Stereoacuity versus fixation disparity as indicators for vergence accuracy under prismatic stress

Correlations between clinical measures and symptoms: Report 1: Stereoacuity with accommodative, vergence measures, and symptoms

AIM

The aim of this study was to explore associations between reduced stereoacuity and clinical measures of accommodation, vergences, and symptoms which could facilitate the development of quick and reliable screening tools.

METHODS

Using a multi-stage random cluster sampling, 1211 high school students (481 males and 730 females) between 13 and 18 years of age, were selected and examined. Visual acuity, stereoacuity and suppression, refractive errors, near point of convergence, heterophoria and fusional vergences, as well as, amplitude of accommodation, accommodative response, facility and relative accommodation were evaluated. Correlations among variables and the validity of Randot stereoacuity to distinguish between children with and without defective clinical measures as well as symptomatic versus asymptomatic children were characterized by the sensitivity and specificity of the tests.

RESULTS

The overall mean stereoacuity was 43.9 ± 25.23 s arc, and 18.9% [95% Confidence Interval, 16.6-21.4%)] of the participants had reduced stereoacuity (defined as ≥60). Stereoacuity values and symptoms scores were worse in children with defective clinical measures. The Receiver Operation Curve showed that maximum sensitivity and specificity was obtained with near point of convergence break (≥10 cm) of (0.70 95% confidence interval: 0.63-0.77) with Randot stereoacuity test (defined as ≥60 s arc). The correlations between reduced stereoacuity and symptoms scores was moderately strong and statistically significant (Pearson's, r = 0.507, p = 0.01). The Receiver Operation Curve showed that maximum sensitivity and specificity obtained with the Convergence Insufficiency Symptoms Survey was 0.57 (95% Confidence interval = 0.53-0.62, p = 0.001), sensitivity of 90.26%, and specificity 15.26% with the Randot stereoacuity test.

CONCLUSION

Reduced stereoacuity, defective clinical measures and symptoms of asthenopia were prevalent among sample of school children studied. Randot stereoacuity test could fairly distinguish between defective and normal clinical measures; though the accuracy to differentiate between symptomatic and asymptomatic school children is poor. These findings highlight the need for validation of a simple and fast screening tool in school settings. Further studies to confirm above findings will be needed.

Reduced stereoacuity as a predictor for clinically significant convergence insufficiency

BACKGROUND

Stereoacuity relies on accurate binocular alignment. Convergence insufficiency (CI) a binocular motor disorder, interferes with near work.

OBJECTIVE

To investigate the association between convergence amplitude (CA) and stereoacuity in a large paediatric cohort.

METHODS

Retrospective chart review included patients aged 6-17 years; excluded patients with amblyopia, manifest strabismus or visual acuity <20/30 in either eye. Stereoacuity, measured by Randot test was defined as normal (≤40arcsec), subnormal (50-400arcsec) and poor (>400 arcsec). CA, measured using base out prism bar was defined by fusion break point (BP) and recovery point (RP), as none (BP=0), poor (BP <20 prism diopter (PD)), borderline (BP <30 PD or RP <20 PD), good (BP ≥30 PD and RP ≥20 PD) and excellent (does not break at 40PD).

RESULTS

In 2200 subjects included, we found an increased prevalence of normal stereoacuity as convergence ability improves (χ² test, p<0.001) with a negative correlation between stereoacuity and BP (Pearson correlation -0.13, p<0.001).CI was significantly associated with below normal stereopsis OR 1.86 (95% CI 1.3 to 2.7, p<0.001). Conversely, prevalence of CI was similar, whether or not CI-symptoms were reported. Follow-up data of at least 2.5 years from presentation was available for a small subgroup of 21 patients treated for CI. Convergence improved in 14 (66%), rate of normal stereoacuity increased from 29% at baseline to 76% at last follow-up (p=0.006).

CONCLUSIONS

CA affects stereoacuity function in children. Evaluation of CA is required in all cases with poor stereoacuity, especially when other etiologies are amiss. The role of convergence improvement exercise on stereoacuity warrants further investigation.

Evaluation of fixation disparity curve parameters with the modified near mallett unit in symptomatic and asymptomatic university students

BACKGROUND

Research suggests that fixation disparity data are extremely useful in the assessment of the binocular and accommodative systems.

OBJECTIVES

The purpose of this study was to evaluate fixation disparity curve (FDC) parameters with a modified near Mallett unit in symptomatic and asymptomatic students of Paramedical Sciences School of Mashhad University of Medical Sciences in 2007.

PATIENTS AND METHODS

In this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. Fixation disparity curve parameters were determined for each subject and compared in symptomatic and asymptomatic groups.

RESULTS

There were more subjects with exo fixation disparity than eso fixation disparity in the study sample. The means for fixation disparity, fixation disparity curve x-intercept, and slope with the modified Mallett unit were each significantly different by Mann-Whitney U test in the symptomatic and asymptomatic groups. Also there was a significant difference in the distributions of fixation disparity curve types in the two symptom groups by Chi-square test.

CONCLUSIONS

The X-intercepts (point at which the FDC crosses the X-axis) were on average more in the base-in direction, Y-intercepts (point at which the FDC crosses the Y-axis) were shifted in the exo direction, and slopes were steeper in the symptomatic group.

Stereoacuity as an indicator of prism adaptation

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 (p < 0.05). 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 (p < 0.05).

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.

Evaluation of fixation disparity curve with the modified near Mallett unit and the wesson fixation disparity card in symptomatic and asymptomatic subjects

PURPOSE

The purpose of this study is the evaluation of fixation disparity curve (FDC) parameters with an instrument that includes a central fusion lock (the modified near Mallett unit) and another without one (the Wesson fixation disparity card) to determine which is useful for diagnosis of symptomatic from asymptomatic subjects.

MATERIALS AND METHODS

In this analytical-descriptive study, 100 students were selected randomly and divided into symptomatic and asymptomatic groups. FDC parameters were determined with the Wesson card and the modified near Mallett unit for each subject and compared in symptomatic and asymptomatic groups. Data were analyzed by Wilcoxon, Mann-Whitney U, Spearman correlation coefficient and Chi-square tests.

RESULTS

The mean slope, y-intercept, and x-intercept with the Mallett unit and the Wesson card were significantly different in the symptomatic and asymptomatic groups (p < 0.001). Significant correlations were obtained between the Mallett unit and the Wesson card in y-intercept, x-intercept and slope (p < 0.001). There was a significant difference in the distributions of fixation disparity curve types in the two symptom groups with the Mallett unit (p = 0.01) and the Wesson card (p = 0.002) by Chi-square test.

CONCLUSIONS

Among symptomatic participants in this study, both Type I and Type III FD are more common than previously thought, but depend upon the method used to measure it. The x-intercepts were on average displaced in the base-in direction, y-intercepts were shifted in the exo direction, and the slopes were steeper with the Wesson FD card compared with the modified near Mallett unit. This may be related to the design of two devices. These differences were more significant in the symptomatic group.

A comparison of self-reported and measured autostereogram skills with clinical indicators of vergence and accommodative function

Failure to perceive the disparity-defined form in autostereograms by those with clinically normal stereoacuity can occur when achieving or maintaining the precise vergence angle required to place the intended left and right images on corresponding areas of the two retinas is difficult. Since vergence and accommodation must be maintained at different depth planes to permit sensory fusion of an autostereogram, poor autostereogram skill has been suggested by different investigators to be related either to the presence of a binocular vision anomaly (ie a poorly tuned binocular system) or to a binocular system that is well-coordinated. The purpose of this study was to clarify the relationship between binocular visual performance and autostereogram skill.

The prism that aligns fixation disparity does not predict the self-selected prism

BACKGROUND

Associated phoria is defined as the prism that aligns monocular markers in a fixation disparity (FD) test. We examined whether the 'FD-aligning prism' predicts the 'self-selected prism' (SSP), i.e. the prism that the observer judges to be most comfortable when viewing fully fusionable images.

METHODS

For the FD-aligning prism, we employed a test target with a central fusion lock. To determine the SSP, identical images were shown to both eyes. The images were displayed on a cathode-ray tube monitor at a distance of 50 cm, separated for the two eyes by means of liquid crystal shutter goggles. The observer adjusted counter-rotating prisms, recorded with a potentiometer. The procedure was repeated about 1-6 weeks later.

RESULTS

Among the 20 non-strabismic subjects over two sessions, there were 16 instances in which the FD-aligning prism indicated a vergence position further in the eso direction than the SSP (< or =15.9 cm m(-1) = prism dioptre). In five instances, the FD-aligning prism indicated a vergence position further in the exo direction (< or =3.6 cm m(-1)). From the first to the second session, changes of > or=1.0 cm m(-1) occurred for the FD-aligning prism in 8/20 observers (< or =11.5 cm m(-1)), and for the SSP in 11/20 observers (< or =5.1 cm m(-1)).

DISCUSSION

We hypothesise that the discrepancy between the SSP and the FD-aligning prism may be attributable to the fact that the SSP was determined while the eyes were presented with identical images, whereas the FD-aligning prism was determined while the images for the right and left eyes were different in the area of the Nonius lines. The unusual experience of binocular rivalry around the Nonius lines may have influenced the vergence of the eyes. Considering the spontaneous changes observed within an interval of 1-6 weeks, in both the FD-aligning prism and the SSP, we feel that research on the repeatability of methods for prescribing prisms is indicated.

Stereoacuity is affected by induced phoria but returns toward baseline during vergence adaptation

PURPOSE

In normal binocular single vision adaptation to an induced deviation occurs over a period of several minutes. This study investigates the effect on stereoacuity during vergence adaptation.

METHODS

Stereoacuity, using the Frisby near stereotest, was measured in 20 participants aged 20.2 +/- 1.9 years with normal binocular single vision. Measurements were taken immediately on inducing a phoria with 12(Delta) base-out prisms (split), and after 3, 6, and 9 minutes of adaptation. A measure of stereoacuity was also taken with the same size prisms that were used concomitantly to control for the reduced visual acuity.

RESULTS

Stereoacuity was found to decrease following introduction of the 12(Delta) base-out prismatic glasses (9.4 +/- 2.5 arcsec compared with 24.4 +/- 21.4 arcsec) and then increase over the 9 minute period of adaptation (ANOVA, p = 0.0002). Using post-hoc tests with Bonferroni correction, the decrease of stereoacuity on introduction of the prism was significant (p = 0.0039), and although an increase in stereoacuity appeared to occur after 3 and 6 minutes of wearing the 12(Delta) glasses (14.9 +/- 4.3 arcsec, 12.3 +/- 4.5 arcsec), this did not return to the baseline value until 9 minutes, when the stereoacuity had increased to 12.6 +/- 10.6 arcsec (p = 0.1982).

CONCLUSIONS

In these participants, inducing a deviation with base-out prisms negatively affected near stereoacuity. However, as adaptation occurred, the level of stereoacuity was seen to increase back toward the baseline measurement.