Amblyopia

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Amblyopia represents diminished vision occurring during the years of visual development secondary to abnormal visual stimulation. It is usually unilateral but it can be bilateral. The diminished vision is beyond the level expected from the ocular pathology present.


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Contents

Disease Entity

Amblyopia is a relatively common disorder and a major cause of visual impairment in children. It represents an insult to the visual system during the critical period of development whereby an ocular pathology (ex. strabismus, anisometropia, high refractive error, or deprivation) interferes with normal cortical visual development. Approximately 3-5% of children are affected by amblyopia.

Disease

Amblyopia represents diminished vision occurring during the years of visual development secondary to abnormal visual stimulation. It is usually unilateral but it can be bilateral. The diminished vision is beyond the level expected from the ocular pathology present.

Etiology

Bilateral amblyopia is less common than unilateral amblyopia. Bilateral cases are caused by bilateral image blur (anterior visual pathway). Examples of etiologies for bilateral amblyopia include bilateral media opacities (ex. corneal opacities, infantile or childhood cataracts, or vitreous hemorrhages), or ametropia (bilateral high astigmatism or high hypermetropia). Unilateral causes of amblyopia also include the same types of media opacities seen in bilateral cases. However, the most common causes of unilateral amblyopia are strabismus and anisometropia, or a combination of the two1.

The etiologies of amblyopia can be easily remembered with the following pnemonic: S.O.S. Spectacles (anisometropia or high myopic or hyperopic refractive error), Occlusion (media opacities, retinal disease, optic nerve pathology, corneal disease, etc.), and Strabismus.

Risk Factors

A positive family history of strabismus, amblyopia, or media opacities would increase the risk of amblyopia in the child. Children who have conditions that increase the risk of strabismus, anisometropia, or media opacities (ex. Down syndrome) would also be at increased risk for the development of amblyopia. The risk of developing amblyopia, from a condition that is known to cause amblyopia, diminshes as the child approaches 8-10 years of age. As a corollary to this, the depth of amblyopia is typically less severe the older the child is at the time of onset of the amblyogenic factor.

General Pathology

In cases of bilateral amblyopia, the basic pathology is a significant blurred retinal image in each eye causing a disruption of normal visual development. This disruption must occur during the critical period of visual development (the first 8-10 years of life). The depth of damage depends on the severity of the blur, the length of time of the abnormal vision, and the age of onset of the insult. The pathology involved in unilateral amblyopia can be twofold. Retinal image blur in one eye can inhibit cortical activity from one eye, preventing normal visual development. Alternatively, misaligned eyes can prevent the normal process of fusion from taking place. This can result in suppression of the deviating eye, diminishing the acuity of the eye and loss of binocularity.

Pathophysiology

Abnormal visual stimulation during the critical period of visual development results in brain damage. Structural and functional damage occurs in the lateral geniculate nucleus and the striate cortex cortex of the visual center in the occipital lobe in the form of atrophy of connections, loss of cross-linking between connections and loss of laterality of connections.

Primary prevention

The key to prevention is detection. There are numerous techniques to detect amblyopia, all with varying degrees of specificity, sensitivity, complexity and cost. These include a complete ophthalmic examination, photoscreening, visual evoked potentials, acuity charts, and tests of stereopsis and binocular function. Children who are at higher risk for amblyopia should be watched closely for early signs of this condition. In general, the quicker amblyopia is detected and addressed, the less negative effect it has on the visual system. Vision screening is advocated on the state level to screen as many children as possible for this disease prior to the age of kindergarten. Early intervention results in better overall vision. This is why the American Association for Pediatric Ophthalmology and Strabismus, the American Academy of Pediatrics, and the American Academy of Ophthalmology all support pre-kindergarten vision screening for children.

Diagnosis

Amblyopia should be considered as a possible diagnosis in children with asymmetric visual behavior or acuity. It can also complicate the course of children with strabismus, or unilateral ocular or adnexal pathology such as a cataract, eyelid capillary hemangioma or corneal scar. Bilateral amblyopia can also occur and should be thought of when a bilateral ocular condition occurs and despite treatment, some degree of diminsihed acuity persists. A careful history, thorough physical examination and knowledge of possible etiologies of amblyopia can help the clinician to diagnose this condition.

History

Parents will often bring their child to the Ophthalmologist because of the underlying cause of the amblyopia (ptosis, strabismus, leukocoria, eyelid hemangioma), without realizing that amblyopia is present. In fact, anisometropic amblyopia usually goes undetected until picked up by a vision screening. The overwhelming majority of children with unilateral amblyopia do not complain of decreased acuity because they do not notice it unless one eye is occluded. The history taking process should include any family history of vision problems (specifically amblyopia and strabismus). Parents should be asked if the child was premature, and if they have ever noted any eye misalignment. Any prior testing (ex. school or Pediatrician vision screening, neuroimaging etc.) should be noted. If any abnormality in the child's visual behavior has been noted, the duration is important. Also, some children may already have received care for amblyopia somewhere else. If this is the case, type of treatment and duration should be determined. Old records can be helpful.

Physical examination

Examination should consist of the following:

A. Acuity testing- age appropriate. Single optotypes (without crowding bar) are not recommended as a good acuity testing technique in amblyopes because this test will tend to underestimate the degree of amblyopia (crowding phenomenon).

B. Record the power of any current spectacles

C. Subjective refraction if age appropriate

D. Tests of stereopsis and binocular function (ex. Worth 4 dot testing)

E. External examination (looking for ptosis, lid hemangioma or other lesion which could affect visual development)

F. Presence of absence of an afferent pupil defect

G. Anterior segment examination (looking for any media opacity, or irregularity)

H. Motility and ocular alignment

I. Funduscopic examination

J. Cycloplegic retinoscopy

Signs

The presence or absence of signs of amblyopia would depend on what the underlying etiology for the amblyopia is. Deprivational amblyopia could manifest with ptosis, an eyelid hemangioma, or a cataract for example. Strabismic amblyopia may show a constant or intermittent ocular deviation. Anisometropic amblyopia often shows no obvious signs when observing the patient, but cycloplegic retinoscopy will reveal the anisometropia. On clinical examination, unilateral amblyopia will show asymmetric visual behavior or acuity testing results (although not all patients with asymmetric acuity have amblyopia). Severe cases may have a mild afferent pupillary defect. The crowding phenomenon is important to be aware of when testing visual acuity in an amblyope. The amblyopic eye of these patients will visualize individual letters better than a whole line of letters. Therefore if the visual acuity tester uses individual letters (sans crowding bar), then they may underestimate the degree of amblyopia that is present or miss it entirely. A neutral density filter significantly reduces vision in organic disease, but generally does not in pure amblyopia.

Symptoms

Patients with unilateral amblyopia are often asymptomatic. Occasionally, patients will complain that one eye is blurry, or younger children may report discomfort in the affected eye. Torticollis occurs infrequently. Poor depth perception or clumsiness may be noted.

Clinical diagnosis

In cases of unilateral amblyopia, the diagnosis requires two components. First, the patient must have a condition that can cause unilateral amblyopia. Examples would include strabismus, anisometropia, or a deprivational cause (ptosis, cataract, etc.). Second, the patient must have residual asymmetric acuity beyond the level expected from the underlying condition or that persists after treatment of the underlying condition. For example, a child with anisometropic hyperopia receives proper spectacle correction. Acuity in the more hyperopic eye improves but is still below that of the less hyperopic eye. This asymmetry of acuity represents amblyopia. In cases of bilateral amblyopia, a condition must be present during the critical years of visual development which produces constant, significant visual blur. Examples of such conditions would include bilateral vitreous hemorrhages, bilateral cataracts, bilateral corneal pathology, bilateral high hypermetropia, or bilateral high astigmatism.

Diagnostic procedures

A normal, comprehensive ophthalmic examination is usually all that is necessary to diagnose amblyopia. Components of this examination include (but are not limited to): acuity testing, cycloplegic refraction and retinoscopy, tests of stereopsis and binocular vision, evaluation of pupillary responses, anterior segment examination, cover-uncover and alternate-cover testing, and dilated funduscopic examination. See the Physical Examination section above.

Laboratory test

Laboratory testing is not a typical feature of amblyopia diagnosis. Certainly if the etiology of the amblyopia was unclear, or if vision was deteriorating despite treatment, neuroimaging would be considered.

Differential diagnosis

There are cases of decreased acuity in children in which amblyopia is not present. Ocular pathology or refractive error (or even improper spectacle correction) may cause decreased acuity without any superimposed amblyopia. Prechiasmal lesions or optic nerve insult can also produce unilateral decreased acuity.

Management

Although there is much practitioner variability in the treatment of amblyopia, the general idea is to first treat the underlying cause for the amblyopia. Examples of this treatment would include prescribing glasses for anisometropia, strabismus surgery or spectacles to eliminate strabismus, or removal of a unilateral cataract to eliminate the media opacity. In unilateral or asymmetric cases of amblyopia, if there is a residual visual deficit after the underlying etiology is present then amblyopia is said to exist. This can be addressed with occlusion therapy, pharmacologic therapy, or some other less commonly used modalities. Much of the data on the success of various treatment modalities for amblyopia through the years has come from retrospective, single site chart-review type studies. Over the last decade, there has been an explosion of amblyopia research. The need for prospective randomized trials in the treatment of amblyopia has begun to be met by the Pediatric Eye Disease Investigator Group (PEDIG). This is an NEI-funded network including both University-based and community-based clinicians. The power of such a group lies in its ability to conduct multiple trials in a cost-effective fashion, with simple protocols implemented as part of routine practice2. Patients are enrolled at multiple clinical sites in a prospective randomized fashion, with standardized visual testing protocols4. Important data derived from these studies is present throughout this section on amblyopia.

General treatment

The key to optimal treatment of amblyopia is early detection and intervention. In symmetric bilateral cases, treatment consists of addressing the etiology of the diminished vision. Often there is residual bilateral amblyopia which may improve over time9. In asymmetric cases or unilateral cases, active treatment with patching, pharmacologic agents, or some less commonly used modalities can often improve the residual visual deficit.

Medical therapy

In anisometropic patients, some improvement in amblyopia can occur with glasses alone. Starting treatment in this manner may lessen the burden of subsequent amblyopia therapy for those with denser levels of amblyopia and in some cases may obviate the need for patching or pharmacologic penalization. Patching of the sound eye to improve the acuity of the amblyopic eye is the most commonly used technique to treat amblyopia. Patching compliance is a major concern, with high rates of poor compliance or noncompliance in some studies. Compliance with therapy can be bolstered by parental education and improving parental attitudes towards patching therapy. The number of prescribed patching hours per day varies widely between practitioners. In general most doctors recommend heavier patching regimens for worse degrees of amblyopia. The thought behind this is that heavier patching would improve results and the rapidity of obtaining them. However this practice has been called into question by recent PEDIG studies.

A study of severe amblyopes randomized the patching regimen to 6 hours of prescribed patching per day versus 12 hours per day. At the 4-month outcome visit, acuity improvements and rapidity of improvement were essentially identical between the groups14. A similar study of moderate amblyopes comparing 2 hours of prescribed patching per day to 6 hours per day, also found no difference in results15. Some clinicians also prescribe 'near activities' in conjunction with patching but this was not found to be beneficial in a recent study8.

Pharmacologic penalization of the sound eye is another commonly used modality to treat amblyopia. Dosing can be a drop in the sound eye daily, or on weekends only. A recent study showed results with weekend-only dosing to be similar to daily dosing for moderate amblyopes11. In children who wear hyperopic spectacles, Atropine usage is sometimes combined with replacing the hyperopic lens over the sound eye with a plano lens. This wass felt to 'enhance' treatment,  but a recent study showed only a minimal benefit of this additional step in therapy6. Atropine is the most commonly used pharmacologic agent. A common assumption is that atropine use in the amblyopic patient can only be effective if it induces a fixation switch. This assumption has been called into question by a recent study. Often the decision whether to treat the amblyopic child with patching or pharmacologic agents, is based on the practitioner's practice patterns and parental wishes.

A head-to-head study showed that 6 hours a day of patching therapy produced a slightly more rapid and beneficial effect than daily instillation of Atropine 1%, in moderate amblyopes younger than 7 years of age. However, the final difference at 6 months was not statistically significant and a parental questionnaire showed families preferred pharmacologic therapy over patching13.

Other modalities of medical amblyopia management include foils placed on the sound eye lens of the glasses, contact lenses used as occlusion or for blurring, and the use of levodopa. Levodopa is a prodrug that crosses the blood-brain barrier and may be effective through a neuromodulatory effect on the plasticity of the visual system5.

Medical follow up

Followup during treatment is typically somewhere between every 1-3 months. When treatment is discontinued, followup is necessary to ensure there is no regression of effect12.

Surgery

Amblyopia itself is not a surgical condition, but there are times when surgery may treat the underlying cause of the amblyopia. Refractive surgery may be used to correct anisometropia. Eye muscle surgery can correct strabismus. Cataract, ptosis, vitrectomy, or corneal surgery may alleviate causes of deprivation.

Surgical follow up

Even though surgery may be performed to alleviate some of the etiologies of amblyopia, most cases will still require followup to treat the amblyopia that is present. For example, in a child with strabismic amblyopia, eliminating the ocular misalignment does not automatically fully correct the amblyopia which resulted from the strabismus.

Complications

Overly aggressive amblyopia therapy (especially in younger patients) can produce reverse amblyopia of the sound eye. A new strabismus or a decompensation of an exisiting strabismus can also occur. Patches can be irritating to the skin, and the skin underlying the patch can become hypopigmented relative to the rest of the facial skin. There is also a potential social stigma associated with wearing the patch to school in some cases. Atropine use can cause side effects related to the use of this medication: flushing, rapid heart rate, mood changes (uncommon) and photophobia (common) would be examples of side effects occurring with the use of this medication. Reverse amblyopia can also occur with Atropine use as can decompensation of existing strabismus or development of a new strabismus. Cases of reverse amblyopia are infrequent and usually mild. Most cases resolve with discontinuation of treatment.

Prognosis

The keys to treatment success are younger age at detection/treatment, short course until intervention, and compliance with treatment. The effectiveness of intensive screening protocols to detect amblyopia at a young age has been shown to result in a better acuity of the amblyopic eye at age 7.5 years. Most patients do improve with treatment, but often residual amblyopia remains. With cessation of amblyopia treatment there is a risk of recurrence. In one study, the risk of recurrence was higher with better visual acuity at the time of cessation of treatment, a greater number of lines improved during the previous treatment, and a prior history of recurrence. Orthotropia or excellent stereoacuity at the time of patching cessation did not appear to have a protective effect on the risk of recurrence. In a prospective study of cessation of treatment in children aged 3 to <8 years with successfully treated amblyopia due to anisometropia, strabismus or both, the risk of amblyopia recurrence was found to be 24%. Patients treated with 6 to 8 hours of daily patching had a 4-fold greater odds of recurrence if patching was stopped abruptly rather than when it was reduced to 2 hours per day prior to cessation. Careful and prolonged follow-up during the amblyogenic years, is needed for all children who have been previously treated for amblyopia to prevent a recurrence. In general, the younger amblyopes are treated, the better the likelihood of improvement.

Most textbooks do not recommend trying amblyopia therapy in the second decade of life but some improvement can be obtained in select cases. A study of amblyopia therapy in children aged 7-17 years found that amblyopia improves to some degree with optical correction alone in about one fourth of patients. However most required additional treatment for amblyopia10. For patients aged 7 to 12 years, 2 to 6 hours per day of patching with near visual activities and atropine improved visual acuity even if the amblyopia had been previously treated. For patients 13 to 17 years, improvement was only noted in those children who had not been previously treated. The degree of improvement in these older children was much more modest than results from other studies of younger children, so the importance of early detection and treatment remains.

Additional Resources

AAPOS Frequently Asked Questions about Amblyopia

Prevent Blindness America - Amblyopia

Visit geteyeSmart.org for a patient-friendly summary of Amblyopia.

References

1.Wright KW and Spiegel PH. Pediatric Ophthalmology and Strabismus. 1st ed. pp 195-229. 1999.

2.Bacal DA. Amblyopia Treatment Studies. Curr Opin Ophthalmol. 15:432-436. 2004.

3.Williams C, Northstone K, Harrad RA, et al. Amblyopia treatment outcomes after screening before or at age 3 years:followup from randomized trial. BMJ 2002; 324:1549-1551.

4.Holmes JM, Beck RW, Repka MX, et al. The amblyopia treatment study visual acuity testing protocol. Arch Ophthalmol 2003, 119:1345-1353.

5.Leguire LE, Rogers GL, Walson PD, et al. Occlusion and levodopa carbidopa treatment for childhood amblyopia. J AAPOS 1998, 2:257-264.

6.Pediatric Eye Disease Investigator Group. Pharmacologic plus optical penalization treatment for amblyopia: results of a randomized trial. Arch Ophthalmol 2009;127(1):22-30.

7.Pediatric Eye Disease Investigator Group. Patching vs atropine to treat amblyopia in children aged 7 to 12 years: a randomized trial. Arch Ophthalmol 2008;126(12):1634-1642. {{Article

8.Pediatric Eye Disease Investigator Group. A randomized trial of near versus distance activities while patching for amblyopia in children aged 3 to less than 7 years. Ophthalmology 2008;115(11):2071-8.

9.Pediatric Eye Disease Investigator Group. Treatment of bilateral refractive amblyopia in children three to less than 10 years of age. Am J Ophthalmol 2007;144(4):487-96.

10.Pediatric Eye Disease Investigator Group. Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol 2005;123(4):437-47.

11.Pediatric Eye Disease Investigator Group. A randomized trial of atropine regimens for treatment of moderate amblyopia in children. Ophthalmology 2004;111(11):2076-85.

12.Pediatric Eye Disease Investigator Group. Risk of amblyopia recurrence after cessation of treatment. J AAPOS 2004;8(5):420-8.

13.Pediatric Eye Disease Investigator Group. A randomized trial of atropine vs. patching for treatment of moderate amblyopia in children. Arch Ophthalmol 2002;120(3):268-278.

14.Pediatric Eye Disease Investigator Group. A randomized trial of prescribed patching regimens for treatment of severe amblyopia in children. Ophthalmology 2003;110:2075-2087.

15.Pediatric Eye Disease Investigator Group. A randomized trial of patching regimens for treatment of moderate amblyopia in children. Ophthalmology 2003;121:603-611.

Original article contributed by: Darron A. Bacal, M.D.
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