L L

M M

A B

FIGURE 10-13

A, Adduction on contraction of medial rectus muscle with eye in primary position. B, Abduction on contraction of lateral ­rectus muscle with eye in primary position. L, Lateral; M, medial.

MOVEMENTS FROM PRIMARY POSITION

Horizontal Rectus Muscles

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 a nasal direction–adduction. The lateral rectus also lies parallel to the sagittal axis and perpendicular to the vertical axis; contraction causes rotation in a temporal direction–abduction (Figure 10-13).

Vertical Rectus Muscles

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, movement around the horizontal axis causes elevation. The muscle insertion is lateral to the origin, so movement around the vertical axis causes adduction; the oblique insertion on the superior surface of the globe causes intorsion on contraction (Figure 10-14, A). The primary action of the superior rectus is said to be elevation; adduction and intorsion are secondary actions.

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 (Figure 10-14, B).

Oblique Muscles

The primary action of the superior oblique muscle is intor- sion.5,17,51-53 This action results from the oblique insertion on the posterosuperior lateral aspect of the globe (Table 10-4); contraction rotates the eye around the sagittal axis,

causing intorsion. The secondary actions are depression and abduction.5,17,51-53 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 (Figure 10-15, A, page 194).

The primary action of the inferior oblique—­ extorsion—occurs because the muscle wraps around the lower portion of the globe and the insertion is superior and lateral to the origin. Secondary actions are elevation and abduction­ .5,17,51-53 Because the insertion is on the posterior eye and above the origin, contraction pulls the back of the eye down, elevating the front. Abduction occurs because the insertion on the back of the eye is pulled toward the medial side; thus the anterior pole is moved laterally in abduction (Figure 10-15, B).

Some authors offer the contrasting view that the primary action of the superior oblique is depression, that of the inferior oblique is elevation, and the torsional actions are secondary.54

MOVEMENTS FROM SECONDARY POSITIONS

As the position of the globe changes, the relationship between the muscle origin and insertion changes relative to the axes, and contraction of a muscle has a different effect than when the eye is in primary position. If the eye is elevated, contraction of the horizontal rectus muscles no longer causes strictly adduction or abduction, but also causes a slight elevation; if the eye is depressed, contraction causes further depression.55

Vertical Rectus Muscles

With the eye abducted approximately 23 degrees from primary position, the vertical rectus muscles parallel the sagittal axis, and lie perpendicular to the horizontal axis; thus only vertical movement will occur. In this position, contraction of the superior rectus will cause only elevation, and contraction of the inferior rectus will cause only depression50 (Figure 10-16, B, page 195).

As the eye adducts, it approaches a position where the plane of the vertical rectus muscles is at a right angle to the sagittal axis; this occurs at approximately 67 degrees of adduction (which may be physically impossible because of the connective tissue constraints of the orbit). If the muscle plane of the vertical rectus muscles is at a right angle to the sagittal axis, and thus parallel to the horizontal axis, contraction of the superior or inferior rectus muscle will not cause vertical movement50 (Figure 10-16, A).

Oblique Muscles

As the eye adducts 51 to 55 degrees, the plane of the oblique muscles becomes parallel to the sagittal axis and perpendicular to the horizontal axis. In this position the superior oblique will cause only depression, and the inferior oblique will cause only elevation. When the eye is abducted 35 to 39 degrees, the plane of the oblique muscles makes a right angle with the sagittal axis and ­parallels the horizontal axis, and the obliques cannot cause vertical movement50 (Figure 10-17).

This analysis is used in the clinical assessment of extraocular muscle function. As the eye increases in abduction, the elevating and depressing abilities of the vertical rectus muscles increase as the elevating and depressing abilities of the oblique muscles decrease. As the eye increasingly moves into adduction, the elevating and depressing abilities of the oblique muscles increase as the elevating and depressing abilities of the vertical rectus muscles decrease.

L

M

Y-axis

A

L

M

X-axis

Y-axis

B

FIGURE 10-16

Relationship between line of muscle movement and Fick’s axes when eye is in a secondary position. A, When eye is adducted 67 degrees (putting plane of muscle parallel to x-axis), ­contraction of superior rectus muscle cannot cause elevation. B, When eye is abducted 23 degrees (putting plane of muscle perpendicular to x-axis), contraction of superior rectus muscle causes only elevation. L, Lateral; M, medial.

L

M

Y-axis

A

L

M

X-axis

Y-axis

B

FIGURE 10-17

Relationship between line of muscle movement and Fick’s axes when eye is in a secondary position. A, When eye is abducted 35 degrees (putting plane of muscle parallel to x-axis), ­contraction of superior oblique muscle cannot cause ­depression. B, When eye is adducted 55 degrees (putting plane of muscle perpendicular to x-axis), contraction of superior oblique muscle almost exclusively causes depression. L, Lateral; M, medial.

AGONIST AND ANTAGONIST

MUSCLES

In any position of gaze, innervation of all extraocular muscles is controlled carefully by the central nervous system, and each muscle is in some stage of contraction or relaxation. No single muscle acts alone; muscles work together as agonists, antagonists, or synergists. In all these movements, fine motor control should provide for smooth, continuous movements. According to

Sherrington’s law of reciprocal innervation, contraction of a muscle is accompanied by a simultaneous and proportional relaxation of the antagonist.56 In adduction the increased contraction of the medial rectus muscle is accompanied by the increased relaxation of the antagonist, the lateral rectus muscle.

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. Jampel­ demonstrated this by stimulating the superior rectus and inferior oblique muscles simultaneously and noting that the eye moved directly up.57

When the superior oblique and inferior rectus muscles were stimulated simultaneously, the eye moved directly downward.57 The superior oblique and the inferior rectus are synergists in depression. The superior oblique is the antagonist for the inferior oblique in vertical movements and torsional movements but is synergistic for abduction.

In primary position the muscles are in a balanced state, each exerting contraction sufficient to keep the eye centered in the palpebral fissure. If one muscle is inactive, the eye will be deviated from primary position in the direction of the pull of the antagonist of the dysfunctional muscle. If the medial rectus muscle is paralyzed, the eye, in primary position, will be positioned temporally because of the unopposed action of the lateral rectus muscle.

Clinical Comment: Extraocular

Muscle Assessment

Assessment of eye position and movements can be an important tool in determining the integrity of the extraocular­ muscles and associated nerves. The practitioner­ first notes the position of each eye while directing the

patient to fixate on a target straight ahead. An eye that is deviated toward the nose would indicate an underactive lateral rectus muscle; the medial rectus is unopposed by the lateral rectus. Figure 10-18, A, shows the direction of pull of each muscle when the eye is in primary position.

Ocular motility testing provides further information on the contractile abilities of the muscles. Evaluation of horizontal eye movement is straightforward. If the eye cannot adduct, the problem lies with the medial rectus; if the eye cannot move into the abducted position, the problem lies with the lateral rectus muscle.

With the more complex movements of the other muscles, the most reliable way to determine a dysfunctional muscle is to put the eye into a position in which one muscle is the primary actor. In the adducted position the oblique muscles

are the primary elevator and depressor; in the abducted position the vertical rectus muscles are the primary elevator and depressor. This arrangement can be represented by the “H” diagrams in Figure 10-18, B. Thus when doing ocular motility testing, it is important to move the eyes to such a position as to isolate the vertical abilities of these muscles. The usual manner of performing ocular motility testing follows:

1.Using a small target, usually a bead, the patient is instructed to follow the target.

2.The horizontal ability is determined first by moving the bead to the far right and to the far left, noting any inability of either eye to follow.

3.In left gaze the bead is elevated to determine the ability­ of the left superior rectus (left eye is abducted) and the right inferior oblique (right eye is adducted). The bead is depressed to determine the ability of

the left inferior rectus and the right superior oblique muscles.

4.In right gaze the bead is elevated to determine the ability of the right superior rectus (right eye is abducted) and the left inferior oblique (left eye is adducted). The bead is depressed to determine the

ability of the right inferior rectus and the left superior oblique muscles.

The extensive network of connective tissue septa and fibroelastic­ pulley system associated with the extraocular­ muscle may affect eye movement, and MRI may be an additional­ tool to determine the extent of muscle involvement­ once a strabismic condition is diagnosed.

B

FIGURE 10-18

A, Direction of eye movement on contraction of each muscle, with eye in primary position. For example, if superior rectus ­muscle contracts, eye will move up and in and intort. Curved arrows represent torsional movements. B, Muscles that cause vertical movement when eye is either adducted or abducted. In adduction, for example, the muscle that causes elevation is the inferior oblique, and the muscle causing depression is the superior oblique. IO, Inferior oblique; IR, inferior rectus; LR, lateral rectus; MR, medial rectus; SO, superior oblique; SR, superior rectus.