Case Analysis and Classification
Several analytical approaches are presented in the optometric literature. Each has its own unique characteristics, advantages, and disadvantages. Each of these systems also has shortcomings that are significant enough to have prevented wide acceptance of any one approach by the profession. Rather, it is common for optometrists, during their early years of practice, to develop their own personal approach to case analysis that is often a combination of the various systems they have been taught during their education.
The four approaches that are most widely discussed in our literature are graphical analysis, the Optometric Extension Program (OEP) analytical analysis approach, Morgan’s system of normative analysis, and fixation disparity analysis. This chapter briefly describes these four case analysis approaches. This discussion leads directly to a detailed presentation of the case analysis approach that is used throughout this text.
Review of Currently Available Analytical Approaches
GRAPHICAL ANALYSIS
Graphical analysis is a method of plotting clinical accommodation and binocular findings to determine whether a patient can be expected to have clear, single, and comfortable binocular vision.1 The test findings that are commonly plotted include the dissociated phoria; base-in to blur, break, and recovery; base-out to blur, break, and recovery; negative relative accommodation (NRA); positive relative accommodation (PRA); amplitude of accommodation; and near point of convergence (Fig. 2.1).
Advantages
The primary advantage of the graphical analysis system is that it allows one to visualize the relationship among several optometric findings and is, therefore, an excellent system to introduce the concepts of case analysis. The width of the zone of clear, single, binocular vision; the relationship between the phoria and fusional vergence; the accommodative convergence to accommodation (AC/A) ratio; and the relationship of the NRA and PRA findings to fusional vergence and/or accommodation are all clearly portrayed on the graph. For the student learning about accommodation and binocular vision for the first time, the ability to view a visual representation can be a very powerful learning tool. Over the years, graphical analysis has become a standard teaching approach in many optometric curricula.
Graphical analysis also facilitates identification of erroneous findings. When data are plotted on the graph, a characteristic pattern becomes evident. If an individual finding deviates from this typical pattern, it may indicate that it is erroneous and unreliable.
Although the primary purpose of graphical analysis is simply the visual representation of accommodative and binocular data,2 various guidelines for analyzing these findings have developed over the years. The most popular of these guidelines has been Sheard’s criterion. Sheard3,4 postulated that for an individual to be comfortable, the fusional reserve should be twice the demand (phoria). For example, in the case of a 10 Δ exophoria, the positive fusional convergence should be 20 Δ to meet Sheard’s criterion. This postulate can also be used to determine the amount of prism necessary to make the patient comfortable or to determine whether lenses or vision therapy would be appropriate.
Disadvantages
The system does have shortcomings, however, which, for the most part, have relegated graphical analysis to the classroom.
 ▪ Figure 2.1 Sample graphical analysis worksheet showing the test findings that are commonly plotted: (A) The dissociated phoria, (B) base-in to blur, (C) base-in to break, (D) base-out to blur, (E) base-out to break, (F) negative relative accommodation, (G) positive relative accommodation, (H) amplitude of accommodation, and (I) near point of convergence. |
The graphical system fails to identify some binocular vision, accommodation, and oculomotor problems. When using the graphical analysis approach, important data such as accommodative facility, fusion facility, fixation disparity, and monocular estimation method (MEM) retinoscopy findings are not included in the analysis. This is significant because of the 15 most common accommodative, ocular motor, and binocular vision anomalies discussed in later chapters, 5 (e.g., accommodative excess, accommodative infacility, ill-sustained accommodation, fusional vergence dysfunction, and ocular motor dysfunction) cannot be identified using graphical analysis. For example, an individual with a condition called accommodative infacility may have a normal amplitude of accommodation, NRA, and PRA. When the data are plotted according to established graphical analysis guidelines and analyzed according to Sheard’s criterion, the result is a normal graph and failure to identify a problem. Accommodative infacility can only be diagnosed when facility testing is performed and analyzed. This type of information, however, is not part of the routine in the graphical system. A condition such as accommodative infacility would, therefore, not be diagnosed using a traditional graphical analysis approach.
Graphical analysis relies heavily upon criteria—such as those by Sheard3,4 and by Percival5—to determine whether a problem exists. These criteria, however, can only be considered guidelines. Although Sheard’s criterion has been readily accepted by optometry since its introduction, there has been little research evidence, until recently, to support its validity. A study by Dalziel6 found that a vision therapy program that was effective in improving fusional vergence to meet Sheard’s criterion was effective in relieving symptoms. Sheedy and Saladin7,8 studied the relationship between asthenopia and various clinical analysis measures of oculomotor balance. The objective was to determine which measures would best discriminate symptomatic from asymptomatic patients. Sheard’s criterion was found to be the best for the entire population and exophoria, but the slope of the fixation disparity curve was found to be best for esophores. Worrell et al9 evaluated patient acceptance of prism prescribed by Sheard’s criterion. They prescribed two pairs of glasses for each subject. The glasses were identical in every way, except that one contained a prism based on Sheard’s criterion. The results of this study showed that patients with esophoria preferred the glasses with the prism, whereas those with exophoria preferred the glasses without the prism. Although these studies are somewhat supportive of Sheard’s criterion, there are certainly suggestions that, in some cases, it fails to identify patients who are symptomatic and may not always be the most effective method for determining appropriate management.
Another shortcoming of graphical analysis is that it may be too precise a method for clinical purposes and is cumbersome to use. Although most optometry students begin their study of case analysis with a presentation of graphical analysis, few continue to graph data throughout their careers. The actual mechanics of plotting the data are cumbersome and time-consuming. An experienced clinician rarely needs to actually plot optometric data to reach a decision about diagnosis and management.
ANALYTICAL ANALYSIS
The second case analysis approach is referred to as the analytical analysis system. Developed by the OEP, this approach has several rigid requirements and steps10:
Administration of a 21-point examination using precise instructional sets
Checking (comparison of data to a table of expected findings)
Chaining (grouping the data)
Case typing (identifying the condition)
In the analytical analysis approach, the specific 21 tests (points), as described by the OEP, must be used and the instructional sets must be precisely followed. Any deviation from the suggested routine invalidates the results and the analytical system.
Results of the examination must then be compared with a table of expected values developed by the OEP (Table 2.1). This is followed by a procedure referred to as chaining, or grouping of the data. Chaining simply means that those findings found to be high are entered above a horizontal line, whereas data that are low are placed below the horizontal line. The data are also grouped together according to specific rules. The following illustrates an example of chaining:
The results of this chaining or grouping of all the high and low data are then analyzed. This process is referred to as case typing. Two basic types or classifications exist in the OEP system: the B-type (accommodative problem) and the C-type (convergence problem). The B-type case is further divided into seven stages or subtypes.
Advantages
Analytical analysis incorporates several unique concepts into its system that are derived from the underlying philosophy of vision of the OEP. Two examples are described next.
Table 2.1 OPTOMETRIC EXTENSION PROGRAM EXPECTED FINDINGS | ||||||||||||||||||||||||||||||||
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Concept 1: The status of the visual system can deteriorate over time. The OEP stresses the concept that vision problems develop over time and that the deterioration occurs as an adaptation to a stressful condition (e.g., excessive reading or near work).11 Analytical analysis allows one to evaluate the current stage or deterioration of the vision problem, and the therapy prescribed depends on this determination. If this treatment using lenses or vision therapy is not instituted, continued reading can be expected to result in adaptations that take the form of fusional vergence and accommodative problems, refractive error, and strabismus. This concept is dramatically different from traditional thinking, which suggests that vision disorders occur as random variations or as a failure in development.12
Concept 2: Vision problems can be prevented. OEP philosophy postulates that vision problems develop as an adaptation to near point demands.11 Because analysis of the data can indicate the current stage of development of a vision problem, subtle changes can be detected early. With appropriate intervention using lenses, prism, and vision therapy, many vision problems can be prevented, according to OEP philosophy.
Disadvantages
The analytical approach is mainly used by members of the OEP and has not gained widespread use for several reasons.
A major problem with this system is that the student or practitioner must be familiar with specific OEP testing protocols. Unless these protocols are precisely followed, the system becomes unusable. Because most schools of optometry do not teach this system of testing, students are generally unfamiliar with the instructional sets.
An understanding and acceptance of OEP philosophy is a basic requirement. The OEP is primarily a postgraduate education organization. Students at the various schools and colleges of optometry generally receive only introductory information about the OEP. It is not difficult to understand, therefore, why so few students feel comfortable with this approach.
The OEP literature is written using a basic language that is often very different from the classic optometric language taught in optometry schools. Basic definitions of terms such as accommodation, convergence, blur, break, recovery, and phoria are all significantly different. For example, Manas13 defines exophoria as “[a] developmental relationship within the visual behavior pattern, between areas of that pattern, operationally active to preserve the integrity of performance of the convergence pattern.” If an optometrist wants to use analytical analysis, it requires a period of time learning this new language. For a student or practitioner who has just spent several years learning one optometric language, the additional effort required is an obstacle that must be overcome before involvement with the OEP analysis system is possible.
MORGAN’S SYSTEM OF CLINICAL ANALYSIS (NORMATIVE ANALYSIS)
Morgan’s system is based on his 1944 study, in which he presented the concept that it is important to analyze the results of groups of data.14 In Morgan’s approach, little significance is attributed to variation from the norm on any one given test. Morgan found that he was able to divide all data into groups based on the direction in which the tests tend to vary. To analyze optometric data using Morgan’s analytical approach, one must first compare the findings with Morgan’s table of expected findings (Table 2.2) and then look for a trend in the group A and group B findings (Table 2.3). The important concept in this system is that no single finding is considered significant by itself. However, when a group as a whole varies in a given direction, it is considered clinically significant. If the group A findings are high and the group B data are low, a convergence problem is present. If the group B data are high and the group A findings are low, an accommodative fatigue problem is indicated.15 The data in group C are used to suggest whether lenses, prism, or vision therapy should be recommended as treatment.
Morgan’s approach, therefore, is an attempt to present an analytical system that is easily applied and that does not go beyond the exactness and significance of the data involved.15
Advantages
The primary advantage of this approach is the concept that it is important to look at groups of findings rather than individual data. Morgan15 stresses that if one finding falls outside the “normal range,” it does not necessarily indicate that the patient has a problem. He states that “statistical data applies to populations and not necessarily to individuals.”
Another advantage of this system is its flexibility and ease of use, compared with the complexity and rigidity associated with graphical and analytical analyses.
Table 2.2 MORGAN’S TABLE OF EXPECTED FINDINGS | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Disadvantages
The primary limitation of Morgan’s approach is that the groups developed by Morgan in the 1940s have not been updated to include some of the more recent optometric tests that have been shown to be important clinical findings. As a result, it fails to identify some binocular vision, accommodation, and oculomotor problems. When using Morgan’s analysis, important data, such as accommodative facility, fusion facility, fixation disparity, MEM retinoscopy, and ocular motility findings, are not included in the analysis.
Table 2.3 MORGAN’S THREE GROUPS | |||||||
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FIXATION DISPARITY ANALYSIS
Fixation disparity is a small misalignment of the eyes under binocular conditions.16 This misalignment from exact bifoveal fixation is very small, with a magnitude of only a few minutes of arc. Several clinical methods have been developed to evaluate fixation disparity at near, including the Mallett unit, the Bernell lantern slide, the Wesson card, and the Borish card. For fixation disparity testing at distance, the Mallett unit (distance unit) and the American Optical vectographic slide were the only primary commercially available instruments for many years. Today, many of the computer-based visual acuity (VA) testing instruments include a distance fixation disparity target.17 The associated phoria, or the amount of prism necessary to neutralize the fixation disparity, is determined using the Mallett unit, the American Optical vectographic slide, the Bernell lantern slide, the Borish card, and computer-based VA testing instruments. The Wesson card permits a more complete analysis of the fixation disparity. Using this instrument, a fixation disparity curve can be generated and four diagnostic characteristics of the curve can be analyzed. These four characteristics are the type, slope, x-intercept, and y-intercept. Chapter 15 presents an in-depth discussion of fixation disparity.
The use of fixation disparity data has been suggested as a useful method for the analysis and diagnosis of problems of the oculomotor system.16,17,18,19 The primary advantage of fixation disparity analysis is that the assessment takes place under binocular and, therefore, more natural conditions. Studies have indicated that analyzing binocular vision using fixation disparity is useful in determining those patients who are likely to have symptoms.18 Some authors16,17,18,19 have suggested that fixation disparity data may be the most effective method for determining the amount of prism to prescribe for binocular vision disorders.
Advantages
The primary advantage of fixation disparity analysis is that the data are gathered under binocular vision conditions. Other analytical systems depend on phoria vergence testing performed under dissociated conditions that may not truly reflect the way the system operates under binocular conditions. For example, in about one-third of patients, a condition referred to as paradoxical fixation disparity is present.19 This is a condition in which the fixation disparity is in the direction opposite to the phoria.
Disadvantages
Fixation disparity testing is a technique for evaluating binocular vision and does not provide direct information about accommodation or ocular motor disorders.
All of the systems described earlier have failed to gain widespread acceptance by the profession because of the limitations described. The rest of this chapter is devoted to the presentation of the case analysis system that is utilized throughout this text. This approach draws from the major contributions of the four systems described, while it attempts to eliminate most of their disadvantages. Its use allows the optometrist to operate with much more flexibility than available with strict adherence to any of the other approaches.
INTEGRATIVE ANALYSIS APPROACH
The integrative analysis approach is an analysis system that attempts to make use of the most positive aspects of other case analysis approaches while avoiding the problems associated with them.
It requires three distinct steps:
1. Comparing the individual tests to a table of expected findings
2. Grouping the findings that deviate from expected findings
3. Identifying the syndrome based on steps 1 and 2.
This format uses the concepts of the OEP analytical analysis system: checking, chaining, and typing. However, the primary disadvantages of analytical analysis—that is, the rigidity of the 21-point examination and the OEP language problems—are avoided. The integrative analysis approach also makes use of the following important characteristics of other systems:
Some of the unique concepts of the OEP system are utilized, including the following:
The status of the visual system can deteriorate over time.
Vision problems can be prevented.
Morgan’s suggestion that it is important to look at groups of findings rather than individual data is a key element in the integrative analysis approach.
Fixation disparity data performed under binocular conditions are included.
The integrative analysis approach includes an analysis of ocular motor, accommodative facility, vergence facility, MEM retinoscopy, and fixation disparity data. No other analysis system makes use of all of these data.
Specifics
To utilize this case analysis system, the optometrist must be knowledgeable about the following:
Expected findings for each optometric test administered
The relationship of one finding to another or how to group the data that are gathered
A classification system that categorizes the most commonly encountered vision problems or syndromes.
Expected Findings for Optometric Tests
Tables 1.4, 1.7, 1.12 and 1.14 list various commonly administered optometric tests and expected findings. These tables are a compilation of data from Morgan’s table of expected findings along with newer data for accommodative facility, ocular motor, vergence facility, step vergence, MEM retinoscopy, and fixation disparity testing. In a recent large-scale study of normative values for clinical measures of vergence and accommodation (n = 1,056), Wajuihian20 determined norms for clinical measures of black, African children 13 to 18 years of age. He found that the data for near phoria and fusional vergences, accommodative response, and relative accommodation were quite similar to the norms listed in Chapter 1.
Grouping Optometric Data
The concept of the importance of looking for trends comes from both the OEP analysis and Morgan’s system. The integrative analysis approach is simply an expansion of this concept and divides optometric data into six groups, rather than the three proposed by Morgan (Table 2.3). Tests or data are placed in a group if they directly or indirectly evaluate the same function.
TESTS EVALUATING POSITIVE FUSIONAL VERGENCE
Positive fusional vergence (PFV)—smooth vergence testing
PFV—step vergence testing
PFV—vergence facility testing
NRA
Binocular accommodative facility (BAF) with plus lenses
Near point of convergence
MEM retinoscopy and fused cross-cylinder
TESTS EVALUATING NEGATIVE FUSIONAL VERGENCE
Negative fusional vergence (NFV)—smooth vergence testing
NFV—step vergence testing
NFV—vergence facility testing
PRA
BAF with minus lenses
MEM retinoscopy and fused cross-cylinder
TESTS EVALUATING THE ACCOMMODATIVE SYSTEM
Monocular accommodative amplitude
Monocular accommodative facility with plus and minus lenses
MEM retinoscopy
Fused cross-cylinder
NRA/PRA
BAF testing
Binocular accommodative amplitude
TESTS EVALUATING VERTICAL FUSIONAL VERGENCE
Supravergence and infravergence
Fixation disparity
TESTS EVALUATING THE OCULAR MOTOR SYSTEM
Fixation status
Subjective assessment of saccades using grading scales
Developmental eye movement (DEM) test
Visagraph
Subjective assessment of pursuits using grading scales
MOTOR ALIGNMENT AND INTERACTION TESTS
Cover test at distance
Cover test at near
Phoria at distance
Phoria at near
Fixation disparity
AC/A ratio
CA/C ratio
Classification System of Common Accommodative and Nonstrabismic Binocular Vision Problems
Once the test findings are grouped and a trend is identified, the specific syndrome can be selected from the list of the 15 common accommodative, ocular motility, and binocular vision problems described in this section. This classification is a modification of the well-known Duane-White classification21 suggested by Wick.18 The rationale for this classification is described in detail later in this chapter.
BINOCULAR ANOMALIES
Heterophoria with Low AC/A Ratio
Orthophoria at distance and exophoria at near—convergence insufficiency
Exophoria at distance, greater exophoria at near—convergence insufficiency
Esophoria at distance, orthophoria at near—divergence insufficiency
Heterophoria with Normal AC/A Ratio
Orthophoria at distance, orthophoria at near—fusional vergence dysfunction
Esophoria at distance, same degree of esophoria at near—basic esophoria
Exophoria at distance, same degree of exophoria at near—basic exophoria
Heterophoria with High AC/A Ratio
Orthophoria at distance and esophoria at near—convergence excess
Esophoria at distance, greater esophoria at near—convergence excess
Exophoria at distance, less exophoria at near—divergence excess
Vertical Heterophoria
Right or left hyperphoria
Accommodative Anomalies
Accommodative insufficiency
Ill-sustained accommodation
Accommodative excess
Accommodative infacility
Ocular Motor Problems
Ocular motor dysfunction
Analysis of Specific Groups
POSITIVE FUSIONAL VERGENCE GROUP DATA
Optometric data that can be used to determine the status of a patient’s PFV are included in this category. These include all data that directly or indirectly assess PFV at both distance and near.
Positive Fusional Vergence: Smooth Vergence Testing
As base-out prism is added, the patient is instructed to keep the target single and clear as long as possible and to report when the target blurs or becomes double. This requires the patient to converge to maintain bifoveal fixation and maintain accommodation at a given level (either distance or near). It is also important to realize that as prism is added and the patient converges, the accommodative response gradually increases because of increased vergence accommodation. The amount of vergence accommodation stimulated depends on the convergence accommodation to convergence (CA/C) ratio. (The CA/C ratio is discussed in depth in Chapter 16.) The patient must relax accommodation to counterbalance this increased vergence accommodation. When the patient can no longer do this, a blur occurs. As more base-out prism is added beyond the blur limit, diplopia occurs when fusion is no longer possible.
An important aspect of this test is that the prism is added in a slow, gradual manner. Because the technique requires the patient to maintain accommodation at a given level, accommodative convergence cannot be used to assist convergence. The patient must, therefore, use PFV. If the patient attempts to use accommodative convergence, he or she will report a blur.
Positive Fusional Vergence: Step Vergence Testing
Step vergence testing is similar to the smooth vergence testing described earlier, except that it is performed outside the phoropter with a prism bar. Because a prism bar is used instead of Risley prisms, the actual prismatic demand is presented in a steplike manner. This is in contrast to the smooth demand introduced using Risley prism. Studies have determined that the expected findings for this test are different from smooth fusional vergence testing for children.20,22,23
Positive Fusional Vergence: Vergence Facility Testing
The patient is instructed to keep a vertical line of 20/30 letters single and clear as base-out prism is suddenly introduced (12 base-out and 3 base-in). To accomplish this, the patient must maintain his or her accommodative level at 2.50 D, using 12 Δ of PFV to restore bifoveal fixation. Because of the lag of accommodation, the actual accommodative response will generally be less than 2.50 D. The usual accommodative response for a 2.50 D accommodative stimulus is about 1.75 to 2.00 D. If sufficient fusional vergence is available, the response will be a single clear image. A report of diplopia indicates that the patient cannot restore binocularity using PFV. Another possible response is a single but blurred target, suggesting the use of accommodative convergence to compensate for the inability to use the fusional vergence mechanism to restore bifoveal fixation.
The important differentiation between vergence facility testing and standard testing of PFV is that prism is introduced in large increments and over a longer period of time. A patient is forced to make rapid changes in fusional vergence to sustain these changes over time. A patient having adequate smooth fusional vergence ranges may experience difficulty on the vergence facility test.
Negative Relative Accommodation
This test evaluates PFV in an indirect manner. The NRA is comparable to the assessment of smooth fusional vergence ranges, because lenses are introduced in a slow, gradual manner. However, with the NRA, the patient is being asked to maintain convergence at a particular level while changing the accommodative response. As plus lenses are added in +0.25 D increments, the patient is instructed to keep the target single and clear. To accomplish this, he or she must relax accommodation. However, any relaxation of accommodation is accompanied by a decrease in accommodative convergence. The amount of accommodative convergence change depends on the AC/A ratio.
If the patient allows his or her eyes to diverge as accommodation is relaxed, he or she will report diplopia. To counteract this decrease in accommodative convergence, the patient must use an appropriate amount of PFV. Thus, the result obtained during the NRA test can depend on the status of the PFV system. Of course, the endpoint in the NRA can also be limited by the patient’s ability to relax accommodation as plus lenses are introduced.
To determine which factor—accommodation or PFV—is causing the blur, the patient’s accommodative status can be tested monocularly. If he or she can clear +2.50 monocularly but only +1.50 binocularly, PFV is the causative factor. Another way to differentiate is simply to cover one eye after the patient reports blur on the NRA test. If the target clears under monocular conditions, the fusional vergence system is at fault.
Binocular Accommodative Facility with Plus Lenses
This test is similar to the NRA, because it requires maintenance of convergence at a specific level while the accommodative response changes. As +2.00 lenses are introduced binocularly, the patient is instructed to maintain single and clear binocular vision. To accomplish this, the patient must relax about 2.00 D of accommodation to keep the target clear (the actual accommodative response will be about 10% less than the stimulus). The relaxation of 2.00 D of accommodation, however, causes a reflex decrease in accommodative convergence. The amount of divergence will be directly related to the AC/A ratio. Assuming a 5:1 AC/A ratio, if the patient relaxes 2.00 D of accommodation, his or her eyes will tend to diverge by 10 Δ. If this occurs, the patient will see two images.
Because the instructions require the patient to maintain single clear vision, he or she must use 10 Δ of PFV to compensate for the decrease in accommodative convergence. The endpoint of this test can be caused by one of two factors. Either the patient has inadequate PFV or is unable to relax his or her accommodative system (ACC). To differentiate, one simply needs to cover one eye. If the print clears under monocular conditions, the limiting factor is the fusional vergence system.
Near Point of Convergence
The patient is asked to maintain single vision as a target is moved toward his or her nose. To accomplish this, the patient can use a combination of various types of convergence, including accommodative convergence, PFV, and proximal convergence. If PFV is deficient, it may affect the patient’s ability to achieve the expected finding on this test. A receded near point of convergence is, therefore, an indirect measure of PFV.
Monocular Estimation Method Retinoscopy and Fused Cross-cylinder
Both tests are performed under binocular conditions and are designed to assess the accommodative response. The normal finding is approximately +0.25 to +0.50 for MEM retinoscopy and +0.50 to +0.75 for the fused cross-cylinder test. However, when a patient presents with exophoria and low PFV group findings, the MEM and fused cross-cylinder tests often yield less plus than expected.
Decreased plus on these tests is interpreted as overaccommodation for the particular stimulus. This is a common response in a patient with exophoria and reduced PFV. The individual is substituting accommodative convergence for the lack of PFV. By overaccommodating, the patient has additional accommodative convergence available to help overcome the exophoria.
SUMMARY
These seven tests constitute the PFV group. In the presence of exophoria and symptoms, the data in the PFV group will generally be lower than expected, and the MEM and the fused cross-cylinder tests will tend to show overaccommodation (less plus than expected). All of the findings in this group provide information about the patient’s PFV system and the ability to compensate for exophoria. Occasionally only the facility findings will be low, whereas the amplitude findings are normal. This would be the type of situation missed with the graphical analysis approach.
NEGATIVE FUSIONAL VERGENCE GROUP DATA
This group includes optometric data that reflect the status of a patient’s NFV. It includes tests that directly or indirectly assess NFV at both distance and near.
Negative Fusional Vergence: Smooth Vergence Testing
As base-in prism is gradually added, the patient is instructed to keep the target single and clear as long as possible and to report if the target blurs or becomes double. The test requires the patient to diverge to maintain bifoveal fixation and maintain accommodation at a given level. It is also important to realize that as prism is added and the patient diverges, the accommodative response gradually decreases as a result of decreased vergence accommodation. The amount of decrease in vergence accommodation depends on the CA/C ratio. The patient must stimulate accommodation to counterbalance this decreased vergence accommodation. When the patient can no longer do this, a blur occurs. By requiring clarity, we are forcing the patient to use NFV to compensate for the base-in prism.
An important aspect of this test is that the prism is added in a slow, gradual manner.
Negative Fusional Vergence: Step Vergence Testing
Although the introduction of the prism demand is different from smooth vergence testing, the instructional set and the explanation of the requirements of the test are similar to that described for smooth vergence testing.
Negative Fusional Vergence: Vergence Facility Testing
The patient is instructed to keep a vertical line of 20/30 letters single and clear as 12 Δ base-out and 3 Δ base-in prism is abruptly introduced. To accomplish this, the patient must maintain his or her accommodative level at 2.50, while using 3 Δ of NFV to restore bifoveal fixation. If sufficient fusional vergence is available, the response will be a single clear image. A report of diplopia would indicate that the patient could not restore binocularity using NFV. A report of a single blurred target indicates the use of a decrease in accommodative convergence to aid the fusional vergence mechanism.
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