Answers

Short answers

1. Right constant convergent strabismus with anisometropic hypermetropia. Anisometropic and strabismic (mixed) amblyopia.

2. A family history of strabismus; premature birth; low birth weight at full term; low socioeconomic status; visual deprivation (for example, cataract); and refractive error—particularly long sightedness and a difference in refractive error between the two eyes.

3. Constant unilateral strabismus; refractive error (>1 DS or >1.5 dioptre cyl (DC) difference in refractive error between the two eyes (amblyogenic anisometropia)); reduced visual acuity (unilateral); absence of any other ocular or visual pathway pathology.

4. Spectacles should be prescribed for fulltime wear for three months to promote in-focus stimulation and reduce or eliminate convergent strabismus. Paediatric prescribing guidelines from the Royal College of Ophthalmologists should be consulted. Patching for four hours a day should be considered if vision does not improve adequately after three to four months.

Long answer
Amblyopia is characterised by reduced visual functions that do not result from a clinically demonstrable anomaly of the visual system. Amblyopia usually, but not invariably, affects one eye. The developing visual system is highly sensitive to visual experience,^{1 2} and interruption by any obstacle—such as anisometropia (a significant difference in refractive errors between the two eyes) or strabismus (misalignment of one eye), or both—before about 7 years of age results in a reduction of visual capacity known as amblyopia.^{1 3} Depth perception, the ability to appreciate three dimensional space and thus use both eyes together, is often reduced or absent because of the misalignment of the two eyes or poor visual acuity in one eye.

Amblyopia is an important public health concern. Around 90% of work in children’s eye services is thought to be related to amblyopia,⁴ and the condition carries an increased lifetime risk (at least three times that of the general population) of serious vision loss in the other eye.⁵ However, although sparsely researched, the disability associated with amblyopia seems to be minimal and does not affect the patient’s occupation, education, or social status.⁶ Strabismus, a misalignment of the eyes, is one of the most common eye problems in children, affecting about 4% of children under 6 years.⁷ About 2-3% of the general population has amblyopia.⁸

Primate models of amblyopia have repeatedly shown that the primary visual cortex (area V1) is dysfunctional, and they have recently identified extrastriatal dysfunction in locations such as the motion processing areas.⁹ Functional imaging studies confirm processing abnormalities in area V1 of humans and hint at deficits within higher cortical areas.¹⁰ However, the eye looks normal (pathology is absent) on ophthalmoscopy (examination of the back of the eye using a light (ophthalmoscope)).

Screening for strabismus and amblyopia is recommended between 4 and 5 years of age in the United Kingdom,¹¹ and between 3 and 4 years in the United States.¹² A cosmetically obvious strabismus will probably be noticed by parents or carers, who will consult their general practitioner. However, cases of anisometropia and small angle strabismus will not be detected unless monocular visual acuity and refraction tests are performed, which emphasises the importance of vision screening. Some types of strabismus have a strong hereditary component, with 26% of patients having first degree relatives with strabismus.¹³

The treatment of amblyopia in humans has two principal components—refractive correction by spectacles and occlusion (by "patching") or "penalisation" (by pharmacological or optical means) of the fellow eye. Spectacles (which most children with amblyopia need) take a long time to improve vision^{14 15 16}—a process called "refractive adaptation."¹⁵ However, although wearing spectacles and patching may each improve vision, their individual contributions to outcome cannot be distinguished in routine clinical practice or research because they are often prescribed at the same time. The time taken for refractive adaptation to occur was defined only recently.¹³ Improvement seems to take up to 22 weeks, and gains range from 0 to 6 logMAR lines, with an average of 2.5 lines (each line represents 0.1 logarithmic change in letter size on the chart).¹⁵ Similar gains in visual acuity have been seen with refractive adaptation for all types of amblyopia studied (anisometropic, strabismic, and mixed), and some children achieve equal visual acuity in each eye during the process.^{15 16}

Until recently the relation between the occlusion dose and the response was poorly defined because most trials did not monitor the dose.^{17 18} Studies that have used objective occlusion monitoring have shown that compliance is rarely full and varies across the treatment course.^{4 19 20 21} This emphasises the need to differentiate between treatment prescribed and treatment received by the child.

The Monitored Occlusion Treatment of Amblyopia Study showed that 95% of children benefited from refractive adaptation; the dose-response function for occlusion therapy was monotonic and linear up to 400 hours; 75% of the improvement related to occlusion occurred in the first six weeks; and a proportion of the visual deficit (~25%) seems not to be corrected with current treatments.¹⁸ The hours of patching needed each day for maximum visual improvement have recently been investigated. Results from the Randomised Occlusion Treatment of Amblyopia Study indicate that for most children two hours of patching is not sufficient, whereas more than six hours provides no extra gain—four hours a day seems to be optimum.²² However, many clinicians have adopted patching guidelines based on two trials from the Pediatric Eye Disease Investigator Group,^{17 18} which found that two hours’ occlusion a day is as good as six hours for moderate amblyopia,¹⁷ and that six hours is as good as 12 hours for severe amblyopia. Although this gives the clinician a guide in the short term, further randomised trials with hours of patching received are needed.

Although it has long been thought that treatment for amblyopia is more successful at earlier stages of visual development,²³ the evidence is unconvincing and contradictory.^{24 25} Recent studies indicate that treatment can be effective past the end of the sensitive period for visual development (around 7 years). However, the patching dose rate (number of hours the patch is worn each day) seems to be the predictive factor of whether older children (over 6 years) can be treated successfully. Thus the child under 4 years responds more rapidly and with smaller doses than the older child, but the final level of attainment for all ages between 3 and 8 years is the same.²²

Cite this as: BMJ 2009;338:b68