Exploring the association between asthenopia, vergence measures and the interpupillary distance

To prospectively evaluate the effectiveness of home-based computer vergence therapy for the treatment of binocular vision disorders in adults at least 3 months after an acquired brain injury. Eligibility criteria included presence of binocular dysfunction characterized by receded near point of convergence (≥6 cm break), insufficient positive fusional vergence at near (failing Sheard's criterion or <15△ blur or break), insufficient negative fusional vergence at near (<12△ blur or break), and/or reduced vergence facility at near (<15 cycles per minute with 12△BO/3△BI). Participants were prescribed 12 weeks of home-based computer vergence therapy. Phoria (cover test), negative fusional vergence, positive fusional vergence, near point of convergence, vergence facility, and symptoms (convergence insufficiency symptom survey [CISS]) were assessed at baseline and after 4, 8, and 12 weeks of prescribed therapy. ANOVA was used to evaluate change in each measure. Percentage successful was also determined. Nineteen participants were enrolled (mean age 45.4 ± 12.9 years); six participants were lost to follow-up. Baseline findings were orthophoria at distance, 7.2△ exophoria at near, near point of convergence break = 17.5 cm, near point of convergence recovery = 21.8 cm, negative fusional vergence = 12.3△, positive fusional vergence blur = 8.4△, vergence facility = 3.9 cycles per minute, and CISS = 32.1. ANOVA showed a statistically significant improvement for near point of convergence break (p = 0.002) and recovery (p < 0.001), positive fusional vergence blur (p < 0.0001), break (p < 0.0001), and recovery (p < 0.0001), negative fusional vergence blur (p = 0.037), break (p = 0.003), and recovery (p = 0.006), vergence facility (p < 0.0001), and CISS (p = 0.0001). The percentage of patients who were classified as "successful" or "improved" was 69% for near point of convergence (<6 cm or decrease of ≥4 cm), 77% for positive fusional vergence (>15△ and passing Sheard's criterion or increase of ≥10△), 77% for negative fusional vergence (≥12△ or increase of ≥6△), 62% for positive fusional vergence and near point of convergence composite, and 92% for vergence facility (15 cycles per minute or increase of 3 cycles per minute). The majority of participants who completed the study experienced meaningful improvements in signs and symptoms.

Purpose: The aim was to determine whether baseline measurements of binocular vision are associated with symptoms of convergence insufficiency (CI) both before and after CI treatment with base‐in prism in symptomatic presbyopes. We further determined the factors that predicted CI treatment outcomes.Methods: A total of 29 symptomatic CI subjects aged 45 to 68-years were examined and the following binocular vision measurements were obtained at the baseline: near heterophoria by cover test, associated phoria at near (AP), near point of convergence (NPC) and positive fusional vergence at near (PFV). Each subject wore each of two pairs of progressive addition spectacles for a three‐week period, one with prism and one with the same prescription but no prism. The wear order was randomised across subjects. Subjects completed the CI symptom survey (CISS) before and at the end of the three weeks, thus baseline, placebo and treatment CISS scores were tabulated. The reduction in symptoms (CISS score improvement) was calculated as the difference between the placebo and treatment CISS scores.Results: Neither baseline CISS score nor treatment CISS score had statistically significant correlations with any of the binocular vision measurements. CISS score improvement was statistically significantly associated with near heterophoria, AP and NPC break but not with PFV. CISS score improvement was significantly predicted by both near heterophoria and NPC break but not by AP and PFV. With other variables held constant, the CISS score improved by 1.57 for each additional prism dioptre of exophoria at near and by 0.81 for each additional centimetre that the NPC break was receded.Conclusion: There was no significant association between the CISS score with any of the baseline binocular vision measurements before or after prism treatment. Our findings suggest that a greater near heterophoria and/or more receded NPC before treatment are associated with more reduction in CI symptoms by prism treatment.

Nearpoint of convergence (NPC) values of 8 to 10 cm are widely used to diagnose binocular dysfunctions such as convergence insufficiency. However, there are no published age-related normative values in the literature to substantiate these values. Subjects were 297 schoolchildren in kindergarten, third grade, and sixth grade who had passed a school-based Modified Clinical Technique vision screening. Each child had the NPC break and recovery taken three times using a standardized protocol developed by the Convergence Insufficiency and Reading Study group. The examiners used an Astron International (ACR/21) Accommodative Rule with a movable column of 20/30 letters as the target. For each grade, the distribution of NPC break was right skewed, with a concentration of values between 1 and 6 cm. At least 85% of the subjects in each grade had an NPC break < or = 6 cm. NPC break values (mean +/- SD) were 3.3 +/- 2.6 cm for kindergartners, 4.1 +/- 2.4 cm for third graders, and 4.3 +/- 3.4 cm for sixth graders, and the means were found to be statistically different (analysis of variance, p = 0.031). NPC recoveries (mean +/- SD) for the three groups were 7.3 +/- 4.8 cm, 8.7 +/- 4.2 cm, and 7.2 +/- 3.9 cm, respectively, which were also significantly different (analysis of variance, p = 0.027). The recovery distributions were more symmetric and less skewed than those for break. For each grade level, there was a strong positive relationship between NPC recovery and NPC break, but the difference between NPC recovery and break had a low correlation with the NPC break. Kindergartners had somewhat better NPC breaks than third or sixth graders, whereas no clear age trend was present for NPC recovery. A supporting study using a random sample of clinic patients (aged 10-12 years) suggests that patients with NPC breaks > 6 cm are more than twice as likely to be symptomatic than patients with NPC breaks < or = 6 cm. Based on these results and the NPC break distributions in this study, a clinical cutoff value of 6 cm is suggested for patients of elementary school age. A cutoff value in the 6- to 10-cm range is recommended for children of elementary school age in a screening context. The exact value within this range depends on the level of concern with identifying patients who have visual signs and symptoms associated with a receded NPC.

The CONCUSS randomised clinical trial compared the effectiveness of immediate office-based vergence/accommodative therapy with movement (OBVAM) to delayed therapy for the treatment of concussion-related convergence insufficiency (CONC-CI) in participants 11-25 years old with persisting postconcussive symptoms 4-24 weeks post injury. Symptomatic CONC-CI was diagnosed using clinical signs via near point of convergence (NPC) and positive fusional vergence (PFV) and symptoms via the Convergence Insufficiency Symptom Survey (CISS). Participants were randomised to immediate OBVAM (twice weekly for 6 weeks) or delayed OBVAM (starting 6 weeks after baseline enrolment). After 6 weeks (outcome time 1 assessment), the therapeutic outcomes of NPC, PFV and CISS were assessed and compared between the two groups. After the outcome time 1 assessment, the delayed group received twice-weekly OBVAM sessions for 8 weeks, while the immediate group received an additional 2 weeks of twice-weekly OBVAM sessions. The outcome time 2 assessment compared groups after each group received all 16 OBVAM sessions. In the immediate group, 46/52 (88%) were classified as successful or improved at the outcome time one assessment based on the primary outcome measure, a composite of NPC and PFV, compared with 4/52 (8%) in the delayed group (p<0.001). The mean NPC decreased (improved) by 7.9 cm in the immediate group and 1.8 cm in the delayed group (mean difference at outcome time 1 assessment: 5.1 cm (95% CI: 3.9 to 6.3; p<0.001)). The mean PFV increased (improved) by 17.5Δ in the immediate group and 2.5∆ in the delayed group (mean difference at outcome time 1 assessment: 15.0∆ (95% CI:11.7 to 18.3); p<0.001). At the outcome time 1 assessment, 41/52 (79%) of the participants in the immediate group had improved symptoms based on CISS scores ≤ preinjury scores or decreased by 10 points or more, compared with only 7/52 (13%) of participants from the delayed group (p<0.001). When comparing dosing in the immediate group, for 12 OBVAM sessions, 88% were classified as successful or improved using the composite measurement of NPC and PFV, which increased to 94% after 16 OBVAM sessions. For the outcome time 2 assessment, when both groups had received 16 OBVAM sessions, no significant difference was observed for NPC, PFV or CISS (p=1.0). OBVAM therapy is effective in improving the NPC, PFV and symptoms in CONC-CI. Immediate initiation of OBVAM compared with delayed initiation shortens the period of symptoms experienced and fosters an earlier return to activities. clinicaltrials.gov identifier: NCT05262361.

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The percentage of children who are symptomatic has been shown to increase with the number of signs of convergence insufficiency (CI). Our goal was to investigate whether there is a relationship between the severity of the clinical signs of CI and symptom level reported in children with a three-sign symptomatic CI. The Convergence Insufficiency Treatment Trial enrolled 221 children with symptomatic CI from ages 9 to 17 years. Inclusion criteria included the following three signs of CI: (1) exophoria at near at least 4Δ greater than at distance, (2) insufficient positive fusional vergence (PFV) at near, and (3) a receded near point of convergence (NPC) of 6 cm break or greater. The relationships between the severity of each sign of CI (mild, moderate, and severe) and the level of symptoms as measured by the Convergence Insufficiency Symptom Survey (CISS) at baseline were evaluated. Mean CISS scores were not significantly different between mild, moderate, and severe exophoria (p = 0.60), PFV blur (p = 0.99), Sheard's criterion (p = 0.89), or NPC break (p = 0.84). There was also no difference between the frequency of subjects scoring at mild, moderate, or severe levels on the CISS and the severity of each sign of CI. Correlations between individual clinical signs and the CISS score were very low and not statistically significant. Among symptomatic children with a CISS score of 16 or higher and three clinical signs of CI, there is no further association between the severity of the clinical signs and their level of symptoms.

BackgroundConvergence insufficiency (CI) is a common binocular vision disorder which often causes symptoms when doing near work. However, the best screening test for CI is unknown. The purpose of this study was to evaluate the ability of common tests of binocular and accommodative function to identify children with CI in a school screening setting.MethodsChildren aged nine to 14 were invited to participate. Positive fusional vergences, near point of convergence (NPC), accommodative amplitude, accommodative facility, Modified Thorington, and the Convergence Insufficiency Symptom Survey were evaluated.ResultsOf the 282 children tested, approximately 20-per cent had 2–3 signs of CI. One half of 2–3 signs of CI and 66-per cent of three signs of CI subjects were symptomatic. Approximately 61-per cent of subjects with symptomatic 2–3 signs of CI had an accompanying low accommodative amplitude. The largest area under the receiver operating characteristic curve was obtained using NPC break measurements. NPC break ≥ 6-cm for CI and NPC break ≥ 7-cm for symptomatic CI were the cut points that maximised the sum of sensitivity and specificity.ConclusionNPC break performed best in identifying children with CI.

Background: Vision tests are used in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Our objective was to determine the one-year test-retest reliability of these tests. Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA). Results: We examined nine female and seven male athletes with a mean age of 22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.53 when an outlier was excluded in a sensitivity analysis. There was good to moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccades (ICC=0.61). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.21 after removing one outlier. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%. Conclusions: Five tests had good to moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Background: Vision tests are used in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Our objective was to determine the one-year test-retest reliability of these tests. Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA). Results: We examined nine female and seven male athletes with a mean age of 22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.55 when an outlier was excluded in a sensitivity analysis. There was good to moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccade (ICC=0.56). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.21 after removing one outlier. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%. Conclusions: Five tests had good to moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Background: Vision tests are used in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Our objective was to determine the one-year test-retest reliability of these tests. Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA). Results: We examined nine female and seven male athletes with a mean age of 22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.53 when an outlier was excluded in a sensitivity analysis. There was good to moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccades (ICC=0.61). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.45 after removing two outliers. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%. Conclusions: Five tests had good to moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Background: Vision tests are increasingly being suggested for use in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Therefore, our objective was to determine the one-year test-retest reliability of these tests. Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines, etc.). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA). Results: We examined nine female and seven male athletes with a meanage of22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.55 when an outlier was excluded in a sensitivity analysis. There was good to moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccade (ICC=0.56). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.21 after removing one outlier. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%. Conclusions: Four tests had moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Background: Vision tests are increasingly being suggested for use in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Therefore, our objective was to determine the one-year test-retest reliability of these tests. Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines, etc.). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA). Results: We examined nine female and seven male athletes with a meanage of22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.55 when an outlier was excluded. There was moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccade (ICC=0.56). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.45 after removing one outlier. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%. Conclusions: Four tests had moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Background: Vision tests are used in concussion management and baseline testing. Concussions, however, often occur months after baseline testing and reliability studies generally examine intervals limited to days or one week. Our objective was to determine the one-year test-retest reliability of these tests.Methods: We assessed one-year test-retest reliability of ten vision tests in elite Canadian athletes followed by the Institut National du Sport du Quebec. We included athletes who completed two baseline (preseason) annual evaluations by one clinician within 365±30 days. We excluded athletes with any concussion or vision training in between the annual evaluations or presented with any factor that is believed to affect the tests (e.g. migraines). Data were collected from clinical charts. We evaluated test-retest reliability using Intraclass Correlation Coefficient (ICC) and 95% limits of agreement (LoA).Results: We examined nine female and seven male athletes with a mean age of 22.7 (SD 4.5) years. Among the vision tests, we observed excellent test-retest reliability in Positive Fusional Vergence at 30cm (ICC=0.93) but this dropped to 0.53 when an outlier was excluded in a sensitivity analysis. There was good to moderate reliability in Negative Fusional Vergence at 30cm (ICC=0.78), Phoria at 30cm (ICC=0.68), Near Point of Convergence break (ICC=0.65) and Saccades (ICC=0.61). The ICC for Positive Fusional Vergence at 3m (ICC=0.56) also decreased to 0.45 after removing two outliers. We found poor reliability in Near Point of Convergence (ICC=0.47), Gross Stereoscopic Acuity (ICC=0.03) and Negative Fusional Vergence at 3m (ICC=0.0). ICC for Phoria at 3m was not appropriate because scores were identical in 14/16 athletes. 95% LoA of the majority of tests were ±40% to ±90%.Conclusions: Five tests had good to moderate one-year test-retest reliability. The remaining tests had poor reliability. The tests would therefore be useful only if concussion has a moderate-large effect on scores.

Objective To investigate the relationship between Strabismus Angle and Convergence Parameters of intermittent exotropia.Methods Based on the size of strabismus angle,48 intermittent exotropia patients with different strabismus angle degrees were enrolled and divided into 3 groups.Respectively,strabismus angle,positive and negative fusion range,positive and negative fusional vergence,near point of convergence and AC/A rate were collected.Statistical analysis of these test results and the strabismus angle of intermittent exotropia were performed.All statistical correlation analyses were performed using SPSS15.0 software.Results The average strabismus angle degree of 48 intermittent exotropia patients at 5m distance∶-13.19±8.40△ average at 33cm distance∶-18.79±6.80△ Intermittent Exotropia average∶ 7.29±4.73,Negative Fusion Range average∶-4.28± 4.55,Near Point Of Convergence average:Break Point∶ 7.50±2.43,Recovery Point:8.74±2.84.Far Positive Fusional Vergence average∶ Blur point∶ 12.34±6.90,Break Point∶ 18.48±7.63,Recovery Point∶ 10.76±7.19; Far Negative Fusional Vergence average∶ Blur point∶ 14.30±6.22,Break Point∶ 21.52±6.94,Recovery Point∶ 8.34±7.92,Near Positive Fusional Vergence average∶ Blur point∶ 18.03± 8.16,Break Point∶ 28.13±7.85,Recovery Point∶ 20.25±18.70; Near Negative Fusional Vergence average∶ Blur point∶ 14.21±7.04,Break Point∶ 21.55±6.96,Recovery Point∶ 8.06±7.99,AC/A Rate average∶ 1.89±1.03.Only negative fusion and Negative Fusional Vergence had a negative correlation with Strabismus Angle in Intermittent Exotropia,the difference was significant (P ＜0.01).Conclusions In Intermittent Exotropia,there are significant differences between Strabismus Angle with Negative Fusion and Negative Fusional Vergence; and there are no significant differences between Strabismus Angle with Positive Fusion Range,Positive Fusional Vergence,Near Point Of Convergence,AC/A Rate. Key words: Intermittent Exotropia; Strabismus Angle; Positive and Negative Fusion Range; Positive and Negative Fusional Vergence; Near Point Of Convergence; AC/A Rate

To establish long-term reliability measures for vergence testing in a control population of adolescents. Healthy participants between 12 and 17.5 years with normal binocular vision were recruited from 10 clinical sites. Cover test, near point of convergence (NPC), positive and negative fusional vergences, vergence facility (12∆ base-out/3∆ base-in) and vergence jumps (using the Oculomotor Assessment Tool) were performed at the initial visit and repeated at 90 days. The mean and standard deviation were calculated for the overall group for NPC, vergence facility and vergence jumps and by prism dioptre step value for PFV and NFV (1Δ or 2Δ if below 20∆ or 5Δ above 20∆). Agreement was assessed using Bland-Altman plots and 95% limits of agreement (LOA). Ninety-three participants (mean age 14.3 ± 1.7 years, 52% female) were enrolled and 91 (98%) completed the initial and 90-day outcome evaluation. The mean differences were significantly greater than zero for vergence facility (p < 0.05) and the first and second 30 s of vergence jumps (p < 0.01). The 95% LOA were narrow for NPC (±2.5) and negative fusional vergence (±5.9), suggesting good repeatability. LOA were larger for positive fusional vergence (±17.8), vergence facility (±9.8) and vergence jumps (±16.2). Analysis of the positive fusional vergence data indicates that the different step sizes (1∆ or 2∆ vs. 5∆) in the horizontal prism bar contribute to considerably larger variability in these measures. In participants with normal binocular vision and no concussion history, good reliability yielded comparable results 90 days apart for all vergence measures. The results provide values that can be used to interpret the effect of intervention for vergence disorders in clinical practice and research studies. An important outcome of this study is the understanding that 5∆ steps on the typical horizontal prism bar contribute to high variability in positive fusional vergence measures when findings are ≥20∆.