Ocular Astigmatism Research Articles

Changes in objective refraction after pharmacological pupil dilation

Abstract Purpose To analyze the variation of the objective refraction induced by pharmacological pupillary dilation in a normal population. Methods The objective refraction obtained by an auto‐kerato‐refractometer was analyzed before and after maximum pupillary dilation obtained by 0.5% Tropicamide and 10% Phenylephrine. 200 phakic eyes of 100 patients aged from 15 to 76 years were prospectively included. The refractive data were expressed in a dioptric space by 3 Cartesian coordinates (X, Y, Z). Results After pupil dilation, the mean refraction ranges from ‐0.08(+0.11)×90.2° to +0.41(+0.10)×90.3° this equal to an astigmatic vectorial variation of (+0.03)×89.6°. The spherical equivalent increases significantly (hyperopization of +0.49D: +0.24D vs +0.74D p<0.001) contrary to the mean keratometry (43.48D vs 43.49 p=0.40) and the cylinder (0.64D vs 0.64D p=0.49) which are unchanged. Change in spherical equivalent is correlated with the age (r=‐0.34 p<0.001) but not with the change in keratometry (r=0.06 p=0.20) or the initial spherical equivalent (r=‐0.05 p=0.23). The mean change in axis between both exams is 15.4° for the ocular astigmatism and 12.2° for the corneal astigmatism. The modification in spherical equivalent of both eyes of a patient are not different (p=0.26) and are strongly correlated (r=0.85 p<0.001) contrary to the modification in axis and in cylinder. Conclusion Pharmacological pupil dilation modifies primarily the spherical equivalent without significant modification of keratometry and astigmatism. Finally, hyperopization is probably due to the cycloplegic effect of eye drops.

Age-related changes in the human visual system and prevalence of refractive conditions in patients attending an eye clinic

To retrospectively report the trends of change in several parameters of the human visual system over a wide age range in patients attending an eye clinic. University of Valencia, Valencia, Spain. The clinical records of 2654 patients were retrospectively reviewed, and the age, sex, spherocylindrical refraction, visual acuity, keratometry, and intraocular pressure were obtained. Descriptive values for each parameter and the correlations with age and between different parameters were calculated. Vectorial components of refraction, including blur, were also derived from clinical refractive data and then analyzed. Several parameters changed significantly with age, particularly in patients in their sixties and older. An increase in the blur component was mainly associated with astigmatic progression and a trend toward against-the-rule orientation and had the highest correlation with total astigmatism (r= -0.319; P<.001) and visual acuity (r= -0.442; P<.001). Refractive conditions had the most homogeneous distribution in the first decade of life and the most heterogeneous distribution in the group between 61 years and 70 years. Best corrected visual acuity began to decrease after the 50s, while changes in the blur component were not patent until the 60s to 70s. This could be explained by the poorer optical quality of the human eye in adulthood and elderly persons. Clinically, these changes could be attributed to changes in ocular astigmatism and have an impact on the best visual acuity achievable with optical compensation.

In chicks wearing high powered negative lenses, spherical refraction is compensated and oblique astigmatism is induced

Chicks develop refractive errors in response to defocusing lenses. Mean ocular refractions (MOR) and astigmatism were measured in chicks that developed myopia in response to unilaterally applied negative lenses (-30 D, n = 19; -15 D, n = 16). The MOR of the control eyes emmetropized with age and both goggle powers were fully compensated given enough time. The initial slopes of plots of MOR with age are not significantly different between inducing lens powers. Astigmatism in control eyes decreased significantly with age. In the presence of -30 D lenses, astigmatism significantly increased with age due to an exponential increase in JCC45 (oblique Jackson crossed cylinder). Although not statistically significant, a similar but smaller trend was observed for the -15 D lenses. Thus, oblique astigmatism results during myopia induction by high-powered lenses. This induced astigmatism is correlated with MOR. These results are consistent with emmetropization of astigmatism, which is disrupted by high-powered spherical lenses.

Evaluation of Internal Refraction with the Optical Path Difference Scan

To evaluate internal refraction and its relation to other optical properties of the eye across a large range of refractive errors, which can provide useful information for the assessment and design of intraocular lenses and corneal ablation patterns. Cohort study. Three hundred ninety-four eyes of 197 healthy subjects with a mean age of 27+/-7 years (range, 18-42). All eyes underwent optical path difference scans to evaluate corneal topography, whole and internal ocular refraction (determined by the subtraction of corneal refraction from whole ocular refraction), and total and higher-order aberrations (HOAs) were assessed. After the conversion of any spherocylindrical refractive errors to vectorial data (sphere equivalent, blurring strength; cylinder, J(0) [power of Jackson cross cylinder at 90 degrees and 180 degrees] and J45 [power of Jackson cross cylinder at 45 degrees and 135 degrees]), the distribution of internal refraction among refraction groups (high myopes, low to moderate myopes, hyperopes, mixed astigmats, and emmetropes) and relationships between internal refraction, corneal refraction, and wavefront aberrations were analyzed. The compensation relation and its rate between corneal and internal astigmatism was assessed by the compensation factor (CF). Whole ocular power and astigmatism, corneal power and astigmatism, internal power and astigmatism, CF for astigmatic data, location of the highest internal refraction zone, and wavefront aberrations. The highest refraction zone was mostly (90%) located in the center in myopes, whereas it was located at the nasal side (71%) in hyperopes. There was a significant correlation between whole ocular and internal powers (P<0.01), but no correlation was observed between corneal and internal powers (P>0.05). Internal astigmatism was mostly against the rule. The mean CF J(0) was 0.63+/-3.78 and CF J(45) was 0.57+/-2.47. The magnitude of the internal astigmatism under the 3-mm zone was correlated with the magnitude of corneal astigmatism (P<0.05). The distribution of astigmatic CF differed among refraction groups. There were significant correlations between internal power and spherical aberration (P<0.05) and internal cylinder under the 5-mm zone and HOAs (P<0.001). There is a remarkable tilt in internal refraction in hyperopes. Although there is a tendency of undercompensation of the corneal astigmatism by internal astigmatism in the entire group of eyes, the compensation differs among refraction groups.

Changes in astigmatism with accommodation

An experiment is described in which changes in ocular astigmatism were recorded over an accommodation stimulus range of 0-4 D, in the right eyes of a group of 31 normal subjects aged 17-37 years. The visual axes were horizontal and the observation conditions were monocular. Although subjects had a range of spherical ametropia, all had initial astigmatism of 1.0 DC or less. It was found that, for most subjects in the group, astigmatism changed in the with-the-rule direction, the mean change being -0.036 DC at axis 176 deg for each dioptre of accommodation response. It is speculated that these changes might reflect increases in lens tilt about a horizontal axis, caused by the combined effects of a slacker zonule and gravity.

Astigmatism in infant monkeys reared with cylindrical lenses

To determine whether developing primate eyes are capable of growing in a manner that eliminates astigmatism, we reared infant monkeys with cylindrical spectacle lenses in front of one or both eyes that optically simulated with-the-rule, against-the-rule, or oblique astigmatism (+1.50−3.00×90, × 180, × 45 or × 135). Refractive development was assessed by retinoscopy, keratometry and A-scan ultrasonography. In contrast to control monkeys, the cylinder-lens-reared monkeys developed significant amounts of astigmatism. The astigmatism was corneal in nature, bilaterally mirror symmetric and oblique in axis, and reversible. The ocular astigmatism appeared to be due to a reduction in the rate of corneal flattening along the steeper meridian while the other principal meridian appeared to flatten at a more normal rate. However, regardless of the orientation of the optically imposed astigmatism, the axis of the ocular astigmatism was not appropriate to compensate for the astigmatic error imposed by the treatment lenses. Our results indicate that visual experience can alter corneal shape, but there was no evidence that primates have an active, visually regulated “sphericalization” mechanism.

Effect of partially coherent light on the subjective measurement of ocular astigmatism

In this paper, we study the effect of meridional accommodation response of the eye, in the measurement of ocular astigmatism, using different targets (single slit, multiple slit and interference fringes) under different illumination conditions (white light, quasimonochromatic light and partially coherent light) in a subjective optometer. We have found that, under natural illumination conditions (incoherent white light), the accommodation of a subject viewing different orientations of a target varies in a manner that compensates for his or her ocular astigmatism. The compensation effect is minimum when interference fringes are used as targets. We also form interference images of different orientations, directly on the retina of the eye, using the meridional refraction properties of the eye and an optical field that has coherence only along specific meridians. Use of interference images formed directly on the retina yields values of cylinder that compares well with objective measurements obtained using an autorefractor.

Lau optometer: A new subjective instrument to measure the astigmatism of the human eye

We have developed an optometer to measure the ocular astigmatism of human eyes and tested its performance. In this instrument we make use of an optical field that has coherence only along specific meridians to form interference images of different orientations on the subject's retina. The subject sees fringes of different orientations sequentially, at a fixed vergence, by rotating an eyepiece consisting of a probe cylinder. From the angular positions of the eyepiece at which each of the fringes are seen best by the subject, his or her astigmatism is calculated. The instrument has the potential to probe meridional vision and accommodation characteristics of human eyes.

Effect of partially coherent light on the subjective measurement of ocular astigmatism

Abstract In this paper, we study the effect of meridional accommodation response of the eye, in the measurement of ocular astigmatism, using different targets (single slit, multiple slit and interference fringes) under different illumination conditions (white light, quasimonochromatic light and partially coherent light) in a subjective optometer. We have found that, under natural illumination conditions (incoherent white light), the accommodation of a subject viewing different orientations of a target varies in a manner that compensates for his or her ocular astigmatism. The compensation effect is minimum when interference fringes are used as targets. We also form interference images of different orientations, directly on the retina of the eye, using the meridional refraction properties of the eye and an optical field that has coherence only along specific meridians. Use of interference images formed directly on the retina yields values of cylinder that compares well with objective measurements obtained using an autorefractor.

Lau optometer: a new subjective instrument to measure the astigmatism of the human eye

Abstract We have developed an optometer to measure the ocular astigmatism of human eyes and tested its performance. In this instrument we make use of an optical field that has coherence only along specific meridians to form interference images of different orientations on the subject's retina. The subject sees fringes of different orientations sequentially, at a fixed vergence, by rotating an eyepiece consisting of a probe cylinder. From the angular positions of the eyepiece at which each of the fringes are seen best by the subject, his or her astigmatism is calculated. The instrument has the potential to probe meridional vision and accommodation characteristics of human eyes.

Topographical analysis of corneal astigmatism in patients with tilted-disc syndrome.

To evaluate the corneal topography in patients with tilted-disc syndrome to determine the relationship between optic disc dysversion and corneal astigmatism and the pattern of astigmatism in these patients. The study included 23 eyes of the 13 tilted-disc syndrome patients with spheric refractive errors ranging between +1.00 D and -9.00 D (mean -4.00 +/- 3.4 D) and astigmatic errors ranging between -0.50 and -4.50 D (mean -1.95 +/- 0.93 D). Corneal topography was performed by computer-assisted videokeratoscope topographic modelling system 2 (TMS-2) and incidence of corneal astigmatism, corneal topographic patterns, and mean values of the topographic indices were determined. Corneal topographic analysis showed corneal astigmatism in 22 out of 23 patients with tilted discs. Corneal astigmatism was symmetric bow tie pattern in 10 eyes (45.45%), asymmetric bow tie pattern in 11 eyes (50%) and irregular in 1 eye (4.5%). Among the patients with bow tie pattern group (21 eyes), 14 eyes had with-the-rule astigmatism, 1 eye had against-the-rule astigmatism, and 6 eyes had oblique astigmatism. In 18 eyes, astigmatism was corneal, whereas combined corneal and lenticular in 4 eyes and lenticular in 1 eye. In the majority of tilted-disc cases, ocular astigmatism is mainly corneal. Morphogenetic factors in the development of the tilted disc might possibly influence the corneal development in such a way to result in corneal astigmatism.

Analysis of astigmatism in anterior segment surgery

Purpose To demonstrate techniques for analyzing astigmatism in the context of anterior segment surgery. The techniques are applied to refractive and corneal astigmatism and to astigmatism associated with the surgery. Method The analysis is based on the 3-dimensional first-order ray optics known as linear optics, in which astigmatism is axial. Anterior segment surgery, whose only direct effect is to change corneal curvature, is optically equivalent to placing a thin lens immediately in front of the (preoperative) eye; the surgery can be described as anterior thin-lens equivalent. In principle, however, surgery may also change the relative axial positions of refracting elements. In such cases, anterior segment surgery is anterior thick-system equivalent and the analysis of astigmatism is considerably more complicated, with astigmatism manifesting in power and in other ways. Fundamentally, the method is based on the ray transference, a 4 × 4 matrix that contains the 4 fundamental optical properties of a system. Each property may contain an antistigmatic component, and it is this component that is analyzed. Results The data are not consistent with the anterior thin-lens model of anterior segment surgery but rather with the anterior thick-lens model. Many components of refraction and corneal power, their means, and their surgically induced changes were calculated and are presented in tabular and graphical form. Graphical representations include polar profiles and stereo-pair scatterplots in dioptric power space. Results are also presented for all the fundamental properties of the surgery. Conclusion Ocular astigmatism can manifest in more ways than just refraction and corneal power. Traditional approaches, which treat astigmatism as cylinder in some sense and only as manifested in power, are unsatisfactory. In particular, current approaches weight astigmatism too heavily relative to stigmatism and to other optical phenomena and are inadequate for thick systems such as the eye. The ray transference and the 4 fundamental properties it contains provide the basis for the only methods that can adequately cope with analysis of astigmatism in general and in surgically induced astigmatism in particular.

Double-pass measurements of retinal image quality in monofocal contact lens wearers

The double-pass method is applied to determine the optical image quality in monofocal contact lens (CL) wearers. This is an objective non-invasive technique that permits in vivo testing of the optical performance of CL wearers' eyes. Retinal image quality was measured for three subjects wearing two types of monofocal CLs: a rigid gas permeable (RGP) CL and a soft contact lens (SL), for pupil diameters of 3 mm and 5 mm. Results show the importance of ocular astigmatism regarding retinal image quality. In eyes presenting corneal astigmatism, the best results are obtained when wearing RGP CLs, because the lens compensates the corneal astigmatism. The modulation transfer function (MTF) is considerably smaller when no lens or soft lenses are worn, even for small amounts of astigmatism (0.5 D). When the astigmatism is corrected, the retinal image quality obtained with both types of CLs and with no lens is similar.

Topographic Evaluation of Toric Soft Contact Lens Correction

The anterior surface of the eye provides the greatest refracting surface to the optical system of the eye because of the large difference in refractive index between air and tissue. When a contact lens is placed on the eye, its anterior surface becomes the greatest refracting surface. Toric soft contact lenses have been designed to neutralize ocular astigmatism. We observed that topographic analysis of toric soft contact lenses on an astigmatic eye demonstrated residual astigmatism on the anterior surface of soft toric contact lenses. In this paper we show toric soft contact lens surface astigmatism by videokeratoscopy and interpret the origins of the astigmatism.

Optical Aspects of Tolerances to Uncorrected Ocular Astigmatism

We approached the question of the visual tolerance to uncorrected astigmatism by exploring the changes in the retinal diffraction images brought about by astigmatism, as a function of spherical focus. Consideration of the Rayleigh and Maréchal wavefront criteria and of the changes in ocular modulation transfer function (MTF) broadly supports a value of 0.25 DC as a practical tolerance limit for uncorrected astigmatism. This study underlines the limitations of a geometrical optical approach to this problem in particular, and shows that the concept of the circle of least confusion may be misleading.

Changes in Ocular Astigmatism Over the Whole Range of Accommodation

The refraction of 20 subjects was measured over the whole range of accommodation using an objective automated refractometer in order to investigate the astigmatic change whose origin is presumed to be at the lens. For this purpose, the corneal vertex was used as the plane of reference to avoid changes in the optical effectively of a cylindrical lens at near. The change in the astigmatism with accommodation was observed in each subject. Subjects were then divided into three groups: (1) seven subjects showed an increase in astigmatism with accommodation; (2) eight subjects showed constant astigmatism up to the accommodative nearpoint; and (3) five subjects showed other types of change, such as reduced astigmatism in the intermediate accommodative state. These variations suggest that accommodative astigmatism is affected by many elements such as lens, zonule, and muscle. Two of 20 subjects could see the vertical line of the visual target composed of many directional lines clearer than the horizontal line at distance. Conversely, they could see the horizontal line clearer than the vertical line at near. We also studied the correspondence between the dark focus of accommodation and the dioptric distance where astigmatism is at a minimum, as suggested previously. The results showed that no such direct relation existed.

Bitoric hard and soft lenses in marked ocularastigmatism

A patient with significant ocular astigmatism of corneal and lenticular origin was fitted with bitoric gas permeablehard lenses. As a result of poor centration and subsequent staining a bitoric medium water content soft hydrophilic lens was fitted with good results.

The Jackson Cross Cylinder: A Reappraisal

The Jackson cross cylinder is a frequently used but poorly understood tool for clinical refraction. A critical analysis of the principles of cross cylinder testing reveals several important features which should be understood for the proper use of this device. For example, a rarely discussed shortcoming of the cross cylinder is that it can fail to detect even large amounts of ocular astigmatism in certain situations. A little-known advantage of the cross cylinder, on the other hand, is its use in refining cylinder power at an incorrect axis. This particular property of the cross cylinder is useful when the correcting cylinder axis must be rotated toward 90°,180°, or toward the old axis. The cross cylinder can be used to determine the optimal cylinder power for this new axis setting. Guidelines are also considered for the appropriate step sizes in axis rotation that should be used in cylinder axis refinement.

Why is ocular astigmatism regular?

Why is ocular astigmatism regular?

Electrophysiological evidence that abnormal early visual experience can modify the human brain.

Visual resolution in humans is nearly equal for vertically and horizontally oriented detail, but for some subjects there is a substantial difference in resolving power for these orientations. Although subjects who exhibit this difference invariably have ocular astigmatism, optical explanations of the effect can be ruled out. Direct evidence has been found for an electrophysiological correlate to the psychophysical finding. Subjects who have reduced resolution for a pattern of a particular orientation also show a decreased evoked potential response elicited by a target of the same orientation. The results are consistent with the hypothesis that a deficiency of specific features in the early visual input can alter the organization of the visual pathways.