A review of astigmatism and its possible genesis

Prevalence of astigmatism in 6- to 72-month-old African American and Hispanic children: the Multi-ethnic Pediatric Eye Disease Study

PURPOSE

To determine the age-, gender-, and ethnicity-specific prevalence of astigmatism in African American and Hispanic children aged 6 to 72 months.

DESIGN

Population-based, cross-sectional study.

PARTICIPANTS

The Multi-Ethnic Pediatric Eye Disease Study is a population-based evaluation of the prevalence of vision disorders in children ages 6 to 72 months in Los Angeles County, California. Seventy-seven percent of eligible children completed a comprehensive eye examination. This report provides the results from 2994 African American and 3030 Hispanic children.

METHODS

Eligible children in 44 census tracts were identified during an in-home interview and scheduled for a comprehensive eye examination and in-clinic interview. Cycloplegic autorefraction was used to determine refractive error.

MAIN OUTCOME MEASURES

The proportion of children with astigmatism defined as cylindrical refractive error ≥ 1.50 diopters (D) in the worse eye. The astigmatism type was defined as with-the-rule (WTR) (+ cylinder axis 90 ± 15 degrees) and against-the-rule (ATR) (+ cylinder axis 180 ± 15 degrees); all other orientations were considered oblique (OBL). The prevalence of astigmatism and its types were also determined for worse eye cylindrical refractive error ≥ 3.00 D.

RESULTS

Prevalence of astigmatism ≥ 1.50 D was higher in Hispanic children compared with African American children (16.8% vs. 12.7%, respectively; P<0.0001). Hispanic children also showed a higher prevalence of astigmatism ≥ 3.00 D than African American children (2.9% vs. 1.0% respectively; P<0.0001). The prevalence of astigmatism ≥ 1.50 D showed a significant decreasing trend with age (P<0.0001). The prevalence of WTR, ATR, and OBL astigmatism ≥ 1.50 D was 13.9%, 0.6%, and 2.2%, respectively, in Hispanic children, and 7.8%, 2.2%, and 2.7%, respectively, in African American children.

CONCLUSIONS

We observed ethnicity-related differences in astigmatism prevalence in preschool children. The age-related decrease in astigmatism prevalence in preschool children likely reflects emmetropization.

Relationships between keratometry and collagen fibrillar structure of cornea by circular polarization biomicroscopy

PURPOSE

To evaluate the relationships between keratometry and collagen fibrillar structure of cornea by circular polarization biomicroscopy.

METHODS

Thirty-four subjects (63 eyes) were included in this study. Intraocular pressure (IOP) was measured, and keratometry was done with an automated keratometer. When the corneal astigmatism was larger than 0.75 diopter, corneal topography was done. After pupil dilation, the examiner placed a circular polarizing filter in front of the subject's eye. The images were captured by a camera connected to the slit lamp. Interfocal distance and angle were measured with ImageJ. The correlations between corneal astigmatism and interfocal distance-angle were examined by double angle vector diagram. Scalar analyses were done for the correlations between subject age and interfocal distance, between IOP and interfocal distance, and between mean cornea power and interfocal distance.

RESULTS

The mean age of the 34 subjects was 59 ± 16 years. Mean interfocal distances and angles determined by double angle vector diagram were 4.67 mm and -20.5 degrees in the right eye and 3.73 mm and +24.2 degrees in the left eye. The correlation between age and interfocal distance and between IOP and interfocal distance were statistically insignificant (P = 0.913 and 0.361, respectively). The interfocal distance and angle showed no statistically significant correlation with corneal astigmatism by vector analysis (P = 0.221 and 0.850, respectively). The fibrillar structure pattern in circular polarizing biomicroscopy showed no statistically significant relationship with the bow tie pattern in corneal topography (P = 0.762). However, analysis of the correlation between mean corneal power and interfocal distance showed a statistically significant, but weak positive correlation (r = 0.326, P = 0.019).

CONCLUSIONS

Interfocal distance determined through circular polarizing biomicroscopy showed a positive correlation with mean corneal power. But age, IOP, and corneal astigmatism did not show a significant correlation with interfocal distance and angle.

Refractive astigmatism in eyes with pseudoexfoliation syndrome

PURPOSE

To determine the effect of pseudoexfoliation syndrome on ocular refractive astigmatism.

METHODS

One hundred eyes of 100 patients with pseudoexfoliation syndrome and 100 eyes of 100 age- and sex-matched control subjects were included in the study. Refractive astigmatism was measured prior to ocular examination performing cycloplegic autorefractive measures using an autorefractokeratometer (Charops, MRK3100).

RESULTS

All patients in the study and control group had myopic astigmatism. Refractive and ocular residual astigmatism were found to be significantly higher in the pseudoexfoliation syndrome group in comparison to the control group (p<0.05). The mean keratometry reading was significantly steeper in the study group than in the control eyes.

CONCLUSIONS

Significant refractive and ocular residual astigmatism were found in pseudoexfoliation syndrome.

Corneal primary aberrations compensation by oblique light incidence

The eye is not a centered system. The line of sight connects the fovea with the center of the pupil and is usually tilted in the temporal direction. Thus, off-axis optical aberrations, mainly coma and oblique astigmatism, are introduced at the fovea. Tabernero et al. [J. Opt. Soc. Am. A 24(10), 3274-3283 (2007)] showed that a horizontal tilt of the crystalline lens generates a horizontal coma aberration that is compensated by the oblique light incidence on the eye. Here we suggest that corneal astigmatism may also play a role in compensation of oblique aberrations, and we propose a simple model to analyze such a possibility. A theoretical Kooijman eye model with a slight ( approximately 0.6 D) with-the-rule astigmatism is analyzed. Light rays at different incidence angles to the optical axis are considered, and the corresponding point spread functions (PSFs) at the retina are calculated. A quality criterion is used to determine the incidence angle that provides the narrowest and highest PSF energy peak. We show that the best image is obtained for a tilted incidence angle compatible with mean values of the angle kappa. This suggests that angle kappa, lens tilt, and corneal astigmatism may combine to provide a passive compensation mechanism to minimize aberrations on the fovea.

Axes of astigmatism in fellow eyes show mirror rather than direct symmetry

PURPOSE

Most astigmats have a similar level of astigmatism in each eye. However, there is controversy over whether the astigmatic axes in fellow eyes typically show direct or mirror symmetry. We carried out a statistical analysis designed to address this issue.

METHODS

The median absolute difference in the astigmatic axes of fellow eyes was calculated for a sample of 50 995 astigmats (subjects with at least 0.25 D of astigmatism in each eye). This was done, firstly, for a 'direct symmetry model' in which the difference in axis was calculated as |AxisR - AxisL| and secondly, for a 'mirror symmetry model' in which the difference in axis was calculated as |AxisR - (180 - AxisL)|.

RESULTS

Under the direct symmetry model, the median absolute difference in the axis of astigmatism between fellow eyes was 20 degrees. Under the mirror symmetry model, the median absolute difference in the axis of astigmatism between fellow eyes was significantly lower, at 10 degrees (p < 10e-100). Comparable results were found when the analysis was restricted to subjects with: lower levels of astigmatism (< or =1.00 D), higher levels of astigmatism (>1.00 D), against-the-rule astigmatism, with-the-rule astigmatism or oblique astigmatism (all p < 10e-100).

CONCLUSION

Our results show that mirror, rather than direct, symmetry is the norm.

Amblyogenic anisometropia in the contralateral eye in unicoronal craniosynostosis

PURPOSE

To describe the prevalence and nature of anisometropic refractive errors in children with unilateral coronal craniosynostosis.

METHODS

Retrospective chart review of children with unilateral coronal craniosynostosis who underwent ophthalmic evaluation, including refraction.

RESULTS

Refraction was available for 25 of 61 children with unilateral coronal craniosynostosis. Of the 25, 56% had amblyogenic anisometropia. Of these, 79% had the greater refractive error contralateral to the coronal synostosis, mostly (91%) consisting of greater with-the-rule astigmatism in the contralateral eye. Seventy-two percent of Hispanic children had amblyogenic anisometropia, compared with only 14% of non-Hispanic children (p= 0.02). Although Hispanic children tended to receive both surgery and ophthalmic evaluation at older ages than non-Hispanic children, these factors did not appear to account for the difference between ethnicities. There was no detectable association between anisometropia and the timing of fronto-orbital advancement surgery.

CONCLUSIONS

Children with unilateral coronal craniosynostosis are at increased risk of developing amblyogenic levels of anisometropia in the eye contralateral to the synostosis and require early evaluation and refraction even in the absence of strabismus.

Distance refractive error among Aboriginal people attending eye clinics in remote South Australia

PURPOSE

To determine the prevalence of distance refractive error among Aboriginal people attending eye clinics in remote South Australia.

METHODS

A clinic-based cross-sectional study was conducted that involved opportunistic sampling of Aboriginal people attending eye clinics in remote South Australia. There were 189 individuals who were invited to participate in the study all of whom underwent ophthalmic examination. This examination included measurement of pinhole-corrected visual acuity and non-cycloplegic autorefraction.

RESULTS

Automated refractive error examinations were performed on 148 people within this sample. The mean age was 44.8 +/- 14.5 years and women comprised 57.7% of the sample. The overall mean refractive error was -0.01 +/- 1.8 D (SD). The prevalence of myopia (spherical equivalent (SE) < -0.5 D), high myopia (SE less than or equal to -6.0 D), hypermetropia (SE > 0.5 D), astigmatism (cylinder at least -0.5 D) and anisometropia (difference in SE of >0.5 D) was 31.1%, 0.7%, 33.1%, 55.8% and 45.9%, respectively. Further analyses revealed significant age-related trends with both myopia and hypermetropia. There were no gender associations with any form of refractive error. Of those people with clinically significant refractive error, 51/148 (34%), only four people owned distance spectacles.

CONCLUSIONS

There continues to be a level of uncorrected distance refractive error within these patients. This represents a need to screen for refractive error among Aboriginal people in remote locations and to provide them with appropriate spectacle correction.