C L I N I C A L
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E X P E R I M E N T A L
OPTOMETRY RESEARCH PAPER
Comparison of four different binocular balancing techniques Clin Exp Optom 2014; 97: 422–425 Hamed Momeni-Moghaddam* MSc David A Goss† OD PhD * Health Promotion Research Center, Zahedan University of Medical Sciences, Zahedan, Iran † School of Optometry, Indiana University, Bloomington, Indiana, USA E-mail:
[email protected]
Submitted: 21 January 2014 Revised: 17 March 2014 Accepted for publication: 24 March 2014
DOI:10.1111/cxo.12198 Background: Several techniques of balancing have been proposed for the equalisation of the accommodative state between the two eyes. This study was designed to compare the results of different balancing techniques. Methods: In this study, 60 subjects participated. Refractive errors were determined using retinoscopy and results were refined with monocular subjective refraction to determine the ametropia with the most plus lens and this was used as the starting point for balancing. The monocular refractive values before the balance were the same for all balancing procedures. Four balancing techniques, three dissociated and one associated, (alternate occlusion, prism-dissociated blur balance, prism-dissociated red-green balance and Humphriss immediate contrast method) were used for the final refinement of findings. Results were analysed as the interocular spherical difference (IOSD) after the completion of each balance procedure. The interocular spherical difference was defined as the right eye sphere minus the left eye sphere. Data were analysed in SPSS.17 software using the paired samples t-test, the Pearson correlation, repeated measures ANOVA and ‘intraclass correlation coefficient’ (ICC) tests. Results: The lowest mean difference was related to the alternate occlusion with the prismdissociated blur balance techniques. The lowest 95% limits of agreement were related to the prism-dissociated red-green balance with the Humphriss immediate contrast methods. The highest correlation and intraclass correlation coefficient were related to the prism-dissociated red-green balance with the Humphriss immediate contrast methods. Conclusions: The four balancing methods yield very similar results. The balance findings with red-green dissociated method and the Humphriss immediate contrast technique can be considered interchangeable and the other pairs of comparisons very nearly so.
Key words: accommodation, binocular balance, binocular refraction, refractive errors, subjective refraction The end-point of monocular subjective refraction is the equalisation or balancing of the subjective endpoints for the two eyes. This is done with the assumption that each eye has been fully corrected when it attains its maximum visual acuity at distance. The technique, which serves to equalise the accommodative response in the two eyes is also known as the binocular balance or spherical equalisation.1 During monocular refraction, the practitioner considers only one eye at a time but most patients will normally use both of their eyes so that clear and comfortable binocular single vision is the ultimate goal. Occlusion can stimulate accommodation in the nontested eye and according to Hering’s rule an equivalent increase in accommodation in the eye being tested, will result. Hence refracting under monocular conditions may not result in maximum plus as obtained on the binocular spherical endpoint. This can
lead to possible over-minusing or underplussing, particularly in patients with hyperopia, pseudo-myopia and antimetropia.2 Equalisation of accommodative response is usually achieved through equalisation of visual acuity but the purpose of the binocular balancing is not necessarily to produce equal acuity in each eye. Because some patients have better visual acuity in one eye, the patient may be uncomfortable if the acuities are artificially equalised.2 Balancing of accommodation is not necessary where accommodative ability is lacking, such as in absolute presbyopia or pseudophakia. Also, it is thought that no separate binocular balancing is needed when subjective refraction is performed while maintaining peripheral fusion because such binocularity helps control accommodation.3 A variety of techniques have been developed to attempt to equalise accommodative
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response. The common methods for equalisation are: 1. alternate occlusion technique (successive alternate occlusion) 2. prism-dissociated blur balance technique 3. prism-dissociated duochrome (bichrome) balance technique and 4. Humphriss immediate contrast (HIC) method.4 Although several techniques of balancing have been proposed for the equalisation of the accommodative response between the two eyes, there is little comparison of the various binocular balancing techniques in the literature. West and Somers5 compared five methods of balancing (vectographic, equal acuity through low plus blur, redgreen equalisation, Jackson cross-cylinder and equal loss of acuity with plus blur) in 25 patients who ranged in age from seven to 58 years and reported that these balancing techniques have adequate repeatability with © 2014 The Authors
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Comparison of binocular balancing techniques Momeni-Moghaddam and Goss
only a small difference in the mean results. In contrast to the West and Somers study,5 the present study was designed to provide additional data comparing the results of more common techniques in practice using a larger sample. MATERIALS AND METHODS The subjects were 60 students from Zahedan University of Medical Sciences. They were randomly selected from the list of students. Before starting the study, the protocol was introduced to the subjects, who agreed to participate in this study and met inclusion criteria. Their informed consent was obtained before starting the study. In addition, the subjects were assured that their information was kept confidential in accordance with the tenets of the Declaration of Helsinki. The study was approved by the University ethics committee. The inclusion criteria were: 1. visual acuity 6/6 or better in each eye at six metres and 40 cm with or without correction to assess the subject’s sensitivity to small amounts of lens changes (0.25 D) 2. stereoacuity of at least 30 seconds of arc using the TNO stereotest 3. no strabismus at six metres or 40 cm with the cover test 4. no more than 1.00 D spherical equivalent of anisometropia 5. no anisocoria more than 1.0 mm 6. a lag of accommodation with retinoscopy in the range of 0.25 to 0.75 D and 7. no history of ocular trauma, ocular disease, amblyopia, aphakia or pseudophakia. Refractive errors were determined by static retinoscopy using a Heine ß-200 retinoscope. Retinoscopic findings were then refined by monocular subjective refraction to determine the ametropia based on the most plus lens and this was used as the starting point for balancing. Four balancing techniques were used for the final refinement of findings. In the prism-dissociated blur balance technique, both eyes were fogged by +1.00 D. The subject’s binocular visual acuity was reduced to less than to 6/7.5. A line of letters one line above the visual acuity found at the end of previous step was selected. Then equal amounts of prism were placed over each eye, 3Δ base-up over the right eye and 3Δ base-down over the left eye for dissociation of the two eyes. The prism power should be sufficient to make sure the subject could
see two lines of letters. The subject was instructed to compare two lines and to state whether the images were equally blurred, in which case equalisation of accommodation was assumed to exist or the images were unequally blurred, in which case, +0.25 D was given to the eye with clearer vision (to blur the image), until the vision of both eyes looked equal (endpoint). If the image still looked better in the eye with clearer vision after adding two steps of plus (+0.50), -0.25 D was added to the other eye (to make visual acuity in this eye better). When equality of blur in the two eyes was reached, the fog was reduced in 0.25 D steps in front of both eyes until finally the binocular maximum plus best visual acuity was determined.6 The prism-dissociated red-green balance technique was carried out in a dark room and sufficient plus power in +0.25 D steps was placed over both eyes until the subject reported that the letters on the red background were more prominent, while separate images were seen in the two eyes using 3Δ base up over the right eye and 3Δ base down over the left eye. In other words, each eye saw one chart under a slight fog, the lower image by the right eye and the upper by the left eye. At first the subject was directed to look at the upper image. When the letters on the red side were clearer, minus lenses were added in 0.25 D steps. The end-point was the lens power that gave the first equal response with letters on both red and green sides appearing equally clear. That procedure was repeated for the lower image.4 Then, the dissociating prisms were removed. Finally, in the fused condition, the eyes were simultaneously fogged by +1.00 D and then unfogged in 0.25 D steps to maximum binocular visual acuity. A self-produced binocular lens rack was used for lens changes to reduce testing time. The alternate occlusion method was done with a handheld cover paddle. At the end of monocular subjective refraction both eyes were fogged with +1.00 D and subjects were instructed to look at an isolated line of letters (6/15). Alternately, each eye was occluded for approximately a half second while asking the subject to compare clarity of vision in the right and left eyes. If the subject reported the vision in one eye was clearer, +0.25 D was given to the eye with clearer vision and alternate occlusion was repeated until approaching equality. The endpoint was when the clarity of both eyes was equal. As with the prism-dissociated blur balance technique,
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
the aim was to determine the lens, with which both eyes had equally clear images. If an exact balance could not be achieved, the lens was used that provided the closest match.6 Then the fog was slowly reduced binocularly until the best visual acuity was reached.4 To implement the Humphriss immediate contrast method, at the end of the monocular subjective refraction, the left eye was fogged with +1.00 D. The subject was directed to view the right eye’s best visual acuity line and spherical refraction was performed on the right eye. When +0.25 D sphere was added to the right eye, if the visual acuity was unchanged or improved, the plus power was increased in steps of 0.25 D and to ensure that the amount of fogging was still effective, additional plus power was added to the left eye. If it worsened with +0.25 DS, the plus lens was removed. The goal was to achieve maximum plus power or minimum minus. Then the right eye was fogged and the above step was performed over the left eye.7 The astigmatic correction was the same for the different methods. For confirmation of the results, three trials were completed of the different techniques and to minimise any possible bias secondary to learning effects, trials were done in random order. Also, all subjects were seen on at least three different days at similar times of day (9:00 am). For lowering the possibility of interexaminer variability, all data were collected by the same examiner and in the same examination conditions. A six-metre examination room was used in conjunction with a projected visual chart (LUNEAU L29) and Oculus trial frame. All testing was performed with full room illumination, which should be at least 12–20 footcandles (130 to 215 lux)3 except for the red-green method, which was done in a darkened room. The monocular refraction values before the balance were the same for all four balancing procedures. Results were analysed as the interocular spherical difference (IOSD) after the completion of each balance procedure. The interocular spherical difference was defined as the right eye sphere minus the left eye sphere. Data were analysed in SPSS.17 software (SPSS for Windows, SPSS Inc., Chicago, Illinois, USA) using the paired samples t-test, the Pearson correlation, repeated measures analysis of variance (ANOVA) and intraclass correlation coefficient (ICC) tests. The intraclass correlation coefficient results can
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Comparison of binocular balancing techniques Momeni-Moghaddam and Goss
SE
Cylinder
Sphere
Mean ± SD
Mean ± SD
Mean ± SD
(95% CI)
(95% CI)
(95% CI)
-0.86 ± 1.28
-0.32 ± 0.44
-0.70 ± 1.24
(-1.19 to -0.53)
(-0.28 to -0.05)
(-1.02 to -0.37)
-0.89 ± 1.28
-0.31 ± 0.40
-0.74 ± 1.27
(-1.23 to -0.56)
(-0.41 to -0.20)
(-1.07 to -0.41)
0.23
0.70
0.07
Variable Eye Right Left p-value
Table 1. Mean and standard deviation for sphere, cylinder and spherical equivalent (SE) separately in two eyes.
Correlation
ICC
95% limits of agreement (1.96 × SD of differences)
Mean difference (95% CI) 0.004 ± 0.327
r = -0.486
0.452
-0.644 to 0.636
p < 0.001
p < 0.001
(±0.640)
(-0.080 to 0.088)
r = -0.469
0.442
-0.658 to 0.568
-0.045 ± 0.313
p < 0.001
p < 0.001
(±0.613)
(-0.126 to 0.035)
r = -0.331
0.315
-0.708 to 0.576
-0.066 ± 0.328
p = 0.010
p = 0.006
(±0.642)
(-0.151 to 0.018)
r = -0.591
0.588
-0.679 to 0.579
-0.050 ± 0.321
p < 0.001
p < 0.001
(±0.629)
(-0.133 to 0.033)
r = -0.488
0.477
-0.750 to 0.610
-0.070 ± 0.347
r < 0.001
p < 0.001
(±0.680)
(-0.160 to 0.019)
r = -0.667
0.667
-0.507 to 0.547
-0.020 ± 0.269
p < 0.001
p < 0.001
(±0.527)
(-0.090 to 0.048)
Variables ALT-PB ALT-PRG ALT-HIC PB-PRG PB-HIC PRG-HIC
Table 2. Comparisons of interocular sphere difference for the different balancing techniques. Alternate occlusion (ALT), prism-dissociated blur balance (PB), prismdissociated R-G balance (PRG), Humphriss immediate contrast method (HIC).
be interpreted as follows: zero to 0.2 is an indication of poor agreement: 0.3 to 0.4 fair; 0.5 to 0.6 moderate; 0.7 to 0.8 strong and more than 0.8 indicates almost perfect agreement.8 Other variables determined for each pair of different balancing techniques were the mean difference between balancing techniques, the standard deviation (SD) and the coefficient of agreement or 95 percent confidence internal (COA = 1.96 × SD). RESULTS Of the 60 students of Zahedan University of Medical Sciences, who were participants in this study, 36 (60.0 percent) were male. The range of age was 18 to 28 years and the subjects had a mean age of 21.53 ± 2.06 years.
The mean and standard deviation of refractive errors based on the monocular subjective refraction before the balance tests in the right and left eyes are shown in Table 1. The paired samples t-test did not show a significant difference in the mean of sphere, cylinder and spherical equivalent between the two eyes (p > 0.05). A repeated measures ANOVA was used for comparison of the mean of interocular spherical difference among the four different methods and did not show differences in the means (F[3, 177] = 1.427, p = 0.2). The mean differences in interocular sphere difference for each comparison pair and the 95 percent limits of agreement are presented in Table 2.
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Table 2 shows that the lowest mean difference in interocular spherical difference among the different methods was related to the alternate occlusion with the prismdissociated blur balance technique but the lowest 95 percent limits of agreement was related to the prism-dissociated duochrome with the Humphriss method. Also, the highest correlation was found for the prism-dissociated duochrome and the Humphriss methods and the lowest correlation for the alternate occlusion and the Humphriss immediate contrast methods. The highest intraclass correlation coefficient result was related to the prismdissociated duochrome and the Humphriss immediate contrast methods. Intraclass correlation coefficients indicate moderate agreement between prism-dissociated duochrome and the Humphriss methods and prism-dissociated duochrome with the prism-dissociated blur balance techniques but fair agreement between other pairs. DISCUSSION The results of this study showed that the lowest mean difference was found with the alternate occlusion with the prismdissociated blur balance techniques but the lowest 95 percent limits of agreement was related to the prism-dissociated red-green balance with the Humphriss immediate contrast methods. Also, the highest correlation and intraclass correlation coefficient were related to the prism-dissociated red-green balance with the Humphriss methods. It should be mentioned that because the refraction was done in 0.25 D steps, the interocular sphere differences were either zero or a multiple of 0.25 D. Each of the balancing methods has its advantages and disadvantages. For example, the limitations of the alternate occlusion technique are first, the images viewed by the two eyes are not seen at the same time and second, its endpoint of equal acuity has limitations in patients with unequal maximum visual acuities in the two eyes; third, it may leave the accommodative status of both eyes at the level of unoccluded eye at the beginning of the balance. Due to some of the disadvantages listed above compared to other balancing techniques, some practitioners do not recommend it.4,9 The dissociated blur balance technique is the traditional form4 of spherical equalisation. Because the balance is done under dissociation or bi-ocular conditions, this © 2014 The Authors
Clinical and Experimental Optometry © 2014 Optometrists Association Australia
Comparison of binocular balancing techniques Momeni-Moghaddam and Goss
method cannot be performed if dissociation is not achieved. In other words, this technique will be contraindicated if the subject cannot perceive two images, one for each eye. This method assumes that both eyes have equal maximum visual acuities (6/6 or better), limiting its usefulness in cases of unequal acuity. Studies have also shown that the patient responses may not be similar when fogged or unfogged. This means that the balance found under blur will not necessarily be the same as the balance required with clear vision.4,6 The dissociated duochrome (bichrome) balance technique is also called the dissociated red/green balance method. This method avoids the problems associated with achieving equal acuity in the two eyes because its endpoint is independent of the patient’s final acuities. Other positive aspects of this technique are balancing at or very close to the maximum acuity in each eye and its consideration as a fast balancing method. Some negative aspects of this technique are that it does not always result in equality between the red and green backgrounds plus all the disadvantages of the duochrome test.3,4 These disadvantages are: 1. some patients who always prefer one colour (often red) give poor results with the duochrome regardless of the changes made to the spherical refraction 2. due to small pupils and the increased absorption of low wavelength light by the lens, older patients tend to prefer red and can give unreliable duochrome results 3. colour-defective patients can use the test, although the red side of the test will appear duller to protans and 4. not all charts provide appropriate red and green wavelengths of light. One possible advantage of the red-green method may be that it may be slightly faster as there may be fewer lens changes. The Humphriss immediate contrast method is really a binocular subjective refraction method but is often used as a binocular balancing technique. This method is useful for balancing accommodation in a trial frame because it does not need multiple lens changes, as does the prism dissociation balance technique. Fogging of one eye results in suspension of foveal fusion, while peripheral binocularity is maintained. The images from the right and left eyes are not compared simultaneously, so it is not a true binocular balance technique and theoretically, accommodation could change between the time the right eye
is tested and the time the left eye is tested. Because accommodation may be more stable under binocular conditions than under monocular conditions and because the best sphere assessment is under binocular conditions without dissociation of the two eyes, it is assumed to be an accommodative balance technique.6 The Humphriss method had the most agreement and correlation to the prismdissociated red-green balance; however, it should be noted that agreement or correlation between two methods only indicates how closely their results are related rather than the quality of those methods. The 95 percent agreement is usually about 0.50 D for most methods of refraction.10 Table 2 shows that the 95 percent limits of agreement for one of the pairs of balance methods (the prism-dissociated duochrome balance with the Humphriss methods) was about 0.50 D, so these two methods can be considered interchangeable because that value is the same as the 95 percent limits of agreement for most refractive procedures.10 The other pairs are also nearly interchangable because that value is not too much higher than 0.50 D. The 95 percent limits of agreement in an ascending trend for different pairs were PRG-HIC < ALTPRG < PB-PRG < ALT-PB < ALT-HIC < PB-HIC where PRG = prism-dissociated R-G balance, HIC = Humphriss immediate contrast, ALT = alternate occlusion and PB = prism-dissociated blur balance. Despite some methods correlating more or less with others, the findings showed that there was no statistical difference among the results obtained with the four balancing methods, so it appears that any of these four methods may give reasonable results. One limitation to the current study is that to lower the possibility of inter-examiner variability, one examiner did all testing, who was not masked regarding previous findings.
0.50 D, they can be interpreted as giving nearly the same findings. ACKNOWLEDGMENTS
The authors would like to thank participants who made this study possible. REFERENCES 1. Rosenfield M, Logan N, Edwards K. Optometry: Science, Techniques and Clinical Management, 2nd ed. Atlanta: Elsevier Health Sciences, 2009. p 209–227. 2. Harvey W, Franklin A. Eye Essentials: Routine Eye Examination. Oxford: Butterworth-Heinemann, 2005. p 99–112. 3. Grosvenor T. Primary Care Optometry, 5th ed. Oxford: Butterworth-Heinemann, 2007. p 114, 209–224. 4. Benjamin WJ. Borish’s Clinical Refraction, 2nd ed. Oxford: Butterworth-Heinemann, 2006. p 836– 847. 5. West D, Somers WW. Binocular balance validity: a comparison of five common subjective techniques. Ophthalmic Physiol Opt 1984: 4: 155–159. 6. Elliott DB. Clinical Procedures in Primary Eye Care, 3rd ed. Oxford: Butterworth-Heinemann. 2007. p 119–124. 7. Carlson NB, Kurtz D. Clinical Procedures for Ocular Examination, 3th ed. New York: McGrawHill Medical, 2003. p 98–140. 8. Portney LG, Watkins MP. Foundations of Clinical Research. Applications to Practice, 2nd ed. Upper Saddle River, New Jersey: Prentice Hall Health, 2000. p 115–140. 9. Eskridge JB, Amos JF, Bartlett JD. Clinical Procedures in Optometry. Philadelphia: JB Lippincott Company, 1991. p 687–691. 10. Goss DA, Grosvenor T. Reliability of refraction—a literature review. J Am Optom Assoc 1996; 67: 619–630.
CONCLUSIONS The results obtained with the four balancing methods showed close agreement given the repeatability of refraction. Based on a value of 0.50 D for the 95% limits of agreement for most methods of refraction,10 the prismdissociated red-green balance and the Humphriss immediate contrast balance yielded results, which can be considered interchangeable. Because the 95 percent limits of agreement for the other pairs of comparisons were not much higher than
© 2014 The Authors Clinical and Experimental Optometry © 2014 Optometrists Association Australia
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