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 ).
Amblyopia can be classified as follows:
Organic amblyopia from pathological or anatomical abnormalities of the retina ( Spalton et al., 1984 ). The organic amblyopias can be further subdivided as follows:
From retinal eye disease, e.g., receptor dystrophy, neonatal macular haemorrhage.
Nutritional amblyopia from nutritional deficiencies.
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 ).
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.
Functional amblyopia in which no organic lesion exists. The functional amblyopias can be further subdivided into:
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).
Strabismic amblyopia as a result of neural changes in strabismus. Strabismic amblyopia and stimulus deprivation amblyopia used to be called amblyopia ex anopsia .
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.
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.
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 .
|Type of amblyopia||Morphoscopic visual acuity (MVA)||Angular visual acuity||Visual acuity with 2.0 ND filter||Cover test||Fixation||Visual field||Amsler charts||Other|
|Strabismic||Reduced, usually unilateral, better with letters at end of line||>MVA||≥MVA||Constant strabismus if not microtropia (rarely, intermittent exotropia)||Eccentric, sometimes variable||Normal, except where suppression is very dense||Lang’s one-sided scotoma in microtropia (see below)||_|
|Stimulus deprivation||Reduced, usually unilateral||>MVA||≥MVA||Usually no strabismus, may be unsteady fixation||Central, may be unsteady||Normal||_||Likely to report relevant history (e.g., cataract or ptosis)|
|Anisometropic and refractive||Reduced, unilateral if anisometropic||=MVA, or very slightly better||Slightly <MVA||Normal, or may show anisophoria if high anisometropia||Central, often unsteady in high refractive errors||Normal||May show large central blur||High refractive error present in one or both eyes|
|Retinal eye disease, idiopathic or congenital||Reduced, sometimes bilateral||=MVA||<MVA||Normal||Central, often unsteady||Depends on organic cause, sometimes central scotoma||Depends on organic cause, sometimes central scotoma||Often history of ocular pathology & poor or absent foveal reflex|
|Toxic and nutritional||Reduced bilateral, not always equal||=MVA||<MVA||Normal||Central, sometimes eccentric if advanced||Central scotoma, especially for red||Central scotoma, especially with red chart||Possibly systemic signs, symptoms, or history|
|Psychogenic (visual conversion reaction; hysterical)||Reduced, variable, inconsistent at different distances, prone to suggestion||Variable||Variable and unpredictable||Normal||Central, may be unsteady||Static perimetry: illogical response kinetic perimetry: star or spiral field||Normal, or illogical response||May have other signs of visual conversion reaction (p. 18)|
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.
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
Basic Visual Functions
Colour vision is normal but the pupillary function of eyes with strabismic amblyopia is subtly different to that of eyes with anisometropic amblyopia ( Barbur, Hess, & Pinney, 1994 ). The spatial contrast sensitivity of amblyopic eyes is close to normal for low spatial frequencies (coarse detail), but there is a marked loss of contrast sensitivity at high spatial frequencies (fine detail). This loss increases with severity of amblyopia and does not result from optical factors, unsteady fixational eye movements, or eccentric fixation ( Flynn, 1991 ). Ocular pursuit is abnormal in strabismic amblyopia ( Bedell, Lee Yap, & Flom, 1990 ).
Visual processing occurs in inter-linked parallel pathways and the two principal systems are the P-system (parvocellular, sustained) and the M-system (magnocellular, transient). The type of visual deficit in amblyopia has led many to suggest that the P-system is affected, and the M-system relatively unaffected (e.g., Nelson, 1988a ), although this is likely to be an over-simplification ( Kelly & Buckingham, 1998 ). Hess and Pointer (1985) showed that in anisometropic amblyopia there is reduced sensitivity centrally and peripherally, whereas in strabismic (and mixed strabismic and anisometropic) amblyopia, the loss of acuity is predominantly restricted to the foveal region.
Fahle and Bachmann (1996) found that a small heterogeneous sample of amblyopes had better than normal function in their amblyopic eyes at a specific task of spatiotemporal integration at high velocities. One explanation for this might be if amblyopes have a P-deficit and normal or supranormal M-function. An electrophysiological study of anisometropic amblyopes found reduced P- but normal M-function ( Shan, Moster, Roemer, & Siegfried, 2000 ). A psychophysical study of strabismic amblyopia found deficits in both channels, but a relatively greater P-deficit ( Davis et al., 2006 ). The reduction in P- relative to M-sensitivity was significantly greater in the late-onset group and there were more subtle but complex deficits in the fellow eye.
An analysis of sensory processing in amblyopia highlights fundamental differences between strabismic and anisometropic amblyopia ( Hess, 2002 ). In additional to the contrast sensitivity deficit in amblyopic eyes, there is a milder deficit in the fellow eye ( Leguire, Rogers, & Bremer, 1990 ). This may relate to a finding of bilateral changes in foveal structure in individuals with amblyopia ( Bruce, Pacey, Bradbury, Scally, & Barrett, 2013 ).
Amblyopic eyes make misperceptions of spatial structures ( Sireteanu, Baumer, & Iftime, 2008 ) and this has been attributed to errors in the neural coding of orientation in the primary visual cortex ( Barrett, Pacey, Bradley, Thibos, & Morrill, 2003 ). Despite normal visual acuity, reading is impaired in the nonamblyopic eye and binocularly ( Kanonidou, Proudlock, & Gottlob, 2010 ). Other dysfunctions associated with amblyopia include impaired perception of mirror symmetry ( Levi & Saarinen, 2004 ), poor fine motor skills ( Webber, Wood, Gole, & Brown, 2008 ), and temporally unstable perception ( Sireteanu et al., 2008 ). The perception of images of real-world scenes is impaired in amblyopic eyes and binocularly ( Mirabella, Hay, & Wong, 2011 ).
Visual acuity can be classified as follows:
Minimum resolvable , the smallest angular separation between targets that can be recognised; e.g., grating acuity in preferential looking acuity cards ( Fig. 3.1 ). Electrophysiological techniques of measuring visual acuity may also use grating stimuli.
Minimum recognisable , the capacity to recognise a form and its orientation; e.g., Snellen letters.
Hyperacuity , the judgement of relative positions; e.g., vernier acuity.
Under ideal conditions, minimum resolvable and minimum recognisable acuity can approach the limit of 0.5–1 minute of arc, which is predicted from the optics of the eye and spacing of foveal cones. Hyperacuity can exceed this anatomical limit by 5–10 times, with optimal thresholds in the order of 3–6 seconds of arc.
Three basic principles can be used to characterise the visual acuity loss in functional amblyopia. First, the types of visual acuity listed previously reflect an increasing degree of cortical processing. Second, amblyopia can be described as a neural deficit and there is a failure in amblyopia to coordinate information from different parts of the spatial frequency spectrum ( Jennings, 2001b ). Third, it seems that the neural deficit is more complex in strabismic amblyopia than in anisometropic amblyopia. The following statements follow from these three principles. Compared with other measures of acuity, grating acuity is relatively unaffected in functional amblyopia. For a given level of grating acuity, strabismic amblyopes have a relatively greater loss of Snellen acuity than do anisometropic amblyopes. For a given level of grating acuity, strabismic amblyopes have a much greater loss of vernier acuity than anisometropic amblyopes.
Considering minimum recognisable acuity, reading letters in a line (morphoscopic acuity) is a more complex neural task than reading letters individually (angular acuity). It is therefore not surprising that most people perform a little worse when reading crowded as opposed to single letters, and this crowding phenomenon is more pronounced in strabismic amblyopia ( Levi, 2008 ). Real passages of text contain a greater degree of crowding than letter charts, and this may explain why amblyopes who have been successfully treated in terms of Snellen acuity may still have impaired capacity for reading passages of text ( Zurcher & Lang, 1980 ).
It should not be concluded from the aforementioned that all strabismic amblyopes show a much greater crowding phenomenon than anisometropic amblyopes; there is probably a continuum between the groups ( Giaschi, Regan, Kraft, & Kothe, 1993 ).
Amblyopia is associated with abnormal accommodative function ( Ciuffreda, Hokoda, Hung, Semmlow, & Selenow, 1983 ), which is clinically detected as a reduced amplitude of accommodation. Without refractive correction, about 80% of anisometropic amblyopes exhibit asymmetric accommodation, about half of whom show aniso-accommodation and a quarter of whom fail to accommodate at near, which has been described as anti-accommodation ( Toor et al., 2018 ). The anti-accommodation resolved with spectacle correction, although these were the cases whose amblyopia required patching in addition to refractive correction ( Toor, Horwood, & Riddell, 2019 ).
Other Deficits in Amblyopia
The visual deficit in amblyopia extends beyond the basic functions processed in V1 and involves extensive regions of extrastriate cortex (above), meaning there are likely to be significant suprathreshold processing deficits ( Li, Dumoulin, Mansouri, & Hess, 2007 ). Hand–eye coordination (prehension) is impacted in amblyopia and improvement in these functions is likely to require treatments that restore binocularity ( Grant, Melmoth, Morgan, & Finlay, 2007 ).
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 ).
History and Symptoms
It might seem surprising that so few researchers have paid any attention to the age of onset of amblyopia, but this may be because it can be difficult to determine this with any certainty. There is electrophysiological and psychophysical evidence of differences between patients with early onset (before 18 months) and late onset amblyopia ( Davis et al., 2003 ). As a general rule, the longer the strabismus has been present, the less likely it is to respond to treatment. In strabismic amblyopia, the age at which the strabismus was first noticed (e.g., in photos) can be used as a proxy for the onset of the amblyopia.
It is important to note any previous treatment in the form of glasses, occlusion, or other therapy: when was this given, what was its effect, and why was it discontinued? In the case of spectacles, the prescription should be known and the extent to which they have been worn. Most children are frightened of criticism and overestimate the amount they have worn their glasses. This should be countered by being noncritical and encouraging candour.
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.