Astigmatism in monkeys with experimentally induced myopia or hyperopia.

Astigmatism is the most common refractive error, occurring frequently in both healthy (for a review, see reference 1) and diseased eyes.2–4 For instance, in a recent multicentered, school-based study (ages 5–17 years), significant amounts of astigmatism were more prevalent (≥1.0 D, 28.4%) than myopia (≥−0.75 D, 9.2%) or hyperopia (≥+1.25 D, 12.8%).5 However, astigmatic errors, particularly large astigmatic errors, are frequently associated with significant spherical ametropias. Although emmetropes or subjects who have low degrees of spherical ametropia usually also exhibit small amounts of astigmatism, subjects who have high amounts of myopia or hyperopia frequently exhibit high amounts of astigmatism.6–11 In fact, it has been reported that the magnitudes of astigmatism and myopia are linearly correlated in children12 and young adults.7, 13 Why is astigmatism associated with spherical ametropias? Although mechanical factors such as eyelid tension2, 3, 14 and ocular rigidity15 have been implicated in the genesis of astigmatism, little is known about the etiology of astigmatism or the mechanism(s) underlying the association between astigmatism and spherical ametropia. However, our recent studies in monkeys16, 17 indicate that visual experience can alter corneal shape resulting in astigmatism. Specifically, when we reared infant monkeys with cylinder lenses in front of their eyes, we found that the optically imposed astigmatism not only altered emmetropization resulting in axial ametropias,17 but it also promoted the development of significant amounts of astigmatism.16 However, the fact that the axis of the ocular astigmatism was not in the appropriate direction to neutralize the astigmatic errors imposed by the treatment lenses suggested that the eye does not have an active vision-dependent “sphericalization” process analogous to emmetropization.18 Instead, the ocular astigmatism found in the cylinder lens-reared monkeys, which was associated with both axial myopia and hyperopia,17 seemed to be a byproduct of the vision-dependent mechanisms that regulate axial elongation and the emmetropization process. If this is true, then eyes that experienced altered refractive-error development as a result of other visual manipulations should also show a high frequency of astigmatism. A variety of rearing strategies have been shown to predictably alter refractive development. For example, in many different animal species, including humans, form deprivation consistently produces axial myopia (e.g., humans,19 rhesus monkeys,20 marmosets,21 tree shrews,22 and chickens23). Moreover, optically imposing either myopia or hyperopia in young animals with spherical lenses results in compensating axial ametropias that are dependent on the sign and power of the treatment lenses (e.g., rhesus monkeys,24 marmosets,25 tree shrews,26 and chickens27). However, in part because experimentally induced refractive errors have traditionally been expressed in a spherical-equivalent format (i.e., the averaged correction for the two principal astigmatic meridians), only a few studies in chickens have previously noted an association between astigmatism and either axial hyperopia or myopia.28, 29 The purpose of this study was to determine if vision-dependent alterations in axial growth also lead to the development of astigmatism in primates. Specifically, we examined the frequency and characteristics of astigmatism in infant monkeys that developed axial ametropias in response to a variety of different types of altered visual experience.