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

PICKWELL’S BINOCULAR VISION ANOMALIES

mechanism. The position of the image on the retina determines the direc-

tion in which it is perceived: its visual location. Because the eyes and

head move, the brain must also take these movements into account in

localizing an object with respect to the egocentre. Eccentric fixation can

cause a disturbance of the retinal system when the fixation point moves

away from the centre of localization at the fovea: past pointing occurs

(Ch. 13).

The motor localization system uses two types of information to register

eye position. One of these is feedback from the extraocular muscles them-

selves (inflow), which is called muscle proprioreception. The other is

information that is copied within the brain from the centres that control

movement to those that monitor eye position (efferent copy: outflow). Pro-

prioreception is a back-up to efferent copy (Bridgeman & Stark 1991). If

the eyes do not move correctly in response to the nerve impulses sent to

the muscles, as in paretic strabismus, then the motor localization system

will be disturbed.

The past pointing test may be used to demonstrate these motor disturb-

ances. The test is applied monocularly to each eye in turn, as described in

Chapter 13. Past pointing will be demonstrated in the eye having the

affected muscle and in the field of action of this muscle. The degree of past

pointing will increase as the eye turns further into the direction of its

action, and will not occur in the opposite direction of gaze. This test can be

made more effective by holding a card horizontally (for horizontal devia-

tions) at the level of the patient’s chin, so that it occludes the patient’s

pointing hand from view while the target appears above the card. Past

pointing only occurs with paresis of recent onset and lessens as the patient

adapts to the deviation. Past pointing can also occur in eccentric fixation

(p 196), where it is typically of a lesser degree than in incomitant deviations

of recent onset.

Testing the vestibular system

In supranuclear lesions the vestibulo-ocular reflex (VOR) will be maintained

but in those caused by infranuclear lesions it will not. The VOR is phylo-

genetically the oldest slow eye movement system and, because it does not

require visual feedback, it has a short latency. Horizontal VOR is well

developed at birth; the vertical VOR develops a little later.

The VOR can be tested to investigate whether a gaze palsy is supranuclear

or infranuclear, and to investigate a lack of abduction in an infant. The two

most common ways of testing the VOR are as follows. In the doll’s head

test, the patient is asked to fixate an object while the examiner rapidly rotates

the patient’s head, first in the horizontal and then in the vertical plane. A

normally functioning VOR will result in eye movements equal and opposite

to head movements. In the spinning baby test the infant is held upright by

the practitioner sitting on a rotating chair. The practitioner rotates him/

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herself and the baby through 180° or 360°, observing the patient’s eyes. The

eyes make a conjugate movement in the direction opposite (compensatory)

INCOMITANT DEVIATIONS

17

to that of the body rotation and then, after about 30°, they make a fast

movement to the primary position; the cycle repeating. Once the rotation

is stopped there may be some postrotational nystagmus, but only for a few

seconds in a sighted infant. An older infant may maintain fixation on the

examiner’s face, in which case the test can only be done in the dark with

eye movement recording equipment.

Algorithms for aiding the diagnosis of vertical muscle

pareses

The diagnosis of the paretic muscle(s) is fairly straightforward in horizon-

tal deviations but is more complicated in cyclovertical deviations (Spector

1993). Some workers have developed algorithms (e.g. three-steps tests) to

help practitioners detect the paretic muscle(s) in cyclovertical deviations,

and three of these will now be described. The principles behind the vari-

ous tests in these methods can be understood from the earlier sections of

this chapter.

The Lindblom 70 cm rod method

A very simple method of qualitatively investigating cyclovertical incomi-

tancies is to use a 70 cm rod held horizontally 1 m in front of the patient

(Lindblom et al 1997). This test can be carried out with most patients using

a normal 30 cm ruler. The patient is asked to describe whether the rod is

single or double and if double whether tilted (indicating faulty oblique

muscle(s)) or parallel (indicating faulty vertical recti muscle(s)). If tilted

then the patient’s perception typically resembles an arrow (e.g. ) that

points towards the side of the eye with the paretic muscle. The muscle can

be further identified by investigating the effect of upand down-gaze on the

patient’s perception. If the patient reports a shape resembling an X then it

suggests that there may be a double oblique paresis, usually both superior

obliques. This method is surprisingly easy to use and is included in the

worksheet in Appendix 8.

This test is delightfully straightforward and a modified version can be used

in patients who have more subtle deviations and who are not diplopic in

the primary position. For these cases, the Lindblom method can be carried

out with two Maddox rods, one in front of each eye placed so that the

patient sees two horizontal lines when viewing a spot-light.

Parks’ method

There are three questions that are considered in Parks’ method, each of

which narrows down the paretic extraocular muscle. These questions are

summarized in Box 17.1: each question is followed by a list of the muscles

that are suggested as possibly paretic by the answer to the question. A right

hypotropia is treated as a left hypertropia and a left hypotropia as a right

hypertropia. A clinical worksheet that is designed to help practitioners carry

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out this test and to interpret the result is reproduced in Appendix 8. Vazquez

Another three-step method was described by Scobee (1952), particularly for

hyperphoria, and is given in Box 17.2. Again, each question is followed by a

list of the muscles that are suggested as possibly paretic by the answer to the

question. The clinical worksheet in Appendix 8 includes Scobee’s method.

The second question in Scobee’s method is based on the fact that the

vertical recti have a greater effect when the eyes are abducted, as during

distance vision, and the vertical actions of the oblique muscles are max-

imal when the eyes are adducted, as during near vision. The third question

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is based on a point discussed on page 278: the secondary deviation is greater

than the primary deviation.

PICKWELL’S BINOCULAR VISION ANOMALIES

rectus). This inhibitional palsy can be greater than the original palsy

(Stidwill 1998, p 151) and the effect of this may be to make the patient’s

ocular motility less incomitant with time. This is sometimes referred to

as a ‘spreading of the comitance’ and occurs between 2 and 9 months

after the original palsy (Stidwill 1998, pp 151–152).

If the patient sometimes fixates with either eye, then secondary sequelae

are less frequently found (Mallett 1969). The literature is not clear about

the situation when the patient has a palsy but is most of the time het-

erophoric, as often happens with superior oblique palsies. It seems likely

that these cases exhibit less marked secondary sequelae, possibly following

the scenario outlined in (2) above.

Differentiating incomitancies that are long-standing

from those of recent onset

The first priority with incomitant deviations is to decide if there is an active

pathological cause requiring immediate medical attention, or if it is a long-

standing deviation. Table 17.2 summarizes the factors that help in this

evaluation.

The aetiology of the incomitancy

It will be seen that one of the differences between deviations of recent patho-

logical cause and more long-standing deviations is that there may be other

symptoms of the general pathological condition. It is therefore useful to be

aware of the primary conditions that can give rise to incomitant deviations

and of the other signs and symptoms that may accompany them. There is a

very large number of these conditions and they are summarized in Table

17.3. In this table, the conditions are divided into three categories to make

them easier to remember rather than as a strict taxonomic classification.

Some of these aetiologies may have incomitant diplopia as an early sign

before the patient is really aware of the seriousness of other signs and

symptoms. This is not a very frequent occurrence but means that we must

be all the more vigilant and continue to bear the possibility in mind. In

other conditions, the deviation may occur as part of the possible progress

of the disease for which the patient is already under treatment. The

patient’s medical adviser needs to be made aware of this development. An

awareness of relevant anatomy is helpful (Evans 2004e, h).

Some of the most common of the conditions that may have ocular mus-

cle palsy as an early sign or as part of the progress of the condition are

described below. The other general symptoms that can accompany the

diplopia are also given as a means of helping to confirm the diagnosis of

the deviation as of recent pathological cause.

(1) Diabetes: ocular palsy from diabetes usually affects the third cranial

nerve and may sometimes involve the pupil reflex and reduce the

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amplitude of accommodation. The symptoms of diabetes may include,

in addition to the diplopia, generalized headache, increased thirst and