CHAPTER 15

Examination of the Patient—V

DEPTH PERCEPTION

tereopsis is an epiphenomenon of normal bin- to normal levels which are reached by the sixth Socular vision (see Chapter 2). Its presence or month of life. Interestingly, this rapid rate of matu- absence is an important indicator of the state of ration far exceeds that of visual acuity.11 The dura- binocularity in patients with ocular motility disor- tion of the plasticity period of in hu- ders. Barring a few notable exceptions (see Chap- mans still needs to be established. For a review of ter 16), patients with essential infantile esotropia the literature, see Teller31 and Birch.5 are stereoblind or, at best, have markedly reduced stereopsis, and the potential for regaining it is practically nil. In childhood with a later Stereopsis and Strabismus onset or in adults with acquired strabismus it is an important therapeutic goal to reestablish stere- Patients with a large manifest deviation do not opsis. Whether this can be accomplished depends have useful stereopsis in casual seeing. Neverthe- on many variables, among them the age of onset less, they can function quite well in space, making and the duration of the strabismus and the com- use of nonstereoscopic clues to depth perception, pleteness of ocular realignment. especially if the strabismus is of early origin. They may have trouble with fast-moving objects, such as flying balls, and this experience may be frustrat- Development of Stereopsis ing to young children. However, when the strabis- mus is acquired later in life the loss of stereopsis Depth perception on the basis of binocular dispar- is felt acutely and may present a real handicap. It ity is not fully developed at birth. Several studies appears as if stereopsis is useful in the comprehen- using different paradigms such as line stereograms sion of complex visual presentations and those and a preferential looking procedure, random dots requiring good hand-eye coordination. Although with a forced-choice preferential looking tech- the importance of stereopsis is often stressed, stud- nique, and random dots with visually evoked re- ies addressing the functional effects of stereo- sponses have shown remarkably consistent find- scopic deficits are sparse.8 ings: stereopsis is absent in almost all infants less It is always interesting and useful to determine than 3 months old, after which it rapidly develops whether a patient with strabismus has stereopsis 298 Examination of the Patient—V 299 or the potential for such. Some patients may re- there should be fiducial marks that permit the spond to disparate stimulations with a degree of examiner to check whether both eyes are used stereopsis if the targets are placed at the objective simultaneously. angle, as in a major amblyoscope. Some patients (e.g., intermittent exotropes) may respond with Major Amblyoscope or Stereoscope good even when a stereoscope is used, although they seemingly may be unable The targets may be opaque or transparent and may to superimpose dissimilar targets. Such patients be used in a major amblyoscope or stereoscope. require strong fusional stimuli to keep their eyes Both devices have mechanically separated fields aligned and to fuse. When they do, they gain of view, are set optically at infinity, and use ex- motor and sensory fusion, often with a high degree changeable targets. The advantage of the major of stereopsis. amblyoscope is that its arms can be set at the Some ophthalmologists use stereoscopic tests patient’s angle of deviation, thus allowing control to determine whether patients with small or inter- of the retinal area being stimulated. Similarly the mittent deviations have foveal suppression. If the stereoscope may be used with prisms, but this stereoscopic threshold is low enough, they con- procedure may not be accurate, and the distortions clude that there is no foveal suppression.27 A posi- induced by prisms may become bothersome. tive result is certainly conclusive, but a negative The number and variety of targets are limited result does not necessarily mean that foveal im- only by the ingenuity of the designer and user, but ages are completely suppressed. There are patients standard sets of targets and cards are commercially who fuse all but disparate retinal stimuli, which available for the different major amblyoscopes are selectively suppressed. and stereoscopes. Targets of special interest in the A positive stereoscopic response of a patient present context are those that contain objects with with a neuromuscular anomaly of the eyes at any differing amounts of disparity (e.g., the Keystone fixation distance and in any part of the binocular DB6 card), so that they appear at different relative field is of paramount importance prognostically depth distances. The object seen in depth, which and in directing treatment. This finding makes it has the least disparity, denotes the patient’s stereo- mandatory that every effort be made, both nonsur- scopic threshold. gically and surgically, to restore to the patient full binocular cooperation with stereopsis at all fixa- Stereogram tion distances and in every part of the field. Testing for stereopsis should always be done A useful clinical application can be made of the after operations have properly aligned the eyes. simple stereogram consisting of eccentric circles, The findings may give indications whether and one set seen with each eye (see Fig. 2–15). If the how to follow up the operation by nonsurgical patient reports that two fiducial marks and two treatment. circles are seen, but not in depth, one should inquire whether the two circles are concentric. They cannot be seen concentrically unless they Testing for Stereopsis are also seen stereoscopically. If they are seen eccentrically, one may now ask whether the inner Equipment for testing stereopsis ranges from sim- circles are closer to the right or left of the outer ple equipment to complex laboratory apparatus. circle. The patient’s answer determines whether Only tests that the ophthalmologist can conve- the disparate elements are suppressed in the right niently apply in the office are discussed in this or the left eye. section. A test for stereopsis must incorporate two essential features. The two eyes must be dissoci- Titmus Stereo Test ated; that is, each eye must be presented with a separate field of view, and each of the two fields cards dissociate the eyes optically. A or targets must contain elements imaged on corre- vectograph consists of Polaroid material on which sponding retinal areas. Thus a frame of reference the two targets are imprinted so that each target is is provided, and disparately imaged elements can polarized at 90Њ with respect to the other. When be fused and seen stereoscopically. In addition, the patient is provided with properly oriented Po- 300 Introduction to Neuromuscular Anomalies of the Eyes

FIGURE 15–1. The Titmus Stereo Test.

laroid spectacles, each target is seen separately Last, the Titmus test contains nine sets of four with the two eyes. This principle is used in the circles arranged in the form of a lozenge. In this Titmus Stereo Test (Fig. 15–1). In this test a gross sequence the upper, lower, left, or right circle stereoscopic pattern representing a housefly is pro- is disparately imaged at random with thresholds vided to orient the patient and to establish whether ranging from 800 to 40 seconds of arc. If the child there is gross stereopsis (threshold: 3000 seconds has passed the other tests, he or she is now asked of arc). In testing young children, one must ask to ‘‘push down’’ the circle that stands out, begin- questions the child will understand. For example, ning with the first set. When the child makes one may ask the child to take hold of the wings mistakes or finds no circle to push down, the of the fly. If the child sees them stereoscopically, limits of stereopsis are presumably reached. the child will reach above the plate. It is amusing If there is doubt whether the patient actually to watch the child’s startled look when he or she does see stereoscopically, one may occlude one does so. It is indeed an eerie feeling not to have eye and inquire whether there is a difference in a tactile sensation of a seen object. Some children, appearance, say, of the housefly, with one or both though they have stereopsis, will touch the wings eyes open. And since only horizontal disparity on the plate because they ‘‘know’’ they are there. produces stereopsis, one can also turn the plate The examiner must explain to these children that 90Њ, which should block out the stereoscopic ef- he or she does not inquire about what they know, fect. but what they see. Because of its simplicity, the Titmus Stereo The Polaroid test also contains three rows of Test is widely used. On the basis of this test alone, animals, one animal in each row imaged dispa- however, one is not always justified in stating rately (thresholds: 100, 200, and 400 seconds of simply that ‘‘the patient has no stereopsis,’’ that arc, respectively). The child is asked which one is, that there is no sensitivity for disparate stimuli. of the animals stands out. The animal figures con- One must keep in mind that the vectograph test is tain a misleading clue. In each row one of the used for testing near vision. Some patients sup- animals, correspondingly imaged in two eyes, is press disparate stimuli at near but respond to them printed heavily black. A child without stereopsis in distance fixation, or vice versa, usually when will name this animal as the one that stands out. the deviation is intermittent at one fixation dis- Examination of the Patient—V 301 tance and constant at the other. If such a pattern when fixating alternately and utilize this clue to is suspected, it is always wise to supplement the pass the Titmus test despite the fact that they may vectograph test with a projected vectograph test at be stereoblind on any of the tests using random distance fixation (Polaroid Vectographic Project- dots.29 Archer1 described a test based on dynamic O-Chart, American Optical Reichert) or with the circles designed to mimic the Titmus circles as B-VAT (Mentor) projection device. closely as possible, while eliminating lateral dis- In recent years much emphasis has been placed placement cues as well as the possibility of pass- on the use of stereoacuity testing as a screening ing the test by alternation. method to detect anomalies of binocular function.9, 29, 30 Normal stereoacuity is said to preclude sup- Random-Dot Stereograms pression, , or heterotropia, and a sub- normal test result may indicate the presence of To avoid any such visual clues, two tests are such anomalies. In applying the Titmus test as a available that use random-dot stereograms.2 The screening device, Simons and Reinecke29 found physiologic principle underlying these tests has that, with the exception of the fine stereoacuity been discussed in Chapter 2. Random-dot stereo- circles 5 to 9, this test often is unreliable in differ- grams are devoid of any monocular clues, and the entiating patients with amblyopia and heterotropia patient has no way of guessing what the ster- from those with normal vision. Moreover, the Tit- eofigure is and where it is located on the test mus test is capable of indicating an artifactual plate.13 Reinecke and Simons28 introduced the ran- stereocapability when none actually exists (see dom-dot E test (RDT) (Fig. 15–2), which contains also Ko¨hler and Stigmar14). Some of the circles of three cards and Polaroid spectacles. One card is a the Titmus test may be selected even by stereoblind bas-relief model of the stereotest figure and is observers because they look ‘‘different’’ and not used to show the child what to look for. One of because they are seen stereoscopically. Some pa- the two other test cards contains the E stereo tients notice an image jump in the disparate por- figure, and the other is stereoblank with an identi- tions of the test target (e.g., the wings of the fly) cal random-dot background. The test is performed

FIGURE 15–2. Random-dot E test set. (From Simons K, Reinecke RD: Amblyopia screening and stereopsis. In Symposium on strabismus: Transactions of the New Orleans Academy of Ophthalmol- ogy. St. Louis, Mosby–Year Book, 1978, p 15.) 302 Introduction to Neuromuscular Anomalies of the Eyes

FIGURE 15–3. A, The TNO test. B, The random-dot stereogram offers no monocular clues as to the presence of a large circle in the center of the upper right quadrant and a smaller circle in the center of the lower left quadrant. (From Noorden GK von: Present status of sensory testing in strabismus. In Symposium on Strabismus: Transactions of the New Orleans Academy of Ophthalmol- ogy. St. Louis, Mosby–Year Book, 1978, p 51.)

by holding both test cards 50 cm in front of the TNO Test patient, who is then requested to indicate which card contains the letter E. The test is simple to Another procedure, the TNO test, is based on a perform, and the patient will give a ‘‘pass’’ or similar principle but has the advantage of eliciting ‘‘fail’’ response. It can be quantitated by increas- quantitative responses without changing the test- ing the testing distance from the patient. Many ing distance. This test uses a pair of red-green modifications of the RDT have become available spectacles and a test booklet (Fig. 15–3). Each in the meantime.6 Random-dot stereopsis can be test plate in the booklet consists of a stereogram measured also for distance with the Mentor B- in which the half-images have been superimposed VAT II-SG computerized testing system (Mentor and printed in complementary colors (anaglyphs). O & O, Norwell, MA). This is particularly useful The test plates, when viewed binocularly with red- in intermittent exotropia.32 green spectacles by a normal subject, will elicit Examination of the Patient—V 303 perception of an image in depth. The TNO test is normals from abnormals. These authors further graded to provide retinal disparities ranging from stated that because of the fact that more than 40% 15 to 480 seconds of arc. Comparative studies of normal children demonstrate stereoacuity of have shown that this test compares favorably with less than 40 seconds of arc, random-dot testing is the Titmus test when used as a screening not a real measure of a biological function. device.26, 33 Together with the Lang test (see be- Awaya et al.4 studied the effects of aniseikonia low) it is the preferred test in our clinic. It must on stereopsis measurements. With their aniseiko- be emphasized, however, that even random-dot nia test, they found that aniseikonia of 7% to 13% testing of stereopsis is not a fail-safe method to is still compatible with binocular fusion. However, assess visual acuity and binocular function in pre- aniseikonia of greater than 5% is incompatible school and school-age children, since normal lev- with testing higher levels of stereoacuity with the els of stereoacuity have been observed in anisome- Titmus and TNO tests. tropic and visual deprivation amblyopia.3, 7, 21 How should stereopsis, determined with any of the Lang Test tests, be recorded? Cards and that attempt to qualify stereopsis are graded in differ- Occasionally, young children will refuse to wear ent ways. Some use artificial scales (such as the Polaroid or red-green spectacles, and observing Sheppard scale); many speak of percentage of the position of the eyes while the patient is being stereopsis, assuming a certain threshold to mean tested for stereopsis may be desirable. To over- 100%. All this is misleading and arbitrary. The come these difficulties, Lang17, 18 reported a new only proper way to record stereopsis is by the test (the Lang test) based on panographic presenta- amount of disparity incorporated into the target. It tion of a random-dot pattern. Glasses are not is unequivocal, and it should be generally under- needed to recognize the stereoscopic images of a stood when it is stated that a patient has stereopsis star, a car, and a cat (Fig. 15–4) embedded in with a threshold of 400 or 100 or 40 seconds of random dots on the test card. A separate image is arc or whatever the threshold may be. provided to each eye through cylindrical lenses Lam and coworkers15 evaluated the response of imprinted on the surface lamination of the test normal subjects to various visual function tests, card (Fig. 15–5) When held at a testing distance including stereopsis. They found a wide range of 40 cm in the frontoparallel plane in front of the of responses in completely normal subjects, thus patient (Fig. 15–6), the disparity of the car and raising the question which level of stereopsis re- star is 600 seconds and of the cat 1200 seconds flects normalcy (see also Fisher9). It appears as a of arc.17 A revised version of this test (Lang II difficult task to identify a cutoff value separating test)19 with smaller disparities and a less dense

FIGURE 15–4. Stereoscopic images embedded in random dots of the Lang test. (From Lang J: A new stereotest. J Pediatr Ophthalmol Strabismus 20:72, 1983.) 304 Introduction to Neuromuscular Anomalies of the Eyes

and the object is seen in depth does recognition take place.

Two-Pencil Test The two-pencil test, though somewhat crude, indi- cates how well a child is able to cope with a simple visual-motor task that is at least partially based on intact stereopsis. The two-pencil test was popularized by Lang but must have been known at least 388 years ago (1613) as shown by a sketch by Peter Paul Rubens to illustrate Aguilonius’ textbook on optics12 (Fig. 15–7). In this illustra- tion, perhaps the oldest one available that shows the superiority of binocular over monocular vi- sion, the cherub teasingly holds a vertical rod in front of the scholar who tries to touch the rod with his index finger from the side while keeping his left eye closed. He will not accomplish this task easily, of course, because his stereopsis can- not function with one eye closed, and the three cherubs anticipate the scholar’s apparent lack of skill with great merriment. FIGURE 15–5. Cylinder gratings provide separate im- We agree with Lang16 that the test is better ages for each eye. (From Lang J: A near stereotest. J Pediatr Ophthalmol Strabismus 20:72, 1983.) performed by approaching the rod from above, since this makes better use of horizontal disparity detectors and approximates daily manual tasks that require good stereopsis, such as pouring milk into arrangement of random dots has become available. a glass or hitting a nail with a hammer. There is One of the stimuli in the Lang II test is perceived no question that monocular clues to depth percep- binocularly and serves as a control mark. The tion (see p. 25) also are involved in completing subject can see it also in the absence of stereopsis. this test. However, the drastic change of perfor- Test results obtained with the older and newer mance when one eye is covered or, for instance, version of the Lang test have been reported to be when a child is fusing through bifocals but has a comparable.25 The advantage of the Lang I test is manifest deviation when looking through the up- that it can be performed in children as young as 6 per segments suggests that stereopsis must be in- months of age. If the baby stares for a few seconds volved to a large extent in this visual task. The at the card one can infer the presence of stereopsis, test is performed as shown in Figure 15–8. Its following the same reasoning underlying the pref- threshold values have been estimated to be be- erential looking testing technique. tween 3000 and 5000 seconds of arc, depending Stereo tests that use random dots are an accu- on the subject’s interpupillary distance and arm rate and established method to measure stereoacu- length.20 ity; however, the results obtained with different Finally, we must mention recent developments tests will vary widely.21 As stated in Chapter 2, aimed at testing stereopsis objectively in infants. testing based on random dots exposes the child to With the current emphasis on early diagnosis and visual demands that are different from and more treatment of strabismus, such efforts are of more difficult than those prevailing under more casual than theoretical interest. The principle of such conditions of seeing. For instance, random-dot tests is based on the ability to elicit optokinetic tests contain no information about the shape or nystagmus2, 10 or saccadic eye movements22 by nature of the object hidden in the visual noise of electronically generated stereograms moving back random dots. Only when the images from the right and forth on a television screen. Although such and left eye are combined at the neural level methods are still largely confined to the laboratory, Examination of the Patient—V 305

FIGURE 15–6. The Lang test. The child points to the stereoscopic image. (From Lang. J: A new stereotest. J Pediatr Ophthalmol Strabismus 20-72, 1983.)

FIGURE 15–7. Illustration by Peter Paul Rubens in Aguilonius’ textbook on optics. (From Jaeger W: Die Illustrationen von Peter Paul Rubens zum Lehrbuch der Optik des Franciscus Aguilonius/1613. Heidelberg, Verlag Brausdruck, 1976, p 36.) 306 Introduction to Neuromuscular Anomalies of the Eyes

one would hope that simplified equipment will eventually become available for use in a clinical environment.

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