Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology Current Thought and A Practical Guide_Wilson, Saunders, Trivedi_2008

vision is re-assessed through the prism. If the deviation has increased, the prism is increased accordingly and the process repeated until the deviation either stabilizes, exceeds the amount available in a temporary prism, or over-corrects the strabismus. The goal of prism adaptation is to determine the target angle for surgery, and to predict the sensory and motor outcome with realignment of the eyes.

Both diagnostic occlusion (described in Sect. 10.2.1) and prism adaptation can be used to confirm the full angle of strabismus measured with the cover test. One method may be better than the other, depending on the case. For example, patients with some control over the deviation, particularly if the deviation was observed to build up gradually with prism and alternate cover testing, are best evaluated with diagnostic occlusion. Prism adaptation should be used with caution in these cases, as those with good motor fusion, including patients with strabismus and anomalous correspondence (see Sect. 10.3.2) may undergo phoria adaptation in which the tonic vergence system adapts to the prism, temporarily resetting the baseline eye alignment and resulting in an

“eating up” of the prism [27]. In these cases, prism adaptation is less a test to determine the full angle than a test of the limits of tonic and fusional vergence amplitudes. These patients will be at increased risk for over-correction if surgery is done for the prismadapted angle.

Cases of distance-near disparity, where the magnitude of the deviation changes by 10 prism diopters or more with a change in fixation distance, may be investigated using diagnostic occlusion or prism adaptation. Diagnostic occlusion is typically used more for the exodeviation that is greater at distance, and often reveals a larger near deviation approaching or even exceeding the distance angle. Prism adaptation, on the other hand, is advised for surgical esodeviations that are greater at near. When prism adapted for the larger near angle, the distance esodeviation may build or the prism may over-correct [2, 25]. This method is very useful in predicting the likelihood of over-correction if surgery is done for the larger angle.

In cases of horizontal strabismus in which the distance and near deviations differ by more than 10 prism diopters, it is often necessary to calculate the AC/A ratio. Just as one would never assume that all exodeviations greater at distance are caused by a

high AC/A, one should never take for granted that the ratio is high in all cases of near esotropia exceeding the distance amount. Accommodative convergence is only one type of convergence contributing to the near alignment. Abnormalities of proximal, fusional, and tonic convergence are also possible, and it is important to distinguish between them, as this will impact management options.

There are three methods that have been used to assess the AC/A ratio. The first is the Clinical Method, which simply entails a comparison of the distance and near measurements in primary position. Though quick and easy, it is the least accurate of all the methods and is not recommended. It does not control for the effect of proximal or fusional convergence, does not quantify the ratio, and should not be used to predict if bifocals would be effective.

The Heterophoria Method is based on the dis- tance-near disparity in measurement, and the geometry of convergence using the interpupillary distance as the base of a triangle whose apex is at the fixation point. Like the Clinical Method, this method neither accounts for nor controls proximal or fusional convergence. It is based on the faulty assumption that accommodative convergence contributes 100% of the convergence necessary to maintain bi-foveal fixation at near, and therefore results in an artificially high AC/A ratio. The Heterophoria Method predicts a normal AC/A would be equal to the interpupillary distance. It further predicts that a normal AC/A will increase over time with growth and increased separation of the orbits. Both of these predictions have been shown to be false [7].

The Gradient Method is the only direct measure of the amount of convergence generated per unit of accommodative effort, which is the very definition of the AC/A ratio. The Gradient is also the most accurate method, as it is the only one that controls for both proximal and fusional convergence because measurements are taken at a fixed testing distance. The Gradient Method not only yields a precise measure of the ratio but can be used to predict whether optical methods may be used successfully to manage the strabismus, and how much the spectacles must be altered to achieve the desired effect. The Gradient AC/A can be calculated by repeating measurements through plus lenses at near or minus lenses at distance [4], subtracting the difference in alignment and dividing by the change in accommodative demand.

A test for torsion should always be included in the motor evaluation of a patient with any acquired vertical strabismus. Expected in cases of superior oblique palsy, it should be remembered that a cyclotropia can also occur in other paretic or restrictive vertical strabismus. Torsion testing is also advisable in any patient with diplopia not resolved with offset of the deviation.

For diagnostic purposes, the double Maddox rod test is the recognized standard. Because this test is very disruptive to fusion, it is excellent for revealing and quantifying the full measure of even small cyclotropic deviations. It is also ideal for quantifying large degrees of torsion. The Awaya cyclo-deviation near test (S. Awaya, M.D., New Cyclo Tests, Handaya Co., Sendai, Miyagi, Japan) though not quite as dissociating as the double Maddox rod, has the advantage of ease of interpretation. Small degrees of torsion can be difficult for the examiner to read on a trial frame but are easily identified on the Awaya. There are two main disadvantages to this test. Firstly, the maximum amount of torsion detectable is 12º, and secondly, it can only record net torsion. If a cyclotropia is present in both eyes, this test will report the sum of the two.

Patients with measurable cyclodeviations are not always symptomatic, however. It is therefore necessary to determine to what extent the cyclodeviation is interfering with fusion. For this, a less dissociating test should be done in addition to the Maddox rod or

Awaya test. The simplest way to do this is to mount individual loose Bagolini striated glasses in a trial frame in the same manner as is done with the Maddox rods [36]. For this test, unlike the Maddox rods, the room should remain illuminated and feature contoured peripheral cues to stimulate and assist fusion (see Sect. 10.3 for more information on preferred visual environments for fusion). If significant torsion is detected in this way, it may have to be addressed surgically in order to restore fusion.

Cyclodeviations can also be detected and investigated using the amblyoscope. The amblyoscope is less dissociating than the Double Maddox rod or Awaya test but is more so than the Bagolini glasses. There are two principle advantages of the amblyoscope over the other tests for torsion. Firstly, the cyclodeviation can be offset, leaving no doubt that the patient is capable of fusion. Secondly, the amblyoscope is the only tool with the ability to quantify torsional fusional amplitudes.

Tests such as the Lancaster red-green, the Hess screen, or the Lees screen yield a graphic representation of strabismus in multiple fields of gaze, facilitating quick comparisons over sequential visits in cases of evolving strabismus. These types of tests use foveal projection to map out the strabismus, so that the position of the resulting fields corresponds to the position of the eyes. (This is in contrast to diplopia, in which the double image is located in the field opposite to that which the eye is directed.) Primary and secondary deviations, incomitance, patterns, overand under-actions, paresis, and restriction can all be diagnosed at a glance with these types of tests; however, prerequisites for testing include normal retinal correspondence, the absence of suppression, foveal fixation in both eyes, and the ability to comprehend and comply with the test instructions, all of which preclude the use of these tests in a large percentage of the pediatric population. These tests offer the advantage of controlled and repeatable fixation distance and positions of gaze, and in the hands of an experienced practitioner, they are much faster to complete and record than prism and cover testing in multiple fields.

10.2.4 Assessment of Control

Though quantifying the magnitude of eye misalignment is important, it is often the assessment of control that determines whether treatment is needed at all, and if so, when it should be done. Control can be evaluated in several ways: the cover/uncover test, the use of control scales, and measurement of fusional amplitudes are the most widely used. Stereoacuity may also correspond to control, but as it is not a direct measure of control, it is covered in the Sect. on sensory testing (see Sect. 10.3.1).

A qualitative measure of control can be made using the cover/uncover test. Though highly subjective, this method is simple and practical. Control is typically graded as good, fair, or poor as the patient maintains fixation in the position of greatest concern. Momentary occlusion of the non-dominant eye is used to ascertain how easily control is disrupted. Once dissociated, the patient is observed to determine how readily fusion is regained. This may be quantified to some extent by estimating the time interval between the

loss and recovery of fusion.The examiner should also look for assists such as blinking, saccades, change in fixation distance, or head movement that the patient may recruit in order to regain control.

Several scoring systems have been described specifically for use in evaluating control in intermittent exotropia, though presumably they could be used in any type of strabismus. These systems typically feature varying combinations of two or more of the following: input from the parent or caregiver; observation during the exam; and results of a dissociating test such as a cover test. Results are combined to reveal a score indicating level of control, with higher numbers representing poorer control, and zero representing excellent control. Studies have shown these systems to be sensitive, reliable, and repeatable [14, 30].

Measurement of motor fusion amplitudes can be used to quantify control [5]. Diminished control is evidenced by a low “break point” (point at which vergence demand exceeds fusional reserves, resulting in a manifest deviation), or a remote “recovery point” (point at which fusion can be recovered as vergence demand is lessened). These values are determined by introducing a rotary prism or prism bar at the lowest value, held with the base in the direction opposite to that which was used to measure the strabismus (i.e., base-in for an esodeviation). This simulates an increase in the strabismic angle. The prism is slowly increased until fusion cannot be maintained (break point), then slowly reduced until fusion is regained (recovery point). The break point may be marked by appreciation of diplopia. While this should never be used as the sole indicator of the break point by the examiner, the absence of diplopia at the break is significant. Breaking fusion with suppression is another indication of poor control.

What would be considered a normal break point varies with the magnitude and direction of the underlying strabismus but ideally should be at least equal to the degree of misalignment to ensure comfortable control. A recovery point more than 5 prism diopters away from the break point would be considered remote [40]. With the exception of monofixation syndrome, constant deviations are so because they far exceed their fusional reserves. A phoria is such because fusional vergence far exceeds the degree of latent misalignment. But those with intermittent deviations are likely near their motor fusion threshold and have the most to gain by vergence amplitude testing.

Poorcontrolisoneofthedefiningfeaturesoftropic deviations so it is often erroneously assumed that control does not need to be evaluated in these situations; however, the presence of a manifest deviation does not preclude fusional vergence ability, and it is useful to identify it, no matter how limited or fragile. The cover/uncover test and the control scales are generally not useful for constant strabismus; therefore, the fusional vergence amplitudes method should be used. This is done by offsetting the deviation with a prism bar, rotary prism, or on the amblyoscope. The prism amount is then gradually reduced until the patient breaks fusion and becomes tropic. The process is then repeated by increasing the prism until fusion is once again broken, with or without diplopia. This method defines the alignment range over which the patient’s fusional ability may be sufficient to maintain orthotropia.

10.3 The Sensory Evaluation

The sensory exam is the oracle of the orthoptic evaluation: it can clarify the current sensory state, give the examiner a glimpse into the past, and predict possible sensory and motor futures. However, the ultimate purpose of the sensory exam is to determine if the patient has fusion, or the potential for fusion. The answer may change or refine treatment goals, and may figure prominently in the decision whether to treat at all.

There is more than one type of fusion, and it is possible to demonstrate one type without others. Tests are available to detect and measure motor fusion (retinal image disparity-driven vergence), sensory fusion

(two-dimensional, flat fusion), and stereopsis (retinal disparity sensitivity resulting in three-dimensional fusion); however, it is not always possible or even necessary to test for all of these forms of fusion on every exam. The sensory exam can be done very efficiently if the right tests are chosen, and most sensory tests can be used to answer more than one diagnostic question. The direction of the sensory exam will depend on (1) the patient’s history, symptoms, and capabilities, (2) the results of the motor examination, and (3) the examiner’s own experience and expectations for the case.

A thorough sensory evaluation can be done with a modest collection of tools: a set of prisms (may be in

the form of a box of prisms, a prism bar, or a rotary prism); a red filter; a stereotest; and the Worth 4-dot or Bagolini glasses. An after-image test is helpful in some cases but not absolutely necessary. A complete exam can also be done − and in some instances done better and faster − with a single piece of equipment: a haploscopic device such as an amblyoscope or synoptophore.

10.3.1 The History

The sensory exam begins with the often difficult task of deciphering the patient’s description of symptoms. It is an important step because the patient’s account may provide significant clues to sensory status.Along with the results of the motor evaluation, the history will help the examiner select the appropriate sensory tests to employ. A careful interview is also imperative in order to assist the patient or caregiver in generating realistic expectations so as to maximize satisfaction with treatment.

One of the most frequent complaints associated with strabismus is diplopia. Though it is not the only unpleasant and debilitating consequence of strabismus, it is currently the only one assigned its own diagnostic code in the International Classification of Diseases, 9th edn. (ICD-9). When investigating a complaint of diplopia, in addition to the obvious questions (e.g., “Is it constant or intermittent?”, “Does it disappear when you close one eye?”, “Are the images side by side or up and down?”) there are a few less obvious questions that will clarify much.

“What do you mean by ‘double vision’?”

Many − particularly older children − use this phrase because they have been told they have double vision, or because they lack the vocabulary to describe what they are really experiencing. The phrase “double vision” is also occasionally used erroneously to describe the eye misalignment, and not necessarily the sensory consequences of that misalignment.

Some patients will deny double vision, but when asked to elucidate their symptoms they describe classic diplopia. For example, torsional diplopia is particularly difficult for the patient to describe, because the fixation point may be single while images in the periphery are twisted or tilted. These patients may actually complain of disorientation or dizziness

when in motion. Some patients complain of blurry vision, ghosting, or glare. There are also those who deny diplopia yet unwittingly engage in diplopiaavoidance behavior, such as head posturing or closing one eye.

The most challenging cases of diplopia are those in which the patient appreciates a double image but is unable to locate the object in space relative to the fixation point.This is sometimes referred to as lost localization [19]. Taking a history from these patients is as frustrating for the patient as it is for the examiner.

These patients are often looked upon with suspicion and told that their visual experience is not possible.

In fact, not only is this type of diplopia genuine, it is a sign of early-onset strabismus with anomalous correspondence and loss of suppression in the nondominant eye.

Diplopia is not the only symptom of strabismus.

When asked, many patients will describe difficulties resulting from a loss of stereopsis or fusion. For example, some report difficulty walking down stairs, up a curb, or even across a level surface if it features a boundary created by a change in color, texture, or pattern that might suggest or create the illusion of depth.

Some patients express their symptoms as the inability to locate things where they should be or expect them to be. Other patients report stationary objects in their peripheral visual field suddenly and unexpectedly moving. Still others have problems reading due to moving print.

Visual confusion, another consequence of strabismus, is the result of stimulation of corresponding retinal points with dissimilar images. It may be just as common as diplopia, but it is rarely reported because it can be so difficult for the patient to describe. As a result, clinicians rarely ask about it. Patients with visual confusion may recount a bizarre jumbling or confusion of images that resembles retinal rivalry.

Finally, those with alternating suppression or fixation switch diplopia may report images jumping suddenly in front of them from the peripheral field, or losing their place and skipping lines when reading.

“Does the double vision change with alterations in the ambient illumination?” Symptoms that change in different visual environments are an important clue to etiology. For example, it is natural for double vision secondary to strabismus to be more noticeable under mesopic (dim illumination) or scotopic (darkness) conditions, particularly when viewing a brightly il-

luminated stimulus. Double vision should be worse when driving at night, for example, because there are fewer peripheral visual cues that assist both fusion and suppression. Diplopia that worsens under photopic (bright illumination) conditions, on the other hand, is uncommon and may actually signify macular pathology, with or without an overlying strabismus (see also Sect. 10.3.5).

“Is there any way you can hold your head to either make the images come together or make the double vision become less bothersome?” It is surprising how many patients have never tried this. The answer to this one question goes a long way in establishing a diagnosis and determining fusion capability.

Finally, “Does the double image appear to move?”

Images that are not only double, but appear to move or float, particularly when they are near the “real” image, signal poor fusion ability. This symptom is indicative of a central disruption of fusion. The patient should be questioned about head trauma or brainstem vascular event.

10.3.2 Sensory Fusion

One of the objectives of the sensory evaluation is to determine if the patient is capable of sensory fusion, the ability to integrate two separate, but similar, visual images into a single impression. Sensory fusion is more than the absence of diplopia, and it should not be assumed that because a patient claims to be diplopia-free or is orthophoric, images from the two eyes are necessarily being fused into one perception. The detection of sensory fusion is singularly important in two clinical situations: (1) the strabismic patient who does not complain of diplopia; and (2) the strabismic patient who has recently been re-aligned, whether by surgical or optical methods. In the former case, the lack of diplopia may be explained by the presence of anomalous correspondence; in the latter, evidence of developing sensory fusion at the new angle of alignment may provide some assurance that the eyes will remain aligned over the long term. Testing for sensory fusion is also worthwhile in the young, otherwise normal patient to document the presence of binocular vision.

Sensory fusion testing is a step in the sensorimotor exam that should not be overlooked in patients with

strabismus secondary to very poor vision or large visual field defects. Though these patients are not likely to have good quality fusion post-operatively, they are actually at increased risk for bothersome diplopia.

The more the visual field is missing or degraded, the harder it is for the visual system to match up the fields, and the more likely it is to result in bothersome diplopia once the eyes are aligned.

The Worth 4-dot test was originally designed to detect sensory fusion, but because it is so dissociating, it is actually a test that challenges the sensory fusion mechanism. It is, however, quite effective in detecting foveal suppression, estimating the depth and extent of the suppression scotoma (see Sect. 10.3.6), and detecting anomalous correspondence (Sect. 10.3.7). There are some weaknesses to the test that must be kept in mind, but if used judiciously, and performed and interpreted correctly, it can yield a wealth of sensory information.

One disadvantage of the test is that it is too easy for the patient to discover the “correct” answer with repeated use over consecutive examinations, peeking around the glasses, or an inopportune slip of the tongue on the part of the examiner (as the answer lies in the very name of the test). It will be most illuminating the first time it is employed. If the patient becomes too familiar with the test, the responses become less reliable. In giving the anticipated answer, whether that may be one indicating fusion, suppression, or diplopia, the child may receive positive feedback from the examiner. This positive reinforcement will increase the likelihood of that answer being given again at a future visit, even if it does not reflect the true sensory status at that time. Therefore, the Worth 4-dot must be used in moderation, particularly with children. The other major drawback is that the Worth

4-dot utilizes a color-based (anaglyphic) format. This can lead to false-positive or false-negative responses in patients with uncorrected refractive error [37]. Finally, the test may yield false or variable responses due to discrepancies in luminance and contrast between the red and green filter.

The Worth 4-dot test should be performed initially in normal room illumination. This test is slightly less dissociating under these conditions; it will therefore be easier for the patient to appreciate sensory fusion

(or suppression). Begin at distance fixation using the same flashlight that is used for near testing. It is not recommended to use the wall-mounted light boxes or

computer-based Worth 4-dot stimuli for this purpose, as these targets form a retinal image large enough to project beyond the limits of a foveal suppression scotoma. The fovea measures about 1.5 mm in diameter, so to test for foveal fusion or suppression, the image should subtend an angle no larger than 1.5º. For example, the Worth flashlight held at distance subtends an angle of 1.25º [31]. The average wall-mounted or computer-based Worth tests feature light stimuli that are up to seven times larger than the flashlight stimuli. To subtend an angle of 1.5º, the light box would need to be mounted over 40 ft. away from the patient!

Figure 10.7 illustrates one useful strategy for utilizing theWorth 4-dot to obtain the maximum amount of sensory information. The appropriate first question to the patient should be, “How many lights do you see?” The answer to this question will determine the next step in the exam. If the patient gives a fusion response at distance, no further testing with this particular instrument may be necessary as foveal fusion with peripheral retinal suppression is unlikely. If the patient reports five lights, which suggests diplopia, he should be questioned carefully whether all five are illuminated simultaneously, or whether the lights alternate from two to three. Patients with alternating suppression do not always appreciate this difference.

The patient should be asked to first concentrate on the lights presented to the dominant eye to determine if the other set of lights disappears. The patient should then be encouraged to switch fixation (either spontaneously or with the assistance of a cover−uncover) and the results under two test conditions compared.

Those who habitually fixate with one eye may not demonstrate a measurable suppression scotoma in that dominant eye. Consequently, they will appreciate suppression when fixating with the dominant eye, and diplopia when fixating with the habitually deviating eye. These patients are at increased risk for fixation switch diplopia [23]. If true diplopia (rather than alternating suppression) is found, prism can be introduced to attempt to restore fusion; however, it is easier and less confusing for both patient and examiner if this step is done using a red filter and light source rather than the Worth 4-dot test.

The next step is to repeat the Worth 4-dot at near fixation in the same matter, and compare the distance and near responses. Strabismic patients who report suppression at distance and near fixation on theWorth

tests have a very large suppression scotoma. Patients with suppression at distance but fusion at near may have anomalous correspondence. If diplopia secondary to macular pathology is suspected based on the patient’s history, then the Worth 4-dot test should be repeated at 33 cm, and again at distance and near with the room lights extinguished. The diplopic patient with macular pathology may sometimes report diplopia at distance and fusion at near under photopic conditions, and the reverse under scotopic conditions. In contrast, the patient with diplopia secondary to strabismus will be more likely to appreciate diplopia under scotopic conditions whether tested at 6 m and 33 cm.

10.3.3Detecting and Quantifying Motor Fusion

Of all the types of fusion, disparity vergence is arguably the most important because this is the type of fusion that maintains eye alignment. Without motor fusion, a patient cannot enjoy the benefits of sensory fusion or stereopsis. Without motor fusion, patients with accommodative strabismus have little hope of weaning out of their hyperopic spectacles or bifocals. It may also be true that without motor fusion, a successful surgical outcome may not remain so over time. Motor fusion is also the type most easily tested in a patient not capable of a subjective response. Yet, of the three major types of fusion, motor fusion is the one least likely to be tested and monitored on a regular basis. A test of motor fusion is most valuable in the following types of patients: (1) the infant who is incapable of demonstrating binocular vision in other, more subjective ways; (2) the strabismic patient who may have anomalous correspondence; (3) the patient with accommodative strabismus (esoor exotropic) who is attempting to wean out of glasses; (4) the orthotropic patient with asthenopic symptoms; and (5) the adult with long-standing strabismus who is scheduled for surgery.

The basic procedure for detecting and measuring motor fusion using a prism bar or rotary prism is reviewed in Sect. 10.2.4. For a quick and simple method to detect motor fusion, introduce a 16−20∆

base-out prism over the dominant eye while fixation is secured and maintained on a near target [22]. This is sometimes referred to as “jump convergence.” A convergence response to re-acquire the target should be observed, followed by a divergence movement when the prism is removed. A 20Δ prism is used because smaller prisms may induce a convergence response that is too difficult to see, and larger prisms may exceed the patient’s normal fusional convergence amplitudes, resulting in a false-negative finding. This simple test is the preferred method for detecting binocular vision in the infant and young child. Motor fusion amplitudes can also be measured on the amblyoscope, but this method is more difficult for children. In order to recognize the fusion break point, the patient must be able to appreciate and communi-

cate subtle changes in the fixation target’s appearance or position.

The jump convergence method is also useful in the strabismic patient who may have anomalous correspondence if the angle of strabismus is <20Δ. Those with larger deviations are best tested on a haploscope. The jump convergence test evaluates the retinomotor value of a peripheral retinal point relative to the “pseudo-fovea” of the deviating eye. (“Pseudo-fovea” is a term given to the non-foveal retinal element of the deviating eye that has acquired a correspondence with the fovea of the fixating eye under binocular conditions.) Anomalous motor fusion is an important feature of anomalous correspondence. Patients with anomalous correspondence and motor fusion are at elevated risk for an early surgical under-correction or

recurrence of the pre-operative angle as the fusional vergence mechanism seeks to restore correspondence between the fovea of the fixing eye and the pseudofovea of the non-dominant eye. These cases are also likely to “eat up” prism with prism adaptation, building to the limits of their convergence amplitudes (see Sect. 10.2.3). If surgery is then performed for this larger angle, an over-correction is highly likely.

Both the strabismic patient attempting to wean out of spectacles and the asthenopic patient should be tested with a prism bar or rotary prism, as their fusional vergence range may be too low to be detected with jump vergence. The base of the prism should be placed in the direction opposite to that used to measure the deviation so as to increase the vergence demand. For example, divergence amplitudes are necessary to control an esotropia. To measure divergence, the prism will be held base-in and gradually increased in power until fusion is broken. This will result in a manifest esotropia, with the patient reporting either diplopia or suppression. In this example, the base-in prism effectively simulates an increasing esodeviation, and the break point estimates just how much esodeviation the patient can manage before becoming esotropic.

The visual system, both sensory and motor components, operates on a “use it or lose it” principle.

The brain is unlikely to expend the energy to maintain a pathway, network, or system that is not being utilized regularly. It is a waste of effort and resources, and the central nervous system is nothing if not efficient. Adults with long-standing strabismus illustrate this point well. Motor fusion amplitudes in the same direction as the strabismus (i.e., convergence in esotropic patients) are likely to be subnormal, and it is important to detect this prior to surgery if the goal is to achieve a small over-correction in the immediate post-operative period. While the child with a small surgical over-correction usually recovers, the adult with the same small over-correction may not. Measuring the fusional amplitudes pre-operatively will help to determine the risk of persistent post-operative diplopia secondary to a small over-correction. For example, a patient with a long-standing exotropia may demonstrate convergence amplitudes that are above the norm, even though they may be insufficient to control the deviation completely; however, this same patient will have poor or absent divergence amplitudes because there has been no need for the

vergence system to maintain that ability. This patient will have difficulty overcoming even a small angle over-correction.

10.3.4Detecting and Quantifying Stereopsis

Stereopsis is the ability of the visual cortex to utilize retinal image disparity to localize objects in space relative to the horopter. (The horopter is the curve described by the distribution of matching retinal elements.) It is the most complex and highly developed type of binocular vision, and the type most susceptible to insult to the visual system. Restoration of stereopsis in the formerly strabismic patient is typically considered the pinnacle of success, and is often the single criterion of binocularity included in final outcome measures in surgical studies. It is also the type of binocularity most often tested on a routine exam.

Because it is so sensitive to decreased vision, suppression, and ocular misalignment, it has also been used as a vision screening tool. Stereopsis has also been used effectively to monitor control of an intermittent deviation, particularly intermittent exotropia, for the purpose of determining appropriate timing for surgery [15, 17]. As the patient spends more time in the tropic state, suppression gains a foothold and becomes deeper. Suppression is present under binocular conditions, even when the eyes are in the phoric state [32], so a suppression scotoma has the potential to reduce stereoacuity by blocking the disparity signal. Decreased stereopsis, therefore, may indicate an expanding or deepening suppression scotoma, which typically results from a loss of control over an intermittent deviation with an onset during childhood

[42].

Stereopsis can be detected and measured a number of different ways. Each stereotest has its own strengths and weaknesses. Because there is no one ideal method, the choice of test will depend on the question the examiner is seeking to answer. Most of the methods require glasses (polarized or red-green) or a septum to control the image presented to each eye. The rightand left-eye images are typically identical in contour and size, but with one image displaced by a small amount horizontally. It is not necessary that

each eye be presented with an identifiable image [13].

The depth effect can be achieved with visual noise alone, though these types of tests are typically more challenging for the patient.

It is very difficult to isolate pure disparity sensitivity in a clinical setting. Most tests available today suffer from monocular clues to depth that may lead to false-positive results or artificially enhanced stereoacuity. The Titmus Stereotest is probably the most well-known example of a stereotest flawed by these artifacts. A stereoblind patient may demonstrate deceptively good stereoacuity due to the nonstereoscopic, image displacement cues. In spite of this, the Titmus test continues to be the most popular and accessible test of stereoacuity worldwide. Like the Worth 4-dot test, if used judiciously and meticulously by an examiner familiar with its limitations, this stereotest can be reasonably effective. The infamous Titmus Housefly, in particular, can actually be quite useful. Because it is designed with such a large degree of disparity (3000 s of arc at 40 cm), even those with the most insubstantial disparity sensitivity may be able to appreciate the image in depth. It is also effective as a qualitative test of stereoacuity in young children who may be incapable of understanding instructions. Without any coaching at all, the dramatic and unpleasant image provokes curiosity and occasionally fear in the young child. A strong reaction is often indicative of a positive response. In fact, it is often when the examiner must instruct the patient that the Titmus test loses some of its effectiveness! It is very difficult to explain the test without inadvertently leading the patient to the correct answer. Some children have a need to please, and may be better at reading the examiner than they are at reading the test! Moreover, those who have worn the polarized glasses and stared at the fly two or three times per year over a decade or more tend to respond affirmatively because they have learned the “correct” answer. Like the Worth 4-dot test, the Titmus fly will be most accurate the first time it is used, and with increasing exposure will come a higher false-positive rate. Those who have never enjoyed stereopsis do not understand what the sensation is like, so asking the patient if the stimulus is “three-dimensional” or “elevated” from the page is meaningless; however, these patients are sometimes able to tell that there is something different about the stimulus and may assume that this difference is what we mean by depth or “3D” effect.

Another point that must be emphasized is that this test is calibrated for use at 40 cm, not the customary

33 cm otherwise used for near fixation. Advancing the book to 33 cm will increase the retinal disparity produced, and overestimate the degree of stereopsis.

For those patients with either very poor or very fine stereopsis, the effect on stereoacuity produced by the shorter test distance is probably negligible; however, for those in monofixation range, repositioning the test book by several centimeters could mean the difference between fine and gross stereopsis and possibly change the diagnosis.

If in doubt of a patient’s positive response, the examiner can rotate the book 90º and repeat the test. This changes the disparity from horizontal to vertical, and collapses the 3D effect. Those with true disparity sensitivity will notice a dramatic flattening of the image occasionally accompanied by the sensation of vertical diplopia; those without disparity sensitivity will report no change in the image.

The only stereotest of true depth available for use in the clinic is the Frisby Stereoacuity Test, a series of clear plastic plates divided into four square segments with random blue shapes printed on one side. One of the square segments features a hidden circle pattern that is printed on the back side of the plate, thereby displacing it off of the plane of fixation (horopter).

This test has several advantages. Firstly, it does not require dissociation by glasses or septum to create the illusion of an image positioned off of the plane of fixation. Such dissociation could potentially break fusion and result in artificially poorer stereoacuity.

Secondly, because the plates can be rotated to change the location of the circle and turned over to change the disparity from crossed to uncrossed, it is not possible for the patient to memorize the correct answer.

Finally, the Frisby test can measure over a wide range of disparities simply by changing test distance, and is one of the few tests available that has been modified to test distance stereoacuity (Frisby-Davis 2).

Like the Titmus test, there are monocular clues to depth on the Frisby test. Both the plates and the patient’s head must be kept steady to avoid the motion parallax cue to depth. The plate should not be placed on a flat surface. In addition, recent comparison studies have found that while the Frisby test may be very good at detecting stereopsis, other types of random dot stereotests may be better at quantifying and monitoring changes over time [28].

10.3.5 Investigating Diplopia

Examination of the diplopic patient has several goals:

(1) to determine if the diplopia corresponds to, and can be explained by the strabismus; (2) to estimate the impact of the diplopia on the patient’s quality of life; and (3) to determine if the diplopic patient is capable of single binocular vision. The pattern and type of diplopia should fit with the pattern and type of strabismus revealed by the motor exam. Placing a red filter over the dominant eye will assist the patient in determining the precise location of most types of diplopic images. Exceptions include torsional diplopia, monocular diplopia, or diplopia secondary to lost localization (see Sect. 10.3.1). If there is a discrepancy between the motor exam and the type of diplopia found on sensory exam, the diplopia may be paradoxical. Paradoxical diplopia typically occurs with a change in alignment in the patient with anomalous correspondence. Post-operative paradoxical diplopia is a common but temporary complication of strabismus surgery in adults with a history of childhoodonset strabismus, good visual acuity, and anomalous correspondence on pre-operative sensory testing[9].

Anomalous correspondence may persist in the early post-operative period and produce a double image that corresponds to the pre-operative position of the fovea of the deviating eye.

Documenting the impact of diplopia on quality of life can be important for several reasons. For example, the examiner will inquire about any restriction of activities or deterioration in performance of activities that require depth perception. The answer to these questions may play a role in the decision to intervene with treatment. There are also situations where documentation of the impairment must be provided to a third party prior to treatment. This is one reason why plotting the field of binocular single vision can be useful. In the past this was most commonly accomplished using the Goldmann perimeter, but these manual perimeters have all but been replaced by automated models that are difficult to adapt for use in recording diplopia. Moreover, the Goldmann perimeter charts diplopia on a concave surface steeper than the curve of the horopter, which may not be as authentic a representation of visual space as a flat surface. Finally, the Goldmann apparatus cannot detect or plot torsional diplopia without special adaptations to the stimulus, which may be costly or cumbersome.

To address these concerns, the Matta map was developed (N. Matta, C.O., Family Eye Associates, Lancaster, Pa.). This method of diagramming diplopia is

amodification of the Lancaster red-green test utilizing the screen and one torch without the red-green glasses. While keeping the head in primary position seated 1 m from the screen, the patient views the examiner’s red line image as it is projected to each of the markers on the Lancaster screen that represent the nine positions of gaze. The patient indicates whether a single or double image is seen at each location, and the examiner maps out the field of single vision on graph paper. Like the Goldmann method, the Matta map can be used as a static test (as described) or as

akinetic test to precisely define the boundaries of single vision.

The cervical range of motion (CROM) method has also been described as an alternative to Goldmann diplopia fields (see also Sect. 10.2.2) [16, 24]. When using the CROM to map a diplopia field, the patient is asked to binocularly fixate an accommodative target with the head held in whatever position allows single vision. The head is then rotated away from this position as fixation is maintained until diplopia is noted.

The head position in degrees can be read from the dials. One of the advantages of the CROM is that it can be used with real targets in free space as a stimulus, rather than lights projected onto the inside of a bowl (Goldmann) or a screen (Matta Map). The targets and backgrounds used for both the Goldmann and Matta methods impart more of a challenge for both fusion and suppression, and therefore may overestimate the diplopia experienced on a day-to-day basis. In addition, the CROM can be used to measure the field of single vision at distance fixation and is more precise in its quantification of the range of motion. On the other hand, the CROM device is more cumbersome and less comfortable for the patient than the other two methods, and it does not produce the simple graphic representation of the field of single vision. It is also

amore expensive piece of equipment than the Matta map.

Just as one should not assume that the diplopiafree patient is fusing, neither should it be assumed that all diplopic patients are capable of sensory fusion. The most common method used to determine if the diplopic patient is capable of fusion is to offset the deviation with prism in “free space” (natural viewing conditions). The problem with this technique