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- Refractive errors are abnormalities in the optical components of the eyes that cause light not to be focused on the retinal plane. In order for a person to have clear vision, light entering the eye must precisely focus on the retina.
- 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 based on the optic effects (Appendix, Figure 1):
- Myopia (near-sightedness): Objects are focused in front of the retinal plane. Near vision is clearer, and distance vision is more blurry. Optical correction is with concave lenses (negative power).
- Hyperopia (farsightedness): Objects are focused behind the retinal plane and optical correction contains convex lens (plus power).
- Astigmatism: unequal curvature of the cornea causing the cornea to be more curved in one direction than another (aspherical). The shape of the cornea is more like a football than a basketball in astigmatism.
- Images are blurred at near and far distances.
- Astigmatism can occur concomitantly with myopia and hyperopia.
- Other terms related to refractive error include the following:
- Emmetropia: no refractive error. Objects are focused on the retinal plane.
- Anisometropia: unequal refractive error between the two eyes that increases the risk of amblyopia
- Accommodation: the changing of the shape of the eye’s lens to focus clearly at near or all of the time in hyperopes
Prevalence of refractive errors varies during childhood as the optical components change with development. At birth, usual median refractive error is low hyperopia, approximately +2.00 diopters. In adults, the median is emmetropia; approximately 30% require optical correction.
- In children aged 5 to 17 years, the prevalence of visually significant refractive error varies on type.
- Myopia = 0.7–5.0%
- Hyperopia = 4.0–9.0%
- Astigmatism = 0.5–3.0%
- The prevalence and type of refractive error varies among ethnic groups. For example, people of Native American, Chinese, and Japanese descent have an increased prevalence of myopia.
- Both genetic and environmental factors are important in refractive status. ~60% of myopia can be predicted by parental degree of refraction.
- There is an increased prevalence of visually significant refractive errors in individuals with prematurity, autism, and cerebral palsy.
- 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.
- Environmental factors associated with refractive error include prematurity, eye surgery, and trauma.
- Early detection and correction of refractive errors is important to prevent amblyopia and strabismus. A child should be able to perform a visual acuity examination by age 4 years.
- Children with significant refractive errors are often asymptomatic. All children should be screened for visual acuity in each eye.
- Glasses may not improve vision alone because of amblyopia (maldevelopment of the brain’s ocular cells). Patients with suspected amblyopia should be rechecked even if wearing glasses.
- The three most important determinants of refractive error include the cornea, lens, and axial length of the eye. The cornea and lens bend light to meet the retina in order to create a sharply focused image. The optical power of the cornea and lens must match the actual eye length (distance from cornea to retina). If the cornea and lens do not bend light to hit the retina, there is a refractive error and can blur the vision.
- Small amounts of hyperopia are normal for children. With small hyperopic errors, a child’s eye can easily bring objects into clear focus by adjusting the shape of the lens (accommodation). These children have no problems seeing at far or near distances.
- With larger amounts of hyperopia (>+3.50 diopters), a child’s vision may be blurred at distance and near because accommodation may be limited or cause esotropia (see “Strabismus”).
- The refractive components evolve as the eye develops over childhood. The cornea, lens, and eye length should simultaneously develop to lead to emmetropia. Factors determining the normal and abnormal growth of eye are not completely understood.
- There are likely genetic and environmental factors determining the growth of the eye. There may be an association with increased level of education and increased incidence of myopia. Epidemiologic data suggest 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.
- Anisometropia is associated with nasolacrimal duct obstruction.
- 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.