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Extraocular muscles: anatomy and clinical investigation.

MODULE 14 PART 1: BINOCULAR VISION COURSE CODE: C-13990 O/D

[ILLUSTRATION OMITTED]

Human visual perception is a sensory-motor system in which the sensory input produced by light on the eye needs to be combined with ocular position and movement in order to provide us with a meaningful visual experience. This series of articles aims to explore eye movements and binocularity in terms of the motor function, binocular alignment and the consequences of misalignment of the visual axes. Interpretation of sensory signals will be discussed in so far as they can be an important diagnostic tool for ocular motor problems. This article describes the extra-ocular muscles (EOMs), their contribution to this sensory-motor relationship, the investigation of eye movements, and the disease processes that can affect the EOMs and their actions.

Movement of the eyes so that the object of interest is placed onto the fovea, the part of the retina capable of resolving the greatest detail, is necessary for high-grade visual acuity (VA). The mapping of the spatial relationship between the foveate object and the more peripheral parts of our visual image helps us to interpret our visual world. Moving the eyes as a co-ordinated pair allows images falling on each retina to be combined to form a sensible, meaningful impression. Images in corresponding positions are perceived as equidistant from the 'straight ahead' and in the same direction. They are normally fused into a single percept. Similar images on almost corresponding points are also seen as single but, being slightly disparate, facilitate the perception of relative depth. Increasing disparity results in diplopia. Accurate information on eye movements needs to reach the visual cortex for us to know whether a seen object is moving. If the image of an object moves across the retina and either the head or eyes are moving, it may indicate that the object is stationary and we are moving. The integration of the sensory retinal signals and vestibular signals with eye movement information occurs at cortical and brainstem levels to produce a stable visual perception of our world. (1) If these signals do not unite appropriately it causes oscillopsia, vertigo and disorientation.

EOM Structure

The orbit

Each orbit is approximately pyramidal in shape, with its widest point some 15mm behind the orbital rim, and its four wails converging to an apex. (2) The walls of the cone are not equal in length, the floor being the shortest. Figure 1 shows how the medial walls of the two orbits are approximately parallel to each other, the sagittal plane of the skull and the line of sight, whereas the lateral walls are at almost 90[degrees].

Pathways of the EOMs

Each eye has six EOMs, the combined actions of which either hold the eye steady or rotate it. The six muscles consist of the four recti (medial, superior, inferior and lateral) and the two obliques (superior and inferior). Close to the apex of the orbit, the annulus of Zinn encircles the optic nerve and contains the origins of the four recti muscles. Figure 2 shows the recti muscles travelling forwards almost parallel to the neighbouring orbital wall. The lengths of the muscles vary, with the medial rectus (MR) being the shortest and the lateral rectus (LR) the longest. The part of the muscle anterior to the equator of the globe lies adjacent to the eyeball before fanning out into a concave insertion into the sclera. The distances of the insertions from the corneal limbus vary, with the MR being closest (averaging 4.7mm) and the superior rectus (SR) being the furthest (averaging 6.7mm). The superior oblique's (SO) anatomical origin is at the apex of the orbit, outside the annulus of Zinn, from where it travels forwards parallel to the medial wall. At the anterior medial corner of the orbit it passes through the trochlea before reflecting back to fan out and insert into the globe behind the equator, underneath the SR muscle. The origin of the inferior oblique (IO) is at the infero-medial corner of the orbital rim, close to the naso-lacrimal duct. The muscle travels between the inferior rectus (IR) and the orbital floor to insert posterior to the equator, with the medial edge of the insertion lying very close to the position of the macula.

Functional muscle origins

During eye movements, only the anterior part of the recti muscle moves with the eye. The rest is held stable by the annulus of Zinn at one end and the 'muscle pulleys' at the other. The pulleys are adjacent to the orbital wall and approximately 5-6mm behind the equator of the globe. They form sleeves around the rectus muscle fibres and contain some smooth muscle of their own. They move very slightly during eye movements but they form a fairly stable anchor that acts as the functional origin of the muscle. (3) The SO and IO both have their functional origins close to the anterior rim of the orbit. The trochlea forms the SO functional origin, and the IO shares its muscle pulley with the IR. (4)

Orbital Fascia

Within the orbit the globe is held in position by various fascia, which allow the eye ball's free rotation but limit the extent of its movement and retraction. The eyeball sits suspended within Tenon's capsule, which covers the globe from the corneal limbus to the optic nerve and forms a cavity within which the eyeball can rotate. (5) Tenon's capsule is pierced by each of the EOMs, which reflects back to form a fascial sheath around the muscle. The fascial sheaths around the IR and IO extend upwards on either side of the globe to form the ligament of Lockwood, which acts like a hammock supporting the eyeball. The fascial sheaths of the LR and MR extend to the orbital wall to form the check ligaments.

Ocular Kinematics

Rotating the globe

The globe sits in its fascial socket in a fairly constant position within the orbit ie its centre of rotation is approximately fixed although it rotates freely around this position. The direction of rotation of a single eye movement is traditionally described in terms of the three elements shown in Figure 3, based on the co-ordinates defined by Fick; (6) horizontal (z-axis), vertical (x-axis) and torsional (y-axis).

[FIGURE 1 OMITTED]

Monocular eye movements are described as ductions. Adduction is an inward rotation, so the line of sight rotates towards the nose, and abduction is an outward rotation. The term 'elevation' to describe an upward rotation and 'depression' for a downward rotation tend to be used in favour of the more logical supraduction and infraduction. When the eye is in the primary position, the ocular dynamics of each muscle are dependent on the direction of the muscle pull between its functional origin and its insertion, as shown in Figure 4. From this position the MR adducts and the LR abducts the eye, but the four vertically acting muscles have horizontal and torsional actions as well. Imagining the direction of pull from the muscle positions shown in Figure 4, it is clear that as the eye rotates, the direction of action of the muscle changes with respect to the line of sight. For example, when the SR is in the primary position, the insertion is directly above the line of sight and the muscle pulls from a medial angle of 23[degrees], so SR contraction will rotate the line of sight upwards and inwards. When the eye abducts by 23[degrees] the line of sight is in the same direction as the muscle so the globe will elevate with no adduction. Krewson (7) used vector analysis to calculate how the direction of action of each muscle varies with changing direction of gaze. Figure 5 shows his results for the vertically acting muscles during eye movements along the horizontal. The x-axis shows the horizontal angle of gaze, the primary position is zero, adduction (inward rotation) is plotted to the right, and abduction (outward rotation) to the left. For each plot the y-axis shows the percentage of muscle activity contributing to the direction of pull specified (dashed line, vertical action; solid line, torsion; dotted line, horizontal action). These graphs are based on some over simplified assumptions (8) but they clearly indicate how the direction of action of an EOM varies with direction of gaze. Their great value is when considering what is 'missing' following a single muscle palsy. For example, if the SO is affected, the graph shows that in the primary position the majority of the muscle action produces intorsion and depression, with a small amount of abduction. If the muscle is palsied the eye will deviate in the opposite direction; extorted, elevated and slightly adducted (eso). As the eye rotates into abduction, a greater proportion of muscle action contributes to abduction and intorsion so, when palsied, on abduction the eye will become more 'eso' and extorted with less height. On adduction the eye will become more elevated. Torsion is a very difficult eye movement to observe and quantify in a clinical setting so the tendency is to rely on observing the horizontal and vertical ocular position. For each of the six directions of gaze shown in Figure 6, one EOM makes a greater contribution to the eye movement than the rest. Equally, if that muscle were palsied, the figure illustrates the direction of gaze in which you would see the greatest under-action and, from the patient's perspective, the largest separation in the two images.

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Monocular Eye Movements

There are six EOMs on each eye and any eye movement occurs as a result of a synergistic co-contraction of some EOMs and relaxation of others. An EOM contraction will cause an eye movement, the contracting muscle being known as the agonist. Sherrington's laws applies not only to the eye but also to other muscle pairings throughout the body, and states that any innervation to a muscle to contract will be accompanied by an equal inhibitory input to the direct antagonist to relax. Each globe has six EOMs in three antagonistic pairs. The MR and LR oppose each other for horizontal eye movements along Fick's z-axis. For vertical rotation, Fick's y-axis, the muscle pair changes with the angle of horizontal gaze. When the eye is adducted the IO and SO oppose each and when abducted the SR and IR oppose each other, as illustrated in Figure 7.

The Field of fixation

In contrast to the field of view, where the eyes are stationary, the field of fixation is the full extent to which the eyes can rotate, defined as 'the area within which central fixation is possible by moving the eye but not the head'. (5) The size of the field of fixation varies with direction of gaze, between individuals and with age but averages depression 54[degrees], adduction 46[degrees], abduction 42[degrees] and elevation 34[degrees].

Binocular eye movements

Under normal circumstances the two eyes will always move as a pair, as dictated by Herings law of equal innervation. (5) This states that whenever a nervous impulse initiates an eye movement in one eye, an equal innervation is sent to the corresponding muscles in the other eye. During versional eye movements both eyes rotate in the same direction; an innervation to initiate an ocular rotation of the right eye to the right (abduction, contraction of the right LR) will be accompanied by an equal innervation to rotate the left eye to the right (adduction, contraction of the left MR). During vergences, the innervations move the eyes in opposite directions; an innervation to initiate an ocular rotation of the right eye to the left (adduction, right MR) will be accompanied by an equal innervation to rotate the left eye to the right (adduction, left MR). Following contraction of a given muscle the contralateral synergist is the name given to the muscle in the fellow eye that will be innervated to move the eyes binocularly, as shown in Figure 8.

The binocular field of fixation

The binocular field of fixation shows the area of eye movement in which bifoveal binocular viewing is maintained. Peripherally there is an area where the eyes can rotate but fixation is not binocular, either because fusion has broken down resulting in diplopia or because one eye has moved beyond its monocular field of fixation (either because of occlusion of one eye by facial features or because it cannot rotate that far). Clinically, nothing can be gained from examining diplopia beyond the area in which the two monocular fields of fixation overlap. When discussing ocular under-actions and over-actions we are talking about the movement of one eye relative to the other. Beyond the area in which the two monocular fields of fixation overlap it is much more difficult to assess relative movement. In 1899 Asher (5) published the extent of his own binocular field of fixation (Figure 9) as 28[degrees] into elevation, 45[degrees] into depression, 20[degrees] into abduction and 35[degrees] into adduction. Presumably fusion had broken down on up-gaze because of a physiological 'V'-pattern.

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Sensory input to vision

In general the EOMs are thought of as a motor unit purely concerned with ocular rotation on the command of the neural signals, but they do have a role in sensory visual perception. The EOMs may produce proprioceptive feedback to the visual system on eye position but there is certainly input from the IIIrd, IVth and VIth nerve nuclei to the visual cortex. The EOMs bear a fairly constant load, so a given innervation almost invariably leads to a predictable ocular rotation. Manual displacement of a globe will stretch muscles in a way that doesn't correspond to the nervous innervation, and produces an illusion of visual motion and inaccuracies in visual perception of spatial position. Whether these illusions can be explained by a conflict of retinal signals with the neural signals or whether they are due to the mismatch with proprioceptive feedback remains a subject of debate. Whichever, they show the importance of ocular motor input to visual perception and how a mismatch in these signals can be disturbing for the subject.

Types of eye movement

Eye movements fall into different categories depending on the type of classification. They can be classified in terms of:

1. Their direction. Horizontal, vertical and cyclo-rotational (or torsional) eye movements are defined by the rotation of the globe around Fick's axes. Retraction of the globe describes the small movements of the eye back into the orbit, limited by the presence of Tenon's capsule. Protraction describes movement of the globe forwards, out of the orbit, as in proptosis.

2. Their binocular relationship. Ductions describe the movements of one eye without any consideration to the movements of the other. Conjugate eye movements mean that the eyes move as a pair in the same direction, a normal binocular versional eye movement. Disjugate means the eyes move in opposite directions as in a vergence eye movement.

3. Their velocity and visual function. Versional eye movements can be smooth pursuit or saccadic eye movements. During pursuit, the eyes attempt to maintain fixation by rotating at the same velocity as the fixation target whereas during saccades, the eyes are moved rapidly to a new fixation position. 'Sustained' describes the effort used to hold the eyes in a position of steady fixation. Steady fixation is something of a misnomer as there are constant small fluctuations in eye position; rapid micro-saccades, relatively slow drifts, and a very fast small amplitude tremor.

[FIGURE 5 OMITTED]

4. Voluntary control. Eye movements can be voluntary (a subject makes a conscious decision to rotate the eyes in the absence of any visual stimulus) or reflex (such as cyclorotations in response to head tilt) with a whole range of subjective control between these two extremes.

The relative contribution of the different types of muscle fibre in the EOM to the different types of eye movement (primarily the Fibrillenstruktur or 'twitch' fibres and Felderstruktur or 'slow' fibres) remains unknown. The fact that there is only one type of nervous input to any EOM indicates that all muscle fibres contribute to saccades, pursuit and vergence eye movements. (9,10) The supranuclear centres that innervate the ocular motor nerve nuclei must dictate the type of movement rather than structural characteristics of the muscles themselves.

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Clinical characteristics of ocular motor anomalies due to disorders of the EOMs

Complete absence of a muscle can occur but in general congenital defects of the fascial system are more common. Examples include Brown's syndrome, the adherence syndromes (11) and strabismus fixus. There are a number of acquired conditions that can affect the EOMs, some of which can mimic the congenital conditions. Examples include entrapment of an EOM due to a blow out fracture, dysthyroid eye disease and ocular myositis. Myasthenia Gravis affects the myoneural junction so, although orbital in origin, it has the characteristics of a neurogenic palsy. Duane's retraction syndrome, fibrosis of the EOMs, and Chronic Progressive External Ophthalmoplegia have neurogenic and myogenic elements. Orbital conditions such as orbital cellulitis or a space occupying lesion can have the clinical characteristics of a myogenic palsy but there are normally accompanying orbital signs or proptosis.

Differential diagnosis of mechanical versus neurogenic palsies

The forced duction test

The eye is manually rotated under anaesthetic. If the muscle is palsied the eye will rotate smoothly and completely in the affected direction, if it is a mechanical limitation the eye will remain restricted.

Characteristics of the anomalous eye movements

Affected eye: If the limitation is due to a mechanical restriction and the unaffected eye is fixating (primary deviation), as the eyes move into the affected direction of gaze the affected eye will suddenly slow to a stop. In contrast to a neurogenic palsy, the onset of the restriction tends to be sudden and dramatic, the ocular misalignment rapidly increasing as the fixating eye continues to move into the affected direction of gaze.

Contralateral synergist: If the unaffected eye is occluded so that the affected eye attempts to fixate the target (secondary deviation), the effort entailed in trying to move the affected eye against the mechanical restriction means that the over-action of the unaffected eye can be very large (due to Hering's law). This movement is seen when removing the occluder and tends to be much greater than the initial under-action of the affected eye. The difference between the primary and secondary angles of deviation is usually much greater in a mechanical than in a neurogenic palsy.

The antagonists: As the eyes move into the opposite direction of gaze, the mechanical restriction has no effect, so the eyes remain in alignment. In a neurogenic palsy, one would expect to see an over-action of the direct

antagonist (due to Sherrington's law) and an under-action of its contralateral synergist, the secondary inhibitory palsy. There is not normally an antagonist sequelae in a mechanical palsy.

In summary a mechanical defect results in a sudden onset under-action of the affected muscle, a large over-action of the contralateral synergist and no sequelae in the antagonists.

Evaluation of Diplopia

Investigating the characteristics of diplopia are invaluable in an optometry clinic where there is normally no Hess Chart or other more sophisticated diagnostic tools. A diplopia chart formalises the description of the diplopia in each direction of gaze and can be sketched as shown in Figure 10. Ideally the chart is plotted using a slit beam as a fixation target so that it can demonstrate torsion, but a spotlight can be used to show horizontal and vertical misalignment. Using red and green goggles (traditionally putting the red filter before the right eye) relative separation of the diplopic images can be sketched. The eye movement characteristics will be reflected in the separation of the diplopic images, the distal or furthest image corresponding to the under-acting eye. In a mechanical palsy there will be a large separation of the images in the affected direction of gaze, with all other directions of gaze normally being unaffected. Crossing of diplopia can occur if there is a limitation in more than one direction of gaze (as often happens in a blow out fracture or thyroid eye disease). It provides a useful indicator of a mechanical palsy as, with the exception of a total IIIrd nerve palsy, crossing of diplopia does not normally occur in a neurogenic condition. It refers to a swapping in ocular misalignment between up-gaze and down-gaze. For example, if there is a right hypertropia in up-gaze, the image from the left eye will be higher than the image from the right eye. Moving into down gaze the eyes swap positions so the diplopia will become right over left.

Past pointing

If you ask the patient to fixate with the affected eye, and then ask them to point at the target, their finger may overshoot, locating the target as further away than it actually is. This is known as "past pointing" and occurs because rotating an eye with a mechanical limitation into the affected direction of gaze requires far more effort than normal. This mismatch between the innervation and the ocular position is interpreted by the visual system as a larger eye movement than it actually is. Assuming the subject normally fixates with the unaffected eye, the more recent the onset of the condition the more dramatic the past pointing. (12)

Intraocular pressure (IOP)

As the globe rotates towards an obstruction or away from a tethering, eye movements may be limited and there will be pressure on the globe, which can cause a temporary rise in IOP. (13) This will not occur with a neurogenic palsy.

Electro-diagnostics

In a subject with a mechanical lesion, electro-myography (EMG) will show vigorous activity in a muscle where there is limited ocular movement. (14) This will not be the case in a neurogenic palsy

Conditions affecting the extra-ocular muscles Brown's syndrome

This is a mechanical limitation in which the clinical picture resembles an IO palsy, with a limitation of the affected eye on elevation in adduction. The function of the contralateral SR is normal and, in contrast to an IO palsy, there is no over-action of the SO (direct antagonists). There tends to be a 'V' pattern in up-gaze. Causes of the congenital defect include a defect in the sheath surrounding the tendon, a short tendon, obstruction in the movement of the tendon through the trochlea, an abnormal trochlea, or tethering of the IO to the orbital wall. (15) An audible click on attempting elevation in adduction, followed by an improvement in the eye movement, indicates that the aetiology is an obstruction to the movement of the muscle through the trochlear pulley. The acquired condition can result from trauma or inflammation in the region of the trochlea. (16) The condition can vary in its severity over time, tending to reduce with age.

[FIGURE 7 OMITTED]

Duane's syndrome

This condition is characterised by narrowing of the palpebral fissure and retraction of the globe during attempted adduction. The degree to which abduction and adduction are affected can vary but most commonly there is severely limited abduction and normal or slightly limited adduction. There may be an up or down shoot of the affected eye on attempted adduction and may be widening of the palpebral fissure on attempted abduction. The lateral rectus is often fibrotic and non-elastic, but this is probably secondary to innervational problems. The favoured aetiology is a brainstem defect (17) leading to co-innervation of the MR and LR so that they co-contract on attempted adduction and are both inhibited on attempted abduction. The condition can be associated with a number of other brainstem anomalies. (18) Orbital trauma can mimic the condition.

[FIGURE 8 OMITTED]

Adherence syndromes

These conditions arise through abnormal fascial connections between the reflections of Tenon's capsule along the EOMs, causing tethering of the globe. (11) These are rare (5) but can occur between the IR and IO, LR and IO and the SR and SO, which result in limitations of eye movements on elevation, adduction and depression respectively.

Strabismus fixus

This is a rare condition in which there is generalised EOM fibrosis and the eyes are fixed in an extremely convergent or, more rarely, divergent position.

Fibrosis of the EOMs

A rare condition in which most or all of the EOMs, including the levator palpebrae superioris (LPS), become fibrosed. There is a downward displacement of both eyes, marked ptosis, and convergence when attempting vertical or horizontal eye movements. It is possible that this is not primarily a muscle problem but the fibrosis is secondary to a supra-nuclear dysfunction. (19)

High myopia

Distance esotropia: The enlarged globe can stretch the LR and compress it against the orbital wall resulting in an eso deviation that tends to increase on distance fixation but, in the extreme, can develop into a strabismus fixus. (20)

The heavy eye syndrome: either a hypo deviation in the more myopic eye in high degrees of anisometropia or downward slippage of the horizontal muscles around the enlarged globe. (20)

The alphabet patterns The so-called "A", "V", "X", "Y", and "[lambda]" patterns describe the way the horizontal angle of deviation varies as the eyes change their direction of gaze from elevation, through the primary position into down-gaze. (21) For example, a 'V' is more exo or less eso in up-gaze and less exo or more eso in down-gaze. There is no clear aetiology for these conditions, but the most favoured explanations are: 1. Under- or over-action of the oblique muscles. 2. A misplacement of muscle insertions. Misplaced insertions may refer to a symmetrical upward or downward displacement of the MR or LR insertions or a difference between the planes of action of the oblique muscles.

[FIGURE 9 OMITTED]

Dysthyroid Eye Disease

Associated with systemic thyroid dysfunction, dysthyroid eye disease occurs in patients who are hyperthyroid, hypothyroid or euthyroid. It is characterised by peri-orbital oedema, enlargement of the EOMs, lid retraction and optic neuropathy. The clinical picture is proptosis, lid retraction, limited eye movements and conjunctival oedema. Secondary to these changes there may be corneal involvement and raised IOP. The optic neuropathy may, in part, be due to compression within the congested orbit but this is not the only explanation. The swelling in the EOM causes tension and a loss of elasticity in the short term, with muscle fibres being replaced by connective tissue in the longer term. The restricted eye movements usually develop slowly; most commonly affecting the IR first, followed by the MR and SR. The condition develops asymmetrically so limited elevation in one eye results in diplopia, which is often the first symptom. Referral into the hospital eye service is necessary so that the condition can be closely monitored. If there is any sign of optic neuropathy or corneal involvement the referral should be urgent. (22)

Orbital myositis

This is an acute onset inflammation of unknown aetiology involving any of the orbital tissues. The presenting symptoms are normally orbital pain, which increases on eye movements, ptosis, conjunctival hyperaemia, proptosis and diplopia. The diplopia results from limited ocular motility due to inflammation directly affecting an EOM or neighbouring structures. These patients ought to be referred rapidly for anti-inflammatory medical intervention and for differential diagnosis from other forms of treatable orbital inflammatory diseases (23) or a potentially sinister space-occupying lesion (SOL).

Orbital injury

A sudden increase in pressure around the circumference of the orbital rim can result in a 'blow out fracture' in which there is damage to an orbital wall. The floor is most commonly affected but occasionally it can be the medial wall. The damage could result from the sudden rise then drop in intra-orbital pressure or it could be a more direct buckling of the wall itself. (24) As the pressure releases the damaged bones move and the orbital contents prolapse into the maxillary antrum or become trapped in the fracture. Immediately after the injury there will be swelling of the orbital tissues and lids, which may result in proptosis. As the swelling subsides there is often enophthalmos and diplopia. The diplopia is due to a limitation of eye movements, most commonly on elevation (due to muscle entrapment) and depression (due to congestion of orbital contents in front of the fracture site). Similar symptoms have been associated with haemorrhage or oedema in the muscle itself. (25) Medical opinions vary on the best time for surgical intervention (26) but, if not previously seen within the hospital eye service, these patients should be referred with an urgency dependent on the duration of the condition.

Ocular Myasthenia Gravis

Ocular myasthenia gravis can mimic any EOM palsy. The reason that the EOMs are particularly vulnerable is uncertain but the defect is in the acetylcholine receptors at the neuro-muscular junction. (27) The neuroganic muscle palsy tends to have a sudden onset and can then quickly resolve. Diplopia and ptosis are often the presenting symptoms, worse following continued activity of the affected muscle and at warmer temperatures. The ice-pack test can be a useful diagnostic tool, with function improving as the temperature of the affected area reduces. Referral to a neurologist should be rapid as early medical intervention can improve the prognosis of this systemic condition. (28)

Chronic progressive external ophthalmoplegia

This is a rare bilateral condition in which ocular movements gradually reduce and the eyelids droop. Probably genetic in origin, it can be any of a group of muscular dystrophies or neurodegenerative conditions. (29,30) Ptosis is usually the first symptom. In the extreme cases, the eyes can become completely immobile with atrophic EOMs.

Orbital Space Occupying Lesion (SOL)

A SOL within the orbit can restrict eye movements by direct pressure on the globe or by affecting the path of the EOMs. There are normally other signs of congested orbital space, such as proptosis or an obvious lump on one side of the globe. Unless the SOL has previously been investigated, the condition should be urgently referred to the hospital eye service, especially in children. A rhabdomyosarcoma is a thankfully rare but rapidly progressive primary orbital tumour of childhood. (23)

Treatment

Treatment of anomalies to the EOMs or their fascia is usually limited to surgery, injections of botulinum toxin or, where indicated above, medical intervention. Congenital defects of the fascial system are very often asymptomatic with good quality binocular vision in the primary position. In these cases it is often best to 'leave well alone'.

Conclusion

The EOMs form just one part of the sensory-motor apparatus of the visual system. The dynamic of any one muscle is complex, let alone that of the entire binocular unit. Arguably, for a clinical optometrist, a definitive diagnosis is important only as far as it has an impact on practice management and diagnosis, as it affects practice management and referral criteria. This article has described the recognition of different anomalies that affect the EOMs and their fascia, and differential diagnosis between these and other conditions that affect binocular function and ocular motility, particularly the neurogenic palsies. Most congenital EOM anomalies are stable and asymptomatic. Acquired defects need specialist investigation with a degree of urgency dependent on the condition and its duration. The rest of this series looks in more detail at neurogenic palsies and management of concomitant deviations.

Acknowledgments

The author would like to thank Dr Simon Grant of City University, London, for his assistance in the preparation of this article.

MSc in Clinical Optometry

City University and OT have joined forces allowing readers to achieve CET points through to a full Masters in Clinical Optometry. MSc courses running at City University include: Principles of Therapeutics (apply anytime--web-based), Independent Prescribing (June 20-22 2010), Glaucoma (July 18-20 2010), Posterior Segment Eye Disease (September 5-7 2010) and Binocular Vision (November 14-16 2010). For further information please contact Dr Michelle L Hennelly by emailing (m.hennelly@city.ac.uk) or call 0207 040 8352.

Module questions

Course code: C-13990 O/D

1. Which of the following best describes the functional origin of the superior rectus muscle?

a. The upper part of the annulus of Zinn near the apex of the orbit

b. Adjacent to the orbit roof, 5-6 mm behind the equator of the globe

c. The trochlea, at the antero-medial corner of the orbit

d. Anterior to the globe's equator, 6.Tmm from the corneal limbus

2. According to Sherrington's law, which of the following correctly identifies the direct antagonist to the right inferior oblique muscle?

a. Right inferior rectus

b. Left superior rectus

c. Right superior oblique

d. Left inferior oblique

3. In a patient with a recent onset right superior oblique palsy, which of the following would be observed with a change in horizontal direction of gaze?

a. Increasing right 'hyper' deviation on adduction with more 'eso' on abduction

b. Increasing right 'hypo' deviation on adduction with more 'exo' on abduction

c. Increasing right 'hyper' deviation an abduction with more 'eso' an adduction

d. Increasing right 'hypo' deviation on abduction with more 'exo' on adduction

4. Which of the following best describes the binocular field of fixation?

a. The area of overlap of the monocular peripheral fields, measured whilst maintaining steady fixation on a central target

b. The area of single vision, measured binocularly whilst maintaining steady fixation on a central target

c. The overlap of the right and left field of fixation

d. The area in which bifoveal binocular single vision can be maintained on a moving target

5. Which of the following statements correctly describes a classification of an eye movement?

a. A cyclo-rotation in response to head tilt is a voluntary eye movement

b. Ductions are binocular eye rotations around one of Fick's axes

c. "Disjugate" refers to a binocular vergence movement

d. A 'drift' is where fixation is maintained on a slowly moving target

6. Which of the following is NOT normally due to an anomaly of the fascial system?

a. Duane's retraction syndrome

b. Brown's syndrome

c. Strabismus fixus

d. Johnson's adherence syndrome

7. Which of the following characteristics is typical of a mechanical rather than neurogenic ocular muscle palsy?

a. Sudden onset under-action of the affected muscle

b. Large over-action of the direct antagonist

c. Gradually increasing under-action of the contralateral synergist

d. A significant secondary inhibitory palsy

8. Which of the following would be equally likely in a neurogenic and a mechanical muscle palsy?

a. Crossing of diplopia between up-gaze and down-gaze

b. Transient increase in intraocular pressure on eye movement into the direction of the limitation

c. Restricted movement on forced duction test

d. Distal diplopic image belonging to the under-acting eye

9. Which of the following statements about 'past pointing' is FALSE?

a. When pointing to a target the patient locates it as further away than it actually is

b. The more longstanding a muscle palsy the more dramatic the 'past pointing'

c. The mis-location of the target is more dramatic when attempting to look in the direction of the paretic muscle

d. The mis-location can be explained as a greater than expected innervation to move the eye to fixate the target

10. Which of the following ocular motor defects have NOT been associated with high myopia?

a. Distance esotropia

b. Near esotropia

c. Hypotropia

d. Strabismus fixus

11. Which of the following mechanical limitations of eye movement does NOT require urgent referral to the hospital eye service?

a. A child with an orbital space occupying lesion

b. Dysthyroid eye disease with signs of corneal involvement

c. A patient with a red eye, having diplopia and pain on eye movements

d. A child with an apparent inferior oblique paresis that improves following an audible 'click'

12. What is the most likely diagnosis of an adult with a recent onset inferior rectus palsy that improves after 10 minutes with a pack of ice over the affected eye?

a. Dysthyroid Eye disease

b. Ocular myositis

c. Myasthenia Gravis

d. Chronic Progressive External ophthalmoplegia

References

See www.optometry.co.uk and search 'references'

Alison Finlay PhD MCOptom DBO

Dr Alison Finlay worked as an orthoptist for some years before training in Optometry at City University, London, and subsequently the Institute of Optometry. She worked in High Street practice before and during research towards her PhD at Imperial College London, which was awarded in 1998. For the following eight years she held an academic position at City University teaching paediatric optometry and binocular vision. She now works in hospital running paediatric optometry and low vision clinics.
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Title Annotation:CONTINUING EDUCATION & TRAINING
Author:Finlay, Alison
Publication:Optometry Today
Geographic Code:4EUUK
Date:Jul 2, 2010
Words:6060
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