INTRODUCTION — This topic will review refractive errors in children. Vision assessment, strabismus, cataracts, and amblyopia are discussed separately. (See "Vision screening and assessment in infants and children" and "Evaluation and management of strabismus in children" and "Cataract in children" and "Amblyopia in children: Classification, screening, and evaluation".)

Refractive errors in adults are discussed separately. (See "Visual impairment in adults: Refractive disorders and presbyopia".)

NORMAL REFRACTION — Refraction is the bending of light rays as they pass from one transparent medium to another medium with a different density. During vision, light that is reflected from an object is refracted by the cornea and lens and focused on the retina.

In emmetropia (an eye with no refractive error), parallel light rays from a distant object are brought into focus precisely on the retina and a clear image is perceived (movie 1). Perfect emmetropia rarely exists. The majority of individuals have some degree of refractive error, although most do not require correction.

REFRACTIVE ERRORS — Refractive errors are present when the optical image does not accurately focus on the retina. There are three types: myopia (movie 2), hyperopia (movie 3), and astigmatism (movie 4).

Refractive errors requiring correction are uncommon in preschool children [1]. However, nearly 20 percent of children develop refractive errors that require the use of eyeglasses before late adolescence. Risk factors for refractive error include retinopathy of prematurity and family history of high refractive error [2].

Myopia — Myopia (nearsightedness) occurs when the refracting power of the eye is too strong. It commonly occurs when the anterior-posterior diameter of the eye is too long relative to the refracting power of the cornea and lens. The focal point of the image is anterior to the retina, and the image that reaches the retina is blurred (movie 2). Patients with typical levels of myopia have better near vision than distance vision when they are uncorrected.

The prevalence of myopia increases throughout childhood, with the greatest incidence occurring during and after puberty, when the eye undergoes its adolescent growth phase [3]. In the United States, the prevalence of myopia among preschool children is 1 to 5 percent; among school-age children, it increases to approximately 9 percent; and among adolescents, the prevalence is approximately 30 percent [1,4-6].

The prevalence of myopia varies with ethnicity and is particularly high among East Asians, with reported prevalences of 70 to 85 percent among adolescents in China and Taiwan [3,7,8]. Among the school-age population in the United States, Asian children have the highest prevalence of myopia (18.5 percent), followed by Hispanic children (13.2 percent) [5]. The rates of myopia in White (4.4 percent) and African-American children (6.6 percent) were not significantly different.

Myopia is corrected with a concave spherical lens to focus the light rays on the retina (movie 5). Mild myopia often does not require correction. However, myopia of any magnitude should be corrected if it interferes with a child's education or social function. Severe myopia (approximately >5 diopters) should be corrected, even in an apparently asymptomatic child, because of the risk of developing bilateral refractive amblyopia (as known as isoametropic amblyopia). The absolute threshold for when a child should be corrected varies by the caretaker's preference, age of the child, and other factors. In general, treatment should be provided for school-age children with myopia >1.5 to 2 diopters. School children with low levels of myopia can usually compensate by sitting in the front portion of the classroom where they can more easily view material presented on the class whiteboard. (See 'Corrective lenses' below and "Amblyopia in children: Classification, screening, and evaluation", section on 'Refractive amblyopia'.)

Axial length generally increases during childhood, resulting in more severe myopia in adolescence and adulthood. Higher levels of myopia are associated with increased risk of sight-threatening complications later in life (eg, myopic macular degeneration and retinal detachment) [9]. Strategies to prevent or slow the progression of myopia include the following [10]:

●Antimuscarinic eye drops – Several randomized trials have demonstrated that topical antimuscarinic agents (eg, atropine, pirenzepine) are effective in slowing the progression of myopia in children [10-15]. This treatment has been widely used in Asia and is increasingly prescribed for children with myopia in the United States. In a randomized trial of 400 children with myopia, low-dose atropine (0.01% ophthalmic solution) was more effective in slowing the progression of myopia and resulted in fewer visual side effects than higher doses of atropine [15].

●Orthokeratology – Orthokeratology involves the placement of a rigid contact lens on the cornea of a patient with myopia during sleep. The contact lens temporarily alters the shape of the cornea, improving unaided vision during the day when the patient is not wearing the contact lens. Some studies have shown a reduction in myopia progression with orthokeratology [16,17].

●Multifocal lenses – In randomized controlled trials, multifocal lenses (either bifocal or progressive) have yielded a small effect in slowing of myopia progression [10].

●Outdoor activity – Increasing time spent outdoors is a simple strategy to reduce the risk of developing myopia and/or slow its progression and is supported by a number of observational studies and clinical trials [18-22]. In a cluster randomized trial of 1913 school children (mean age of 6.6 years) in China randomized (by school) to an additional daily 40-minute outdoor class or usual activity, the cumulative incidence rate of myopia over three years was lower in the intervention group compared with the control group (30 versus 40 percent) [22].

Hyperopia — Hyperopia (also termed hypermetropia or farsightedness) is the opposite of myopia. Hyperopia occurs when the refracting power of the eye is too weak. The hyperopic eye is too short relative to the refracting power of the cornea and lens. The focal point of the image is posterior to the retina, and the image is blurred when it reaches the retina (movie 3). High degrees of hyperopia are associated with amblyopia and accommodative esotropia. (See "Causes of horizontal strabismus in children", section on 'Accommodative esotropia' and "Amblyopia in children: Classification, screening, and evaluation", section on 'Refractive amblyopia'.)

Mild hyperopia is the normal refractive state for infants and children. However, most children with mild hyperopia do not require refractive correction, because they have the ability to accommodate or focus by contracting the ciliary body (figure 1). This steepens the curvature of the lens (ie, increases its power), which brings the focal point forward and focuses the image appropriately on the retina. Patients who cannot accommodate (eg, after cycloplegic drops) have better distance vision than near vision.

Hyperopia is corrected with a convex spherical lens to focus the light rays on the retina (movie 6). Mild hyperopia generally does not require optical correction in children. Correction may be warranted in asymptomatic children with higher degrees of symmetric hyperopia (ie, >4 diopters) because it can interfere with school-related activities and because there is a risk of developing refractive amblyopia and/or accommodative esotropia. Any degree of hyperopia may warrant correction if the child is symptomatic. (See 'Corrective lenses' below.)

Astigmatism — Astigmatism occurs when the optical system of the eye, particularly the cornea, is not perfectly spherical. The refractive power of the eye is different in different meridians, and the light rays cannot be brought to a single point (movie 4). Astigmatism may occur in combination with myopia or hyperopia. Children with moderate or more severe astigmatism typically have reduced visual acuity at both distance and near fixation.

Astigmatism is corrected with a cylindrical lens (movie 7). Astigmatism should be corrected in symptomatic children and in asymptomatic children with large degrees of astigmatism (ie, approximately >1.5 to 2 diopters) [23]. The threshold for correcting astigmatism is lowered as the child gets older and visual demands increase. (See 'Corrective lenses' below.)

ANISOMETROPIA — Anisometropia occurs when the two eyes have different refractive power. Anisometropia can cause amblyopia. (See "Amblyopia in children: Classification, screening, and evaluation", section on 'Refractive amblyopia'.)

The threshold for anisometropia that is enough to cause amblyopia varies depending upon the type of refractive error:

●Myopia – 2 diopters

●Hyperopia – 1 diopter

●Astigmatism – 1.5 diopters

Anisometropia is treated with refractive correction. If the anisometropia is ≥3 diopters, the size of the retinal image produced by the correction may differ significantly from one eye to the other (ie, aniseikonia), which can cause difficulty in fusing the images. Hyperopic corrections tend to magnify the images, and myopic corrections tend to minify them. Aniseikonia is minimized with contact lenses, which are closer to the optical center of the eye than glasses and thereby produce less image magnification or minification.

Anisometropia also can be treated with refractive surgery in selected children. (See 'Refractive surgery' below.)

TREATMENT — Small refractive errors usually do not require treatment; the threshold for when a child should be corrected depends upon the severity of the refractive error, age of the child, caretaker's preference, and other factors. Symptomatic refractive errors, even if small, should be treated.

Corrective lenses — Most typical refractive errors can be adequately corrected with glasses or contact lenses. When glasses are prescribed, myopia and hyperopia are corrected with spherical lenses (concave and convex lenses, respectively). Concave lenses are minus or divergent; convex lenses are plus or convergent. Astigmatism is neutralized with cylindrical lenses. The details of how contact lenses correct refractive errors are beyond the scope of this topic review.

Refractive surgery — Refractive error can be reduced with extraocular surgical procedures and intraocular surgical procedures. The extraocular procedures include excimer laser procedures (photorefractive keratectomy [PRK], laser in situ keratomileusis [LASIK], and laser-assisted subepithelial keratectomy [LASEK]) and intrastromal corneal rings (INTACS). The excimer laser emits an ultraviolet beam that has sufficient energy to break intermolecular bonds within the cornea ("photoablation"), which results of changing the shape and thus the refracting power of the cornea.

The intraocular procedures include phakic intraocular lens implantation and refractive lensectomy or lens exchange. Phakic intraocular lens implantation is a procedure in which an artificial lens is inserted into the eye (anterior chamber or posterior chamber), while preserving the natural crystalline lens. Refractive lensectomy is a procedure that is essentially the same as cataract surgery, except that the crystalline lens is being replaced because of high refractive error rather than lens opacity.

Refractive surgical procedures are typically performed in adults (see "Laser refractive surgery"). However, in select cases of severe refractive error, refractive surgery may be performed in children to prevent amblyopia, treat amblyopia that is not responding to standard therapy, or as a component of the treatment of amblyopia (eg, in children with severe anisometropia or bilateral severe abnormal refraction [isoametropia] who cannot or will not wear refractive correction) [24].

SUMMARY

●Refractive errors are present when the optical image is not focused on the retina by the natural optical system of the eye. Risk factors for refractive error include retinopathy of prematurity and family history of high refractive error. (See 'Refractive errors' above.)

●Myopia (nearsightedness) occurs when the refracting power of the eye is too strong, most commonly when the anterior-posterior diameter of the eye is too long relative to the refracting power of the cornea and lens (movie 2). The prevalence of myopia increases throughout childhood, particularly during and after puberty. Patients with severe myopia can develop amblyopia, though this occurs less commonly in patients with myopia compared with those with hyperopia. Myopia is corrected with a concave spherical lens (movie 5). Strategies to prevent or slow the progression of myopia in children include antimuscarinic eye drops (eg, atropine, pirenzepine), multifocal lenses (either bifocal or progressive), and outdoor activities. (See 'Myopia' above.)

●Hyperopia occurs when the refracting power of the eye is too weak, most commonly when the eye is too short relative to the refracting power of the cornea and lens (movie 3). Low and moderate degrees of hyperopia can be overcome by the patient through accommodation, and low degrees of hyperopia are typical in infants and young children. High degrees of hyperopia are associated with amblyopia and accommodative esotropia. Hyperopia is corrected with a convex spherical lens (movie 6). (See 'Hyperopia' above.)

●Astigmatism occurs when the optical system of the eye, particularly the cornea, is not perfectly spherical (movie 4). Astigmatism is corrected with a cylindrical lens (movie 7). Astigmatism should be corrected in symptomatic children and in asymptomatic children when it is ≥1.75 to 2 diopters. (See 'Astigmatism' above.)

●Anisometropia occurs when the two eyes have different refractive power. Uncorrected anisometropia may lead to amblyopia. Anisometropia is treated with refractive correction. (See 'Anisometropia' above and "Amblyopia in children: Classification, screening, and evaluation", section on 'Refractive amblyopia'.)

●Small refractive errors often do not require treatment. The threshold for when a child should be corrected depends upon the severity of the refractive error, age of the child, caretaker's preference, and other factors. Symptomatic refractive errors, even if small, should be treated. (See 'Treatment' above.)