eMedicine Specialties > Ophthalmology > Neurologic Disorders

Amblyopia

Kimberly G Yen, MD, Assistant Professor of Ophthalmology, Department of Ophthalmology, Cullen Eye Institute, Baylor College of Medicine

Updated: Dec 31, 2008

Introduction

Background

Amblyopia refers to a decrease of vision, either unilaterally or bilaterally, for which no cause can be found by physical examination of the eye. The term functional amblyopia often is used to describe amblyopia, which is potentially reversible by occlusion therapy. Organic amblyopia refers to irreversible amblyopia.^1,2,3

Most vision loss from amblyopia is preventable or reversible with the right kind of intervention. The recovery of vision depends on how mature the visual connections are, the length of deprivation, and at what age the therapy is begun. It is important to rule out any organic cause of decreased vision because many diseases may not be detectable on routine examination.

Pathophysiology

Although many types of amblyopia exist, it is believed that their basic mechanisms are the same even though each factor may contribute different amounts to each specific type of amblyopia. In general, amblyopia is believed to result from disuse from inadequate foveal or peripheral retinal stimulation and/or abnormal binocular interaction that causes different visual input from the foveae.⁴

Three critical periods of human visual acuity development have been determined.^5,6 During these time periods, vision can be affected by the various mechanisms to cause or reverse amblyopia. These periods are as follows:

• The development of visual acuity from the 20/200 range to 20/20, which occurs from birth to age 3-5 years.
• The period of the highest risk of deprivation amblyopia, from a few months to 7 or 8 years.
• The period during which recovery from amblyopia can be obtained, from the time of deprivation up to the teenage years or even sometimes the adult years.

Whether different visual functions (eg, contrast sensitivity, stereopsis) have different critical periods is not known. In the future, determination of these time frames may help modify treatment of amblyopia.

Frequency

United States

Prevalence of amblyopia is difficult to assess and varies in the literature, ranging from 1-3.5% in healthy children to 4-5.3% in children with ophthalmic problems. Most data show that about 2% of the general population has amblyopia.

Amblyopia was shown in the Visual Acuity Impairment Survey sponsored by the National Eye Institute (NEI) to be the leading cause of monocular vision loss in adults aged 20-70 years or older. Prevalence of amblyopia has not changed much over the years.

Mortality/Morbidity

Amblyopia is an important socioeconomic problem. Studies have shown that it is the number one cause of monocular vision loss in adults. Furthermore, persons with amblyopia have a higher risk of becoming blind because of potential loss to the sound eye from other causes.

Race

No racial preference is known.

Sex

No gender preference is known.

Age

Amblyopia occurs during the critical periods of visual development. An increased risk exists in those children who are developmentally delayed, were premature, and/or have a positive family history.

Clinical

History

• Ocular history
  • Elicit any previous history of patching or eye drops as well as past compliance with these therapies.
  • Document previous ocular surgery or disease.
• In addition to the routine information, obtaining a family history of strabismus or other ocular problems is important because the presence of these ocular problems may predispose a child to amblyopia.

Physical

• Visual acuity
  • Diagnosis of amblyopia usually requires a 2-line difference of visual acuity between the eyes; however, this definition is somewhat arbitrary and a smaller difference is common.
  • Crowding phenomenon: A common characteristic of amblyopic eyes is difficulty in distinguishing optotypes that are close together. Visual acuity often is better when the patient is presented with single letters rather than a line of letters.
• Diagnosis is not an issue in children old enough to read or with use of the tumbling E.
• Testing in preverbal children
  • If the child protests with covering of the sound eye, amblyopia can be diagnosed if it is dense.
  • Fixation preference may be assessed, especially when strabismus is present.
  • Induced tropia test may be performed by holding a 10-prism diopter before one eye in cases of an orthophoria or a microtropia.
  • In infants who cross-fixate, pay attention to when the fixation switch occurs; if it occurs near primary position, then visual acuity is equal in both eyes.
  • Caution should be used when obtaining Teller acuity in children, as grating acuity may be less reduced than Snellen acuity, especially in strabismic amblyopia.
• Contrast sensitivity: Strabismic and anisometropic amblyopic eyes have marked losses of threshold contrast sensitivity, especially at higher spatial frequencies; this loss increases with the severity of amblyopia.
• Neutral density filters: Patients with strabismic amblyopia may have better visual acuity or less of a decline of visual acuity when tested with neutral density filters compared to the normal eye. This was not found to be true in patients with anisometropic amblyopia or organic disease.
• Binocular function: Amblyopia usually is associated with changes in binocular function or stereopsis.
• Eccentric fixation: Some patients with amblyopia may consistently fixate with a nonfoveal area of the retina under monocular use of the amblyopic eye, the mechanism of which is unknown. This can be diagnosed by holding a fixation light in the midline in front of the patient and asking them to fixate on it while the normal eye is covered. The reflection of the light will not be centered.
• Refraction: Cycloplegic refraction must be performed on all patients, using retinoscopy to obtain an objective refraction. In most cases, the more hyperopic eye or the eye with more astigmatism will be the amblyopic eye. If this is not true, one needs to investigate further for ocular pathology.
• Rest of examination: Perform a full eye examination to rule out ocular pathology.

Causes

Many causes of amblyopia exist; the most important causes are as follows^2,1 :

• Anisometropia
  • Inhibition of the fovea occurs to eliminate the abnormal binocular interaction caused by one defocused image and one focused image.
  • This type of amblyopia is more common in patients with anisohypermetropia than anisomyopia. Small amounts of hyperopic anisometropia, such as 1-2 diopters, can induce amblyopia. In myopia, mild myopic anisometropia up to -3.00 diopters usually does not cause amblyopia.
  • Hypermetropic anisometropia of 1.50 diopters or greater is a long-term risk factor for deterioration of visual acuity after occlusion therapy.
• Strabismus
  • The patient favors fixation strongly with one eye and does not alternate fixation. This leads to inhibition of visual input to the retinocortical pathways.
  • Incidence of amblyopia is greater in esotropic patients than in exotropic patients.
• Strabismic anisometropia: These patients have strabismus associated with anisometropia.
• Visual deprivation: Amblyopia results from disuse or understimulation of the retina. This condition may be unilateral or bilateral. Examples include cataract, corneal opacities, ptosis, and surgical lid closure.⁷
• Organic: Structural abnormalities of the retina or the optic nerve may be present. Functional amblyopia may be superimposed on the organic visual loss.

Differential Diagnoses

Other Problems to Be Considered

Consider organic problems that may be subtle or difficult to diagnose, such as optic nerve hypoplasia.

Workup

Imaging Studies

• If suspicion exists of an organic cause for decreased vision and the ocular examination is normal, then further investigations into retinal or optic nerve causes should be initiated. Studies to perform include imaging of the visual system through CT scan, MRI, and fluorescein angiography to assess the retina.

Other Tests

• Although differences in the electrophysiologic responses of normal eyes versus amblyopic eyes have been reported, these techniques remain investigational and the differences are controversial.

Procedures

• See Physical.

Histologic Findings

Histologic studies of the lateral geniculate nucleus in kittens with deprivation amblyopia have shown that cells receiving input from the deprived eye were shrunken and atrophied, while cells receiving input from the nondeprived eye were expanded.

Treatment

Medical Care

• The clinician must first rule out an organic cause and treat any obstacle to vision (eg, cataract, occlusion of the eye from other etiologies).
• Remove cataracts in the first 2 months of life, and aphakic correction must occur quickly.
• Treatment of anisometropia and refractive errors must occur next.^8,9,10,11
  • The amblyopic eye must have the most accurate optical correction possible. This should occur prior to any occlusion therapy because vision may improve with spectacles alone.
  • Full cycloplegic refraction should be given to patients with accommodative esotropia and amblyopia. In other patients, a prescription less than the full plus measurement that was refracted may be prescribed given that the decrease in plus is symmetric between the two eyes. Because accommodative amplitude is believed to be decreased in amblyopic eyes, one needs to be cautious about cutting back too much on the amount of plus. Refractive correction alone has been shown to improve amblyopia in up to 77% of patients in a nationwide trial. 
  • Patients with bilateral refractive amblyopia do well with spectacle correction alone, with most children aged 3-10 years achieving 20/25 or better within a year.^12,13
• The next step is forcing the use of the amblyopic eye by occlusion therapy. Occlusion therapy has been the mainstay of treatment since the 18th century. The following are general guidelines for occlusion therapy:
  • Patching may be full-time or part-time. Standard teaching has been that children need to be observed at intervals of 1 week per year of age, if undergoing full-time occlusion to avoid occlusion amblyopia in the sound eye. The Amblyopia Treatment Studies (ATS) have helped to provide new information on the effect of various amounts of patching.^14,15
  • Always consider lack of compliance in a child where visual acuity is not improving. Compliance is difficult to measure but is an important factor in determining the success of this therapy.
  • In addition to adhesive patches, opaque contact lenses, occluders mounted on spectacles, and adhesive tape on glasses have been used.
  • Establishing the fact that the vision of the better eye has been degraded sufficiently with the chosen therapy is important.
  • The Amblyopia Treatment Studies have helped to define the role of full-time patching versus part-time patching in patients with amblyopia. The studies have demonstrated that, in patients aged 3-7 years with severe amblyopia (visual acuity between 20/100 and 20/400), full-time patching produced a similar effect to that of 6 hours of patching per day. In a separate study, 2 hours of daily patching produced an improvement in visual acuity similar to that of 6 hours of daily patching when treating moderate amblyopia (visual acuity better than 20/100) in children aged 3-7 years. In this study, patching was prescribed in combination with 1 hour of near visual activities.
  • Data from the Amblyopia Treatment Studies are also available for older patients. For patients aged from 7 years to younger than 13 years, the Amblyopia Treatment Studies have suggested that prescribing 2-6 hours a day of patching can improve visual acuity even if the amblyopia has been previously treated. For patients aged from 13 years to younger than 18 years, prescribing 2-6 hours a day of patching might improve visual acuity when amblyopia has not been previously treated; however, this is likely to be of little benefit if amblyopia was previously treated with patching. Long-term results from these studies are still pending.¹⁶
  • The Amblyopia Treatment Studies have also found that about one fourth of children with amblyopia who were successfully treated experience a recurrence within the first year after discontinuation of treatment. Data from these studies suggest that patients treated with 6 or more hours a day of patching have a greater risk of recurrence when patching is stopped abruptly rather than when it is reduced to 2 hours a day prior to cessation of patching. Randomized studies have still yet to be performed.¹⁷
• Penalization therapy
  • In the past, penalization therapy was reserved for children who would not wear a patch or in whom compliance was an issue. The Amblyopia Treatment Studies, however, have demonstrated that atropine penalization in patients with moderate amblyopia (defined by the study as visual acuity better than 20/100) is as effective as patching. The Amblyopia Treatment Studies were performed in children aged 3-7 years.^18,19,20,21
  • The Amblyopia Treatment Studies have also demonstrated that weekend use of atropine provided an improvement in visual acuity similar to that of daily use of atropine when treating moderate amblyopia in children aged 3-7 years.
  • Atropine drops or ointment is instilled in the nonamblyopic eye. This therapy is sometimes used in conjunction with patching or occlusion of the glasses (eg, adhesive tape, nail polish) by individual practitioners. In the Amblyopia Treatment Studies that evaluated patching versus atropine penalization, atropine penalization and patching were used in conjunction with 1 hour of near visual activities.
  • This technique may also be used for maintenance therapy, which is useful, especially in patients with mild amblyopia.
• Other options include optical blurring through contact lenses or elevated bifocal segments.²²
• The endpoint of therapy is spontaneous alternation of fixation or equal visual acuity in both eyes.²³
  • When visual acuity is stable, patching may be decreased slowly, depending on the child's tendency for the amblyopia to recur.
  • Because amblyopia recurs in a large number of patients (see Prognosis), maintenance therapy or tapering of therapy should be strongly considered. This tapering is controversial, so individual physicians vary in their approaches.
• Treatment of strabismus generally occurs last. The endpoint of strabismic amblyopia is freely alternating fixation with equal vision. Surgery generally is performed after this endpoint has been reached.

Surgical Care

Surgical therapy for strabismus generally should occur after amblyopia is reversed. Disadvantages to surgical therapy prior to correction of amblyopia include difficulty in telling if amblyopia is present because there is no longer a strabismus to assess fixation preference and higher potential to being lost to follow-up, as the child cosmetically looks better. The improved cosmesis gives the parents a false sense of security about the vision improving.

Activity

Close supervision during occlusion therapy is necessary to make sure children do not peek. Various methods of preventing children from removing patches have been considered, from a reward system for older children to arm splints and mittens for infants.

Medication

Pharmacologic treatment with levodopa has been investigated and has showed transient improvement of vision in amblyopic eyes. However, the exact role of such pharmacologic agents has not been determined. Levodopa currently is not being used clinically.

Atropine penalization (with either ointment or drops) is an alternative method of blurring vision in the sound eye of patients who refuse patching. It may be applied once a day to patients in the preferred eye only.

Cycloplegics

These agents are used to blur vision in one eye to treat amblyopia in the contralateral eye.

Atropine sulfate 0.5 - 2.0% (Isopto, Atropair, Atropisol)

A topically applied muscarinic antagonist, which blocks the action of acetylcholine. This results in paralysis of the iris sphincter and resultant pupillary dilation. Paralysis of the ciliary muscles also occurs, which inhibits accommodation and relieves pain in iridocyclitis. The medication is dispensed in a topical formulation, either an ointment or a solution.

Dosing

Adult

1 gtt in the fornix in affected eye 1-3 times qd, then taper as necessary

Pediatric

1 gtt in the fornix in affected eye qd

Interactions

Coadministration with other anticholinergics have additive effects; pharmacologic effects of atenolol and digoxin may increase with atropine; antipsychotic effects of phenothiazines may decrease with this medication; tricyclic antidepressants with anticholinergic activity may increase effects of atropine

Contraindications

Documented hypersensitivity; thyrotoxicosis; narrow-angle glaucoma; tachycardia; infants; albino patients; Down syndrome

Precautions

Pregnancy

C - Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus

Precautions

Adverse effects include flushing, fever, tachycardia, urinary retention, and delirium; adverse effects are usually dose-related from systemic absorption; local irritation may occur; emphasize that the medication should be used topically only and kept out of reach of children; practice proper hand washing after administration; caution in coronary heart disease, congestive heart failure, cardiac arrhythmias, hypertension, peritonitis, ulcerative colitis, hepatic disease, and hiatal hernia with reflux esophagitis; in prostatic hypertrophy, prostatism can have dysuria and may require catheterization

Follow-up

Further Outpatient Care

• Outpatient follow-up care needs to continue beyond the primary completion of amblyopia treatment because visual deterioration occurs in many children. In a multicenter study conducted as part of the Amblyopia Treatment Studies, one fourth of patients experienced recurrence of amblyopia within the first year after treatment, with the risk of recurrence greater if the treatment was stopped abruptly rather than tapered. One study by Levartovsky et al showed deterioration in 75% of children with anisometropia of 1.75 diopters or more after occlusion therapy.²⁴  Recidivism can occur, even several years after the initial treatment period, and is as high as 53% after 3 years.

Deterrence/Prevention

• Vision screening programs: Studies have shown these programs to be technically easy and that they help reduce cost as well as incidence of amblyopia because of early treatment and detection. Current programs include use of the photorefractor and school vision screening programs.
• Amblyopia after trauma
  • Young patients who have trauma to their eyes often are at risk for occlusion amblyopia. Possible reasons include lid edema, hyphema, occlusive dressing, vitreous hemorrhage, and traumatic cataract.
  • This amblyopia often is superimposed on a visual deficit caused by any structural abnormality and needs to be taken into account when treating these children.
  • Vision needs to be monitored closely in children after ocular trauma, especially in those aged up to 6 years and in nonverbal children. Occlusive therapy needs to be instituted if there is any suggestion of decreased vision in the injured eye.

Complications

• The main complication of not treating amblyopia is long-term irreversible vision loss. Most cases of amblyopia are reversible if detected and treated early, so this vision loss is preventable.

Prognosis

• After 1 year, about 73% of patients show success after their first trial of occlusion therapy. Studies have shown that the number of patients who retain their level of visual acuity decreases over time to 53% after 3 years.
• Risk factors for failure in amblyopia treatment include the following:
  • Type of amblyopia: Patients with high anisometropia and patients with organic pathology have the worse prognosis. Patients with strabismic amblyopia have the best outcome.
  • Age at which therapy began: Younger patients seem to do better.
  • Depth of amblyopia at start of therapy: The better the initial visual acuity in the amblyopic eye, the better the prognosis.

Patient Education

• Parents need to be educated about the importance of treatment and compliance as well as the visual implications because the treatment of amblyopia often lies in the hands of the parents.
• For excellent patient education resources, visit eMedicine's Eye and Vision Center. Also, see eMedicine's patient education article How to Instill Your Eyedrops.

Miscellaneous

Medicolegal Pitfalls

• The clinician must educate parents and patients of the implications of not treating amblyopia. A delay in treatment beyond the critical period results in irreversibility of amblyopia and could result in liability claims against the physician.

Special Concerns

• Early treatment and detection is the key to visual rehabilitation in amblyopic patients. Methods of identifying children at risk or who have amblyopia are needed. Further research into the mechanism of amblyopia is needed to be able to address and treat the direct etiology.

References

1. American Academy of Ophthalmology. Amblyopia. In: Basic and Clinical Science Course: Pediatric Ophthalmology and Strabismus. 1997: 259-65.

2. Kushner, BJ. Amblyopia. In: Nelson LB, ed. Harley's Pediatric Ophthalmology. 1998:125-39.

3. von Noorden GK. Binocular Vision and Ocular Motility: Theory and Management. 1996;216-54.

4. Lempert P. Retinal area and optic disc rim area in amblyopic, fellow, and normal hyperopic eyes: a hypothesis for decreased acuity in amblyopia. Ophthalmology. Dec 2008;115(12):2259-61. [Medline].

5. Daw NW. Critical periods and amblyopia. Arch Ophthalmol. Apr 1998;116(4):502-5. [Medline].

6. Kirschen DG. Understanding Sensory Evaluation. In: Rosenbaum AL, Santiago AP, eds. Clinical Strabismus Management: Principles and Practice. 1999: 22-35.

7. Lin LK, Uzcategui N, Chang EL. Effect of surgical correction of congenital ptosis on amblyopia. Ophthal Plast Reconstr Surg. Nov-Dec 2008;24(6):434-6. [Medline].

8. Flynn JT. Amblyopia: its treatment today and its portent for the future. Binocul Vis Strabismus Q. Summer 2000;15(2):109. [Medline].

9. Flynn JT, Woodruff G, Thompson JR, et al. The therapy of amblyopia: an analysis comparing the results of amblyopia therapy utilizing two pooled data sets. Trans Am Ophthalmol Soc. 1999;97:373-90; discussion 390-5. [Medline].

10. Flynn JT. 17th annual Frank Costenbader Lecture. Amblyopia revisited. J Pediatr Ophthalmol Strabismus. Jul-Aug 1991;28(4):183-201. [Medline].

11. Brown SM. Verisyse IOL implantation in a child with anisometropic amblyopia. J Cataract Refract Surg. Jul 2008;34(7):1057-8. [Medline].

12. Cotter SA, Edwards AR, Wallace DK, et al. Treatment of anisometropic amblyopia in children with refractive correction. Ophthalmology. Jun 2006;113(6):895-903. [Medline].

13. Wallace DK, Chandler DL, Beck RW, et al. Treatment of bilateral refractive amblyopia in children three to less than 10 years of age. Am J Ophthalmol. Oct 2007;144(4):487-96. [Medline].

14. Holmes JM, Kraker RT, Beck RW, et al. A randomized trial of prescribed patching regimens for treatment of severe amblyopia in children. Ophthalmology. Nov 2003;110(11):2075-87. [Medline].

15. Repka MX, Beck RW, Holmes JM, et al. A randomized trial of patching regimens for treatment of moderate amblyopia in children. Arch Ophthalmol. 2003;121:603-11. [Medline].

16. Scheiman MM, Hertle RW, Beck RW, et al. Randomized trial of treatment of amblyopia in children aged 7 to 17 years. Arch Ophthalmol. Apr 2005;123(4):437-47. [Medline].

17. Holmes JM, Beck RW, Kraker RT, et al. Risk of amblyopia recurrence after cessation of treatment. J AAPOS. 2004;8:420-8. [Medline].

18. Pediatric Eye Disease Investigator Group. A randomized trial of atropine vs. patching for treatment of moderate amblyopia in children. Arch Ophthalmol. 2002;120:268-278. [Medline].

19. Repka MX, Wallace DK, Beck RW, et al. Two-year follow-up of a 6-month randomized trial of atropine vs patching for treatment of moderate amblyopia in children. Arch Ophthalmol. 2005;123:149-157. [Medline]. [Full Text].

20. Repka MX, Cotter SA, Beck RW, et al. A randomized trial of atropine regimens for treatment of moderate amblyopia in children. Ophthalmology. 2004;111:2076-85. [Medline].

21. Scheiman MM, Hertle RW, Kraker RT, et al. Patching vs atropine to treat amblyopia in children aged 7 to 12 years: a randomized trial. Arch Ophthalmol. Dec 2008;126(12):1634-42. [Medline].

22. Collins RS, McChesney ME, McCluer CA, et al. Occlusion properties of prosthetic contact lenses for the treatment of amblyopia. J AAPOS. Dec 2008;12(6):565-8. [Medline].

23. Repka MX. How much amblyopia treatment is enough?. Arch Ophthalmol. Jul 2008;126(7):990-1. [Medline].

24. Levartovsky S, Oliver M, Gottesman N, Shimshoni M. Factors affecting long term results of successfully treated amblyopia: initial visual acuity and type of amblyopia. Br J Ophthalmol. Mar 1995;79(3):225-8. [Medline].

25. Wallace DK, Edwards AR, Cotter SA, Beck RW, Arnold RW, Astle WF, et al. A randomized trial to evaluate 2 hours of daily patching for strabismic and anisometropic amblyopia in children. Ophthalmology. Jun 2006;113(6):904-12. [Medline].

Keywords

amblyopia, lazy eye, functional amblyopia, organic amblyopia, decreased vision, poor visual acuity, vision loss, monocular vision loss, blindness in one eye, strabismus, strabismic anisometropia, anisometropia, amblyopia treatment studies, ATS, refractive error, occlusion therapy, patching

Contributor Information and Disclosures

Author

Kimberly G Yen, MD, Assistant Professor of Ophthalmology, Department of Ophthalmology, Cullen Eye Institute, Baylor College of Medicine
Kimberly G Yen, MD is a member of the following medical societies: Association for Research in Vision and Ophthalmology
Disclosure: Nothing to disclose.

Medical Editor

Gerhard W Cibis, MD, Clinical Professor, Director of Pediatric Ophthalmology Service, Department of Ophthalmology, University of Kansas, Kansas City
Gerhard W Cibis, MD is a member of the following medical societies: American Academy of Ophthalmology, American Association for Pediatric Ophthalmology and Strabismus, and American Ophthalmological Society
Disclosure: Nothing to disclose.

Pharmacy Editor

Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine
Disclosure: Nothing to disclose.

Managing Editor

J James Rowsey, MD, Former Director of Corneal Services, St Luke's Cataract and Laser Institute, Florida
J James Rowsey, MD is a member of the following medical societies: American Academy of Ophthalmology, American Association for the Advancement of Science, American Medical Association, Association for Research in Vision and Ophthalmology, Florida Medical Association, Pan-American Association of Ophthalmology, Sigma Xi, and Southern Medical Association
Disclosure: Nothing to disclose.

CME Editor

Lance L Brown, OD, MD, Ophthalmologist, Affiliated With Freeman Hospital and St John's Hospital, Regional Eye Center, Joplin, Missouri
Disclosure: Nothing to disclose.

Chief Editor

Hampton Roy Sr, MD, Associate Clinical Professor, Department of Ophthalmology, University of Arkansas for Medical Sciences
Hampton Roy Sr, MD is a member of the following medical societies: American Academy of Ophthalmology, American College of Surgeons, and Pan-American Association of Ophthalmology
Disclosure: Nothing to disclose.

The authors and editors of eMedicine gratefully acknowledge the assistance of Ryan I Huffman, MD, with the literature review and referencing for this article.

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