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Relationship between the axis and degree of high astigmatism and obliquity of palpebral fissure
Relationship Between the Axis and Degree of High Astigmatism and Obliquity of Palpebral Fissure M. Lourdes Garcia, MD, PhD,a,b David Huang, MD, PhD,c Sue Crowe, COTa and Elias I. Traboulsi, MDa Purpose: To investigate a possible relationship between the slanting of palpebral fissures and the magnitude and axis of astigmatism in children with astigmatism. Methods: Cross-sectional study at a referral center of 53 children with astigmatism of more than ⫹1.50 D in at least 1 eye. Visual acuity testing, cycloplegic refraction, slit-lamp biomicroscopy, and ophthalmoscopy were done on every patient. Corneal topography was obtained in 40 cooperative patients. External photographs of the midface were taken in 45 children. The degree of slanting of the palpebral fissures was evaluated based on the photographs. The statistical analysis tool used was repeated measures analysis of variance. Patients in whom photographic analysis was not available were excluded from the part of the statistical analysis dealing with eyelid slant. Results: Palpebral fissure slant (P ⫽ .013) and gender (P ⫽ .0005) were highly correlated with the obliquity of cylinder axis. There was a possible correlation between gender and eyelid slant (P ⫽ .0594), with females having slightly larger degrees of upward palpebral fissure slanting and male more downward slanting of their fissures compared to published angles in an age-matched population. We found a statistically significant correlation between the degree of total astigmatism and a larger abnormal slant (P ⫽ .0192) and between the axis and magnitude of corneal astigmatism and abnormal slant (P ⫽ .0092). Higher degrees of eyelid slant (⬎ 8° or ⬍ ⫺4°) increased the risk of high cylinder magnitude (⬎ 3.00 D) by an odds ratio of 4.17 (95% CI: 1.03, 19.95). Conclusions: Children with astigmatism with large degrees of slanting of their palpebral fissures are at higher risk for high astigmatism (⬎ 3.00 D). The axis of the astigmatism is highly correlated with the slanting of the palpebral fissure. (J AAPOS 2003;7:14-22) stigmatism is a condition in which the parallel rays of light entering the eye through the refractive media are not focused on a single point. There are 2 components of astigmatism: corneal and lenticular. Corneal astigmatism results from unequal corneal curvature in different axes while lenticular astigmatism is caused by variations in lens contour. Astigmatism of more than 0.75 D is present in about 20% of Caucasian adults.1 Corneal astigmatism changes from predominantly with the rule (WTR) in the very young to against the rule (ATR) and oblique (OBL) in older subjects.2 In a study of
A
From the Department of Pediatric Ophthalmology and the Center for Genetic Eye Diseases,a Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio; the IOBA (Instituto de Oftalmobiologia Aplicada),b Universidad de Valladolid, Valladolid, Spain; and the Department of Refractive Surgery,c Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio Presented in part at the American Academy, Dallas, Texas, October, 2000. This study was conducted at the Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio. Submitted January 18, 2002. Revisions accepted August 5, 2002. Reprint requests: Elias I. Traboulsi, MD, The Cole Eye Institute, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. Copyright © 2003 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2003/$35.00 ⫹ 0 doi:10.1016/S1091-8531(03)00055-7
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military personnel, age 20 to 30 years, ATR and WTR astigmatism (equal to or more than 0.25 D) were noted in approximately the same frequency, whereas OBL astigmatism was found in only 8% of eyes with astigmatism.3 The reported prevalence of astigmatism varies depending on diagnostic criteria and studied population.4-6 A population-based survey of ocular disorders among subjects 40 years of age and older4 found that astigmatism of more than 0.5 D is more common in whites than in blacks, and men are affected more than women, with prevalence values ranging form 15.8% to 48.9%. Astigmatism present in the first year of life decreases as the infant grows and reaches adult levels between 18 months and 3.5 years of age.6 Although the cause of astigmatism remains unclear, there is evidence that eyelid and/or corneal pathology alter the shape of the anterior surface of the cornea and result in refractive errors of unequal power in different axes. Although some studies have suggested that genetic factors do not contribute to astigmatism,7 a recent report of 476 individuals from 125 families with astigmatism showed some evidence for the effect of a major autosomal dominant gene, especially in severe cases and when multiple family members were affected.1 The authors suggested Journal of AAPOS
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that the putative gene has more effect on the presence of astigmatism than on its severity. Astigmatism is a prominent feature of some genetic syndromes and appears to be associated with eyelid anomalies.8,9 As many as 82% of patients with Down syndrome have upward slanting of the palpebral fissures, and 60% have astigmatism, which is greater than 3.00 D in 20% of cases.8 In a study of the ocular findings in Treacher Collins syndrome, an overall correlation was found between the severity of facial and eyelid abnormalities, including downward slanting of the palpebral fissures, and the presence of astigmatism; the axis of the negative cylinder was usually in the same quadrant as the horizontal palpebral fissure axis.9 We explored the possibility of a quantitative relationship between the slanting of the lid fissure and the magnitude and axis of astigmatism in nonsyndromic patients.
PATIENTS AND METHODS From December 1999 through June 2000, all children under 18 years of age who had astigmatism greater than ⫹1.5 D were eligible for the study at a referral center. The Institutional Review Board (IRB) approved all investigations. Informed consent was obtained from the parents of all participants. Participation was offered to all but no statistics are available on the total number of patients with this degree of astigmatism who were examined during that period. Patients with ptosis, keratoconus, spina bifida, Down syndrome, and contact lens wear were excluded. Cycloplegic refraction and external photographs of 53 children were done at the time of their office visit. Cycloplegic refraction was performed by retinoscopy by the same examiner (E.I.T.) on every child. Corneal topography was attempted on every subject, but some children were too young or restless to complete the test, therefore data on corneal topography were available only on 40 cooperative subjects. Corneal topography was performed by the same examiner (M.L.G.) on every child using the Humphrey Atlas Corneal Topography System (Zeiss Humphrey, Model 992, Dublin, Calif). The examiners did not have access to eyelid slant angle measurements at the time the refractive status of the children was determined. Adequate photographs, and therefore palpebral fissure measurements, were available on 45 of the 53 patients; in the remaining 8 patients the quality of the photographs did not permit good measurements of palpebral fissure slanting and the patients were not brought back for another set of photographs. All measurements were performed by one of the authors. The examiner did not have refractive data at the time of measuring the eyelid slanting of the children. Facial photographs were taken from a distance of 1 m, with the head of the patient in a chin rest, with the eyes open and the face relaxed. Photographs were scanned and angles were measured using Scion Image software (Scion Corporation, Frederick, Md). The degree of slanting of the palpebral fissures was determined on the basis of the photographs. Two lines
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FIG 1. Measurements of palpebral fissure slanting and cylinder axis tilting. The slanting of the palpebral fissure was considered positive if the axis was above the horizontal and negative if it was below. The horizontal was defined as a prolongation of a line joining the 2 inner canthi. The tilting of the cylinder axis was evaluated in degrees from the vertical meridian. For the right eye, the tilting of the cylinder axis was considered positive if the axis of the plus cylinder was less than 90° (incyclo-positioned) and negative if it was greater than 90° (excyclo-positioned). TABLE 1. Distribution of palpebral fissure slanting and high astigmatism Level Group Group Group Group Group Group Group
1 2 3 4 5 6 7
(⬍ ⫺4°) (⫺1.1° - ⫺4°) (1.9 - ⫺1°) (2° - 4.9°) (5° - 7.9°) (8° - 10.9°) (⬎ 11°)
Count
Percentage
Astigmatism >3D
3 4 13 25 35 8 2
3.33 4.44 14.44 27.77 38.88 8.88 2.22
2 (66.7%) 1 (25.0%) 6 (46.1%) 11 (44.0%) 9 (25.7%) 5 (62.5%) 2 (100%)
were drawn on the scanned image. One line connected the inner and outer canthus of the eye. A second line joining the two inner canthi and extending laterally has been considered equivalent to the Frankfurt horizontal.10 The Frankfurt horizontal is defined as a theoretical line connecting the lowest point of the lower margin of each bony orbit and the highest point on the upper margin of the cutaneous external auditory meatus (the porion). The slant of the palpebral fissure was defined by the angle between the 2 lines (Figure 1). 10 For analysis purposes, patients were stratified into 7 groups according to the degree and direction of slanting of their palpebral fissure, considering an average upward slanting of 3.5° normal for this age group (Table 1). 10 Measurements from both right and left eyes were included. The angle of slanting of the palpebral fissure was empirically considered positive if the axis was above the horizontal and negative if it was below. The horizontal was defined as the Frankfurt horizontal, as described in the previous paragraph. Palpebral fissure slanting was measured 3 times for every eye and the mean value was used for statistical analysis.
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TABLE 2. Refractive measurements Spherical equivalent (D) Magnitude of total astigmatism RE (D) Magnitude of total astigmatism LE (D) Magnitude of corneal astigmatism RE (D) Magnitude of corneal astigmatism LE (D) Tilting of retinoscopy cylinder RE (degrees) Tilting of retinoscopy cylinder LE (degrees) Tilting of topographic cylinder RE (degrees) Tilting of topographic cylinder LE (degrees)
Mean
Standard deviation
Minimum
Maximum
⫺0.95 3.08
3.86 1.15
⫺15.00 1.00
⫹10.75 6.25
3.04
1.40
0.75
7.25
2.81
1.15
0.58
6.33
2.51
1.00
0.47
5.37
⫺0.46
13.51
⫺45
⫹25
⫺0.52
14.01
⫺45
⫹30
⫺0.05
12.80
⫺46
⫹20
⫺4.62
16.81
⫺59
⫹28
LE, left eye; RE, right eye.
Astigmatism was classified as WTR when the axis of the correcting minus cylinder was 180° ⫾ 30°, ATR when the axis was 90° ⫾ 30°, and OBL when the axis was between 31° and 59°, or 121° and 149°.3 The tilting of the cylinder axis was defined as the difference in degrees from the vertical meridian. For the right eye, the tilting of the cylinder axis was considered positive if the axis of the plus cylinder was less than 90° (incyclo-positioned) and negative if it was more than 90° (excyclo-positioned). For the left eye the tilting of the cylinder axis was considered to be negative if the axis of the plus cylinder was less than 90° (excyclo-positioned) and positive if it was more than 90° (incyclo-positioned; Figure 1). Data collected for every patient included visual acuity, demographic and refractive information, and palpebral fissure slanting measurements. Random effects linear models with a compound symmetry covariance structure were used to investigate the relationship of palpebral fissure slant, gender, axis and strength of astigmatism, and other patient covariates. This modeling structure is used to account for the lack of independence due to most subjects having measurements from both eyes included in the analysis. The software used for statistical analysis was SAS/ STAT, Version 8.0, (SAS Institute, Inc, Cary NC). Statistical graphics were constructed using S-PLUS 6.0 (Insightful, Inc, Seattle, Wash). To include data from right and left eye in the same analysis, the extent of symmetry between eyes was assessed for the 53 subjects, with respect to sphere, cylinder, retinoscopic, and topographic astigmatic axes, as well as visual acuity and palpebral fissure slanting. Astigmatic axes or palpebral fissures axes were considered to display mirror symmetry if right and left eye axes occur on opposite sides of the 90° meridian and were of similar magnitude. In 8 patients the quality of the photographs did not permit good measurements of palpebral fissure slanting.
These patients were not included in the analysis of correlation of palpebral fissure slanting with other parameters, but were considered for correlation of refractive status with gender, race, or visual acuity. In 2 patients the astigmatism was against the rule; because the number of subjects was too small, they were excluded from analysis.
RESULTS A total of 27 white and 26 black children were examined and included 29 males and 24 female. Mean age was 8.0 years (SD, 3.4 yrs; range, 1.0-17.0 yrs). Best-corrected visual acuity was better than or equal to 20/30 in at least 1 eye in 45 of the 53 patients (85%). The remaining 15% of patients had some degree of amblyopia in both eyes. There were 13 (24.5%) children who had amblyopia in at least 1 eye. Data on corneal topography were available on 40 (75.4%) subjects. Adequate photographs, and therefore palpebral fissure measurements, were available on 45 of the 53 (84.9%) patients. Descriptive statistics of refractive measurements for both eyes in our patients are displayed in Table 2. Mean spherical equivalent was – 0.95 D (SD, 3.86 D; range, ⫺15.00 D to ⫹10.75 D; median (95% CI), ⫺0.25 (⫺1, 0)). The spherical equivalent was distributed with a symmetrical t distribution with 4 df. Mean sphere for the right eye was ⫺0.90 D (SD, 3.91 D; range, ⫺13.75 D to ⫹10.75 D). In 29 right eyes the spherical component was myopic, in 16 it was hyperopic, and in 8 it was 0. Mean sphere magnitude for the left eye was ⫺0.95 D (SD, 4.02 D; range, ⫺15.00 D to ⫹10.75 D). In 29 left eyes the spherical component was myopic, in 17 it was hyperopic, and in 7 it was nil. Mean magnitude of cylinder obtained by retinoscopy was 3.08 D for the right eye (SD, 1.15 D; range, 1.00 D to 6.25 D) and 3.04 D for the left eye (SD, 1.40 D; range, 0.75 D to 7.25 D). A total of 52 eyes
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FIG 2. Subject symmetry. Best corrected visual acuity, spherical, and cylindrical errors of refraction showed no significant differences between right and left eyes. The graphic shows mirror symmetry of palpebral fissure slants and both retinoscopy and topography astigmatic axes between right and left eyes.
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FIG 3. Corneal astigmatism versus palpebral fissure slant. The correlation between the magnitude of astigmatism (refractive and corneal) and palpebral fissure slant was nonlinear. The data best fit a curve in which total and corneal astigmatism were correlated with the square of palpebral fissure slanting. This correlation was statistically significant (P ⫽ .0267). TABLE 3. High astigmatism (⬎3.0) versus abnormal palpebral slant (⬍ ⫺4° or ⬎ ⫹8°) contingency table Palpebral fissure Normal slant (Groups 2, 3, 4, 5) Abnormal slant (Groups 1, 6, 7) Total
Astigmatism < 3.0
Astigmatism > 3.0
Total
50 (64.9%)
27 (35.1%)
77 (85.6%)
4 (4.4%)
9 (10.0%)
13 (14.4%)
54 (60.0%)
36 (40.0%)
90 (100%)
(47.3%) of 31 patients had a cylinder greater than or equal to 3 D. Mean magnitude of cylinder using corneal topography was 2.81 D for the right eye (SD, 1.15 D; range, 0.58 D to 6.33 D) and 2.51 D for the left eye (SD, 1.00 D; range, 0.47 D to 5.37 D). The axis of the retinoscopic cylinder was tilted between ⫺45° and ⫹25° from the 90° meridian in the right eyes, and between ⫺45° and ⫹30° in the left eyes. The axis of the topographic cylinder was tilted between ⫺46° and ⫹20° from the 90° meridian in the right eyes, and between ⫺59° and ⫹28° in the left eyes. Palpebral fissure slanting from the horizontal axis varied between ⫺6.20° and ⫹11.77° for right eyes (average, 4.48°; SD, 3.59°), and between ⫺5.36° and ⫹13.08° for left eyes (average, 4.38°; SD, 3.65°). Normal palpebral fissure slanting for this age group would be 3.5°10 (group 4), but only 28% of our patients fitted in this group. Most patients had upward slanting of the palpebral fissure with group 5 (5° ⫺ 7.9°) including 35 eyes or 38.9% (Table 1). A total of 20 eyes (22.2%) had a horizontal or downwards slanted palpebral fissure (Table 1). We found mirror symmetry of palpebral fissure slants and both retinoscopy and topography astigmatic axes be-
tween right and left eyes. Best corrected visual acuity, spherical, and cylindrical errors of refraction showed no significant differences between right and left eyes (Figure 2). Mean total or refractive astigmatism as evaluated by retinoscopy was 3.08 D (95% CI: 2.71, 3.39); mean corneal topographic astigmatism magnitude was 2.61 D (95% CI: 2.29, 2.92); and the mean of nontopographic astigmatism (vector difference of refractive and topographic astigmatism) was 0.50 D (95% CI: 0.26, 0.69). Corneal astigmatism was highly correlated with total astigmatism ® ⫽ 0.73), but was not significantly correlated with noncorneal astigmatism (P ⫽ .25). The magnitude of total astigmatism was marginally dependent upon age (P ⫽ .0659) and increased about 0.09 D per year (CI: – 0.009, 0.180). It was independent of gender and race. Neither gender, race, nor age was correlated with the magnitude of corneal astigmatism as measured by corneal topography. For analysis purposes patients were stratified into 7 groups according to the degree and direction of eye slant (Table 1). Mean palpebral fissure slanting for this age group would be around 3.5°, with a standard deviation of 2° according to literature.10 Group 4 was comprised of patients with normal palpebral fissure slanting (2°– 4.9°).10 Groups 3 and 5 included those with a fissure slant within 2 SD. The remaining groups had increasing degrees of upward or downward slanting from the normal range (Table 1). The degree of slanting was compared to the axis of astigmatism, amount of total astigmatism, and corneal astigmatism in all groups. We found a statistically significant higher proportion of high (⬎ 3 D) corneal (P ⫽ .0092) and refractive astigmatism (P ⫽ .0192) in groups 1,
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FIG 4. Axis of astigmatism versus gender. Females had a mean axis slant of 6.09° (95% CI: 1.27, 10.91) whereas males had a mean axis slant of ⫺5.98° (95% CI: ⫺10.36, ⫺1.61).
6, and 7. The odds of a patient with normal or moderate palpebral fissure slanting (groups 2, 3, 4, 5) having more than 3 D of refractive astigmatism is 0.5 while the odds of a patient with abnormally slanted eyes (groups 1, 6, 7) being highly astigmatic is 2.25. The odds ratio of having more than 3.00 D of astigmatism was 4.16 (95% CI: 1.03, 19.95) in patients in groups 1, 6, and 7 compared to groups 2 to 5; ie, children with astigmatism with highly abnormal palpebral fissure slanting have a higher probability of having astigmatism greater than 3.0 D than children with astigmatism with normal slants (Table 3).
The correlation between the magnitude of astigmatism (refractive and corneal) and palpebral fissure slant was nonlinear. The data best fit a curve in which total and corneal astigmatism were correlated with the square of palpebral fissure slanting; with the magnitude of astigmatism increasing as palpebral fissure slanting deviates from the horizontal. This correlation was not statistically significant for refractive astigmatism (P ⫽ .16), but was so for corneal astigmatism (P ⫽ .0267; Figure 3). This model corresponds to the equation: corneal astigmatism (D) ⫽ 2.3955 – 0.04525 PFS (degrees)2, where PFS is palpebral fissure slant.
FIG 5. Axis of astigmatism versus palpebral fissure slant. This graphic shows a linear relationship between the axis of astigmatism and palpebral fissure slant. For every degree of change in eye slant, the axis of astigmatism changes by 1.36° (95% CI: 0.55 – 2.16; P ⫽ .0013).
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TABLE 4. Palpebral fissure slant and gender Fissure slant Up (> 3.5°) Males (50 eyes) Females (40 eyes) Total (90 eyes)
29 (58.0%) 32 (80.0%) 51 (56.7%)
Down (< 3.5°) 21 (42.0%) 8 (20.0%) 29 (32.2%)
The axis of the astigmatic cylinder axis was dependent upon the gender of the patient (P ⫽ .0005). Females had a mean axis slant of 6.09° (95% CI: 1.27, 10.91) whereas males had a mean axis slant of ⫺5.98° (95% CI: ⫺10.36, ⫺1.61; Figure 4). Race and age were not associated with the axis of astigmatism. In Figure 5, palpebral fissure slant is compared to axis of astigmatism and a clear linear relationship is shown. For every degree of change in eye slant, the axis of astigmatism changes by 1.36° (95% CI: 0.55 – 2.16; P ⫽ .0013). The direction and magnitude of eyelid slant was not significantly correlated with age or race. There was a possible correlation between gender and eyelid slant (P ⫽ .0594). Females had slightly larger degrees of upward slanting (average, 5.52; SD, 3.43) and were more likely to have upward slanting (80%) than males who had lesser degrees of upward slanting (average, 3.56; SD, 3.52) in only 58% of cases (95% CI for difference of means: ⫺0.024, 3.94; P ⫽ .06; Table 4). Only females had large upward slanting while mainly males had large downward slanting, indicating that gender might be a confounding factor, children with larger downward fissure slanting having excyclo-positioned axes just because they are males, and children with upwards slanting having incyclo-positioned astigmatic axes because they are females (Figure 6).
Fitting gender and eye slant simultaneously results in a lineal mixed-effects model with a compound symmetry covariance matrix indicating that both palpebral fissure slant (P ⫽ .005) and gender (P ⫽ .0007) are statistically significant predictors of the refractive astigmatism axis. Using this model, restricted maximum likelihood estimates are shown: Cylinder tilt ⫽ ⫺10.7 ⫹1.1 Palpebral Fissure Slant ⫹12.2383 female, where cylinder tilt and palpebral fissure slant are measured in degrees and female is 1 if the patient is female and 0 if the patient is male. Best corrected visual acuity was not correlated with total astigmatism (r ⫽ ⫺0.09), corneal astigmatism (r ⫽ ⫺0.06), or axis of astigmatism (r ⫽ ⫺0.10). The incidence of lower best-corrected visual acuity (visual acuity ⬍ 20/ 25) was not significantly correlated with age, sex, race, slant group, or cylinder magnitude.
DISCUSSION In this study we explored the possibility of a relationship between the slanting of the palpebral fissures and the magnitude and axis of cylinder in astigmatic children. We found evidence of this association and also differences in the slanting of the palpebral fissure between boys and girls. We chose for our study patients with at least 1.5 D of astigmatism because of the propensity of these patients to develop amblyopia. The discovery of associated features would allow an earlier diagnosis and correction. Another reason for setting this limit is that measurements of axis are more reliable in this range, allowing the detection of smaller degrees of tilting of the cylinder axis. The selection of individuals with astigmatism of at least 1.5 D may have led to higher than expected palpebral fissure
FIG 6. Palpebral fissure slant versus gender. Females had slightly larger degrees of upward (average, 5.52; SD, 3.43) and males more downward slanting of their fissures (average, 3.56; SD, 3.52; P ⫽ .0594).
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FIG 7. Relationship between astigmatic axis and palpebral fissure slanting. This boy has a marked downwards slanting of his palpebral fissures. Corneal topography shows excyclo-placed astigmatic axes and high magnitude corneal astigmatism.
slant (mean: 3.5°; SD: 2) for this age group.10 We found mirror symmetry between the 2 eyes for palpebral fissure slant, retinoscopic astigmatic axis, and topography astigmatic axis. Using the mixed effects model allows for the aggregation of right and left eyes in the statistical analysis. Some studies have agreed with these findings for astigmatic axis,11 while others reported different results,12 warning about the necessity to assess this mirror symmetry before actually comparing data from right and left eyes in the same calculations. Solsona11 retrospectively analyzed 51,000 patients with astigmatic corrections equal or higher than 0.75 D and found 67.5% of the population to display mirror symmetry within 10°. McKendrick,12 however, studied 192 adults and found no evidence of a predominance of either direct or mirror symmetry of the astigmatic axes between the 2 eyes, either for astigmatic corrections of more than or less than 0.5 D. We did not find a statistically significant difference in best corrected visual acuity between patients who had more severe astigmatism and those with milder degrees of astigmatism. This can be explained by the fact that all our patients were appropriately treated, wearing glasses with the accurate refractive correction. It is possible that new patients with higher refractive errors might have poorer visual acuities than those with milder defects. We did not stratify our patients into those 2 categories because most were returning patients. It is also true that we selected patients who had at least a moderate amount of astigmatism (⬎ 1.5 D). It is possible that if we had compared our patients to an emmetropic population, we might have found some differences. In our sample we found that roughly six-sevenths of total astigmatism was attributable to the anterior surface of the cornea in children, while the other one-seventh was attributable to other parts of the eye (posterior surface of the cornea and lens curvature or differences in lens refractive index). We found a statistically significant relationship between palpebral fissure slanting and corneal astigmatism. This
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association was significant for both amount and axis of the astigmatism, revealing that the steeper corneal axis was oriented perpendicular to the horizontal of the palpebral fissure (Figure 7). We excluded from our analysis patients with ATR astigmatism, given the small number of subjects that would preclude any conclusion. According to our results, astigmatic males with a palpebral fissure slant of 0° are expected to have the cylinder axis tilted –10.72° (excyclo-positioned) from 90° while females are expected to have cylinder axis tilted 1.52° (incyclo-positioned). The tilting of the axis then increases for both genders at a rate of 1.36° (95% CI: 0.55 – 2.16) of incyclo-positioning per increased degree of palpebral fissure slant (Figure 7). We calculated this using a lineal mixed-effects model with a compound symmetry covariance matrix, to compensate for the fact that we had 2 observations per study participant. Typically linear regression would require independent observations; this model accounts for the dependence as it is shown in the standard errors and confidence intervals for the estimates. We found differences in palpebral fissure slanting and cylinder axis tilting between males and females, with males having more downward fissure slanting and excyclo-positioned astigmatic axes, and females more upward fissure slanting and incyclo-positioned astigmatic axes. But there were no differences between the genders in the relationship between palpebral fissure slanting and cylinder axis tilting, showing an increase in tilting proportional to fissure slanting. To the best of our knowledge, this association has not been previously reported. Palpebral fissure slanting is a universal feature in Asians; our study did not include patients of Asian descent, and further studies should explore the relationship between lid fissure slanting and astigmatism in this population. It would be interesting to study that population where lid fissure slant is significant to determine whether the association found is more mechanical or more genetic. There are 2 possible explanations for the correlation between lid fissure slanting and astigmatism. First, developmental factors may lead to an orchestrated configuration of lid and ocular surface resulting in this final conformation, with a more refractive corneal medium in the axis perpendicular to the eyelid fissure. Second, the mechanical effects of a slanting eyelid on the developing corneal surface may lead to alterations in its curvature. The association with ptosis and lid tumors is well described in infants. Astigmatism usually resolves or is significantly reduced after regression or resection of the tumor.13 Studies on adult patients who develop corneal astigmatism after eyelid surgery14 have proven that these mechanical effects can be temporary, resolving after some months without further treatment in most cases. In adults the cornea appears to be able to recover its original conformation with time if the cause of the astigmatism is just a deforming external mechanical factor. This was also true for children under 4 years of age who underwent eyelid
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surgery for ptosis repair, but not for those over 4 years, in whom astigmatism increased after the surgery.15 We have shown that patients with an abnormal slanting of the palpebral fissure are at higher risk of having high oblique astigmatism, and therefore may be more prone to develop amblyopia if not appropriately treated. The recognition of this association may lead to an earlier detection and treatment of significant errors of astigmatism, preventing the development of amblyopia in this group of patients. Additional studies in groups of patients that were not included in the present study, such as those from other ethnic and racial background and those with ATR astigmatism, will provide additional insight into the relationship between the configuration of the lid fissures and corneal curvature and refractive power. We thank Jason Connor, MS, at the Department of Biostatistics and Epidemiology at the Cleveland Clinic Foundation, for the statistical analysis and expert construction of the graphics. References 1. Clementi M, Angi M, Forabosco P, Di Gianantonia E, Tenconi R. Inheritance of astigmatism: evidence for a major autosomal dominant locus. Am J Hum Genetics 1998;63:825-30. 2. Fledelius HC, Stubgaard M. Changes in refraction and corneal curvature during growth and adult life. A cross–sectional study. Acta Ophthalmol (Copenh) 1986;64:487-91. 3. Satterfield DS. Prevalence and variation of astigmatism in a military population. J Am Optom Assoc 1989;60:14-8.
4. Katz J, Tielsch JM, Sommer A. Prevalence and risk factors for refractive errors in an adult inner city population. Invest Ophthalmol Vis Sci 1997;38:334-40. 5. Angi MR, Pucci V, Forattini F, Formentin PA. Results of photorefractometric screening for amblyogenic defects in children aged 20 months. Behav Brain Res 1992;49:91-7. 6. Saunders KB. Early refractive development in humans. Survey Ophthalmol 1995;40:207-16. 7. Teikari JM, O’Donnell JJ. Astigmatism in 72 twin pairs. Cornea 1989;8:263-6. 8. Da Cunha RP, Moreira JB. Ocular findings in Down syndrome. Am J Ophthalmol 1996;122:236-44. 9. Wang FM, Millman AL, Sidoti PA, Goldberg RB. Ocular findings in Treacher Collins syndrome. Am J Ophthalmol 1990;110:280-6. 10. Hall JG, Froster-Iskenius UG, Allanson JE. Handbook of normal physical measurements. Oxford University Press, Oxford, New York, 1989:132-220. 11. Solsona F. Astigmatism as a congenital bilateral and symmetrical entity. (Observations based on the study of 51,000 patients). Br J Physiol Opt 1975;30:119-27. 12. McKendrick AM, Brennan NA. Distribution of astigmatism in the adult population. J Opt Soc Am 1996;13:206-14. 13. Plager DA, Snyder SK. Resolution of astigmatism after surgical resection of capillary hemangiomas in infants. Ophthalmology 1997; 104:1102-6. 14. Holck DEE, Dutton JJ, Wehrly SR. Changes in astigmatism after ptosis surgery measured by corneal topography. Ophthalmic Plast Reconstr Surg 1998;14:151-8. 15. Cadera W, Orton RB, Hakim O. Changes in astigmatism after surgery for congenital ptosis. J Pediatr Ophthalmol Strabismus 1992;29:85-8.
An Eye on the Arts – The Arts on the Eye
The second time he was caught came a month before the leap of the Escapist. Tommy settled into his seat at the back of the last car and opened his copy of Walter B. Gibson’s Houdini on Magic. Cousin Joe had given it to him the week before; it was signed by the author, the creator of the Shadow, with whom Joe still played cards from time to time. Tommy had his shoes off, his eye patch on, and half a pack of Black Jack in his mouth. He heard a clatter of heels and looked up in time to see his mother, in her sealskin coat, stumble into the train car, out of breath, mashing her best black hat down onto her head with one arm. She was at the opposite end of a relatively full car, and there was a tall man positioned directly in her line of sight. She sat down without noticing her son. This stroke of good fortune took a moment to sink in. He glanced down at the book in his lap. The dark gray wad of gum lay in a small pool of saliva on the left-hand page; it had fallen out of his mouth. He put it back in and lay down across the pair of seats in his row, his face hidden in the hood of his coat and behind the screen of his book. His sense of guilt was exacerbated by the knowledge that Harry Houdini had idolized his own mother and doubtless never would have deceived or hidden from her. At Elmont, the conductor came by to check his ticket, and Tommy scrabbled up onto one elbow. The conductor gave him a skeptical look, and though Tommy had never seen him before, he tapped the patch with a fingertip and tried to echo the nonchalance of Cousin Joe. “Ophthalmologist,” he said. The conductor nodded and punched his ticket. Tommy lay back down. —Michael Chabon (from The Amazing Adventures of Kavalier & Clay)
A
From the Department of Pediatric Ophthalmology and the Center for Genetic Eye Diseases,a Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio; the IOBA (Instituto de Oftalmobiologia Aplicada),b Universidad de Valladolid, Valladolid, Spain; and the Department of Refractive Surgery,c Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio Presented in part at the American Academy, Dallas, Texas, October, 2000. This study was conducted at the Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio. Submitted January 18, 2002. Revisions accepted August 5, 2002. Reprint requests: Elias I. Traboulsi, MD, The Cole Eye Institute, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. Copyright © 2003 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2003/$35.00 ⫹ 0 doi:10.1016/S1091-8531(03)00055-7
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military personnel, age 20 to 30 years, ATR and WTR astigmatism (equal to or more than 0.25 D) were noted in approximately the same frequency, whereas OBL astigmatism was found in only 8% of eyes with astigmatism.3 The reported prevalence of astigmatism varies depending on diagnostic criteria and studied population.4-6 A population-based survey of ocular disorders among subjects 40 years of age and older4 found that astigmatism of more than 0.5 D is more common in whites than in blacks, and men are affected more than women, with prevalence values ranging form 15.8% to 48.9%. Astigmatism present in the first year of life decreases as the infant grows and reaches adult levels between 18 months and 3.5 years of age.6 Although the cause of astigmatism remains unclear, there is evidence that eyelid and/or corneal pathology alter the shape of the anterior surface of the cornea and result in refractive errors of unequal power in different axes. Although some studies have suggested that genetic factors do not contribute to astigmatism,7 a recent report of 476 individuals from 125 families with astigmatism showed some evidence for the effect of a major autosomal dominant gene, especially in severe cases and when multiple family members were affected.1 The authors suggested Journal of AAPOS
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that the putative gene has more effect on the presence of astigmatism than on its severity. Astigmatism is a prominent feature of some genetic syndromes and appears to be associated with eyelid anomalies.8,9 As many as 82% of patients with Down syndrome have upward slanting of the palpebral fissures, and 60% have astigmatism, which is greater than 3.00 D in 20% of cases.8 In a study of the ocular findings in Treacher Collins syndrome, an overall correlation was found between the severity of facial and eyelid abnormalities, including downward slanting of the palpebral fissures, and the presence of astigmatism; the axis of the negative cylinder was usually in the same quadrant as the horizontal palpebral fissure axis.9 We explored the possibility of a quantitative relationship between the slanting of the lid fissure and the magnitude and axis of astigmatism in nonsyndromic patients.
PATIENTS AND METHODS From December 1999 through June 2000, all children under 18 years of age who had astigmatism greater than ⫹1.5 D were eligible for the study at a referral center. The Institutional Review Board (IRB) approved all investigations. Informed consent was obtained from the parents of all participants. Participation was offered to all but no statistics are available on the total number of patients with this degree of astigmatism who were examined during that period. Patients with ptosis, keratoconus, spina bifida, Down syndrome, and contact lens wear were excluded. Cycloplegic refraction and external photographs of 53 children were done at the time of their office visit. Cycloplegic refraction was performed by retinoscopy by the same examiner (E.I.T.) on every child. Corneal topography was attempted on every subject, but some children were too young or restless to complete the test, therefore data on corneal topography were available only on 40 cooperative subjects. Corneal topography was performed by the same examiner (M.L.G.) on every child using the Humphrey Atlas Corneal Topography System (Zeiss Humphrey, Model 992, Dublin, Calif). The examiners did not have access to eyelid slant angle measurements at the time the refractive status of the children was determined. Adequate photographs, and therefore palpebral fissure measurements, were available on 45 of the 53 patients; in the remaining 8 patients the quality of the photographs did not permit good measurements of palpebral fissure slanting and the patients were not brought back for another set of photographs. All measurements were performed by one of the authors. The examiner did not have refractive data at the time of measuring the eyelid slanting of the children. Facial photographs were taken from a distance of 1 m, with the head of the patient in a chin rest, with the eyes open and the face relaxed. Photographs were scanned and angles were measured using Scion Image software (Scion Corporation, Frederick, Md). The degree of slanting of the palpebral fissures was determined on the basis of the photographs. Two lines
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FIG 1. Measurements of palpebral fissure slanting and cylinder axis tilting. The slanting of the palpebral fissure was considered positive if the axis was above the horizontal and negative if it was below. The horizontal was defined as a prolongation of a line joining the 2 inner canthi. The tilting of the cylinder axis was evaluated in degrees from the vertical meridian. For the right eye, the tilting of the cylinder axis was considered positive if the axis of the plus cylinder was less than 90° (incyclo-positioned) and negative if it was greater than 90° (excyclo-positioned). TABLE 1. Distribution of palpebral fissure slanting and high astigmatism Level Group Group Group Group Group Group Group
1 2 3 4 5 6 7
(⬍ ⫺4°) (⫺1.1° - ⫺4°) (1.9 - ⫺1°) (2° - 4.9°) (5° - 7.9°) (8° - 10.9°) (⬎ 11°)
Count
Percentage
Astigmatism >3D
3 4 13 25 35 8 2
3.33 4.44 14.44 27.77 38.88 8.88 2.22
2 (66.7%) 1 (25.0%) 6 (46.1%) 11 (44.0%) 9 (25.7%) 5 (62.5%) 2 (100%)
were drawn on the scanned image. One line connected the inner and outer canthus of the eye. A second line joining the two inner canthi and extending laterally has been considered equivalent to the Frankfurt horizontal.10 The Frankfurt horizontal is defined as a theoretical line connecting the lowest point of the lower margin of each bony orbit and the highest point on the upper margin of the cutaneous external auditory meatus (the porion). The slant of the palpebral fissure was defined by the angle between the 2 lines (Figure 1). 10 For analysis purposes, patients were stratified into 7 groups according to the degree and direction of slanting of their palpebral fissure, considering an average upward slanting of 3.5° normal for this age group (Table 1). 10 Measurements from both right and left eyes were included. The angle of slanting of the palpebral fissure was empirically considered positive if the axis was above the horizontal and negative if it was below. The horizontal was defined as the Frankfurt horizontal, as described in the previous paragraph. Palpebral fissure slanting was measured 3 times for every eye and the mean value was used for statistical analysis.
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TABLE 2. Refractive measurements Spherical equivalent (D) Magnitude of total astigmatism RE (D) Magnitude of total astigmatism LE (D) Magnitude of corneal astigmatism RE (D) Magnitude of corneal astigmatism LE (D) Tilting of retinoscopy cylinder RE (degrees) Tilting of retinoscopy cylinder LE (degrees) Tilting of topographic cylinder RE (degrees) Tilting of topographic cylinder LE (degrees)
Mean
Standard deviation
Minimum
Maximum
⫺0.95 3.08
3.86 1.15
⫺15.00 1.00
⫹10.75 6.25
3.04
1.40
0.75
7.25
2.81
1.15
0.58
6.33
2.51
1.00
0.47
5.37
⫺0.46
13.51
⫺45
⫹25
⫺0.52
14.01
⫺45
⫹30
⫺0.05
12.80
⫺46
⫹20
⫺4.62
16.81
⫺59
⫹28
LE, left eye; RE, right eye.
Astigmatism was classified as WTR when the axis of the correcting minus cylinder was 180° ⫾ 30°, ATR when the axis was 90° ⫾ 30°, and OBL when the axis was between 31° and 59°, or 121° and 149°.3 The tilting of the cylinder axis was defined as the difference in degrees from the vertical meridian. For the right eye, the tilting of the cylinder axis was considered positive if the axis of the plus cylinder was less than 90° (incyclo-positioned) and negative if it was more than 90° (excyclo-positioned). For the left eye the tilting of the cylinder axis was considered to be negative if the axis of the plus cylinder was less than 90° (excyclo-positioned) and positive if it was more than 90° (incyclo-positioned; Figure 1). Data collected for every patient included visual acuity, demographic and refractive information, and palpebral fissure slanting measurements. Random effects linear models with a compound symmetry covariance structure were used to investigate the relationship of palpebral fissure slant, gender, axis and strength of astigmatism, and other patient covariates. This modeling structure is used to account for the lack of independence due to most subjects having measurements from both eyes included in the analysis. The software used for statistical analysis was SAS/ STAT, Version 8.0, (SAS Institute, Inc, Cary NC). Statistical graphics were constructed using S-PLUS 6.0 (Insightful, Inc, Seattle, Wash). To include data from right and left eye in the same analysis, the extent of symmetry between eyes was assessed for the 53 subjects, with respect to sphere, cylinder, retinoscopic, and topographic astigmatic axes, as well as visual acuity and palpebral fissure slanting. Astigmatic axes or palpebral fissures axes were considered to display mirror symmetry if right and left eye axes occur on opposite sides of the 90° meridian and were of similar magnitude. In 8 patients the quality of the photographs did not permit good measurements of palpebral fissure slanting.
These patients were not included in the analysis of correlation of palpebral fissure slanting with other parameters, but were considered for correlation of refractive status with gender, race, or visual acuity. In 2 patients the astigmatism was against the rule; because the number of subjects was too small, they were excluded from analysis.
RESULTS A total of 27 white and 26 black children were examined and included 29 males and 24 female. Mean age was 8.0 years (SD, 3.4 yrs; range, 1.0-17.0 yrs). Best-corrected visual acuity was better than or equal to 20/30 in at least 1 eye in 45 of the 53 patients (85%). The remaining 15% of patients had some degree of amblyopia in both eyes. There were 13 (24.5%) children who had amblyopia in at least 1 eye. Data on corneal topography were available on 40 (75.4%) subjects. Adequate photographs, and therefore palpebral fissure measurements, were available on 45 of the 53 (84.9%) patients. Descriptive statistics of refractive measurements for both eyes in our patients are displayed in Table 2. Mean spherical equivalent was – 0.95 D (SD, 3.86 D; range, ⫺15.00 D to ⫹10.75 D; median (95% CI), ⫺0.25 (⫺1, 0)). The spherical equivalent was distributed with a symmetrical t distribution with 4 df. Mean sphere for the right eye was ⫺0.90 D (SD, 3.91 D; range, ⫺13.75 D to ⫹10.75 D). In 29 right eyes the spherical component was myopic, in 16 it was hyperopic, and in 8 it was 0. Mean sphere magnitude for the left eye was ⫺0.95 D (SD, 4.02 D; range, ⫺15.00 D to ⫹10.75 D). In 29 left eyes the spherical component was myopic, in 17 it was hyperopic, and in 7 it was nil. Mean magnitude of cylinder obtained by retinoscopy was 3.08 D for the right eye (SD, 1.15 D; range, 1.00 D to 6.25 D) and 3.04 D for the left eye (SD, 1.40 D; range, 0.75 D to 7.25 D). A total of 52 eyes
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FIG 2. Subject symmetry. Best corrected visual acuity, spherical, and cylindrical errors of refraction showed no significant differences between right and left eyes. The graphic shows mirror symmetry of palpebral fissure slants and both retinoscopy and topography astigmatic axes between right and left eyes.
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FIG 3. Corneal astigmatism versus palpebral fissure slant. The correlation between the magnitude of astigmatism (refractive and corneal) and palpebral fissure slant was nonlinear. The data best fit a curve in which total and corneal astigmatism were correlated with the square of palpebral fissure slanting. This correlation was statistically significant (P ⫽ .0267). TABLE 3. High astigmatism (⬎3.0) versus abnormal palpebral slant (⬍ ⫺4° or ⬎ ⫹8°) contingency table Palpebral fissure Normal slant (Groups 2, 3, 4, 5) Abnormal slant (Groups 1, 6, 7) Total
Astigmatism < 3.0
Astigmatism > 3.0
Total
50 (64.9%)
27 (35.1%)
77 (85.6%)
4 (4.4%)
9 (10.0%)
13 (14.4%)
54 (60.0%)
36 (40.0%)
90 (100%)
(47.3%) of 31 patients had a cylinder greater than or equal to 3 D. Mean magnitude of cylinder using corneal topography was 2.81 D for the right eye (SD, 1.15 D; range, 0.58 D to 6.33 D) and 2.51 D for the left eye (SD, 1.00 D; range, 0.47 D to 5.37 D). The axis of the retinoscopic cylinder was tilted between ⫺45° and ⫹25° from the 90° meridian in the right eyes, and between ⫺45° and ⫹30° in the left eyes. The axis of the topographic cylinder was tilted between ⫺46° and ⫹20° from the 90° meridian in the right eyes, and between ⫺59° and ⫹28° in the left eyes. Palpebral fissure slanting from the horizontal axis varied between ⫺6.20° and ⫹11.77° for right eyes (average, 4.48°; SD, 3.59°), and between ⫺5.36° and ⫹13.08° for left eyes (average, 4.38°; SD, 3.65°). Normal palpebral fissure slanting for this age group would be 3.5°10 (group 4), but only 28% of our patients fitted in this group. Most patients had upward slanting of the palpebral fissure with group 5 (5° ⫺ 7.9°) including 35 eyes or 38.9% (Table 1). A total of 20 eyes (22.2%) had a horizontal or downwards slanted palpebral fissure (Table 1). We found mirror symmetry of palpebral fissure slants and both retinoscopy and topography astigmatic axes be-
tween right and left eyes. Best corrected visual acuity, spherical, and cylindrical errors of refraction showed no significant differences between right and left eyes (Figure 2). Mean total or refractive astigmatism as evaluated by retinoscopy was 3.08 D (95% CI: 2.71, 3.39); mean corneal topographic astigmatism magnitude was 2.61 D (95% CI: 2.29, 2.92); and the mean of nontopographic astigmatism (vector difference of refractive and topographic astigmatism) was 0.50 D (95% CI: 0.26, 0.69). Corneal astigmatism was highly correlated with total astigmatism ® ⫽ 0.73), but was not significantly correlated with noncorneal astigmatism (P ⫽ .25). The magnitude of total astigmatism was marginally dependent upon age (P ⫽ .0659) and increased about 0.09 D per year (CI: – 0.009, 0.180). It was independent of gender and race. Neither gender, race, nor age was correlated with the magnitude of corneal astigmatism as measured by corneal topography. For analysis purposes patients were stratified into 7 groups according to the degree and direction of eye slant (Table 1). Mean palpebral fissure slanting for this age group would be around 3.5°, with a standard deviation of 2° according to literature.10 Group 4 was comprised of patients with normal palpebral fissure slanting (2°– 4.9°).10 Groups 3 and 5 included those with a fissure slant within 2 SD. The remaining groups had increasing degrees of upward or downward slanting from the normal range (Table 1). The degree of slanting was compared to the axis of astigmatism, amount of total astigmatism, and corneal astigmatism in all groups. We found a statistically significant higher proportion of high (⬎ 3 D) corneal (P ⫽ .0092) and refractive astigmatism (P ⫽ .0192) in groups 1,
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FIG 4. Axis of astigmatism versus gender. Females had a mean axis slant of 6.09° (95% CI: 1.27, 10.91) whereas males had a mean axis slant of ⫺5.98° (95% CI: ⫺10.36, ⫺1.61).
6, and 7. The odds of a patient with normal or moderate palpebral fissure slanting (groups 2, 3, 4, 5) having more than 3 D of refractive astigmatism is 0.5 while the odds of a patient with abnormally slanted eyes (groups 1, 6, 7) being highly astigmatic is 2.25. The odds ratio of having more than 3.00 D of astigmatism was 4.16 (95% CI: 1.03, 19.95) in patients in groups 1, 6, and 7 compared to groups 2 to 5; ie, children with astigmatism with highly abnormal palpebral fissure slanting have a higher probability of having astigmatism greater than 3.0 D than children with astigmatism with normal slants (Table 3).
The correlation between the magnitude of astigmatism (refractive and corneal) and palpebral fissure slant was nonlinear. The data best fit a curve in which total and corneal astigmatism were correlated with the square of palpebral fissure slanting; with the magnitude of astigmatism increasing as palpebral fissure slanting deviates from the horizontal. This correlation was not statistically significant for refractive astigmatism (P ⫽ .16), but was so for corneal astigmatism (P ⫽ .0267; Figure 3). This model corresponds to the equation: corneal astigmatism (D) ⫽ 2.3955 – 0.04525 PFS (degrees)2, where PFS is palpebral fissure slant.
FIG 5. Axis of astigmatism versus palpebral fissure slant. This graphic shows a linear relationship between the axis of astigmatism and palpebral fissure slant. For every degree of change in eye slant, the axis of astigmatism changes by 1.36° (95% CI: 0.55 – 2.16; P ⫽ .0013).
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TABLE 4. Palpebral fissure slant and gender Fissure slant Up (> 3.5°) Males (50 eyes) Females (40 eyes) Total (90 eyes)
29 (58.0%) 32 (80.0%) 51 (56.7%)
Down (< 3.5°) 21 (42.0%) 8 (20.0%) 29 (32.2%)
The axis of the astigmatic cylinder axis was dependent upon the gender of the patient (P ⫽ .0005). Females had a mean axis slant of 6.09° (95% CI: 1.27, 10.91) whereas males had a mean axis slant of ⫺5.98° (95% CI: ⫺10.36, ⫺1.61; Figure 4). Race and age were not associated with the axis of astigmatism. In Figure 5, palpebral fissure slant is compared to axis of astigmatism and a clear linear relationship is shown. For every degree of change in eye slant, the axis of astigmatism changes by 1.36° (95% CI: 0.55 – 2.16; P ⫽ .0013). The direction and magnitude of eyelid slant was not significantly correlated with age or race. There was a possible correlation between gender and eyelid slant (P ⫽ .0594). Females had slightly larger degrees of upward slanting (average, 5.52; SD, 3.43) and were more likely to have upward slanting (80%) than males who had lesser degrees of upward slanting (average, 3.56; SD, 3.52) in only 58% of cases (95% CI for difference of means: ⫺0.024, 3.94; P ⫽ .06; Table 4). Only females had large upward slanting while mainly males had large downward slanting, indicating that gender might be a confounding factor, children with larger downward fissure slanting having excyclo-positioned axes just because they are males, and children with upwards slanting having incyclo-positioned astigmatic axes because they are females (Figure 6).
Fitting gender and eye slant simultaneously results in a lineal mixed-effects model with a compound symmetry covariance matrix indicating that both palpebral fissure slant (P ⫽ .005) and gender (P ⫽ .0007) are statistically significant predictors of the refractive astigmatism axis. Using this model, restricted maximum likelihood estimates are shown: Cylinder tilt ⫽ ⫺10.7 ⫹1.1 Palpebral Fissure Slant ⫹12.2383 female, where cylinder tilt and palpebral fissure slant are measured in degrees and female is 1 if the patient is female and 0 if the patient is male. Best corrected visual acuity was not correlated with total astigmatism (r ⫽ ⫺0.09), corneal astigmatism (r ⫽ ⫺0.06), or axis of astigmatism (r ⫽ ⫺0.10). The incidence of lower best-corrected visual acuity (visual acuity ⬍ 20/ 25) was not significantly correlated with age, sex, race, slant group, or cylinder magnitude.
DISCUSSION In this study we explored the possibility of a relationship between the slanting of the palpebral fissures and the magnitude and axis of cylinder in astigmatic children. We found evidence of this association and also differences in the slanting of the palpebral fissure between boys and girls. We chose for our study patients with at least 1.5 D of astigmatism because of the propensity of these patients to develop amblyopia. The discovery of associated features would allow an earlier diagnosis and correction. Another reason for setting this limit is that measurements of axis are more reliable in this range, allowing the detection of smaller degrees of tilting of the cylinder axis. The selection of individuals with astigmatism of at least 1.5 D may have led to higher than expected palpebral fissure
FIG 6. Palpebral fissure slant versus gender. Females had slightly larger degrees of upward (average, 5.52; SD, 3.43) and males more downward slanting of their fissures (average, 3.56; SD, 3.52; P ⫽ .0594).
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FIG 7. Relationship between astigmatic axis and palpebral fissure slanting. This boy has a marked downwards slanting of his palpebral fissures. Corneal topography shows excyclo-placed astigmatic axes and high magnitude corneal astigmatism.
slant (mean: 3.5°; SD: 2) for this age group.10 We found mirror symmetry between the 2 eyes for palpebral fissure slant, retinoscopic astigmatic axis, and topography astigmatic axis. Using the mixed effects model allows for the aggregation of right and left eyes in the statistical analysis. Some studies have agreed with these findings for astigmatic axis,11 while others reported different results,12 warning about the necessity to assess this mirror symmetry before actually comparing data from right and left eyes in the same calculations. Solsona11 retrospectively analyzed 51,000 patients with astigmatic corrections equal or higher than 0.75 D and found 67.5% of the population to display mirror symmetry within 10°. McKendrick,12 however, studied 192 adults and found no evidence of a predominance of either direct or mirror symmetry of the astigmatic axes between the 2 eyes, either for astigmatic corrections of more than or less than 0.5 D. We did not find a statistically significant difference in best corrected visual acuity between patients who had more severe astigmatism and those with milder degrees of astigmatism. This can be explained by the fact that all our patients were appropriately treated, wearing glasses with the accurate refractive correction. It is possible that new patients with higher refractive errors might have poorer visual acuities than those with milder defects. We did not stratify our patients into those 2 categories because most were returning patients. It is also true that we selected patients who had at least a moderate amount of astigmatism (⬎ 1.5 D). It is possible that if we had compared our patients to an emmetropic population, we might have found some differences. In our sample we found that roughly six-sevenths of total astigmatism was attributable to the anterior surface of the cornea in children, while the other one-seventh was attributable to other parts of the eye (posterior surface of the cornea and lens curvature or differences in lens refractive index). We found a statistically significant relationship between palpebral fissure slanting and corneal astigmatism. This
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association was significant for both amount and axis of the astigmatism, revealing that the steeper corneal axis was oriented perpendicular to the horizontal of the palpebral fissure (Figure 7). We excluded from our analysis patients with ATR astigmatism, given the small number of subjects that would preclude any conclusion. According to our results, astigmatic males with a palpebral fissure slant of 0° are expected to have the cylinder axis tilted –10.72° (excyclo-positioned) from 90° while females are expected to have cylinder axis tilted 1.52° (incyclo-positioned). The tilting of the axis then increases for both genders at a rate of 1.36° (95% CI: 0.55 – 2.16) of incyclo-positioning per increased degree of palpebral fissure slant (Figure 7). We calculated this using a lineal mixed-effects model with a compound symmetry covariance matrix, to compensate for the fact that we had 2 observations per study participant. Typically linear regression would require independent observations; this model accounts for the dependence as it is shown in the standard errors and confidence intervals for the estimates. We found differences in palpebral fissure slanting and cylinder axis tilting between males and females, with males having more downward fissure slanting and excyclo-positioned astigmatic axes, and females more upward fissure slanting and incyclo-positioned astigmatic axes. But there were no differences between the genders in the relationship between palpebral fissure slanting and cylinder axis tilting, showing an increase in tilting proportional to fissure slanting. To the best of our knowledge, this association has not been previously reported. Palpebral fissure slanting is a universal feature in Asians; our study did not include patients of Asian descent, and further studies should explore the relationship between lid fissure slanting and astigmatism in this population. It would be interesting to study that population where lid fissure slant is significant to determine whether the association found is more mechanical or more genetic. There are 2 possible explanations for the correlation between lid fissure slanting and astigmatism. First, developmental factors may lead to an orchestrated configuration of lid and ocular surface resulting in this final conformation, with a more refractive corneal medium in the axis perpendicular to the eyelid fissure. Second, the mechanical effects of a slanting eyelid on the developing corneal surface may lead to alterations in its curvature. The association with ptosis and lid tumors is well described in infants. Astigmatism usually resolves or is significantly reduced after regression or resection of the tumor.13 Studies on adult patients who develop corneal astigmatism after eyelid surgery14 have proven that these mechanical effects can be temporary, resolving after some months without further treatment in most cases. In adults the cornea appears to be able to recover its original conformation with time if the cause of the astigmatism is just a deforming external mechanical factor. This was also true for children under 4 years of age who underwent eyelid
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Journal of AAPOS Volume 7 Number 1 February 2003
Garcia et al
surgery for ptosis repair, but not for those over 4 years, in whom astigmatism increased after the surgery.15 We have shown that patients with an abnormal slanting of the palpebral fissure are at higher risk of having high oblique astigmatism, and therefore may be more prone to develop amblyopia if not appropriately treated. The recognition of this association may lead to an earlier detection and treatment of significant errors of astigmatism, preventing the development of amblyopia in this group of patients. Additional studies in groups of patients that were not included in the present study, such as those from other ethnic and racial background and those with ATR astigmatism, will provide additional insight into the relationship between the configuration of the lid fissures and corneal curvature and refractive power. We thank Jason Connor, MS, at the Department of Biostatistics and Epidemiology at the Cleveland Clinic Foundation, for the statistical analysis and expert construction of the graphics. References 1. Clementi M, Angi M, Forabosco P, Di Gianantonia E, Tenconi R. Inheritance of astigmatism: evidence for a major autosomal dominant locus. Am J Hum Genetics 1998;63:825-30. 2. Fledelius HC, Stubgaard M. Changes in refraction and corneal curvature during growth and adult life. A cross–sectional study. Acta Ophthalmol (Copenh) 1986;64:487-91. 3. Satterfield DS. Prevalence and variation of astigmatism in a military population. J Am Optom Assoc 1989;60:14-8.
4. Katz J, Tielsch JM, Sommer A. Prevalence and risk factors for refractive errors in an adult inner city population. Invest Ophthalmol Vis Sci 1997;38:334-40. 5. Angi MR, Pucci V, Forattini F, Formentin PA. Results of photorefractometric screening for amblyogenic defects in children aged 20 months. Behav Brain Res 1992;49:91-7. 6. Saunders KB. Early refractive development in humans. Survey Ophthalmol 1995;40:207-16. 7. Teikari JM, O’Donnell JJ. Astigmatism in 72 twin pairs. Cornea 1989;8:263-6. 8. Da Cunha RP, Moreira JB. Ocular findings in Down syndrome. Am J Ophthalmol 1996;122:236-44. 9. Wang FM, Millman AL, Sidoti PA, Goldberg RB. Ocular findings in Treacher Collins syndrome. Am J Ophthalmol 1990;110:280-6. 10. Hall JG, Froster-Iskenius UG, Allanson JE. Handbook of normal physical measurements. Oxford University Press, Oxford, New York, 1989:132-220. 11. Solsona F. Astigmatism as a congenital bilateral and symmetrical entity. (Observations based on the study of 51,000 patients). Br J Physiol Opt 1975;30:119-27. 12. McKendrick AM, Brennan NA. Distribution of astigmatism in the adult population. J Opt Soc Am 1996;13:206-14. 13. Plager DA, Snyder SK. Resolution of astigmatism after surgical resection of capillary hemangiomas in infants. Ophthalmology 1997; 104:1102-6. 14. Holck DEE, Dutton JJ, Wehrly SR. Changes in astigmatism after ptosis surgery measured by corneal topography. Ophthalmic Plast Reconstr Surg 1998;14:151-8. 15. Cadera W, Orton RB, Hakim O. Changes in astigmatism after surgery for congenital ptosis. J Pediatr Ophthalmol Strabismus 1992;29:85-8.
An Eye on the Arts – The Arts on the Eye
The second time he was caught came a month before the leap of the Escapist. Tommy settled into his seat at the back of the last car and opened his copy of Walter B. Gibson’s Houdini on Magic. Cousin Joe had given it to him the week before; it was signed by the author, the creator of the Shadow, with whom Joe still played cards from time to time. Tommy had his shoes off, his eye patch on, and half a pack of Black Jack in his mouth. He heard a clatter of heels and looked up in time to see his mother, in her sealskin coat, stumble into the train car, out of breath, mashing her best black hat down onto her head with one arm. She was at the opposite end of a relatively full car, and there was a tall man positioned directly in her line of sight. She sat down without noticing her son. This stroke of good fortune took a moment to sink in. He glanced down at the book in his lap. The dark gray wad of gum lay in a small pool of saliva on the left-hand page; it had fallen out of his mouth. He put it back in and lay down across the pair of seats in his row, his face hidden in the hood of his coat and behind the screen of his book. His sense of guilt was exacerbated by the knowledge that Harry Houdini had idolized his own mother and doubtless never would have deceived or hidden from her. At Elmont, the conductor came by to check his ticket, and Tommy scrabbled up onto one elbow. The conductor gave him a skeptical look, and though Tommy had never seen him before, he tapped the patch with a fingertip and tried to echo the nonchalance of Cousin Joe. “Ophthalmologist,” he said. The conductor nodded and punched his ticket. Tommy lay back down. —Michael Chabon (from The Amazing Adventures of Kavalier & Clay)