secondary intorsion around the y-axis, and adduction around the z-axis. In this position, the main pulling power of the superior rectus is medial to the center of rotation, which accounts for the adduction action of the muscle. The relative vertical strength of the superior rectus muscle 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 its elevating action is maximal. To minimize the elevation action of the superior rectus, the visual axis should be perpendicular to the muscle axis at a position of 67° of adduction. In this position, the superior rectus would become a pure intortor. Because the globe cannot be adducted this far, the superior rectus maintains significant elevating action in maximal voluntary adduction.
The actions of the inferior rectus muscle mirror the vertical and torsional actions of the superior rectus. Because the inferior rectus is attached to the globe inferiorly, its action from primary position is primarily depression and secondarily extorsion. However, as with the superior rectus, its tertiary action is adduction (see Fig 5-5). Its action as a depressor is maximally demonstrated in 23° of abduction and reduced, though not absent, in adduction.
The direction in which the 2 oblique muscles lie is from the anteromedial aspect of the globe to the posterolateral area, forming an angle of approximately 51° with the visual axis (Figs 5-6, 5-7). Because of the large angle formed in primary position, the primary action of the superior oblique is intorsion, with its secondary action being depression and its tertiary action, abduction. Because the muscle plane is aligned with the visual axis in extreme adduction, the superior oblique muscle action can be seen as mainly depression. When the eye abducts about 40° from primary position, the visual axis becomes perpendicular to the muscle plane, and the muscle action is one of mainly intorsion. A clinical application of this principle is the fact that in a complete third nerve palsy, the eye develops a large-angle exotropia. With the eye in this exotropic position, the superior oblique is a pure intortor. This intorsion can be observed by asking the patient to attempt to gaze downward and inward with the involved eye: the eye will intort even though it cannot be voluntarily moved into that field of gaze.
The vertical and torsional actions of the inferior oblique mirror those of the superior oblique (see Fig 5-7). In primary position, the primary action is extorsion, and the secondary action is elevation. However, as with the superior oblique, the tertiary action is abduction. The inferior oblique’s action as an elevator is best seen in adduction and, as an extortor, in abduction.
Binocular Eye Movements
When binocular eye movements are conjugate and the eyes move in the same direction, such movements are called versions. When the eye movements are disconjugate and the eyes move in opposite directions, such movements are known as vergences (eg, convergence, divergence, vertical vergence, and cyclovergence).
Versions
Right gaze (dextroversion) is movement of both eyes to the patient’s right. Left gaze (levoversion) is movement of both eyes to the patient’s left. Elevation, or upgaze (sursumversion), is an upward rotation of both eyes. Depression, or downgaze (deorsumversion), is a downward rotation of both eyes. In dextrocycloversion, both eyes rotate so that the superior portion of the vertical corneal meridian moves to the patient’s right. Similarly, levocycloversion is movement of both eyes so that the superior portion of the vertical corneal meridian rotates to the patient’s left.
The term yoke muscles is used to describe 2 muscles (1 in each eye) that are the prime movers of their respective eyes into a given position of gaze. For example, when the eyes move or attempt to move into right gaze, the right lateral rectus muscle and the left medial rectus muscle are simultaneously innervated and contracted. These muscles are said to be “yoked” together.
Each EOM in one eye has a yoke muscle in the other eye. Because the effect of a muscle is usually best seen in a given direction of gaze, the concept of yoke muscles is used to evaluate the contribution of each EOM to eye movement. See Figure 5-2, which shows the 6 cardinal positions of gaze and the yoke muscles whose primary actions are in that field of gaze.
Hering’s law of motor correspondence states that when the eyes move into a gaze direction, simultaneous innervation leads to yoke pairs of muscles having equal force. The most useful application of this law is in evaluation of binocular eye movements and, in particular, the yoke muscles involved.
Hering’s law has important clinical implications, especially when the practitioner is dealing with a paralytic or restrictive strabismus. Because the amount of innervation to both eyes is always determined by the fixating eye, the angle of deviation varies according to which eye is fixating. When the unaffected eye is fixating, the amount of misalignment is called the primary deviation. When the paretic or restrictive eye is fixating, the amount of misalignment is called the secondary deviation. The secondary deviation is larger than the primary deviation because of the increased innervation necessary to move the paretic or restrictive eye to the position of fixation. This extra innervation is transmitted to the yoke muscle(s) in the fellow eye, which causes excessive action of this muscle and thus a larger angle of deviation. In cases where Hering’s law does not appear to hold, the diagnosis may be a dissociated vertical deviation (see Chapter 11) or a dissociated horizontal deviation (DHD) (see Chapter 9).
Application of Hering’s law is also useful in the explanation of muscle sequelae of a right superior oblique muscle palsy (see Chapter 11). If a patient uses the right eye to fixate on an object that is located up and to the left, the innervation of the right inferior oblique muscle required to move the eye into this gaze position is reduced because the right inferior oblique does not have to overcome the normal antagonistic effect of the right superior oblique muscle (Sherrington’s law). According to Hering’s law, less innervation is also received by the yoke muscle of the right inferior oblique muscle, the left superior rectus muscle. This decreased innervation could lead to the incorrect impression that the left superior rectus muscle is paretic, or what is referred to as an inhibitional palsy of the contralateral antagonist (Fig 5-8).
Figure 5-8 Palsy of right superior oblique muscle. A, With the palsied right eye fixating, little or no vertical difference appears between the 2 eyes in the right (uninvolved) field of gaze (1 and 4). In primary position (3), a left hypotropia may be present because the right elevators require less innervation to stabilize the eye in primary position, and thus the left elevators will receive less than normal innervation. When gaze is up and left (2), the RIO needs less than normal innervation to elevate the right eye because its antagonist, the RSO, is palsied. Consequently, its yoke, the LSR, will be apparently underacting, and pseudoptosis with pseudopalsy of the LSR will be present. When gaze is toward the field of action of the palsied RSO muscle (5), maximum innervation is required to move the right eye down during adduction, and thus the yoke LIR will be overacting. B, With unaffected left eye fixating, no vertical difference appears in the right field of gaze (1 and 4). In primary position (3), the right eye is elevated because of unopposed elevators. When gaze is up and left (2), the RIO shows marked overaction because its antagonist is palsied. The action of the LSR is normal. When gaze is down and left (5), normal innervation required by the fixating normal eye does not suffice to fully move the palsied eye into that field of gaze. (See also
Chapter 7, Fig 7-10.) (Reproduced with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:24–25.)
Vergences
Convergence is movement of both eyes nasally relative to a given starting position; divergence is movement of both eyes temporally relative to a given starting position. The medial rectus muscles are yoked muscles for convergence; the lateral rectus muscles are yoked for divergence. Incyclovergence is a rotation of both eyes such that the superior portion of each vertical corneal meridian rotates nasally; excyclovergence is a rotation of both eyes such that the superior pole of each vertical corneal meridian rotates temporally. Vertical vergence movement, though less frequently encountered, can also occur: 1 eye moves upward and the other downward. (See also Chapter 6.) See Table 5-2 for a classification of the vergence system. The roles of its various components are described below.
Table 5-2
Accommodative convergence of the visual axes Part of the synkinetic near reflex. A fairly consistent increment of accommodative convergence (AC) occurs for each diopter of accommodation (A), giving the accommodative convergence/accommodation (AC/A) ratio.
Abnormalities of this ratio are common, and they are an important cause of strabismus (see Chapter 8). With an abnormally high AC/A ratio, the excess convergence tends to produce esotropia during accommodation on near targets. An abnormally low AC/A ratio tends to make the eyes less esotropic, or even exotropic, when the patient looks at near targets. For techniques of measuring this ratio, see the discussion of the AC/A ratio under Convergence in Chapter 7.
Fusional convergence A movement to converge and position the eyes so that similar retinal images project on corresponding retinal areas. Fusional convergence is accomplished without changing the refractive state of the eyes and is activated when a target in the midline is seen with bitemporal retinal image disparity. See also Chapter 6.
Proximal (instrument) convergence An induced convergence movement caused by a psychological awareness of near; this movement is particularly apparent when a person looks through an instrument such as a binocular microscope.
Tonic convergence The constant innervational tone to the EOMs when a person is awake and alert. Because of the anatomical shape of the bony orbits and the position of the rectus muscle origins, the alignment of the eyes under complete muscle paralysis is divergent. Therefore, convergence tone is necessary in the awake state to maintain straight eyes even in the absence of strabismus. A practical application of this can be seen in esotropic patients under general anesthesia, who may show less esotropia or even exotropia with suspension of tonic convergence, which may lead to the incorrect assumption that a lesser amount of surgery should be done.
Voluntary convergence A conscious application of the near synkinesis.
Fusional divergence An optomotor reflex to diverge and align the eyes so that similar retinal images project on corresponding retinal areas. Fusional divergence is accomplished without changing the refractive state of the eyes and is activated when a target in the midline is seen with binasal retinal