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

There are three good reasons for treating orthoptic anomalies: if they are

causing problems; if they are likely to deteriorate if left untreated; or if

treatment may be required but less effective when the patient is older.

Cosmetically apparent strabismus causes psychosocial problems and after

surgery most cases have an improvement in appearance, self-esteem and

self-confidence (Menon et al 2002). The minimum size of strabismus that

is cosmetically apparent varies from one person to another (Larson et al

2003) but is less for exotropia (typically 8 ) than for esotropia (typically

14.5 ) (Wiessberg et al 2004).

Some cases of strabismus do not result in any overt problems (e.g. symp-

toms, poor cosmesis) and are unlikely to deteriorate or to be harder to treat

later. However, strabismic patients are likely to have a marked reduction in

stereoacuity and this is undesirable, even though the patient may not be

aware of the deficit. Reduced stereoacuity can impair performance in

everyday activities, such as driving (Bauer et al 2000) and motor tasks

(Hrisos et al 2006), and binocular reaction times are faster than monocu-

lar (Justo et al 2004). Binocular reading speed is impaired in amblyopia,

even when binocular visual acuity is normal (Stifter et al 2005). So if the

strabismus can be treated then this deserves consideration.

Strabismus with a small stable angle is often associated with deep har-

monious anomalous retinal correspondence (HARC) and this can give the

patient quite good ‘pseudobinocular vision’, sometimes with a reasonable

degree of stereopsis. If these cases are asymptomatic, have a good cosmesis

for the strabismus, and have good visual function then it is hard to justify

the disruption to the child and family that accompanies treatment with

exercises. Although successful treatment might open up a few more career

possibilities, it must be acknowledged that the vast majority of cases

would gain little benefit from treatment. Additionally, these well adapted

cases will be difficult to treat and there is always a possibility that treat-

ment might make the situation worse. Some parents are keen to eliminate

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a strabismus at any cost, but they should be fully informed of the likely

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PICKWELL’S BINOCULAR VISION ANOMALIES

risks and benefits and few practitioners would take on the treatment of this

type of case, other than treating amblyopia in children of a suitable age.

The investigation and management of comitant strabismus can be

broadly divided into two parts: sensory and motor factors. Patients who

have not managed to achieve a sensory adaptation to their strabismus

(usually because they are too old) will have diplopia and this is discussed

below. Suppression and abnormal retinal correspondence are adaptations

in the visual sensory mechanisms that occur in strabismus. These adapta-

tions have been reviewed in general outline in Chapter 12 and are further

described in this chapter with particular reference to clinical investigation

and treatment. As explained in Chapter 12, these two binocular sensory

adaptations are interrelated and usually both occur in small-angle strabis-

mus. In strabismus over 25 , suppression seems to dominate.

In cases of strabismus where treatment is appropriate, sensory factors

(suppression, HARC, amblyopia, eccentric fixation) are generally treated

first. Throughout this period some form of occlusion is maintained during

the intervals between treatment to prevent diplopia and confusion. In

cases where the patient is diplopic, treatment of the motor deviation can

be started straight away. The motor deviation sometimes spontaneously

resolves when sensory factors have been corrected. When this does not

occur, refractive correction or fusional reserve exercises are required to

treat the motor component. Treatment of sensory factors should only be

attempted in cases where the practitioner is sure that the motor deviation

will respond to treatment (see below).

Diplopia

It has already been noted that most patients with strabismus develop a sen-

sory adaptation (HARC or suppression) to avoid diplopia and confusion. In

some cases this is not possible, usually because the patient is too old, and

the patient develops diplopia and confusion (Ch. 12). The distinction

between diplopia and confusion is illustrated in Figure 12.1, and both

phenomena usually occur together (p 172). Throughout this section, the

word ‘diplopia’ is typically used to describe the problems of diplopia and

confusion.

Investigation of diplopia

Diagnosis: the Worth Four Dot Test

In most cases, diplopia is simply diagnosed on the basis of symptoms of

double vision during everyday life. Occasionally, it may be necessary to

formally investigate the diagnosis of diplopia in patients (e.g. patients

who may be denying diplopia in order to enter certain vocations). In these

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cases, the Worth Four Dot Test can be used to diagnose diplopia. The test

is carried out in room illumination and patients should not be shown the

INVESTIGATION AND MANAGEMENT OF COMITANT STRABISMUS

14

R

R

R

G

G

G

G

W

P

R

A

B

C

G

G

G

G

G

G

G

G

R

R

G

D

E

F

In addition to strabismus, other conditions can lead to reports of ‘double

vision’ and the investigation of diplopia is summarized in Figure 14.2.

Covering one eye will determine whether the diplopia is monocular or

binocular, and pinhole and Amsler tests will further help to determine the

aetiology (Finlay 2000). Monocular diplopia accounts for one quarter of

cases of diplopia presenting to an eye hospital and for nearly all of these

cases a genuine cause can be found; usually lenticular or corneal pathology

(Morris 1991). Monocular diplopia can result from an epiretinal membrane,

for example after cataract surgery (Foroozan & Arnold 2005). Monocular

diplopia in children can be due to refractive errors, cataracts, corneal disease

or occasionally retinal disease (Taylor 1997). Sensory causes of monocular

diplopia and polyopia (more than two images) include brain trauma, cere-

brovascular accidents and migraine.

When diplopia is binocular, then orthoptic tests should be used to

detect the presence of strabismus, in all or any positions of gaze. The direc-

tion of the diplopia (horizontal, vertical, oblique, torsional) should be

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determined by questioning the patient. By introducing a red filter in front

Monocular

Binocular

diplopia

diplopia

Pinhole test

Orthoptic tests

Diplopia is

Diplopia is

No manifest

Strabismus

eliminated

still present

strabismus

is present

• Refractive error

• Epiretinal membrane

• Physiological diplopia

• Determine:

• Media opacities

• Lenticular polyopia

• Sensory causes

comitancy,

• Corneal tear film

• Sensory causes

crossed,

• Dislodged intraocular

uncrossed,

implant

vertical,

torsional,

paradoxical

Figure 14.2 Diagram summarizing the investigation of diplopia. Investigative tests are in

boxes with dashed outlines, diagnoses are in the blue area at the bottom. (Modified from

von Noorden 1996, p 209.)

of one eye, or by covering an eye, the practitioner can determine whether

any horizontal diplopia is crossed (heteronymous, suggesting an exotropia)

or uncrossed (homonymous, suggesting an esotropia). If diplopia occurs

after surgery, it should be determined whether it is in accordance with the

postoperative deviation or paradoxical (crossed with esotropia and uncrossed

with exotropia), in which case there is a persistence of the preoperative

sensory adaptation (von Noorden 1996, p 208). The practitioner should

detect and investigate any incomitancy as outlined in Chapter 17 and any

comitant deviation as outlined elsewhere in this chapter and in Chapters

15–16.

Monocular diplopia or binocular triplopia can occur through a persist-

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ence of the sensory state preceding a surgical intervention. The strabismic

eye sees two images of a fixation point, as a result of competition between

INVESTIGATION AND MANAGEMENT OF COMITANT STRABISMUS

14

the innate normal retinal correspondence (NRC) and long-standing anom-

alous retinal correspondence (ARC) that existed before surgery. During

binocular viewing, the NRC in the dominant eye can cause triplopia (von

Noorden 1996, pp 269–270). Rarely, binocular diplopia can result from a

change in fixation preference when a previously dominant eye becomes

the more myopic causing a change in ocular dominance. Such cases are

resolved by correction of the myopia.

Intractable diplopia (see below) from strabismus suggests that either the

patient was unable to develop sensory adaptations (e.g. because they were

too old when the strabismus occurred) or that there has been a change in

their sensory or motor status.

A particularly troublesome form of binocular sensory diplopia occurs in

non-strabismic patients who have developed a macular or retinal lesion

(e.g. epiretinal membrane) causing metamorphopsia. Bifoveal fusion may

be impossible, yet peripheral fusion is likely to be normal. A useful way of

investigating this dragged fovea syndrome is with the lights on–off test (De

Pool et al 2005). The patient fixates a small isolated target (e.g. dot or single

6/18 letter) that should be white in the centre of a black computer screen.

With the room lights on, this will be seen doubled. When the room lights are

extinguished, then within 2–10 s the letter should become single. Occa-

sionally, patients need a partial prism correction to achieve this central

fusion. There is no complete cure, but some cases benefit from monovision

and others require occlusion (De Pool et al 2005). These authors cautioned

that surgery for the epiretinal membrane should not be thought of as a cure,

since this can trigger or worsen the problem. A low-density Bangerter foil

(frosted stick-on filter) sometimes helps (Silverberg et al 1999).

It is possible that sensory diplopia might also occur as one of the anom-

alous visual effects that can accompany Meares–Irlen syndrome or migraine.

These visual perceptual distortions probably result from hyperexcitability of

the visual cortex (Wilkins 1995, p 157). Covering one eye halves the sensory

input to the visual cortex and thus reduces the probability of these effects

(Wilkins 1995, pp 21–23). Hence, sensory diplopia from this source could

conceivably present as binocular diplopia that resolves on covering one eye,

although the patient does not have a strabismus. The treatment of the non-

binocular types of diplopia classified in Figure 14.2 was summarized by Evans

(2001c) and the treatment of binocular cases is now described.

Can the patient achieve binocular single vision?

In most cases, the complaint of binocular diplopia suggests that there is

the potential for binocular single vision, especially if the patient can con-

sciously control the diplopia by adopting a compensatory head posture.

Exceptions are the intractable cases described later in this section. In every

case, prisms should be used to establish whether the diplopia can be elim-

inated before surgery is considered. Loose prisms, rotary prisms (e.g. in a

refractor head), or prism bars can be used. It should be noted that errors

can occur when prisms are stacked (e.g. several loose prisms placed in a

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trial frame; Firth & Whittle 1994).

The effect of prisms on diplopia from comitant strabismus can be inves-

tigated using the Mallett OXO test (Fig. 14.3). If the diplopia is predomin-

antly vertical, then the horizontal OXO should be used. Even incomitant

cases, if the incomitancy is subtle, sometimes benefit from the prism sug-

gested by testing with the Mallett unit in the primary position (Ch. 17).

For patients with good visual acuity, the usual small OXO can be used.

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Patients with poor acuity can use the large OXO that is included on modern

near Mallett units.

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PICKWELL’S BINOCULAR VISION ANOMALIES

Box 14.1 Some causes of intractable diplopia

Secondary deviation from unsuccessful strabismus surgery; in some cases a

surgeon advises against further surgery

Late-onset strabismus, which may in some cases be inoperable

Acquired anisometropia (e.g. secondary to complicated cataract or refractive

surgery); some cases may not be suitable for contact lenses

Retinal distortion following detachment or macular lesion

Refractive surgery

Sensory fusion disruption syndrome (see below)

Horror fusionis (see below)

to prescribe monovision with refractive surgery before a temporary trial of

monovision with contact lenses (Vogt 2003). The cause of diplopia in

these cases has been attributed to fixation switch diplopia, when the patient

is forced to fixate with a previously strabismic eye (Kushner 1995).

Horror fusionis and sensory fusion disruption syndrome

Heterotropic patients with horror fusionis cannot demonstrate fusion, even

when the deviation is corrected with prisms or in a haploscopic instru-

ment. These patients report a ‘jumping over’ phenomenon: as the prism is

increased and the diplopic images move together they suddenly ‘jump’ and,

for example, crossed diplopia suddenly changes to uncrossed diplopia. The

same phenomenon occurs when the angle of deviation is approached from

the other direction. It appears that the patient is unable to achieve motor

fusion. Caloroso & Rouse (1993, pp 162–163) said that the condition should

be differentially diagnosed from aniseikonia, undetected small angle HARC

and deep foveal suppression (when horror fusionis would not be present for

large targets). Many affected patients are congenital esotropes and Kirschen

(1999) stated that horror fusionis was only seen in occasional patients who

had had a strabismus since early childhood. Treatment is usually aimed at

alleviating any intractable diplopia, and this may require occlusion or

hypnosis.

Heterotropic patients with sensory fusion disruption syndrome (Case study

14.1) can achieve motor superimposition of their diplopic images, but sens-

ory fusion cannot be attained. If appropriate prisms are placed before the

eyes then the patient reports that the targets are ‘on top of each other but

not together’. One of the images is often seen in constant motion (Kirschen

1999). The condition usually follows closed head trauma, sometimes associ-

ated with coma (Case study 14.1). Treatment includes monovision (London,

cited by Evans 1994) occlusion (full or central; Kirschen 1999) or hypnosis.

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It is essential that horror fusionis and sensory fusion disruption syn-

drome are identified before surgery, since surgery would not be able to

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14

The management options for intractable diplopia are limited and include

occlusion, monovision and hypnosis. These options are discussed below.

Occasionally, patients with long-standing diplopia are encountered who

seem to have ‘grown used’ to the diplopic image and are happy to tolerate

this. Although they can appreciate the diplopia at any time, they seem to

have adapted by concentrating their attention on the dominant image, and

the diplopic image seems not to interfere with their everyday perception.

Even patients with horror fusionis may use the input from each eye in a

rudimentary way to maintain a controlled angle of strabismus (Bucci et al

1999). Such patients can become symptomatic if, for example, prism in their

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glasses is changed (Case study 14.2).