立体视正常值变化观察
作者：Ping Situ David Elliott
单位：Ping Situ(加拿大滑铁卢大学眼视光学院)；David Elliott(英国布雷德福大学)
关键词：立体视；立体视检查；方法；可重复性
眼视光学杂志000407 [摘 要] 目的:比较Frisby、Randot、Howard-Dolman三种立体视检查方法的可重复性，确定此三种临床用立体视检查方法的正常值变化的95%可信区间。方法:选择26位无斜视、弱视和眼手术史的健康个体，平均年龄为24.42±4.26岁(14～32岁)，所有的受检者具有正常的双眼视且双眼视力为6/6。测量视远瞳距和习惯视力后，以随机的顺序用Frisby、Randot、Howard-Dolman三种检查方法测量立体视，测量时保持光照度为670lx。约1周后在相同的测量条件下同样用三种检查方法测量立体视，平均间隔时间为7.8±2.4天。 结果:三种临床立体视检查方法的可重复性系数分别为±2.7″(Frisby)、±8.0″(Randot)和±9.3″(Howard-Dolman)，再次试验的平均值低于首次试验的平均值，t检验提示这改变差异无显著性意义(P>0.05)。重复试验的相关系数分别为0.91(Frisby)、0.56(Randot)、0.60(Howard-Dolman)。三种检查方法之间的相关系数分别为0.31、0.31、0.33。结论：三种临床立体视检查方法的相关性较差。Frisby检查法有最好的重复性和正常值有较小的范围。用该法测得成年人立体视改变的95%可信区间为±2.7″。如果用Frisby法测得年轻成年人的立体视的变化超过这个范围，可认为是有显著意义的临床改变。
, 百拇医药
[中图分类号] R778.3 [文献标识码] A
[文章编号] 1008-1801(2000)04-0216-03
The detection of change in stereoacuity
Ping Situ
(Centre for Contact Lens Research,School of Optometry, University of Waterloo,Waterloo, Ontario, Canada, N2L 3G1)
David Elliott
(Department of Optometry,University of Bradford,Bradford,BD7 1DP,UK.)
, 百拇医药
Abstract：Objective：To compare the repeatability of the Frisby, Randot and Howard-Dolman stereotests, and to determine the 95% confidence limits for the change of each test.Methods：Stereoacuity using the three clinical stereoacuity tests was measured in 26 healthy adult subjects with normal binocular vision. The subjects were retested approximately one week later.Results：The coefficients of repeatability for the three tests were ±2.7″ (Frisby) ±8.0″ (Randot) and ±9.3″ (Howard-Dolman) and test-retest correlation coefficients were 0.91 (Frisby), 0.56 (Randot) and 0.60 (Howard-Dolman).Conclusion：The Frisby test showed the best repeatability and smallest range of normal values. For young adults, the 95% confidence limits for change were calculated to be ± 2.7″. If a young adult's stereoacuity measured with the Frisby test changes by more than this amount, then this should be considered a significant clinical change.
, http://www.100md.com
Key words：stereoacuity; stereoacuity tests/methods; test-retest repeatability
[CLC number] R778 [Document code] A
[Article ID] 1008-1801(2000)04-0216-03
Introduction
Measurements of stereoacuity can be used clinically to determine the presence of binocular single vision and how well developed or established it is, and to monitor possible change with treatment or time. The detection of change in stereoacuity or deviation from normality is clinically important, as it is liable to influence the likely management of a particular condition. Changes in score on a stereoacuity test (or any other test) can occur due to chance and despite no real change having occurred. For a clinician to decide that a real change has occurred, confidence limits for change need to be determined for a particular test[1,2]. These confidence limits can be determined as the outer limits within which the vast majority (95%) of ‘normal’ subjects will change from test to retest[1,2].These limits are influenced by the inherent variability of a test, in that tests which are inherently variable are likely to provide larger confidence limits for change[3]. Such tests will be less capable to detect changes in score or deviations from normality[3]. The purpose of this study was to determine the confidence limits for change for three commonly used clinical stereoacuity tests: the Howard-Dolman, Frisby, and Randot stereotests.
, http://www.100md.com
Although several papers have assessed stereoacuity tests in terms of their screening ability[4～9], very few have determined their ability to detect change in stereopsis. Frisby and colleagues determined correlation coefficients between test and re-test for the Frisby, TNO and Titmus circles stereotests[10]. However, they did not provide information regarding the confidence limits for change for the three tests. In addition, correlation coefficients, although frequently used to assess repeatability, only give a measure of association but not necessarily of agreement[11]. Moreover, they are dependent upon the range values in the sample, as well as their association. A more appropriate indicator of repeatability is gained from the coefficient of repeatability (COR)[11]. The COR describes the 95% confidence limits for any discrepancy between test and retest data. These confidence limits also provide information for longitudinal assessment of function so the significance of any change in performance can be assessed[1,2].
, http://www.100md.com
1 Materials and methods
1.1 Frisby stereotest
This is a real depth test consisting of a series of three perspex plates of varying thickness (6mm,3mm,1.5mm). Each plate has four 6mm squares of random dot patterns printed on one side. One of the squares contains a small central circular area of dots on the back of the plate. Provided the patient's head and test plate are parallel and do not move during testing, so the disparity produced by the circular patch can only be detected using stereoacuity. At a 40cm testing distance, the disparities of the plates are 340″, 170″ and 85″ for a PD of 64 mm. The amount of stereopsis presented to a subject can be altered by changing the viewing distance. Stereoacuity was determined using a bracketing technique with the plate being presented in a number of random positions and different distances. Threshold was recorded as the largest distance at which correct responses were given in at least 3 out of four presentations.
, 百拇医药
1.2 Randot test
The Randot test with random dot stereograms in vectograph form views using polaroid glasses. It consists of three parts, of which the first two parts assess gross stereopsis only and were not used. Contoured circles at ten levels of crossed disparity provide a finely graded sequence for critical testing. The circles can be presented in uncrossed disparity by turning the booklet over. The range of disparity provided is from 400″ to 20″ when used at 40cm. Threshold was recorded as the largest distance where the last level of stereopsis was chosen correctly.
, 百拇医药
1.3 Howard-Dolman test
This is another real-depth test. The version used consisted of one stationary rod and one moveable rod, which were viewed through a rectangular aperture against an evenly illuminated white background at a testing distance of 3 meters. Subjects were required to align the moved rod with the stationary rod. The offset from alignment in seconds of arc of retinal disparity was calculated. The process was repeated six times and the average was taken.
, http://www.100md.com
1.4 Subjects
26 healthy subjects who had no history of strabismus, amblyopia or ocular surgery participated in the study. The mean age (±1SD) was 24.42±4.26 years (range from 14-32 years). All subjects had binocular visual acuity 6/6.
1.5 Procedure
After informed consent was obtained, the subject's distance interpupillary distance and habitual visual acuity were measured. Stereoacuity was then measured using the three tests in a random order at a room illumination of 670 lux. The tests were repeated under identical conditions approximately one week later (mean test-retest interval was 7.8±2.4 days).
, http://www.100md.com
1.6 Data Analysis
The absolute value of stereothreshold was used in all calculations (In this way, values of -4″,-4″ (crossed disparities),+4″,+4″ (uncrossed disparities) produced a mean stereothreshold of 4″ rather than 0″). The repeatability of each test was assessed using the coefficient of repeatability (COR). The COR describes the 95% confidence limits for any discrepancy between test and retest data, and for normally distributed data it is calculated as 1.96 multiplied by the standard deviation of the discrepancy.
, 百拇医药
2 Results
Table 1 shows the mean test and retest stereoacuity thresholds for the three tests. The mean values of retest data are slightly lower than the test scores, suggesting a slight learning effect. However, two-tailed t-tests indicated that these changes were not significant (P> 0.05). Coefficients of repeatability (COR) and test-retest Pearson correlation coefficients are shown in Table 2. The CORs were calculated as 1.96 times the standard deviation of the discrepancy between test and retest. Table 3 displays Pearson correlation coefficients between the three stereoacuity tests.
, http://www.100md.com
Tab.1 Mean stereoacuity ± SD of test and retest results for the three ste-
reoacuity tests from 26 normal adult subjects
Howard-Dolman
Frisby
Randot
test
8.30″±5.75″
7.43″±3.18″
10.87″±4.45″
, 百拇医药
retest
7.68″±4.55″
6.88″±3.36″
9.68″±4.24″
Tab.2 Coefficient of repeatability (COR) and test-retest Pearson correlation
coefficients for the three stereoacuity tests from 26 normal adult sub-jects
Howard-Dolman
Frisby
Randot
, 百拇医药
COR
±9.3″
±2.7″
±8.0″
correlation coefficient
0.60
0.91
0.56
Tab.3 Inter-test Pearson correlation coefficients
among the three stereoacuity tests
Randot
, 百拇医药
Frisby
Howard-Dolman
0.31
0.31
Randot
-
0.33
3 Discussion
Our results of mean stereoacuity are generally similar to those from other studies that used selected normal subjects and similar methods. Heron and co-workers[9] found similar median stereoacuity values of 8.0″ and 20.0″ in young adults for the Frisby and Randot tests respectively, and Simmerman[12] found a mean value for young adults on the Frisby stereotest of 10.7″. Obviously, much poorer level of stereoacuity can be obtained if the subject group is unselected and includes subjects with strabismus or poor binocular vision. For example, Frisby and colleagues obtained a mean stereothreshold of 79″ in an unselected group of young adults, some of whom had stereothresholds of 800″ and 400″[10]. These data were also truncated in that the best score a subject could gain was 40″[10]. Brown and colleagues measured stereoacuity using a Howard-Dolman apparatus and found an average stereoacuity slightly higher than ours of 16.8″ in a group of subjects aged 21-28 years[13]. Although they selected only patients with healthy eyes and good binocular vision as we did, they used a complicated psychophysical procedure to measure threshold with a minimum step size of 3.55″ which may have led to a higher threshold.
, 百拇医药
The stereothresholds from the three tests were similar at 7-11″. However, the range of stereoacuities was much greater for the Howard-Dolman and Randot compared with the Frisby test (Table 1). The 95% confidence ranges for the three tests can be calculated as follow:the lower 95% confidence limit=mean-(1.96 times SD),the upper 95% confidence limit=mean+(1.96 times SD). These ranges from the test results were 0″ to 19.6″ (Howard-Dolman),2.2″ to 19.6″ (Randot) and 1.2″ to 13.7″. These findings are similar to those of Heron and colleagues[9]. They measured stereoacuity in normal children and adults, and concluded that among the Frisby, TNO, Titmus, and Randot tests, the Frisby stereotest showed the least variability. The Frisby stereotest is therefore more likely to detect subtle deviations from normality compared to the other two tests. The larger 95% confidence ranges of values for the Randot and Howard-Dolman tests compared to the Frisby test from normal subjects is likely due to their greater inherent variability (Table 2).
, http://www.100md.com
The Frisby test was shown to have the best repeatability (COR of ±2.7″) and the highest test-retest correlation (r=0.91) in the study. This is similar to previous findings of Frisby and co-workers who found a higher test-retest correlation for the Frisby test (0.76) compared to the Titmus circles (0.51) and TNO stereotests (0.29)[10]. The better repeatability is also shown in the smaller inter-subject variability as discussed above. No previous studies have reported the 95% confidence limits for change for stereotests. We found values of ± 9.3″ (Howard-Dolman),±8.0″ (Randot) and ±2.7″ (Frisby)(Table 2). This indicates that for a change in stereoacuity to be 'real' (meaning not just due to chance variability) it must be greater than 9.3″ for the Howard-Dolman, greater than 8″ for the Randot and greater than only 2.7″ for the Frisby test. Obviously, these figures are only valid if the tests are measured using the methodology described here and for a young adult group. It is highly likely, for example, that the 95% confidence limits for change would be much larger for all three tests for a group of young children (Heron et al.) If a value for a 'real' change in stereoacuity for children of a certain age were required,the experiment described would have to be repeated with children of that age group with normal healthy eyes and binocular vision. It should also be noted that the technique used here with both the Frisby and Randot tests of varying the working distance is only practical for non-presbyopic subjects. With presbyopic subjects,such as when assessing stereopsis before and after cataract surgery[14], a different reading add would be required for each new distance used. The Frisby test is particular affected in this regard as it only has three step sizes (340″,170″ and 85″ at 40cm with a PD of 64 mm) if working distance is not varied.
, 百拇医药
Generally the three clinical tests used in the study are poorly correlated, with correlation coefficients of around 0.30(Table 3). This is similar to the findings of earlier results[7～10], and is likely due in part to different design characteristics and psychophysical methods in each test[9]. In addition, the poor correlations are due to the tests' variability, for example, if the Randot and Howard-Dolman provide relatively low test-retest correlations (0.56 and 0.60 respectively, i.e. they correlate moderately with themselves), how can they correlate highly with another test? Previous authors have also suggested that the tests might measure different aspects of stereo performance, which resulted in a poor correlation between the tests. Therefore, they have argued against judging stereoacuity on the basis of one test alone.
, 百拇医药
In summary, among the three stereotests used in the study, the Frisby test showed the best repeatability and smallest range of normal values. Furthermore, it has several other advantages: no glasses needed, independent on visual acuity (large dot size) and impossible to solve in the absence of binocular single vision[5]. A disadvantage is that it is difficult to use with presbyopic patients, such as before and after cataract surgery[14], because of the practical difficulties of changing reading add for each change in working distance. For young adults, the 95% confidence limits for change were calculated to be ±2.7″. If a young adult's stereoacuity measured with the Frisby test changes by more than this amount, then this should be considered a significant clinical change.
, 百拇医药
Biography:Ping Situ(1962-), B.Med.MSc. centre for contact lens research,Scool of Optometry,University of Waterloo.
References
[1] Reeves BC, Hill AR, Aspinall PA. The clinical significance of change[J]. Ophthal Physiol Opt. 1987，7:441-446.
[2] Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effects of scaling[J]. Invest Ophthalmol Vis Sci，1991，32:422-432.
, http://www.100md.com
[3] Elliott DB, Bullimore MA. Assessing the reliability, discriminative ability, and validity of disability glare tests[J]. Invest Ophthalmol Vis Sci，1993，34:108-119.
[4] Okuda FC, Wanter BS. Evaluation of the TNO random-dot stereogram test[J]. Am Orthopt J，1977，27:124-130.
[5] Hinchliffe HA. Clinical evaluation of stereopsis[J]. Brit Orthopt J，1978，35:46-57.
[6] Cooper J, Feldman J, Medlin D. Comparing stereoscopic performance of children using the Titmus, TNO and Randot stereo tests[J]. J Am Optom Assoc，1979，50: 821-825.
, http://www.100md.com
[7] Simons K. A comparison of the Frisby, Random-dot E, TNO, and Randot circles stereotests in screening and office use[J]. Arch Ophthalmol，1981，99，446-4529.
[8] Hall C. The relationship between clinical stereotests[J]. Ophthal Physiol Opt，1982，2:135-143.
[9] Heron G, Dholakia S, Collins DE, McLaughlan H. (1985) Stereoscopic threshold in children and adults[J]. Am J Optom Physiol Opt，1985，62:505-515.
, 百拇医药
[10] Frisby JP, Nielsen P, Parker J. Clinical tests of stereoacuity: Do they measure the same thing? In: Mein J, Moore S, eds. Orthoptics, Research and Practice[C]. Transactions of the Fourth International Orthoptic Congress, Berne,1979. London: Kimpton,1981.211-214.
[11] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement[J]. The Lancet，1986，1:307-310.
[12] Simmerman JS. Absolute threshold of stereopsis using the Frisby ster-eotest[J]. J Am Optom Assoc，1984，55:50-53.
, 百拇医药
[13] Brown B, Yap MKH, Fan WCS. Decrease in stereoacuity in the seventh decade of life[J]. Ophthal Physiol Opt，1993，13:138-142.
[14] Elliott DB, Patla AE, Furniss M, Adkin A. Improvements in clinical and functional vision and quality of life after second eye cataract surgery[J]. Optom Vis Sci，2000，77:13-24.
Received date:2000-08-29, 百拇医药
单位：Ping Situ(加拿大滑铁卢大学眼视光学院)；David Elliott(英国布雷德福大学)
关键词：立体视；立体视检查；方法；可重复性
眼视光学杂志000407 [摘 要] 目的:比较Frisby、Randot、Howard-Dolman三种立体视检查方法的可重复性，确定此三种临床用立体视检查方法的正常值变化的95%可信区间。方法:选择26位无斜视、弱视和眼手术史的健康个体，平均年龄为24.42±4.26岁(14～32岁)，所有的受检者具有正常的双眼视且双眼视力为6/6。测量视远瞳距和习惯视力后，以随机的顺序用Frisby、Randot、Howard-Dolman三种检查方法测量立体视，测量时保持光照度为670lx。约1周后在相同的测量条件下同样用三种检查方法测量立体视，平均间隔时间为7.8±2.4天。 结果:三种临床立体视检查方法的可重复性系数分别为±2.7″(Frisby)、±8.0″(Randot)和±9.3″(Howard-Dolman)，再次试验的平均值低于首次试验的平均值，t检验提示这改变差异无显著性意义(P>0.05)。重复试验的相关系数分别为0.91(Frisby)、0.56(Randot)、0.60(Howard-Dolman)。三种检查方法之间的相关系数分别为0.31、0.31、0.33。结论：三种临床立体视检查方法的相关性较差。Frisby检查法有最好的重复性和正常值有较小的范围。用该法测得成年人立体视改变的95%可信区间为±2.7″。如果用Frisby法测得年轻成年人的立体视的变化超过这个范围，可认为是有显著意义的临床改变。
, 百拇医药
[中图分类号] R778.3 [文献标识码] A
[文章编号] 1008-1801(2000)04-0216-03
The detection of change in stereoacuity
Ping Situ
(Centre for Contact Lens Research,School of Optometry, University of Waterloo,Waterloo, Ontario, Canada, N2L 3G1)
David Elliott
(Department of Optometry,University of Bradford,Bradford,BD7 1DP,UK.)
, 百拇医药
Abstract：Objective：To compare the repeatability of the Frisby, Randot and Howard-Dolman stereotests, and to determine the 95% confidence limits for the change of each test.Methods：Stereoacuity using the three clinical stereoacuity tests was measured in 26 healthy adult subjects with normal binocular vision. The subjects were retested approximately one week later.Results：The coefficients of repeatability for the three tests were ±2.7″ (Frisby) ±8.0″ (Randot) and ±9.3″ (Howard-Dolman) and test-retest correlation coefficients were 0.91 (Frisby), 0.56 (Randot) and 0.60 (Howard-Dolman).Conclusion：The Frisby test showed the best repeatability and smallest range of normal values. For young adults, the 95% confidence limits for change were calculated to be ± 2.7″. If a young adult's stereoacuity measured with the Frisby test changes by more than this amount, then this should be considered a significant clinical change.
, http://www.100md.com
Key words：stereoacuity; stereoacuity tests/methods; test-retest repeatability
[CLC number] R778 [Document code] A
[Article ID] 1008-1801(2000)04-0216-03
Introduction
Measurements of stereoacuity can be used clinically to determine the presence of binocular single vision and how well developed or established it is, and to monitor possible change with treatment or time. The detection of change in stereoacuity or deviation from normality is clinically important, as it is liable to influence the likely management of a particular condition. Changes in score on a stereoacuity test (or any other test) can occur due to chance and despite no real change having occurred. For a clinician to decide that a real change has occurred, confidence limits for change need to be determined for a particular test[1,2]. These confidence limits can be determined as the outer limits within which the vast majority (95%) of ‘normal’ subjects will change from test to retest[1,2].These limits are influenced by the inherent variability of a test, in that tests which are inherently variable are likely to provide larger confidence limits for change[3]. Such tests will be less capable to detect changes in score or deviations from normality[3]. The purpose of this study was to determine the confidence limits for change for three commonly used clinical stereoacuity tests: the Howard-Dolman, Frisby, and Randot stereotests.
, http://www.100md.com
Although several papers have assessed stereoacuity tests in terms of their screening ability[4～9], very few have determined their ability to detect change in stereopsis. Frisby and colleagues determined correlation coefficients between test and re-test for the Frisby, TNO and Titmus circles stereotests[10]. However, they did not provide information regarding the confidence limits for change for the three tests. In addition, correlation coefficients, although frequently used to assess repeatability, only give a measure of association but not necessarily of agreement[11]. Moreover, they are dependent upon the range values in the sample, as well as their association. A more appropriate indicator of repeatability is gained from the coefficient of repeatability (COR)[11]. The COR describes the 95% confidence limits for any discrepancy between test and retest data. These confidence limits also provide information for longitudinal assessment of function so the significance of any change in performance can be assessed[1,2].
, http://www.100md.com
1 Materials and methods
1.1 Frisby stereotest
This is a real depth test consisting of a series of three perspex plates of varying thickness (6mm,3mm,1.5mm). Each plate has four 6mm squares of random dot patterns printed on one side. One of the squares contains a small central circular area of dots on the back of the plate. Provided the patient's head and test plate are parallel and do not move during testing, so the disparity produced by the circular patch can only be detected using stereoacuity. At a 40cm testing distance, the disparities of the plates are 340″, 170″ and 85″ for a PD of 64 mm. The amount of stereopsis presented to a subject can be altered by changing the viewing distance. Stereoacuity was determined using a bracketing technique with the plate being presented in a number of random positions and different distances. Threshold was recorded as the largest distance at which correct responses were given in at least 3 out of four presentations.
, 百拇医药
1.2 Randot test
The Randot test with random dot stereograms in vectograph form views using polaroid glasses. It consists of three parts, of which the first two parts assess gross stereopsis only and were not used. Contoured circles at ten levels of crossed disparity provide a finely graded sequence for critical testing. The circles can be presented in uncrossed disparity by turning the booklet over. The range of disparity provided is from 400″ to 20″ when used at 40cm. Threshold was recorded as the largest distance where the last level of stereopsis was chosen correctly.
, 百拇医药
1.3 Howard-Dolman test
This is another real-depth test. The version used consisted of one stationary rod and one moveable rod, which were viewed through a rectangular aperture against an evenly illuminated white background at a testing distance of 3 meters. Subjects were required to align the moved rod with the stationary rod. The offset from alignment in seconds of arc of retinal disparity was calculated. The process was repeated six times and the average was taken.
, http://www.100md.com
1.4 Subjects
26 healthy subjects who had no history of strabismus, amblyopia or ocular surgery participated in the study. The mean age (±1SD) was 24.42±4.26 years (range from 14-32 years). All subjects had binocular visual acuity 6/6.
1.5 Procedure
After informed consent was obtained, the subject's distance interpupillary distance and habitual visual acuity were measured. Stereoacuity was then measured using the three tests in a random order at a room illumination of 670 lux. The tests were repeated under identical conditions approximately one week later (mean test-retest interval was 7.8±2.4 days).
, http://www.100md.com
1.6 Data Analysis
The absolute value of stereothreshold was used in all calculations (In this way, values of -4″,-4″ (crossed disparities),+4″,+4″ (uncrossed disparities) produced a mean stereothreshold of 4″ rather than 0″). The repeatability of each test was assessed using the coefficient of repeatability (COR). The COR describes the 95% confidence limits for any discrepancy between test and retest data, and for normally distributed data it is calculated as 1.96 multiplied by the standard deviation of the discrepancy.
, 百拇医药
2 Results
Table 1 shows the mean test and retest stereoacuity thresholds for the three tests. The mean values of retest data are slightly lower than the test scores, suggesting a slight learning effect. However, two-tailed t-tests indicated that these changes were not significant (P> 0.05). Coefficients of repeatability (COR) and test-retest Pearson correlation coefficients are shown in Table 2. The CORs were calculated as 1.96 times the standard deviation of the discrepancy between test and retest. Table 3 displays Pearson correlation coefficients between the three stereoacuity tests.
, http://www.100md.com
Tab.1 Mean stereoacuity ± SD of test and retest results for the three ste-
reoacuity tests from 26 normal adult subjects
Howard-Dolman
Frisby
Randot
test
8.30″±5.75″
7.43″±3.18″
10.87″±4.45″
, 百拇医药
retest
7.68″±4.55″
6.88″±3.36″
9.68″±4.24″
Tab.2 Coefficient of repeatability (COR) and test-retest Pearson correlation
coefficients for the three stereoacuity tests from 26 normal adult sub-jects
Howard-Dolman
Frisby
Randot
, 百拇医药
COR
±9.3″
±2.7″
±8.0″
correlation coefficient
0.60
0.91
0.56
Tab.3 Inter-test Pearson correlation coefficients
among the three stereoacuity tests
Randot
, 百拇医药
Frisby
Howard-Dolman
0.31
0.31
Randot
-
0.33
3 Discussion
Our results of mean stereoacuity are generally similar to those from other studies that used selected normal subjects and similar methods. Heron and co-workers[9] found similar median stereoacuity values of 8.0″ and 20.0″ in young adults for the Frisby and Randot tests respectively, and Simmerman[12] found a mean value for young adults on the Frisby stereotest of 10.7″. Obviously, much poorer level of stereoacuity can be obtained if the subject group is unselected and includes subjects with strabismus or poor binocular vision. For example, Frisby and colleagues obtained a mean stereothreshold of 79″ in an unselected group of young adults, some of whom had stereothresholds of 800″ and 400″[10]. These data were also truncated in that the best score a subject could gain was 40″[10]. Brown and colleagues measured stereoacuity using a Howard-Dolman apparatus and found an average stereoacuity slightly higher than ours of 16.8″ in a group of subjects aged 21-28 years[13]. Although they selected only patients with healthy eyes and good binocular vision as we did, they used a complicated psychophysical procedure to measure threshold with a minimum step size of 3.55″ which may have led to a higher threshold.
, 百拇医药
The stereothresholds from the three tests were similar at 7-11″. However, the range of stereoacuities was much greater for the Howard-Dolman and Randot compared with the Frisby test (Table 1). The 95% confidence ranges for the three tests can be calculated as follow:the lower 95% confidence limit=mean-(1.96 times SD),the upper 95% confidence limit=mean+(1.96 times SD). These ranges from the test results were 0″ to 19.6″ (Howard-Dolman),2.2″ to 19.6″ (Randot) and 1.2″ to 13.7″. These findings are similar to those of Heron and colleagues[9]. They measured stereoacuity in normal children and adults, and concluded that among the Frisby, TNO, Titmus, and Randot tests, the Frisby stereotest showed the least variability. The Frisby stereotest is therefore more likely to detect subtle deviations from normality compared to the other two tests. The larger 95% confidence ranges of values for the Randot and Howard-Dolman tests compared to the Frisby test from normal subjects is likely due to their greater inherent variability (Table 2).
, http://www.100md.com
The Frisby test was shown to have the best repeatability (COR of ±2.7″) and the highest test-retest correlation (r=0.91) in the study. This is similar to previous findings of Frisby and co-workers who found a higher test-retest correlation for the Frisby test (0.76) compared to the Titmus circles (0.51) and TNO stereotests (0.29)[10]. The better repeatability is also shown in the smaller inter-subject variability as discussed above. No previous studies have reported the 95% confidence limits for change for stereotests. We found values of ± 9.3″ (Howard-Dolman),±8.0″ (Randot) and ±2.7″ (Frisby)(Table 2). This indicates that for a change in stereoacuity to be 'real' (meaning not just due to chance variability) it must be greater than 9.3″ for the Howard-Dolman, greater than 8″ for the Randot and greater than only 2.7″ for the Frisby test. Obviously, these figures are only valid if the tests are measured using the methodology described here and for a young adult group. It is highly likely, for example, that the 95% confidence limits for change would be much larger for all three tests for a group of young children (Heron et al.) If a value for a 'real' change in stereoacuity for children of a certain age were required,the experiment described would have to be repeated with children of that age group with normal healthy eyes and binocular vision. It should also be noted that the technique used here with both the Frisby and Randot tests of varying the working distance is only practical for non-presbyopic subjects. With presbyopic subjects,such as when assessing stereopsis before and after cataract surgery[14], a different reading add would be required for each new distance used. The Frisby test is particular affected in this regard as it only has three step sizes (340″,170″ and 85″ at 40cm with a PD of 64 mm) if working distance is not varied.
, 百拇医药
Generally the three clinical tests used in the study are poorly correlated, with correlation coefficients of around 0.30(Table 3). This is similar to the findings of earlier results[7～10], and is likely due in part to different design characteristics and psychophysical methods in each test[9]. In addition, the poor correlations are due to the tests' variability, for example, if the Randot and Howard-Dolman provide relatively low test-retest correlations (0.56 and 0.60 respectively, i.e. they correlate moderately with themselves), how can they correlate highly with another test? Previous authors have also suggested that the tests might measure different aspects of stereo performance, which resulted in a poor correlation between the tests. Therefore, they have argued against judging stereoacuity on the basis of one test alone.
, 百拇医药
In summary, among the three stereotests used in the study, the Frisby test showed the best repeatability and smallest range of normal values. Furthermore, it has several other advantages: no glasses needed, independent on visual acuity (large dot size) and impossible to solve in the absence of binocular single vision[5]. A disadvantage is that it is difficult to use with presbyopic patients, such as before and after cataract surgery[14], because of the practical difficulties of changing reading add for each change in working distance. For young adults, the 95% confidence limits for change were calculated to be ±2.7″. If a young adult's stereoacuity measured with the Frisby test changes by more than this amount, then this should be considered a significant clinical change.
, 百拇医药
Biography:Ping Situ(1962-), B.Med.MSc. centre for contact lens research,Scool of Optometry,University of Waterloo.
References
[1] Reeves BC, Hill AR, Aspinall PA. The clinical significance of change[J]. Ophthal Physiol Opt. 1987，7:441-446.
[2] Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effects of scaling[J]. Invest Ophthalmol Vis Sci，1991，32:422-432.
, http://www.100md.com
[3] Elliott DB, Bullimore MA. Assessing the reliability, discriminative ability, and validity of disability glare tests[J]. Invest Ophthalmol Vis Sci，1993，34:108-119.
[4] Okuda FC, Wanter BS. Evaluation of the TNO random-dot stereogram test[J]. Am Orthopt J，1977，27:124-130.
[5] Hinchliffe HA. Clinical evaluation of stereopsis[J]. Brit Orthopt J，1978，35:46-57.
[6] Cooper J, Feldman J, Medlin D. Comparing stereoscopic performance of children using the Titmus, TNO and Randot stereo tests[J]. J Am Optom Assoc，1979，50: 821-825.
, http://www.100md.com
[7] Simons K. A comparison of the Frisby, Random-dot E, TNO, and Randot circles stereotests in screening and office use[J]. Arch Ophthalmol，1981，99，446-4529.
[8] Hall C. The relationship between clinical stereotests[J]. Ophthal Physiol Opt，1982，2:135-143.
[9] Heron G, Dholakia S, Collins DE, McLaughlan H. (1985) Stereoscopic threshold in children and adults[J]. Am J Optom Physiol Opt，1985，62:505-515.
, 百拇医药
[10] Frisby JP, Nielsen P, Parker J. Clinical tests of stereoacuity: Do they measure the same thing? In: Mein J, Moore S, eds. Orthoptics, Research and Practice[C]. Transactions of the Fourth International Orthoptic Congress, Berne,1979. London: Kimpton,1981.211-214.
[11] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement[J]. The Lancet，1986，1:307-310.
[12] Simmerman JS. Absolute threshold of stereopsis using the Frisby ster-eotest[J]. J Am Optom Assoc，1984，55:50-53.
, 百拇医药
[13] Brown B, Yap MKH, Fan WCS. Decrease in stereoacuity in the seventh decade of life[J]. Ophthal Physiol Opt，1993，13:138-142.
[14] Elliott DB, Patla AE, Furniss M, Adkin A. Improvements in clinical and functional vision and quality of life after second eye cataract surgery[J]. Optom Vis Sci，2000，77:13-24.
Received date:2000-08-29, 百拇医药