DONAHUE, SEAN P. MD, PhD
Children are not simply little adults. They have unique needs based upon their visual demands and their developing visual system. One cannot simply extrapolate the spectacle needs of adults onto young children. Doing so creates cognitive dissonance for parents, who feel their child's visual system will be damaged by not wearing glasses, but who hear their child insisting that he doesn't see any better while wearing the glasses. As a general rule (anisometropia excepted) if a child appreciates the improvement obtained with spectacles, he or she will wear them. The opposite is also true: The child who doesn't want to wear spectacles (or who forgets them repeatedly) likely obtains no significant benefit from them, and should not be forced to wear them.
Most practice patterns with respect to spectacle prescribing for young children are based on experience, rather than evidence. Obtaining evidence of the usefulness of spectacles for children with mild and moderate myopia, hyperopia, or astigmatism, and an otherwise healthy visual system would be difficult, if not impossible. Therefore, most guidelines are obtained by surveys of practitioners, and are based upon experience acquired over many years. Fortunately, it appears as though optometrists and ophthalmologists whose practices are dedicated to children usually have relatively similar practice patterns. The biggest variable appears to be the practitioner's degree of expertise with examining and treating preschool children. Continuing education of ophthalmologists and optometrists, and additional research regarding the natural history of refractive development are needed to further improve quality of care.
Unique Visual Needs of Young Children
Children have unique characteristics that influence their use of spectacles, In adults, one typically makes a decision to prescribe based upon the difference between uncorrected and corrected visual acuity. This is not useful for most preschool children. Most children younger than 4 years of age cannot provide a reasonable, reliable, and repeatable objective visual acuity in a busy office with standard techniques of measurement. Although such measurements can be done, their high variability limits their clinical usefulness in making a decision about prescribing. Even after a child becomes verbal, the measured acuity often underestimates the true acuity, because the child may tire, or simply have no interest in reading small letters on an eye chart. One must therefore also consider the level of refractive error, as determined with cycloplegia. Therefore, cycloplegic refraction is mandatory in determining the spectacle needs of children.
Most pediatric ophthalmologists use cyclopentolate 1% to obtain cycloplegia for the examination. When the detection of latent hypermetropia is crucial (a child with new onset esotropia, or residual strabismus in a previously well-controlled accommodative esotrope), 2 drops of cyclopentolate are administered 5 min apart. Refraction should be carried out 30 min following the second drop. Although this method provides adequate cycloplegia in nearly all children, it may not provide sufficient mydriasis in those with darkly pigmented irides; 1% tropicamide and 2.5% phenylephrine are therefore used in addition to 2% cyclopentolate in black and Hispanic children. Most hospital pharmcotherapeutic committees prohibit mixing of medication, so a noncommercial mix of agents (or a spray) is not used in most academic practices. Some pediatric ophthalmologists will instill one drop of topical proparacaine before cyclopentolate to decrease the stinging (and possibly enhance absorption). I do not, as proparacaine also stings, and the combination means that the child needs 4 drops rather than 2. Similar hospital policy issues exist with a combination of topical anesthesia and cycloplegic as described above. Tropicamide alone can produce cycloplegia, but its half-life is so short as to make it not useful in a busy pediatric office. Atropine can be used for difficult refraction but in my experience is almost never necessary.
In addition to being more difficult to examine, children also have different visual demands than adults. The working distance of most preverbal children is very different from that of adults. Generally, children have minimal or no need for sharply focused distance acuity (although we invariably describe visual function on the basis of distance acuity). This is especially true for children of the age of 3. The preschool child typically has a working distance of 1 to 2 m. Thus, in contrast to older children and adults, preschool children have minimal need for mild symmetric myopia correction.
Children also have different accommodative abilities than adults. There is vast literature, dating back to the early 1900s, that describes the extremely high levels of accommodation that young children possess.1,2 Healthy children in their first decade of life typically possess 12 D or more of accommodative function.3 Accordingly, even moderate uncorrected hypermetropia does not degrade acuity in young children.4 As a result, there is minimal need to correct moderate hyperopia, except when it is associated with strabismus.
The final unique characteristic of a child's visual system is its increased risk of amblyopia, from anisometropic, strabismic, or high spherical or cylindrical refractive errors. In contrast to anisometropia or strabismic adults who do not jeopardize their visual systems by failing to correct the nondominant eye, the young child is at risk of permanent vision loss unless the eyes are straight and have symmetrical and adequately focused retinal images. However, the level of refractive error that produces amblyopia for each particular child is different, and depends on other factors, such as the family history.5 Thus, no firm evidence-based recommendations can be made regarding the threshold levels of refractive error that need to be corrected to protect against the development of amblyopia.
The above characteristics of children mean that spectacle prescribing for children is an art, requiring interpretation of the child's refractive error and visual acuity within the global evaluation of the child. This is especially true for children who are not yet able to provide an accurate objective visual acuity measurement, and for whom the only information available is the cycloplegic refractive error, and the visual behavior of the child. The remainder of this manuscript will detail the thought processes many pediatric ophthalmologists use to determine when to prescribe spectacles.
Prescribing for Myopia
Because of the minimal risk of amblyopia with symmetrical myopia, prescribing for symmetric myopia should solely be based upon anticipated visual acuity needs. Two fundamental observations underscore the minimal need to prescribe spectacles for symmetric low levels of myopia in young children. First the visual acuity demands of very young children are unlikely to exceed 20/40 before the late elementary school years. Although the fovea is adequately developed and capable of 20/20 acuity by 6 years of age, most of the items a child views are not small enough to require such fine resolution. The second factor impacting prescribing for myopia in children is their proximity to the visual target. Unlike adolescents and adults, who are required to view distant targets with high resolution, most children have a working distance that is close to them. Infants, for example, have a very proximate working distance; a newborn infant typically only needs to see her mother's face, which often is only 25 cm away. The ocular structures of infant eyes are also not capable of high spatial resolution. Hence, only extreme myopia (approximately minus 4 D or more) is probably necessary to treat in this age group. Late in the first year of life, the eyes become anatomically capable of better spatial resolution, but until a child begins to walk, he is rarely interested in objects more than 2 to 3 feet away. Therefore, −3.00 D of myopia is a threshold one may consider correcting in the very young child.5 Guidelines from the American Academy of Ophthalmology's Preferred Practice Pattern6 and the Pediatric Eye Disease Investigator Group5 both set 3.00 D of myopia as a threshold for correction. A similar magnitude was established as a criterion to detect using preschool vision screening by the Vision Screening Committee of the American Association of Pediatric Ophthalmology and Strabismus.7
Children in kindergarten or first grade typically do not use a chalkboard at school, but do most things at school at a desk, and are beginning to read. Thus, arguably even up to 1.5 D of myopia may not be important to correct for children in this age group. However, older children, beginning in the mid elementary school years, when acuity can be tested accurately, warrant full correction of myopia. The optometry community probably has less tolerance for undercorrection of myopia in preschool children than does the pediatric ophthalmology community. Reasons for this are unclear. However, there is no well-documented evidence that either under- or overcorrection of myopia stimulates or retards its progression. In fact, a recent well-controlled study failed to find any effect, even when myopia was overcorrected.8
Most pediatric ophthalmologists do not prescribe bifocals in young myopic children to retard myopia progression despite the COMET conclusions. This is because most feel that a small difference in myopia as an adult is of little clinical relevance compared with the added cost and cosmetic issues associated with bifocal wear. Similar thought processes limit the use of atropine and pirenzipine.
Correction of Astigmatism
Mild to moderate meridional astigmatism of <1.5 D produces minimal degradation of visual acuity in the young child and is not felt to be amblyogenic when symmetric.5 Oblique astigmatism degrades visual acuity more, and may be amblyogenic with slightly less magnitude. High levels of astigmatism are typically found in the American Indian population and therefore should be screened for.9 If astigmatism is balanced by spherical refractive error (compound myopic astigmatism or compound hyperopic astigmatism), the spherical equivalent places the Conoid of Sturm nicely on the retina. Also, depending upon the degree of accommodation used, the necessary portion of the visual environment may be sufficiently focused to prevent amblyopia or significantly decreased acuity. This may explain why some patients tolerate moderate levels of astigmatism without spectacles.
Preverbal children with symmetric astigmatism <1.5 D typically do not need correction unless the astigmatism is associated with high myopia or high hyperopia. The AAPOS vision screening committee has set a threshold of 1.5 D of meridional cylinder as a target condition to detect with preschool vision screening.7 The Vision in Preschoolers (VIP) study group had a similar threshold10 as does the Pediatric Eye Disease Investigator Group.5 The Pediatric Preferred Practice Pattern for Children aged 2 to 3 years suggests prescribing at a slightly higher magnitude (2.0 D).6 Early elementary school-age children with 1.0 to 1.5 D of astigmatism may benefit from correction, and a trial of spectacles is probably warranted for such children. However, the parents should be informed that not wearing spectacles will not harm a child's vision, and if children choose not to wear the glasses, they should not force them to do so. For children in the late elementary school years, a postcycloplegic manifest refraction to compare best corrected visual acuity with uncorrected acuity can help guide the decision of whether or not to prescribe spectacles for lower levels of astigmatism. In all such situations, one would prescribe the full cylinder that can be tolerated.
Correction of Anisometropia
Anisometropia can be a very powerful amblyogenic factor, and anisometropic amblyopia is extremely difficult to detect with traditional screening of preliterate children. However, the treatment of what appears to be asymptomatic anisometropia detected either on a routine eye examination or following referral from a photorefractive screening causes a dilemma for the ophthalmologist and optometrist, because severe levels of anisometropia often cause amblyopia but mild and moderate levels often do not.11,12 In addition, some children, especially those having a family history of amblyopia may develop amblyopia even with relatively small levels of cylindrical or spherical ametropia. Finally, the natural history of anisometropic refractive error over time is not well established. For example, a child with moderate anisometropia may have the ametropia completely resolve before school entry. Whether or not such anisometropia needs to be treated is unclear. Anisometropia usually produces amblyopia by the age of 3 years12; thus, if the uncorrected acuity is normal at that age, treatment is likely unnecessary. In addition, recent evidence from the amblyopia treatment study series has demonstrated that many preschoolers with mild to moderate anisometropic amblyopia can have restoration of good visual acuity and stereopsis simply with spectacle correction alone, even at late ages.13,14 Hence, the importance of detecting and treating very small levels of anisometropia in very young children, even when amblyopia is already present, is now open to question.
The threshold for treating anisometropia is also controversial. The vision screening committee of AAPOS recommends that preschool screening detect children having >1.5 D anisometropia.7 A similar threshold was chosen by the VIP study group.10 Thresholds within 0.5 D of this value are suggested by the PEDIG5 and the American Academy of Ophthalmology Preferred Practice Pattern.6
Evidenced-based data support these thresholds. Weakley11 evaluated acuity results from several hundred anisometropic children seen in his practice and concluded that >1.0 D of spherical anisometropic hyperopia and >1.5 D of cylindrical hypermetropia produced an increased risk of amblyopia development. A retrospective review by Kutschke et al.15 found that anisometropic amblyopia was never associated with <1.5 D of anisometropia unless a coexisting strabismus was present, and that 1.0 D appeared to be a threshold at which anisometropia began to be associated with amblyopia.
An additional difficulty with treating anisometropic amblyopia is that the dominant fellow eye typically has minimal refractive error, and therefore, many children do not appreciate any improvement and do not wish to wear the glasses. This is the primary instance in pediatric ophthalmology in which spectacle compliance is often difficult and needs to be forced; in most other situations, compliance with spectacles wear is not difficult, even for young children, providing the above guidelines are adhered to (and the prescription is correct!).
Treatment of anisometropia should consist of symmetric reduction of hypermetropia of up to 2.0 D, prescribing the full amount of cylinder unless the child has an associated accommodative esotropia. In this situation, all hypermetropia should be corrected along with the full cylindrical correction. This practice has been well established by clinical care, and is used in the PEDIG study protocols.5
Prescribing for Hypermetropia
Prescribing spectacles for hypermetropia also presents unique challenges. Uncorrected hypermetropia can produce accommodative esotropia, strabismic amblyopia, and isoametropic (refractive) amblyopia. Fortunately, the practitioner is aided by evidence-based guidelines from population studies, as well as surveys.
Most young children are mildly hypermetropic; hence, moderate hypermetropia does not need to be corrected.6 The threshold for treatment of hypermetropia, however, is controversial. Some evidence is available to help guide this decision. The prevalence of hypermetropia has been estimated in several studies.16,4,17 It is difficult to compare the studies, as the definition of hypermetropia varies based upon whether the hypermetropia is thought to be potentially pathologic4 or if it is simply being distinguished from ametropia.4,16,18 Nevertheless, these studies generally show that fewer than 1% of healthy children have >4 D of hypermetropia4,15; other studies not referenced here have reached similar conclusions.
A recent study examined the relationship of increasing hypermetropia with degradation of visual acuity, and failed to demonstrate any significant reduction in acuity until hypermetropia exceeds 4 D.4 This threshold represents only a very small portion of the population.4,16
Prescribing spectacles for hypermetropia has also been postulated to improve reading ability. An excellent study by Helveston demonstrated that in the absence of acuity degradation there is no relationship between reading ability, school performance, and level of hypermetropia.19 Thus, children with moderate levels of hypermetropia do not need spectacles simply to improve their near vision or reading ability.
The treatment of moderate to high hypermetropia has been demonstrated to decrease the risk of strabismus and amblyopia in prospective randomized studies. Atkinson et al. compared treatment vs. no treatment of otherwise healthy hypermetropes.20 Children with hypermetropia >+3.50 D had a 13 times greater risk of developing strabismus or amblyopia than did children who had no significant hypermetropia. Prescribing spectacles for the hypermetropia decreased the risk substantially, but these children remained at a four times greater risk than the general population. These results suggest that levels of hypermetropia >4.00 D should warrant consideration of correction, especially if there is a family history of strabismus or amblyopia, or if there is a poorly controlled phoria without correction.
Guidelines for treatment of hypermetropia have also been determined from practice patterns and surveys. Most surveys have demonstrated that optometrists have a lower threshold for prescribing spectacles for children than do ophthalmologists. Reasons for this are unclear. Lyons et al. performed a survey of 212 optometrists and 102 ophthalmologists (both comprehensive and pediatric).21 They were asked whether they would prescribe spectacles for a 6-year-old child having between +3.00 and +4.00 D hypermetropia. Optometrists prescribed spectacles in 33% of instances, whereas only 5% of ophthalmologists did. For 2-year-old children, most ophthalmologists and optometrists began to prescribe spectacles at a level of +5.00 D of hyperopia.
A separate but similar survey of Pediatric Ophthalmologists demonstrated that threshold levels for prescribing hypermetropia vary by child age and by the level of hyperopia.22 Fifty percent of Pediatric Ophthalmologists would prescribe spectacles for children younger than the age of 2 years when hyperopia reached +5.00 D. For children older than age 2 years, 50% prescribed at 4.00 D of hyperopia.
The American Academy of Ophthalmology has guidelines for prescribing spectacles in their Preferred Practices Patterns (PPP).6 The PPP “Childhood Eye Examinations” indicates that “cycloplegia is mandatory” in determining the refractive needs of children. For children aged 3 years and younger, they suggest prescribing at +4.50 D of hypermetropia. For children aged 4 years or older, they indicate that spectacles should be prescribed if necessary to improve acuity, or alleviate esotropia. No numerical threshold guidelines are given in this situation.
A final set of guidelines are provided from papers regarding preschool vision screening techniques. The American Academy of Pediatric Ophthalmology and Strabismus (AAPOS) Vision Screening Committee has published standards on what should be detected with preschool vision screening. They suggest that vision screening instruments and tests should detect hypermetropia >+3.50 D in any meridian.7 Likewise, the Vision in Preschoolers study, which is primarily optometry based, defines hypermetropia >+3.50 D as a condition that is important to detect.10
Concern abounds about the effect of spectacle correction of hypermetropia on the eventual emmetropization of the eye. Studies both support and oppose this notion. The issues regarding this complicated topic are deferred to Dr. Mutti's paper, which is part of this transcript.
When a decision is made to correct hypermetropia, how much should be corrected? Full correction in the nonstrabismic child should be avoided as the accompanying blur at distance can be a factor that hinders compliance. The Pediatric Eye Disease Investigator Group has mandated symmetric reduction of up to 1.5 D of spherical hypermetropia when treating anisometropic amblyopia in the amblyopia treatment studies, with full correction of all hypermetropia for the strabismic child.23–25
An exception to these threshold levels for prescribing for hypermetropia is for children with developmental delay or Down Syndrome. Some children, especially those with severe development delay, are minimally interactive, and have very little need for spectacle correction. Children with significant cortical visual impairment, severe structural ocular abnormalities, and marked mental retardation are examples. Many such children will not appreciate the improvement provided by the spectacles and will not tolerate them on their face. In my experience, well-minded parents often become exhausted in futile attempts to keep such glasses on these children, fearing that not wearing them will damage the child's vision. In contrast, children with Down syndrome are often hypo-accommodators, and have low accommodative amplitudes. Therefore, they may benefit from spectacle correction at lower thresholds.
In summary, a consensus appears to exist to prescribe spectacles for hypermetropia in children when hypermetropia exceeds 3.5 D and acuity cannot be adequately determined. As accurate determination of uncorrected visual acuity is often quite difficult until approximately age 4 years, a better method considers a child's visual demands, based upon the child's age, his or her baseline level of cycloplegic refractive error, and whether there is a family history of amblyopia or strabismus. A discussion with the parents that reassures them that the lack of wearing spectacles will not harm the child in the absence of anisometropia is also important. Finally, parents should be warned that children who develop eye crossing should be seen immediately, as such an observation mandates spectacle correction of full hypermetropia.
This work was supported by Research to Prevent Blindness, New York, New York.
Sean P. Donahue
Department of Ophthalmology
1211 21st Avenue S
104 Medical Arts Building
Nashville, TN 37212-1348
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