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

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PICKWELL’S BINOCULAR VISION ANOMALIES

together with important practical issues such as the duration of occlusion,

and recidivism (regression of a benefit from treatment). Key conclusions

concerning the treatment of amblyopia are drawn at the end of this chapter.

Prognosis

In evaluating the prognosis for the treatment of amblyopia and eccentric

fixation, consideration should be given to the following factors:

(1) Type of amblyopia. Where the amblyopia appears to be the consequence of

uncorrected refractive error, then refractive correction is the obvious first

step. In strabismic amblyopia, patients are most likely to benefit from

improvement of the acuity if the strabismus is eliminated: by glasses in an

accommodative deviation (Ch. 15). If the amblyopia is treated but the

strabismus remains, then recidivism (regression of acuity) after treatment

is more likely. One study suggests that even patients whose amblyopia

results from structural lesions (media opacities, macular lesions, optic

nerve abnormalities) can benefit from full-time occlusion in about 50% of

cases (Bradford et al 1992). In general, the prognosis is best for pure ani-

sometropic amblyopia (Beardsell et al 1999) and worst for mixed ani-

sometropic and strabismic amblyopia (Woodruff et al 1994a). The effect

of type of amblyopia is discussed further below.

(2) Age of the patient. It is often said that the older the patient when treat-

ment begins the less likely the treatment is to be successful (Epelbaum

et al 1993). The review below indicates that, while this is the case for

strabismic amblyopia where the rate of acuity improvement is greatest

for younger patients (Fulton & Mayer 1988), in orthotropic anisome-

tropic amblyopia age is no barrier to treatment.

(3) Age of onset of the amblyopia. The shorter the time since the onset of the

factors causing the amblyopia the more likely it is that the acuity can

be restored. Additionally, the length of time that the amblyopia has

been present is critical. Nonetheless, there is evidence that at least some

human amblyopes retain cortical plasticity into adulthood (see below).

(4) Acuity. Not surprisingly, patients who start with worse acuity have a

worse prognosis for achieving good acuities (Woodruff et al 1994a,

Levartovsky et al 1995), and lower acuities usually require long periods

of occlusion. Acuities worse than 6/36 in patients over the age of 6

years are unlikely to respond to treatment.

(5) Cooperation and interest. Active exercises are only appropriate if the

patient and the parent’s interest can be held. In the cases where occlu-

sion is being considered, patient cooperation is the most critical factor

in predicting success (Lithander & Sjostrand 1991).

It can be difficult to persuade teenage patients to wear an occluder. On the

other hand, active stimulation (see below) and physiological diplopia

methods seem more acceptable and it is easier for these to be understood

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and applied by teenagers (Pickwell & Jenkins 1982). With any treatment,

the patient (and parent) need to understand what is required.

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PICKWELL’S BINOCULAR VISION ANOMALIES

when their correction improves the clarity of the retinal images, balances

the accommodative effort between the two eyes, or reduces the angle of a

strabismus.

Anisometropia over 1.50 D usually requires correction (Weakley 2001)

and higher anisometropia is likely to be associated with worse amblyopia

(Townshend et al 1993) and worse binocularity (Rutstein & Corliss 1999).

As discussed in Chapter 11, contact lenses (Evans 2006a) are a better optical

solution to anisometropia than spectacles. Anisometropic amblyopia

responds less well to treatment when it is associated with astigmatism

(Kutschke et al 1991), particularly against-the-rule astigmatism (Somer

et al 2002). Many patients with amblyopia do not have accurate optical

correction (Scheiman et al 2005b) and this may reflect a tendency for cli-

nicians to sometimes ‘write off’ the amblyopic eye. This is undesirable for

reasons that will now be discussed.

It has been known for a long time that the accurate correction of clinically

significant refractive errors is an essential feature of treatment for amblyopia

(Gibson 1955) and many authors have noted that the visual acuity can

improve with spectacles alone (Pickwell 1976) and have recommended a

period of spectacle wear before occlusion is started (Pickwell 1984b, Moseley

et al 1997, 2002, Mulvihill et al 2000). This effect has recently been quanti-

fied and used to recommend a period of refractive adaptation of 18 weeks

before occlusion is commenced (Moseley et al 2002, Stewart et al 2004a). The

1

mean improvement in visual acuity with refractive adaptation was about 2/2

lines. One report suggests that the improvement does not differ significantly

for different amblyopia types (Stewart et al 2004a), but another that the con-

tribution by refractive adaptation was greater in anisometropic amblyopia

(Stewart et al 2004b). Approximately one-fifth of all types of amblyopia in

this study improved so much with refractive correction alone that they no

longer met the criteria for amblyopia (Stewart et al 2004a). The importance of

refractive correction for these cases was further demonstrated by an improve-

ment in the non-amblyopic eye’s visual acuity in many cases.

The Pediatric Eye Disease Investigator Group (PEDIG) studied 84

orthotropic anisometropic amblyopes aged 3–6 years (Cotter et al 2006).

Amblyopic eye acuities ranged from 6/12 to 6/75 and eccentric fixation was

not tested, so it is possible that some of the patients had microtropia (Ch. 16).

Spectacles were given with the ‘optimal refractive correction’ and partici-

pants were measured at 5-week intervals until visual acuity stabilized or

amblyopia resolved. Amblyopia improved by 2 or more lines in 77% and

resolved in 27%. It took up to 30 weeks until stabilization occurred.

Treatment outcome was not related to age but people with better initial

acuity or with less anisometropia did better.

As noted above, many of these anisometropic cases would benefit from

contact lenses and optical penalization is possible with contact lenses (p 209).

Contact lenses will avoid problems of bullying that can be associated with

wearing spectacles and patches (Horwood et al 2005). Refractive surgery

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also seems capable of treating amblyopia in adults with bilateral refractive

amblyopia and anisometropic amblyopia (Roszkowska et al 2006).

AMBLYOPIA AND ECCENTRIC FIXATION

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Occlusion

Since c. AD 900 (von Noorden 1996, p 512) the principal treatment for

amblyopia has been to patch the non-amblyopic eye (direct occlusion;

Pickwell 1977b). The conventional view among eyecare practitioners is that

if patching is carried out within the sensitive period then some improve-

ment in the visual acuity of the amblyopic eye is likely to occur (Snowdon &

Stewart-Brown 1997). This sensitive period during which treatment is

thought to be possible is said to end at 6–7 years (Fells & Lee 1984) or by

about 8 years (Snowdon & Stewart-Brown 1997). Other authors argue that

the visual system is still ‘plastic’ and can respond to treatment up to (Day

1997) and beyond (Levi 1994) the age of 12 years. The evidence concern-

ing the effect of age on treatment will now be reviewed.

Is occlusion treatment effective and, if so, at what age?

In the early 1990s it would have been almost inconceivable for an eyecare

practitioner to question the efficacy or desirability of patching an amblyopic

child whose age was within the sensitive period. But these questions were

raised by a systematic review in the UK by the NHS Centre for Review and

Dissemination (Snowdon & Stewart-Brown 1997).

Healthcare treatments should be evaluated with randomized controlled

trials (RCTs). RCTs are designed to gather objective data so that the expec-

tations of patients (and parents) and practitioners do not influence the

outcome. The above review found that there had been no RCTs of patching

as a treatment for amblyopia and that there was also a lack of good research

on the natural history of this condition. In a summary, the review stated

that the available evidence ‘falls very short of showing that treatment works’

(Snowdon & Stewart-Brown 1997). The review also argued that the disabling

effect of unilateral amblyopia was poorly quantified and may have been

overestimated and noted that amblyopia treatment may have an adverse

effect on the quality of life of the sufferer and their family, which had also

not been quantified (recent evidence on this issue was discussed earlier in

this chapter).

A related question is, if amblyopia should be treated with patching, then

when should this patching occur? Much research on amblyopia concentrates

on strabismic amblyopia, yet there are many differences between this and

anisometropic refractive amblyopia (Mallett 1988a, Birch & Swanson 2000).

In particular, anisometropic refractive amblyopia often improves without

patching, when the patient is just given glasses or contact lenses to correct

the anisometropia (Sen 1982, Mallett 1988b). It will become clear from the

literature review below that the influence of age on the outcome of patch-

ing differs for anisometropic and strabismic amblyopia.

A study of 22 strabismic amblyopes aged 7–10 years found that only three

patients (aged 9, 10 and 10 years) were resistant to occlusion and that none

reported diplopia (Brown & Edelman 1976). The authors concluded that

occlusion should be tried in amblyopes over the age of 7 years. A literature

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review of 23 published studies of occlusion therapy for amblyopia concluded

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PICKWELL’S BINOCULAR VISION ANOMALIES

that success rates at all ages under 16 years were quite similar and even

over this age many patients still responded well to treatment (Birnbaum

et al 1977).

Kivlin & Flynn (1981) carried out a retrospective review of 67 patients with

orthotropic anisometropic amblyopia, more than a third of whom were

8 years or older. Patients with less than 3 D of anisometropia were more

likely to succeed with glasses alone, but even some cases of higher ani-

sometropia improved with spectacles alone, so a trial with spectacles was

recommended before patching. The correlation of outcome with age was

weak and only reached statistical significance in one of four refractive

groups (Kivlin & Flynn 1981).

Sen (1982) investigated the effect of occlusion treatment of anisometropic

amblyopia by comparing a group of 56 children aged 6–12 years with 46

patients aged 13–20 years. Both groups benefited from treatment and the

difference in the benefit received by each group was not statistically sig-

nificant. De Vries (1985) investigated 17 patients with anisometropic ambly-

opia, ranging in age (at presentation) from 2–9 years. Younger subjects did

tend to have a better outcome than older subjects but this effect was very

weak and failed to reach the usual criterion for statistical significance.

A retrospective study of 350 children with unilateral amblyopia treated

with occlusion (initially full-time, then part-time when 6/18 was reached)

included a wide age range of 1–11.5 years (Oliver et al 1986). Older children

were less compliant. When considering compliant cases only, children less

than 8 years old had a slightly more favourable prognosis but the difference

between different age groups was small and only reached statistical signifi-

cance after 1 year of follow-up. None of the patients reported any diplopia.

The mean improvement in compliant cases over the age of 8 years was four

lines, typically occurring within 3 months. The authors concluded that the

reported high failure rates in older children can be ascribed to a lack of com-

pliance rather than to age-related factors. The authors did not subclassify

their data according to amblyopia type.

A study of 30 esotropic amblyopes aged 3–10 years found that the rate of

acuity improvement with occlusion was slower in older children (Fulton &

Mayer 1988).

A study that investigated anisometropic amblyopia (Hardman Lea et al

1989) excluded cases with high astigmatism. These authors looked at 36

children, aged 3–7.5 years and initially gave full spectacle correction followed

by patching if visual acuity failed to improve. They found that the period

for the possible improvement of acuity extends uniformly, without tailing

off, at least up to the age of nearly 8 years, which was the oldest age that

they studied.

It should be noted that amblyopia treatment with patching is not always

effective, regardless of the age when treatment is started. For example, many

of the subjects in the research study by Hardman Lea et al (1989) showed a

minimal or complete lack of any improvement with patching, and the age

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of these subjects was fairly evenly distributed across the age of subjects in

their study. Esotropic patients who do not respond well to treatment of

AMBLYOPIA AND ECCENTRIC FIXATION

13

their amblyopia are particularly likely to have a history of high hyperme-

tropia at age 1 year (Ingram et al 1990). Neither the reported age of onset

nor delay in presentation influenced the final visual outcome.

Wick et al (1992) examined the records from 19 patients with anisome-

tropic amblyopia who had been treated aggressively with patching and with

active vision therapy. The patients showed a marked improvement in visual

acuity and the authors concluded that treatment of anisometropic ambly-

opia ‘can yield substantial long-lasting improvement in visual acuity and

binocular function for patients of any age’.

Hiscox et al (1992) reviewed 368 patients who had been treated with

patching for various types of amblyopia. Most were aged between 3 and

7 years and the success rate was found to vary little with the starting age.

One study that took account of recidivism (see below) suggests that at

long-term follow-up the outcome of treatment was no worse in those who

were over 8 years at the start of treatment than in younger participants

(Levartovsky et al 1992). This study did not differentiate between different

types of amblyopia.

Rutstein & Fuhr (1992) reviewed the records of 64 patients with strabismic

and/or anisometropic amblyopia. The authors divided their patients into

those aged 7 years or less and those over 7 years. They concluded ‘These

findings indicate that visual acuity can be improved by patching therapy

in most patients older than 7 years, but the acuity improvement is some-

what less than in younger patients’. Their analysis does not provide any

information on whether the reduced effect of treatment with age was lin-

ear or whether there is an abrupt reduction in the effect of treatment at the

age of 7.

In a study of 38 children aged 5–10 years with orthotropic anisometropic

amblyopia who had been treated by occlusion, the outcome was not

related to patient age (Noda et al 1993).

Epelbaum et al (1993) investigated the effect of occlusion on 407 children

with strabismic amblyopia aged 21 months to 12 years. Recovery of acuity of

the amblyopic eye was maximum when the occlusion was initiated before

3 years of age and was about nil by the time the patient was 12 years old.

Flynn et al (1998) pooled the data from 961 patients reported in 23 studies

published between 1965 and 1994. The subjects under investigation had a

wide range of ages, from 0–3 years to more than 21 years. The majority of

the subjects had strabismic amblyopia and for this group younger patients

were significantly more likely to benefit from treatment. The mean age of

1

the subjects for whom patching was successful was about 4 /2 years and the

1

mean age for those where it was unsuccessful was about 7 /2 years. But for 108

anisometropic amblyopes there was no significant relationship between

age and the success of treatment.

Flynn et al (1999) extended their original sample of 961 subjects with 961

from another study group. The new sample was younger and, although there

was a weak effect of age on treatment success in the sample as a whole, this

did not reach significance for the anisometropic or strabismic groups when

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these were considered separately. Flynn et al (1999) defined success as an

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PICKWELL’S BINOCULAR VISION ANOMALIES

improvement in visual acuity to 6/12 or better and about half of their

subjects (in both the strabismic and anisometropic groups) achieved this

level.

A study that included a sophisticated battery of tests of visual function

evaluated the effect of patching in 50 patients with strabismic or ani-

sometropic amblyopia who were aged 4–10 years (Simmers et al 1999). The

authors concluded that ‘we could find no relationship between age and

degree of visual function improvement’.

Mintz-Hittner & Fernandez (2000) studied a series of 36 consecutive

compliant children with amblyopia who were aged between 7 and 10.3

years. The patients were treated aggressively with either full-time ‘standard’

occlusion, total penalization or full-time occlusive contact lenses. All patients

achieved a final acuity of between 6/6 and 6/9, a marked improvement on

initial acuities that ranged from 6/15 to 6/120 with a mean of 6/45. Although

not specifically analysed by the authors, an inspection of the graphs in their

paper suggests that age had no effect on treatment regardless of type of

amblyopia. The authors concluded that, given compliance, therapy for

anisometropic and strabismic amblyopia can be successful even if initiated

after age 7 years.

Cleary (2000) studied 136 children aged 8 years or younger with either

strabismic (77 cases) or mixed (59 cases) amblyopia. Cleary (2000) excluded

cases with purely refractive amblyopia and prescribed full cycloplegic refrac-

tive findings to all subjects. She excluded those who were non-compliant

to spectacle wear and compared the outcome of the 119 cases who complied

with occlusion with the 17 who failed to comply with occlusion. The ambly-

opic eye improved in 74% of those complying with occlusion but only in

59% of those non-compliant to occlusion. The acuity of the amblyopic eye

worsened in only one participant, in the non-compliant group. Maximal

improvement occurred within the first 3 months of occlusion. The author

found that ‘as in previous studies, age at initiation of treatment was not

influential on outcome’. The maximal improvement occurred in response

to 400 hours of occlusion or less, and to full-time occlusion.

Amblyopic adults who lose vision in their non-amblyopic eye through

age-related macular degeneration experience an improvement in the acuity

of their amblyopic eye of 2–3 lines, typically over a 1–12 month period (El

Mallah et al 2000). A larger study of adult amblyopes who lost vision in their

non-amblyopic eye reported an improvement of 2 lines or more in 10% of

cases (Rahi et al 2002a). The aetiology of the improvement was unclear but

one factor in at least some cases was a new refractive correction.

Cobb and colleagues carried out a retrospective analysis of 112 children

(aged 3–12 years) with anisometropic amblyopia who were treated with

spectacle correction and, if necessary, occlusion (Cobb et al 2002). Age was

not significantly related to outcome and the lack of an effect of age was

present not only in orthotropic anisometropes but also in microtropic ani-

sometropes. Higher degrees of anisometropia were associated with a worse

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outcome, in contrast to an earlier study, which found no relationship

between degree of anisometropia and final acuity (Kutschke et al 1991).

AMBLYOPIA AND ECCENTRIC FIXATION

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A large RCT compared intensive vision screening from age 8–37 months

with one vision screening episode at 37 months only (Williams et al 2002).

When the children reached 7.5 years of age the prevalence of amblyopia

was significantly lower in the group who had received intensive vision

screening (0.6%) compared with the group with less intensive screening

(1.8%). The authors acknowledge that the reason for the improved results

in the intensive group cannot be determined from the study but could be

related to age, compliance or the effects of repeated testing. They did not

differentiate between the results for subtypes of amblyopia but another

paper revealed that the vision screening could not detect most cases of

orthotropic amblyopia until 37 months of age (Williams et al 2001).

A study of 209 children aged 3–7 years found that occlusion obtained

similar results to atropine (see below) treatment (Pediatric Eye Disease

Investigator Group 2002a, 2003c). There was a mean acuity improvement

with occlusion over 6 months of about 3 lines and there was no effect of

age nor of type of amblyopia for either treatment approach.

A randomized controlled trial of 177 children aged 3–5 years with uni-

lateral visual impairment (6/9 to 6/36) compared treatment with spectacles

alone with treatment with spectacles and occlusion (Clarke et al 2003). On

average, those who received occlusion only improved by about 1 line more

than those with only spectacles, but for those with worse acuity (6/18 to

6/36) the improvement was a more clinically significant 2 lines. The authors

question whether treatment is worthwhile in cases with mild (6/9 to 6/12)

acuity loss (Clarke et al 2003). Children whose treatment was deferred from

age 4 to 5 years had the same acuity after treatment but fewer needed occlu-

sion at all.

In a pilot study, 66 amblyopes aged 10–17 years were treated with daily

occlusion of 2–8 hours a day (median 2 hours) including at least 1 hour of

near vision activities (Pediatric Eye Disease Investigator Group 2004a).

After 2 months of treatment, 27% improved by 2 or more lines and no

effect of age was apparent. Although the authors state that approximately

one-third of the sample each had strabismic, anisometropic and mixed

amblyopia, the results are not reported for different types of amblyopia.

In a study of 55 compliant adolescents (aged 11–15 years), full-time occlu-

sion was prescribed until there was no further improvement for three con-

secutive monthly examinations (Mohan et al 2004). The mean improvement

was 4.5 lines and this was seen in those with strabismic, anisometropic or

mixed amblyopia. In another small study, 16 compliant amblyopes aged

9–14 years responded well to occlusion, with all except one improving by

2 lines (Park et al 2004). Most cases were anisometropic but some were

strabismic and these also improved. No patient developed diplopia, and

binocular status improved in some cases.

In the monitored occlusion treatment of amblyopia study (MOTAS), 94

participants aged 3–8 years were studied comprising approximately equal

numbers of strabismic, anisometropic and mixed amblyopia (Stewart et al

2004b). More than 75% of the acuity deficit was corrected for half the partici-

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pants. Some patients were intractable to treatment: in 10% of participants,

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less than 25% of the acuity deficit was corrected. Treatment outcome was

significantly better for children aged 3–4 years than in children aged 6–8

years but the effect of age was not reported for different types of ambly-

opia (Stewart et al 2004b). A later publication noted that age is a factor

when considering simple change in visual acuity; however, if outcome

by residual amblyopia and proportion of deficit corrected are considered,

the effect of age ceases to be significant: ‘in the majority of children, age

was not a factor in obtaining optimum outcome’ (Stewart et al 2005). All

types of amblyopia showed a similar dose–response relationship to occlu-

sion, but not surprisingly the contribution by refractive adaptation was

greater in anisometropic amblyopia (Stewart et al 2004b). In addition to

occlusion dose and age, other factors that improved the prognosis were

milder initial amblyopia, good binocular vision and no eccentric fixation

(Stewart et al 2005).

A smaller study, of 52 children aged 8 years or less, also showed a dose–

response relationship and found a significant improvement only in cases

who wore the patch for more than 3 hours a day (Awan et al 2005). There

was no effect of age within the range studied and only strabismic and mixed

amblyopes were included.

A randomized controlled trial by the PEDIG studied two age groups with

amblyopia from 6/12 to 6/120 (Scheiman et al 2005b). The first group con-

tained 404 patients aged 7–12 years. The control was refractive correction

alone and the full treatment was refractive correction and 2–6 hours of

patching combined with 1 hour near vision activities and atropine. There

was an improvement in the amblyopic eye of at least 2 lines in 53% of the

treatment group and in 25% of the control group. The authors state that

patients improved regardless of previous treatment and of type or severity of

amblyopia. However, the data in the paper indicate a trend towards more

improvement in those with anisometropia (15.6 letters) than in those with

strabismus (11.7 letters) or mixed amblyopia (12.3 letters).

This study also included 103 participants aged 13–17 years with the same

control and full treatment, but with no atropine (Scheiman et al 2005b).

The improvement with treatment was less than in younger participants, and

only reached significance for the subgroup who had not received treatment

before. From the data presented in the paper, the improvement in the ani-

sometropic subgroup receiving full treatment was 2 lines compared with

one line in the control group, which appears to have been statistically sig-

nificant. The strabismic subgroup receiving the treatment actually did a little

worse than those receiving only spectacles. Although a number of patients

reported occasional diplopia when specifically queried, in almost all cases the

diplopia was infrequent and inconsequential. The authors plan to report

long-term follow-up in due course.

A retrospective study evaluated 128 children with all types of amblyopia,

aged 3–12 years, who had been treated by occlusion (Arikan et al 2005). The

mean improvement was about 4 lines and did not differ significantly in

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the three main types of amblyopia. Factors predicting success, when all sub-

groups were combined into one group, were younger age, better initial visual

AMBLYOPIA AND ECCENTRIC FIXATION

13

acuity and full-time occlusion. However, when each subgroup was analysed

separately only initial visual acuity was significantly correlated with final

outcome.

Another randomized controlled trial by the PEDIG studied 180 children

aged 3–7 years with acuity in the amblyopic eye of 6/12 to 6/120 (Wallace

et al 2006). Participants had strabismic, anisometropic or mixed ambly-

opia and had worn optimal refractive correction for 16 weeks or for two

consecutive visits without improvement. Participants either received 2

hours of daily patching with 1 hour of near visual activities or continued

with spectacles alone (if needed). After 5 weeks, the amblyopic eye average

acuity improved significantly more in the occlusion group (1.1 lines) than

in the spectacles-only group (0.5 lines). The interaction between age, type

of amblyopia and treatment outcome was not reported.

The conclusions from this review are drawn in the last section of this

chapter.

Types of occluder

Different types of occluder are listed in the next chapter (see Fig. 14.4),

although the more invasive forms (e.g. eyelid occlusion) are not appropri-

ate in amblyopia treatment. In non-amblyopic normal subjects, different

types of occluder (e.g. frosted, 1.50 fogging lens, opaque occluder) have

different effects on the visual function of the unoccluded eye (Wildsoet

et al 1998), so it is possible that the type of occluder will influence the out-

come of patching in amblyopia. However, the most important aspect of

the occluder is that it must occlude! Children are resourceful and will seek

to achieve the best vision they can, which will mean removing or peeping

around the patch. Practitioners and parents must be alert to this possibil-

ity and continuous reinforcement of instructions is required.

The usual method is total occlusion, which tries to ensure that no light

enters the eye and that the amblyopic eye is brought into use. The most

effective device is an adhesive patch that covers the eye and extends a lit-

tle over the orbit margins. A smaller piece of gauze or lint in the centre of

the patch prevents adhesion to the lids. Plastic, rubber or felt cup devices

are also made to fit between the eye and spectacles. These do not provide

such total occlusion and a young child can soon learn to peep over them.

Alternatives that may be more cosmetically acceptable and can be used if

the child can be trusted to maintain occlusion include the use of a Chavasse,

or frosted lens, in glasses.

Bangerter foils (Appendix 11) are translucent press-on films that are

adhered on to a lens in a similar way to Fresnel lenses. Bangerter foils are

not prismatic but are frosted in a series of increasing degrees of opacifica-

tion. They were developed for amblyopia therapy and are graded accord-

ing to the typical level of decimal visual acuity obtained through the foil.

Bangerter foils seem to work well in occlusion of amblyopes with 6/18 or

better vision and have the advantage in anisometropia of allowing coarse

stereopsis to be maintained (Iacobucci et al 2001). Occlusive contact lenses

207

can achieve success in some cases (Joslin et al 2002).