Binocular Vision and Introduction to Strabismus
Kenneth W. Wright
In normal vision, both eyes are precisely aligned on an object of regard, so the images from that object fall on the fovea of each eye. Precise image orientation on corresponding retinal areas of each eye permits cortical processing, which results in the merging or fusion of the two images. This process is termed binocular fusion.There are two important aspects of binocular fusion: sensory fusion and motor fusion. This chapter discusses the process of binocular vision and provides an introduction to strabismus.
SENSORY FUSION
Sensory fusion is the cortical process of blending the images from each eye into a single binocular stereoscopic image. This fusing occurs as optic nerve fibers from the nasal retina cross in the chiasm to join the uncrossed temporal retinal nerve fibers from the fellow eye. Together, ipsilateral temporal fibers and contralateral nasal fibers project to the lateral geniculate nucleus and then on to the striate cortex. This division of hemifields does not totally respect the midline. There is significant overlap in the foveal area with some of the nasal foveal fibers projecting to the ipsilateral cortex and some of the temporal foveal fibers crossing to the contralateral cortex. Within the striate cortex, afferent pathways connect to binocular cortical cells that respond to stimulation of either eye. Retinal areas from each eye that project to the same cortical binocular cells are called cor- responding retinal points.In Figure 3-1, points “A” left eye and
“A” right eye, and points “B” left eye and “B” right eye, are cor-
3
70
responding retinal points. In humans, approximately 70% of the cells in the striate cortex are binocular cells whereas the minor- ity are monocular cells. Binocular cortical cells, along with neurons in visual association areas of the brain, produce single binocular vision with stereoscopic vision.
FIGURE 3-1. Vieth–Müller circle and the empirical horopter. By mathe- matical theorems, points on the Vieth–Müller circle should project to cor- responding retinal points. Point A stimulates the nasal retina of the left eye and the temporal retina of the right eye, and these retinal areas should mathematically correspond. Psychophysical experiments, however, show that the retinal architecture does not follow the mathematical circle of Vieth–Müller and that points on the empirical horopter stimulate corre- sponding retinal points. The bottom of the figure shows the fusion of the images from each eye into a binocular perception.
When a subject with normal eye alignment fixes on a target, that target falls on both foveas. Mathematical theory predicts that objects peripheral to the fixation target (points A and B in Fig. 3-1) will project to corresponding retinal points if the periph- eral objects lie on a circle that passes through the optical centers of each eye. This mathematically determined circle of points is called the Vieth–Müller circle. As is often the case for mathe- matical explanations of biological phenomena, physiological experiments have shown that the Vieth–Müller mathematical model only partially works for visual perception. Psycho- physical experiments indicate that the locus of points, which project to corresponding retinal points of each eye, is not a circle but actually takes the shape of an ellipse. This elliptical line of points, which project to corresponding retinal points, is the empirical horopterand is shown as a dotted line in Figure 3-1.
Remember, the location of the horopter is determined by the point of fixation. Objects located in front of or behind the empir- ical horopter will project to noncorresponding retinal points.
In Figure 3-2A, note that point “A” is distal to the empirical horopter and stimulates binasal retina. Point “B” in Figure 3-2B, which is proximal to the horopter, stimulates bitemporal retina. These binasal and bitemporal retinal points are non- corresponding retinal points, and images falling on these points are termed disparate images. Disparate images have the poten- tial for either producing stereoscopic vision or causing physio- logical diplopia.
Stereoscopic Vision
The empirical horopter is a theoretical locus of points, and is infinitely thin. All three-dimensional objects lie in front of and behind the horopter line; therefore, virtually all solid objects stimulate noncorresponding retinal points and result in dis- parate retina images. The brain, however, can merge or “fuse”
images from slightly noncorresponding retinal points. This finite area in front of and behind the horopter line where objects stimulate noncorresponding retinal points, yet are still fusible into a single binocular image, is called Panum’s fusional area (Fig. 3-3). Stimulation of noncorresponding retinal points within Panum’s fusional area will produce three-dimensional vision.
This ability for the brain to determine that images are falling on retinal points that are not exactly corresponding (i.e., disparate images) produces stereoscopic vision. Only horizontal retinal
A
B
FIGURE 3-2A,B. Empirical horopter and Panum’s fusional area. Objects that lie in front of or behind Panum’s fusional area will stimulate non- corresponding retinal points. (A) Patient fixating on the star in the center of the empirical horopter. Point A, which is distal to the horopter, stimu- lates the binasal retinal points that are noncorresponding. (B) Patient fix- ating on the same spot; however, point B is proximal to the Panum’s fusional area, and point B stimulates bitemporal retinal points that are noncorresponding. Point A in (A) would cause uncrossed diplopia, whereas point B in (B) would cause crossed diplopia. This type of diplopia is termed physiological diplopia.
image disparities produce stereoscopic vision; vertical dispari- ties do not. Panum’s fusional area is narrow at the center and gradually widens in the periphery reflecting the high resolu- tion–small receptive fields in the central visual field and low resolution–large receptive fields in the periphery. Large dis- placements are required for the peripheral retina to detect a change in receptive field.
Figure 3-3 shows a three-dimensional cube as a fixation target. Note that the cube lies in front of and behind the empiri-
Stereoscopic Image Empirical
Horopter
Panum's fusional
}area
FIGURE 3-3. Diagrammatic representation of stereoscopic vision. Note that any three-dimensional objects will straddle the empirical horopter and parts of that object will be in front of or behind the empirical horopter; this stimulates noncorresponding retinal points that provide stereoscopic vision so long as the three-dimensional objects fall within Panum’s fusional area.
cal horopter, projecting to noncorresponding retinal points. The fovea has high spatial resolution, so even small displacements off the horopter line (i.e., small image disparities) in the central visual field are detected, resulting in fine, high-grade stereo- scopic vision. In contrast, as one moves to the peripheral fields, the receptive field size enlarges and the spatial resolution decreases. The peripheral binocular visual fields are sensitive to large image disparities and provide coarse stereoacuity. This retinal architecture of high central resolution versus low periph- eral resolution explains the excellent stereoacuity from central fields and progressively poorer stereoacuity from peripheral binocular retinal fields.
Physiological Diplopia
If an object is too far off the horopter line and outside of Panum’s fusional area, then the images can no longer be fused and double vision may result (diplopia) (Figs. 3-2, 3-4). This type of double vision is a normal phenomenon and is termed physiological diplopia. Note that, in Figure 3-4, the pencil is in front of Panum’s fusional area and the pencil is, therefore, stimulating the temporal retinas of each eye. Because the temporal retina projects to the nasal visual field (opposite field), the observer per- ceives two pencils with the left image coming from the right eye and the right image coming from the left eye; this is called
“crossed diplopia,” and occurs with bitemporal stimulation.
Physiological diplopia would occur in everyday life; however, it is normally ignored or suppressed. You can experience physio- logical diplopia by simply fixating on a distant object several feet away then placing a pencil a few inches from your nose. While you are looking at the distant object, the pencil will appear double. This is crossed diplopia: when you close your right eye the left pencil disappears, and when you close the left eye the right pencil disappears. You can demonstrate that Panum’s fusional area is narrow centrally and wide in the periphery by moving the pencil held at near to the right or left, while main- taining fixation on a distance target. Observe that the physio- logical diplopia and image quality diminish when the pencil is moved into the peripheral binocular fields. (Remember to keep your fixation on a distant object while the pencil is held at near.) Objects distal to Panum’s fusional area stimulate binasal retinal points and can cause uncrossed diplopia (see Fig. 3-2A). You can experience uncrossed diplopia by fixating on a pencil a few
inches in front of you and observing that the distant objects are double (this may be difficult to see).
Stereoacuity Testing
Stereoscopic perception can be created from two-dimensional figures by presenting each eye with similar figures that are horizontally offset to produce bitemporal or binasal retinal image disparities. Bitemporal retinal stimulation within Panum’s fusional area gives the stereoscopic perception of an image coming toward the observer (Fig. 3-2B), and binasal retinal stimu- lation within Panum’s fusional area gives the perception of an image going away from the observer (Fig. 3-2A). Note that the upper circles in Figure 3-5 are displaced nasally. The displaced FIGURE 3-4. A pencil is seen in front of Panum’s fusional area; this stimulates noncorresponding bitemporal retinal points. Because the tem- poral retina projects to the opposite field (arrows), the patient perceives crossed physiological diplopia.
circles result in bitemporal retinal stimulation within Panum’s fusional area, and the circle will be perceived as a single circle coming up off the page. In contrast, temporal displacement of stereoscopic figures results in binasal retinal stimulation, with the perception of depth away from the observer and into the page. Most clinical stereoacuity tests present nasally displaced images to each eye by using mirror systems, red/green glasses
FIGURE 3-5. Diagrammatic representation of a contour stereogram.
Polarized glasses donned by the patient match the orientation of two polarized plastic plates on the stereo book, so one eye sees one plate and the fellow eye sees the other plate. The polarization is oriented vertically over the left eye and horizontally over the right eye, so the left eye views the left figure with the upper circle shifted to the right, and the right eye views the right figure with the upper circle shifted to the left. This nasal displacement of the circles stimulates bitemporal disparate retinal points and produces the stereoscopic perception that the upper circle is raised off the page. Titmus testing uses nasally displaced figures to produce stereoscopic images that come up off the page, towards the observer.
with corresponding red/green figures, or polarized glasses with corresponding polarized figure plates (see Fig. 3-5). These systems provide different images to each eye separately under binocular viewing and are termed haploscopic devices.
Stereoacuity can be quantified by measuring the amount of image disparity. The angle of disparity can be measured in seconds of arc. The minimum stereoscopic resolution is a dis- parity of approximately 30 to 40 s of arc. Stereoscopic resolution depends upon visual acuity, as poor vision in one or both eyes will decrease stereoacuity. A general guide on the effect of image blur on stereoacuity is seen in Table 3-1. Interpupillary distance also influences stereoacuity. The farther apart the two eyes, the greater the angle of visual disparity and the greater the stereo- scopic potential. Additionally, the closer an object is to the eyes, the greater the angle of disparity; therefore, the better the stereo- scopic view. As objects move away from the observer, the rela- tive interpupillary distance diminishes as does the visual angle, so stereoscopic vision decreases for distance objects.
CONTOURSTEREOACUITYTEST
Contour stereoacuity tests use stereoscopic figures with a con- tinuous contoured edge (Fig. 3-5). The Titmus test is a popular contour stereoscopic test and measures disparities from 3000 s arc (the big fly) to 40 s arc (ninth circle). Some pictures in the test are stereoscopic and others are flat (two-dimensional). The patient is required to identify which figure is stereoscopic.
Contour stereoscopic figures are clinically useful because the stereoscopic effect is obvious and easy to see, but they have the disadvantage of having monocular clues. Monocular clues allow patients who are stereoblind to identify the stereoscopic figures1,3; this occurs because each stereoscopic figure is made
TABLE 3-1. Visual Acuity (VA) and Titmus Stereoacuity.
Circles
9 40 s 20/25
8 50 s 20/30
7 60 s 20/40
6 80 s 20/50
5 100 s 20/60
4 140 s 20/70
3 200 s 20/80
2 400 s 20/100
1 800 s 20/200
Circles/seconds (s) of arc VA.4
up of two drawings, nasally shifted off center (see Fig. 3-5).
Patients with monocular vision can identify which figure is sup- posed to be stereoscopic, because that figure will be horizontally off center. Another type of monocular clue used by patients with alternating strabismus to falsely pass contour stereo tests is
“image jump.”1These patients alternate fixation between the two horizontally displaced figures and identify the figure that jumps back and forth. Monocular clues work for stereoscopic figures with large disparities and if the stereoscopic figure is framed so the displaced figure looks off center. The first three stereoscopic “circles” and first stereoscopic “animal” on the Titmus stereoacuity test can often be identified by using monocular clues, but stereo figures with smaller disparities are difficult to detect using monocular clues.
One way to help verify that the patient has true stereoacu- ity is to retest with the Titmus test book turned 90° and see if the patient still sees the stereoscopic target. With the test book turned 90°, the targets are not stereoscopic, but the monocular clues still work. If the patient again identifies the stereoscopic target, they are using monocular clues, not true stereopsis. For further verification, turn the book 180° (upside down) and see if the patient notes that the stereo targets have returned but are now projecting in an opposite direction away from the patient.
The Titmus “fly” can be useful in preverbal children as young as 1 to 2 years of age. If a child startles to the fly coming out of the page, then this is suggestive of gross stereopsis. Also, if a child clearly picks up the wings of the Titmus “fly” well off the page, this is good evidence for at least some peripheral fusion.
RANDOMDOTSTEREOACUITYTEST
Random stereograms consist of two fields of randomly scattered dots or specks, with one field of dots projected to each eye sep- arately through a haploscopic device. Each field of random dots is identical except for a group of dots that is displaced nasally.
The group of displaced dots can take the form of any recogniz- able shape, such as the square shown in Figure 3-6. The nasally displaced square of dots stimulates bitemporal retinal points and produces the perception that a single square of dots is coming up off the page. Random dot stereoacuity tests have an advan- tage over contour stereo tests, as random dot tests have almost no monocular clues, and a positive response indicates true stereopsis with few false-positive responses.8The problem with
random dot stereoacuity testing, however, is that many young, normal children and some normal adults have trouble seeing the random dot stereoscopic effect and falsely fail the test.
Monocular Depth Perception
Depth perception can occur without stereoacuity. Monocular vision can provide information regarding depth and the distance of an object. Motion parallax, shadows, object overlap, and the relative size of objects give us monocular clues of depth. Motion FIGURE 3-6. Diagrammatic representation of a Randot stereogram. The left eye sees one set of dots and the right eye sees a second set of dots.
The dots are identical, except for the dots within the square that have been horizontally displaced (nasally in the figure). Nasal displacement stimulates bitemporal disparate retinal points and produces the stereo- scopic perception that the square of circles is raised off the page. This clinical test for Randot stereoacuity consists of nasal displacement, so that the stereo images appear to come off the page.
parallax is the perception of a change in position of an object resulting from a change in position from where the object is viewed. For example, a monocular observer viewing a distant object will note that near objects move to the left as the observer moves his head to the right. Monocular clues can be so power- ful that one-eyed patients, or patients with large-angle strabis- mus, can successfully perform a variety of tasks that require keen depth perception. Professional athletes, microsurgeons, even ophthalmologists have been successful using monocular depth perception.2
Bifoveal Fusion
Marshall Parks coined the term “bifoveal fusion” or “bifixa- tion” to indicate the normal state of binocular fusion.7Bifoveal fusion includes high-grade stereoacuity of 40 to 50 s of arc, accu- rate eye alignment, and normal motor fusion. Patients with bifoveal fusion have normal retinal correspondence.
Rivalry
Rivalry, or as it is sometimes termed, retinal rivalry, is a condi- tion where a patient with normal binocular vision is presented with different images to corresponding retinal points of each eye. Instead of seeing two different images superimposed on each other (termed “confusion”), the subject perceives patchy dropout of each image where the images binocularly overlap.
Rivalry can be demonstrated most dramatically by presenting parallel lines to each eye with the lines rotated 90° in one eye (Fig. 3-7). The observer will perceive that some of the lines dis- appear in a spotty fashion as they cross over each other. You can experience rivalry by placing a pencil horizontally 2 inches in front of one eye and your index finger vertically 2 inches in front of the other eye. Note that there is patchy dropout of either the pencil or the index finger where they overlap. The rivalry phe- nomenon is often described as retinal rivalry; however, it is a complex interaction involving cortical inhibition. The presence of rivalry indicates the existence of bifoveal fusion potential.
Motor Fusion
Motor fusion is the mechanism that allows fine-tuning of eye position to maintain eye alignment. It acts as a locking mecha-
nism to keep the eyes aligned on visual targets as they move through space. Motor fusion also controls innate tendencies for the eyes to drift off target. These correctional eye movements that maintain binocular foveal alignment provided by motor fusion are termed fusional vergence movements.
Unlike version movements, in which both eyes move in the same direction, vergence eye movements are in the opposite direction; they are termed “disjunctive” and disobey Hering’s law. Convergence, for example, is invoked when one eye follows an object moving from distance to near and results in both eyes moving to the midline with the right eye moving left and the left eye moving right (Fig. 3-8A). You can experi- ence convergence by fixating on a pencil at arm’s length and slowly bringing the pencil to your nose. As the pencil approaches your nose, the eyes converge to hold alignment on the pencil. Convergence movements are the strongest vergence
A
B C
FIGURE 3-7A–C. Diagonal lines are presented to each eye with the lines oriented 90° to each other (A,B). The combined binocular perception is a patchy pattern, with lines from each eye being seen; however, because of rivalry, crossing lines are not seen (C).
movements, and there are several mechanisms that contribute to convergence (see Vergence Amplitudes, following).
In addition to convergence, there are two other vergence movements: divergence and vertical vergence (Fig. 3-8B,C).
Divergence is used to follow an object moving away and con- sists of the right eye moving right and left eye moving left. Ver- tical vergence is the weakest vergence movement and keeps our eyes from drifting vertically. Vertical vergence is depression of one eye with elevation of the fellow eye.
Measurement of vergence amplitudes and a discussion of the various mechanisms of convergence are presented next.
A
B
C
FIGURE 3-8A–C. Vergence. (A) Convergence of the eyes as the pencil approaches from the distance. (B) Divergence as the patient changes fixa- tion from a near target to a distance target. (C) Vertical vergence, as the patient vertically aligns the eyes to compensate for the vertical phoria or an induced deviation produced by a vertical prism.
INTRODUCTION TO STRABISMUS
Normally our eyes are well aligned so the foveas are aimed on the same visual target; this is termed orthotropia (Fig. 3-9). Stra- bismus is the term for ocular misalignment, or if there is an underlying tendency toward misalignment. Another term for strabismus is “squint.” This term comes from the fact that stra- bismic patients often squint one eye to block out one of the two images that they see. A manifest misalignment is called a heterotropiaor tropia for short. A tropia causes double vision (diplopia) if acquired after 7 to 9 years of age; however, children under 6 to 7 years of age will cortically suppress vision from the deviated eye. Cortical suppression is a neurological mechanism that allows children to eliminate diplopia. Children who alter- nate fixation between eyes (i.e., alternate suppression) will retain equal vision, but constant suppression of the deviated eye can cause decreased vision of the deviated eye, resulting in strabismic amblyopia.
In contrast, a hidden tendency for an eye to drift is termed heterophoria or phoria. Patients with a phoria have a latent tropia and use motor fusion to maintain proper alignment. One can demonstrate the latent deviation of a phoria by disrupting binocular fusion. Occluding or fogging the vision of one eye (either eye) will disrupt fusion, and the eye behind the occluder will deviate (Fig. 3-10). Identifying a phoria indicates that some degree of motor fusion is present. Orthophoria is the state of the eyes where there is no strabismus and not even a tendency for the eyes to drift (i.e., no phoria). Orthophoria is rare to non-
FIGURE 3-9. Normal eye alignment with image falling on both foveas.
existent, as virtually all normally sighted people, with normal bifoveal fusion, have a small phoria but maintain alignment through motor fusion. Thus, most normal people are orthotropic but heterophoric.
Phorias may spontaneously become manifest under condi- tions such as fatigue or illness that can cause central nervous system depression and diminish motor fusion. Central nervous system depressants also diminish motor fusion, and a patient with a large phoria may manifest their deviation after imbibing alcoholic beverages or taking sedatives. (This explains why the cowboy sees double after celebrating in town with one too many whiskies.) A large phoria that is difficult to control may sponta- neously become manifest, and this is called an intermittent tropia.
Strabismus most commonly occurs in infancy or childhood and is usually idiopathic or related to a refractive error. In most of these cases, the eye muscles are normal and the eye can rotate freely. Less often, mechanical restriction of eye movements (restrictive strabismus) or an extraocular muscle paresis (para- lytic strabismus) causes the strabismus. A blind eye may also drift, and this is termed sensory strabismus.
B A
FIGURE 3-10A,B. Alternate cover test in patient with an esophoria. (A) Eyes are straight; the patient has a tendency to cross (esophoria), but fusional divergence maintains proper alignment. (B) Left eye is covered, dissociating fusion and allowing the left eye to manifest the esophoria.
Note that the left eye turns in under the cover.
Ocular misalignment may be horizontal, vertical, torsional, or any combination of these. Strabismus is described by prefixes that tell the direction of the deviation: eso, turning in; exo, turning out; and hyper, vertical deviation. A suffix is added to the prefix to denote if the strabismus is a tropia or phoria. An esodeviation that is a tropia is termed an esotropia (ET) and a phoria is termed an esophoria (E); likewise, an exodeviation is either an exotropia (XT) or exophoria (X). The strabismic patient will have one eye fixing on a target and the fellow eye will deviate. With esotropia, the deviated eye turns in so the target image falls nasal to the fovea (Fig. 3-11). In exotropia, the eye
FIGURE 3-11. Alternating esotropia. Top diagram: right eye is fixing and the image is aligned with the right fovea while the image falls nasal to the left fovea as the left eye is deviated. Bottom diagram: left eye is fixing with the image falling on the left fovea and the image falling nasal to the right fovea as the right eye is deviated.
turns out and the target image is temporal to the fovea (Fig. 3- 12). Note that fixation can switch from eye to eye. According to Hering’s law, as the deviated eye moves into primary position, the fixing eye turns in the same direction to become the devi- ated eye (compare upper and lower drawings of Figs. 3-11 and 3-12).
Vertical strabismus can be categorized as hypertropia or hypotropia. Because of Hering’s law, a left hypertropia is the same deviation as a right hypotropia, depending on which eye is fixing (Fig. 3-13). In contrast to a horizontal deviation, when FIGURE 3-12. Alternating exotropia. Top diagram: right eye is fixing with the left eye and the image falling temporal to the fovea. Bottom diagram:exotropic left eye is fixing with a right exotropia and the image falling temporal to the right fovea.
describing a hyperdeviation we must identify which side the hypertropia is on, either right hypertropia (RHT) or left hyper- tropia (LHT). By convention, we usually refer to a vertical devia- tion as a hypertropia, rather than use the term hypotropia, unless there is an obvious restriction or paresis that keeps one eye in a hypotropic position. This convention has practical importance as it minimizes confusion over which terminology is used, thus reducing the risk of inadvertently operating for a right hypotropia when the patient actually had a right hypertropia.
Cyclotropia, or torsion, refers to a twisting misalignment around the Y axis of Fick. Excyclotropia (extorsion) is a tempo- ral rotation of the 12 o’clock position, whereas incyclotropia (intorsion) means a nasal rotation of the 12 o’clock position.
Normally the fovea should be aligned between the middle and the lower pole of the optic disc (Fig. 3-14, top). If the fovea is below the lower pole of the optic disc by direct view (above the disc in the indirect ophthalmoscopic view), this indicates objec- tive extorsion (Fig. 3-14, bottom left). A fovea oriented above the middle of the optic disc by direct view (below the middle in the indirect ophthalmoscopic view) indicates intorsion (Fig. 3-14, bottom right). Torsion can also be measured by the Maddox rod test, and this is termed subjective torsion. Torsional motor FIGURE 3-13. Alternating left hypertropia. Top left: right eye fixing and a left hypertropia. Top right: retinal image (x) is falling on the fovea (small dot) of the right eye; however, the left fovea is rotated down (left hyper- tropia) so the retinal image (x) is located above the fovea (small dot).
Bottom:left eye fixing with right eye turned down. Now the retinal image (x) falls below the right fovea, which is rotated up (right hypotropia).
fusion is weak to nonexistent; therefore, a tendency for torsional misalignment is manifest as a tropia and, for practical purposes, torsional phorias do not exist. There are consequently no tor- sional vergence eye movements. A small amount of torsional misalignment, however, is tolerated surprisingly well as the brain will accept up to 5° of torsional misalignment. Patients with a tropia less than 10 prism diopters (PD) will often have peripheral fusion and have a phoria coexisting with a small tropia. This condition is called the monofixation syndrome and is associated with peripheral binocular fusion, central fixation with the preferred eye, and central suppression of the foveal area in the fellow eye. Tropias greater than 10 PD preclude fusion, as the disparity of the images is too great to allow for even periph- eral fusion. Patients with a tropia greater than 10 PD will not have motor fusion and will not have a coexisting phoria.
FIGURE 3-14. Ocular torsions through the direct view (left eye). Top:
normal fovea to disc relationship with the fovea located along the lower half of the disc. Lower left: extorsion with the fovea below the lower half of the disc. Lower right: intorsion with the fovea above the lower half of the disc. In actuality, it is the disc that rotates around the fovea.
Prisms and Strabismus
Prisms are important tools for the diagnosis and treatment of strabismus, as they are used to measure and neutralize ocular deviations. A prism bends light toward the base of the prism (Fig.
3-15) because light has both particle and wave characteristics.
As light passes through the prism, the part of the light wave closest to the prism base has more prism to traverse than the part of the wave closest to the apex. This is analogous to a row of soldiers marching through a triangle of sand; the soldiers walk slowly through sand so those at the base of the triangle exit the sand after the soldiers at the apex. The direction of the march- ing soldiers turns toward the base of the triangle as they exit.
The ability of a prism to bend light is measured in prism diopters (PD). Light travels slower through the plastic prism than it does through air, so light toward the base of the prism takes longer to exit than light traversing the apex. The exit time differential causes the light to bend toward the base of the prism.
One prism diopter will shift light 1 centimeter (cm) at 1 meter (m) or a displacement of approximately 0.5°. A 20 PD esotropia
A B C
FIGURE 3-15A–C. Diagram of the effect of a prism over one eye. (A) Patient fixates on the X. (B) A prism is introduced, and the image is dis- placed toward the base of the prism and off the fovea. Note that the patient will perceive the image to jump in the opposite direction. Thus, a patient will perceive the image to jump in the direction of the apex of the prism. (C) Patient refixates to place the image on the fovea by rotat- ing the eye toward the apex of the prism. Note that when a prism is intro- duced, the patient will always refixate by rotating the eye in the direction of the apex of the prism.
would mean the eye turns in approximately 10°. When a prism is placed in front of one eye, it moves the image off the fovea, causing a perceived image “jump.” The retinal image will shift toward the base of the prism, but the perceived image jump is in the opposite direction, toward the apex of the prism; this is because the retinal images are reversed, right/left and up/down (Fig. 3-15A,B). To refixate on the shifted image, the eye will move in the direction of the prism’s apex, thus aligning the fovea with the new image location (Fig. 3-15C).
Prism Neutralization of a Deviation
Prisms can be used to optically neutralize or correct strabismus.
A prism acts to change the direction of the incoming image so the retinal image in each eye falls directly on the fovea. Neu- tralization occurs when enough prism is placed in front of the eye so the two foveas are aligned on the same object of regard.
For example, when a base-out prism (prism held horizontally with the apex directed toward the nose) is placed in front of the deviated eye of a patient with esotropia, the retinal image shifts temporally toward the fovea (Fig. 3-16). If the correct amount of prism is used, the retinal image will fall directly on the fovea of the deviated eye. Thus, as seen in Figure 3-16B, the deviation has been optically neutralized by the prism even though the eye is still anatomically deviated.
The rule for neutralizing a deviation is to orient the prism so the apex is in the direction of the deviation. For esotropia, the apex is directed nasally and, for exotropia, the apex is directed temporally. The apex is directed superiorly over a hypertropic eye and inferiorly over a hypotropic eye.
The prism can also be placed in front of the fixing eye (straight eye) to neutralize the deviation. If the prism is placed base-out in front of the fixing eye (Fig. 3-17), the retinal image will move temporal to the fovea (Fig. 3-17A,B). The fixing eye will see the image shift and will immediately rotate nasally to reestablish foveal fixation (Fig. 3-17). As the fixing eye rotates nasally, the deviated eye rotates temporally causing a version movement to the right (Fig. 3-17B). Therefore, when a base-out prism is placed in front of the fixing eye, both eyes move in the same direction as the apex of the prism, and both foveas shift into alignment (Fig. 3-17B,C). In Figure 3-17C, both eyes have shifted to the right, with the left eye now turned in nasally and the right eye now straight in primary position. The previously
deviated right eye is now straight and in alignment with the fixation target. Thus, one can place a prism in front of either eye or even split the prisms between the eyes to neutralize a strabismic deviation.
Prism-Induced Strabismus
A prism placed over one eye in a patient with straight eyes will induce a deviation and produce strabismus. A base-in prism induces esotropia, as the target image is displaced nasal to the fovea (Fig. 3-18). Likewise, a base-up prism induces a hypertropia and a base-out prism induces an exotropia.
A
B
FIGURE 3-16A,B. Prism neutralization. (A) Patient with an esotropia. (B) A prism is introduced to direct the image onto the fovea of the left eye, thus correcting, or neutralizing, the deviation.
A
B
C
FIGURE 3-17A–C. Neutralization of an esotropia by placing the prism in front of the fixing eye. (A) Esotropia with left eye fixing. (B) Prism is placed base out in front of the fixing eye (left eye), which displaces the image temporally off the fovea. The left eye rotates nasally to refixate to the displaced image. As stated by Hering’s law, both eyes rotate in the direction of the apex of the prism. (C) Patient fixing through the prism, left eye. The left eye has deviated nasally to put the image on the fovea.
The right eye has moved temporally and is also in alignment with the fixation target.
94
A
B
C
FIGURE 3-18A–C. Prism-induced esotropia. Patient with straight eyes and binocular vision is given an esotropia by placing a base-in prism over one eye. (A) Patient orthotropic with images falling on both foveas. (B) Base-in prism is placed before the left eye causing the image to move nasally off the fovea. Patient is fixing with right eye. (C) Patient now fixates with the left eye, viewing through the base-in prism. Left eye moves temporally to place the image on the fovea and, because of Hering’s law, the right eye moves nasally to displace the right retinal image nasally off the fovea.
Prism-Induced Vergence
Normal adult subjects with binocular fusion will see double when a prism is placed in front of one eye. If the prism is rela- tively small, the patient’s fusional vergence eye movements will be able to realign the eyes to keep the images appropriately placed on the foveas. The prism will initially invoke diplopia and the patient will realign the eyes within a second or two to replace the diplopia with single binocular vision. A base-out prism evokes fusional convergence, a base-in prism causes fusional divergence, and a base-up or base-down prism will evoke fusional vertical vergence. Figure 3-19 shows the steps of prism-induced convergence. A base-out prism placed over one eye will displace the retinal image off the fovea onto temporal retina, inducing an exotropia (Fig. 3-19A,B). The eye behind the prism moves nasally to refixate to the fovea and the fellow eye moves temporally in a version movement (Hering’s law) (Fig. 3- 19B). Diplopia occurs briefly until fusional convergence is used to realign the eyes so retinal images can fall directly on each fovea (Fig. 3-19C,D). The key aspect of the convergence move- ment is the nasal fusional movement of the eye without the prism (Fig. 3-19C). Note that, after prism-induced strabismus in a patient with fusion, a compensatory vergence movement will occur in the eye without the prism (Fig. 3-19C). Prism-induced strabismus in a patient without fusion results in a version move- ment of both eyes without a subsequent vergence movement (see Fig. 3-18C).
Fusional Vergence Amplitudes
Vergence movements compensate for phorias and keep the eyes aligned as targets move in depth throughout space. A patient with an exophoria uses convergence; those with esophorias use divergence, and hypertropias are controlled with vertical ver- gence. Convergence is by far the strongest of the vergence move- ments and can be strengthened by eye exercises if convergence is ineffective. Divergence is relatively weak and does not sig- nificantly improve with eye exercises. The strength of vergence movements can be measured in prism diopters and is called fusional vergence amplitudes.
Fusional vergence amplitudes are measured by inducing a deviation to stimulate a motor fusion to correct the induced deviation. Induce an exodeviation to test convergence (base-out
prism), an esodeviation for divergence (base-in prism), and a hyperdeviation for vertical vergence. Start by inducing a small deviation that can be fused and gradually increase the deviation until vision is blurred (blur point), then increase until fusion breaks (break point). A deviation can be induced by placing prisms (usually in the form of a prism bar) over one eye or by A
B
FIGURE 3-19A–B. Four steps of prism convergence. (A) Eyes are well aligned in a patient with good fusional convergence. (B) Exophoria is created by introducing a base-out prism in front of the left eye. Patient ini- tially fixates with the left eye, causing a version movement to the right, thus placing the left fovea on the image.
C
D
FIGURE 3-19C–D. (C) Because of Hering’s law, the right eye also rotates and the image is now off the right fovea. To compensate for this, patient exercises fusional convergence and the right eye rotates nasally to put the image on the fovea; this is a vergence movement in distinction to the version movement seen in (B). (D) Patient is once again fusing, using fusional convergence to maintain eye alignment on the fixation target.
Note that the eye behind the prism is deviated nasally. The base-out prism actually induces an exophoria, even though the eye behind the prism is nasally deviated and looks esotropic.
moving the amblyoscope arms off parallel. Measure near- convergence amplitudes by placing a base-out prism bar over one eye, starting with 4 PD, having the patient fixate on an accom- modative target at a distance of 33 cm. Then, move the bar up slowly to increase the base-out prism. The eye behind the prism bar will progressively turn in to converge as the prism is increased. The greatest prism that the patient can fuse is the fusional vergence amplitude.Prisms larger than this will break fusion and one eye will turn out, usually causing diplopia. Have the patient note when the fixation target blurs (i.e., blur point), and when it becomes double (i.e., break point). Table 3-2 shows normal fusion vergence amplitudes based on the break point.
The maximum base-out prism that can be fused is around 30 PD (convergence), the maximum base-in prism that can be fused is 6 to 10 PD (divergence), and the maximal vertical prism that can be fused is usually 2 to 3 PD (vertical vergence). In certain conditions, divergence and vertical vergence fusional amplitudes can be quite large. Patients with congenital superior oblique palsy, for example, can have vertical fusion vergence amplitudes up to 25 to 30 PD.
Types of Convergence
There are various mechanisms of convergence; these include fusional convergence, accommodative convergence, tonic con- vergence, voluntary convergence, and proximal or instrument convergence.
FUSIONALCONVERGENCE
Fusional convergence is based on binocular vision. Occluding, or severely blurring the image of one eye, will disrupt fusional convergence; however, convergence mechanisms still function when binocular vision is suspended.
TABLE 3-2. Normal Fusion Vergence Amplitudes.
Distance Near
(6 m) (1/3 m)
Convergence 20–25 PD 30–35 PD
Divergence 6–8 PD 8–10 PD
Vertical vergence 2–3 PD 2–3 PD
PD, prism diopter.
ACCOMMODATIVECONVERGENCE AND THENEARREFLEX When an object approaches from distance to near, the images falling on the retina are displaced temporally, then blur and enlarge. These retinal image changes stimulate the near reflex.
The near reflex includes accommodation, convergence, and pupillary miosis. The ciliary muscles contract to increase the lens curvature and focus the image (accommodation). Contrac- tion of both medial rectus muscles occurs to keep the eyes aligned on target (convergence), and the pupil constricts to increase the depth of focus. The synkinetic reflex of accommo- dation and convergence is termed accommodative convergence.
Accommodation is one of the main drivers of convergence. For any individual, a specific amount of accommodation will result in a specific amount of convergence. The quantitative relation- ship between the amount of convergence associated with an amount of accommodation is referred to as the AC/A ratio (accommodative convergence/accommodation). A high AC/A ratio indicates overconvergence whereas a low AC/A ratio indi- cates convergence insufficiency. Patients with a high AC/A ratio are predisposed to developing esotropia (crossed eyes) at near, and a low AC/A ratio causes an exotropia (eye turning out) at near. The normal AC/A ratio is between 4 and 6 PD of conver- gence for every diopter of accommodation. Patients with wide interpupillary distances (PD) will have to have a relatively high AC/A ratio to converge sufficiently and keep both eyes aligned on near targets. The methods for measuring the AC/A ratio are described in Chapter 5.
TONICFUSIONALCONVERGENCE
Tonic fusional convergence is a type of fusional convergence that persists even after monocular occlusion is introduced; this is a form of proprioceptive eye position control, which keeps the eyes converging even after one eye is occluded. Tonic fusional convergence dissipates with prolonged monocular occlusion.
Patching one eye for 30 to 60 min eliminates most tonic fusional convergence. Tonic fusional convergence is referred to as tena- cious proximal fusion by Kushner.5
VOLUNTARYCONVERGENCE
Voluntary convergence is voluntarily invoked. Comedians use this to cross their eyes, and patients will voluntarily converge to produce convergence nystagmus.
PROXIMAL ORINSTRUMENTCONVERGENCE
This type of convergence is induced by a psychological aware- ness of an object at near, or when one views an object through an instrument such as a microscope.
Comitant Versus Incomitant Strabismus
Strabismus can be classified into two broad categories: comitant and incomitant. Comitant strabismus is when the deviation measures the same in all fields of gaze. Most types of congeni- tal and childhood strabismus are comitant. With comitant stra- bismus, both eyes move together equally well and there is no significant restriction or paresis. Comitant strabismus is usually a “good” sign and indicates that the strabismus is not secondary to a neurological problem. Occasionally, however, acquired neu- rological disease processes, such as early-onset myasthenia gravis, chronic progressive external ophthalmoplegia (CPEO), or even a mild bilateral sixth nerve palsy, can initially present as a clini- cally comitant strabismus.
Incomitant strabismus means the deviation is different in different fields of gaze. In the vast majority of cases, incomitance is caused by a limitation of ocular rotations secondary to ocular restriction or extraocular muscle paresis.
Causes of ocular restriction include a tight or stiff muscle and periocular adhesions to the eye. Muscle paresis can be caused by a lack of innervation (i.e., third, sixth, or fourth nerve paresis), traumatic muscle damage, an overrecessed or lost muscle, or neuromuscular junction disease such as myasthenia gravis.
Figure 3-20 shows an example of an incomitant esotropia secondary to limited abduction of the left eye. When the patient in Figure 3-20 looks to the left, the left eye cannot fully abduct; thus, the right eye overshoots and creates an esotropia (ET) that increases in leftgaze (Hering’s law of yoke muscles). In this example, the limited abduction could be due to either restriction (e.g., a tight left medial rectus muscle or a nasal fat adherence scar to the globe) or paresis (e.g., left sixth nerve palsy or left slipped lateral rectus muscle). Methods for diagnosing restriction and paresis are presented in Chapter 5.
Primary Versus Secondary Deviation
Patients with incomitant strabismus secondary to ocular restric- tion or muscle paresis will show a larger deviation when the eye with limited ductions is fixing (secondary deviation) than when the eye with full ductions fixates (primary deviation); this is in accord with Hering’s law. As shown in Figure 3-21, the primary deviationis small because relatively little innervation (1) is needed to keep the eye in primary position when the nonparetic
A B
FIGURE 3-20. Left lateral rectus paresis. In primary position, there is a moderate esotropia. In right gaze, the esotropia (ET) diminishes, and in left gaze the esotropia increases. A tight left medial rectus muscle would give the same pattern of incomitance.
FIGURE 3-21A,B. Left sixth nerve palsy. (A) Normal right eye fixating with little effort. Only 1 innervation is needed to put the eye on target;
there is a small esotropia of 25 PD. (B) Change of fixation to the left eye.
Because the left lateral rectus muscle is weak, it requires 4 innervation to bring the left eye to primary position to view the target. The right medial rectus muscle is the yoke muscle to the weak left lateral rectus muscle, so the right medial rectus muscle also gets 4 innervation. The
4 innervation of the normal right medial rectus results in a large esotropia of 50 PD.
right eye is fixing (Fig. 3-21A). The paretic eye receives the same
1 innervation and turns in slightly because the left lateral rectus muscle is slightly weaker than its antagonist, the left medial rectus muscle. The secondary deviation is larger because the weak left lateral rectus muscle must receive a tremendous amount of innervation (4) to bring the left eye into primary position when the paretic eye fixates (Fig. 3-21B). Both the paretic left lateral rectus and its yoke muscle, the right medial rectus, receive 4 innervation because of Hering’s law. This excess drive to the healthy right medial rectus muscle causes a large secondary nasal deviation of the right eye. This same mechanism of primary and secondary deviations also applies to restrictions.
Primary overaction of oblique muscles can also cause incomitance. What we clinically refer to as primary muscle overaction, however, may actually represent a previous paresis of the antagonist and secondary overaction of the agonist muscle.
References
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4. Donzis PB, et al. Effect of binocular variations of Snellen’s visual acuity on Titmus stereoacuity. Arch Ophthalmol 1983;101:930–932.
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