Definition

Hippocrates in 400 BCE described amblyopia as ‘when the doctor and patient see nothing’ ( Day, 1997 ). Lyle and Wybar (1967) defined amblyopia as ‘a condition of diminished visual form sense which is not associated with any structural abnormality or disease of the media, fundi or visual pathways, and which is not overcome by correction of the refractive error.’ The problem with the ‘no structural abnormality’ clause is that it depends on the depth of the clinical investigations. This may be why many definitions replace this phrase with alternatives such as ‘apparent lesion’ ( Wingate, 1976 ; Millodot, 1993 ) or ‘ophthalmoscopically detectable’ lesion ( Gibson, 1947 ; Spalton et al., 1984 ; Nelson, 1988a ). Another problem with this definition is that in 22% of cases, amblyopia is cured simply by wearing spectacles, albeit over several months (p. 187). Accordingly, some studies have changed the last clause in the aforementioned definition to ‘not directly correctable with glasses’ ( Cordonnier & de Maertelaer, 2005 ).

In view of these problems with the definition of amblyopia, the following broad definition is proposed: a visual loss resulting from an impediment or disturbance to the normal development of vision .

Two quantitative approaches are commonly used to diagnose amblyopia: a difference between the acuity of the two eyes of two lines or more ( Papageorgiou, Asproudis, Maconachie, Tsironi, & Gottlob, 2019 ) and/or acuity in the amblyopic eye of worse than 6/9 ( Jennings, 2001b ). It is implicit in this definition that the child is old enough for the visual acuity norms to be 6/6 ( Table 3.1 ). Stewart and colleagues described two more sophisticated approaches to defining amblyopia, and of measuring the outcome of treatment ( Stewart, Moseley, & Fielder, 2003 ). The first is the difference in final visual acuity of amblyopic and fellow eye ( residual amblyopia ) and the second is the proportion of the deficit corrected. Residual amblyopia is similar in principle to a function previously called the acuity ratio ( Fulton & Mayer, 1988 ).

Amblyopia can be graded as mild (6/9 to 6/12), moderate (6/12 to 6/36), or severe (worse than 6/36) ( Papageorgiou et al., 2019 ).

Classification

Amblyopia can be classified as follows:

• 1.
  

  Organic amblyopia from pathological or anatomical abnormalities of the retina ( Spalton et al., 1984 ). The organic amblyopias can be further subdivided as follows:

  
  
  
  
  • (a)
    

    From retinal eye disease, e.g., receptor dystrophy, neonatal macular haemorrhage.

    
    
  • (b)
    

    Nutritional amblyopia from nutritional deficiencies.

    
    
  • (c)
    

    Toxic amblyopia from poisoning (e.g., arsenic, lead, or quinine). Alcohol amblyopia and tobacco amblyopia are usually considered to be toxic amblyopias, although they are sometimes classified as nutritional amblyopias. The terms tobacco–alcohol amblyopia/neuropathy have been criticised and it has been argued that these are not amblyopia but rather nutritional optic neuropathy ( Grzybowski & Brona, 2017 ).

    
    
  • (d)
    

    Idiopathic or congenital which is amblyopia of unknown aetiology. It may be that, with modern electrophysiological testing and imaging techniques, many of these cases would be found to have subtle pathological causes, cortical or subcortical.

  
  
  
  
• 2.
  

  Functional amblyopia in which no organic lesion exists. The functional amblyopias can be further subdivided into:

  
  
  
  
  • (a)
    

    Stimulus (or visual) deprivation amblyopia from opacities or occlusion of the ocular media (e.g., congenital cataracts or ptosis). A systematic review found no high level (randomised controlled trial; RCT) evidence concerning the treatment of stimulus deprivation amblyopia ( Antonio-Santos, Vedula, Hatt, & Powell, 2020 ). Occlusion amblyopia is an iatrogenic visual loss of the ‘good’ eye from excessive occlusion of this eye to treat primary amblyopia in the other eye (p. 191).

    
    
  • (b)
    

    Strabismic amblyopia as a result of neural changes in strabismus. Strabismic amblyopia and stimulus deprivation amblyopia used to be called amblyopia ex anopsia .

    
    
  • (c)
    

    Anisometropic amblyopia resulting from a blurred image in the more ametropic eye in uncorrected anisometropia, usually hypermetropia. A unidirectional causal relationship is likely to be an over-simplification ( Barrett, Bradley, & Candy, 2013 ). Anisometropic amblyopia often occurs in association with microtropia ( Hardman Lea, Loades, & Rubinstein, 1991 ), when it is best classified as mixed strabismic/anisometropic amblyopia.

    
    
  • (d)
    

    Refractive amblyopia (isometropic amblyopia) from blurred images in bilateral uncorrected refractive errors, usually hypermetropia. Visual acuity generally improves with spectacle correction ( Wallace et al., 2007 ; Ziylan, Yabas, Zorlutuna, & Serin, 2007 ). Refractive amblyopia includes meridional amblyopia , which occurs in high uncorrected astigmatism.

    
    
  • (e)
    

    Psychogenic amblyopia (hysterical amblyopia) , a visual conversion reaction (p. 18) where the amblyopia is of psychological origin.

  
  

It is very important to detect any organic cause, so that appropriate medical treatment can be considered. This chapter is principally concerned with functional amblyopia and will concentrate on the two most common types, strabismic amblyopia and anisometropic amblyopia. Differential diagnosis between organic and functional amblyopia also will be discussed and is summarised in Table 13.1 .

Prevalence

Amblyopia occurs in about 3% of the population ( Attebo et al., 1998 ; Jennings, 2001b ). A population-based study ( Attebo et al., 1998 ) found that the relative prevalence of different types of amblyopia is anisometropic (50%), strabismic (19%), mixed strabismic and anisometropic (27%), and visual deprivation (4%). Hospital eye clinics in the UK receive many more referrals with strabismic than orthotropic anisometropic amblyopia, ( Woodruff, Hiscox, Thompson, & Smith, 1994b ), probably because strabismic amblyopia is more visible to parents and anisometropic amblyopia is often cured by community optometrists.

Seventy-five percent of children attending the hospital eye service do so for amblyopia-related reasons ( Stewart, Shah, Wren, & Roberts, 2016 ). Of the children with anisometropic amblyopia who present to the hospital eye service, those from socially deprived backgrounds present on average about 2 years later than other cases ( Smith, Thompson, Woodruff, & Hiscox, 1994 ). Amblyopia is less likely to be successfully treated in children from poorer socioeconomic groups ( Hudak & Magoon, 1997 ).

Amblyopia is more likely to be present in the left eye, and this asymmetry is exaggerated for anisometropic amblyopia ( Woodruff et al., 1994b ). In another study, this asymmetry was found in the presence of anisometropia, but not in strabismic-only amblyopia ( Repka, Simons, & Kraker, 2010 ).

Detection of Amblyopia and Vision Screening

Second only to refractive error, amblyopia is a leading cause of visual loss in the age group 20–70 years. Amblyopia can preclude some vocations, mostly related to military or transport ( Adams & Karas, 1999 ). Amblyopia is associated with adverse psychosocial effects, even in nonstrabismic cases ( Packwood, Cruz, Rychwalski, & Keech, 1999 ). The treatment of amblyopia is cost-effective ( Konig & Barry, 2004 ; Membreno, Brown, Brown, Sharma, & Beauchamp, 2002 ). There is some evidence that occlusion therapy is distressing for children ( Parkes, 2001 ), although two studies found that amblyopia treatment does not have an adverse psychosocial impact ( Choong, Lukman, Martin, & Laws, 2004 ; Hrisos, Clarke, & Wright, 2004 ).

It is important to discover amblyopia, or the ‘amblyogenic’ factors which may cause it, at as early an age as possible. Children are at risk if their parents or siblings have amblyopia and/or strabismus. Any adult with amblyopia should be cautioned about the need for professional eyecare in relatives who are children.

Most young children in the UK do not routinely visit community optometrists ( Guggenheim & Farbrother, 2005 ) and screening of children at school entry has been advocated ( Hall, 1996 ). Parents sometimes assume that proper eye examinations are unnecessary because their children have had vision screening. However, the standards of screening programmes are variable ( Woodruff et al., 1994b ) and have been criticised ( Wright, Colville, & Oberklaid, 1995 ). The evidence for vision screening in preschool children will now be briefly reviewed.

A thorough screening programme at age 37 months significantly improves the visual outcome in the population at age 7.5 years ( Williams, Harrad, Harvey, Sparrow, & ALSPAC Study Team, 2001 ; Williams et al. 2003 ). The prevalence of amblyopia is almost halved and visual acuity is improved. The problems of vision screening are exemplified by the fact that only 69% of the intervention group attended any of the vision screening appointments and the authors caution that parents must be told that passing vision screening does not guarantee that no abnormality is present.

Inevitably, there is a trade-off between the desirability of early screening ( Williams et al., 2002 ) and the practical issue of the age at which useful screening results can be obtained ( Williams et al., 2001 ). This, together with changing visual status, makes a powerful argument for screening on more than one occasion, so it is surprising that this approach was discontinued in the UK ( Hall, 1996 ). A study highlighted the inaccuracies in screening children aged 4–5 years: over a third of cases thought to require treatment after repeat screening did not actually have acuity loss ( Clarke et al., 2003 ). Conversely, another study argued that screening, at least by photorefraction, should occur at age 9 months ( Anker, Atkinson, Braddick, Nardini, & Ehrlich, 2004 ).

Evidence from other countries supports the benefit of vision screening for reducing the prevalence of amblyopia ( Hoeg et al., 2014 ). The choice in the UK to only screen vision once at school entry ( Hall, 1996 ) seems impossible to justify on any scientific grounds. By comparison, a highly successful screening programme in Sweden which has reduced the prevalence of deep amblyopia from 2% to 0.2%, repeats screening at five different ages, with visual acuity being tested on four of these occasions ( Kvarnstrom, Jakobsson, & Lennerstrand, 2001 ).

A classic study found that 72% of cases of esotropia and/or amblyopia had a refractive error of +2.00DS or more spherical hypermetropia in the more emmetropic eye, or +1.00D or more spherical or cylindrical anisometropia ( Ingram, 1977 ). Infants (mean age 9 months) who are not refractively corrected for significant hypermetropia (more than +4.00D) are four times more likely to have poor acuity at 5.5 years than infants who wore their hypermetropic correction ( Anker et al., 2004 ). Oblique astigmatism significantly increases the risk of developing amblyopia ( Abrahamsson & Sjostrand, 2003 ).

The effect of early correction (before the age of 2.5 years) of significant degrees of hypermetropia (+3.00D or more) and hypermetropic astigmatism (1.00DC or more) was investigated in a retrospective study of the records of 103 strabismic children ( Freidburg & Kloppel, 1996 ). Early refractive correction was associated with significantly better visual acuities at age 8 years or later. Vision screening is only effective if it is followed up with comprehensive eyecare, preferably state-funded. A North American study found that a year after visual screening had detected visual problems, only 30% of cases detected were complying with recommended treatment ( Preslan & Novak, 1998 ).

Recent publications on vision screening in the UK continue to concentrate on amblyopia, with minimal consideration of other conditions ( Solebo & Rahi, 2013 ; Solebo, 2019 ). Children from less advantaged backgrounds have an increased risk of hypermetropia, which could cause inequity in access to care ( Williams et al., 2008 ). Although not directly related to binocular vision anomalies, the rate of myopia in 10–16-year-olds in the UK has doubled in the last 50 years ( McCullough, O’Donoghue, & Saunders, 2016 ) and most university students are now myopic ( Logan, Davies, Mallen, & Gilmartin, 2005 ). Schoolchildren with visual symptoms often do not self-present for eyecare ( Thomson & Evans, 1999 ; Thomson, 2002 ) and in the absence of repeated vision screening it seems advisable for schoolchildren to have regular eye examinations with community optometrists.

A promising development is a computerised vision screener ( Thomson & Evans, 1999 ) that takes about 3 minutes per child and has a sensitivity of 97% and a specificity of 96% ( Thomson, 2002 ). Recently, an iPhone App using photorefraction has shown promising results at detecting the risk factors for amblyopia ( Walker et al., 2020 ). However, earlier evidence indicates video-autorefractors fail to detect about one in five cases of amblyogenic ametropia ( Schimitzek & Haase, 2002 ).

Vernier acuity is probably cortically-mediated and has good potential to detect amblyopia ( Drover, Morale, Wang, Stager, & Birch, 2010 ). Vernier acuity can be tested by preferential looking and seems to have potential for vision screening ( Drover et al., 2010 ).

Prevention of Further Visual Loss in Amblyopia

Another important role for eyecare practitioners is to advise amblyopic patients of ways they can minimise the risk of visual loss in the future. People with amblyopia have almost three times the risk of visual impairment in their better seeing eye compared with people without amblyopia ( Chua & Mitchell, 2004 ). Although amblyopes who lose sight in their nonamblyopic eye often experience an improvement in their amblyopic eye, this is only of a significant degree (two lines or more) in 10% of cases ( Rahi, Logan, Borja et al., 2002 ). Indeed, the lifetime risk of serious visual loss for an individual with amblyopia is at least 1.2%–3.3% ( Rahi, Logan, Timms, Russell-Eggitt, & Taylor, 2002b ).

Another study calculated the lifetime risk of bilateral visual impairment is 18% for amblyopes, compared with 10% for nonamblyopes ( van Leeuwen et al., 2007 ). Therefore, eyecare practitioners should advise amblyopic patients about wearing eye protection when appropriate. It often helps to bring this message home if practitioners cover the patient’s good eye and point out the level of vision in the amblyopic eye.

Development

The most critical period for loss of binocularity and for the development of functional amblyopia is the first 18 months of life ( Levi, 1994 ). After this, the plasticity of the visual system seems to decrease rapidly at first, and then gradually. The conventional view is that it remains sensitive up to the age of about 6 years ( Keech & Kutschke, 1995 ), 6 to 7 years ( Simons & Preslan, 1999 ), 8 years ( Nelson, 1988a ; Levi, 1994 ; Daw, 1997 ), or possibly 10 years ( Vaegan & Taylor, 1979 ). Different visual functions have different sensitive periods: the sensitive periods for cortical visual functions are longer than for retinal functions ( Harwerth, Smith, Duncan, Crawford, & Von Noorden, 1986 ). Data from monkeys suggests that an earlier onset of strabismus tends to be associated with deeper amblyopia ( Kiorpes, Carlson, & Alfi, 1989 ).

It is sometimes assumed that the upper age limit for the onset of amblyopia is the same as the upper age limit for the treatment of amblyopia, but this is not necessarily the case. It is noted later in this chapter that there is strong evidence for considerable plasticity in the visual system, with even adults responding to intensive amblyopia treatment.

The Impact of Strabismic and Anisometropic Amblyopia

Investigation of Amblyopia

When a patient reports the symptom of reduced vision, a full routine eye examination should be carried out. This is outlined in Chapter 2, Chapter 3 . The present chapter covers the particular procedures in the investigation of amblyopia as a part of that routine, and with supplementary tests that aid a diagnosis with respect to the amblyopia. As a part of this investigation, tests for the presence of eccentric fixation may also be required, and these are described later in this chapter. After the section on eccentric fixation, the differential diagnosis of amblyopia and detection of pathology is discussed.

The worksheet in Appendix 6 summarises a clinical approach to the investigation of amblyopia. One aim of this is to differentially diagnose the type of amblyopia, summarised in Table 13.1 . Compared with nonamblyopic eyes, eyes with strabismic amblyopia experience a significant improvement in visual acuity when viewing through a low-density neutral density filter ( Habeeb, Arthur, & ten Hove, 2012 ).

An interesting study found many amblyopic eyes may have a subtle form of optic nerve hypoplasia ( Lempert, 2000, Lempert, 2004 ). Lempert suggests optic nerve hypoplasia may be the primary reason for the reduced acuity, although he notes it is also possible the reduced size of the optic nerve is the result of the amblyopia. Optical coherence tomography studies indicate increased macular thickness in anisometropic amblyopia but not strabismic amblyopia ( Al-Haddad, Mollayess, Cherfan, Jaafar, & Bashshur, 2011 ). The macular thickness reduces (becomes more normal) after amblyopia treatment ( Kavitha et al., 2019 ). The retinal nerve fibre layer thickness appears normal in amblyopia ( Bandyopadhyay, Chatterjee, & Banerjee, 2012 ; Kavitha et al., 2019 ).

Visual Acuity Measurement

Assessment of the unaided vision should be made, but an evaluation of amblyopia can only be made with the optimum refractive correction in place. Acuity will vary with illumination, contrast, and the type of test used ( Table 13.1 ), and every effort should be made to standardise the apparatus and the procedure used. This may need to vary to some extent with the age of the patient, as young children require a different approach. The method used should then be recorded along with the test distance and acuity measurement.

Line (Morphoscopic or Crowded) Acuity

In testing visual acuity, patients with unilateral amblyopia often give up reading when the letters are too small to read easily. If pressed, these patients typically read lower down the chart, and sometimes this can reveal much better acuity than would otherwise have been obtained. It is important to ask the patient to read until the limit of acuity is reached, otherwise no real starting point for any treatment is known and any improvement may be illusory. Modern letter chart designs utilising principles detailed by Bailey and Lovie (1976) are most suitable for accurate visual acuity measurements ( Chapter 3 and Appendix 10 ). The sensitivity to visual acuity changes in amblyopia is increased by decreasing letter spacing ( Laidlaw, Abbott, & Rosser, 2003 ). The chart that these authors recommended has, like most charts, less crowding for letters at the end of the line than for those in the centre of the line. This is undesirable and crowding is perhaps better controlled with individual optotypes in a crowding box (see next section). Great care must be taken to ensure that the patient does not ‘peep’ around the occluder. These precautions are particularly important with children, but also apply to adults. Computerised test charts are preferable because the optotypes can be randomised; if not available, the acuity of the amblyopic eye should be measured first.

Where there is eccentric fixation, a small foveal scotoma may result in patients missing out letters or reading the line backwards more easily than in the normal way from left to right. This may be particularly true of left convergent strabismus ( Fig. 13.1 ). Other factors that may contribute to missequencing of letters are confusion over the visual direction and monocular fixation instability associated with eccentric fixation.

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