Description

• Amblyopia is a reduction in visual acuity as a result of abnormal visual development in the absence of a structural or pathologic abnormality of the eye; it cannot be corrected by eyeglasses or contact lenses.
• The abnormality is in the brain's visual pathways and interpretative areas, not in the eye itself, although it is the eye that triggers the perceptual change.
• Functional amblyopia is potentially reversible with occlusion therapy.
• Organic amblyopia is irreversible.
• The visual acuity deficit is typically unilateral; rarely, it may be bilateral.
• The reduction in vision is usually apparent in the eye with the greater refractive error or with abnormal position of gaze (strabismus).
• Identification of abnormal visual pathway development became the basis of the 1981 Nobel Prize in Physiology or Medicine awarded to David H. Hubel and Torsten Wiesel.

• System(s) affected: Nervous
• Synonym(s): Lazy eye, Squint

Pediatric Considerations
More commonly seen in the pediatric age group. The mean age at presentation is 3–6 years.

Epidemiology

• Predominant age: Onset may be present from birth or can appear in early childhood. If diagnosed in adulthood, it is most likely permanent and uncorrectable. The condition often goes undiagnosed but can be detected at any age.
• Predominant gender: Male > Female

Incidence
Amblyopia carries a projected lifetime risk of visual loss of 1.2%.

Prevalence
~2–3% of children

Risk Factors

• Pre-existing refractive error, such as myopia, hyperopia, or astigmatism
• More common with a pre-existing occlusion of the visual pathway, such as abnormal eyelid position, hemangioma, or malposition of the eye
• Conditions that cause anisometropia (unequal refractive difference between the eyes) or obstruction to clear vision (i.e., cataract, corneal abnormalities) can lead to permanent amblyopia.

Genetics
Increased incidence in children with a parent or sibling with a history of amblyopia

General Prevention

• All infants and children should be screened from birth for normal visual acuity and development. Refractive errors or abnormal position of the eyes should be promptly evaluated by an ophthalmologist and treated if permanent visual loss is to be avoided.
• Children do not grow out of refractive errors or strabismus and should always be evaluated and treated by a specialist in pediatric ophthalmology.

Pathophysiology

• Strabismic amblyopia is a loss of visual acuity due to suppression of the images from an eye that turns in or out in an individual with misalignment of the visual axis.
• Anisometropic amblyopia is present when 1 eye has a significantly different refractive error than the other, especially if that error is hyperopia; it leads to visual blurring and therefore suppression of the image from that eye.
• Refractive amblyopia is due to uncorrected high refractive error, resulting in visual blurring in either or both eyes.
• Deprivation amblyopia (amblyopia ex anopsia) is due to relatively complete visual deprivation in 1 eye, which may be caused by a congenital abnormality such as a corneal scar or cataract.
• Deficiency amblyopia is also known as nutritional optic neuropathy or tobacco–alcohol amblyopia. Deficiencies of vitamin B₁, B₁₂, or riboflavin may be responsible.
• Amblyopia can only occur early in life:
  • When the brain detects unequal images, for any reason, it is forced to ignore one.
  • The ability of the brain to suppress the unwanted image can only occur when the development of neuroadaptive responses is in a critical “plastic” period, usually the 1st several years of life.
  • If amblyopia has not developed after that period, the individual will be unable to “suppress” the unwanted image, and diplopia or double vision will result.

Etiology

• Strabismus causes disparate retinal images whereby 1 eye sees the object of regard in the fovea and the other in a different part of the retina.
• Inability to fuse the 2 images from each eye results in the brain ignoring the less preferred image (this does not necessarily need to be the less clear image).
• Refractive errors such as anisometropia (a difference in refractive error between the 2 eyes) can cause the 2 retinal images to be of unequal clarity.
• An obstruction to the visual axis, such as cataracts, causes unequal clarity of the retinal image. Cataract in an infant or child, therefore, is a medical emergency warranting surgical intervention and prompt removal.
• The result of 1 eye seeing better than the other interrupts the development of fine visual perception, which can contribute to the development of amblyopia.
• Individuals with amblyopia do not have normal degrees of stereo vision and often complain of not appreciating 3D images.

Commonly Associated Conditions

Neurological abnormalities, Down syndrome, cerebral palsy

History

• Squinting of 1 eye in bright light is the most common symptom, hence the alternative term for strabismus, “squint.”
• Rubbing the eyes
• Sitting close to television or computer screen
• Problems in sports
• Preference for front-row seating
• Covering or closing an eye
• Eye turns in or out, wandering eye
• Poor vision in an eye without apparent explanation or a diagnosable organic cause
• Poor vision that does not correct with glasses
• Often, because 1 eye is seeing properly, parents, schools, and physicians are not aware of a potentially amlyopic eye.
• Absence of “red reflex,” such as that seen with flash photography in both eyes

Physical Exam

• All children should have complete visual exams prior to starting school, with each eye tested individually (this is critical, children peak under cover) (1)[B]. Children from families with a known history of amblyopia or strabismus should have dilated exams performed by an ophthalmologist.
• The corneal light reflex test (shining a light into the child's eyes and noting the location of the light in relation to the pupil) may be used to assess ocular alignment in young children. Observing a photograph of the child looking into the camera can often reveal the abnormality as well (2)[C].

Diagnostic Tests and Interpretation

Specialized tests such as Worth Four Dot, Steroscopic testing, and monocular vs. binocular acuity testing can reveal abnormalities in acuity between the 2 eyes.

Lab
There is no lab test for amblyopia.

External photographs can reveal an obstruction to the visual pathway or a malalignment in the positions of the eyes.

Initial Imaging Approach
At the first examination, the face and position of the eyes should be documented with flash photography.

• Any of the above conditions indicate prompt evaluation and ophthalmologic referral. In a young child, the earlier the diagnosis, the better the therapeutic outcome.

• Children with cataract may require cataract surgery.
• Unequal refractive errors need to be promptly treated with glasses or contact lenses to improve sight before amblyopia sets in.
• Any concern regarding the positions of the eyes or visual acuity requires a prompt referral to an ophthalmologist.

• Measurement of visual acuity (this can be accomplished even in infants and nonverbal children
• Measurement of the alignment and position of the eyes (Krimsky, Prism testing, Cover-Uncover tests)

Pathological Findings
The presence of a pathological abnormality of the eye, cataract, retinal lesion, precludes the diagnosis of amblyopia but necessitates prompt evaluation and intervention.

Differential Diagnosis

• The diagnosis of amblyopia can be confused with an organic lesion causing decreased visual acuity, and must always be excluded before amblyopia is considered.
• Intraocular tumors, glaucoma, congenital abnormalities, and trauma can result in and be mistaken for amblyopia.

Medication (Drugs)

Cycloplegia is often utilized to pharmacologically paralyze the ability of the eye to accommodate, necessitating the use of the “weaker” or nondominant amblyopic eye.

Additional Treatment

• Correction of the underlying disorder should be instituted promptly, as the condition may become irreversible if the child is >4–6 years of age.
• Patching of the stronger eye to encourage visual development of the amblyopic eye is warranted. There may be resistance from the child to wearing a patch, which may lessen benefit. Various patching regimens have been studied (from 2 to 23 hours of patching per day for 4–6 months, and alternate patching). Close follow-up is necessary. Should the “good” eye be patched excessively, the risk of development of amblyopia in that eye increases.
• An alternative therapy to patching is pharmacologic blurring, usually achieved with atropine eye drops. Similar efficacy is achieved compared to patching, with improved compliance. The risk associated with systemic effects from the antimuscarinic effects of the drug cannot be ignored (3,4)[A].
• Corrective lenses should be prescribed for refractive errors.
• Correction of anatomic obstructions, including cataracts or ptosis, may improve vision and minimize recurrence.
• Amblyopia never corrects itself spontaneously and will always require treatment. Children do not outgrow amblyopia.
• Deficiency amblyopia: Balanced diet, vitamins, and avoidance of alcohol and tobacco
• All treatment for amblyopia, regardless of cause, needs to be instituted promptly and early, if success is to be achieved. Studies have shown that intervention after the age of 8 greatly lessens the outcome, although treatment of amblyopia has met with limited success in adults.

Issue for Referral
Obstruction of vision in the infant or toddler is a medical emergency. Failure to refer can result in irreversible loss of vision in an otherwise healthy eye.

Complementary and Alternative Therapies

• There are no effective homeopathic remedies for amblyopia.
• Vision training can be an effective adjunct only if the underlying organic causes are addressed and patching therapy instituted.

Surgery/Other Procedures

Surgical correction of an abnormal eye position or intraocular obstruction may be required.

Follow-Up Recommendations

• All children diagnosed with amblyopia need to be followed for years to prevent recurrence.
• Amblyopia treatment is not a one-time therapy; lapses and recurrences are common, and therefore continued monitoring and intervention is required for years.

Patient Monitoring
Once the diagnosis of amblyopia is made, the patient must be seen frequently until complete resolution of the problem occurs.

Diet

There is no known dietary effects on amblyopia.

Patient Education

Advise all parents to have children's eyes examined as infants and again prior to starting school if no abnormalities are noted.

Prognosis

• A treatable condition in most cases if the diagnosis is made early:
  • Patching therapy, pharmacologic blurring, eyeglasses, and surgical correction of abnormal eye positions can result in near-normal vision when instituted early.
  • Visual development occurs during the first several years of life, and amblyopia therapy can be effective until 12 years.
• The risk of recurrence is 24% after 1 year; reinstitution of treatment is warranted.
• 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 exam.
• The best outcome is achieved if treatment is started before age 8. Research has shown that children >12 and some adults can also show improvement with treatment (5)[B].
• Children from 9–11 who wore an eye patch and performed near-point activities (vision therapy) were 4 times as likely to show a 2-line improvement on a standard 11-line eye chart than amblyopic children who did not receive treatment.
• Adolescents aged 13–17 showed limited improvement, not as great as when treatment was instituted at a younger age.
• A 2004 study showed that perceptual learning can improve vision in amblyopic adults (6)[B].
• Virtual reality computer games, especially when each eye receives different images, has shown limited success in improving monocularity in the affected eye.

Complications

• Failure to institute therapy early in life can lead to permanent, irreversible, unilateral visual loss:
  • Unilateral amblyopia causes an increased risk of severe visual impairment due to loss of vision in the nonamblyopic eye.
• Psychosocial complications include difficulty in schooling, work, or physical activity, and an increased risk of depression and anxiety.

See Also

Refractive Errors; Strabismus

ICD-9

• 368.00 Amblyopia, unspecified
• 368.01 Strabismic amblyopia
• 368.02 Deprivation amblyopia
• 368.03 Refractive amblyopia

ICD-10

• H53.009 Unspecified amblyopia, unspecified eye
• H53.001 Unspecified amblyopia, right eye
• H53.002 Unspecified amblyopia, left eye
• H53.003 Unspecified amblyopia, bilateral
• H53.011 Deprivation amblyopia, right eye
• H53.012 Deprivation amblyopia, left eye
• H53.013 Deprivation amblyopia, bilateral
• H53.019 Deprivation amblyopia, unspecified eye
• H53.021 Refractive amblyopia, right eye
• H53.022 Refractive amblyopia, left eye
• H53.023 Refractive amblyopia, bilateral
• H53.029 Refractive amblyopia, unspecified eye
• H53.031 Strabismic amblyopia, right eye
• H53.032 Strabismic amblyopia, left eye
• H53.033 Strabismic amblyopia, bilateral
• H53.039 Strabismic amblyopia, unspecified eye

SNOMED

• 387742006 Amblyopia (disorder)
• 387743001 functional amblyopia (disorder)
• 35600002 Strabismic amblyopia (disorder)
• 90927000 Refractive amblyopia
• 193638002 Stimulus deprivation amblyopia