The Extra-ocular Muscles

Trochlea

Superior oblique

Fig. 13.1 Diagram to show the position of the extra-ocular muscles.

nerve supply – trochlear nerve

primary action – to depress the eye

secondary action – abduction.

•The inferior oblique muscle:

Origin – the medial aspect of the maxillary bone

insertion – posterior lateral sclera lying inferior to rectus muscle

nerve supply – oculomotor nerve

primary action – extorsion of the eye

secondary action – elevation and abduction.

Note that the primary muscle action relates to the main movement when the eye is in the primary position whilst the secondary action relates to any additional movements the muscle makes to the eye.

Blood supply

Muscular branches of the ophthalmic artery and vein are responsible for supply and drainage of blood.

Eye movements

Both eyes must move together in a co-ordinated manner. In order for this to occur each extra-ocular muscle is paired with a muscle in the opposite eye. These pairs of muscles are known as synergistic or ‘yoke’ muscles. For example, to look to the right, the right eye looks outwards, i.e. abducts, while the left eye looks inwards, i.e. adducts. The right lateral rectus muscle abducts the right eye, while the left medial rectus muscle adducts the left eye. These two muscles work together to cause the eyes to look to the right.

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The right lateral rectus muscle and the left medial rectus muscle are therefore yoke muscles.

Nine positions of gaze

To look directly upwards and downwards, two pairs of yoke muscles are required to contract. In the other positions, only one pair of yoke muscles is needed.

The nine positions of gaze, and yoke muscles needed are:

(1)Straight ahead: the primary position of gaze when all the muscles are contracting to maintain the eye in this position.

(2)Upwards to the right: right superior rectus; left inferior oblique.

(3)To the right: right lateral rectus; left medial rectus.

(4)Downwards to the right: right inferior rectus; left superior oblique.

(5)Downwards to the left: left inferior rectus; right superior oblique.

(6)To the left: left lateral rectus; right medial rectus.

(7)Upwards to the left: left superior rectus; right inferior oblique.

(8)Direct elevation: right superior rectus; left inferior oblique and left superior rectus; right inferior oblique.

(9)Direct depression: right inferior rectus; left superior oblique and left inferior rectus; right superior oblique.

Convergence is the position of the eyes when looking at something close. In this case both medial recti contract to turn the eye inwards.

Antagonist muscles

When each muscle in the eye contracts, in order for the eye to move the antagonist or opposite muscle in the same eye must relax to allow the first muscle to work; for example, to look to the right, the right lateral rectus contracts and the right medial rectus must relax. In the left eye, the left medial rectus contracts while the left lateral rectus relaxes.

The antagonist muscles are:

•Medial rectus and lateral rectus

•inferior rectus and superior rectus

•inferior oblique and superior oblique.

Strabismus or squint

Strabismus or squint is a deviation of one or either eye in an inward, outward, upward or downward direction (Fig. 13.2). There are many types of squint. Only the most common will be described here. Orthoptists are highly trained to diagnose which muscle is involved and to treat the deviation in conjunction with ophthalmologists.

Fig. 13.2 Left convergent squint.

A squint causes diplopia. In a child under eight years of age, this double vision can be suppressed but an adult is unable to do this so the diplopia persists. The child suppresses the vision in the squinting eye so he no longer sees double, which is more comfortable for him. Until the age of eight, the visual process has not matured and if an eye is not used for any reason it may lose its ability to see. This results in reduced visual acuity, a condition called amblyopia. Because the visual system is not mature until the age of eight, reduced vision can be improved with treatment up to this age. Thereafter loss of visual acuity due to amblyopia will not improve. Therefore it is important that squint and any resulting amblyopia are diagnosed before the age of eight years. This applies to all squints, whether manifest or latent. School eye tests are important to pick up poor vision before it is too late for treatment to be of help.

Normal vision with both eyes in use is termed binocular single vision. The images from both eyes together are seen as one visual impression by means of the fusion faculty. The aim of treatment for squint is to restore binocular single vision and prevent or reverse amblyopia.

Squints are either non-paralytic or paralytic.

Non-paralytic squint

Non-paralytic squint is the squint of childhood and is sometimes called a concomitant squint. It can be manifest or latent, convergent or divergent, alternating or non-alternating (unilateral):

•A manifest squint is where one or other eye deviates from the primary or straight ahead position. It can often be an obvious squint to the observer.

•A latent squint is where there is a tendency for both eyes to deviate and this is not usually observed unless symptoms such as headaches or diplopia have occurred, in which case the latent squint may have become manifest.

•A convergent squint is a squint in which one eye turns inwards.

•A divergent squint is a squint in which one eye turns outwards.

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•An alternating squint is a squint in which the eyes deviate alternately, whereas it is always the same eye which deviates in a non-alternating squint.

Causes

A non-paralytic squint occurs because there may be obstacles to the correct formation of the image falling on one retina. In other words, there is an obstruction to clear vision in the visual media which results in loss of binocular single vision.

Uncorrected refractive error

Uncorrected refractive error is the commonest cause of squint in childhood. The most usual squint is convergent, the commonest cause being hypermetropia, which causes over-accommodation and therefore over-convergence. In contrast to this, myopia can predispose to a tendency to divergence, because the eyes are already in focus to near vision without the aid of accommodation, convergence is not stimulated and divergence may result. Anisometropia (a different refraction in each eye) causes unequally clear images, which can lead to a squint.

The usual pattern of events to occur (if the onset of squint is under the age of eight years) is:

•squint leads to loss of binocular single vision

•loss of binocular single vision results in diplopia

•diplopia is overcome by suppression of one image

•suppression leads to amblyopia (a reduction in visual acuity in one eye)

•the passage of time leads to loss of binocular function (the two eyes being no longer able to see one image together).

Amblyopia does not develop in an alternating squint as both eyes are used alternatively.

Prolonged eye inactivity

Prolonged inactivity of one eye may be a dissociating factor and may enable the affected eye to deviate through disuse:

•Opacities in the media, i.e. cornea, lens, vitreous or retina, may cause squint. A cataract or retinoblastoma may present as a squint.

•Bandaging of one eye – for example following injury to the eye or a unilateral ptosis – may also cause squint.

Examination

History

A careful history must be taken and should include:

•family history because there may be a hereditary factor

•age at onset which is an important factor in prognosis for reversing amblyopia and restoring binocular single vision

•nature of onset – the onset of squint may occur in one or several of the following ways:

suddenly – the squint occurs without any previous sign

gradually – the squint appears more and more frequently and possibly increasing in size over a period of time

intermittently, e.g. when tired, upset or unwell; the eye may be straight at other times

constantly – the squint is present all the time

changing in size at different times of day

when looking in a particular position of gaze or at a particular distance

either eye may deviate alternatively or it may always be the same eye that deviates.

The onset may be associated with some systemic disease because the child is generally unwell.

Visual acuity

Visual acuity will be recorded using the Snellen chart, Sheridan Gardiner test or other tests. Selection of test will be determined by the age, intellectual ability of the patient and their language.

Determination of refractive error

In children a cycloplegic drop such as G. Cyclopentolate 0.5%, by prescription or PGD, is needed to prevent accommodation, which would give a false result on retinoscopy. A retinoscope directs a light beam onto the retina and movement of the retinoscope produces movement of the light beam across the retina in a particular direction. A lens is selected and held in front of the eye while continuing to move the retinoscope. A change is noted in the amount and direction of the movement of the light beam. Finally, the lens, which actually neutralises or abolishes the movement of the light beam, determines the refractive error.

Physical examination

(1)The presence of the following features is noted:

epicanthus – if a child has broad epicanthic folds, it can give the

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appearance of a convergent squint but if the cover test is negative this is called a pseudo-squint

ptosis or other feature leading to asymmetry of the palpebral apertures

nystagmus

an abnormal head posture, which could be compensating for squint

unequal pupil sizes.

(2)An inspection of the eyes is carried out:

corneal reflections

cover test

ocular movements

measuring the angle of deviation

stereoscopic vision.

The photoscreener has been developed to detect squint, refractive errors and media opacities in young children without cyclopegia (Ottar et al., 1995).

Corneal reflections

Method

A pen torch is held at 1/3 m directly in front of both eyes. The position of the reflection on each eye is then compared.

Results

The results may be:

•normal corneal reflections – symmetrical (Fig. 13.3);

•asymmetrical corneal reflections (Fig. 13.4).

An upturning eye or a down-turning eye can also be detected by observing the corneal reflections (such squints are less common).

Cover test

The cover test is carried out to detect the presence of a squint, and should be used in conjunction with observation of the corneal reflections.

Method

A penlight is held approximately 1/3 m from the child. The child must be looking at the light whilst the cover test is carried out. It is important to repeat the cover test using a detailed target, e.g. a small picture on a tongue depressor, because some squints are only present when looking at detailed objects. The cover test should also be carried out at 6 m where possible because other squints are only present when looking into the distance, i.e. intermittent squints.

The corneal reflections are symmetrical, usually slightly nasal in each eye.

Note: corneal reflections may not be central, but check symmetry.

Fig. 13.3 Normal corneal reflections – symmetrical.

(a)

Note asymmetry of corneal reflections.

The left corneal reflection is in the normal position, i.e. slightly nasal.

The right corneal reflection is displaced temporally, because the right eye is turning inwards.

(b)

Note asymmetry of corneal reflections.

The left corneal reflection is in the normal position, i.e. slightly nasal.

The right corneal reflection is displaced nasally, because the right eye is turning outwards.

Fig. 13.4 Asymmetrical corneal reflections. (a) Right convergent squint and (b) right divergent squint.

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Cover one eye, watching for any movement of the uncovered eye, remove the cover and repeat covering the other eye and watching for any movement of the uncovered eye.

Results

The results may be:

•no manifest squint (Fig. 13.5)

•manifest squint – right convergent squint (Fig. 13.6)

•manfest squint – right divergent squint (Fig. 13.7).

An intermittent convergent squint may not be present when the child is looking at a light but becomes manifest when focusing on a detailed target. Therefore it is important to check the corneal reflections and to carry out the cover test using a light and a detailed target.

An intermittent divergent squint may not be present for near but becomes manifest in the distance. Therefore it is important to carry out a cover test for near and distance. The cover test for a latent squint, or an alternate cover test, is where the occluder covers one eye then the other. Observation of the eye that has just been covered is noted.

Ocular movements

The examiner sits in front of the patient and using a pen torch, and a toy if appropriate, observes both eyes moving in all eight positions of gaze. This will include up, down, both sides and in all four corners, always returning to the straight ahead or primary position. The patient’s head must be held still. Any muscle imbalance, over actions and under actions are then noted.

Measuring the angle of deviation

The angle of deviation can be measured using objective or subjective methods. Objective methods are based on the observer neutralising the patient’s deviation as it takes up fixation. Subjective measurements are where the patient tells you the position of each image from each eye on a calibrated scale of some sort.

The following methods include both objective and subjective measurements:

•Prisms and cover test: this is the most commonly used objective method of measuring the angle of a manifest or a latent squint. The measurements are performed at near (1/3 m), distance (6 m) and occasionally beyond 6 m (far distance). Loose prisms, or a prism bar, are introduced in front of the squinting eye in a manifest squint, or in front of either eye in a latent squint, with the apex of the prism in the direction of the deviation.

The corneal reflections are symmetrical, i.e. no manifest squint detected.

No movement of the left eye when the right eye is covered.

No movement of the right eye when the left eye is covered.

Fig. 13.5 No manifest squint.

•Major amblyoscope (synoptophore): the angle of deviation can be measured both objectively and subjectively. Slides are introduced and the measurement can be read from the scales in degrees or prism dioptres.

•Maddox rod and Maddox wing: The Maddox rod is used in conjunction with a light at 6 m and measures small degrees of squint. The Maddox

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The corneal reflections are asymmetrical, showing a right convergent squint.

Confirm with cover test.

No movement of the left eye when the right eye is covered.

The right eye moves out to fix when the left eye is covered.

Fig. 13.6 Manifest squint – right convergent squint.

rod is made of a series of parallel cylinders, red in colour, which convert a light image into a red line. This is placed before one eye while the patient is fixing on a light at 6 m distance with both eyes open. The eye looking through the Maddox rod sees a red line instead of a white light and the other eye sees the light as it really is. These images are too dis-

The corneal reflections are asymmetrical, showing a right divergent squint.

Confirm with cover test.

No movement of the left eye when the right eye is covered.

The right eye moves in to fix when the left eye is covered.

Fig. 13.7 Manifest squint – right divergent squint.

similar for fusion to take place, so the eyes deviate to the position of squint. Prisms are introduced until the light and the red line are superimposed. The Maddox wing enables measurements of latent and small manifest deviations to be made at a near distance. The Maddox rod and

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wing allow measurement of horizontal and vertical deviations. The wing also allows torsional measurements to be made.

Stereoscopic vision

The ability to see in ‘depth’ or 3D vision is the highest form of binocular sight. The main tests measuring stereo acuity in seconds of arc (the measurement of minimum disparity that gives rise to appreciation of depth) include:

•Frisby test

•Wirt fly test (Titmus test)

•TNO (red/green test)

•Lang stereo test.

See Kanski (2003) for further information.

Treatment

There are several treatment choices for a non-paralytic squint:

•Optical treatment: accurate refraction must be carried out and appropriate glasses ordered. This in itself might correct the squint.

•Occlusion therapy: the visual acuity must be assessed and if amblyopia is present, it must be reversed. This is performed by occluding the ‘good’ eye to encourage the amblyopic eye to work. Occlusion therapy is carried out, according to the state of amblyopia, in the following ways:

total occlusion – the patch is placed over the ‘good’ eye itself

partial occlusion – the lens of the glasses is covered with tape or paper

constant occlusion – total or partial occlusion used from morning till night

part-time occlusion – total or partial occlusion used for a specified number of hours each day.

•Orthoptic exercises are used to stimulate binocular single vision and strengthen fusion by means of binocular instruments such as the synoptophore. This instrument uses pairs of slides, one for each eye, which match together to form a picture; a bird seen with one eye and a cage seen with the other will be seen with both eyes together as a bird in a cage.

•Operative measures:

Early surgery is performed if there is either potential binocular function present; or poor cosmetic appearance with no binocular vision.

Surgery can be delayed if the deviation is cosmetically quite good.

Surgery is not performed if the cosmetic appearance is good in the presence of no binocular function.

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Inflammation

•mastoid disease

•peripheral neuritis

•encephalitis

•multiple sclerosis

•tertiary syphilis

•haemorrhage

•thrombosis

•aneurysm

•neoplasm, e.g. pituitary tumour

•diabetes mellitus

•temporarily following cataract extraction.

Treatment

•Orthoptic assessment to establish which muscle is affected, e.g. examination of eye movements, the Hess chart and diplopia tests.

•Referral to a neurologist or physician for investigation of the cause.

•Teach compensatory head posture, if this is applicable. The patient is taught to hold his head in the position that relieves the diplopia.

•Occlude one eye if the diplopia is intolerable.

•Give temporary Fresnel prism to join the diplopia to binocular single vision if possible.

•Botulinum toxin can be used to paralyse muscles and thereby straighten the eye. It is used to treat strabismus, paralytic strabismus and diplopia post-retinal detachment surgery. Patients need to be aware that the maximum effect is not immediate, but some two to five days later. The effect is only temporary and lasts for between two and three months.

•Wait six to nine months for recovery to occur.

•If recovery does not occur after nine months, surgery can be performed to restore binocular single vision.

Patient’s needs (non-paralytic and paralytic squint)

•Correction of squint for cosmetic reasons (children are especially teased at school).

•Correction of amblyopia and restoration of binocular single vision.

•Relief of diplopia if an adult.

Nursing action (non-paralytic and paralytic squint)

•Assist with examination in the outpatient department.

•Instil prescribed cycloplegic drops, e.g. G. Cyclopentolate hydrochloride.

•Liaise with the orthoptic department if necessary about patching and exercises, and offer teaching and encouragement to the parents and child on the use of patching.

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Causes

•Epicanthus.

•Abnormal angle alpha: this is a larger or smaller than normal angle between the optic axis and the visual axis. This angle is very similar to that of angles kappa and gamma.

•Facial asymmetry.

•Wide or narrow interpupillary distance: a wide interpupillary distance gives the appearance of divergent squint; and a narrow interpupillary distance, convergent squint.