Fig. 10.1
Teller acuity card. Reproduced from Quinn et al. (2002) © Springer Science+Business Media, New York 2002
Fixation preference, introduced by Dr. Kenneth Wright in the 1990s, is performed by observing which eye a child fixates with while a 10 prism diopter base down prism is held over either eye. However, a study in 2009 concluded that using fixation preference was a poor surrogate to determine if a child had amblyopia, with low sensitivity in anisometropic and strabismic amblyopes (Cotter et al. 2009).
Older children, age 3–5 years, can often perform a matching game using HOTV optotypes or LEA symbols (Fig. 10.2). In 2001, the Pediatric Eye Disease Investigative Group (PEDIG) determined that a single HOTV optotype with surround bars had a high testability, reliability, and accuracy in determining visual acuity in children age 3–6 (Homes et al. 2001). The crowding phenomenon is characteristic of amblyopic vision. Patients with amblyopia have difficulty resolving optotypes that are placed close to each other. This phenomenon occurs because of abnormalities in the receptive fields in the visual cortex. Thus, using a single optotype to assess vision will likely overestimate visual acuity and amblyopia will likely be missed. Testing with linear optotypes or a single optotype with crowding bars surrounding the optotype are methods to address this issue.
Literate children can perform Snellen visual acuity testing based on level of development, with children as young as age 2 occasionally able to identify the letters of the alphabet.
Familiarity with all these methods is crucial for the examiner to diagnose amblyopia and monitor a child’s progress with treatment. It is essential to test a child’s vision using an occlusive patch. Numerous children who pass a school or pediatrician’s vision screening eventually come to the attention of a pediatric ophthalmologist with amblyopia who “passed” because the unaffected eye was not sufficiently occluded during testing.
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There are other characteristics of amblyopic vision that are important to be familiar with in order to understand the full impact of amblyopia on a patient. These attributes include impaired contrast sensitivity and abnormal motion processing. Stereopsis is reduced in amblyopic patients. Even reading fluency, defined as speed and accuracy of oral reading, is compromised (Repka et al. 2008).
Anisometropia and strabismus comprise 90% of the causes of amblyopia. Cycloplegic retinoscopy must be performed to determine if there is a significant difference in refractive error between the eyes. Guidelines from the most recent AAPOS publication can help the clinician identify refractive and strabismic risk factors for amblyopia (Table 10.1).
Table 10.1
Amblyopia risk factor criteria
Age, months | Anisometropia | Hyperopia | Astigmatism | Myopia |
|---|---|---|---|---|
12–30 | >2.5D | >4.5D | >2.0D | >−3.5D |
31–48 | >2.0D | >4.0D | >2.0D | >−3.0D |
>48 | >1.5D | >3.5D | >1.5D | >−1.5D |
Role of Strabismus in Amblyopia
If visual deprivation is severe enough, ocular alignment instability, or sensory strabismus, may result. This is an additional impediment to visual rehabilitation of the affected eye with a corneal opacity. The gold standard of identifying strabismus is the cover uncover test. This requires the cooperation of the child to fixate an accommodative target at distance and near. Observe the uncovered eye for movement. If there is movement, there is a tropia, which if constant, and with a strong fixation preference, could indicate risk for development of amblyopia. Strabismus may seem to be present by Hirschberg light reflex test, but may represent an abnormal angle kappa. Angle kappa is defined as the angle between the pupillary and visual axis. The corneal light reflex is displaced temporally with a negative angle kappa, giving the appearance of an esotropia. The corneal light reflex is displaced nasally with a positive angle kappa, which gives the appearance of an exotropia. Occasionally, immature facial features, such as a narrow pupillary distance, epicanthal folds, and a flat nose bridge give the appearance of esotropia, but this is an illusion or pseudostrabismsus. Use specific clinical methods of strabismus determination such as the Hirschberg light reflex test or cover test to determine if this is the case. The Bruckner test is also useful in identifying a manifest strabismus. Observe the red reflex with the direct ophthalmoscope. The fixating eye will have a darker red reflex and the deviated eye will have a brighter or lighter red reflex because light is reflected from a less pigmented area of the retina or even from the optic nerve, depending on the direction of strabismus.
Role of Refractive Error in Amblyopia
Recognizing significant refractive error is a key element in the visual rehabilitation of children with corneal opacities. Glasses are often the first line of treatment. Prescribing glasses in children is an art. A cycloplegic refraction is essential to provide the most accurate prescription. If there is significant hyperopic anisometropia, for example, subtract equal amounts of hyperopia to reflect the natural accommodation of the child in the undilated state. However, if there is a significant esotropia along with a significant refractive error, give all the hyperopic correction, or at most subtract 0.5 D of hyperopia from each lens. If the child with a corneal condition is also pseudophakic, base your refraction on the child’s age. For infants and toddlers, whose visual interactions occur mainly in the near and intermediate range, aim for 2.5–3D of additional hyperopia in the pseudophakic eye. For the child over age 2, correct for infinity in the distance part of the lens and add a bifocal to the affected eye. For young children, avoid prescribing progressive addition lenses since young children have a difficult time finding the ideal head position for various viewing distances.
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Contact lenses are an ideal approach to ameliorating the irregular astigmatism induced by many corneal disorders. A knowledgeable pediatric optometrist is essential to obtain a good fit and educate the child and parents about proper handling and care of the contact lens. Irregular astigmatism from corneal scarring arises from irregular refraction of light by the scarred area and light scatter by the opacity with concurrent light sensitivity, reduced contrast sensitivity, and impaired visual acuity. Spectacles cannot address this irregular refraction. Rigid gas permeable lenses or hybrid lenses reduce irregular astigmatism and promote visual clarity by providing a regular refracting surface. Also, the tear film between the irregular cornea and back of the lens contributes to improved focus. However, in one study, the success of long-term wear of gas permeable lenses in patients less than 6 years of age was only 50%, likely due to loss of the lens or lens discomfort (Kanpolat and Ciftci 1995). A scleral contact lens, also known as a PROSE lens, (Prosethetic Replacement of the Ocular Surface Ecosystem, Boston Foundation Fight for Sight, Needham, MA), is helpful in promoting visual clarity and comfort in patients with scarred corneas. However, tear film debris accumulates, and a recent study found improved vision is reduced after 4 h of use, requiring frequent rinsing and replacing, which can be a challenge in young children (Rathi et al. 2012).
Treatment of Amblyopia
If optical correction is not sufficient to achieve excellent visual acuity, then amblyopia therapy is indicated. Current standard of care is to penalize the dominant or sound eye either by patching or use of atropine. Optical penalization by use of a blur lens, a Bangerter foil, or even an occluder contact lens may be appropriate for certain children.
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The Pediatric Eye Disease Investigative Group (PEDIG), a consortium of private and university-based pediatric ophthalmologists, have conducted multicenter randomized prospective clinical trials since its inception in 1997 to further our understanding of amblyopia and to provide an evidence-based approach to optimal treatment (Repka and Holmes 2012). The first Amblyopia Treatment Study (ATS) compared visual acuity outcomes in children age 3–6 with moderate amblyopia (20/40–20/100) who were randomized to patching at least 6 h a day or to daily atropine. The improvement in vision was similar in both groups (3.16 lines in the patching group and 2.84 lines in the atropine group), but children who patched achieved their final best acuity sooner than those who received atropine (Beck 2002). Until publication of the ATS, many ophthalmologists administered occlusion therapy concurrently with glasses for anisometropic amblyopia. However, this study demonstrated that children with moderate to severe anisometropic amblyopia (20/40–20/250) improved visual acuity by >2 lines in 77% and resolved amblyopia in 27% by wearing glasses alone. The average response time to best visual acuity was 10 weeks and some study subjects continued to improve even up to 30 weeks after initiating wearing glasses (Cotter 2006). Full time occlusion versus patching the dominant eye 6 h a day was examined in severe amblyopes (20/100–20/400). Both treatment groups had improvement of visual acuity by an average of just over 4.5 lines at the 4-month study conclusion (Beck 2003).
Atropine is a powerful anti-cholinergic eye drop that is an important treatment option for children either as a first-line treatment or for those who are non-compliant with patching. Atropine is most efficacious in moderate amblyopes with visual acuity better than 20/100. The regimen of atropine penalization, whether daily or just on the weekend, was studied in a cohort of children younger than 7 with visual acuity in amblyopic eye of 20/40–20/80. Visual acuity improved by an average of 2.3 lines in each group at the 4-month study conclusion. Occlusion amblyopia, in which the sound eye suffers reduced visual acuity during treatment, is rare but needs to be monitored for especially in children receiving atropine penalization therapy. Check the vision, which is blurred from atropine from cycloplegia, by performing a refraction. Then place the correcting lens in front of the dominant eye. If the vision in the dominant eye does not improve to better than the amblyopic eye, or close to 20/20, then occlusion amblyopia is likely. Discontinue treatment and closely follow the patient to ensure restoration of acuity in the previously dominant eye (Repka 2004).
Intuitively, it may seem that patching while performing a near activity that requires concentration and integration of visual and fine motor skills, such as a video game, would be more effective in improving visual acuity. This assumption was investigated in a study that randomly assigned near activities or distance activities during 2 h a day of patching the dominant eye. There was no difference in best-corrected visual acuity at the conclusion of the study between the two groups (2008).
The effect of age on response to amblyopia treatment was examined in a meta-analysis of prior studies. Visual cortex plasticity appears to diminish with age and amblyopia treatment appears to be more effective when instituted at a younger age (Homes et al. 2011). However, older children do respond to treatment as demonstrated in this ATS of children 7–17 years old with residual amblyopia or never before treated amblyopia (20/40–20/400). All participants were given optimal optical correction and then randomized to continued wear of optical correction alone or optical correction and amblyopia treatment. All participants in the treatment group patched the dominant eye 2–6 h a day and in the younger, 7–12-year old, group they also received daily atropine 1%. The study determined who were responders, defined as a participant who achieved greater than or equal to two lines of visual acuity improvement, or non-responders. In the 7–12 year olds, 53% were responders in the treatment group and 25% were responders in the optical correction only group. In the older cohort, about one quarter of participants in the treatment and optical correction group were responders. However, for participants in the older group who never received amblyopia treatment in the past, 47% were responders in the treatment group versus 16% in the optical correction only group. These findings emphasized that ophthalmologists should offer at least optimal optical correction to all older children with amblyopia and offer amblyopia treatment even to teens, especially if they never received amblyopia treatment in the past (Scheiman 2005).
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