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

Most of the rest of this chapter is concerned with the detection of incomitant

deviations and the interpretation of their significance. Most patients in need

of urgent medical attention have symptoms that lead them to consult med-

ical practitioners in the first instance. It is, however, important to be able

to detect incomitancy, as it does not usually respond well to eye exercises,

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and occasionally cases of active pathology present themselves and it is essen-

tial to be able to recognize them.

INCOMITANT DEVIATIONS

17

The internal examination of the eyes may also provide further evidence

of pathology such as those present in vascular conditions or metabolic

disease. Indirect ophthalmoscopy and fundus photography can be used

to provide an objective measure of ocular torsion in which the relative

positions of the fovea and optic disc are noted. Normally, the fovea is 0.3

disc diameters below a horizontal line extending through the geometric

centre of the optic disc. A variation of more than 0.25 disc diameters

between the two eyes indicates cyclodeviation (von Noorden 1996). Visual

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field analysis can also be used in a similar way. A problem with these

PICKWELL’S BINOCULAR VISION ANOMALIES

approaches is their sensitivity to improper head position (Phillips &

Hunter 1999).

Ocular motility test

The examination of ocular motility is an essential part of the detection of

incomitancy. The ocular motility test allows a subjective, and an objective,

check that:

(1) both eyes move smoothly and follow the target

(2) there is a corresponding lid movement accompanying the vertical eye

movements

(3) there is no underaction or overaction of the movement of one eye in

any direction of gaze.

The details of procedure for investigating the motility in routine exami-

nation are given in Chapter 2. This chapter is mainly concerned with

determining the significance of any anomaly and with any additional tests

that may give further information. The site of a muscle palsy can be deter-

mined from an understanding of the actions of the extraocular muscles, as

described at the beginning of this chapter.

In the motility test, the patient is asked to keep the head still and to fol-

low a pen torch with the eyes as it is moved into the different parts of the

visual motor field. The patient is asked to report any diplopia, although

patients with an incomitancy may not report any diplopia, because of sens-

ory adaptations. Often, the most useful information that the motility test

gives relates to the practitioner’s observation of the eye movements.

The fixation light is first moved up and down in the median plane, so

that lid movements and vertical eye movements (e.g. detecting gaze palsies)

can be observed. In the method recommended by Boylan & Clement (1987),

the light is then moved across the field at three levels: at the top, at eye

level and in the lower part of the motor field. This is done with the patient

following the light with both eyes, so that one eye’s position can be judged

relative to the other. A failure of one eye to follow the light in the top of the

field indicates an anomaly of one of the elevators. To the patient’s right

and top, the affected muscle is likely to be either the right superior rectus

or the left inferior oblique. Failure of one eye to turn to the right or to the

left at eye level is likely to show an anomaly of either medial recti muscles

or either lateral recti, and failure in the lower field shows a problem with one

of the depressor muscles (Fig. 17.6). In these directions of gaze, each muscle

has little or no secondary actions. Alternatively, some authors recommend

that a ‘star’ technique is used where the pen torch is moved in the vertical,

horizontal (at eye level) and four oblique positions (Mallett 1988a).

It should be noted that Figure 17.6 does not show muscle actions but the

approximate directions in which the muscles have their greatest ability to

move the eyes, excluding torsional movements. These diagnostic positions

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of gaze are very different from the actions of the extraocular muscles in the

primary position (Table 17.1). This is because the actions of each muscle

17

RLR

LMR

Figure 17.6 The six cardinal diagnostic positions of gaze, indicating the muscles that

should have maximum power to maintain the eyes in these directions. The paired synergists

(muscles, one from each eye, that act together) are shown.

will change as the eye moves. The cardinal diagnostic positions of gaze

are important in interpretation of ocular motility results but knowledge

of the actions of the muscles in the primary position is required to interpret

the results of the cover test carried out in the primary position.

The motility test is the only objective method available for standard

clinical investigation of muscle paresis. Small deviations of one eye when

it is in a tertiary position are not easy to detect. Observation of the corneal

reflection of the fixation light will help, as will the symmetry of the lid

and eye positions comparing dextroversion with laevoversion. Fortunately,

from the detection point of view, underaction of a muscle is usually accom-

panied by an overaction in the paired synergic muscle, which exaggerates

Patients with active pathology usually have diplopia and this helps the

detection. Very small degrees of diplopia may be detected subjectively, which

makes diagnosis more certain. During the motility test, therefore, the patient

must be asked to report any doubling and how this varies in different parts

of the field. A diplopic image due to a paretic muscle is displaced in the same

direction as the rotation that contraction of that muscle normally produces.

The eye that sees the outermost diplopic image when the eyes look in the

direction of maximum separation of the images is therefore the eye with

the paretic muscle, and this eye can be identified by covering one eye.

Subjective analysis of ocular motility can be assisted by the use of red–green

diplopia goggles. A red goggle is worn before the right eye and a green one

before the left. However, goggles prevent the eyes from being observed.

It is useful to be able to record the degree to which a deviation is incomi-

tant, so that any change can be monitored to assess if the condition is getting

better or worse. This can be done by several methods and Appendix 8 is a

worksheet for recording these results. This includes three variations of the

motility test, including cover testing in peripheral gaze as described below.

Incomitant deviations can be difficult to diagnose and these three versions of

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the motility test are often easiest to interpret if they are carried out separately.

PICKWELL’S BINOCULAR VISION ANOMALIES

The cover test

In the primary position, the cover test can be used to compare the size of

the deviation when the patient is fixating with either eye. The deviation is

larger (secondary deviation) when the patient is fixating with the paretic

eye than when they are fixating with the non-paretic eye (p 278).

The cover test also can be used to measure the deviation in different pos-

itions of gaze during the motility test. The cover/uncover test, alternating

cover test or prism cover test can be used in peripheral gaze to provide fur-

ther objective information on the motility test results. The alternating

cover test is often easiest to interpret, but with just two or three alternate

covers in each position of gaze to avoid causing more dissociation than is

necessary.

The Maddox rod or hand frame

Primary and secondary deviations

The difference between primary and secondary deviations was explained earl-

ier in this chapter. This phenomenon can be investigated with a Maddox

rod test in the primary position with the rod first in front of the strabismic

eye (measuring the primary deviation) and then in front of the non-

strabismic eye (measuring the secondary deviation). If the two readings

are different this suggests an incomitancy (Borish 1975, pp 1262–1264).

Unfortunately, I have been unable to find any norms for determining what

represents a significant difference between the two eyes with this test, but

it is included in Appendix 8.

Deviation in different positions of gaze

The Maddox rod can be used to measure the horizontal and vertical devi-

ations in different directions of gaze (Appendix 8). Using a pen torch for

fixation, the patient’s head is kept still while measurements are taken in dif-

ferent parts of the field. It is important that the light is at a fixed distance

and is moved to definite peripheral positions, so that the test is repeatable.

It is suggested that it is held at 50 cm from the eyes, and at the corners of a

square formation in front of the patient and of 50 cm dimensions.

Double Maddox rod test and similar approaches

In the double Maddox rod test, two Maddox rod lenses are placed one in

front of each eye to measure any cyclodeviation (Phillips & Hunter 1999).

The rods are placed exactly vertical in a trial frame and the test should be

carried out in complete darkness (Simons et al 1994). If there is no vertical

deviation then a vertical prism is introduced to separate the horizontal lines

seen by each eye. The orientation of the Maddox rod in the trial frame can

be adjusted until the two lines are parallel. This gives a measure of the

cyclodeviation but does not differentiate between a cyclophoria and a

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cyclotropia. A significant cyclodeviation suggests the involvement of an

oblique muscle.

INCOMITANT DEVIATIONS

17

The tilt of the retinal image is opposite to the tilt of the line as seen by the

observer. So, if patients report that the line seen by the right eye is tilted

towards the nose then they have right excyclodeviation, suggesting that the

right superior oblique may be underacting. In summary, the line is perceived

to be tilted in the direction in which the underacting muscle would rotate the eye.

Paresis of the superior oblique muscle can be very difficult to detect on

motility testing (Brazis 1993) and Simons et al (1994) stated that the dou-

ble Maddox rod test is the standard test for investigating a superior oblique

paresis. Theoretically, the test can demonstrate which eye(s) manifests the

paresis and the degree of excyclotorsion (p 306).

Von Noorden (1996, p 411) cautioned that an excyclodeviation may occur

in the non-paretic eye in patients who habitually fixate with the paretic eye

because of a monocular sensorial adaptation to the cyclodeviation. Sensory

adaptations (harmonious anomalous retinal correspondence (HARC) or

sensory cyclofusion) and motor cyclofusion (Phillips & Hunter 1999) may

explain why some patients with congenital superior oblique palsies have

minimal subjective torsion with the double Maddox rod test (Plager 1999).

Originally, it was recommended that a red Maddox rod should be placed

in front of the right eye and a white one in front of the left eye (von Noorden

1996, p 190). Theoretically, the eye with the underaction would be the one

whose image was cyclorotated, although von Noorden (1996, p 190) noted

that exceptions to this rule were common. Simons et al (1994) explained

these exceptions with an experiment demonstrating that a white rod was

less disruptive to vision than a red rod. They recommended that two red

rods be used, in which case the paretic eye is correctly diagnosed in 94% of

cases. A comparison of five methods of measuring ocular torsional move-

ments found that the double Maddox rod was reliable when used in the

primary position (Capdepon et al 1994), although Kraft et al (1993) found

that the test was best at discriminating superior oblique palsies (normal/

single/double pareses) in down-gaze.

A pair of Bagolini lenses can be used, with axes parallel, in an analogous

way to the double Maddox rod test (von Noorden 1996). When Bagolini

lenses are used the eyes are not dissociated, so the test is unlikely to work

in strabismic cases where there is suppression and will be confounded in

strabismic cases where there is HARC. The result with this test may be simi-

lar to an assessment of the cyclodeviation with the Mallett fixation disparity

test, which might be expected to have the same shortcomings with strabis-

mic patients.

The Maddox wing test can be used to measure cyclodeviations, but only

a fairly limited range of cyclodeviations can be measured. It is important

to keep the patient’s head and the instrument level.

Rabbetts (1972) described a prototype instrument, the cyclophorometer,

that used polarized filters to dissociate the eyes. A similar principle is used

in the commercially available torsionometer, in which the patient views a red

and a green line on card through red/green goggles (Georgievski & Kowal

1996). One of the lines is rotated until they are parallel, and the degree of

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torsion is recorded from the required rotation of the line. The test is less

PICKWELL’S BINOCULAR VISION ANOMALIES

dissociating than the double Maddox rod test but probably more dissociat-

ing than the Bagolini lens test. Even for patients with diplopia, where the

eyes are effectively already dissociated, there is some variation (more than 5°

in 10% of patients) between different methods of measuring cyclodeviations

(double Maddox rod, Maddox wing, synoptophore, torsionometer), although

no particular test is responsible for the variation (Georgievski & Kowal

1996).

Screen tests

Alternatives to the Hess screen, the Foster and Lancaster screens, employ

similar principles to those described for the Hess screen below. These meth-

ods provide the most thorough way of recording the degree of incomitancy

and other information that will help the assessment of the progress of the

condition. During all of these tests it is essential that the patient’s head

does not move.

Screen tests are essentially dissociation tests that are carried out in dif-

ferent positions of gaze. The deviation is plotted in space, and chart plots

can be used to give a precise measure of the deviation in different pos-

itions of gaze. Another important feature of the test is that it is carried out

first with one eye fixating and then the plot is repeated with the other eye

fixating. This is to differentiate between the primary and secondary devi-

ation (p 278).

Hess screen

Modern Hess screens are grey in colour, so that the light from two projector

torches can be seen on the screen. The patient sits at a distance of 50 cm.

The screen is divided into ‘squares’ representing 5° rotations of the eyes. As

the screen is flat, tangential to the line of sight, the squares are distorted

into a pin-cushion pattern. The practitioner holds the red torch and the

patient wears red–green diplopia goggles. On the screen, a red bar image

from this torch can be seen only by the eye with the red goggle before it.

Thus, when the patient is asked to look at the red bar image on the screen,

the eye will be positioned so that the red image falls on the fovea of that eye.

The patient holds the green torch and is asked to shine its bar image on the

screen so that it appears to cross the red bar. This subjective cross will be

formed when the green bar of light is in such a position that its image falls

on the fovea, as the foveae in each eye are corresponding points and have

the same visual direction. Therefore, the positions of the bar images on the

screen mark the points of intersection of the visual axes with the screen.

The degree of any deviation can be estimated from the 5° marking. The gog-

gles are changed round so that the red goggle is in front of the other eye,

so that the plot is repeated with this eye fixating and the first one deviated.

A copy of the plot is made on a paper chart with each eye fixating in turn.

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Some versions are internally illuminated, so that an LED is illuminated

in the appropriate position (Fig. 17.7).