Previous studies have been equivocal regarding the relationship between myopia and astigmatism.1–5 Although an association between the degree of sphere and the degree of astigmatism has been identified,1–5 whether and by what mechanism astigmatism may contribute to the progression of myopia remains unclear. In a recent study of infant monkeys, astigmatic defocus resulted in changes in axial length with a trend toward hyperopia,6 suggesting that there is a link between astigmatism and the signals that affect globe elongation/shortening.
We examined patients with familial high myopia with 5 D or greater of spherical refractive error to determine whether the degree of myopia was associated with the presence and degree of total astigmatism, and to assess the risk factors of astigmatism. These patients, whose myopia developed at an early age, are particularly susceptible to conditions that may independently impair visual development or that may contribute to the progression of the myopia itself such as astigmatism.
To our knowledge, this is the first study in the literature to explore the association of astigmatism and spherical refractive error in a large cohort of patients with nonsyndromic familial high myopia; recently, however, Farbrother et al. showed an association between with-the-rule astigmatism and high myopia in a small group of patients with familial high myopia.7 Understanding the relationship between myopia and astigmatism could influence the aggressiveness of visual screening and refractive error correction in these patients. In addition, clarifying the salient clinical features of patients with familial high myopia may provide insight into important genotype–phenotype relationships that may impact patient prognosis.
MATERIALS AND METHODS
We performed a retrospective study of 217 individuals with a mean age of 37.1 years (range = 0.5–91 years). The patient cohort was from families with two or more subjects from successive generations with a myopic spherical refraction of at least -5 D or greater in one eye and at least -3.5 D or greater in the other eye. Individuals who had a history of an ocular insult such as retinopathy of prematurity or ptosis or connective tissue gene mutations that are associated with the development of myopia were excluded from the study.
For those patients who were locally available, refractive error was measured by one of the authors (T.L.Y.) using cycloplegic retinoscopy in individuals <15 years of age and dry manifest refractions in those older. Otherwise, refractive error data were obtained from the individual’s eye care specialist and were based on the most recent refractive error spectacle correction.
Risk factors for the presence and degree of astigmatism were assessed by generalized linear regression. The association between the degree of myopia and cylinder power was examined by correlation analysis. Data from both eyes were included, and the generalized estimating equations approach was used to adjust for the intereye correlation.
Astigmatism was defined as equal to 1.0 D in accordance with multiple studies within the literature.8–10 We assigned astigmatic type as defined in Katz and Kruger11: with-the-rule (WTR) if the plus cylinder axis was within 30° of 90°, against-the-rule (ATR) if the plus cylinder axis was within 30° of 180°, and oblique if the plus cylinder was not WTR or ATR.
The research project was performed in accordance with the tenets of the Declaration of Helsinki and was approved by the Human Subjects Committee Institutional Review Board (IRB) of the Children’s Hospital of Philadelphia (IRB 2003-9-3455).
The characteristics of the patients are summarized in Table 1. Mean spherical refractive component was -10 D (range = 50–3.5 D). Of the 183 patients for whom age of onset data was available, the mean age of myopia onset was 7 years (range = 0–19 years).
In this high myopia cohort, the prevalence of total astigmatism was 36.1%. The mean cylinder was 0.97 D (range = 0–5.75 D). As shown in Table 2, of the 156 eyes that fulfill our criteria for astigmatism, the majority (77.6%) had a low to moderate severity of astigmatism, i.e., between 1.0 and 2.5 D of cylinder. The dominant form of astigmatism was WTR (55.8%). When the cohort was subdivided according to age (i.e., 0–16, 16.1–30, 30+ years of age), WTR astigmatism was the predominant type across all three age groups (Table 4).
Statistically significant associations were found between the presence of astigmatism and risk factors, including age (p = 0.03) and age of myopia onset (p = 0.0001), but not with either gender or spherical equivalent (Table 3).
In those patients with astigmatism, however, there was a moderate correlation between the degree of sphere and cylinder power (Pearson correlation coefficient: r = -0.34, p < 0.0001) (Fig. 1). This association was similar when each eye was tested separately (right eye: r = -0.42, p < 0.0001; left eye: r = –0.26, p = 0.03). However, we did not find an association between a particular type of astigmatism and the degree of myopic sphere (Table 4).
In addition to the degree of sphere, we found that age was associated with a high cylinder power. Patients <16 years old had 0.62-D higher cylinder power than those >16 years old (p = 0.03).
We have presented clinical data on a unique large cohort of patients with nonsyndromic familial high myopia in an effort to delineate the relationship between the severity of myopia with the severity and type of astigmatism. In this group, there is a high prevalence of astigmatism; however, the degree of myopia was not a risk factor for the presence of astigmatism.
The association among astigmatic patients between higher sphere and the severity of astigmatism is in agreement with previous studies.1–5 Our finding that the majority of myopic astigmats have WTR astigmatic axis is consistent with a recent study that demonstrated the relationship in nonfamilial high myopes.5 A trend toward WTR astigmatism has also been highlighted among members of several Native American tribes, including the Tohono O’Odham and Sioux.12, 13 However, among the general population of school-aged to young adult astigmats, WTR astigmatism appears to be the dominant type.14, 15
In contrast to a previous study, our data did not support an association between more severe myopia and an oblique axis of astigmatism.1
In summary, among severely myopic patients, there is a high prevalence of astigmatism that is predominantly WTR. High myopia is not a risk factor for the presence of astigmatism. In those patients with astigmatism, the degree of myopia is correlated with astigmatism severity.
The authors thank the high myopia families for their participation in this project.
Terri L. Young
Division of Ophthalmology
The Children’s Hospital of Philadelphia
34th and Civic Center Blvd.
Philadelphia, Pennsylvania 19104
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Keywords:© 2005 American Academy of Optometry
high myopia; astigmatism; refractive error; with-the-rule astigmatism; oblique astigmatism