Current Eye Research, 2014; 39(8): 775–779 ! Informa Healthcare USA, Inc. ISSN: 0271-3683 print / 1460-2202 online DOI: 10.3109/02713683.2014.883410

ORIGINAL ARTICLE

Stereoacuity as an Indicator of Prism Adaptation Hamed Momeni-Moghaddam1, Frank Eperjesi2, James Kundart3 and Kazem Mostafavi-Nam1 1

Health Promotion Research Center, Zahedan University of Medical Sciences, Zahedan, Iran, 2Ophthalmic Research Group, School of Life and Health Sciences, Aston University, Birmingham, UK, and 3College of Optometry, Pacific University, Forest Grove, Oregon

ABSTRACT Purpose: The purpose of this study was to determine whether stereoacuity can be used as an indicator of prism adaptation. In particular, we wanted to know whether the time required for stereoacuity to return to the initial level after viewing through a prism can be used to determine the degree of adaptation. Materials and Methods: Eighteen subjects participated in this study. Stereoacuity and dissociated phoria were determined using the TNO stereotest and the Maddox rod, respectively. Prism vergences were measured using a prism bar. For each participant, prism power equivalent to the blur point of base-in (BI) and base-out (BO) fusional vergence at 40 cm was divided and placed in front of both eyes. At 0, 3, 6, 9 and 12 min after prism introduction, the stereoacuity was measured, and at 0 and 12 min, the heterophoria was measured. Results: The repeated measures ANOVA showed a significant difference between the mean stereoacuity for BI and BO prisms at the different measurement times (p50.05). For BO prism, the initial value was different between 0 and 3 min after the prism introduction, whereas for BI prism, a difference in stereoacuity was found between the pre-prism value and the value at 0, 3 and 6 min. The size of the heterophoria with BO and BI prisms was different from 0 to 12 min (p50.05). Conclusion: The time required for stereoacuity to return to baseline level was more than 3 min for BO, and more than 6 min for BI prism. In addition, the time required to return to baseline values was not similar for the stereoacuity and heterophoria. The recovery of stereoacuity is slower when adapting to divergence, as when looking from near to far. This implies that stereopsis responds faster to near targets than to distant one, and may precede complete phoria adaptation. Keywords: Binocular alignment, binocular vision, prism adaptation, stereopsis, vergence adaptation

adaptation phenomenon.4 Following the induction of a phoria using prism, adaptation can cause the phoria and fixation disparity to return to approximate initial values after a specific time. An option in the management of some binocular vision anomalies, such as symptomatic basic eso- and exophoria, is prismatic correction. Prior to prescribing prism, the clinician can evaluate potential for success using the prism adaptation test. In the optometry clinical setting, this usually involves the measurement of the magnitude of the phoria prior to prism introduction and then again a few minutes later. Henson and North reported that it takes 2 to 3 min for

INTRODUCTION Vergence and accommodative systems are subject to adaptation to visual stimuli. Prism (also known as vergence) adaptation takes place in most people with normal binocular vision,1 but it is not exclusively a characteristic of the normal system. Prism adaptation has been reported in individuals with symptomatic convergence insufficiency and those with anomalous correspondence.2,3 Instead of a normal distribution of deviation in the general population centering on orthophoria, the prevalence of moderate phoria is attributed to this

Received 22 February 2013; revised 22 October 2013; accepted 30 December 2013; published online 21 February 2014 Correspondence: Hamed Momeni-Moghaddam, Zahedan Optometry Department, School of Rehabilitation Sciences, Kafami Str., Zahedan, Sistanobaluchestan, Iran. Tel/Fax: +985413228445. E-mail: [email protected]

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776 H. Momeni-Moghaddam et al. prism adaptation to occur in people with normal binocular vision.5 It is likely to take longer in those with compromised binocular vision.6 Prism adaptation consists of fast and slow components.7 Immediately upon disparity being induced with prism, the fast component acts to overcome the disparity. Gradually, the slow component is activated. Consequently, the individual’s heterophoria will often return to its original value. An activation time of around 10 s has been proposed for the slow component.6 Stereoacuity and magnitude of heterophoria have been used as indicators of prism adaptation.8,9 Kromeier et al. noted the benefit of stereoacuity in the evaluation of binocular vision under prismatic stress conditions compared to fixation disparity tests conducted under more natural viewing conditions.10 The purpose of this study was to revisit and evaluate the use of stereoacuity and magnitude of heterophoria as indicators of prism adaptation. We were particularly interested in determining whether stereoacuity could be used to determine the degree of adaptation. It would be simpler and quicker to use stereoacuity to determine adaptation than the conventional prism adaptation test in the clinical setting.

MATERIALS AND METHODS In this study, 18 participants were recruited from the student body of Zahedan University of Medical Sciences. Signed consent was obtained and met the tenets of the Declaration of Helsinki. Inclusion criteria were as follows: (1) Visual acuity at least 20/20 (Snellen) in each eye at 6 m and 40 cm with or without optimal refractive correction. (2) Stereoacuity of at least 30 s of arc (00 ) using the TNO stereotest. (3) Prism fusion ranges (to break point) of at least 20D base-out (BO) to 10D base-in (BI) for near and 10D BO to 5D BI for distance. (4) No strabismus at 6 m and 40 cm as determined using the unilateral cover test. (5) No vertical deviation at 6 m and 40 cm as determined using the Maddox rod test. (6) Normal accommodative function. By this we mean that the accommodative amplitude was equal to the average values of Hofstetter using the push-up method, the lag of accommodation was within the range (+0.25 to +0.75D) as determined using the monocular estimate method, and monocular and binocular accommodative facility 410 and 46 cycles per minute using ± 2.00 D flippers, respectively. (7) No history of ocular trauma or ocular disease. (8) No history of refractive surgery or aphakia.

Refractive errors were determined objectively by retinoscopy, refined by subjective refraction and finalized with the dissociated red-green balance test. Stereoacuity was measured at this point using the TNO stereotest with best refractive correction in a trial frame with red and green anaglyphic filters placed over the refractive correction. The TNO test was held at 40 cm parallel to each participant’s face. For determination of stereoacuity, the graded plates from 480 to 1500 were presented until the subject was unable to identify orientation of the gap in the circle from the random dot background. The lowest discriminated disparity for each participant was recorded. Near horizontal heterophoria was determined with the Maddox rod test. A penlight was used as fixation target on the midline of the participant’s face at 40 cm. With the best refractive correction in the trial frame, a red Maddox rod was placed in front of one eye with its cylinder axes horizontal. If the dot was superimposed on the vertical line, then no deviation was present. Otherwise, the presence of deviation was recorded. For quantifying the deviation, a horizontal prism bar with prism base in the appropriate orientation (BI or BO) was held until the dot was superimposed on the line. This prism power was noted as the amount of dissociated phoria present in prism diopters. A prism bar was used for measurement of fusional reserves at 40 cm with a vertical column of small letters E of approximately 20/30 size as an accommodative target. The subject was asked to look at the target and BI prism was introduced over the refractive correction. The magnitude of the prism was slowly increased until the subject reported sustained blur, break and recovery. This procedure was repeated with BO prism. We observed subjects’ eyes during the measurement for detection of any movement that was suggestive of suppression. For each subject, the prism power equivalent to the blur point during fusional vergence was divided between the two eyes, and prisms were then placed in the trial frame in front of each eye in order to create an induced phoria. Immediately after prism introduction, a vertical Maddox rod was placed over one eye and the induced phoria was measured. After phoria determination stereoacuity was measured, and this was recorded as the measurement at 0 min. Stereoacuity measures were made at 3, 6, 9 and 12 min after prism introduction and phoria measurement. During the interval between stereoacuity measurements, participants were asked to read a newspaper at 40 cm. Immediately following the completion of the stereoacuity measurement at 12 min, the Maddox rod was placed over one eye and the horizontal phoria was measured again. As a rule, measurements were first done with BI prism, followed by BO prism. Current Eye Research

Stereoacuity and Prism Adaptation

prism introduction after 6 min for BO prism and 9 min for BI prism. The repeated measurement ANOVA was used for comparison of stereoacuity before and after prism introduction and showed significant differences for stereoacuity at the different time intervals for BO and BI prisms (p50.05). The Bonferroni test was used for the multiple comparisons. We found a difference between 0 and 3 min for BO, and between 0, 3 and 6 min for BI prism (p50. 05). Table 4 shows mean and SD of the Maddox rod measurements in prism diopters before, at 0 and 12 min after prism introduction. Table 4 shows some interesting results. The introduction of BO prism caused an increase in the mean magnitude of the near exophoria (p50.001) and this change in magnitude decayed with time (p50.001). The introduction of BI prism produced a decrease in the mean magnitude of the near exophoria (p50.001), that is a shift to esophoria, and this change in magnitude decreased with time (p50.001). Interestingly, while stereoacuity returned almost to the level measured before prism introduction after 12 min this was not the case for the magnitude of the near phoria. At 12 min, there was still a significant difference in the phoria magnitude compared to the

Data were analyzed in SPSS.17 software (SPSS for Windows, SPSS Inc. Chicago, IL). Normality of data was assessed with the Kolmogorov-Smirnov test that indicated normal distribution. Hence the repeated measures ANOVA and Pearson correlation test were used for statistical analysis. In all tests, the significance level was considered to be 0.05.

RESULTS The subjects were 18 male students with a mean age 21.06 ± 1.39 years (age range of 19–24 years). The mean and SD spherical equivalents (SE) of the right eye was 0.15 ± 0.33 D and for the left eye 0.10 ± 0.25 D. The Pearson correlation test showed a significant correlation between the mean of SE between the two eyes (r = 0.73, p50.001). Table 1 shows the mean and SD of positive and negative fusional reserves (PFR and NFR, respectively) measured using base-out (BO) and base-in (BI) prisms (blur, break and recovery), respectively. Table 2 shows the value of prism as determined from the blur point, which was split between each eye for each subject. The mean and SD for near heterophoria measured using the Maddox rod test for all subjects was 5.33 ± 1.57 prism diopters. The negative sign indicates exophoria. Table 3 shows mean and SD of stereoacuity as determined using the TNO test before prism introduction and at 0, 3, 6, 9 and 12 min after prism introduction for BO and BI prisms. Table 3 shows some interesting results in that stereoacuity values increase, that is stereopsis becomes poorer immediately after prism introduction for BO and BI prism. The change in stereoacuity is more for BI than BO prism. Our data also indicate that stereoacuity values returned near to those prior to

TABLE 3. Mean and SD of stereoacuity as determined using the TNO test before prism introduction and at 0, 3, 6, 9 and 12 min after prism introduction for base-out (BO) and base-in (BI) prisms. Prism direction

Variable

13.2 ± 2.7 9.1 ± 2.9

25.6 ± 6.2 17.1 ± 6.2

BI, mean ± SD (seconds of arc)

Before prism 28.5 ± 4.8 28.5 ± 4.8 introduction (a) 0 min (b) 51.7 ± 22.5 55.0 ± 27.6 3 min (c) 43.4 ± 15.2 40.9 ± 16.1 6 min (d) 29.3 ± 9.6 40.9 ± 16.0 9 min (e) 32.6 ± 10.6 31.8 ± 11.3 12 min (f) 32.6 ± 10.6 31.8 ± 11.3 Repeated measures F (1.9, 33.1) = 10.338, F (1.8, 31.8) = 8.811, ANOVA p50.001 p = 0.001 Pairwise a,b (p = 0.006) a,b (p = 0.010) comparisons a,c (p = 0.010) a,c (p = 0.030) b,d (p = 0.010) a,d (p = 0.030) Other groups: 40.05 Other groups: 40.05

Blur, mean ± SD Break, mean ± SD Recovery, mean ± SD (prism diopters) (prism diopters) (prism diopters)

PFV NFV

BO, mean ± SD (seconds of arc)

Time

TABLE 1. Mean and SD of blur, break and recovery points of the positive and negative fusional reserves.

FR

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18.2 ± 3.5 13.5 ± 5.2

TABLE 2. Prism power introduced as determined using fusional vergence testing to the blur point and the near phoria in prism diopters prior to prism introduction for each subject. Values depicted in this table were split between each eye. Subject

Variable 1 Power of prism introduced

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

BI 12 4 10 10 10 12 6 4 8 10 12 10 8 10 BO 14 16 12 14 14 14 14 18 12 8 10 12 18 14 Near deviation before prism introduction (exophoria) 6 6 5 7 7 7 2 4 6 2 5 6 6 7 !

2014 Informa Healthcare USA, Inc.

6 8 4

6 12 14 14 12 14 6 4 6

778 H. Momeni-Moghaddam et al. TABLE 4. Mean and SD of the Maddox rod measurements in prism diopters before, at 0 and 12 min after prism introduction. Prism direction Time

BO (mean ± SD)

BI (mean ± SD)

Before prism 5.3 ± 1.5 5.3 ± 1.5 introduction (a) 0 min (b) 20.0 ± 6.8 2.2 ± 5.6 12 min (c) 11.2 ± 8.9 1.6 ± 4.5 Repeated measures F (1.8, 31.8) = 70.701, F (1.7, 29.7) = 32.471, ANOVA p50.001 p50.001 Pairwise a,b (p50.001) a,b (p50.001) comparisons a,c (p = 0.002) a,c (p = 0.001) b,c (p50.001) b,c (p50.001)

value measured prior to prism introduction for BO (p = 0.002) and BI (p = 0.001) prism.

DISCUSSION This study investigated the effects of BO and BI prism on stereoacuity and magnitude of horizontal phoria in 18 normal subjects. Our results indicate that the time required for stereoacuity to return almost to the preprism level was 6 min for BO and 9 min for BI prism. However, although stereoacuity was back to almost the initial value after these time intervals adaptation may not have been fully completed since the magnitude of the phoria remained significantly different from the pre-prism value. In other words, the time taken to return to near baseline values was different for the stereoacuity and heterophoria. Our results are similar to those reported by Spencer et al. with regard to the different adaptation time course for stereoacuity and heterophoria.11 This difference between the time taken to return to baseline levels for stereoacuity and heterophoria probably relates to a quicker recovery of stereoacuity due to a speedier recovery of induced fixation disparity through the fast adaptation component compared to the relative slower action of the slow adaptation component. We believe that the shorter time for stereoacuity to return back to almost the baseline values for BO (6 min) compared to BI prism (9 min) can be attributed to the superior robustness of the convergence mechanism when compared to divergence in normal subjects. Spencer et al.8 reported a decrease in stereoacuity after BO prism introduction that is with a return to the baseline value after 9 min. For our participants, stereoacuity returned to near baseline values for BO after 6 min. This difference is relatively small but could be due to several factors. Spencer et al. used the Frisby test, which measures depth perception without filters while we used the TNO.

Furthermore, Spencer et al. reported that they used the Frisby test in a way which differed from that recommended by the manufacturers. Also, the mean initial heterophoria (prior to prism introduction) reported by Spencer et al.8 was 0.25D exophoria while we found a mean of 5.33D exophoria. Finally, Spencer et al.8 used a single prism of 12D BO for all measurements, whereas we used prism power that was equal to the blur point of fusional vergence at 40 cm for BO and BI. Henson and North, and North et al. reported quicker recovery time for BO at distance compared to BI but no difference at near.5,6,11 Their finding for near is contrary to our observations. We are unsure of the reason for this, but it may be due at least in part to differences in experimental procedure. For example, Henson and North, and North et al. used 6D in front of one eye while we used a mean of 9D (range 4–14) BI and a mean of 13D (range 8–16D) split between the eyes. Another possible reason for this particular difference between our findings and those reported by others for near is that the prism adaptation mechanism may vary according to the size of the prism used to induce the heterophoria. With regards to the time required for adaptation, Thiagarajan et al.12 investigated the effects of vergence adaptation and positive fusional vergence training on oculomotor parameters. They found that following introduction of 12D BO prism, most adaptation took place within the first in 3 to 6 min. This is in contrast to our finding that complete adaptation (based on comparison of the pre- and post-prism phoria magnitude) had not occurred by 12 min. The reasons for this difference are unclear to us may be partly due to procedural differences. With information on the deviation type and magnitude before prism introduction and at 12 min, it is clear that the degree of the prism adaptation for BO prism was 56% versus 42% for BI prism. Therefore, the mean adaptation was 49% which compares well with the 59% reported by Larson and Faubert.13

CONCLUSION This study showed that the time required for stereoacuity to return to its baseline level was 6 min for BO and 9 min for BI prism introduction. Stereoacuity recovery was quicker than phoria recovery. This means that good stereoacuity can be achieved even without orthophoria and that stereoacuity is not a good indicator of prism adaptation. Although stereoacuity might be resilient to defocus and very sensitive to vertical misalignment, it seems less sensitive to horizontal heterophoria. Current Eye Research

Stereoacuity and Prism Adaptation

ACKNOWLEDGEMENTS The authors thank participants who made this study possible.

DECLARATION OF INTEREST The authors indicate no conflicts of interest and funding support.

REFERENCES 1. Evans BJW. Pickwell’s binocular vision anomalies. 5th ed. Boston: Butterworth-Heinemann; 2007. pp 70, 106–107, 144–146, 214–215, 249. 2. Brautaset RL, Jennings JA. Distance vergence adaptation is abnormal in subjects with convergence insufficiency. Ophthalmic Physiol Opt 2005;25:211–214. 3. Larson WL. Prism adaptation without binocular vision. Optom Vis Sci 1990;67:196–200. 4. Rosenfield M. Tonic vergence and vergence adaptation. Optom Vis Sci 1997;74:303–328.

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5. Henson DB, North R. Adaptation to prisminduced heterophoria. Am J Optom Physiol Opt 1980;57: 129–137. 6. Schor CM. The influence of rapid prism adaptation upon fixation disparity. Vision Res 1979;19:757–765. 7. North R, Henson DB. Adaptation to prisminduced heterophoria in subjects with abnormal binocular vision or asthenopia. Am J Optom Physiol Opt 1981;58: 746–752. 8. Spencer S, Firth AY. Stereoacuity is affected by induced phoria but returns toward baseline during vergence adaptation. J AAPOS 2007;11:465–468. 9. Winn B, Gilmartin B, Sculfor DL, Bamford JC. Vergence adaptation and senescence. Optom Vis Sci 1994;71:797–800. 10. Kromeier M, Schmitt C, Bach M, Kommerell G. Stereoacuity versus fixation disparity as indicators for vergence accuracy under prismatic stress. Ophthalmic Physiol Opt 2003;23:43–49. 11. North RV, Sethi B, Owen K. Prism adaptation and viewing distance. Ophthalmic Physiol Opt 1990;10:81–85. 12. Thiagarajan P, Lakshminarayanan V, Bobier WR. Effect of vergence adaptation and positive fusional vergence training on oculomotor parameters. Optom Vis Sci 2010;87: 487–493. 13. Larson WL, Faubert J. An investigation of prism adaptation latency. Optom Vis Sci 1994;71:38–42.

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Stereoacuity as an indicator of prism adaptation.

The purpose of this study was to determine whether stereoacuity can be used as an indicator of prism adaptation. In particular, we wanted to know whet...
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