DESCRIPTION

• To produce clear vision, light entering the eye must precisely focus on the retina. When it does precisely focus on the retina, it is called emmetropia. When abnormalities in the optical components of the eye cause light not to be focused on the retina, that is called refractive error.
  • Uncorrected refractive errors blur vision in one or both eyes.
  • If left untreated in children, uncorrected refractive errors may cause permanent vision loss from amblyopia and strabismus (see “Amblyopia” and “Strabismus”).
• Refractive errors are measured in diopter units.
• Refractive errors can be classified in three groups:
  • Myopia (nearsightedness): Objects are focused in front of the retina. Distance vision is blurry. Optical correction is with concave lenses (minus power).
  • Hyperopia (farsightedness): Objects are focused behind the retinal plane. Optical correction is with convex lens (plus power).
  • Astigmatism: The cornea is steeper in one direction than the other, causing the shape of the cornea to be more like a football than a basketball. Images are blurred at both near and far distances. Astigmatism can occur concomitantly with myopia and hyperopia.
• Anisometropia refers to asymmetric refractive error between the two eyes.

EPIDEMIOLOGY

The refractive status of the eye changes throughout development. Eyes are typically hyperopic at birth, become slightly more hyperopic until the age of 7 years, and then have a myopic shift until reaching adult dimensions around the age of 16 years.

RISK FACTORS

• Large family studies have reported heritability of refractive error to be 50–60%.
• Children with prematurity, autism, and cerebral palsy are more likely to have refractive error.
• Some genetic syndromes or medical problems associated with refractive errors include the following:
  • Myopia is associated with Stickler, Marfan, Down, and Ehlers-Danlos syndromes.
  • Hyperopia is associated with Senior-Loken syndrome, WAGR (Wilms tumor, aniridia, genitourinary malformations, mental retardation) syndrome, and Down syndrome.
  • Astigmatism is associated with Down syndrome, craniofacial abnormalities, and albinism.
  • Eye surgery and trauma can cause refractive error.

PATHOPHYSIOLOGY

• The three most important determinants of refractive error are the cornea, lens, and axial length of the eye. The cornea and lens bend light in order to create a sharply focused image on the retina. If the light does not focus on the retina, there is a refractive error (see Appendix IV, Figure 1).
• Small amounts of hyperopia are normal for children. In children with hyperopia, their eyes can easily bring objects into clear focus by adjusting the shape of the lens (accommodating).
• With larger amounts of hyperopia (>3.50 diopters), a child’s vision may be blurred at distance and near because accommodation may be limited. Large amounts of hyperopia can also cause esotropia (see “Strabismus”).
• During childhood, the cornea, lens, and axial length should develop in concert to lead to emmetropia. Factors determining the normal and abnormal growth of the eye are not completely understood.
• There are genetic and environmental factors that influence the growth of the eye. Epidemiologic data indicate that increased amount of time spent outdoors protects against the development of myopia.

COMMONLY ASSOCIATED CONDITIONS

Refractive errors are frequently associated with other ocular conditions.

• Myopia is associated with childhood glaucoma, deprivation amblyopia, retinopathy of prematurity, retinal dystrophies, coloboma, and retinal detachments.
• Hyperopia is associated with esotropia, Leber congenital amaurosis, and aphakia (absence of lens).
• Astigmatism is associated with ptosis, coloboma, glaucoma, retinopathy of prematurity, lid hemangioma, nystagmus, and limbal dermoid.