The Extraocular Muscles

The Extraocular Muscles

CHAPTER 5.

THE EXTRAOCULAR MUSCLES (EOM

BY THE END OF THE CHAPTER YOU WILL HAVE COVERED THE

FOLLOWING ASPECTS OF THE TOPIC:

DEFINITION AND TYPES OF EOM

INDIVIDUAL EOM THAT MOVE THE EYEBALL

EYEBALL MOVEMENTS

PHYSIOLOGICAL LAWS

SOME CLINICAL CONSIDERATIONS

ARWONS>

• 1. DEFINITION AND TYPES OF EOM.
• The muscles of the globe can be divided into two groups: the involuntary
• intrinsic muscles and the voluntary extrinsic muscles. The intrinsic
• muscles— the ciliary muscle, the iris sphincter, and the iris dilator— are
• located within the eye; these muscles control the movement of internal
• ocular structures. The extrinsic muscles— the six extraocular
• muscles— attach to the sclera and control movement of the globe.
• Extra ocular muscles are a group of muscles that are found outside the
• eyeball and are attached either to the eyeball itself or to the surrounding
• structures.
• This is in contrast to the intraocular muscles, that are found within the
• There are 8 extra ocular muscles grouped into two categories:
• a. 6 muscles are attached to the eyebal/ and function as movers
• of the eye.
• b. 2 muscles are attached to the eyelid and the adjacent
• structures like the lacrimal sac.
• This chapter will be dealing with muscles that move the eyeball. The other

muscles have been covered in the chapter dealing with the eyelids.

STRUCTURE OF THE

EXTRAOCULAR MUSCLES

• The extraocular muscles have a denser blood supply, and their connective
• tissue sheaths are more delicate and richer in elastic fibers than is skeletal
• muscle. Precise fine motor control and quick accurate movement of the
• extraocular muscles occur because of this dense innervation. Singly
• innervated fibers have the classic end plate (en plaque) seen in skeletal
• muscle. Muscle spindles and Golgi tendon organs have been identified in
• human extraocular muscle, although it is unclear whether these structures
• provide any useful proprioceptive information relative to the extraocular
• muscles.18 Afferent information regarding extraocular muscle
• proprioception is thought to be mediated by a receptor that is unique to
• extraocular muscle, the
• myotendinous cylinder (palisade ending)
• 2. INDIVIDUAL EOM THAT MOVE THE EYEBALL.
• There are 6 muscles that are attached to the outer surface of the eyeball. 4
• of these rmuscles have a straight course from their origin to thei insertion
• on the sclera, hence they are called the RECTI muscles (singular — RECTUS
• which means STRAIGHT).
• The 4 recti muscles are:
• Superior rectus
• Inferior rectus
• Lateral rectus
• Medial rectus
• ao7D
• 28
• The names of the muscles suggest the positions where they are attached or
• . Meaning that, the superior rectus muscle
• is attached on the superior aspect of the eye, the lateral rectus on the lateral
• aspect and so on.
• The other two muscles take a slanting of tilted course from their origin to
• insertion and hence called OBLIQUE muscles.
• They are:
• a. Superior oblique
• b. Inferior oblique
• Again, the names suggest the positions where they are inserted on the eye.
• The other two muscles that are NOT attached to the eyeball surface are:
• a. Orbicularis oculi
• b. Levator palpebrae superioris
• 2.1. Muscles origins, insertions and nerve supply.
• The 4 recti originate from a common tendinous ring ( of Zinn ).
• This is a thickened connective tissue ring that bridges the superior orbital
• fissure at the orbital apex.
• All four muscles insert anteriorly on the globe. A line connecting the rectus
• muscle
• insertions forms a spiral, as described by Tillaux. This spiral starts at the
• medial rectus, the insertion that is closest to the limbus, and proceeds to the
• inferior rectus, the lateral rectus, and finally the superior rectus, the insertion
• farthest from the limbus
• a. The Superior Rectus muscle.
• Arises above the optic foramen on the common tendinous ring. The
• superior rectus muscle parallels the roof of the orbit until it passes
• through a connective
• tissue pulley just posterior to the equator of the globe;
• at this point it follows the curve of the globe to its
• insertion .It then passes forward and inserts on to the sclera 7.7mm
• from the limbus and is curved slightly, with the convex side forward .
• A line drawn from the origin to the insertion along the muscle will
• form an angle of approximately 23 degrees with the sagittal axis —
• The muscle is innervated by the superior division of the oculomotor
• (cranial nerve I/I)which enters the muscle on its inferior face.
• Branches pass either through the muscle or around it to innervate the
• levator
• b. The Inferior Rectus muscle.
• Arises from the tendinous ring below the optic foramen. It passes
• forwards and inserts onto the sclera about 6.5mm from the limbus.
• The inferior rectus approximately parallels the superior rectus, making
• an angle of 23 degrees with the sagittal axis. The inferior rectus
• muscle parallels the orbital floor until it passes through a connective
• tissue pulley just posterior to the equator of the globe; at this point it
• follows the curve of the globe to its insertion, which is parallel to the
• insertions of the superior rectus
• The muscle is innervated by the inferior division of the oculomotor
• nerve which enters the muscle on its superior surface
• c. The Lateral Rectus muscle.
• The lateral rectus muscle has its origin on both limbs of the common
• tendinous ring and the spina recti lateralis, a prominence on the greater wing
• of the
• sphenoid bone. Arises from the lateral aspect of the tendinous ring.
• It passes forwards and laterally and inserts onto the sclera about 6.9mm
• from the limbus. The lateral rectus muscle parallels the lateral orbital wall
• until it passes through a connective tissue pulley just posterior to the
• equator of the globe;at this point it follows the curve of the globe to its
• insertion. The insertion parallels that of the medial rectus and is
• approximately 6.9 mm from the limbus, and the length of the tendon is
• approximately 8.8 mm
• The muscle is innervated by the abducent nerve (cranial nerve VI )
• which enters on the medial side of the muscle.
• 29
• d. The Medial Rectus muscle.
• This is the largest of the 6 muscles with its size probably resulting
• from the
• frequency of its use in convergence. It arises from the medial portion
• of the tendinous ring and inserts onto the sclera 5.5mm from the
• limbus.
• The medial rectus muscle parallels the medial orbital wall until it
• passes through a connective tissue pulley just posterior to the equator
• of the globe; at this point it follows the curve of the globe to its
• insertion.
• It is important to note here, that the medial rectus insertion is very
• near to the limbus.
• et surgeons who perform pterygium and other medially located
• orn clei onei not careful during the operation.
• The muscle is innervated by the inferior division of the oculomotor
• Spiral of
• Tillaux
• Fig. 5.1. Insertion of the recti muscles on the sclera ( spiral of Tillaux ).

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MUSCLE

• TENDON LENGTH(mm)
• TENDON WIDTH(mm)
• Superior rectus
• 5.8
• 10.6
• Inferior rectus 5.5 9.8
• Medial rectus 3.7 10.3
• Lateral rectus 8.8 9.2
• e. The Superior Oblique muscle.
• This is along and slender muscle arising from the body of the
• sphenoid bone,
• above and medial to the optic canal, outside the common tendinous
• ring. The muscle courses forward and passes through the trochlea, a U-
• shaped piece of cartilage attached to the orbital plate of the frontal bone
• The superior oblique muscle is the longest and thinnest of the extraocular
• muscles because of its long(2.5 cm) tendon of insertion
• The muscle therefore
• muscle. It
• runs forward between the roof and medial wall of the orbit and gives
• rise toa
• rounded tendon. The tendon of insertion changes direction as it _
• passes through the trochlea to run in a posterior direction and lies inferior to _
• ‘the superior rectus muscle. The insertion of the superior oblique muscle
• attaches in the superoposterior lateral quadrant of the globe33 and is fan
• shaped, concave forward, and oblique
• The trochlea is considered the physiologic or effective origin of the
• superior oblique muscle in determining muscle action because it acts as a
• pulley and changes the direction of muscle pull. A line drawn from the
• physiologic origin to the insertion makes an angle of approximately 55
• degrees with the visual axis.
• The tendon passes through a cartilaginous pulley, the trochlea,
• located on the medial aspect of the orbital roof just behind the orbital
• margin.
• After emerging from the trochlea, the tendon bends downward,
• backward and
• laterally and passes inferior to the superior rectus muscle. It gets
• inert strony onthe slerabehind the equator of the globe
• It is innervated by the trochlear nerve (cranial nerve IV ).
• f. The Inferior Oblique muscle.
• This is the only muscle out of the six to take its origin from the front
• of the orbit.
• It arises from the floor of the orbit just posterior to the orbital margin
• and just
• lateral to the nasolacrimal canal. The insertion of the inferior oblique
• is on the posterior portion of the globe on the lateral side, mostly inferior,
• lying just outer to the macular area The insertion is curved concave
• downward.The muscle makes an angle of approximately 51 degrees with the
• visual axis.
• It then runs below the inferior rectus going posteriorly and laterally.
• It is inserted on the posterior aspect of the sclera behind the orbital
• The muscle is innervated by the inferior division of the oculomotor
• nerve which enters the muscle on its upper surface
• 2.2. Points to note :
• 1. All the recti originate from the common tendinous ring.
• 2. The obliques DO NOT originate from the common ring.
• 3. The recti are inserted anterior to the equator.
• 4. The obliques are inserted posterior to the equator.
• 31
• 5. ; ALL the muscles are supplied by the oculomotor nerve, the
• me cranial nerve, EXCEPT the superior oblique, supplied by the 4
• crania
• nerve AND the lateral rectus supplied by the 6" cranial nerve
• (REMEMBER the mnemonic AL3 SO4 LR6).
• Superior
• oblique
• Superior
• muscle
• rectus
• muscle
• Medial
• KL rectus
• muscle
• Inferior
• rectus
• muscle
• Lateral
• rectus
• muscle
• Inferior
• oblique
• muscle
• Fig. 5.2. Diagram representing the 6 EOM that move the eyeball.

3. EYEBALL MOVEMENTS.

DUCTIONS

• Movements involving just one eye are called ductions
• They include abduction, adduction, supraduction, infraduction and the
• torsional movements. Torsions or cyclorotations are rotations around the
• sagittal axis and are described in relation to a point at the 12-o’clock
• position on the superior limbus.
• Intorsion (incyclorotation) is the rotation of that point nasally, and
• extorsion (excyclorotation) is the rotation of that point temporally.
• Torsional movements may occur in an attempt to keep the horizontal retinal

raphe parallel to the horizon

VERGENCES AND VERSIONS

• Movements involving both eyes are eithefiVergencesionversions, depending
• on the relative directions of movement. In vergence movements the eyes
• move in opposite left-right directions; these are disjunctive movements. In
• convergence each eye is adducted, and in divergence each eye is abducted.
• Version movements are conjugate movements and occur when the eyes
• move in the same direction. Dextroversion is right gaze, and levoversion is
• left gaze. In supraversion both eyes are elevated, and in infraversion both
• eyes are depressed.

GD ait with wes offic

MONOCULAR BINOCULAR

• Eye term Eye Term
• movement movement
• Medial adduction Right dextroversion
• Lateral abduction left levoversion
• Up Elevation, supraduction, up Supraversion or
• sursumversion
• Down Depression, infraduction, down Infraversion or
• deorsumversion
• Rotation of Intorsion, Up and right | dextroelevation
• 120’clock incyclorotation,incyclduction
• medially
• Rotation of Extorsion, excyclorotation, Down and Dextrodepression
• 120’clock excycloduction right
• literary
• Anterior out | Protrusion, exophthalmos Up and left levoelevation
• of orbit
• Posterior Retraction, enophthalmos Down and levodepression
• into orbit left
• Both eyes convergence
• adduct
• Both eyes divergence
• abduct
• Both eyes excyclovergence
• extort
• Both eyes incyclovergence
• intort
• Rotation of dextrocycloversion
• 120’clock
• postion to
• right
• Rotation of Levocycloversion
• 120’clock
• postion to
• left
• For all movement the anterior pole of the globe is the point of reference
• unless otherwise noted.
• 3.1. Axes of rotation of the eyeball.
• The eyeball moves about 3 imaginary axes: The X- axis is the horizontal or
• transverse axis and runs from nasal to temporal. The Y- axis is the sagittal
• axis running from the anterior pole to the posterior pole. The Z-axis is the
• vertical axis and runs from superior to inferior. The anterior pole of the globe
• is the reference point used in the description of any 6yélmovement
• a. The vertical axis:
• This is the axis that passes straight upward or supero-inferiorly. Hold a
• pencil straight in front of your eye to imagine this axis.
• The eyeball rotates sideways about this axis ic. t moves either tothe right
• of to the left about this axis.
• 32
• When the eye moves laterally or temporally the movement is termed
• ABDUCTION and when it moves medially or nasally it is termed ADDUCTION.
• b. The horizontal axis:
• This axis passes through the eyeball temporo- nasally.
• The eyeball rotates upwards or downwards about this axis i.e. it either looks
• upward or downward causing elevation or depression ofthe eyeball
• These movements are also called SUPRADUCTION and INFRADUCTION
• c. The antero-posterior or sagital axis:
• This is the axis that passes straight through the cornea into the pupil to the
• Hold a pencil in front of your eye in a way that the back part of the pencil is
• in front of the eye and the tip is away from the eye to imagine this axis.
• The
• When the eye twists inward it is called INTORSION and when it twists
• outwards it is called EXTORSION.
• In summary there are 6 movements studied here:
• Abduction
• Adduction
• Elevation
• Depression
• Intorsion
• Extortion
• >~oaoTD
• It is important to know these movements well in order to understand the
• actions of the individual muscles.
• 3.2. Actions of individual muscles.
• Out of the 6 muscles that move the eyeball, 2 have one action each.
• 33
• The other 4 muscles have three actions each – one primary action (i.e. the
• main action of the muscle) and two secondary actions.
• The lateral and medial recti have only one action each:
• The medial rectus lies parallel to the sagittal axis and perpendicular to the
• vertical axis; therefore it has only one action, which is rotation around the
• vertical axis in
• anasal direction— adduction.
• Lateral rectus: ABduction
• Medial rectus: ADduction
• The superior rectus:
• The action of the superior rectus is more complex than
• that of the medial and lateral rectus muscles because it
• lies at an angle to each of the axes; with the insertion
• above the origin and on the anterior globe,
• Primary action: Elevation
• Secondary actions: ADduction and Intorsion
• The inferior rectus:
• The primary action of the inferior rectus is depression, because the insertion
• is below the origin and on the anterior of the globe. Secondary actions are
• adduction, because the insertion is lateral to the origin, and extorsion, which
• results from the oblique insertion on the inferior surface of the globe
• Primary action: Depression
• Secondary actions: ADduction and Extorsion
• The superior oblique :
• The primary action of the superior oblique muscle is intorsion. Depression
• occurs because the insertion is posterior and inferior to the physiologic
• origin; contraction of the muscle pulls the back of the eye up, and the
• anterior pole moves down. Because the insertion is lateral to the trochlea,
• contraction of the superior oblique pulls the back of the globe medially, thus
• moving the anterior pole laterally
• Primary action: Intortion
• Secondary actions: Depression and ABduction
• The inferior oblique :
• The primary action of the inferior oblique— extorsion— occurs because the
• muscle wraps around the lower portion of the globe and the insertion is
• lateral to the origin. Secondary actions are elevation and abduction.
• Primary action: Extortion
• Secondary actions: Elevation and ABduction
• It is tedious to remember all these actions for the individual muscles. To
• simplify all these actions, it is worth remembering the following rules:
• 1. ALL the recti Adduct the eye except the lateral rectus (which
• Abducts the eye)
• 2. ALL the obliques ABduct the eye (‘B’ in oblique and ‘B’ in abduct)
• 3. ALL the superiors INtort the eye (remember SIN = Superior INtort)
• 4. ALL the inferiors EXtort the eye (remember INFEX = INFerior
• EXtort)
• If you only remember these laws you will have the actions of all the muscles
• immediately.
• 34
• 3.3. Eyeball positions of gaze.
• The position an eyeball assumes while looking at different directions is
• termed the position of gaze.
• These positions are grouped into 4:
• a.
• Primary position of gaze .
• This is when the eye is looking straight forward
• Secondary positions of gaze.
• This is when the eye is looking straight:
• Upward
• Downward
• Medially
• Laterally
• Tertiary positions of gaze.
• This is when the eye is looking obliquely:
• Up and Inward
• Up and Outward
• Down and Inward
• Down and Outward
• Cardinal positions of gaze.
• These are clinically important positions as they are used to test
• the individual muscles in motility disorders.
• They are:
• Lateral
• Medial
• Up and Inward
• Up and Outward
• Down and Inward
• oblique)
• Down and Outward
• 35
• (testing lateral rectus)
• (testing medial rectus)
• (testing inferior oblique)
• (testing superior rectus)
• (testing superior
• (testing inferior rectus)
• 4. PHYSIOLOGICAL LAWS
• One of the earliest models developed to explain eye movement is the
• isolated agonist model described by duane. it is important to remember that
• during eye movements, al! six extraocular muscles are in some state of
• contraction or relaxation, and it is strictly hypothetical to discuss the
• movement of the eye as if only one muscle contracts.
• At any one time, the position of the eyeball on its fatty bed, irrespective of
• the movement, is determined by the tone of all the 6 extra ocular muscles.
• It is thus clear that for the eyeball to move in any given direction, no single
• Groups of muscles must act together as agonists, antagonists and
• When the superior rectus muscle and the inferior oblique muscle contract at
• the same time, the adduction action of the superior rectus and the abduction
• action of the inferior oblique, as well as the intorsion of the superior rectus
• and the extorsion of the inferior oblique, will counteract each other. The
• resultant eye movement is elevation; the muscles are synergists in elevation.
• There are two physiological laws governing the eyeball movements:
• 4.1. Herring’s law of equal and simultaneous innervation .
• ‘Yoke muscles are those muscles of the two eyes acting together to cause
• binocular movements Equal and simultaneous innervation flows from
• the central nervous system to the muscles of both eyes gazing ina
• particular direction.
• For example, while looking to the right, the
• 4.2. Sherrington’s law of reciprocal innervation .
• For example, while looking to the right there is an increased contraction of
• the right
• lateral rectus muscle and the /eft medial rectus muscle and this is associated

dened avy he ight medetusand te tract

PAIRED ANTAGONIST

• MODEL
• A model by Boeder51 analyzes the actions of the extraocular muscles as
• antagonist pairs

BLOOD SUPPLY

• The extraocular muscles are supplied by two muscular branches from the
• ophthalmic artery: The superior (lateral) branch supplies the superior and
• lateral rectus and the superior oblique muscles, and the inferior (medial)
• branch supplies the inferior and medial rectus and the inferior oblique

muscles,

AGING CHANGES IN THE

EXTRAOCULAR MUSCLES

• Both horizontal rectus muscles are displaced inferiorly with age, with the
• medial rectus displaced more than the lateral rectus. This may be the cause
• of aconstant
• partial depression and may contribute to the impaired ability to elevate the
• eyes often observed in elderly persons, predisposing them to an incomitant
• (nonconcomitant)
• strabismus.43 The superior rectus and inferior rectus muscles do not change
• locations.43 Other age-related changes in extraocular muscles include a
• greater variety in fiber sizes, increased connective tissue in the muscle,
• increased adipose tissue in the bundles, deposits of lipofuscin, and
• degenerative changes
• 5. SOME CLINICAL CONSIDERATIONS.
• 1.1. Strabismus (Squint ).
• This is a condition in which the visual axes of the two eyes are not
• straight in the primary position or the two eyes do not follow each
• other normally in any of the eye movements.
• 36
• Exotropia (divergent strabismus) is characterized by the eye deviating
• outward.
• Inward deviation of the eye is called esotropia ( convergent
• strabismus).
• 5.2. Nystagmus is the involuntary oscillation of the eyes .

37