortex and nucleus at select states of stretching/accommodation. Multiple regression analysis (MRA) is used to develop a stretch-dependent mathematical model relating the lens sagittal height to the radial position of the lens surface as a function of dis-accommodative stretch. A load analysis is performed to compare the finite element results to empirical results from lens stretcher studies. Using the predicted geometrical changes, the optical response of the whole eye during accommodation was analysed by ray-tracing.

RESULTS

Aspects of lens shape change relative to stretch were evaluated, including change in diameter, central thickness and accommodation. Maximum accommodation achieved was 10.29 D. From the multiple regression analysis, the stretch-dependent mathematical model of the lens shape related lens curvatures as a function of lens ciliary stretch well (maximum mean-square residual error 2.5 × 10(-3 ) μm, p < 0.001). The results are compared with those from in vitro studies.

CONCLUSIONS

The finite element and ray-tracing predictions are consistent with Ex Vivo Accommodation Simulator (EVAS) studies in terms of load and power change versus change in thickness. The mathematical stretch-dependent model of accommodation presented may have utility in investigating lens behaviour at states other than the relaxed or fully accommodated states.

Influence of shape and gradient refractive index in the accommodative changes of spherical aberration in nonhuman primate crystalline lenses

PURPOSE

To estimate changes in surface shape and gradient refractive index (GRIN) profile in primate lenses as a function of accommodation. To quantify the contribution of surface shape and GRIN to spherical aberration changes with accommodation.

METHODS

Crystalline lenses from 15 cynomolgus monkeys were studied in vitro under different levels of accommodation produced by a stretching system. Lens shape was obtained from optical coherence tomography (OCT) cross-sectional images. The GRIN was reconstructed with a search algorithm using the optical path measured from OCT images and the measured back focal length. The spherical aberration of the lens was estimated as a function of accommodation using the reconstructed GRIN and a homogeneous refractive index.

RESULTS

The lens anterior and posterior radii of curvature decreased with increasing lens power. Both surfaces exhibited negative asphericities in the unaccommodated state. The anterior surface conic constant shifted toward less negative values with accommodation, while the value of the posterior remained constant. GRIN parameters remained constant with accommodation. The lens spherical aberration with GRIN distribution was negative and higher in magnitude than that with a homogeneous equivalent refractive index (by 29% and 53% in the unaccommodated and fully accommodated states, respectively). Spherical aberration with the equivalent refractive index shifted with accommodation toward negative values (-0.070 μm/diopter [D]), but the reconstructed GRIN shifted it farther (-0.124 μm/D).

CONCLUSIONS

When compared with the lens with the homogeneous equivalent refractive index, the reconstructed GRIN lens has more negative spherical aberration and a larger shift toward more negative values with accommodation.

Full OCT anterior segment biometry: an application in cataract surgery

In vivo three-dimensional (3-D) anterior segment biometry before and after cataract surgery was analyzed by using custom high-resolution high-speed anterior segment spectral domain Optical Coherence Tomography (OCT). The system was provided with custom algorithms for denoising, segmentation, full distortion correction (fan and optical) and merging of the anterior segment volumes (cornea, iris, and crystalline lens or IOL), to provide fully quantitative data of the anterior segment of the eye. The method was tested on an in vitro artificial eye with known surfaces geometry at different orientations and demonstrated on an aging cataract patient in vivo. Biometric parameters CCT, ACD/ILP, CLT/ILT Tilt and decentration are retrieved with a very high degree of accuracy. IOL was placed 400 μm behind the natural crystalline lens, The IOL was aligned with a similar orientation of the natural lens (2.47 deg superiorly), but slightly lower amounts (0.77 deg superiorly). The IOL was decentered superiorly (0.39 mm) and nasally (0.26 mm).

In vivo human crystalline lens topography

Custom high-resolution high-speed anterior segment spectral domain optical coherence tomography (OCT) was used to characterize three-dimensionally (3-D) the human crystalline lens in vivo. The system was provided with custom algorithms for denoising and segmentation of the images, as well as for fan (scanning) and optical (refraction) distortion correction, to provide fully quantitative images of the anterior and posterior crystalline lens surfaces. The method was tested on an artificial eye with known surfaces geometry and on a human lens in vitro, and demonstrated on three human lenses in vivo. Not correcting for distortion overestimated the anterior lens radius by 25% and the posterior lens radius by more than 65%. In vivo lens surfaces were fitted by biconicoids and Zernike polynomials after distortion correction. The anterior lens radii of curvature ranged from 10.27 to 14.14 mm, and the posterior lens radii of curvature ranged from 6.12 to 7.54 mm. Surface asphericities ranged from -0.04 to -1.96. The lens surfaces were well fitted by quadrics (with variation smaller than 2%, for 5-mm pupils), with low amounts of high order terms. Surface lens astigmatism was significant, with the anterior lens typically showing horizontal astigmatism ([Formula: see text] ranging from -11 to -1 µm) and the posterior lens showing vertical astigmatism ([Formula: see text] ranging from 6 to 10 µm).

Anterior segment biometry during accommodation imaged with ultralong scan depth optical coherence tomography

PURPOSE

To measure by ultralong scan depth optical coherence tomography (UL-OCT) dimensional changes in the anterior segment of human eyes during accommodation.

DESIGN

Evaluation of diagnostic test or technology.

PARTICIPANTS

Forty-one right eyes of healthy subjects with a mean age of 34 years (range, 22-41 years) and a mean refraction of -2.5 ± 2.6 diopters were imaged in 2 repeated measurements at minimal and maximal accommodations.

METHODS

A specially adapted and designed UL-OCT instrument was used to image from the front surface of the cornea to the back surface of the crystalline lens. Custom software corrected the optical distortion of the images and yielded the biometric measurements. The coefficient of repeatability and the intraclass correlation coefficient were calculated to evaluate the repeatability and reliability.

MAIN OUTCOME MEASURES

Anterior segment parameters and associated repeatability and reliability upon accommodation. The dimensional results included central corneal thickness (CCT), anterior chamber depth and width (ACD, ACW), pupil diameter (PD), lens thickness (LT), anterior segment length (ASL = ACD + LT), lens central position (LCP = ACD + 1/2LT), and horizontal radii of the lens anterior and posterior surface curvatures (LAC, LPC).

RESULTS

Repeated measurements of each variable within each accommodative state did not differ significantly (P>0.05). The coefficients of repeatability (CORs) and intraclass correlation coefficients for CCT, ACW, ACD, LT, LCP, and ASL were excellent (1.2%- 3.59% and 0.998-0.877, respectively). They were higher for PD (18.90%-21.63% and 0.880-0.874, respectively) and moderate for LAC and LPC (34.86%-42.72% and 0.669-0.251, respectively) in the 2 accommodative states. Compared with minimal accommodation, PD, ACD, LAC, LPC, and LCP decreased and LT and ASL increased significantly at maximal accommodation (P<0.05), whereas CCT and ACW did not change (P>0.05).

CONCLUSIONS

The UL-OCT measured changes in anterior segment dimensions during accommodation with good repeatability and reliability. During accommodation, the back surface of the lens became steeper as the lens moved forward.

FINANCIAL DISCLOSURE(S)

The authors have no proprietary or commercial interest in any of the materials discussed in this article.

Ex vivo measurement of postmortem tissue changes in the crystalline lens by Brillouin spectroscopy and confocal reflectance microscopy

Use of Brillouin spectroscopy in ophthalmology enables noninvasive, spatially resolved determination of the rheological properties of crystalline lens tissue. Furthermore, the Brillouin shift correlates with the protein concentration inside the lens. In vitro measurements on extracted porcine lenses demonstrate that results obtained with Brillouin spectroscopy depend strongly on time after death. The intensity of the Brillouin signal decreases significantly as early as 5 h postmortem. Moreover, the fluctuation of the Brillouin frequency shift inside the lens increases with postmortem time. Images of lens tissue taken with a confocal reflectance microscope between measurements reveal a degenerative aging process. These tissue changes correlate with our results from Brillouin spectroscopy. It is concluded that only in vivo measurements appropriately reflect the rheological properties of the eye lens and its protein concentration.

Contribution of the crystalline lens gradient refractive index to the accommodation amplitude in non-human primates: in vitro studies

The purpose of this study was to determine the contribution of the gradient refractive index to the change in lens power in hamadryas baboon and cynomolgus monkey lenses during simulated accommodation in a lens stretcher. Thirty-six monkey lenses (1.4-14.1 years) and twenty-five baboon lenses (1.8-28.0 years) were stretched in discrete steps. At each stretching step, the lens back vertex power was measured and the lens cross-section was imaged with optical coherence tomography. The radii of curvature for the lens anterior and posterior surfaces were calculated for each step. The power of each lens surface was determined using refractive indices of 1.365 for the outer cortex and 1.336 for the aqueous. The gradient contribution was calculated by subtracting the power of the surfaces from the measured lens power. In all lenses, the contribution of the surfaces and gradient increased linearly with the amplitude of accommodation. The gradient contributes on average 65 ± 3% for monkeys and 66 ± 3% for baboons to the total power change during accommodation. When expressed in percent of the total power change, the relative contribution of the gradient remains constant with accommodation and age in both species. These findings are consistent with Gullstrand's intracapsular theory of accommodation.

Predicting crystalline lens fall caused by accommodation from changes in wavefront error

PURPOSE

To illustrate and develop a method for estimating crystalline lens decentration as a function of accommodative response using changes in wavefront error and show the method and limitations using previously published data (2004) from 2 iridectomized monkey eyes so that clinicians understand how spherical aberration can induce coma, in particular in intraocular lens surgery.

SETTINGS

College of Optometry, University of Houston, Houston, USA.

DESIGN

Evaluation of diagnostic test or technology.

METHODS

Lens decentration was estimated by displacing downward the wavefront error of the lens with respect to the limiting aperture (7.0 mm) and ocular first surface wavefront error for each accommodative response (0.00 to 11.00 diopters) until measured values of vertical coma matched previously published experimental data (2007). Lens decentration was also calculated using an approximation formula that only included spherical aberration and vertical coma.

RESULTS

The change in calculated vertical coma was consistent with downward lens decentration. Calculated downward lens decentration peaked at approximately 0.48 mm of vertical decentration in the right eye and approximately 0.31 mm of decentration in the left eye using all Zernike modes through the 7th radial order. Calculated lens decentration using only coma and spherical aberration formulas was peaked at approximately 0.45 mm in the right eye and approximately 0.23 mm in the left eye.

CONCLUSIONS

Lens fall as a function of accommodation was quantified noninvasively using changes in vertical coma driven principally by the accommodation-induced changes in spherical aberration. The newly developed method was valid for a large pupil only.

FINANCIAL DISCLOSURE

Neither author has a financial or proprietary interest in any material or method mentioned.

Semiautomated analysis of optical coherence tomography crystalline lens images under simulated accommodation

Presbyopia is an age related, gradual loss of accommodation, mainly due to changes in the crystalline lens. As part of research efforts to understand and cure this condition, ex vivo, cross-sectional optical coherence tomography images of crystalline lenses were obtained by using the Ex-Vivo Accommodation Simulator (EVAS II) instrument and analyzed to extract their physical and optical properties. Various filters and edge detection methods were applied to isolate the edge contour. An ellipse is fitted to the lens outline to obtain central reference point for transforming the pixel data into the analysis coordinate system. This allows for the fitting of a high order equation to obtain a mathematical description of the edge contour, which obeys constraints of continuity as well as zero to infinite surface slopes from apex to equator. Geometrical parameters of the lens were determined for the lens images captured at different accommodative states. Various curve fitting functions were developed to mathematically describe the anterior and posterior surfaces of the lens. Their differences were evaluated and their suitability for extracting optical performance of the lens was assessed. The robustness of these algorithms was tested by analyzing the same images repeated times.

Age-dependent variation of the Gradient Index profile in human crystalline lenses

PURPOSE: To reconstruct the gradient index (GRIN) profile of human crystalline lenses ex-vivo using Optical Coherence Tomography (OCT) imaging with an optimization technique and to study the dependence of the GRIN profile with age. METHODS: Cross-sectional images of nine isolated human crystalline lenses with ages ranging from 6 to 72 (post mortem time 1 to 4 days) were obtained using a custom-made OCT system. Lenses were extracted from whole cadaver globes and placed in a measurement chamber filled with preservation medium (DMEM). Lenses were imaged with the anterior surface up and then flipped over and imaged again, to obtain posterior lens surface profiles both undistorted and distorted by the refraction through the anterior crystalline lens and GRIN. The GRIN distribution of the lens was described with three variables by means of power function, with variables being the nucleus and surface index, and a power coefficient that describes the decay of the refractive index from the nucleus to the surface. An optimization method was used to search for the parameters that produced the best match of the distorted posterior surface. RESULTS: The distorted surface was simulated with accuracy around the resolution of the OCT system (under 15 µm). The reconstructed refractive index values ranged from 1.356 to 1.388 for the surface, and from 1.396 to 1.434 for the nucleus. The power coefficient ranged between 3 and 18. The power coefficient increased significantly with age, at a rate of 0.24 per year. CONCLUSION: Optical Coherence Tomography allowed optical, non-invasive measurement of the 2-D gradient index profile of the isolated human crystalline lens ex vivo. The age-dependent variation of the changes is consistent with previous data using magnetic resonance imaging, and the progressive formation of a refractive index plateau.

Objective evaluation of the changes in the crystalline lens during accommodation in young and presbyopic populations using Pentacam HR system

AIM

To quantify the changes in the lens profile with accommodation in different age groups.

METHODS

The Pentacam HR system was used to obtain the images of the anterior eye segment from 23 young and 15 presbyopic emmetropic subjects in unaccommodated (with an accommodation stimulus of 0.0D) and accommodated (with an accommodation stimulus of 5.0D for the young group and 1.0D for the presbyopic group) states. The phakic crystalline lens shape, including curvature of crystalline lens and central lens thickness (CLT), and the measurements of anterior segment length (ASL), central anterior chamber depth (CACD) were investigated. The anterior chamber volume (ACV) was also measured.

RESULTS

The reduction of CACD and ACV were significant in both groups after accommodation stimulus. From the profile of anterior eye segment, a significant decrease in anterior crystalline lens radii of curvature (-2.52mm) and a mean increase in CLT (0.222mm) and ASL (0.108mm) were found in the young group with an accommodation stimulus of 5.0D. However, no statistically significant changes of CLT, ASL, or crystalline lens radii of curvature were found in the presbyopic group.

CONCLUSION

Our data showed that the shallowing of anterior chamber during accommodation was caused by the forward bulging of the anterior lens surface, rather than by anterior shifting of lens position in either young or presbyopic subjects.

Transferences of heterocentric astigmatic catadioptric systems including Purkinje systems

PURPOSE

To develop the linear optics of general catadioptric systems with allowance for both astigmatism and heterocentricity.

METHODS

Reflecting elements partition a catadioptric system into subsystems of four distinct types: (unreversed) dioptric subsystems, anterior catoptric subsystems, reversed dioptric subsystems, and posterior catoptric systems. Differential geometry of an arbitrary astigmatic and tilted or decentered surface is used to determine the anterior and posterior catoptric transferences of a surface.

RESULTS

The transference of a catadioptric system is obtained by multiplication of the transferences of unreversed and reversed dioptric subsystems and anterior and posterior catoptric transferences of reflecting elements. Formulae are obtained for the transferences of the visual system of an eye and of six nonvisual systems including the four Purkinje systems.

CONCLUSIONS

The transference can be calculated for a catadioptric system, and from it, one can obtain other optical properties of the system including the dioptric power and the locations of the optical axis and cardinal structures.

Topical and intravenous pilocarpine stimulated accommodation in anesthetized rhesus monkeys

Many studies have used pilocarpine to stimulate accommodation in both humans and monkeys. However, the concentrations of pilocarpine used and the methods of administration vary. In this study, three different methods of pilocarpine administration are evaluated for their effectiveness in stimulating accommodation in rhesus monkeys. Experiments were performed in 17 iridectomized, anesthetized rhesus monkeys aged 4-16 years. Maximum accommodation was stimulated in all these monkeys with a 2% pilocarpine solution maintained on the cornea for at least 30 min in a specially designed perfusion lens. In subsequent topical pilocarpine experiments, baseline refraction was measured with a Hartinger coincidence refractometer and then while the monkeys were upright and facing forward, commercially available pilocarpine (2, 4, or 6%) was applied topically to the cornea as 2 or 4 drops in two applications or 6 drops in three applications over a five minute period with the eyelids closed between applications. Alternatively, while supine, 10-12 drops of pilocarpine were maintained on the cornea in a scleral cup for 5 min. Refraction measurements were begun 5 min after the second application of pilocarpine and continued for at least 30 min after initial administration until no further change in refraction occurred. In intravenous experiments, pilocarpine was given either as boluses ranging from 0.1mg/kg to 2mg/kg or boluses followed by a constant infusion at rates between 3.06 mg/kg/h and 11.6 mg/kg/h. Constant 2% pilocarpine solution on the eye in the perfusion lens produced 10.88+/-2.73 D (mean+/-SD) of accommodation. Topically applied pilocarpine produced 3.81 D+/-2.41, 5.49 D+/-4.08, and 5.55 D+/-3.27 using 2%, 4%, and 6% solutions respectively. When expressed as a percentage of the accommodative response amplitude obtained in the same monkey with constant 2% pilocarpine solution on the eye, the responses were 34.7% for 2% pilocarpine, 48.4% for 4% pilocarpine, and 44.6% for 6% pilocarpine. Topical 4% and 6% pilocarpine achieved similar, variable accommodative responses, but neither achieved maximum accommodation. IV boluses of pilocarpine achieved near maximal levels of accommodation at least ten times faster than topical methods. Doses effective for producing maximum accommodation ranged from 0.25mg/kg to 1.0mg/kg. IV pilocarpine boluses caused an anterior movement of the anterior lens surface, a posterior movement of the posterior lens surface, and a slight net anterior movement of the entire lens. Considerable variability in response amplitude occurred and maximum accommodative amplitude was rarely achieved with topical application of a variety of concentrations of commercially available pilocarpine. Intravenous infusion of pilocarpine was a rapid and reliable method of producing a nearly maximal accommodative response and maintaining accommodation when desired.

Refractive power and biometric properties of the nonhuman primate isolated crystalline lens

Purpose. To characterize the age dependence of shape, refractive power, and refractive index of isolated lenses from nonhuman primates. Methods. Measurements were performed on ex vivo lenses from cynomolgus monkeys (cyno: n = 120; age, 2.7-14.3 years), rhesus monkeys (n = 61; age, 0.7-13.3 years), and hamadryas baboons (baboon: n = 16; age, 1.7-27.3 years). Lens thickness, diameter, and surface curvatures were measured with an optical comparator. Lens refractive power was measured with a custom optical system based on the Scheiner principle. The refractive contributions of the gradient, the surfaces, and the equivalent refractive index were calculated with optical ray-tracing software. The age dependence of the optical and biometric parameters was assessed. Results. Over the measured age range isolated lens thickness decreased (baboon: -0.04, cyno: -0.05, and rhesus: -0.06 mm/y) and equatorial diameter increased (logarithmically for the baboon and rhesus, and linearly for cyno: 0.07 mm/y). The isolated lens surfaces flattened and the corresponding refractive power from the surfaces decreased with age (-0.33, -0.48, and -0.68 D/y). The isolated lens equivalent refractive index decreased (only significant for the baboon, -0.001 D/y), and as a result the total isolated lens refractive power decreased with age (baboon: -1.26, cyno: -0.97, and rhesus: -1.76 D/y). Conclusions. The age-dependent trends in the optical and biometric properties, growth, and aging, of nonhuman primate lenses are similar to those of the pre-presbyopic human lens. As the lens ages, the decrease in refractive contributions from the gradient refractive index causes a rapid age-dependent decrease in maximally accommodated lens refractive power.