Ductions

Ductions are monocular rotations of the eye. Adduction is movement of the eye nasally; abduction is movement of the eye temporally. Elevation (supraduction or sursumduction) is an upward rotation of the eye; depression (infraduction or deorsumduction) is a downward rotation of the eye. Intorsion (incycloduction) is defined as a nasal rotation of the superior pole of the vertical meridian. Extorsion (excycloduction) is a temporal rotation of the superior pole of the vertical meridian.

The following are important terms relating to the muscles used in monocular eye movements:

• agonist: the primary muscle moving the eye in a given direction

• synergist: the muscle in the same eye as the agonist that acts with the agonist to produce a given movement (eg, the inferior oblique muscle is a synergist with the agonist superior rectus muscle for elevation of the eye)

• antagonist: the muscle in the same eye as the agonist that acts in the direction opposite to that of the agonist (eg, the medial rectus and lateral rectus muscles are antagonists)

Sherrington’s law of reciprocal innervation states that increased innervation of a given EOM is accompanied by a reciprocal decrease in innervation of its antagonist. For example, as the right eye abducts, innervation of the right lateral rectus muscle increases and innervation of the right medial rectus muscle decreases.

Field of action

Field of action refers to the gaze position (one of the cardinal positions) in which the effect of the EOM is most readily observed. For the lateral rectus muscle, the direction of rotation and the position of gaze are both abduction; for the medial rectus muscle, they are both adduction. However, the direction of rotation and the gaze position are not the same for the vertical muscles. For example, the inferior oblique muscle, acting alone, is an abductor and elevator, pulling the eye up and out—but its elevating action is best observed in adduction. Similarly, the superior oblique muscle, acting alone, is an abductor and depressor, pulling the eye down and out—but its depressing action is best observed in adduction.

The clinical significance of fields of action is that a deviation (strabismus) that increases with gaze in some directions may result from weakness of the muscle normally pulling the eye in that direction, from restriction of its action by its antagonist muscle, or from a combination of these 2 factors.

Primary, secondary, and tertiary actions

With the eye in primary position, the medial and lateral rectus muscles move the eye only horizontally and therefore have a primary horizontal action. Anatomical studies have shown compartmentalization of the innervation to the horizontal rectus muscles in some patients; this may explain the finding of small vertical actions of these muscles in these cases (see Chapter 3). The vertical rectus muscles have a direction of pull that is mostly vertical as their primary action, but in primary position, the angle of pull from origin to insertion is inclined 23° to the visual axis (or midplane of the eye), giving rise to a torsional effect as well. Intorsion is the secondary action of the superior rectus, and extorsion is the secondary action of the inferior rectus; because the net positions of the insertions are medial to the center of rotation of the eye, adduction is the tertiary action of both muscles. Because the oblique muscles are inclined 51° to the visual axis, torsion is their primary action, vertical rotation (depression/elevation) is their secondary action, and abduction is their tertiary action. The levator palpebrae superioris is also an EOM, and its sole action is elevation of the upper eyelid. See Table 4-1 for a summary of the EOM actions.

Changing muscle action with different gaze positions

The gaze position can alter the effect of EOM contractions on the rotation of the eye. In each of the cardinal positions, each of the 6 oculorotatory EOMs has different effects on the eye’s rotation, based on the relationship between the visual axis of the eye and the orientation of the muscle plane to the visual axis. In each cardinal position, the angle between the visual axis and the direction of pull of the muscle being tested is minimized, thus maximizing the horizontal effect of the medial or lateral rectus or the vertical effect of the superior rectus, inferior rectus, superior oblique, or inferior oblique. By having the patient move the eyes to the 6 cardinal positions, the clinician can isolate and evaluate the ability of each of the 6 oculorotatory EOMs to move the eye. See also the section Binocular Eye Movements.

Table 4-1 Action of the Extraocular Muscles Referenced to Primary Position

With the eye in primary position, the horizontal rectus muscles share a common horizontal plane that contains the visual axis (Fig 4-2). The clinician can assess the relative strength of the horizontal rectus muscles by observing the horizontal excursion of the eye as it moves medially from primary position to test the medial rectus and laterally to test the lateral rectus. The actions of the vertical rectus and oblique muscles are more complex.

With insertions anterior to the center of rotation of the globe and, in primary position, the 23° angle between the muscle planes and the visual axis (Figs 4-3, 4-4), the superior and inferior rectus muscles have 3 actions: primary vertical, secondary torsion, and tertiary adduction. The relative vertical strength of the vertical rectus muscles can be most readily observed by aligning the visual axis parallel to the muscle plane axis—that is, when the eye is rotated 23° into abduction. In this position, the superior rectus becomes a pure elevator and the inferior rectus a pure depressor. To minimize the vertical action of these muscles, the visual axis should be perpendicular to the muscle axis at a position of 67° of adduction. In this position, the superior rectus action would be pure intorsion, and the inferior rectus action would be pure extorsion. Because the globe cannot adduct this far, the vertical rectus muscles maintain significant elevating and depressing action even in maximal voluntary adduction.

Figure 4-2 The right horizontal rectus muscles. A, Right medial rectus muscle. B, Right lateral rectus muscle.

(Modified with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:3.)

Figure 4-3 The right superior rectus muscle, viewed from above.

(Modified with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:3.)

Figure 4-4 The right inferior rectus muscle, viewed from below.

(Modified with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:5.)

Figure 4-5 The right superior oblique muscle, viewed from above.

(Modified with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:7.)

With insertions posterior to the center of rotation of the globe and, in primary position, the 51° angle between the muscle planes and the visual axis (Figs 4-5, 4-6), the superior and inferior oblique muscles have 3 actions: primary torsion, secondary vertical, and tertiary abduction. In 51° adduction, the muscle plane is aligned with the visual axis, and the vertical action of the oblique muscle can be most readily observed. When the eye abducts 39°, the visual axis becomes perpendicular to the muscle plane, and the muscle action is mainly torsion.

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.