Amblyopia

It is difficult to define and has been described as a disorder "in which the patient sees nothing and the doctor sees nothing".¹
It is the commonest cause of monocular visual loss.¹

• Amblyopia is a disorder in which there is dysfunction of the processing of visual information caused by vision deprivation with or without abnormal binocular interaction.
• The usual dysfunction is reduced recognition visual acuity.
• Clinical eye examination is usually normal (although microscopic abnormalities have been found in retina, lateral geniculate bodies and visual cortex).^2,3

Different definitions have been used in different studies. Consistency is needed in the definition of the degree of visual acuity reduction, the magnitude of interocular differences in acuity, methods of vision testing, the effect of refractive adaption, the absence of an organic cause etc.

Amblyopia is a consequence of degradation of the retinal image during a sensitive period in development, typically up to about the age of 7 years. It is the effect of other pathological processes and there are 3 causes of this degradation of image:

• Strabismus. This accounts for about 1/3 of cases and results in amblyopia because the images at the fovea are different.
• Differences in refraction (anisometropia). These account for another 1/3 of cases. Amblyopia results because one image is more blurred. Another 1/3 of cases results from a combination of strabismus and anisometropia.
• Deprivation or physical obstruction of the image (by cataract, ptosis etc). This is relatively rare.

Amblyopia affects 1-5%of children and is the commonest cause of monocular visual loss.¹ Failure of detection or of treatment efforts in children means prevalence rates are similar in adults as shown by a study of nearly 4000 adults in Australia.⁴

A unilateral (rarely bilateral) decrease in visual acuity in the absence of an organic lesion provides the diagnosis although presentation can be in a number of ways:

• Strabismus:
  • Amblyopia caused by strabismus tends to present earlier as the squint is more easily detected by parents.
  • Screening programmes (although not wisely employed).
• Visual acuity:
  • Detection with visual acuity testing. An accepted definition of amblyopia based on visual acuity is 2 or more Snellen or logMAR lines difference between eyes.³ However many of the visual acuity tests used are insensitive to amblyopia. In addition there is a range of typical visual acuity in a given population and this range changes with age according to development of the nervous system. Thus in 4 year old children the range is from 6/6 to 6/9 and this confounds the diagnosis of amblyopia. Visual acuity in amblyopia is often better when reading single letters than a row ('crowding phenomenon'). This is seen to a certain extent in everybody but more marked in amblyopes. Visual fields and colour vision are normal. There are more advanced visual acuity tests (neutral density filter and grating acuity) available to confirm the diagnosis in more difficult cases.
  • Detection by screening programme. Screening tests to detect refractive errors and strabismus exist and they are sensitive and reliable. Effective treatments exist. However screening is not widely employed and debate surrounds methods, benefit and cost.

Given that early detection and treatment of amblyopia can improve visual outcome, that the at risk population can be defined and that specific and sensitive screening tests exist, one might expect screening to be well established and that severe amblyopia as a public health problem could be eliminated.⁵ However:

• The benefits of treatment have been questioned. For example the functional disability of amblyopia can be small and the treatment can be distressing.⁶
• The methods used have still to be fully evaluated.⁷

In the UK a randomised controlled trial compared visual surveillance by health visitors and family practitioners with orthoptist assessment (measuring visual acuity, ocular alignment, stereopsis and non-cycloplegic photorefraction). The study concluded that photorefraction (detecting refractive errors) and a cover test (to detect strabismus) at age 37 months would have the best sensitivity and specificity of the methods used. Orthoptists were shown to be the most accurate screeners but the costs of this would be prohibitive in some countries. Orthoptic led screening programmes are currently used in the UK.⁸ In the USA a recent study also concluded that non-cycloplegic photorefraction was quick and reliable as a screening tool for refractive amblyopia.⁷

When to screen?

Early diagnosis and treatment before age 7 is supported by early research highlighting the sensitive period in humans and experimental animals.⁹ This led to the view that it was best to screen and test as early as possible. However there is a balance between early diagnosis and successful treatment. Treatments at 3 years and 6 years are equally successful in terms of visual acuity and this has led to some authorities recommending screening at school entry (age 5 years in the UK and 6 years in the USA).³

Is amblyopia worth treating?

Some have questioned the value of treatment as it causes little functional disability and treatments (patching particularly) can sometimes be psychologically distressing.⁶ However this partly perhaps reflects paucity and difficulty of research and therefore evidence. Clinical beliefs that children with amblyopia do improve during treatment are supported by evidence. However, without sound evidence on the natural history of these conditions, this evidence falls short of showing that treatment works. One analysis has shown substantial lifetime gains from childhood treatment of amblyopia.¹⁰ Reduction in unilateral visual acuity preclude entry to certain professions (e.g. fire service and armed forces because they cannot hold a class II professional driving license).¹¹ In addition there is the increased risk of damage to the good eye and consequent major visual handicap.^11,12

Treating the cause of the amblyopia

The aim is generally to increase use of the amblyopic eye usually by depriving the healthy eye of visual input. As well as treating the amblyopia, the cause of the amblyopia needs to be treated:

• Deprivation amblyopia. The cause (e.g. ptosis, cataract) should first be treated.
• Anisometropic amblyopia. Correct refractive error with glasses or contact lenses. Research shows progressive improvement in acuity for up to 18 weeks in some patients after correction of refractive error alone (refractive adaption). Refractive correction is all that is needed in a quarter of children with amblyopia.³
• Strabismic amblyopia. Conventionally amblyopia should be treated first, then the strabismus corrected. Debate surrounds timing of surgery.^13,14 Sometimes re-alignment helps reverse the amblyopia and amblyopia treatment is not needed.¹⁴

Treating the amblyopia

• Patching. This has been used for centuries. However it is obtrusive and compliance can be poor. Recent research indicates less disruptive patching regimens can be just as effective (e.g. 1-2 hours per day only).³
• Atropine blockade (optical penalisation) to reduce accommodation in the good eye is another option. It is less obtrusive, compliance is enhanced and peripheral binocularity is allowed.
• Informed choice between patching and atropine should be offered to parents and carers.
• Ideally it should be treated under age 7 years. Clinically there seems no benefit over age 10 years.⁵
• Other methods of treatment (patching, levodopa and citocholine, visual stimulation) are being investigated. Combination treatments may be used more in the future.³

Does amblyopia treatment work?

Retrospective case studies show that only 50% of children achieve normal vision in the amblyopic eye.³ Subtle ocular and cerebral pathologies may explain these failures as may inaccurate refractive correction and lack of compliance.

• Refer squint early.
• Be aware of local screening programmes.