In children younger than 6 years, dense cataracts occupying the central 3 mm or more of the lens are capable of causing severe visual deprivation amblyopia. Similar lens opacities acquired after age 6 years are generally less harmful. Small anterior polar cataracts, around which retinoscopy can be readily performed, and lamellar cataracts, through which a reasonably good view of the fundus can be obtained, may cause mild to moderate amblyopia or may have no effect on visual development. Unilateral anterior polar cataracts, however, are associated with anisometropia and subtle optical distortion of the surrounding clear portion of the lens, which may cause anisometropic and/or mild visual deprivation amblyopia.
Reverse amblyopia is a form of visual deprivation amblyopia that develops in the fellow eye as a result of patching (occlusion amblyopia) or penalization.
Evaluation
Amblyopia is diagnosed when a patient has a condition known to cause amblyopia and has decreased visual acuity that cannot be fully explained by physical abnormalities of the eye. Characteristics of vision alone cannot reliably differentiate amblyopia from other forms of vision loss. The crowding phenomenon, for example, is typical of amblyopia but not pathognomonic or uniformly demonstrable. Subtle afferent pupillary defects occur rarely and only in severe cases of amblyopia. Amblyopia sometimes coexists with vision loss directly caused by an uncorrectable structural abnormality of the eye such as optic nerve hypoplasia or coloboma. When the clinician encounters a doubtful or borderline case of this type and the patient is a young child, it is appropriate to undertake a trial of amblyopia treatment. Improvement in vision confirms that amblyopia was indeed present.
Multiple assessments of visual acuity are sometimes required in order to determine the presence and severity of amblyopia. (Assessment of visual acuity is discussed in Chapter 1.) In some circumstances, it is appropriate to assume that amblyopia is present and to begin treatment even before decreased vision has been conclusively determined. Examples include initiating occlusion therapy with the presence of a high degree of anisometropia or shortly after surgery for a unilateral cataract.
When determining the severity of amblyopia in a young patient, the clinician should keep certain considerations in mind. Assessment of fixation preference is sensitive for detecting amblyopia, but false-positive results can occur. For example, a child with small-angle strabismus may show a strong fixation preference despite visual acuity in the 2 eyes being equal or nearly so. In addition, the young child’s brief attention span frequently results in measurements that fall short of the true limits of acuity; these measurements can mimic those of bilateral amblyopia or obscure or falsely suggest a significant interocular difference.
Treatment
Treatment of amblyopia involves the following steps:
1.Eliminate (if needed) any obstruction of the visual axis, such as a cataract.
2.Correct any significant refractive error.
3.Force use of the amblyopic eye by limiting use of the better eye.
Cataract Removal
Cataracts capable of producing amblyopia require timely surgery. Removal of unilateral visually
significant congenital lens opacities during the first 4–6 weeks of life is necessary for optimal recovery of vision. Significant cataracts with uncertain time of onset also deserve prompt and aggressive treatment if recent development is at least a possibility. See Chapter 23 for further discussion of childhood cataract.
Refractive Correction
In general, optical prescription for amblyopic eyes should be based on the refractive error as determined with cycloplegia. Because an amblyopic eye’s ability to control accommodation tends to be impaired, this eye cannot be relied on to compensate for uncorrected hyperopia as would a normal child’s eye. Sometimes, however, symmetric reductions in plus lens power may be required in order to foster acceptance of spectacle wear. Refractive correction for aphakia following cataract surgery in childhood must be provided promptly to avoid prolonging the visual deprivation that occurs because of a severe uncorrected refractive error. Anisometropic, isoametropic, and even strabismic amblyopia may improve or resolve with only refractive correction. Given this, many ophthalmologists initiate treatment with refractive correction alone, adding occlusion or penalization later if needed (see the next section). The role of refractive surgery in patients who fail conventional treatment with spectacles and/or contact lenses is under investigation.
Paysse EA, Coats DK, Hussein MA, Hamill MB, Koch DD. Long-term outcomes of photorefractive keratectomy for anisometropic amblyopia in children. Ophthalmology. 2006;113(2):169–176.
Writing Committee for the Pediatric Eye Disease Investigator Group; Cotter SA, Foster NC, Holmes JM, et al. Optical treatment of strabismic and combined strabismic-anisometropic amblyopia. Ophthalmology. 2012;119(1):150–158. Epub 2011 Sep 29.
Occlusion and Penalization
Occlusion therapy (patching) is commonly employed to treat unilateral amblyopia. The sound eye is covered, obligating the child to use the amblyopic eye. Adhesive patches are usually used for occlusion therapy, but spectacle-mounted occluders or opaque contact lenses can be utilized instead if skin irritation or inadequate adhesion is a problem. With spectacle-mounted occluders, close supervision is necessary to ensure that the patient does not peek around the occluder.
Part-time occlusion, defined as occlusion for 2–6 hours per day, has been shown to achieve results similar to those of prescribed full-time occlusion. The relative duration of patch-on and patch-off intervals should reflect the degree of amblyopia. For severe deficits (visual acuity of 20/100–20/400), 6 hours per day is preferred. Maintenance patching of 1–2 hours per day is often prescribed to prevent recurrence of amblyopia after successful patching.
Full-time occlusion of the sound eye is defined as occlusion during all waking hours. In rare cases, with aggressive patching, strabismus may occur because of lack of binocular viewing and tenuous fusion. Therefore, the child whose eyes are consistently or intermittently straight may benefit from being given some opportunity to see binocularly. Part-time occlusion reduces the likelihood of occlusion amblyopia or induced strabismus.
The timing of follow-up should be related to the intensity of treatment and the age of the child. An examination is typically scheduled within 2–3 months after initiation of treatment. Subsequent visits can be scheduled at longer intervals based on early response. Part-time occlusion and penalization methods allow for less frequent observation.
The desired endpoint of therapy for unilateral amblyopia is free alternation of fixation, linear recognition acuity that differs by no more than 1 line between the 2 eyes, or both. The time required for completion of treatment depends on several factors, including the severity of amblyopia, choice and intensity of therapeutic approach, adherence to treatment, and age of the patient.
More severe amblyopia and older age correlate with a need for more intensive or longer treatment. Consistent occlusion during infancy may reverse substantial strabismic amblyopia in less than 1 month. In contrast, an older child who wears a patch only after school and on weekends may require several months to overcome a moderate deficit.
Adherence to occlusion therapy for amblyopia declines with increasing age. However, studies in older children and teenagers with strabismic or anisometropic amblyopia have shown that treatment can still be beneficial beyond the first decade of life. This is especially true in children who have not previously undergone treatment. The effectiveness of part-time patching regimens in older children is being investigated.
Other methods of amblyopia treatment involve pharmacologic and/or optical degradation of the better eye’s vision such that it becomes temporarily inferior to the amblyopic eye’s vision, an approach referred to as penalization. Use of the amblyopic eye is thus promoted without complete occlusion of the fellow eye. An advantage of penalization over occlusion therapy in patients with orthotropia or small-angle strabismus is that penalization allows a degree of binocularity, which is particularly beneficial in children with latent nystagmus.
Studies have demonstrated that pharmacologic penalization can successfully treat moderate degrees of amblyopia. A cycloplegic agent (usually atropine 1% solution) is administered to the better-seeing eye so that it is unable to accommodate. As a result, the better eye experiences blur with near viewing and also, if hyperopia is undercorrected, with distance viewing. This form of treatment has been demonstrated to be as effective as patching for mild to moderate amblyopia (visual acuity of 20/100 or better). Atropine may be administered daily, but weekend administration is as effective for milder amblyopia. Depending on the depth of amblyopia and the response to prior treatment, hyperopic correction of the dominant eye can be reduced to enhance the effect. Regular follow-up of patients whose amblyopia is being treated with cycloplegia is important in order to monitor for reverse amblyopia (see Complications of Therapy). Pharmacologic penalization offers the advantage of being difficult for the child to thwart. It does not work well for myopic patients, however, because clear near vision persists in the penalized eye despite cycloplegia.
Optical penalization involves the prescription of excessive plus lenses (fogging) or diffusing filters for the sound eye. This form of treatment avoids potential pharmacologic adverse effects and may be capable of inducing greater blur. If the child wears glasses, application of a translucent filter, such as Scotch Magic Tape (3M, St Paul, MN) or a Bangerter foil (Ryser Optik AG, St Gallen, Switzerland), to the spectacle lens can be tried. Optical penalization may be more acceptable than occlusion therapy to many children and their parents, but patients must be closely monitored to ensure proper utilization (no peeking) of spectacle-borne devices.
Complications of Therapy
With occlusion therapy and penalization, there is a risk of overtreatment, which can result in reverse amblyopia in the sound eye. Development of strabismus is also a risk. Full-time occlusion carries the greatest risk of reverse amblyopia and of strabismus and thus requires close monitoring. Consequently, most ophthalmologists do not use full-time occlusion in younger children. The parents of a strabismic child should be instructed to watch for a switch in fixation preference and to report its occurrence promptly. Iatrogenic reverse amblyopia can usually be treated successfully with judicious patching of the formerly better-seeing eye or by alternating occlusion. Sometimes, simply stopping treatment leads to equalization of vision.
Adherence issues
Lack of adherence to the therapeutic regimen is a common problem that can prolong the treatment
period or lead to outright failure. If difficulties derive from a particular treatment method, the clinician should seek a suitable alternative. Families who appear to lack sufficient motivation should be counseled concerning the importance of the therapy and the need for consistency in carrying it out. They can be reassured that once an appropriate routine is established, the daily effort required is likely to diminish, especially if the amblyopia improves.
Methods to improve a resistant child’s adherence to treatment vary according to age. In infants and toddlers, physical methods such as wearing arm splints or mittens or making the patch more adhesive with tincture of benzoin may be useful. For children older than 3 years, creating goals and offering rewards tend to work well, as does linking patching to play activities (eg, decorating the patch or patching while the child plays a video game). Authoritative words directed specifically at the child by the clinician may also help.
In some patients, skin irritation due to the adhesive may develop. Switching to a different brand of patch or preparing the skin with tincture of benzoin or ostomy adhesive can eliminate most skinrelated problems.
Unresponsiveness
In some cases, even conscientious application of an appropriate therapeutic program fails to improve vision at all or beyond a certain level. Complete or partial unresponsiveness to treatment occasionally affects younger children but more often occurs in patients older than 5 years. A repeat comprehensive eye examination to look for potential subtle optic nerve or retinal anomalies is indicated when there is a significant deviation from expected response to amblyopia treatment in the face of good adherence to the program. Neuroimaging might be considered in cases that inexplicably fail to respond to treatment.
The decision whether to initiate or continue treatment in a prognostically unfavorable situation should take into account the wishes of the patient and family. Primary therapy should generally be terminated if there is a lack of demonstrable progress over 3–6 months despite good treatment adherence. Amblyopia is not always fully correctable, even at younger ages of treatment.
Recurrence
When amblyopia treatment is discontinued after complete or partial improvement of vision, approximately one-third of patients show some degree of recurrence. Reducing the occlusion regimen to 1–2 hours per day or the frequency of pharmacologic penalization for a few months before cessation decreases the incidence of recurrence. If recurrence occurs, visual acuity can usually be improved again with resumption of therapy. If the need for maintenance therapy is established, treatment must be continued until stability of vision is demonstrated with no treatment other than regular spectacles. This may require periodic monitoring until age 8–10 years. As long as vision remains stable, intervals of up to 12 months between follow-up visits are acceptable. The improvement in vision that is obtained in most children treated between 7 and 12 years of age is sustained following cessation of treatment.