The human eye functions like a camera, using the cornea and the internal lens to focus a clear image on the visual center of the retina (fovea). From there, the image is relayed through the visual pathways in the central nervous system to the corresponding occipital cortex of either side of the brain, where the information can be processed and stored. The coordinated effort of both eyes properly aligned allows the image of one eye to be fused with the image of the other eye, permitting the appreciation of depth perception (stereopsis). Any disruptive factor that prevents the eyes from seeing equally, fusing their images, or working together (coordination), whether it be an imbalance of refractive errors between the eyes (anisometropia), a lens opacity obstructing normal vision (cataract), or an ocular misalignment (strabismus), can potentially cause loss of vision (amblyopia). If amblyopia is not treated in a timely manner, this visual loss can persist throughout adulthood.
It is important for practitioners, particularly pediatricians, to be aware of the normal development of a child's eyes and its relation to normal visual acuity and fusion. Unlike adults, with children we are working against time. The window of opportunity to correct refractive errors and eyeglass-related strabismic conditions, which may cause amblyopia, is limited.
This article explains the developmental and refractive changes occurring in a child's eyes. The indications for eyeglasses are outlined and various conditions are discussed. Recommendations for appropriate screening and referral to pediatric vision specialists are offered.
The three main refractive conditions causing visual deficit are generally related to the size and the shape of the child's eyes. In nearsightedness (myopia), the axial length (from front to back) is longer than normal (emmetropia). In farsightedness (hyperopia), the axial length is shorter (Fig. 1). In these conditions, the object that should be focused on the retina is focused either in front of or behind the retinal surface, resulting in a blurred image. Concave lenses that diverge light rays (for myopia) and convex lenses that converge light rays (for hyperopia) are used to focus the object's image on the appropriate retinal point.
In the third refractive disorder, astigmatism, the corneal surface is not uniformly spherical. Light rays are bent and focused along another axis, giving a distorted image at near and far distances (Fig. 2). This can be corrected by a cylindrical lens set at the appropriate axis. Many children have a combination of either myopia and astigmatism or hyperopia and astigmatism (compound refractive errors). It appears that eyeglasses are being prescribed more often for children. This may be due to positive factors, such as early detection and treatment, or negative factors, such as an increased incidence of refractive errors.
Figure 1 . A schematic representation of the three refractive states in the eye that are dependent on size. In diagram A (emmetropia), the eye is a normal size and no lens is required to focus the image on the retina. In diagram B (hyperopia), the eye is smaller than normal and a convex lens is needed to converge the light rays to focus the image on the retina. In diagram C (myopia), the eye is larger and a concave lens is needed to diverge the light rays to focus the image on the retina.
Eye Size and Refractive Changes
The size of a child's eye changes throughout normal development. The mean axial length of a full-term infant's eye is approximately 16.5 mm. The average increase during the next 18 months is 3.75 mm.1 By 13 years of age, the mean is 23 mm, which is near the adult mean of 24.5 mm.2 The growth of the eye explains why nearsightedness often progresses significantly between the ages of 8 and 13 years.3 It also explains why 75% of newborns have some form of hyperopia and only 25% have myopia.4
The mean refractive error of a full-term infant is approximately +2 diopters (± 2 diopters standard deviation).5 A diopter is a standard unit of measurement in optical lenses where plus designates that a convex lens (of this magnitude) is needed to correct farsightedness and minus that a concave lens is needed to correct nearsightedness. It seems likely that, during normal development, both passive factors (normal growth of the eye) and active factors (feedback of visual information to the brain) combine to guide the refractive error toward 0 (emmetropia) and maintain it at that level.6
Figure 2. A schematic representation of astigmatism. The eye is normal in size, but the contour of the cornea distorts the image on the retina. The distorted retinal image can be corrected by the use of a cylindrical lens situated at the appropriate axis.
Special consideration must be given to children with particular health problems and syndromes. For instance, significant myopia is much more common in premature infants,7,8 particularly those who have had retinopathy of prematurity. These infants also have an increased incidence of anisometropia (refractive imbalance) and astigmatism.7 Appropriate eye wear is available for these infants because they sometimes require correction before 1 year of age (Fig. 3). Children with Down syndrome seem to have a higher incidence of hyperopia, myopia, and astigmatism.9 These children should not be overlooked. The increased intraocular pressure of congenital glaucoma can expand the size of an infant's eye, causing large amounts of myopia, astigmatism, and anisometropia. The infant often loses vision because of the refractive imbalances rather than the high intraocular pressures. Syndromes that affect multiple organ systems may also have a high incidence of myopia. They include homocystinuria and Marfan, Stickler, and Noonan syndromes, among others.10 All of these children deserve special attention.
Figure 3. Eyeglasses come in all shapes and sizes. Ones with cables or curved temples and those with straps are usually reserved for infants and toddlers.
If we are to understand the function of a child's visual system, we must define the concept of accommodation. This occurs when the lens of the eye increases its refractive power by changing its shape to bring an unfocused image into focus on the fovea. This is a concerted effort that works in conjunction with the action of convergence, which is the inward alignment of the eyes on a near object.
A young child has an enormous accommodative reserve. This allows him or her to compensate for substantial amounts of hyperopia. Unfortunately, accommodation works to alleviate farsightedness only, not myopia, so a child who cannot see well at distance cannot bring things into focus by accommodating. This means that young children with small or moderate amounts of hyperopia are asymptomatic, have good vision, and do not require eyeglasses. This, of course, depends on the eyes remaining properly aligned. Some children's hyperopic eyes become excessively turned inward, even with relatively small amounts of hyperopia (accommodative esotropia). These children will require full-time correction with eyeglasses. This may sometimes occur even after eye muscle surgery to correct conditions such as congenital esotropia (Fig. 4).
The accommodative range is a function of advancing age. The amplitude is approximately 14 diopters (± 2 diopters) at 8 years of age. It then decreases by 1 diopter every 4 years until the age of 40. u Adults start to require bifocals and reading glasses at this age because of this phenomenon. A newborn's accommodative reserve is in excess of 14 to 15 diopters, so farsightedness at birth is not a visual problem. It is the norm due to the size of the eyes at this stage of development. This may also explain why accommodative esotropias generally start to appear between 2 and 4 years of age, when a child's accommodative reserve begins to lessen.
Figure 4. (A) A child with a left intermittent esotropia noted by the parents during a 6-month period. The child previously underwent surgery for congenital esotropia and her vision was Orthophorie (straight) for a period of time. (B) Eyeglasses were later required to correct her residual accommodative esotropia. Note the symmetry of the corneal light reflexes.
Vision and Depth Perception
Newborns have vision but do not process it well. They preferentially view face-like stimuli,12 and by 1 month of age can distinguish a circle from a triangle. A normal infant's visual system is capable of resolving a 20/20 target by at least 18 months of age.13 Fusion develops between 4.5 and 6 months of age.14 Significant depth perception has been demonstrated at 3 months of age.15
Most of us tend to underestimate a young child's visual abilities. If an infant is not fixating well, we should not necessarily assume that it is a normal delay. The infant may require eyeglasses or have another ophthalmologic or developmental problem that demands immediate intervention. A pediatric ophthalmologist should be consulted.
INDICATIONS FOR EYEGLASSES
The improvement of visual acuity is a valid reason for prescribing eyeglasses for young children, but is it the most important reason? The answer is no. As previously stated, we are working against time. If certain visual problems are not corrected in children in a prompt fashion, the outcome may be amblyopia or what is commonly referred to as a "lazy eye."
Amblyopia occurs when there is an interruption of the normal visual stimuli in one eye (and sometimes in both eyes). When this occurs, an actual physical change takes place in the neurons of the corresponding visual cortex in the occipital lobe.1617 If this is corrected during the stage of visual development, visual loss can be either prevented or reversed. Treatment modalities may include any combination of occlusion therapy, eyeglasses, or surgery. It is essential that prompt action be taken because, as the child nears visual maturity, which is generally considered to be between the ages of 7 and 9 years, the chances that intervention will produce a favorable visual outcome decline.
In this article, we are concerned with the use of eyeglasses for children, but we must not forget that amblyopia may also be caused by congenital or acquired anomalies such as cataracts, ptosis, capillary hemangiomas of the eyelid, and corneal scars. Even so, amblyopia is more commonly caused by a refractive imbalance (anisometropia) or strabismus.
In anisometropia, one eye is more farsighted, more astigmatic, or more nearsighted than the other eye. The eyes can only focus on an object together, not separately. Therefore, the eye with the least amount of refractive error will be in focus, whereas the other eye will appreciate a blurred image. The brain cannot tolerate this disparity and will suppress the blurred image during the stage of visual development, using various sensory adaptations. Over time, suppression of the blurred image will often develop into fullblown amblyopia.
Occasionally, a child may have such high refractive errors in both eyes that no attempt is made to focus. The retinas rarely perceive a clear image. In this case, the child may actually develop bilateral or ametropic amblyopia.
In strabismic amblyopia, the eyes are not properly aligned. One eye fixates on an object and the other does not. The brain therefore receives disparate images, which cannot be tolerated. The child again uses sensory adaptations that prevent double vision (diplopia). This, in turn, can cause eventual amblyopia.
Another indication for eyeglasses for children is correction of strabismus. Accommodative esotropia occurs when an attempt is made to focus on an object. If a child has hyperopia, there may be an inappropriate amount of convergence, causing esotropia (Fig. 5). This may be alleviated by eyeglasses that correct the refractive error (Fig. 5). Children can outgrow this condition, but there are no guarantees. When there is a greater esotropia at near than at far distances, and this is not overcome by the full farsighted correction, we call this a high accommodative convergence to accommodation (AC /A) ratio. This may be treated by the use of bifocal lenses, which allow the child to maintain normal vision at distance while correcting the deviation for near images (Fig. 5). When surveyed, 87% of pediatric ophthalmologists in the United States reported treating children with this condition. This is not to be confused with the use of bifocals for children with myopia. When surveyed, 97% of pediatric ophthalmologists reported rarely prescribing bifocals for this group of patients, and the pediatric ophthalmologic community does not generally recommend them.18
By correcting strabismus, improving acuity, and preventing amblyopia, we are, in turn, preserving binocular vision. The use of both eyes as a functional unit allows fusion and the appreciation of depth perception (stereopsis). All of these factors are dependent on one another and together they allow normal visual development.
Other indications for correction with eyeglasses, although not as important, deserve mention. We sometimes prescribe eyeglasses for children to restore comfortable vision and enhance visual efficiency. As a child grows older, visual demands become greater and this means that eyeglasses not prescribed at an earlier age may now be considered. Occasionally, prism lenses are used to offset a misalignment until it resolves or surgery is performed. This is usually a temporary treatment. An important consideration for prescribing eyeglasses is the protection of a non-amblyopic eye. If a child has an unrecoverable loss of vision in one eye with acuity less than 20/100, polycarbonate protective lenses are highly recommended to protect the normal eye. Policy statements by the American Academy of Ophthalmology and the American Academy of Pediatrics strongly advise the use of protective eye wear in these cases.
Figure 5. (A) This child had right esotropia between 2 and 4 years of age. Note the asymmetric corneal light reflexes. (B) She was found to be farsighted and was prescribed eyeglasses to correct her deviation (accommodative esotropia). The turn was alleviated at distance fixation, but not at near. (C) The addition of bifocal lenses corrected her esotropia at near (high accommodative convergence to accommodation ratio).
Figure 6. This child has a large amount of bilateral myopia (nearsightedness). Notice that the facial image is minimized through the concave lens. This is one way of differentiating among the types of refractive errors seen in children.
As discussed in the previous sections, there are several categories of refractive error that require correction with eyeglasses. They include myopia, hyperopia, astigmatism, compound refractive errors, and anisometropia. Most children with bilateral myopia (Fig. 6) require eyeglasses mainly to improve acuity. Bilateral amblyopia is less likely here because near vision is not compromised. In high amounts of bilateral hyperopia (Fig. 7), both near and distance vision may be blurred and bilateral amblyopia becomes more of a possibility. If there is no concurrent strabismus, this may go undetected during most of childhood. Bilateral astigmatism may also cause bilateral amblyopia because larger degrees of this tend to blur near and far points of focus. Anisometropia is the most likely condition to cause significant visual loss and an imbalance in the amount of farsightedness tends to be the most common offender. An important subset of anisometropia is unilateral high myopia, which often has a poor visual prognosis even with timely eyeglass or contact lens wear and occlusion therapy. Even so, aggressive treatment is still recommended.
Figure 7. This child has a large amount of bilateral hyperopia (farsightedness). Here, unlike myopia, the facial image is magnified due to the convex lenses.
Accommodative esotropia usually occurs in children between 2 and 4 years old, although it may occur earlier. It must be distinguished from other forms of esotropia, usually by a trial of eyeglasses or occasionally by trying cholinesterase-inhibiting eye drops, which facilitate accommodation and thus lessen convergence. Children whose deviations are adequately controlled by eyeglasses are not candidates for surgery and should not be confused in this regard with other patients with strabismus. In esotropia with a high AC /A ratio, there is an inappropriate amount of convergence for each unit of accommodation, causing the eyes to turn inward at near focus. Again, this can be treated by the addition of bifocal lenses (Fig. 5). Certain forms of exotropia (outward deviation of the eyes) are exacerbated by undercorrected refractive errors. This should be considered another indication for eyeglasses.
REASONS WHY EYEGUSSES MAY NOT BE PRESCRIBED
Young children have low visual demands. If an eye care professional determines that a young child is not in an amblyogenic situation, prescribing eyeglasses may be postponed, even if vision is less than 20/40. As the child nears school age, this situation will likely change. Young children tend to be noncompliant with the use of eyeglasses, especially if they do not notice significant improvement in vision. This is particularly evident in anisometropia because the child's vision with both eyes open is as good as that of the eye that sees better. It is up to the ophthalmologist to determine when a child's noncompliance becomes an issue in the use of eyeglasses. Cost can be an important factor. Eyeglasses can be expensive. Social acceptance can also be a factor.
There is evidence that the eyes tend to gravitate toward emmetropia (a nonprescription state). This process, called emmetropization, occurs passively and actively. By prescribing eyeglasses unnecessarily and at an early point in development, we may interrupt the eyes' eventual growth toward normalcy. Eyeglasses are therefore typically prescribed when indications for correction outweigh reasons not to prescribe.
ALTERNATIVES TO WEARING EYEGLASSES
When a child is old enough, contact lenses are an alternative to eyeglasses. This usually occurs when a child becomes responsible enough to properly care for them and is cognizant of the problems that may develop. Most children can begin wearing contact lenses at approximately 13 years old. Exceptions to this are when a child either has a unilateral high myopia or has undergone a cataract extraction.
Recently, there has been much excitement over the use of laser surgery to correct refractive errors. The two most common procedures are photorefractive keratometry and laser in situ keratomileusis (LASIK). Although frequently performed on adults, these procedures remain experimental in the pediatric population for several reasons. First, they are difficult to perform in young, inattentive children. Patient cooperation, particularly in LASIK, is oí paramount importance to the success of the procedure. The need for general anesthesia when performing the procedure in children adds another element of risk that is not present in the adult population. Second, the eyes of a child are still actively developing and the refractive error is in a state of change. This creates difficulty in determining the amount of surgery necessary to correct the refraction. Finally, we are unaware of the long-term effects of such procedures on a young child's eyes.1920 Therefore, it is prudent, for the time being, to avoid these particular modes of treatment.
CONCLUSIONS AND RECOMMENDATIONS
The adage "an ounce of prevention is worth a pound of cure" certainly applies when referring to a child's eyes. Proper visual screening in the pediatrician's office at an early age could probably prevent much of the amblyopia we see today. A good history should be taken because parents sometimes observe behaviors in a child's daily routine that evade the practitioner in the office.
Preverbal children should have both eyes individually examined for fixation and a normal red reflex. Observing a symmetrical corneal light reflex or doing a cover-uncover test can help determine whether any strabismus is present and differentiate between true strabismus and Pseudostrabismus. The latter occurs because of facial immaturity. Here a wide nasal bridge and prominent epicanthal skin folds give the appearance of eyes turned inward.
When a child is old enough to provide verbal feedback, visual acuities should be measured in the pediatrician's office. In the absence of a preexisting sign or symptom, children should have a formal ophthalmologic examination by 4 to 5 years of age. Several screening devices and cameras are available to pediatricians to facilitate the detection of visual disorders in children. This article does not recommend one over the others. These are useful alternatives to standard testing methods, but none is 100% effective.
Certainly, if there is any doubt, a child should be referred to a specialist. Many pediatricians face litigation because of undetected amblyopia in their patients. The prompt prescribing of eyeglasses may prevent not only irreversible visual loss, but also untold aggravation for the child, his or her parents, and the pediatrician.
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