Diagnostic Testing
After a thorough case history and determination of the refractive error, the first important step in the management of accommodative, ocular motor, and nonstrabismic binocular vision problems is the diagnostic testing routine. In this chapter, we discuss testing procedures for assessing accommodation, binocular vision, and ocular motor skills. The emphasis is on presentation of important issues, considerations, and expected values for the various tests. The setup and administration of these tests is summarized in the Appendix to this chapter.
Determination of Refractive Error
All measures of alignment and accommodation require an accurate full-plus refraction with a binocular balance. It is useful to perform a binocular refraction technique that yields a maximum plus refraction. Such an examination often requires an initial objective determination of the refractive error. This can be accomplished with static retinoscopy, autorefraction, or even starting with the patient’s previous refractive correction. To perform a modified binocular refraction, we recommend the following procedure:
1. Use a 20/30 line (or an acuity line two lines above threshold).
2. With the left eye occluded, add plus (0.25 diopter [D] at a time) to the objective findings until the right eye is barely able to read the 20/30 threshold line. If too much plus is used, the next step will be difficult, so you may want to back off slightly (add -0.25 D, at most).
3. Perform Jackson cross-cylinder (JCC) testing. Adding plus in the earlier step allows the patient to make more accurate JCC responses.
4. Repeat for left eye, with right occluded.
5. Add prism (3 Δ up before the right eye; 3 Δ down before the left) and +0.75 D to each eye.
6. Perform a dissociated balance by adding plus to the clearer target until both are reported to be equally blurred.
7. Remove the dissociating prism and slowly add minus until the patient can just read 20/20. Do not arbitrarily add some amount of minus!
8. Place the Stereo Optical Vectographic Projector Slide in the projector with analyzers in the phoropter. Place “I” target with letters on each side in the patient’s view and ask if both sides are equally clear. If not, add +0.25 D to the clearer side. This is a binocular balance, but not a true binocular refraction where the JCC would be performed under these conditions as well; it is generally not necessary to perform a JCC here unless the patient has a significant astigmatism (>1.00 DC) and a torsional phoria is suspected.
9. Perform associated phoria measures and stereopsis testing.
10. Return to the standard slide and check visual acuity. If the patient cannot see 20/15, check whether -0.25 more oculus uterque (OU) improves the acuity. It is virtually never necessary to add more than -0.50 OU total. Do not arbitrarily add some amount of minus!
The maximum plus refraction technique breaks down when acuity is very unequal (e.g., amblyopia). In these instances, where often no refractive technique works well, use retinoscopy to determine balance after attempting to achieve maximum plus on the “good” eye (make the retinoscopic reflexes appear equal for the two eyes).
With the introduction of computer-based visual acuity test charts, most optometrists will be using these systems in the future, rather than using traditional projectors. Thus, an alternative to the Stereo Optical Vectographic Projector Slide is available with the M&S Smart System® 2 in which there is a series of polarized targets that could be used for the binocular refraction.
Assessment of Nonstrabismic Binocular Vision Disorders
GENERAL CONSIDERATIONS
The evaluation of binocular vision involves several distinct steps (Table 1.1). The first phase of testing is the measurement of the magnitude and direction of the phoria at a distance and near, along with the accommodative convergence to accommodation (AC/A) ratio. Conventional procedures to accomplish this include tests such as cover testing, the von Graefe phoria test, and the modified Thorington test. Fixation disparity testing represents a more recent method of assessing binocular vision and provides additional information that should be considered in the evaluation of binocular vision status. The primary advantage of fixation disparity testing is that it is performed under binocular or associated conditions, in contrast to other tests that are performed under dissociated conditions.
The second step is the assessment of positive and negative fusional vergence using both direct and indirect measures. Direct measures refer to tests such as smooth and step vergence testing, whose primary objective is to assess fusional vergence. Indirect measures refer to tests such as the negative relative accommodation (NRA), positive relative accommodation (PRA), fused cross-cylinder, binocular accommodative facility (BAF), and monocular estimation method (MEM) retinoscopy that are generally thought of as tests of accommodative function. Because these procedures are performed under binocular conditions, they indirectly evaluate binocular function as well. The results of such testing, therefore, can be used to confirm or deny a particular clinical hypothesis of a binocular vision disorder. Chapter 2 describes the analysis of these indirect measures in detail.
The traditional evaluation of fusional vergence involves only measurement of smooth vergence ranges or vergence amplitude using a Risley prism in the phoropter. In recent years, additional ways of evaluating fusional vergence have been suggested. One method is step vergence testing, which is done outside the phoropter, using a prism bar.1,2 Another addition to the traditional approach to assessing fusional vergence is vergence facility testing.3,4,5,6,7,8 This test is also performed outside the phoropter, using a specially designed vergence facility prism (Fig. 1.1). The patient’s ability to make large rapid changes in fusional vergence is assessed with this procedure over a specific period of time.
An important distinction among different methods of evaluating fusional vergence is the assessment of vergence amplitude versus vergence facility. Smooth and step vergence testing are designed to assess the patient’s vergence amplitude, whereas vergence facility testing measures vergence dynamics. Grisham5 found a relationship between vergence dynamics and symptoms in subjects he studied. His research indicated that vergence latency and vergence velocity are of diagnostic importance in a binocular evaluation. It is possible for a patient to have normal fusional vergence amplitudes and still have a problem in the area of facility or vergence dynamics. Using only the traditional smooth vergence evaluation approach would fail to detect such a problem. Gall et al6
found that the use of 3 Δ base-in/12 Δ base-out for vergence facility testing can differentiate symptomatic from nonsymptomatic patients.
found that the use of 3 Δ base-in/12 Δ base-out for vergence facility testing can differentiate symptomatic from nonsymptomatic patients.
Table 1.1 IMPORTANT STEPS IN THE EVALUATION OF BINOCULAR VISION | ||||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ||||||||||||||||||||||||||||||||
 ▪ Figure 1.1 A: Vergence facility prism (3 Δ base-in/12 Δ base-out). B: Vergence facility prism clinical procedure. |
Another consideration in testing fusional vergence amplitude or facility is the issue of performance over time.3 The underlying question is whether the patient is able to compensate for a given amount of prism over an extended period of time. Traditionally, fusional vergence amplitude is measured just once. Research suggests that this may not be sufficient.5,6 Rather, these tests should be repeated several times, and testing that probes facility and ability to respond over time should be incorporated into the evaluation.
The third area that should be evaluated is convergence amplitude. Generally referred to as the near point of convergence (NPC), this test is particularly important in the diagnosis of one of the most common binocular vision disorders—convergence insufficiency. Important issues include the type of target or targets to be used and the issue of performance over time.9,10
The last aspect of the binocular evaluation is sensory status. Suppression and stereopsis are the primary areas to evaluate. Information about sensory status can also be obtained from many of the other tests discussed earlier. On several of these tests, suppression can be monitored. A specific test that can be used to assess suppression is the Worth four-dot test. As a general rule, clinical measures of stereopsis are either not affected or only minimally affected in nonstrabismic binocular vision disorders. Intermittent mild suppression, however, is a common finding.
A complete assessment of binocular vision should include all four of the components just described. A suggested minimum database would include the NPC, the cover test at distance and near, step vergence ranges at distance and near, and stereopsis testing. If a patient presents with symptoms and the minimum database does not yield conclusive information, additional testing using indirect measures of binocular function, along with facility testing and fixation disparity assessment, should be utilized.
ASSESSMENT OF SIZE AND DIRECTION OF THE PHORIA OR FIXATION DISPARITY
Cover Test (in the Absence of Strabismus)
1. Purpose The cover test is an objective method of evaluating the presence, direction, and the magnitude of the phoria.
2. Important issues
(a) Controlling accommodation The most important aspect of the cover test procedure, or any other test of binocular alignment, is control of accommodation. A study by Howarth and Heron11 reaffirmed the significance of the accommodative system as a potential source of variability in clinical heterophoria measurement. Underaccommodation will result in an overestimation of the degree of exophoria or an underestimation of the esophoria. Overaccommodation will yield the opposite results. There are two techniques (Video 1.1) that can be used to maximize control of accommodation during the cover test procedure. These refinements to the basic procedure tend to increase attention on the task. The examiner can use multiple fixation targets to maintain attention and accommodation on the task. This can be easily accomplished by using Gulden fixation sticks that have 20/30 targets on both sides of the stick (Fig. 1.2). Periodically, the fixation stick is turned around to change targets. The patient is asked to identify the target during the cover test.
Another useful procedure is to move the target left to right very slightly (1 to 2 cm) between movements of the cover paddle. The examiner looks for a small pursuit movement in the uncovered eye. If a pursuit movement occurs when the target is moved left to right, it suggests that the patient is attending to the target. Attention on the target tends to encourage accommodation.
(b) Objectivity Because the cover test is an objective technique, it is one of the most valuable methods for assessing the motor characteristics of binocularity. It becomes particularly valuable when working with young children.
(c) Repeatability Johns et al12 found that the alternate cover test with prism neutralization has high intraexaminer and interexaminer repeatability.
(d) Assessing frequency and control of the deviation When an intermittent strabismus is detected using the cover test, an additional assessment must be made of the proportion of time the eye is deviated, or the frequency of the deviation. This can also be referred to as control of the deviation. It is commonly
believed that a worsening of control in intermittent exotropia is an indication for vision therapy or surgical intervention. The problem is that until recently, precise criteria for progression have not been established.
▪ Figure 1.2 A: Gulden fixation stick. B: Gulden fixation sticks with small targets used as a fixation target.
Table 1.2 REVISED NEWCASTLE CONTROL SCORE
Home control
XT or monocular eye closure seen
0
Never
1
<50% of time fixing at distance
2
<50% of time fixing at distance
3
50% of time fixing in distance + seen at near
Clinic control
Near
0
Immediate realignment after dissociation
1
Realignment with aid of blink or refixation
2
Remains manifest after dissociation/prolonged fixation
3
Manifest spontaneously
Distance
0
Immediate realignment after dissociation
1
Realignment with aid of blink or refixation
2
Remains manifest after dissociation/prolonged fixation
3
Manifest spontaneously
Total Newcastle Scale Score: (Home + Near + Distance).
Haggerty et al13 described the Newcastle Control Score that they developed as a tool to assess control of an intermittent exotropia deviation. The scale incorporates both objective (office assessment) and subjective (home assessment by parents) measures of control into a grading system that differentiates and quantifies the various levels of severity in intermittent exotropia. The authors suggest that the scale is a consistent and robust method of rating severity that can be used accurately in clinical practice. Hatt et al,14 however, questioned the reliability of parental observations. The revised Newcastle Control Score15 is illustrated in Table 1.2. Mohney and Holmes16 developed an office-based scale that can describe the wide range of control in patients with intermittent exotropia and avoids many of the weaknesses of prior systems. It provides a quantitative measure of the severity and duration of the manifest component of the exodeviation and is useful for the longitudinal evaluation of patients with intermittent exotropia. Hatt et al17 used this scale with 12 children with intermittent XT and they were evaluated during four sessions (2 hours apart) over a day on two separate days (eight sessions per child). Control was standardized using the scoring system and quantified three times during each examination. They found that the mean of three assessments of control during a clinic examination better represents overall control than a single measure. In recent years, this office control scale has been used in several randomized clinical trials as the primary outcome measure.18,19,20 This scale is illustrated in Table 1.3.
Table 1.3 INTERMITTENT EXOTROPIA CONTROL SCALE
Control Score
Control Score Description
5
Constant exotropia during a 30-sec observation period (before dissociation)
4
Exotropia 50% of the time during a 30-sec observation period (before dissociation)
3
Exotropia 50% of the time during a 30-sec observation period (before dissociation)
2
No exotropia unless dissociated (10 sec): recovery in 5 sec
1
No exotropia unless dissociated (10 sec): recovery in 1-5 sec
0
Pure phoria: 1-sec recovery after 10-sec dissociation
3. Expected values Although the expected finding for the cover test has not been specifically studied, we expect it to be similar to the values found during phoria testing. At distance, the expected value is 1 exophoria, with a standard deviation of ±1 Δ. The mean expected value at near is 3 exophoria, with a standard deviation of ±3 Δ.21
Phoria Measured Using the von Graefe Technique
1. Purpose The von Graefe phoria test is a subjective method of evaluating the presence, direction, and magnitude of the phoria.
2. Important issues
(a) Controlling accommodation Controlling accommodation is also important when evaluating the phoria using the von Graefe procedure. It is vital to emphasize this in the instructional set to the patient. Often clinicians merely ask the patient to look at one image and report when the other is right above or below. To ensure more accurate accommodation, the clinician should state
I want you to look at the lower image, and it is very important to keep it clear at all times. While you keep it clear, tell me when the upper image moves directly above the lower image.
Although the instruction to keep the target clear is not always included in phoria testing, a lack of attention to this issue may lead to variability and poor reliability.
Another issue that should be considered, particularly in young children, is whether the patient understands the task. Clinicians often use the following instructional set to try to explain the objective of the test:
Look at the bottom line and tell me when the top line moves directly above it, like buttons on a shirt.
Although this may be helpful for older children and adults, we have found that children who are 7 years old and younger do not perform well with this analogy. To promote an understanding in young children, we suggest an actual simple demonstration outside the phoropter using one’s fingers. The young child is asked to look at the examiner’s fingers, which are held one directly over the other. We use the following instructional set:
Look at the finger on the bottom and tell me when my top finger is right over my bottom finger. (Demonstrate by misaligning your fingers and then bringing them back to alignment.) Now let’s try it; tell me when to stop.
Using this method allows the examiner to determine whether the child has an understanding of what is expected.
Although the von Graefe procedure is commonly used in clinical practice, a study by Rainey et al22 indicated that this procedure is the least repeatable of the various tests used to measure the phoria.
(b) Reliability Rouse et al23 reported a high level of intraexaminer reliability, both within and between sessions, using the von Graefe method of assessing the phoria in children 10 to 11 years old.
3. Expected values At distance, the expected value is 1 exophoria, with a standard deviation of ±1 Δ (Table 1.4). The mean expected value at near is 3 exophoria, with a standard deviation of ±3 Δ21 for children and young adults; for presbyopes, the mean expected values are 1 esophoria, with a standard deviation of ±1 Δ at distance, and 8 exophoria with a standard deviation of ±3 Δ at near.
Phoria Measured Using the Modified Thorington Technique
1. Purpose This technique is a subjective method of evaluating the presence, direction, and magnitude of the phoria (Video 1.2).
2. Important issues
(a) Controlling accommodation With the modified Thorington test, it is important for the clinician to emphasize that the patient keep the letters on the chart clear during the test procedure. In a study by Rainey et al,22 the results of seven different procedures of assessing the phoria were compared to determine the repeatability of the clinical tests. The authors compared the estimated cover test, prism-neutralized objective cover test, prism-neutralized subjective cover test, von Graefe continuous presentation, von Graefe flash presentation, the Thorington method, and the modified Thorington method. They found that the modified Thorington procedure was the most repeatable method, whereas the von Graefe methods had the poorest repeatability.
Table 1.4 EXPECTED VALUES: BINOCULAR VISION TESTING
Test
Mean Value
Standard Deviation
Cover test
Distance
1 exophoria
±2 Δ
Near
3 exophoria
±3 Δ
Distance lateral phoria
1 exophoria
±2 Δ
Near lateral phoria
3 exophoria
±3 Δ
AC/A ratio
4:1
±2 Δ
Smooth vergence testing
Base-out (distance)
Blur:
9
±4
Break:
19
±8
Recovery:
10
±4
Base-in (distance)
Break:
7
±3
Recovery:
4
±2
Base-out (near)
Blur:
17
±5
Break:
21
±6
Recovery:
11
±7
Base-in (near)
Blur:
13
±4
Break:
21
±4
Recovery:
13
±5
Step vergence testing
Children 7-12 y old
Base-out (near)
Break:
23
±8
Recovery:
16
±6
Base-in (near)
Break:
12
±5
Recovery:
7
±4
Adults
Base-out (distance)
Break:
11
±7
Recovery:
7
±2
Base-in (distance)
Break:
7
±3
Recovery:
4
±2
Base-out (near)
Break:
19
±9
Recovery:
14
±7
Base-in (near)
Break:
13
±6
Recovery:
10
±5
Vergence facility testing near (12 base-out/3 base-in)
15.0 cpm
±3
Vergence facility testing distance (12 base-out/3 base-in)
15.0 cpm
±3
Near point of convergence
Accommodative target
Break:
2.5 cm
±2.5
Recovery:
4.5 cm
±3.0
Penlight and red/green glasses
Break:
2.5 cm
±4.0
Recovery:
4.5 cm
±5.0
(b) Testing outside the phoropter An important advantage of this technique is that it can be used for patients who are difficult to test with a phoropter. For this reason, the modified Thorington technique has value with children younger than 7 or 8 years. As indicated earlier, it has also been shown to be the most repeatable method of assessing the phoria.
Fixation Disparity Assessment
1. Purpose Fixation disparity testing is designed to evaluate binocular vision under associated conditions. This is in contrast to cover testing, the von Graefe phoria test, and the modified Thorington techniques, which are done under conditions in which either one eye is covered or the eyes are dissociated.
2. Important issues
(a) Fixation disparity testing is performed under binocular conditions The main deficiency of the typical phoria measurement is that the evaluation occurs under dissociated conditions. Wick24 states that “the vergence error under binocular conditions is often not the same as it is under monocular conditions.” As a result, there are situations in which a patient may be symptomatic, but the conventional phoria/vergence analysis does not produce a clear understanding of the cause of the patient’s symptoms. Although some clinicians suggest the routine use of fixation disparity testing, we have found that in the majority of cases, phoria/vergence testing is sufficient to reach a tentative diagnosis and management plan. In those situations in which the diagnosis is unclear or a prism prescription is being considered, fixation disparity testing is a useful addition to the examination procedure.
(b) Associated phoria versus forced vergence fixation disparity assessment Various instruments are available for the evaluation of fixation disparity. Instruments, such as the Mallett unit, the Stereo Optical Vectographic Projector Slide, the Borish card, the Bernell lantern, the Wesson card, the Sheedy Disparometer, and some computerized distance visual acuity charts (Chapter 15) can all be used to determine the associated phoria. Some of this equipment is no longer commercially available (Sheedy Disparometer) or uses technology that is no longer being used (Stereo Optical Vectographic Projector Slide—requires a projector chart). The associated phoria is the amount of prism necessary to neutralize any perceived misalignment of the lines.
Studies suggest, however, that the use of forced vergence fixation disparity testing is more likely to yield data that are useful for determining those patients who are likely to have symptoms.25,26 The Wesson card is currently the only commercially available instrument for measuring actual fixation disparity. Based on current information, forced vergence fixation disparity testing should be used when assessing a horizontal deviation. For a vertical deviation, associated phoria testing is sufficient.
(c) Determination of prism correction Fixation disparity is currently considered the method of choice for determining the amount of prism to prescribe for binocular disorders. Other methods tend to yield higher amounts of prism than fixation disparity analysis.
3. Expected values Refer to Chapter 15.
AC/A Ratio
1. Purpose To determine the change in accommodative convergence that occurs when the patient accommodates or relaxes accommodation by a given amount.
2. Important issues
(a) Significance in diagnosis and treatment Determination of the AC/A ratio is important in analysis of optometric data. The AC/A finding is a key characteristic in the final determination of the diagnosis. It is also one of the most important findings used to determine the appropriate management sequence for any given condition. For example, esophoria at near associated with a high AC/A ratio would generally respond well to plus lenses. If the same degree of esophoria were associated with a normal or low AC/A ratio, the recommended treatment approach would include prism correction or vision therapy or both.
(b) Calculated versus gradient AC/A ratio There are two methods for determining a patient’s AC/A ratio. The first, referred to as the calculated AC/A ratio, is determined using the following formula:
AC/A = IPD (cm) + NFD (m) (Hn – Hf)
where,
IPD = interpupillary distance in centimeters
NFD = near fixation distance in meters
Hn = near phoria (eso is plus and exo is minus)
Hf = far phoria (eso is plus and exo is minus)
Example: IPD = 60 mm, the patient is 2 exophoric at distance and 10 exophoric at near (40 cm).
AC/A = 6 + 0.4 (-10 + 2) = 6 + 0.4 (-8) = 6 + (-3.2) = 2.8
When using this formula, one should remember to use the correct signs for esophoria and exophoria. A rule of thumb is that a high AC/A ratio will result in more eso or less exo at near, and a low AC/A ratio will lead to less eso or more exo at near.
The second method, called the gradient AC/A, is determined by measuring the phoria a second time using -1.00 or -2.00 lenses. The change in the phoria, with the additional minus, is the AC/A ratio. For example, if the near phoria is 2 esophoria through the subjective finding and, with -1.00, it is 7 esophoria, the AC/A ratio is 5:1.
There may be significant differences between the two methods of determining the AC/A ratio. For instance, both divergence excess and convergence excess patients have high calculated AC/A ratios, but many of these patients have approximately normal gradient AC/A ratios.22 The same phenomenon may occur with convergence insufficiency. The calculated AC/A ratio will be low, but the gradient AC/A may be normal.24 The reason for these differences is the effect of proximal convergence and the lag of accommodation. The calculated AC/A ratio is usually larger than the gradient because of the effect of proximal vergence, which affects the near phoria measurement. Because the gradient ratio is measured by testing the near phoria twice at a fixed distance, proximal vergence is held constant and theoretically does not alter the final result. The lag of accommodation also accounts for differences between the calculated and gradient AC/A ratio measurements. Although the stimulus to accommodation is 2.50 D at near, the accommodative response is typically less than the stimulus. This difference between the stimulus and response of the accommodative system is called the lag of accommodation. The lag of accommodation is generally +0.25 to +0.75 D. Because the patient will tend to underaccommodate for any given stimulus, the gradient AC/A tends to be lower than the calculated AC/A ratio.
(c) Controlling accommodation A source of measurement error in the AC/A evaluation is failure to control accommodation. The clinician should emphasize, in the instructional set, that clarity of the target is essential. It is easy to understand how variation in accommodative response from one measurement to another would adversely affect results. The gradient AC/A requires two measurements of the near phoria, first with only the subjective in place and then with -1.00 over the subjective. If a patient accurately accommodates for the first measurement, but underaccommodates for the second, the result will be an underestimation of the true AC/A ratio. It is, therefore, critical to ask the patient to maintain clarity, and it is advisable to ask the patient to read the letters periodically.
(d) Response versus stimulus AC/A ratio When evaluating the accommodative or binocular systems, we usually present the stimulus at 40 cm. This creates an accommodative demand of 2.50 D. This is referred to as the stimulus to accommodation. Although the stimulus to accommodation is 2.50 D, the accommodative response will generally be about 10% less than the stimulus.27 The expected finding for MEM retinoscopy, for example, which assesses the accommodative response, is a lag of accommodation of about +0.25 to +0.50 D. It is important to be aware of the difference between the response and stimulus to accommodation, realizing that most patients will underaccommodate by about 10%. An instance where this becomes important is when comparing the calculated AC/A ratio with the gradient AC/A ratio. The gradient AC/A ratio will tend to underestimate the AC/A ratio. For example, suppose the phoria is measured as 10 exophoria at near and, when repeated with -1.00 lenses, the phoria is 6 exophoria. Based on this information, the gradient AC/A ratio would be 4:1. However, if we assume that the patient underaccommodates by 10%, the phoria has changed by 4 Δ while accommodation has changed by 0.75 D. This would be an AC/A ratio of about 4.45:1.
3. Expected values The expected AC/A ratio is 4:1, with a standard deviation of ±2.
CA/C Ratio
1. Purpose To determine the change in accommodation that occurs when the patient converges or relaxes convergence by a given amount.
2. Important issues
(a) Significance in diagnosis and treatment The convergence accommodation to convergence (CA/C) ratio is still not commonly assessed in the clinical situation. Determination of the CA/C ratio is important in the analysis of optometric data. The CA/C finding is sometimes an important characteristic in the final determination of the diagnosis. It may also play a key role when one determines appropriate management. For example, divergence excess and other cases of high exophoria at distance may benefit from the use of added minus lenses. Analysis of the CA/C ratio helps in this determination.
(b) Clinical determination of the CA/C ratio To measure the CA/C ratio clinically, one has to use either a blur-free target or pinholes to eliminate blur as a stimulus. There is still no widely accepted method for determining the CA/C ratio. One possible approach is to use a target called the Wesson DOG (difference of gaussian) card28 along with dynamic retinoscopy. To use this technique, ask the patient to view this target at four different distances as you perform retinoscopy. You can determine the amount of accommodation with different vergence levels.
(c) Stimulus versus response CA/C Unlike the accommodative system, in which there may be a significant difference between the stimulus and response, the vergence stimulus and vergence response are generally identical. There is, therefore, no need to differentiate between a stimulus and response CA/C ratio.29
3. Expected values The expected CA/C value for young adults is 0.50 D per meter (m) angle. In vision research, 1 m angle equals 10% of the distance IPD in millimeters (mm); thus, for a patient with a 50-mm distance IPD, 1-m angle is 5 Δ, and for a patient with a 69-mm distance IPD, 1-m angle is 6.9 Δ. For clinical purposes, it is satisfactory to consider 1-m angle to be about 6 Δ. Because there is little difference between vergence stimulus and vergence response, there is very little difference between the stimulus and response CA/C ratio. The CA/C ratio is inversely related to age.
DIRECT ASSESSMENT OF POSITIVE AND NEGATIVE FUSIONAL VERGENCE
Smooth Vergence Testing
1. Purpose Smooth vergence testing is designed to assess the fusional vergence amplitude and recovery at both distance and near. This is considered a direct measure of fusional vergence.
2. Important issues
(a) Amplitude versus facility Smooth vergence testing is the most common method used for assessing the amplitude of the fusional vergence response for both positive and negative fusional vergence. The blur finding is a measure of the amount of fusional vergence free of accommodation. The break indicates the amount of fusional vergence and accommodative vergence. The recovery finding provides information about the patient’s ability to regain single binocular vision after diplopia occurs. Although smooth vergence testing provides important information about the amplitude of fusional vergence, studies6 have shown that it is possible to have normal fusional amplitudes and still have a problem referred to as fusion vergence dysfunction. Additional testing must be performed to assess fusional facility.
(b) Reliability Rouse et al23 reported only fair intraexaminer reliability, both within and between sessions using the von Graefe smooth vergence testing procedure in children aged 10 to 11 years. Their results suggest that differences up to 12 Δ occur with follow-up visits even without intervention. Thus, when evaluating the effects of treatment, such as vision therapy, a change of greater than 12 Δ is needed to be confident that the change is real and not the result of measurement variability.
(c) Smooth versus step vergence Smooth and step vergence testing are both designed to evaluate fusional vergence amplitude. The primary value of step vergence testing is that it is administered outside the phoropter. This is an important advantage when examining young children. Before the age of 8 or 9, children tire quickly and may move around, making testing with a phoropter difficult. Because it is impossible to see the child’s eyes behind the phoropter, the clinician cannot be sure whether the patient is responding appropriately. Studies1,2 have demonstrated that expected findings are different for smooth versus step vergence. Two studies have also compared fusional vergence ranges with rotary prism (smooth) versus step vergence with a prism bar.30,31 Antona et al30 compared phoropter rotary prism vergence ranges with phoropter prism bar fusional vergence ranges for 61 optometry students in Spain. The results suggested that the two tests should not be used interchangeably. Goss and Becker31 did a similar study and also concluded that fusional vergence ranges determined by prism bars out of the phoropter cannot be used interchangeably with those determined by phoropter rotary prisms for the purpose of follow-up on individual patients or for the purpose of comparison with norms. Thus, clinicians should use one method or the other in the initial examination and when following the patient’s progress, reevaluate using the same method.
3. Expected values Table 1.4 lists the expected values for the blur, break, and recovery for positive and negative fusional vergence using smooth vergence testing.
Step Vergence Testing (Videos 1.3 and 1.4)
1. Purpose Step vergence is a method of evaluating fusional vergence amplitude outside the phoropter.
2. Important issues Testing is done outside the phoropter. When a young child who is either very active or not responding reliably is evaluated, step vergence testing represents a useful alternative. The child’s eyes
can be seen because testing is done with a prism bar, and the test becomes more objective. Instead of relying on the patient’s responses, the examiner can observe when the child loses binocularity.
Vergence Facility Testing (Video 1.5)
1. Purpose Vergence facility testing is designed to assess the dynamics of the fusional vergence system and the ability to respond over a period of time. This ability to make rapid repetitive vergence changes over an extended period of time can be referred to as a measure of stamina and is the characteristic that we assess clinically. Another characteristic that we indirectly evaluate using vergence facility testing is sustaining ability. This refers to the ability of the individual to maintain vergence at a particular level for a sustained period of time, rather than to rapidly alter the level.
2. Important issues
(a) Amplitude versus facility Melville and Firth32 investigated the relationship between positive fusional vergence ranges and vergence facility. They found no correlation between these values and suggest that this indicates that the two tests assess different aspects of the vergence system. A more recent study by McDaniel and Fogt33 also found a lack of correlation between the two test findings and concluded that patients with vision-related asthenopic symptoms who have normal compensating disparity vergence ranges should undergo vergence facility testing. Because it is possible to have normal fusional vergence amplitudes and vergence facility problems, both aspects should be evaluated with a symptomatic patient. We suggest using vergence facility testing when a patient presents with symptoms characteristic of a binocular disorder and other testing does not reveal any problems. Such a patient may have normal fusional vergence amplitudes but reduced facility.
(b) Strength of prism to use and target to use Until fairly recently, there had been a lack of systematically gathered normative data and little consensus in the literature about the strength of the prism that should be used for this test. Buzzelli4 recommended the use of 16 base-out and 4 base-in. Another common recommendation3 was 8 base-out and 8 base-in. Gall et al6 performed the first systematic study of vergence facility and found that the magnitude of choice is 3 Δ base-in/12 Δ base-out. This combination of prisms yielded the highest significance for separating symptomatic from nonsymptomatic subjects. They also found that this combination of prisms produced repeatable results (R = 0.85) when used for near-vergence facility testing.
In another study, Gall et al7 compared the use of three different vertically oriented targets for vergence facility testing. The targets tested were a vertical column of 20/30 letters, a back-illuminated anaglyphic target, and the Wirt circles oriented vertically. The study was designed to determine whether it is important to use a target with a suppression control for vergence facility testing. They found that vergence facility is nearly independent of the target and that a simple vertical row of 20/30 letters is an appropriate target.
(c) Testing Distance A recent study found that vergence facility testing at distance may be a more sensitive method than testing at near for convergence insufficiency. Trieu et al tested 28 subjects between the ages of 9 and 30 years of age with symptomatic convergence insufficiency and performed vergence facility at distance and near.34 They found the mean and standard deviation vergence facility measures in cycles per minute were 5.40 (±4.28) at distance and 8.97 (±4.71) at near in this group of symptomatic convergence insufficiency patients. Distance vergence facility was significantly lower (p < 0.001). An interesting finding is that a quarter of the sample passed vergence facility testing at near, but could not even complete one cycle at distance because of an inability to fuse with 12 Δ base-out.
At first glance, one might expect patients with convergence insufficiency to have more difficulty performing vergence facility at near than at distance, because the amount of exophoria is larger and the positive fusional vergence ranges are lower than at distance. However, vergence facility testing with 12 Δ base-out and 3 Δ base-in presents a vergence demand that is significantly different from the traditional vergence demand assessed with smooth or step vergence when the goal is to determine the maximum vergence amplitude. First, during traditional testing the vergence stimulus is either a ramp stimulus (smooth vergence using Risley prisms) or a small step stimulus (either 2 Δ or 5 Δ using a prism bar). In contrast, with vergence facility testing the vergence demand is a much larger 12 Δ convergence step stimulus. To accomplish this larger step convergence demand at near, an individual has four vergence components to utilize: tonic, accommodative, proximal, and disparity vergence. However, at distance, accommodative and proximal vergence are unavailable, and tonic vergence does not
change in the short term, so disparity vergence is left as the primary mechanism to fuse the stimulus. The second major difference in the task at distance compared with near is that the individual must voluntarily initiate a convergence movement during vergence facility testing. In contrast to smooth or step vergence testing in which the task begins with a single, fused target and the demand is slowly increased, with vergence facility testing the task begins with diplopia and the patient must actively diverge and converge alternately. For patients with convergence insufficiency, the ability to initiate a disparity convergence movement is typically impaired and vergence facility testing at distance may be particularly sensitive to this issue. Improvement in vergence facility testing results at distance may also be a valuable method for determining progress with vision therapy as proposed by Tannen et al.35
Based on these data, we suggest that clinicians consider performing vergence facility at distance when convergence insufficiency is suspected.
INDIRECT ASSESSMENT OF POSITIVE AND NEGATIVE FUSIONAL VERGENCE
Near Point of Convergence (Video 1.6)
1. Purpose The purpose of the NPC is to assess the convergence amplitude. A remote NPC was found to be the most frequently used criterion by optometrists for diagnosing convergence insufficiency.36
2. Important issues
(a) Target to be used Different targets have been suggested for NPC testing. Recommendations vary, including an accommodative target, a light, a light with a red glass before one eye, and a light with red/green glasses. Some suggest that a variety of targets should be used to determine whether there are differences with various targets. We recommend repeating the NPC twice—first using an accommodative target and then using a transilluminator or penlight with red/green glasses.
(b) Does repetition yield additional useful clinical data? The NPC test traditionally is performed by slowly moving a target toward the eyes until the patient reports diplopia or the examiner notices a break in fusion.37 This is recorded as the breakpoint. The target is then slowly moved away from the patient until fusion is reported or the examiner notices realignment of the eyes, signaling recovery of fusion.
Several modifications to this traditional approach have been suggested in the literature to make the test more sensitive. Wick24 and Mohindra and Molinari38 recommend that the NPC test be repeated four to five times. Their suggestions are based on the claim of Davies39 that asymptomatic patients manifest little change in the near point with repeated testing, whereas symptomatic patients have significantly less convergence with repeated testing. Thus, this recommendation is designed to improve the diagnostic sensitivity of the break of the NPC test. Scheiman et al10 found a recession of the NPC after repetition in both normal subjects and convergence insufficiency patients. In the subjects with normal binocular vision, however, the amount of recession was small, less than 1 cm. In the convergence insufficiency group, the amount of recession was 1.5 cm after 5 repetitions and about 4 cm after 10 repetitions.10 These findings suggest that the NPC test would have to be repeated about 10 times to yield useful clinical information. Maples and Hoenes40 also investigated the changes in the NPC after repetition and found that the NPC break and recovery do not change appreciably with multiple repetitions of the test.
(c) Does the use of the red glass or red/green glasses yield any additional useful clinical data? Another criterion utilized for assessment of convergence ability is the recovery point, or the point at which an individual regains fusion (after fusion has been lost) during the push-up convergence testing. Capobianco41 reported that a recovery point greatly different from the break indicates greater convergence problems. She also suggested repeating the test with a red glass before one eye. She stated that greater recession with the red glass suggests a more significant convergence problem. Several authors24,38,42,43 have suggested that this procedure be part of the standard assessment of convergence amplitude.
Scheiman et al10 found a statistically significant difference between the break and recovery with an accommodative target and the results with a penlight and red/green glasses in patients with convergence insufficiency. For convergence insufficiency subjects, the mean break with an accommodative target was 9.3 cm and, with a penlight and red/green glasses, the mean break was 14.8 cm. The recovery finding with the accommodative target was 12.2 cm, and with a penlight and red/green glasses it was 17.6 cm. For both the break and recovery, therefore, there was a difference of about 5.5 cm between the
accommodative target and penlight and red/green glasses. Statistically significant differences were not found for an accommodative target compared to a penlight or a penlight compared to a penlight and red/green glasses.
In the subjects with normal binocular vision, there were no significant differences for any of the conditions just described. The mean break was between 2.4 and 2.9 cm, and the mean recovery was between 4.2 and 5 cm.
(d) The value of assessing convergence ability using a jump convergence format Pickwell and Stephens44 described another method of assessing convergence ability, which they termed jump convergence. In this procedure, the subject first fixates a target at 6 cm and then changes fixation to a target at 15 cm. Pickwell and Stephens44 reported that this jump convergence test appears to have more clinical significance and is a more sensitive way of determining the presence of convergence problems than the NPC. In the original study, the authors compared the effectiveness of the standard near point test (pursuit convergence) and the jump convergence procedure in a group of 74 subjects with inadequate convergence; 50 of the 74 showed normal pursuit convergence but reduced jump convergence. Only five subjects passing the jump convergence test failed the pursuit convergence procedure. The authors concluded that “this evidence clearly suggests that the jump convergence test is more likely to detect inadequacy of convergence than the measurement of the NPC.” In a second study, Pickwell and Hampshire9 found that in a sample of 110 subjects with inadequate convergence, poor jump convergence was more frequently associated with symptoms than was poor pursuit convergence. One problem with the jump convergence test is the lack of expected values for this test. In their 2003 study, Scheiman et al10 found a mean of 30 cpm (standard deviation = 10) for subjects with normal binocular vision and 23 cpm (standard deviation = 11) for subjects with convergence insufficiency.10
3. Expected values Although this test is commonly used to diagnose convergence insufficiency, there had been no normative data for children or adults until recently. Hayes et al45 studied 297 schoolchildren and recommended a clinical cutoff value of 6 cm. Maples and Hoenes40 reported a similar value with a cutoff value of 5 cm. Scheiman et al10 studied an adult population and suggested that when using an accommodative target, a 5-cm cutoff value should be used for the break and a 7-cm cutoff value should be used for the recovery. Using a penlight and red/green glasses, the cutoff value for the break is 7 cm and that for the recovery is 10 cm.
NRA and PRA
1. Purpose NRA and PRA tests were designed to be used as part of the near point evaluation of accommodation and binocular vision. The primary objective of these tests is to determine whether the patient requires an add for near work. In a prepresbyopic patient, the two findings should be approximately balanced (NRA = +2.50, PRA = -2.50). An NRA value higher than the PRA suggests that a patient may benefit from an add (Chapter 10). The test is also used with the presbyopic population in the same manner to determine if an add is necessary and to finalize the magnitude of the required add. The NRA can also be used to determine whether a patient has been overminused during the subjective examination. The NRA is performed through the subjective prescription, which should eliminate all accommodation at distance. Because the test distance is 40 cm, the patient will accommodate approximately 2.5 D to see the target clearly. Therefore, the maximum amount of accommodation that can be relaxed is 2.50 D. Thus, an NRA finding greater than +2.50 suggests that the patient was overminused.
In this text, we stress another use for the NRA and PRA tests. These tests can be used to indirectly analyze both accommodation and vergence. This is explained in detail in Chapter 2.
2. Important issues
(a) Instructional set It is important to ask the patient to keep the target clear and single during these tests. Traditionally, the instructional set is, “As I add lenses in front of your eyes, keep these letters clear for as long as you can. Tell me when the letters are blurry.” We believe it is important to also ask the patient to report diplopia, because these tests also indirectly probe the ability to maintain fusion using positive and negative fusional vergence.
(b) High NRA finding A high NRA finding indicates that the patient has been overminused during the subjective.
(c) At what level should the PRA be discontinued? The maximum value that should be expected with the NRA is +2.50, for the reasons explained earlier. However, there is no consistent endpoint for the PRA. The endpoint for the PRA will vary depending on the patient’s amplitude of accommodation, AC/A ratio, and the negative fusional vergence. Table 1.5 illustrates the variables that determine the endpoint for the PRA.
Table 1.5 DETERMINING VARIABLES THAT AFFECT THE ENDPOINT FOR THE PRA
Test
Patient 1
Patient 2
Patient 3
Patient 4
Amplitude of accommodation
12 D
12 D
12 D
2 D
AC/A ratio
2:1
4:1
8:1
2:1
Base-in vergence (near)
12/20/12
10/20/10
8/12/8
12/20/12
Expected PRA finding
-6.00
-2.50
-1.00
-2.00
AC/A, accommodative convergence to accommodation; PRA, positive relative accommodation.
In the first patient, we would expect the patient to be able to keep the target single and clear until about -6.00. As we add minus lenses binocularly, the patient must accommodate to maintain clarity. This is not a problem because the amplitude of accommodation is 12 D. At the same time, the patient must maintain single binocular vision. As the patient accommodates, the AC/A ratio causes convergence that must be counteracted using negative fusional vergence. For every 1 Δ of accommodation, the patient must use 2 Δ of negative fusional vergence. Because patient 1 has 12 D of accommodation and 12 Δ of negative fusional vergence, he or she will be able to maintain clear single binocular vision until about -6.00 D. Using the same reasoning, the PRA endpoint will decrease as the AC/A increases, as demonstrated earlier for patients 2 and 3, who have higher AC/A ratios and lower negative fusional vergence ranges. Patient 4 has findings identical to patient 1, except that the amplitude of accommodation is only 2 D. Even though this patient has a low AC/A ratio and normal negative fusional vergence, blur would be expected at -2.00 because of the low amplitude of accommodation.
In contrast to the NRA, where the maximum expected endpoint is always +2.50, the maximum endpoint for the PRA varies with multiple factors. Because the primary objective of the NRA and PRA tests is to determine whether the two values are balanced, it makes sense to stop the PRA test after reaching a value of -2.50.
3. Expected values The expected values for NRA are +2.00, ±0.50; for PRA, the expected values are -2.37, ±1.00.
Evaluation of Sensory Status
GENERAL CONSIDERATIONS
Although sensory fusion anomalies can be very dramatic in cases of strabismus, in cases of nonstrabismic binocular vision disorders, sensory anomalies are much less severe. Most patients with nonstrabismic binocular anomalies have normal or only mildly reduced stereopsis. Intermittent mild suppression is common in heterophoria, but is less intense and the size of the suppression scotoma is smaller than in strabismus.
Although sensory status is not as significant an issue in heterophoria, the presence of suppression or loss of stereopsis is still important in determining the prognosis and sequence of treatment. In many cases, the presence of suppression can be determined by performing the binocular vision testing described earlier. During the NPC, near lateral phoria, and fusional vergence testing, patients may be unable to appreciate diplopia in spite of misalignment of the visual axes, indicating suppression.
EVALUATION OF SUPPRESSION
Worth Four-Dot Test
1. Purpose The Worth four-dot test is a subjective test designed to evaluate the presence and size of the suppression scotoma. It is considered one of the most accurate methods of evaluating suppression.46
2. Important issues
(a) Determining the size of the suppression scotoma The size of the suppression scotoma can be determined by moving the Worth four-dot flashlight away from the patient. As the flashlight is moved away from the patient, the target subtends a smaller angle. For instance, at 33 cm, the target subtends an angle of approximately 4.5 degrees. At 1 m, the angle subtended is approximately 1.5 degrees. When performing the Worth four-dot test, the flashlight is initially held at 33 cm, and the patient, wearing red/green glasses, is asked to report the number of dots seen. If the patient reports four dots, the clinician should slowly move the flashlight from 33 cm to about 1 m. If the patient reports four
dots at 33 cm, but two or three dots at 1 m, a small suppression scotoma is present. If a three- or two-dot-response is present, even at 33 cm, the suppression scotoma is larger. The size of the suppression scotoma is important because there is an inverse relationship between the size of the suppression scotoma and the level of stereopsis. As the suppression scotoma becomes larger, the stereopsis decreases.
(b) Determining the intensity or depth of the suppression It is important to evaluate the intensity of the suppression scotoma. It is possible to have a small suppression scotoma that is more intense and, therefore, more difficult to treat than a larger, less intense, suppression scotoma. To assess the depth of the suppression, the clinician can perform the Worth four-dot test with normal room illumination and again with the room lights turned off. Normal illumination simulates the patient’s normal visual conditions and is more likely to yield a suppression response. As the conditions are made artificial, the patient has more difficulty maintaining the suppression. The suppression is considered more intense, therefore, if it is present even with the room lights off.
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