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

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PICKWELL’S BINOCULAR VISION ANOMALIES

Kutschke 1995) to 8 years (Nelson 1988a, Levi 1994, Daw 1997), possibly to

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 et al 1986). Data from monkeys sug-

gest that an earlier onset of strabismus may tend to be associated with

deeper amblyopia (Kiorpes et al 1989).

Visual function in strabismic and

anisometropic amblyopia

Basic visual functions

Colour vision is normal but the pupillary function of eyes with strabismic

amblyopia is subtly different from that of eyes with anisometropic amblyopia

(Barbur et al 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 the severity of the amblyopia and does not result from optical

factors, unsteady fixational eye movements or eccentric fixation (Flynn

1991). Intermediate spatial frequency letter contrast sensitivity is less affected

by amblyopia than high-contrast visual acuity (Moseley et al 2006). Ocular

pursuit is abnormal in strabismic amblyopia (Bedell et al 1990).

Visual processing occurs in interlinked parallel pathways and the two

principal subsystems 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 is

relatively unaffected (e.g. Nelson 1988a), although this is likely to be an

oversimplification (Kelly & Buckingham 1998). However, Hess & 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 & Bachmann (1996) found that a small heteroge-

neous sample of amblyopes had better than normal function in their ambly-

opic eyes at a specific task of spatiotemporal integration at high velocities.

One explanation for this finding might be if amblyopes have a P-deficit and

normal or supranormal M-cellular function. An electrophysiological study

of anisometropic amblyopes found reduced P- but normal M-function (Shan

et al 2000). A more recent analysis of sensory processing in amblyopia

highlights fundamental differences between strabismic and anisometropic

amblyopia (Hess 2002). Amblyopic eyes make misperceptions of spatial

structures and this has been attributed to errors in the neural coding of

orientation in the primary visual cortex (Barrett et al 2003).

Visual acuity

Visual acuity can be classified as follows:

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(1) Minimum resolvable, the smallest angular separation between targets that

can be recognized; e.g. grating acuity, as measured with the Teller or

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PICKWELL’S BINOCULAR VISION ANOMALIES

of amblyopia as a part of that routine and with supplementary tests that

may be needed to reach 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 clinical worksheet in Appendix 6 summarizes a clinical approach to

the investigation of amblyopia. One aim of the investigation of amblyopia

is to differentially diagnose the type of amblyopia, and this is summarized

in Table 13.1.

An interesting study carried out biometry on amblyopic and control

patients and found evidence suggesting that many amblyopic eyes may have

a subtle form of optic nerve hypoplasia (Lempert 2000, 2004). Lempert

suggests that the optic nerve hypoplasia may be the primary reason for the

reduced acuity, although he notes that it is also possible that the reduced

size of the optic nerve is the result of the amblyopia. A recent study did

find reduced nerve fibre layer thickness in amblyopic eyes compared with

fellow eyes, but only in anisometropic and not strabismic amblyopia (Yen

et al 2004).

History and symptoms

In the case of strabismic amblyopia, the age at which the strabismus was

first noticed should be known. Sometimes past photographs can help.

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 diffi-

cult 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.

It is important to have a full appreciation of 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 pre-

scription 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 non-critical

and encouraging underestimation.

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 standardize 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

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method used should then be recorded along with the test distance and

acuity measurement.

AMBLYOPIA AND ECCENTRIC FIXATION

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D

A

N

N

H

D

A

Figure 13.1 Reading Snellen letters with an eccentrically fixating eye: the patient is

fixating the letter N in a line of Snellen letters. The image of the next letter on the right of

N, which is H, will fall on the foveal scotoma and not be seen easily by the patient, who may

therefore miss it and read the following letter, A. If the patient reads from right to left

instead of beginning at the left as normal, the difficulty does not occur; the image of the

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letter D, for example, does not fall on the scotoma.

Many studies have suggested that strabismic amblyopia may be detected by

tests assessing random dot stereopsis (Cooper & Feldman 1978, Schweers &

Baker 1992, Hatch & Laudon 1993, Walraven & Janzen 1993). Contoured

stereopsis tests (e.g. Titmus circles) are probably less sensitive for detecting

strabismic amblyopia (Schweers & Baker 1992). The outcome of occlusion

treatment is typically defined as improvement in visual acuity but this

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is usually accompanied by improvement in stereoacuity, both in small angle

or intermittent strabismus and in anisometropic cases (Lee & Isenberg 2003).

AMBLYOPIA AND ECCENTRIC FIXATION

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Eccentric fixation

Eccentric fixation is a monocular condition in which a point on the retina

other than the fovea is used for fixation. Fixation is usually eccentric in

patients whose amblyopia is purely strabismic and the acuity tends to be

worse in higher degrees of eccentric fixation (Hess 1977). A recent study

found eccentric fixation in none of 20 cases of orthotropic anisometropic

amblyopia, 19% of cases of strabismic amblyopia and 58% of cases of

mixed amblyopia (Stewart et al 2005). Of the cases with eccentric fixation,

76% had mixed and 24% had strabismic amblyopia. The presence of

eccentric fixation worsened the prognosis for successful treatment of the

amblyopia.

The rate of reduction in visual acuity with increasing eccentric fixation

is, in most cases of strabismic amblyopia, more rapid than the normal

decline in visual acuity in the peripheral retina (Hess 1977). It is important

to know the degree and the stability of the eccentricity, since this will aid

in the differential diagnosis of the amblyopia (Table 13.1) and can be used

to monitor the effect of treatment. However, Cleary & Thompson (2001)

noted that instability was common, so that precise measurement is often

not possible.

There is some controversy about why eccentric fixation occurs (Jennings

2001b). The initial ‘sensory theory’ was that there is a central scotoma and

the patient fixates with the area giving best acuity rather than the fovea

(Worth 1903). There has also been a suggestion that it arises from a change

of the central area of localization as a central scotoma develops in the

amblyopic eye (Duke-Elder 1973).

A more recent and more likely theory is that the habitually strabismic

position of the amblyopic eye leads to an error in localizing the straight

ahead when it is required to fixate monocularly (Schor 1978). This ‘motor

theory’ in essence suggests that the eccentric fixation results from an adap-

tive after-effect. In other patients, it may be a sequel to an enlargement of

Panum’s fusional area that follows decompensated heterophoria at an

early age and leads eventually to microtropia (Pickwell 1981).

These different theories on the aetiology of eccentric fixation are not

mutually exclusive. One or more may apply to a particular patient and the

others in other patients. As far as management is concerned, it does not

really matter which theory is correct and both theories and experimental

data have disproved the notion that occlusion might worsen eccentric fix-

ation (Jennings 2001b). The presence of eccentric fixation may make it

more likely that recidivism will occur (see below).

Investigation of eccentric fixation

No method of assessing the fixation seems to be satisfactory for all patients

(Cleary & Thompson 2001) and the clinical worksheet in Appendix 6 can

be used for a detailed workup. Ophthalmoscopic methods are most widely

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used and are therefore described in detail below.

Fixation star

Foveal reflex

foveal reflex and the fixation target. The degree of eccentric fixation can be recorded, using

a disc diameter (dd) or drawing the ophthalmoscope graticule (if present) as the unit of

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measurement. If the ophthalmoscope graticule is used then record the make and model of

ophthalmoscope. There may be a slight vertical element to the eccentric fixation.

AMBLYOPIA AND ECCENTRIC FIXATION

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investigation is maintaining the amblyopic eye’s position when the target

is moved from the eccentric area to the fovea.

After-image transfer method

This test assumes that an after-image in one eye will be transferred to the

normally corresponding point in the other; that is, a foveal after-image in

the non-amblyopic eye is ‘transferred’ to the fovea of the amblyopic eye.

For this test, a photographic flash gun, which has been masked to provide

a very bright strip of light, is used. The amblyopic eye is occluded and the

patient is asked to fixate the centre of the strip with the non-amblyopic eye.

The flash produces a central line after-image. The occluder is then changed to

the other eye and the patient is asked to look at a small fixation target such

as a Snellen letter with the amblyopic eye. After a few seconds, the after-

image appears, having been transferred at a cortical level to the amblyopic

eye. The patient is asked to indicate the position of the after-image in rela-

tion to the fixation point. In eccentric fixation, the after-image will appear

slightly to the side of the fixation letter.

In some cases, it can be seen a long way away: at an angular distance

approaching the degree of the angle of the strabismus. This indicates deep

HARC (Ch. 14).

Entoptic phenomena (Haidinger’s brushes and

Maxwell’s spot)

Haidinger’s brushes and Maxwell’s spot are entoptic phenomena that

occur as a result of the characteristics of the central foveal area of the retina.

These approaches are no longer in common clinical use and will not be

described here.

Perimetry method

This method depends on the fact that the physiological blind spot is the

same angular distance from the fixation point in both eyes in normal

subjects; which is only true for isometropic eyes of similar axial length

(Brockbank & Downey 1959).

Amsler charts

A patient with a foveal scotoma will report an interruption in the pattern of

the squares corresponding to the scotoma. In amblyopia with central fix-

ation (e.g. anisometropic amblyopia), this disturbance in the lines will be at

the point of fixation and extend for about 1 cm or more, depending on the

extent of the amblyopia. In eccentric fixation, the scotoma will be to one

side of the point fixated by the patient. Mallett (1988a) called this Lang’s one-

sided scotoma and stated that it was only present in microtropia and is on

the opposite side to that which would be expected from the strabismus

(e.g. temporal in the esotropia that is characteristic of microtropia). The

Amsler charts are also used to show early signs of organic amblyopia, where

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there is typically a small dense central scotoma.

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PICKWELL’S BINOCULAR VISION ANOMALIES

Past pointing test and anomalous foveal localization

This will give an indication if the localization of objects in space has been dis-

turbed with the amblyopic eye. The procedure is first tried with the patient’s

good eye, so that the practitioner can see the normal ability of the patient to

perform the test and can train the patient. The amblyopic eye is covered and

the patient is asked to place a finger on the forehead just above the uncovered

eye. A pen torch is held before the eye at a distance of about 25 cm. The prac-

titioner explains that on the word ‘Go’ the patient moves the finger to touch

the light. The patient is allowed to practise if there is any difficulty. The

occluder is then changed to the good eye and the test is repeated with the

amblyopic eye, the patient being required to touch the light with the tip of

the finger. If this cannot be done but the finger goes a few centimetres to one

side, past pointing is demonstrated. This indicates that the eccentric area,

upon which the object of regard is imaged, is not being used to estimate the

principal visual direction. The innate association between the principal visual

direction and the fovea is maintained (see below). The patient may be more

uncertain in this test with their amblyopic eye (Fronius & Sireteanu 1994).

The maintenance of the normal relationship between the principal visual

direction and the fovea in most cases of eccentric fixation explains why,

when patients fixate a target in the ophthalmoscope light and the exam-

iner detects eccentric fixation, the patients feel that they are looking to one

side of the fixation target. Rarely, the amblyopia is so profound that the

eccentric area usurps the origin of the principal visual direction and patients

perceive the ophthalmoscope fixation target as being ‘straight ahead’

although they are viewing it eccentrically. This is ‘eccentric fixation with

anomalous foveal localization’ (Mallett 1988a) and these patients are

unlikely to exhibit past pointing. In anomalous foveal localization the visual

acuity is usually worse than 6/60 and the prognosis for treatment is poor.

Past pointing can also occur in an incomitant deviation of recent onset

(p 291). This is usually of a greater degree than past pointing in eccentric

fixation.

Evaluation, prognosis and management of

amblyopia

The first and most important stage in the evaluation of amblyopia is to con-

firm that the correct diagnosis is amblyopia; in particular to rule out the possi-

bility of pathology (Table 13.2). A useful approach is to look for negative signs

of pathology (e.g. normal: ophthalmoscopy, pupil reactions, visual fields) and

positive signs of an amblyogenic factor (e.g. anisometropia and/or strabismus).

In strabismus, there are three aspects that sometimes require treatment.

These are the motor deviation (in strabismic amblyopia), binocular sensory

adaptations to the strabismus, and monocular sensory adaptations.

The treatment of reduced acuity in amblyopia has been the subject of con-

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siderable controversy, triggered partly by a sceptical review by Snowdon &

Stewart-Brown in 1997. Since then, there have been several thorough studies

AMBLYOPIA AND ECCENTRIC FIXATION

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Table 13.2 The differential diagnosis of amblyopia

Step

What to do

Detect any ocular

Check pupil reactions, particularly looking for an APD

pathology

Carry out careful ophthalmoscopy. In younger children,

dilated funduscopy might be necessary to obtain a good

view, commonly after cycloplegic refraction. Keep

checking ophthalmoscopy at regular intervals

As soon as the child is old enough, check visual fields

Look for neurological

Carefully check pupil reactions

problems

Assess and record optic disc appearance in both eyes, if

Look for incomitancy and/or strabismus (which may be

a sign of neurological problems)

As soon as the child is old enough, check visual fields

Enquire about general health (e.g. neurological signs,

including headache)

Look for amblyogenic

Look for a cause of the amblyopia: strabismus,

factors

anisometropia, high ametropia (cycloplegic may be

necessary)

The lack of an amblyogenic factor greatly increases the

odds of pathology being present

The absence of an amblyogenic factor does not exclude

the possibility of pathology but makes it less likely

Is the amblyopia

Treat the amblyopia decisively so that you can be sure

responding to

that, if the patient is not responding to treatment, this

treatment?

is not due to lack of effort

If amblyopia does not respond to treatment, review the

diagnosis

Failure to respond to treatment might indicate a

pathological cause, so refer for a second opinion

If the visual acuity in a presumed amblyopic eye

worsens, it is probably something other than amblyopia

and requires early referral for further investigation

and a consensus is emerging. The management of amblyopia is one of the

major roles for eyecare practitioners who examine children, so the literature in

this field is reviewed below in some detail. This section includes a discussion of

refractive correction and a detailed review of occlusion. Alternative or comple-

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mentary treatment approaches (e.g. atropine penalization) are also reviewed,