Approach
Amblyopia is best treated at an early age, when the visual system shows greatest plasticity. Amblyopia should, therefore, be diagnosed as soon as possible. The primary care provider with access only to simple, inexpensive tests can detect a child with risk factors for the development of amblyopia.
In the US, screening tests are advised throughout childhood, but are particularly recommended in children ages 3-5 years.[24][26] If a child is considered to be at risk of amblyopia after the initial exam, refer to an orthoptist, a pediatric ophthalmologist, or a pediatric optometrist.[27] These specialists can perform a more complete ophthalmic exam to confirm the diagnosis.
History
History should include questioning about the presence of risk factors. Age <9 years is an important risk factor because the developing brain of a young child is most susceptible to abnormal visual stimulation. The older the child, the more resistant that child is to the effects of optical defocus, suppression, and visual deprivation. Other risk factors include the presence of strabismus and refractive errors. Siblings or parents may have a history of amblyopia or strabismus, so it is important to ask about a family history of lazy eye, the need for glasses before age 5 years, eye muscle surgery, or childhood patching. Any diagnosis of opacities in the visual axis, such as congenital cataract, nonclearing vitreous hemorrhage, macular hemorrhage, or history of prolonged occlusion of one or both eyes (e.g., severe ptosis), should be noted because these are associated with the development of form-deprivation amblyopia. Developmental delay and prematurity are also risk factors for amblyopia.
In preverbal children and infants older than 3 months, parents may have noticed that the child does not seem to track their face or follow objects. Parents may have noticed nystagmus, which before age 6 months can be due to abnormal sensory input or idiopathic with normal structure of the visual system. Bilateral form-deprivation amblyopia causes a bilateral sensory nystagmus. It portends an unfavorable visual outcome, suggesting that amblyopia treatment has already been delayed past the optimal time period. Older children may complain of blurred vision, but most older children with amblyopia, especially if unilateral, are unaware of their visual deficit. Eye strain is not a common presentation of amblyopia.[4]
Primary care provider ocular assessment
The following tests may be performed by the primary care physician as part of a routine eye check. This assessment may also be performed when the possibility of a visual defect is being considered. This initial exam may detect risk factors for amblyopia. Amblyopia without strabismus, unequal or high refractive error, or visual axis obstruction is rare.
Red reflex
A direct ophthalmoscope exam should show a symmetric and bright reflex in each pupil. Frequently, the normal reflex appears bright yellow instead of bright red. The binocular red reflex test (Brückner test) is a useful screening test for nonspecialists. The direct ophthalmoscope light is directed at both pupils from an arm's length away, while the child sits in a darkened room. This "1-second" nonthreatening test can detect media opacities (diminished reflex or opacity within the reflex), strabismus (diminished reflex or absent reflex in the eye that is not directed toward the direct ophthalmoscope light), or high refractive error. Patients with an absent or irregular red reflex, or opacity within the reflex, should be referred for ophthalmology assessment promptly, as this could indicate visual axis obstruction from causes such as cataract or intraocular tumor. Significant hyperopia presents as an inferiorly placed brighter crescent in the red reflex, whereas significant myopia presents as a superiorly placed brighter crescent.[1] Asymmetric crescents could indicate unequal refractive errors between the eyes, which also require further evaluation.
Vision testing
Normal visual acuity increases with age, and the ability to assess visual acuity depends on the child's maturity, attentiveness, and mood, and on the type of test used. For example, a 2-month-old infant has a visual acuity of about 20/400, which cannot be practically tested in the office setting but can be measured with forced preferential looking tests and visual evoked potentials in research settings. Visual acuity increases dramatically in the first year of life. By age 12 months, visual acuity improves to about 20/40. Therefore, a test of vision that is appropriate to the child's age and developmental ability is required.
Infants 3 to 6 months of age: these infants may be more interested in fixing and following a face. Parents should confirm that the child can track their face at home when attentive and that the child has a social/responsive smile.
Infants older than 6 months (preverbal): parents should confirm that the child can track their face at home when he or she is attentive. These children should also be able to fix and follow a small toy. To test, the infant must be cooperative and attentive to achieve a reliable response. Objects that make noise can be used to get the infant's attention, but sound cues should not be present during the assessment of the infant's ability to fix and follow. The fixation and following test, performed under monocular conditions, can be used until the child is verbal. The infant's behavioral response to alternate occlusion of each eye may also be assessed. The infant should object or not object equally in both eyes.
Verbal children (more than about 3 years of age): most children can cooperate with visual acuity testing using an eye chart. The following eye charts are considered most accurate and acceptable by the World Health Organization: Sloan letters, Lea symbols, "H, O, T, V" letters, and the tumbling E chart.[1][24] Snellen letters and Allen figures are not considered as accurate.[1][24] Children who do not know the letters can be tested with a number or picture chart, or taught the "H, O, T, V" letter chart. Shy children who do not want to speak aloud can play a matching game instead. Testing can be performed at 10 feet (3 meters) with appropriately sized targets, as children may be distracted performing a test at the recommended testing distance of 20 feet (6 meters). Most 3- or 4-year-old children can cooperate with visual acuity testing to 20/40, and 5-year-old children to 20/30. Patients should be tested under monocular conditions. In other words, each eye must be tested independently because a normal binocular visual acuity could miss complete blindness in one eye. Children frequently peek, so patches or special occlusive glasses are more reliable than hand-held occluders (a child's hand is perhaps the least effective occluder, as the child may peek between the fingers).
Indications for referral to an ophthalmologist include:[24]
Visual acuity in either eye worse than 20/50 in a 3-year-old
Visual acuity in either eye worse than 20/40 in a 4-year-old to 5-year-old
More than 2-line difference in visual acuity between the two eyes. (In practice, most pediatric ophthalmologists treat amblyopia as long as vision continues to improve, even if the magnitude of difference between the two eyes decreases to just one line or several letters on the eye chart.)[Figure caption and citation for the preceding image starts]: Snellen lettersFrom the collection of Tina Rutar, MD [Citation ends].
[Figure caption and citation for the preceding image starts]: Allen figures on cardFrom the collection of Tina Rutar, MD [Citation ends].
Ocular alignment
Starting at about 3 months of age (or 3 months corrected gestational age), the child should have straight eyes most of the time. Intermittent strabismus is of less concern than constant strabismus at ages 3-6 months, when the visuomotor system is still immature. In a study of nearly 3000 healthy infants, authors observed brief episodes of intermittent esotropia (convergence spasm) up to age 4 months and exodeviations (outward drift of one eye) up to age 6 months.[28] Normal ocular alignment can be confirmed by:
Detecting a symmetric light reflex centered on the pupil in both eyes: a light is shone on the eyes from an arm's length away while the patient looks at a small toy held adjacent to the light. If the reflection of the light is symmetrically centered on the cornea in each eye, no manifest strabismus is present. If it is decentered in one eye, manifest strabismus is likely. [Figure caption and citation for the preceding image starts]: Esotropia: left eye fixating (note decentered light reflection on right cornea)From the collection of Daniel J. Salchow, MD [Citation ends].
Cover/uncover testing: a cooperative child can also undergo this test, which is a more reliable assessment of normal ocular alignment. While the child is fixating (staring) at an object, the examiner should cover one eye and look for a refixation movement of the uncovered eye. Then, the cover should be removed and placed over the other eye to again look for a refixation movement of the uncovered eye. A refixation movement in either eye indicates strabismus. Children with strabismus require referral to an ophthalmologist for further assessment. If the child constantly fixates with one eye, and the other eye remains deviated, the child has decreased vision in the deviated eye, which is often due to strabismic amblyopia. Strabismic children who alternate fixation between the two eyes and can maintain fixation through a blink with either eye probably do not have amblyopia.
Pupils
Children or infants with abnormalities in pupillary shape, pupils of unequal size, or poor and unequal reaction to light should be referred to an ophthalmologist.
Ocular structures
Children or infants who have structural abnormalities of the eye, eyelids, or orbit detected through external inspection should also be referred to an ophthalmologist for further investigation. A penlight is sufficient as a screening tool for external inspection of the eye.
Ophthalmology assessment
If there are concerns after the initial exam by the primary care physician, the child is referred to an ophthalmologist (or optometrist), generally one with subspecialty training in pediatrics and strabismus, who then performs a complete ophthalmic exam. The exam involves specific ophthalmologic testing. Important elements of the ophthalmologist's exam include the following:
Visual acuity
Verbal children are best tested with multiple targets or with crowding bars around individual targets.[29] Some young children get distracted or confused when presented with multiple targets, like a series of pictures in a line or grid. An amblyopic eye sees better when an individual target is presented on a blank background than with other targets in the vicinity of the target of interest. Thus, measurements using individual targets overestimate visual acuity in the amblyopic eye. To avoid this overestimate, crowding bars may be used. These consist of stripes that surround an individual target and thereby simulate the presentation of multiple targets.
Fixation pattern
This reveals a central or eccentric fixation pattern.
Patients with a clear visual axis to the fovea and with a normally positioned fovea fixate with the center of the eye, whereas patients with amblyopia, an opacity within the visual axis, or displacement or disease involving the fovea may fixate eccentrically, as if they were looking at an object from the side. Marked eccentric fixation is detected by observing the noncentral position of the corneal reflection in the amblyopic eye while the amblyopic eye fixates on a light. A strabismic patient who can freely alternate fixation between the eyes does not have amblyopia. When either eye is deviated, the fixating other eye should be able to maintain fixation through a blink.
Stereopsis and binocular vision testing
Stereopsis refers to the perception of 3-dimensionality, or depth. This is typically reduced in children with amblyopia.
The Titmus, Frisby, and Lang tests are commonly used to measure stereopsis. During the Titmus test, patients wear polarized glasses and are asked to identify 3-dimensional images that appear to pop-up toward them. The degree of stereopsis is reported as the number of arc seconds, with lower numbers (40 arc seconds) indicating better stereopsis than higher numbers (3000 arc seconds). Both the Frisby and Lang stereotests can be performed in free-space without the need for dissociative glasses.
Ocular alignment (using corneal light reflex, cover/uncover and alternate cover testing)
In strabismic patients, the magnitude of the deviation is measured using prisms.
The ophthalmologist pays special attention to anomalous head postures, which can indicate peeking, refractive error, strabismus, or nystagmus.
Pupillary exam
This is performed to help rule out ocular pathologies that may contribute to decreased vision (e.g., an abnormal pupillary response may be due to optic nerve hypoplasia or a retinal pathology).
Anterior segment exam
This helps rule out ocular pathologies that may contribute to decreased vision (e.g., an anterior segment exam may reveal cataracts).
The anterior segment exam is typically performed with a slit lamp.
A portable slit lamp or magnifier is used for young children. [Figure caption and citation for the preceding image starts]: Slit lampFrom the collection of Tina Rutar, MD [Citation ends].
[Figure caption and citation for the preceding image starts]: Author performing slit lamp examFrom the collection of Tina Rutar, MD [Citation ends].
[Figure caption and citation for the preceding image starts]: Portable slit lampFrom the collection of Tina Rutar, MD [Citation ends].
Cycloplegic retinoscopy
This is an essential part of the complete ophthalmic exam of a child.
Eye drops, typically cyclopentolate 0.2%/phenylephrine 1% for infants, and cyclopentolate 1% (occasionally with phenylephrine 2.5%) for toddlers and children, are used to dilate the pupil and relax the ciliary muscle. A relaxed ciliary muscle impairs the strong focusing ability of the child's eye, allowing the ophthalmologist to objectively determine the child's refractive state.
Many causes of amblyopia can only be assessed with a reliable cycloplegic retinoscopy.
While performing retinoscopy, the ophthalmologist also assesses the quality of the light reflex to insure nothing in the visual axis interferes with a clear image reaching the retina.[Figure caption and citation for the preceding image starts]: Retinoscope with plus and minus spherical lenses for refractionFrom the collection of Tina Rutar, MD [Citation ends].
Dilated fundoscopic exam
This helps rule out ocular pathologies that may contribute to decreased vision (e.g., macular lesions). Sometimes, these structural ocular defects are solely responsible for vision loss. However, when they occur asymmetrically or monocularly, coexistant amblyopia may explain a portion of the vision loss.
Dilated fundoscopic exam is typically performed after cycloplegic retinoscopy.
Eye drops are used to dilate the pupil and relax the ciliary muscle, typically cyclopentolate 0.2%/phenylephrine 1% for infants, and cyclopentolate 1% (occasionally with phenylephrine 2.5%) for toddlers and children.[Figure caption and citation for the preceding image starts]: Indirect ophthalmoscope with 28 diopter lens for performing fundus examFrom the collection of Tina Rutar, MD [Citation ends].
[Figure caption and citation for the preceding image starts]: Author performing indirect ophthalmoscopyFrom the collection of Tina Rutar, MD [Citation ends].
Amblyopia is diagnosed when visual impairment is detected in the presence of amblyopia risk factors, such as strabismus, high or unequal refractive errors, or media opacities. When amblyopia due to refractive error is suspected, the child's refractive error should first be corrected with spectacles or lenses in a trial frame. If the visual acuity normalizes, the diagnosis is refractive error alone. However, if visual acuity remains subnormal, the diagnosis is refractive amblyopia.
Investigations
Amblyopia is diagnosed after a complete and thorough ophthalmology exam by an ophthalmologist or optometrist. Visual evoked responses are an emerging technology that may be used in the future to aid diagnosis in uncertain cases.
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