Binocular vision is obtained with the simultaneous use of both eyes and the fusion, at the level of the brain, of their respective images. To achieve this, the eyes must be correctly aligned on a fixation point, whereby bifoveal fixation occurs by stimulating the corresponding retinal spots in both retinas.1 To ensure binocular vision, fusional vergences compensate for heterophoria2 to ultimately achieve a single binocular vision image and avoid diplopia.3
The eyes move via the extraocular muscles, and the movements that allow for correct aiming and are responsible for binocular vision are called vergencial movements. They are divided into four components: tonic vergence, accommodative vergence, proximal vergence, and fusional vergence.4 In addition, in the evaluation of fusional vergence, a range of outcomes is determined by the following: blur, which measures the amount of merge fusion free of accommodation; break, which indicates the amount of fusional vergence and accommodative vergence; and recovery, which measures the patient's ability to recover binocular vision after diplopia.3
Non-strabismic binocular dysfunctions are vision disorders that affect the binocular system and visual performance of the patients. These dysfunctions tend to cause difficulties in activities related to near vision and induce symptoms such as blurred vision, difficulty reading, headache, diplopia, and, in many cases, inability to maintain comfortable viewing for a long time.5,6 In recent decades, the prevalence of these dysfunctions has been signally increased.7,8 Montés-Micó9 found 56.2% of patients presented with symptoms of binocular dysfunction, 61.4% with accommodative disorders, and 38.6% with vergence disorders. The study population was recruited from an ophthalmologic clinic. Several symptoms and signs can be used to diagnose these dysfunctions. However, there is a lack of consensus among researchers about which diagnostic criteria are useful for defining each anomaly.10,11 The clinical signs are the objective manifestations observed in ophthalmic and optometric examination and are considered in or out of normative values. The most commonly used normative values were established by Morgan12 and Scheiman and Wick.13 In their publications, these authors referred to both children and adults, but they did not specify the ages of the patients within the adult population.
Scientific literature supports that, with age, a decrease occurs in visual acuity, contrast sensitivity, stereoacuity,14–18 and accommodation.19 Accommodation is a physiological process. When a change occurs in lens shape, it increases or decreases the diopter power of the eye and produces a clear image on the retina of objects located at different distances.20 Loss of accommodation begins in adolescence. The accommodation amplitude is up to 15.00 diopters (D) in children and approximately 10.00 D in adolescents, and begins to decrease in the second and third decades of life. At this moment, the accommodative reserve is insufficient, and there are difficulties in performing near vision tasks. From 50 to 55 years, the accommodative capacity is completely stopped.19,20 This implies changes in the global vergence system that affect the ability to maintain binocular vision.4,21 The ciliary muscle, responsible for accommodation, and extraocular musculature, responsible for convergence, present the same innervation. Convergence stimulates accommodation and divergence relaxes it.22,23
Both systems, accommodative and vergencial, work together to maintain stable vision. The relationship between the two systems is given through AC/A (change in convergence caused by a certain change in accommodation) and CA/C (change in accommodation induced by a change in convergence).24 Other authors found an increase of exophoria25,26 in a presbyopic population. This situation increases patient-referred symptomatology. Visual therapy has been described as a treatment option in adults with decompensated heterophoria.27,28
The objective of our study was to establish relationships between patient age and horizontal heterophoria,25,26 range of horizontal vergences, base-in or negative fusional vergence (NFV), base-out or positive fusional vergence (PFV), and vergence facility testing.13,29 The study was designed to assess the dynamics of the fusional vergence system and the ability to respond over a period of time.
Patients and Methods
This observational, prospective, cross-sectional, correlational study was conducted from March 1 to December 31, 2017, at the Faculty of Pharmacy, Optics and Optometry Titling facilities of the University of Seville, Spain.
The research followed the tenets of the Declaration of Helsinki. Informed consent was obtained from the patients after explaining the nature and possible consequences of the study, and the University of Seville institutional review board approved the research.
The selected population was composed of students, professors, and administrative and service personnel of the University of Seville. A recruitment letter was sent via email to the entire university community (143 patients) of the Faculty of Pharmacy at the University of Seville. All patients were informed verbally and in writing. Six people refused to participate and 3 did not sign the informed consent, leaving a total of 134 participants who gave their consent to participate in this research.
Questions included were: (1) Have you had any history of previous ocular pathology? (2) Did you use glasses or contact lenses during infancy? (3) Have you had any type of eye surgery? (4) Have you had a history of ocular pathologies in your family? (5) Do you currently suffer from any type of disease at all? (6) Do you take medication? If yes, describe in detail. Screening corneal topography was done if possible alteration of the anterior segment was suspected. Twenty-two patients were excluded (Figure 1) due to not meeting the inclusion criteria for the study.
Study flow chart.
Measurements and Procedures
Horizontal Heterophoria. The magnitude of the horizontal heterophoria (prism diopters [PD]) was performed at distance and near (6 and 0.4 m) with an occluder, a prism bar, and a near accommodative target using the alternating prism cover test.30,31
Horizontal Fusional Vergences. Horizontal fusional vergences (PD), base-in or NFV, and base-out or PFV, were measured using the rotary prisms of the phoropter (Essilor). The two methods used (rotary prism in the phoropter and prism bars) to measure fusional vergences showed fairly good inter-session repeatability for measuring NFV but repeatability was reduced for PFV measurements.3 The 20/30 line on the Snellen chart was used as the distance fixation target.32 It was projected to 6 m to obtain far values. The near vergences were tested with a standard fixation card mounted in a phoropter at 0.4 m.3,29 The patients fixated on a letter (either far or near). A licensed and experienced optometrist performed all optometric examinations (MCS-G). Prisms were introduced at a rate of 1 PD per second. Patients indicated when they saw the text blurred (blur point) or doubled (break point). Patients were instructed to report when they clarified the image. The prismatic power was then decreased until the patient merged the image again (recovery point). NFV was measured with the base-in prism and PFV was measured with the base-out prism. For the NFV distance, there was no blur point.3 Vergence range was determined at distance fixation first and near fixation second. NFV was always measured first, because there seems to be a prismatic adaptation if PFV is measured first.33
Vergence Facility Testing. Vergence facility testing (cpm) was quantified with a combination of 3 PD base-in and 12 PD base-out prisms. Repeatability of test results was good at near.34 Vergence facility was measured by changing between base-in and base-out prisms (first base-in) with a prism flip-per, requiring the patients to converge and diverge. The fixation point was a near Snellen chart located 0.4 m from the patient. It presented a visual acuity equivalent to 20/30 (with both eyes, Snellen scale). The measurement involved introducing the base-in prism first. The patient clarified the image. Next, we changed to the base-out prism. The process alternated for 1 minute. The number of complete cycles (one base-in and one base-out prism) was the value of the vergence facility.34
Data were analyzed using SPSS software for Windows (version 24; SPSS, Inc). The normality of variables was verified using the Shapiro–Wilk test. Next, the relationship between the variables (distance horizontal heterophoria without compensation, distance horizontal heterophoria with compensation, near horizontal heterophoria without compensation, near horizontal heterophoria with compensation, distance break base-in or NFV, distance recovery base-in or NFV, near blur base-in or NFV, near break base-in or NFV, near recovery base-in or NFV, distance blur base-in or PFV, distance break base-out or PFV, distance recovery base-out or PFV, near blur base-in or PFV, near break base-in or PFV, near recovery base-in or PFV, and vergence facility testing), and age was studied, calculating the Pearson coefficient r and carrying out a simple linear regression analysis, showing the values of the coefficient of determination R2 and unstandardized coefficient b. The values of binocular vision were compared in the groups in which we differentiated the patients according to the age ranges. The t test was used. Effect size was calculated with partial square eta coefficient and Cohen's d. Finally, patients were classified in and out the norm, distance and near PFV were classified according to the normative values of Morgan12 (based on a study with 800 patients that valued the heterophoria, next point of convergence, and positive and negative fusional vergences), horizontal heterophoria and vergence facility were classified according to the normative values of Scheiman and Wick13 (to our knowledge, they were first to establish these normative values), and compared between non-presbyopic and presbyopic groups, using the chi-square test. All statistical tests were performed with 95% confidence level (P < .05).
The sample consisted of 112 patients with a mean age of 39.8 ± 14.97 years (range: 18 to 65 years) and was composed of 61 women (54.5%) and 51 men (45.5%). The non-presbyopic group included patients aged 18 to 39 years (n = 49) and the presbyopic group included patients aged 40 to 65 years (n = 63). All patients had a visual acuity of at least 20/20 (in both eyes, Snellen scale) with their best correction in distance and near. Correction was considered in near for all participants. Room illumination was 120 cd/m2.35 All patients had absence of ocular motility defects, manifest strabismus, nystagmus, corneal ectasias, suppression, diplopia, or amblyopia (visual acuity worse than 20/25 in both eyes, Snellen scale), and any ocular or systemic disease that could affect the results. Patient ocular status was obtained via a questionnaire.
Relationship Between Horizontal Phoria and Vergence System Versus Age
We studied the horizontal phoria at distance and near fixation, negative and positive vergences both in far and near, and vergence facility compared by age. Age was treated as a continuous and quantitative variable in correlation analysis. A statistically significant relationship was obtained between age and the variables listed in Table 1. Linear regression models are also shown in Figure 2.
Correlation Between Horizontal Fusional Vergences and Vergence Facility Variables vs Age
Linear regression graphs of (A) distance negative fusional vergence (NFV) recovery versus age; (B) distance positive fusional vergence (PFV) recovery versus age; (C) near PFV blur versus age; (D) near PFV break versus age; (E) near PFV recovery versus age; and (F) vergence facility versus age.
Comparison of the Presbyopic and Non-presbyopic Groups
A comparison was then made of all study variables according to the defined presbyopic and non-presbyopic groups. Significant differences in exophoria were obtained in near heterophoria with compensation (3.15 ± 8.90 and 6.87 ± 6.76, P < .05), distance PFV recovery (10.35 ± 5.29 and 7.48 ± 4.35 PD, P < .01), near PFV blur (14.21 ± 7.30 and 11.08 ± 6.40 PD, P < .05), near PFV break (22.12 ± 8.70 and 17.67 ± 7.77 PD, P < .01), near PFV recovery (14.24 ± 7.80 and 9.55 ± 6.71 PD, P < .01), and vergence facility (10.70 ± 4.96 and 8.07 ± 3.41 cpm, P < .01). A statistically significant relationship (t test) between the six variables was found between the non-presbyopic group (18 to 39 years) and the presbyopic group (40 to 65 years). Results are shown in Table 2. The boxplot graphs for near heterophoria with compensation, distance PFV recovery, near PFV (blur, break, and recovery), and vergence facility are represented in Figure 3.
Descriptive Analysis of Horizontal Heterophoria, Horizontal Fusional Vergences, and Vergence Facility
Boxplot graphs for nonpresbyopic and presbyopic groups: (A) near heterophoria with compensation; (B) distance positive fusional vergence (PFV) recovery; (C) near PFV blur; (D) near PFV break; (E) near PFV recovery; and (F) vergence facility.
For the statistically significant variables, the size of the effect was calculated. Near heterophoria with compensation had a medium effect size of 0.48 and the mean difference was 3.74 PD (CI: 0.86 to 7.40). Distance PFV recovery had a medium effect size of 0.59 and the mean difference was 2.86 PD (CI: 1.01 to 4.71). Near PFV blur had a medium effect size of 0.45 and the mean difference was 3.13 PD (CI: 0.08 to 6.18). Near PFV break had a medium effect size of 0.54 and the mean difference was 4.45 PD (CI: 1.32 to 7.58). Near PFV recovery had a medium effect size of 0.64 and the mean difference was 4.69 PD (CI: 1.93 to 7.45). Vergence facility had a medium effect size of 0.61 and the mean difference was 2.63 cpm (CI: 0.81 to 4.45). Linear regression along with the trendline are represented for distance NFV recovery, distance PFV recovery, blur, break, and near PFV recovery and vergence facility versus age in Figure 2.
Classification According to the Normative Values of Morgan and Scheiman and Wick
The values of the near PFV were classified according to the normative values established by Morgan,12 and vergence facility was classified according to the normative values established by Scheiman and Wick.13 We compared these values between the non-presbyopic and presbyopic groups. For distance PFV recovery, 12.2% in the non-presbyopic group and 24.1% in the presbyopic group had values below the norm, and 20.4% in the non-presbyopic group and 6.9% in the presbyopic group had values above the norm (chi-square = 5.57, P = .05). For near PFV blur, 42.9% in the non-presbyopic group and 51.3% in the presbyopic group had values below the norm, and 16.7% in the non-presbyopic group and 5.1% in the presbyopic group had values above the norm (chi-square = 2.77, P = .25). For near PFV break, 24.5% in the non-presbyopic group and 43.3% in the presbyopic group had values below the norm, and 30.6% in the non-presbyopic group and 13.3% in the presbyopic group had values above the norm (chi-square = 6.57, P = .03). For near PFV recovery, 2.0% in the non-presbyopic group and 15.0% in the presbyopic group had values below the norm and 26.5% in the non-presbyopic group and 6.7% in the presbyopic group had values above the norm (chi-square = 11.93, P = .03). For vergence facility, 65.9% in the non-presbyopic group and 86.0% in the presbyopic group had values below the norm, and 9.1% in the non-presbyopic group and 0% in the presbyopic group had values above the norm (chi-square = 6.43, P = .04).
In this study, we proposed an evaluation of horizontal heterophoria, range of horizontal vergences, base-in NFV, base-out PFV, and vergence facility testing that define the state of binocular vision in a sample with two age intervals (non-presbyopic and presbyopic groups), using tests that present the highest repeatability to establish relationships between age and binocular vision variables. Our results matched those of previous studies, which indicate how age affects the binocular vision variables.36–38 Palomo et al37 established a relationship between age and binocular vision only at distance fixation. Other authors have measured binocular vision values individually.4,38 In addition, the normative values described referred to an adult population without specifying age ranges.12,13
Our analysis indicated that the values of near horizontal heterophoria with compensation, distance PFV recovery, blur, and break and near PFV recovery in vergence facility, which determine the status of binocular vision, decrease with age. The results obtained for the statistically significant variables were analyzed according to the normative values in adults. According to the normative values of Scheiman and Wick,13 near horizontal heterophoria with compensation standard range of exophoria of 3 ± 3 PD. In our study, exophoria was 3.15 PD in the non-presbyopic group and 6.87 PD in the presbyopic group, and the presbyopic group was clearly outside the norm. Normative values of Morgan12 (horizontal fusion vergences) were used to compare with our values. Distance PFV recovery ranges from 6 to 14 PD, and in our study it was 10.35 ± 5.39 PD in the non-presbyopic group and 7.48 ± 4.35 PD in the presbyopic group (which was outside the norm). Near PFV blur ranges from 12 to 22 PD, and in our study it was 14.21 ± 7.30 PD in the non-presbyopic group and 11.08 ± 6.40 PD in the presbyopic group (which was outside the norm). Near PFV break ranges from 15 to 27 PD, and in our study it was 22.12 ± 8.70 PD in the non-presbyopic group and 17.67 ± 7.77 PD in the presbyopic group. Near PFV recovery ranges from 4 to 18 PD, and in our study it was 14.24 ± 7.80 PD in the non-presbyopic group and 9.55 ± 6.71 PD in the presbyopic group. Although the presbyopic group is within the standard, the difference between groups is notable.
Morgan did not include vergence facility in his study, but Schieman and Wick reported it as 15 ± 3 cycles per minute (cpm). In our study, it was 10.70 ± 4.96 PD in the non-presbyopic group and 8.07 ± 3.41 PD in the presbyopic group, which means no patients in the presbyopic group were within the standard range.
Stable vision maintenance required collaboration of accommodative and vergencial systems.24 The AC/A ratio was not studied because of the age of the patients (18 to 65 years). In this sense, the negative lens used in AC/A measurement stimulates accommodation. Presbyopic patients do not have accommodation to clarify the text under this situation. For this reason, the variable was not studied. With age, a decrease in accommodation occurs,19 which produces an increase in the AC/A ratio and a decrease in the CA/C ratio. Near objects are blurred by a decrease in accommodation amplitude.36,39,40 These changes imply rearrangements in the other components of vergence to achieve a unique and stable binocular vision.4
Most studies conclude that there is no evidence of change in either proximal vergence or tonic vergence that can counteract the increase in accommodative convergence.36 Therefore, it must be the fusional convergence that varies. As shown in our results, near horizontal heterophoria with compensation increased exophoria by 3.74 PD, distance PFV recovery decreased by 2.86 PD, near PFV blur decreased by 3.13 PD, near PFV break decreased by 4.45 PD, near PFV recovery decreased by 4.69 PD, and vergence facility decreased by 2.63 PD. Most variables correspond to the value of the near PFV, which is directly related to accommodation.
In exophoria, visual axes tend to go outward without manifesting deviation, because the fusion mechanism (PFV) is responsible for coordination at a fixation point. Convergence-accommodation mechanism relaxation supposes that visual axes diverge. Hence, age increases near exophoria value,25,26 due to convergence-accommodation mechanism inefficiency. We also observed a decrease in vergence facility associated with age, a result that is in line with another study.38 In addition, this result is justified because vergence facility evaluates the dynamic ability of the fusional vergence system34 (ie, the patient's ability to merge images).
Our results indicated an increase in exophoria and a decrease in near positive horizontal fusional vergences and vergence facility through age. Thus, we believe that normative values defined for the entire adult population should not be generalized. They must be interpreted according to patient age because accommodation in a young population is not equal to a presbyopic population. Changes in the normal values should be considered for each age range. We suggest that by increasing the population under study, a normative value in relation to age can be established.
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Correlation Between Horizontal Fusional Vergences and Vergence Facility Variables vs Age
|Distance NFV recovery, PD with age||−0.25||< .01||0.038||y = 5.86 – 0.03 x|
|Distance PFV recovery, PD with age||−0.30||< .01||0.094||y = 12.85 – 0.1 x|
|Near PFV blur, PD with age||−0.32||< .01||0.088||y = 18.17 – 0.14 x|
|Near PFV break, PD with age||−0.27||< .01||0.075||y = 25.84 – 0.15 x|
|Near PFV recovery, PD with age||−0.32||< .01||0.111||y = 18.34 – 0.17 x|
|Vergence facility, PD with age||−0.36||< .01||0.150||y = 13.85 – 0.12 x|
Descriptive Analysis of Horizontal Heterophoria, Horizontal Fusional Vergences, and Vergence Facility
|Variable||Group Age Range||P|
|18 to 39 Years||40 to 65 Years|
|Age (years)||49||25.29||6.04||21||62||52.18||7.59||25||< .01a|
|Distance HH (PD)|
| Without compensation||49||0.39X||4.96||30||61||0.52X||1.97||15||.86|
| With compensation||33||0.70X||3.45||20||34||0.45X||1.14||6||.70|
|Near HH (PD)|
| Without compensation||48||5.73X||9.33||50||59||6.29X||5.90||24||.71|
| With compensation||33||3.15X||8.90||36||48||6.87X||6.76||29||< .05a|
|Base-in or NFV|
| Distance (PD)|
| Near (PD)|
|Base-out or PFV|
| Distance (PD)|
| Recovery||49||10.35||5.29||24||58||7.48||4.35||18||< .01a|
| Near (PD)|
| Blur||42||14.21||7.30||26||39||11.08||6.40||24||< .05a|
| Break||49||22.12||8.70||34||60||17.67||7.77||26||< .01a|
| Recovery||49||14.24||7.80||31||60||9.55||6.71||24||< .01a|
|Vergence facility (PD)||44||10.70||4.96||21||43||8.07||3.41||14||< .01a|