Ординатура / Офтальмология / Английские материалы / Pickwell's Binocular Vision Anomalies 5th edition_Evans_2007

Table 12.1 Example of calculation of angle of anomaly in HARC and UARC

Angle

HARC: habitual angle

HARC: total angle

UARC

Objective angle

15

R SOT

20

R SOT

40

R SOT

Subjective angle

0

5

25

Angle of anomaly

15

15

15

HARC is NRC with diplopia or suppression of the binocular field of the stra-

bismic eye. Another option, unharmonious anomalous retinal correspond-

ence (UARC), is exceedingly rare and is best understood with an example.

Imagine a young child who develops a small, stable strabismus and associ-

ated HARC. The purpose of the HARC is to prevent diplopia and confusion

and to allow some rudimentary degree of binocular vision in the presence of

strabismus. As mentioned above, the angle of anomaly will be equal to the

objective angle of strabismus. Now, assume that after many years in his

adapted state the patient suffers, for example, trauma and an extraocular

muscle paresis resulting in a change in the angle of the strabismus, with con-

sequent diplopia. If the HARC was shallow then the patient would revert to

NRC. In this case the subjective angle (angle of diplopia) would be equal to

the new objective angle and the angle of anomaly would be zero.

However, if the HARC associated with the old strabismus was very deep

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then the patient might continue with this HARC in the presence of the new

strabismus. It is unlikely that a long-standing stable HARC could covary

OVERVIEW OF SENSORY CHANGES IN STRABISMUS

12

with a new change in the angle of the strabismus. Instead, the patient might

develop ‘a strabismus on top of a strabismus’. The objective angle would be

the angle of the new strabismus, the subjective angle would be the differ-

ence between the angle of the old strabismus and the new strabismus, and

the angle of anomaly would be neither zero nor equal to either of the sub-

jective angles (Table 12.1).

It will be appreciated that this sequence of events is extremely unlikely

(although UARC can also occur secondary to surgery), so why is UARC

given such prominence in some textbooks? The reason is that many early

methods of investigating retinal correspondence created very artificial

conditions that tended to cause HARC to break down. It was sometimes

concluded that these techniques were detecting UARC. Of course, if the

patients really had UARC then they would complain of the symptom of

constant diplopia. It would not make sense for the visual system to

undergo extensive remapping only to leave constant diplopia.

The foregoing description of anomalous retinal correspondence can only

be a very brief overview. There are many different theories on the aetiology

of this condition and these have been thoroughly reviewed by Jennings

(1985). Another detailed description of this condition was given by Nelson

(1988b). Chapter 14 includes details of the investigation and treatment

of HARC.

Monocular sensory changes in strabismus

There are two other sensory changes that may be present in strabismus, and

these are monocular. They occur in the strabismic eye of a patient with uni-

lateral strabismus and remain when the fellow eye is covered. Indeed, the

dominant eye needs to be covered to detect and investigate them. They are

amblyopia and eccentric fixation. These sensory changes, which occur in

strabismus developing at an early age, are more fully described in Chapter 13

but will be introduced here.

Amblyopia

Amblyopia is an impairment of form vision with no obvious organic cause.

In strabismus, amblyopia may assist in lessening the effects of confusion but

there are other types of amblyopia that do not necessarily accompany stra-

bismus (Ch. 13).

Eccentric fixation

This is a failure of an eye in monocular vision to take up fixation with the

fovea. There are several theories, but little consensus, on its aetiology.

These theories are discussed in Chapter 13. Usually, there are no accompa-

nying changes to the localization system in the monocular vision of an

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eccentrically fixating eye (Ch. 13).

Lyle & 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 over-

come by correction of the refractive error’. The problem with the ‘no struc-

tural abnormality’ clause is that it depends on the depth of the clinical

investigations. This may be why many definitions replace the phrase ‘not

associated with any structural abnormality or disease’ with alternatives

such as ‘apparent lesion’ (Wingate 1976, Millodot 1993) or, more specific-

ally, ‘ophthalmoscopically detectable lesion’ (Gibson 1947, p 30, Spalton

et al 1984, p 18.8, Nelson 1988a). Presumably, in stimulus-deprivation

amblyopia this type of definition refers to the uncorrectable visual loss

remaining after the lesion (e.g. cataract) has been removed. Another prob-

lem with this definition is that 22% of cases of amblyopia are cured simply

by wearing spectacles, albeit over an 18-week period (Stewart et al 2004a).

This may be why recent studies have changed the last clause in the above

definition to ‘not directly correctable with glasses’ (Cordonnier & de

Maertelaer 2005).

In view of these problems with the definition of amblyopia, the follow-

ing 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:

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a difference between the acuity of the two eyes of two lines or more and/or

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tropia. Anisometropic amblyopia often occurs in association with

microtropia (Hardman Lea et al 1991)

AMBLYOPIA AND ECCENTRIC FIXATION

13

(d) Refractive amblyopia (isometropic amblyopia), from blurred images

in bilateral uncorrected refractive errors, usually hypermetropia.

This includes meridional amblyopia which occurs in the principal

meridian(s) of high uncorrected astigmatism

(e) Psychogenic amblyopia (hysterical amblyopia), a visual conversion

reaction where the amblyopia is of psychological origin.

It is obviously very important that any organic cause be detected, so that

appropriate medical treatment can be considered. This chapter is princi-

pally concerned with functional amblyopia and will concentrate on the

two most common types of amblyopia, strabismic amblyopia and anisome-

tropic amblyopia. Differential diagnosis between organic and functional

amblyopia will also be discussed and is summarized 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 depriv-

ation 4%. A recent study found an almost equal prevalence of strabismic

and anisometropic amblyopia in a clinical population and this may reflect

a referral bias, with strabismic cases more readily recognized by parents and

hence more likely to be referred to clinics (Pediatric Eye Disease Investigator

Group 2002c). This is also likely to explain why hospital eye clinics in the UK

seem to receive many more referrals with strabismic than with orthotropic

anisometropic amblyopia, and this reflects inadequacies in vision screening

(Woodruff et al 1994b).

Children with anisometropic amblyopia present on average about 2 years

later to hospital eye clinics if they come from a socially deprived background

(Smith et al 1994a). A North American study found that 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).

Detection of amblyopia

Amblyopia is the leading cause of visual loss in the age group 20–70 years.

Amblyopia can preclude some vocations, which are mainly related to the

military or transport (Adams & Karas 1999). Amblyopia is associated with

adverse psychosocial effects, even when amblyopes with strabismus are

excluded (Packwood et al 1999). The treatment of amblyopia is cost-effective

(Membreno et al 2002, Konig & Barry 2004). There is some evidence that

occlusion therapy is found to be distressing by children (Parkes 2001),

although two recent studies found that amblyopia treatment does not have

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an adverse psychosocial impact (Choong et al 2004, Hrisos et al 2004).

13

PICKWELL’S BINOCULAR VISION ANOMALIES

It is important to discover amblyopia, or the ‘amblyogenic’ factors that

may cause it, at as early an age as possible. This is particularly true of those

types of amblyopia in which refractive error plays a large part in the cause:

accommodative strabismus, anisometropic amblyopia and astigmatic ambly-

opia. Children are at risk if their parents or siblings have amblyopia and/or

strabismus. Any adult with amblyopia should be cautioned of the need for

professional eyecare in relatives who are children.

The majority of young children in the UK do not routinely visit primary

care optometrists (Guggenheim & Farbrother 2005) and screening of chil-

dren 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 criticized (Wright et al

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 71/ years (Williams et al 2001,

2

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 actually attended any of the vision

screening appointments and the authors caution that parents must be told

that passing a vision screening event does not guarantee that no abnor-

mality is present. This study raises an important issue for vision screening:

there is a trade-off between the desirability of early screening (Williams

et al 2002) and the practical question of at what age 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 is being 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 et al 2004).

Infants (mean age 9 months) who are not refractively corrected for

significant hypermetropia ( 4.00 D) are four times more likely to have

poor acuity at 5.5 years than infants who wore their hypermetropic

correction (Anker et al 2004). A partial correction (leaving about

1.00 D of hypermetropia) is usually prescribed, which is likely to allow

emmetropization to occur. The full correction may be required in some

cases to prevent strabismus. A powerful argument for vision screening

of refractive errors arose from the finding that 72% of cases of esotropia

and/or amblyopia had a refractive error of 2.00 DS or more spheri-

cal hypermetropia in the more emmetropic eye, or 1.00 D or more spher-

ical or cylindrical anisometropia (Ingram 1977). However, the role of

video-autorefractors in screening may be limited, since they fail to detect

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about one in five cases of amblyogenic ametropia (Schimitzek & Haase

2002).

AMBLYOPIA AND ECCENTRIC FIXATION

13

The effect of early correction (before the age of 2.5 years) of significant

degrees of hypermetropia ( 3.00 D or more) and hypermetropic astigmatism

(1.00 DC or more) was investigated in a retrospective study of the records

of 103 strabismic children (Freidburg & Kloppel 1996). Early refractive cor-

rection was associated with significantly better visual acuities at the age

of 8 years or later. Oblique astigmatism significantly increases the risk of

developing amblyopia (Abrahamsson & Sjostrand 2003).

In lower degrees of hypermetropia where the child is compensating well

without correction then it is sometimes acceptable to monitor the child

closely and not prescribe glasses, unless symptoms (e.g. intermittent

esotropia, problems at school) or signs (e.g. decompensated esophoria,

reduced acuity) develop. This strategy is only appropriate for straightfor-

ward compensated cases where the parents are observant, understand the

risks and are prepared to attend for very frequent examinations. As always,

full clinical records need to be kept.

The choice to only screen vision once, at school entry (Hall 1996), seems

unique to the UK and is 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 screen-

ing at five different ages, with visual acuity being tested on four of these

occasions (Kvarnstrom et al 2001). A promising development is a compu-

terized vision screener (Thomson & Evans 1999) that takes about 3 min per

child and has a sensitivity of 97% and a specificity of 96% (Thomson 2002).

Prevention of further visual loss in amblyopia

Another important role for primary eyecare practitioners is to advise ambly-

opic patients of ways that they can minimize the risk of visual loss to

themselves in the future. About 1.2% of people with severe amblyopia will

eventually become visually impaired (Jakobsson et al 2002). People with

amblyopia have almost three times the risk of visual impairment in their

better-seeing eye to less than 6/12 compared with people without ambly-

opia (Chua & Mitchell 2004). Although amblyopes who lose sight in their

non-amblyopic 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 et al 2002a). Indeed, the lifetime risk of serious visual loss for an

individual with amblyopia is at least 1.2–3.3% (Rahi et al 2002b). So eye-

care practitioners should advise amblyopic patients about wearing eye pro-

tection. It often helps to bring this message home if practitioners cover the

patient’s good eye and point out the level of vision in their 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

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gradually, so that it remains sensitive up to the age of about 6 (Keech &