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

AMBLYOPIA AND ECCENTRIC FIXATION

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PICKWELL’S BINOCULAR VISION ANOMALIES

severe amblyopia (6/30 to 6/120) found a similar improvement in children

with 6 hours compared with full-time occlusion (Holmes et al 2003b). A

randomized controlled trial of nearly 200 children below age 7 years with

moderate amblyopia compared 2 hours with 6 hours daily patching

(Pediatric Eye Disease Investigator Group 2003b). All groups in these stud-

ies were asked to carry out 1 hour a day of near visual activities during

patching. The improvement in visual acuity was not significantly different in

the two groups. However, this study has been criticized (Greenberg 2004,

von Noorden & Campos 2004).

The development of an objective electronic device for monitoring occlu-

sion dosage has made it possible to evaluate the treatment-dose response

in amblyopia therapy, in the MOTAS study (Stewart et al 2004b). Increasing

dose rate beyond 2 hours per day hastened the response but did not improve

outcome. In a later paper, these authors noted that ‘increasing evidence

suggests that small doses of occlusion are as beneficial as substantial or

maximum doses’.

Alternate and inverse occlusion

Occlusion amblyopia occasionally develops in a previously good eye follow-

ing occlusion and occasionally the strabismus can transfer to the previously

good eye (Assaf 1982). This is most likely in the young (under age 3 years)

if patched very aggressively (Assaf 1982). If the patient is over 3 years of

age, any loss of acuity in the occluded eye is likely to return when the

occlusion treatment ceases. Under 3 years, the occluder or patch should be

worn over the non-amblyopic eye for 3 days and then changed to the

amblyopic eye for 1 day to allow the development of the non-amblyopic

eye. Under 2 years, a 2:1 occlusion regimen may be more appropriate.

Some authorities have advocated occlusion of the amblyopic eye, ‘inverse

occlusion’, but this does not seem to be effective (von Noorden 1965). It has

been recommended for the rare cases of abnormal foveal localization but it

is unlikely that the amblyopia will respond to treatment in these cases.

Duration of occlusion

Not all cases of amblyopia improve with occlusion, and failure to respond to

treatment may in some cases be attributable to subtle optic nerve hypoplasia

(Lempert 2000).

Most compliant children are cured (Lithander & Sjostrand 1991) or at

least improve, most (Oliver et al 1986) within 3 months. One study found

that three periods of full-time occlusion, each of up to 4 weeks, were

required before it could be safely concluded that no improvement was

occurring (Keech et al 2002). More than one-quarter of patients had no ini-

tial improvement but then exhibited an improvement of 3 or more lines.

In a study of 209 children aged 3–7 years treated with occlusion or

atropine penalization, the average improvement in acuity was 3 lines

(Pediatric Eye Disease Investigator Group 2003d). Of the 157 patients who

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improved by at least 3 lines, 15% achieved their maximum improvement

in 5 weeks and 52% by 16 weeks.

AMBLYOPIA AND ECCENTRIC FIXATION

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In the MOTAS study, 80% of the improvement with occlusion occurred

within 6 weeks (Stewart et al 2004b). This study design included refractive

adaptation: all participants had worn spectacles for 18 weeks before occlu-

sion commenced.

When the acuity ceases to improve, the occluder may be removed in the

case of the refractive amblyopes. Where there is a strabismus, the next step

in the total treatment needs to be considered. Sometimes, patients with

cosmetically acceptable and asymptomatic strabismic amblyopia wish to

have the acuity in their amblyopic eye improved but do not desire any fur-

ther treatment of their strabismus. They may simply wish to have a better

‘eye in reserve’. These cases may subsequently manifest a regression of their

acuity (see below).

Follow-up appointments

When a patient is being treated, improvement is expected and if this does not

occur an alternative treatment or referral will be required. This means that

patients who are being treated need to be monitored closely so that if they are

not responding to treatment an alternative management strategy can be

started without delay. Younger patients should be reviewed more frequently.

With full-time occlusion, the interval between appointments in weeks

should equal the patient’s age in years (e.g. 4 years old, 4-week review).

Recidivism

Recidivism refers to a relapse of acuity following apparently successful treat-

ment. In one study of patients with strabismic amblyopia who achieved 6/12

or better with treatment, only 40% maintained this on prolonged follow-

up (Sparrow & Flynn 1977). The authors recommended close follow-up and

repeat patching when necessary.

Kivlin & Flynn (1981), in their study of 67 anisometropic amblyopes,

found that, of the 35 cases who completed therapy and achieved 6/12 or

better, three patients (9%) fell permanently below this level of vision at

follow-up examinations. Bradford et al (1992) carried out a retrospective

review of a case series of 51 patients with structural anomalies (described

above). Amblyopia recurred in 31% of patients and was successfully treated

with resumption of full-time occlusion.

Some evidence suggests that recidivism might be more likely in patients

who are older when treatment is started (Malik et al 1975, Oliver et al 1986).

However, one study suggests that, at long-term follow-up, the outcome of

treatment was no worse in those who were over 8 years old at the start of

treatment than in younger participants (Levartovsky et al 1992). This study

did not differentiate between different types of amblyopia.

Recidivism is more likely to be exhibited by patients with worse initial

acuities and by those with mixed strabismic and anisometropic amblyopia

(Levartovsky et al 1995). In anisometropic amblyopia, the risk of recidivism

appears to be greater for higher degrees of hypermetropia (Levartovsky

et al 1998). However, these authors did not report in detail on the compliance

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with continued spectacle wear and it would be interesting to see if contact

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lens wear (Winn et al 1988) prevents recidivism with higher anisometropia.

El Mallah et al (2000) found that patients with age-related macular degener-

ation often showed an improvement in the acuity of an amblyopic eye,

typically of 2–3 lines. This improvement was stable over time, suggesting

that recidivism may not occur if the amblyopic eye remains in constant use.

Some authors recommend maintenance occlusion, part-time occlusion

after the cessation of full-time occlusion, in the belief that this will prevent

acuity loss. However, when 32 children who had been treated with occlusion

at age 11–15 years were followed up for a mean of 18 months, only 9%

exhibited recidivism and this was just as likely in those who had received

maintenance occlusion (Mohan et al 2004).

Leiba et al (2001) carried out a retrospective study of 54 patients who were

treated successfully as children for unilateral amblyopia with occlusion and

had follow-up data for 17–25 years. The visual acuity was maintained or

improved in two-thirds of patients. This study did not differentiate between

subtypes of amblyopia. A 10-year follow-up of 24 participants with ambly-

opia found that visual acuity was essentially stable (Ohlsson et al 2002b).

There was a high (36%) recidivism rate in a study that found a rapid

improvement in visual acuity with atropine penalization (Kaye et al 2002).

It seems plausible that the patients who manifest a rapid improvement in

visual acuity are most prone to recidivism.

Rutstein & Corliss reported on 61 patients (22 with strabismic and 26

with anisometropic amblyopia) who were monitored for at least 4 years

after the completion of occlusion therapy (Rutstein & Corliss 2004). The non-

amblyopic eye tended to improve over time but in no group was the aver-

age improvement more than 1 line. For strabismic amblyopia the amblyopic

eye improved by 0.36 lines with treatment and regressed over a mean of

9 years by just under 1 line. For anisometropic amblyopia the amblyopic

eye improved by 0.30 lines with treatment and regressed over a mean of

7 years again by just under 1 line.

Active amblyopia therapy

So far, the use of occlusion in everyday life has been described. During a

period of occlusion, the treatment can be made more effective (Francois &

James 1955) by asking the patient to undertake a detailed visual task, to

interest the patient and stimulate vision. Gould and colleagues used the

term active visual stimulation to describe these detailed visual tasks and rec-

ommended such treatments for periods of 45–60 min, reducing as acuity

improves (Gould et al 1970). These authors described 35 cases aged 6–17 years

and reported that, with active therapy, the patients over the age of 10 years

improved by 3.6 lines compared with 2.7 lines in younger participants.

Active therapy should be presented to younger children as games or com-

petitions and video games can be helpful. Care must be taken to ensure that

the patient is working at the limit of acuity by maintaining an appropriate

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working distance. The distance should also be monitored when the patient

is watching television. With video games, parents can be asked to help

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with different levels of acuity. Patients are asked to use a fine-tipped pen to

trace over the slides. The unit is used for periods of about 30 min, once or

twice weekly. Mallett (1985) argues that patients of any age can be treated

and this approach has been shown to be effective in a small randomized

controlled trial of amblyopes over the age of 10 years (Evans et al, in prepa-

ration). A computerized version of the IPS treatment (Field 2005) seems to

be better at maintaining patient interest.

Suppression treatment

Suppression treatment as a means of improving the acuity in anisometropic

amblyopia can be used as an alternative to occlusion. Although occlusion

is sometimes necessary, it is not always the preferred method: it could be

argued that covering one eye is undesirable if the goal is to establish bin-

ocular vision. In cases where there is no strabismus and the effect of the

refractive correction has been tried, further improvement in acuity can

sometimes be produced by exercises that ensure that the amblyopic eye is

used with its fellow. It is useful to investigate the depth and extent of the

suppression and, where it appears appropriate, to give treatment for any

suppression found. Exercises described in Chapter 14 may be required, but

often the simpler treatment outlined for suppression in heterophoria in

Chapter 10 will help considerably. Most teenage patients would prefer this

approach to wearing an occluder (Pickwell & Jenkins 1983).

Physiological diplopia

In some cases of convergent strabismus, physiological diplopia methods

can be used as an alternative to occlusion (Pickwell 1976), sometimes in

combination with other treatments and in teenage as well as younger chil-

dren (Pickwell & Jenkins 1983). The physiological diplopia method is simple

to understand but in practice it may require a lot of patient supervision.

A near vision fixation distance is found where the visual axes cross in con-

vergent strabismus, and an appreciation of physiological diplopia is used to

encourage the use of binocular vision. This brings the amblyopic eye into

use with the other eye at one fixation distance and results in an improve-

ment in acuity over several weeks.

The difficulty in applying this method is usually in the early stages. In

theory, it is not difficult to see that there is a point in front of the eyes of

a patient with convergent strabismus where the visual axes cross and that

an object placed there ought to have a foveal image in both eyes; the object

is being fixated binocularly. In practice, this can be found by using the cover

test and moving the object closer or further away until no strabismus move-

ment is seen. Where the patient increases the angle of deviation as the object

is moved towards the eyes (the convergence excess element), this can some-

times be inhibited by explaining to the patient that the eye is turning

inwards too much and encouraging less convergence. This may take time.

With some patients, convergence needs to be inhibited by positive spher-

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ical additions. When binocular fixation of the object has been achieved,

the patient is encouraged to see a second object in physiological diplopia.

AMBLYOPIA AND ECCENTRIC FIXATION

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This should be introduced at a greater distance than the fixation object. Its

image will then fall on the nasal retina in the area where there are likely to

be binocular sensory adaptations (see Fig. 10.4). With further encourage-

ment, appreciation of physiological diplopia of the second object can be

seen, while fixation of the first is maintained. A week of home exercise

supervised by a parent should consolidate this. The parent may be taught

how to apply the cover test to check that fixation has been maintained.

The patient then progresses to wire reading, which is described on page 152.

This method is appropriate only to angles of strabismus less than 12

or in which it can be reduced to this by refractive correction. It requires a

high degree of interest and cooperation. The starting acuity should be 6/24

or better, and in some cases can go on improving for several months if

the method is successful in establishing binocular vision for close work.

The method is further described in Chapter 14 in relation to its use in the

treatment of binocular sensory adaptations.

After-image transfer method

A central after-image is created in the dominant eye and transferred to the

amblyopic eye as described earlier in the chapter. The patient is then asked

to try to locate the after-image at the point of fixation and to see smaller

fixation letters. The procedure is repeated when the after-image fades, and

the acuity is measured after several repeats (Caloroso 1972, Mallett 1975).

This approach probably improves acuity by encouraging central fixation. It

appears that the best results are obtained when the starting acuity is 6/24 or

better and in those cases when the binocular vision and acuity has deterio-

rated following previous improvement achieved by other orthoptic proce-

dures (Jenkins et al 1979).

Other approaches

It has been suggested that a deep red filter placed over the unoccluded eye

reduces eccentric fixation but one study suggested that a neutral density

filter is as good as a red filter (Matilla et al 1995). Indeed, a recent random-

ized controlled trial suggests that occlusion while a blue filter is worn over

the amblyopic eye may be an effective treatment (Metzler et al 1998).

The Euthyscope is a special ophthalmoscope that was used in a pleoptic

technique to create a ring after-image centred on the fovea of the ambly-

opic eye in an attempt to encourage foveal fixation (Schmidt & Stapp 1977).

Recent evidence of the risks of excessive exposure to light (Young 1994)

may be a cause for concern with this type of treatment.

One early active stimulation technique required the patient to look at a

rotating grating of black and white lines for a few minutes and to maintain

clear vision of the lines: the CAM disc method (Banks et al 1978). Despite

some promising results (Watson et al 1985), in most studies this method has

not been shown to produce good results (Douthwaite et al 1981). Several stud-

ies indicate that it produces the same results as occlusion (Nyman et al 1983),

which may also be required (Tytla & Labow-Daily 1981). It may be a little

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more effective in anisometropic amblyopia than in strabismic amblyopia

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(Lennerstrand & Samuelson 1983), although there is little evidence to suggest

that the presence of the rotating grating has any effect other than a placebo.

A recent approach uses perceptual learning (practising certain visual

tasks) in an attempt to train amblyopic eyes to better recognize low-level

features of simple visual stimuli (Polat et al 2004). These authors studied

77 adult amblyopes, most of whom were in their 30s or 40s, who received

approximately 45 treatment sessions, each lasting about 30 min. 14 of these

received a control treatment. An improvement of 2 or more lines occurred

in 68% and was maintained. The authors believe that one outcome of their

training is to reduce the crowding effect and the benefit was independent

of age and of amblyopia type, although patients with eccentric fixation were

excluded. A case study indicates that this approach may help to improve

vision in an amblyopic eye after vision has been lost in the non-amblyopic

eye (Fronius et al 2005). A small study suggests that a modification of this

approach may be successful in some children with amblyopia, who seem to

improve by about as much as adults (Li et al 2005).

Levi and colleagues showed that Vernier acuity can respond to training

in adult amblyopes and in some cases Snellen acuity also improved (Levi

et al 1997). Another interesting development is the use of virtual reality sys-

tems in amblyopia treatment for children, using interactive two-dimensional

and three-dimensional games and videos (Eastgate et al 2006). Preliminary

results are promising (Waddingham et al 2006).

Certain neurotransmitters have been implicated in neuronal plasticity.

Based on this finding, the drug levodopa has undergone preliminary testing

as a treatment for amblyopia. Levodopa combined with occlusion (Leguire

et al 1998) may lead to long-term improvement in visual acuity after recidi-

vism (Leguire et al 2002), although another study contradicts this view

(Bhartiya et al 2002). Corticosteroids may also have a beneficial effect

on amblyopia, although this hypothesis requires more investigation

(Constantinescu & Gottlob 2001).

Conclusions concerning treatment of amblyopia

First, conclusions will be drawn from the review earlier in this chapter on

whether occlusion is effective and if so at what age. There are fundamen-

tal differences between anisometropic and strabismic amblyopia, so it is

appropriate to consider the effect of age on treatment of these conditions

separately. Nine studies in the review analysed the effect of age on the out-

come of occlusion in strabismic amblyopes, six of which found no effect

of age. Ten studies in the review analysed the effect of age on the outcome

of occlusion in anisometropic amblyopia, and all ten found no effect of

age. Some caution is needed, since the age range of most studies is quite

limited. Nonetheless, these results suggest that anisometropic amblyopia

can be treated at any age, although with strabismic (or mixed) amblyopia

treatment may become less effective with increasing age.

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This fact appears to be better appreciated in German-speaking countries,

where orthoptists are more willing to treat amblyopia in older children