When binocular eye movements are conjugate and the eyes move in the same direction, such movements are called versions. When the eye movements are dysconjugate and the eyes move in opposite directions, such movements are known as vergences.
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. Dextrocycloversion is rotation of the superior pole of the vertical meridian of both eyes to the patient’s right. Similarly, levocycloversion is movement of both eyes so that the superior pole of the vertical meridian rotates to the patient’s left.
Figure 4-6 The right inferior oblique muscle, viewed from below.
(Modified with permission from von Noorden GK. Atlas of Strabismus. 4th ed. St Louis: Mosby; 1983:9.)
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 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. See Figure 4-1, which shows the 6 cardinal positions of gaze and the yoke muscles whose primary actions are in those fields of gaze.
Hering’s law of motor correspondence states that when the eyes move into a gaze direction, there is a simultaneous and equal increase in innervation to the yoke muscles for that direction. Hering’s law has important clinical implications when the practitioner is evaluating a paralytic or restrictive strabismus. Because the amount of innervation supplied to both eyes is always determined by the fixating eye, the angle of deviation varies according to which eye is fixating. When the sound eye is fixating (prism over the affected eye when the prism alternate cover test is performed), the amount of misalignment is called the primary deviation. When the affected eye is fixating (prism over the sound eye when the prism alternate cover test is performed), 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 affected eye to the position of fixation. This extra innervation is shared by the yoke muscle in the sound eye, which causes excessive action of that muscle and a larger angle of deviation.
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 yoke muscles for convergence, and the lateral rectus muscles are yoked for divergence. With vertical vergence movements, one eye moves upward, and the other moves downward. Incyclovergence is a rotation of both eyes such that the superior pole of the vertical meridian is rotated nasally; excyclovergence is a rotation of both eyes such that the superior pole of the vertical meridian rotates temporally. Vergence movements are described in the following sections; see Table 4-2 for a classification of these movements.
Accommodative convergence of the visual axes
Part of the near reflex (also called near synkinesis, near triad), which consists of accommodation, convergence, and miosis. A certain amount of accommodative convergence (AC) occurs with each diopter of accommodation (A), giving the accommodative convergence/accommodation (AC/A) ratio.
Abnormalities of this ratio are common and are important causes of strabismus (see Chapter 8). With an abnormally high AC/A ratio, the excess convergence tends to produce esotropia during near fixation that is greater than esotropia at distance. An abnormally low AC/A ratio tends to make the eyes less esotropic, or even exotropic, when the patient looks at near targets. Techniques for measuring this ratio are discussed in Chapter 7 under Convergence.
A movement to converge and position the eyes so that similar retinal images project on corresponding retinal areas. Fusional convergence is activated when a target in the midline is seen with bitemporal retinal image disparity. See also Chapter 5.
Proximal (instrument) convergence
An induced convergence movement caused by a psychological awareness that the object of fixation is near. This movement is particularly apparent when a person looks through an instrument such as a binocular microscope.
Table 4-2 Classification of Vergence Movements
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 position of the eyes during complete muscle paralysis is divergent. Therefore, convergence tone is normally necessary in the awake state for the eyes to be aligned. For example, an esotropic patient under general anesthesia may become less esotropic or even exotropic with suspension of tonic convergence.
A conscious application of the near reflex.
A movement to diverge and position the eyes so that similar retinal images project on corresponding retinal areas. Fusional divergence is activated when a target in the midline is seen with binasal retinal image disparity. See also Chapter 5.
Fusional vertical vergence
A superior movement of one eye and inferior movement of the other to reduce vertical disparity so that similar retinal images project on corresponding retinal areas.
Intorsion of both eyes (incyclovergence) or extorsion of both eyes (excyclovergence) to reduce torsional disparity so that similar retinal images project on corresponding retinal areas. While it can be enhanced by special training, cyclovergence is normally very limited, and fusion of torsional disparity is mostly accomplished by sensory adaptation.
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.