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 stereopsis 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 strabismus 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 stereoscopic acuity 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 Vectograph 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.