Prevalence of non-strabismic binocular vision anomalies and age-related changes in binocular vision among middle-aged and older adults: a systematic review

Abstract

Background

With the global aging population, binocular vision anomalies in middle-aged and older adults have gained increasing attention. Non-Strabismic Binocular Vision Anomalies (NSBVAs) are particularly prevalent in this population, yet clinical research remains insufficient. To bridge this gap, we conducted a systematic review to evaluate NSBVAs prevalence and subtypes in middle-aged and older adults and analyze age-related changes in binocular vision. The aim was to clarify how aging affects binocular function and inform clinical management.

Methods

Studies were identified using PubMed, Web of Science and existing reviews, with the search strategy incorporating terms related to binocular vision alterations in middle-aged and older adults (40 years and older). Cross-sectional studies and observational cohort studies investigating binocular vision alterations in this population were included. The study adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 statement guidelines. Two independent researchers carried out all study procedures, including literature search, quality assessment using the Critical Appraisal Skills Programme Quality Appraisal Scale, and blinded study selection and data extraction. The main outcomes and measures were the measurement parameters of binocular vision function and the prevalence rates of NSBVAs in middle-aged and older adults.

Results

Among the eight studies included in this review, all examined binocular vision measures in middle-aged and older adults. Age demonstrates a significant influence on these measures. However, data regarding the prevalence of binocular vision anomalies within this population remain limited. Specifically, only four studies reported the prevalence of specific diagnosed conditions, while an additional two studies documented the prevalence of abnormal binocular vision test results that did not correspond to defined diagnostic criteria. All eight studies were assessed as being of moderate to high methodological quality.

Conclusion

The existing literature indicates that age significantly influences binocular vision function, with NSBVAs being particularly prevalent among middle-aged and older adults. However, prior studies predominantly focus on the prevalence of abnormal individual test results in this group, while limited data are available regarding actual diagnostic conditions. Establishing normative reference values for binocular vision parameters is therefore essential to determine a more accurate prevalence rate of NSBVAs in this population.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-025-04275-6.

Introduction

With the global population aging, research on age-related changes in visual function has become increasingly crucial, particularly in the context of binocular vision. The aging process often leads to significant alterations in the collaborative functioning of the eyes for achieving optimal visual perception [1]. These alterations encompass declines in accommodation, convergence, and binocular coordination, which can detrimentally affect visual efficiency and contribute to asthenopia, headache, blur, and diplopia, as well as difficulties in daily tasks such as reading and driving [2–4]. While much attention has been devoted to prominent age-related visual impairments such as presbyopia [5–7], the impact of presbyopia on binocular vision is primarily reflected in a significant decline in accommodative amplitude. Given the close relationship between accommodation and vergence, age-related changes in vergence should not be overlooked. There is increasing awareness of how dysfunctions in binocular vision can affect the quality of life of middle-aged and older adults [8–11]. However, systematic reviews focusing on non-strabismic binocular vision anomalies (NSBVAs) in middle-aged and older adults remain scarce. Most existing studies have concentrated on specific age groups or single visual functions, lacking comprehensive integrative analyses of the prevalence, underlying mechanisms, and clinical implications of NSBVAs in individuals aged 40 years and above. Furthermore, significant discrepancies in diagnostic criteria, measurement methods, and sample selection across studies have rendered results difficult to compare horizontally, further impeding the academic community's in-depth understanding of age-related binocular vision changes. This review aims to explore the current research on age-related binocular vision changes, with a particular focus on NSBVAs. By examining the prevalence, underlying mechanisms, and clinical implications of these changes, the review seeks to provide a comprehensive understanding of how aging affects binocular vision. Additionally, it will delve into the risk factors and diagnostic challenges of binocular vision disorders in middle-aged and older adults, highlighting the need for more targeted interventions and clinical guidelines. Ultimately, this review will underscore the importance of addressing binocular vision changes in middle-aged and older adults, offering insights that can inform both clinical practice and future research directions.

Methods

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines; see S1 Checklist [12]. The protocol for this review has been registered with PROSPERO (CRD420250650568).

Search strategy

An exhaustive literature search was performed using PubMed and Web of Science, employing the following search strategy: ("aged"[mesh] OR elderly OR "middle aged"[mesh] OR old OR older OR geriatric OR Senior) AND ("binocular vision anomalies" OR "binocular vision alterations" OR "binocular vision change" OR "binocular vision disorders" OR "binocular vision dysfunction" OR "binocular vision problems" OR "accommodative disorders" OR "accommodative dysfunction" OR "accommodative problems" OR "convergence insufficiency" OR "divergence insufficiency" OR "convergence excess" OR "divergence excess" OR "fusional vergence dysfunction" OR "basic esophoria" OR "basic exophoria" OR "eye movement disorders"). The search was independently conducted by two authors (CYL and CKL).

Selection criteria

The inclusion criteria were: (1) original research articles such as cross-sectional, cohort studies, or clinical trials; (2) studies focusing on diagnosing and analyzing the epidemiology of NSBVAs in participants aged 40 and older with presbyopia or early presbyopia; (3) studies examining age-related changes in binocular vision.

The exclusion criteria were: (1) individuals with strabismus, ocular pathology (e.g., glaucoma, retinal diseases), history of ocular surgery or trauma, abnormal ocular development, or best-corrected visual acuity (logMAR) ≤ 0.2 in both eyes; (2) studies focused on treatment or binocular vision refraction testing that were not pertinent to diagnosing NSBVAs; (3) non-original research like expert opinions, guidelines, conference abstracts, letters, or study protocols.

Data extraction and quality assessment

For each included study, we extracted the following information: first author, publication year, journal name, country, study design, sample size, participant age, measures of binocular vision, prevalence of binocular vision anomalies, and limitations of the studies. The quality of the studies was evaluated using the Critical Appraisal Skills Programme (CASP) Quality Appraisal Scale for cohort and descriptive/cross-sectional studies; see S2 Checklist and S3 Checklist [13]. The CASP Checklist for Cohort Studies includes three sections: Section A ("Are the study results valid?"), Section B ("What are the results?"), Section C ("Will the results be applicable locally?"), and Section D, "Is the study methodologically sound?”. Meanwhile, the CASP Checklist for Descriptive/Cross-Sectional Studies focuses solely on Section A ("Are the study results valid?"). The quality of the included articles was categorized into three equally weighted levels: low (0–3 "yes" responses), moderate (4–7 "yes" responses), and high (8–12 "yes" responses) for observational cohorts and cross-sectional studies.

This study employed cross-verification and discussion for CASP consistency assessment to address inherent subjective judgments and textual interpretation issues. The procedure involved: two sufficiently reliable reviewers (CYL and CKL) first establishing a unified judgment standard through joint evaluation of 8 literatures with resolved interpretive discrepancies; subsequently performing blinded independent CASP evaluations, literature grading, and data extraction for target studies, documenting rationales per item while categorizing discordances as "missing information", "divergent criterion interpretation", or "discrepant subjective judgments"; Any discrepancies arising between them were resolved by a domain expert third reviewer (QQT). No articles were excluded from this analysis.

Results

Literature search results

The key outcomes in this systematic review include the results of binocular vision examinations and the prevalence rates of NSBVAs among individuals with presbyopia and early presbyopia. Following the removal of duplicates, a total of 684 records were screened; 32 full-text articles were evaluated for eligibility, and ultimately, 8 relevant studies were included. The process of study selection is illustrated in Fig. 1, which presents the PRISMA flow diagram.

Fig. 1.

PRISMA flow diagram of study selection

Study quality assessment

Table 1outlines the characteristics of all included studies and presents the agreed ratings from the CASP Quality Appraisal Scale. Based on this classification, we identified five cross-sectional studies of high quality: Tan et al. [11], Hashemi et al. [14], Hashemi et al. [15], Sánchez-González et al. [10], and Ostadimoghaddam et al. [16]; two cross-sectional studies of moderate quality: Hashemi et al. [17] and Hashemi et al. [8]; and one cohort study of high quality: Leat et al. [9].

Table 1.

Demographic characteristics and quality appraisal of included studies

Author(s)	Design	Sample size	Age (years) Mean (SD)	Gender (F/M)	Yes/total
Tan et al. [11]	CS	73	70.2 (6.7)	57/16	11/11†
Hashemi et al. [14]	CS	Total:3132, including: age 0–39: 1974; age 40–49: 490; age 50–59: 357; age 60–69: 124; age 70–79: 119; age 80–89: 63; age ≥ 90: 5	30.5 (14.0) (range 10–69)	age 40–49:169/321; age 50–59: 182/175; age 60–69: 54/70; age 70–79: 54/65; age 80–89: 34/29; ≥ 90: 3/2	9/11†
Hashemi et al. [17]	CS	Total:1683, including: age 10–39: 1315; age 40–49: 367	27.8 (11.9) (range 10–49)	Total:1156/527	5/11#
Hashemi et al. [15]	CS	Total:1793, including: age 60–64:860; age 65–69:572; age 70–74:252; age 75–79: 84; age ≥ 80: 25	65.9 (5.0)	1082/711	11/11†
Hashemi et al. [8]	CS	2,227	66.6 (5.4) (range 60–95)	1323/904	7/11#
Sánchez-González et al. [10]	CS	112	Total: 39.8 (14.97) (range 18–65) NPG: 25.29 (6.04) (range 18–39) PG: 52.18 (7.59) (range 40–65)	61/51	10/11†
Ostadimoghaddam et al. [16]	CS	2433	31.2 (15.1) (range 10–86)	1650/783	9/11†
Leat et al.[9]	OC	500	≥ 60, include three patients aged 59 years (59, 59.6, and 59.9 years)	284/216	8/12†

F/M, the ratio of females to males; CS, cross-sectional study; OC, observational cohort study; NPG, non-presbyopic group; PG, presbyopic group. The quality of the included articles was categorized into three equally weighted levels for observational cohorts and cross-sectional studies: low quality: 0–3 "yes" responses; ^#Moderate quality: 4–7 "yes" responses; ^†High quality: 8–12 "yes" responses

Study characteristics

A total of 8674 adults aged 40 years and older, comprising both pre-presbyopic and presbyopic individuals, were assessed for NSBVAs. The systematic review reveals a significant association between advancing age and binocular vision function.

In the studies reviewed, Tan et al. [11] identified NSBVAs, particularly convergence insufficiency, in adults with age-related cataracts. Hashemi et al. [8, 14, 15] reported on the prevalence of convergence insufficiency, fusional vergence dysfunction (FVD) and heterophoria in Iranian populations over 60 years old, correlating these conditions with age. Sánchez-González et al. [10] conducted a comparative study on presbyopia, establishing relationships between age and horizontal heterophoria, horizontal fusional vergence amplitudes, and vergence facility testing. The only cohort study included in the review, by Ostadimoghaddam et al. [16], examined the normal range of near point of convergence (NPC) across different age groups, providing the first description of NPC norms in presbyopic populations and exploring age-related changes in NPC. Additionally, Leat et al. [9] determined the prevalence of binocular vision and ocular motility disorders in an elderly outpatient population. Collectively, these studies highlight the significant impact of aging on binocular vision function, particularly in older adults.

All studies included conducted comprehensive binocular vision assessments, with variations in diagnostic criteria tailored to the specific binocular vision functions under investigation. For the three studies primarily focusing on convergence insufficiency, Tan et al. [11] adapted diagnostic criteria from Scheiman and Wick [18], employing a three-sign criterion recommended by Ma et al. [19] for grading convergence insufficiency. Similarly, Hashemi et al. [14, 15] utilized the four diagnostic criteria established by the CITT-ART Investigator Group [20] for diagnosing convergence insufficiency. In contrast, Hashemi et al. [17] applied the guidelines proposed by Scheiman and Wick [18] for diagnosing FVD. Sánchez-González et al. [10] assessed horizontal heterophoria, horizontal fusional vergence amplitudes, and vergence facility, referencing the normative values most used and established by Morgan [21] as well as Scheiman and Wick [18]. Ostadimoghaddam et al. [16] investigated normative values for the NPC, while Hashemi et al. [8] examined heterophoria, and Leat et al. [9] defined any binocular vision or ocular motility abnormalities based on test results falling outside the normal range.

The prevalence of NSBVAs in presbyopic populations was reported as follows: convergence insufficiency was studied by Tan et al. [11], Hashemi et al. [14, 15]; heterophoria was investigated by Tan et al. [11], Hashemi et al. [8], and Leat et al. [9]; eye-movement disorders were examined by Leat et al. [9]; FVD was reported by Tan et al. [11] and Hashemi et al. [17]. Tan et al. [11] also provided data on overall NSBVAs and convergence excess prevalence. Leat et al. [9] reported overall binocular vision and eye-movement disorder prevalence, along with abnormal NPC analysis. Sánchez-González et al. [10] and Ostadimoghaddam et al. [16] did not report prevalence data.

Table 2 provides a summary of the binocular vision examination results and the prevalence of NSBVAs identified in each study. However, only two studies have reported the results on the prevalence of overall binocular vision abnormalities. In elderly populations, Tan et al. [11] studied 73 patients with senile cataracts (mean age 70.2 years) and found that 32.9% had NSBVAs. Leat et al. [9] reported prevalence rates of 27%, 30%, and 38% for individuals aged 60–69, 70–79, and over 80 years, respectively.

Table 2.

Characteristics of binocular vision in the middle-aged and elderly from the included studies

Study	Phoria (△) Mean (SD)	PFV/NFV (blur/break/recovery, △) Mean (SD)	VF (cpm) Mean (SD)	NPC (break/recovery, cm) Mean (SD)	Symptom survey Mean (SD)	Stereoacuity(″) Mean (SD)	Prevalence
Tan et al. [11]	NBV: distance: − 0.9 (2.2); near: − 3.9 (3.8); NSBVAs: distance: − 1.2 (2.6); near: − 5.2 (7.7)	NBV: distance PFV: x/20.4 (11.2)/15.4 (9.5); distance NFV: x/10.5 (7.2)/7.2 (5.8); near PFV: x/30.6 (12.6)/23.5 (11.6); near NFV: x/14.0 (5.7)/10.3 (4.0) NSBVAs: distance PFV: x/13.1 (9.2)/10.0 (8.9); distance NFV: x/9.0 (6.6)/5.2 (3.6); near PFV: x/11.8 (3.6)/8.4 (4.7); near NFV: x/11.4 (4.7)/8.7 (4.6)	NBV: distance: 6.8 (5.8); near: 11.5 (5.3) NSBVAs: distance: 4.2 (4.5); near: 6.4 (4.8)	NBV: 8.6 (3.7)/10.6 (4.4); NSBVAs: 10.6 (3.6)/13.1 (4.7)	CISS: NBV: 20.0 (11.1); NSBVA: 17.7 (11.1)	NBV: 172.1 (165.6); NSBVAs: 137.3 (149.0)	NSBVAs: 32.9%: including CI (24.7%); basic exophoria (4.1%); convergence excess (2.7%); FVD (1.4%)
Hashemi et al. [14]	Without CI: − 2.05 (2.60); definite CI: − 10.35 (2.87)	Near PFV break: without CI: 15.21 (8.02); definite CI: 11.21 (4.71)	–	NPC break: without CI: 8.36 (4.88); definite CI: 11.15 (4.33)	–	–	CI: age 40–49: 4.62%; age 50–59: 3.21%; age 60–69: 8.57%
Hashemi et al. [17]	–	–	–	–	–	–	FVD: 3.2%: including age 10–19: 2.35%; age 20–29: 2.18%; age 30–39: 3.47%; age 40–49: 5.45%
Hashemi et al. [15]	Distance: without CI: − 2.9 (2.5); high suspect CI: − 3.2 (2.8); definite CI: − 3.5 (2.1) Near: without CI: − 5.8 (3.0); high suspect CI: − 10.1 (3.2); definite CI: − 11.3 (3.4)	Near PFV break: without CI: 17.7 (7.2); high suspect CI: 9.5 (5.9); definite CI: 6.2 (4.9)	–	NPC break: without CI: 8.7 (4.5); high suspect CI: 13.7(5.2); definite CI: 15.5 (5.1)	–	–	CI: 21.5%; age 60–64: 20.6%; age 65–69: 21.4%; age 70–74: 27.0%; age 75–79: 16.7%; age ≥ 80: 19.0%
Hashemi et al. [8]	–	–	–	–	–	–	Phoria: 35.6%: including distance exophoria (8.9%); distance esophoria (0.34%); distance hyperphoria (0.29%); near exophoria (32.4%); near esophoria (0.61%); near hyperphoria (0.37%)
Sánchez-González et al. [10]	distance (without Add): − 0.52 (1.97); distance (with Add): − 0.45 (1.14); near (without Add): − 6.29 (5.90); near (with Add): − 6.87 (6.76)	distance NFV: x/9.80 (3.71)/4.43 (2.52); near NFV: 10.45 (4.61)/17.94 (5.44)/11.66 (5.13); distance PFV: 12.36 (6.33)/17.86 (7.12)/7.48 (4.35); near PFV: 11.08 (6.40)/17.67 (7.77)/9.55 (6.71)	near: 8.07(3.41)	–	–	–	–
Ostadimoghaddam et al. [16]	–	–	–	NPC break: age 40–49: 10.04 (5.08); age 50–59: 11.30 (6.82); age 60–69: 11.98 (5.75); age ≥ 70: 13.06 (5.20)	–	–	–
Leat et al. [9]	–	–	–	–	–	–	Any BV and eye-movement disorder: 31.6%; Vertical phoria: 14.4%; Distance exophoria: 2.4%; Near exophoria: 12.0%; NPC > 10: 13.8%; Distance esophoria: 5.0%; Near esophoria: 3.4%; Incomitancy: 3.6%; Anomaly of pursuits: 7.4%; Decompensating distance exophoria: 1.0%; Decompensating near exophoria: 1.6%

SD: standard deviation; NBV, normal binocular vision; NSBVAs: non-strabismic binocular vision anomalies; NPC, near point of convergence; VF, vergence facility; CISS, convergence insufficiency symptom survey; CI: convergence insufficiency; FVD, fusional vergence dysfunction; NFV, negative fusional vergence; PFV, positive fusional vergence; BV, binocular vision; Add, near addition; ^△, prism diopter; ″, arcsecond; cpm, cycle per minute; cm, centimetre

In a study by Tan et al. [11], elderly patients underwent comprehensive binocular vision assessments before cataract surgery, including tests like stereoacuity, ocular deviation, fusional vergence, vergence facility, and NPC. Using criteria from Scheiman and Wick [18], the prevalence of NSBVAs was diagnosed and reported. Hashemi et al. [8] conducted a population-based cross-sectional study on elderly individuals (mean age 66.6 years). They defined exophoria as exodeviation > 3Δ at 6 m and > 9Δ at 40 cm, and esophoria as esodeviation > 1Δ at 6 m and any amount at 40 cm. The study reported prevalence rates of 8.9% for distance exophoria, 0.34% for distance esophoria, and 0.29% for distance hyperphoria. For near vision, the rates were 32.4% for exophoria, 0.61% for esophoria, and 0.37% for hyperphoria. Leat et al. [9] reviewed medical records of individuals aged 60 and older from optometric clinics, reporting an increased prevalence of abnormalities in binocular vision tests over 10 years, such as vertical phoria, near exophoria, remote NPC, and pursuit abnormalities.

Discussion

Prevalence of NSBVAs in middle-aged and older adults

Strabismic binocular vision disorders are also significant and should not be overlooked. Examples include 6th nerve palsy [22], which impairs eye movement due to dysfunction in the cranial nerve controlling the lateral rectus muscle, and decompensated 4th nerve palsy [23], often causing vertical diplopia and head tilt due to superior oblique muscle weakness. Sagging eye syndrome [24] has been identified as an age-related disorder where weakened extraocular muscles lead to ocular misalignment and binocular diplopia. While these conditions have clinical importance, this review focused on non-strabismic binocular vision disorders. Limiting the scope allowed for a more focused analysis of their mechanisms, diagnosis, and treatment. Previous studies have demonstrated significant variations in the prevalence of NSBVAs. This variability can likely be attributed to multiple confounding factors such as age, diagnostic criteria, examination methods, study design, population characteristics, and medication use [9, 15].

Age significantly influences binocular vision function, with varying prevalence rates of NSBVAs across different age groups. Studies show that in children, the prevalence of NSBVAs ranges from 26 to 31.5% [25, 26]. Specifically, Hussaindeen et al. [25] found a 26% prevalence among 305 children (mean age 12.7 years), while another study [26] by the same team reported 31.5% in urban and 29.6% in rural areas for children aged 12.7 years. Jang et al. [27] noted 28.5% of elementary school students exhibited NSBVAs. In young adults, García‐muñoz et al. [28] reported an 8% prevalence among 175 college students aged 18–35 years. Atiya et al. [29] found a higher rate of 55% among 75 ophthalmology interns, likely due to their high visual demands. Cai et al. [30] identified 36.71% of university students had NSBVAs, consistent with Ma et al.’s 40% [31] rate for individuals aged 21–38 years but lower than Kumar et al.’s 81% [32] for young adults. In two studies by Tan et al. [11] and Leat et al. [9], the reported prevalence of binocular vision anomalies in older adults ranged from 27 to 38%—comparable to prevalence data in children and non-presbyopic adults. This suggests that while high near-work demands in children and young adults may increase binocular vision issues, age-related declines in physiological accommodative amplitude among middle-aged and older adults similarly contribute to such abnormalities. Notably, Leat et al. [9] did not provide specific diagnostic outcomes, which limits the accuracy of reflecting true NSBVAs prevalence in older populations.

It is reasonable to infer that the insufficient attention to binocular vision anomalies in middle-aged and older adults in clinical and research settings stems primarily from inconsistent diagnostic criteria. Current studies on this demographic still rely on diagnostic standards derived from databases of children and non-presbyopic adults. Only Tan et al. [11] reported results from systematic binocular vision examinations, whereas most other studies focused on isolated test abnormalities rather than systematic diagnosis or reporting of binocular vision anomalies in older adults.

In the subsequent section, we will systematically review and critically analyze the findings from prior literature concerning changes in binocular vision function parameters among middle-aged and older adults.

Age-related alterations in binocular vision

Previous studies [33–35] have shown that age affects binocular vision, primarily due to presbyopia-related declines in accommodation. Accommodation decreases with age, particularly in amplitude, positive relative accommodation, and monocular/binocular facility. Assessing only accommodative function in presbyopic individuals is insufficient because accommodation and vergence are interdependent. A comprehensive evaluation of binocular vision is necessary to understand age-related effects on these key functions. Most prior studies [2, 36] focused on specific parameters such as heterophoria, the accommodative convergence to accommodation ratio (AC/A), and the convergent accommodation to convergence ratio (CA/C). By integrating findings from these studies, we can systematically explore aging's impact on binocular vision aspects like horizontal phoria, NPC, fusion reserve, vergence facility, and eye movements. This approach clarifies presbyopia's physiological mechanisms and provides a basis for preventing and treating NSBVAs.

Heterophoria and age

Previous studies [2, 37, 38] report a tendency for phoria to shift toward exophoria with age, sometimes progressing to strabismus. In Table 2, all four studies reported that the clinical measurement values of near exophoria in middle-aged and older adults were greater than -3^△, i.e., exceeding than the normal reference values for non-presbyopic adults established by Scheiman and Wick. This suggests that the near phoria of middle-aged and older adults aged 40 years and above tends to shift toward exophoria compared with that of non-presbyopic adults. Exophoria is significantly higher in presbyopic individuals compared to non-presbyopic adults, with the mean exophoria approximately doubling. Accommodation effort decreases with age [3], reducing both accommodation and convergence due to the AC/A and CA/C coupling, leading to increased near exophoria. However, some studies find no substantial association between exophoria and age, attributing this to variations in measurement methods [2, 39]. Age-related distance exophoria may result from increased negative fusional vergence (NFV) and decreased positive fusional vergence (PFV) [10, 36], relaxing convergent accommodation. Weakened medial and lateral rectus muscle function with age may also contribute to impaired ocular coordination and increased exophoria.

NPC and age

Multiple studies [16, 36, 40–42] have consistently demonstrated that NPC exhibits a progressive recession with advancing age, at an average rate of approximately 0.10 cm per year, with the most significant changes occurring during the transition from non-presbyopic to presbyopic stages [2, 16]. Similarly, the NPC break values reported in Table 2 were significantly greater than the 2.5 cm established by Scheiman and Wick, indicating that the NPC of middle-aged and older adults shifts farther away compared to non-presbyopic adults. This age-related recession is primarily attributed to diminished accommodative convergence resulting from the natural decline in accommodative amplitude and effort characteristic of presbyopia. Furthermore, research by Abraham et al. [43] involving participants aged 10–35 years and Fray's study [40] with a mean age of 38 years both corroborate this trend, suggesting that the observed NPC recession may be further influenced by developmental changes including a gradual shift toward exophoria and increasing interpupillary distance [41, 43] that occur throughout childhood and early adulthood.

Fusional reserves (ranges) and age

Fusional reserve includes two components: convergence and divergence. Clinically, base-out (BO) prism measures convergence capacity at distance and near, indirectly assessing PFV through the blur, break, and recovery points. Base-in (BI) prism evaluates divergence and NFV. Fusional reserve correlates with age and horizontal phoria. Definitions of reduced fusional reserve vary; most studies [14, 27, 44–47] define it as test values below a standard criterion, where the "blur point/break point/recovery point" varies across different studies; or it inconsistent with the Sheard criterion, which stipulates that the compensatory fusional vergence should be at least twice the measured phoria [48].

For various studies, the fusional reserve declines with age primarily due to a decrease in the blur point, break point, and recovery point of near PFV [10, 36], as well as the recovery point of distance PFV [10, 39]. In Table 2, the break point and recovery point measurements of near PFV reported in four studies were respectively less than the normal reference values of 19△ and 14△ established by Scheiman and Wick for non-presbyopic adults. Notably, in Tan et al.'s study [11], the mean break point and recovery point of near PFV in elderly individuals classified as having normal binocular vision were as high as 30.6△ and 23.5△, suggesting that elderly adults with greater inherent convergence reserves can maintain better binocular vision despite age-related effects. Age-related changes in the convergence system include reduced amplitude of convergence movements due to extraocular muscle aging and diminished accommodation effort from crystalline lens aging, which affects convergence through accommodative convergence and CA/C coupling. Sánchez-González et al.’s study [10] further noted that the age-related reduction in binocular convergence is linked to both accommodative and fusional convergence but not significantly to proximal or tonic convergence.

In Palomo Álvarez et al.’s study [39], the recovery point of distance NFV decreased, while the break point remained stable. This contradicts the hypothesis that accommodative convergence diminishes with age. Although the break point was not significantly correlated with age, individuals over 70 had a notably lower mean break point than those under 70. This discrepancy may result from the limited sample size.

In a study of non-presbyopic adults, Yekta et al. [36] suggested that age-related distance exophoria may affect divergence by increasing the break and recovery points of distance NFV. However, Ostadimoghaddam et al. [16] found no significant correlation between near PFV/NFV and age or gender, indicating stable fusional reserve at close distances. This challenges the idea that reduced accommodation and exophoria changes with age are primary factors influencing fusional reserve. Future research should clarify this relationship to improve understanding of convergence and divergence declines, enabling better prediction of visual issues in middle-aged and older adults and targeted therapy programs.

Vergence facility (VF) and age

Previous studies [10, 49] have consistently demonstrated that VF decreases with age. Similarly, in Table 2, the VF measurement values reported in two studies were both lower than the normal reference value of 15 CPM established by Scheiman and Wick for non-presbyopic adults. This decline is primarily attributed to the diminishing dynamic range of the fusional vergence system, which evaluates the ability of both eyes to maintain binocular fusion. The age-related reduction in fusional vergence performance may stem from the aging of the crystalline lens, which contributes to a decline in accommodative function.

AC/A, CA/C and age

While previous research [50] has proposed that the AC/A ratio stabilizes early in life and remains constant with age, a growing body of evidence [2, 16, 51] demonstrates significant age-related variations, likely attributable to physiological changes such as crystalline lens alterations. Due to the lag in accommodation, the response AC/A ratio is slightly higher than the stimulant AC/A ratio. Although the eight studies included in this review did not directly report measurements of AC/A or CA/C values in middle-aged and older adults, it can be inferred from the above findings that due to the development of presbyopia, middle-aged and older adults exhibit a physiological decline in accommodative amplitude in terms of accommodative function. In terms of vergence function, they primarily show decreased vergence facility and convergence amplitude. The most common type of binocular vision anomaly in middle-aged and older adults is convergence insufficiency, which is often associated with a low AC/A ratio. However, other studies have shown that accommodation may have a more significant impact on binocular vision than convergence. Studies [10, 52] show that the response AC/A ratio increases with age, while the CA/C ratio decreases. The AC/A ratio correlates positively with age [2, 16, 50], with the most significant changes occurring around presbyopia onset (40–50 years). As presbyopia approaches, the response AC/A ratio approximately triples. These age-related changes result from a decline in accommodation amplitude, while convergence remains stable.

The AC/A ratio is linked to accommodation effort. Factors such as monocular paralysis, age-related decline in ciliary muscle function, and accommodative lag due to refractive errors indirectly affect the AC/A ratio [50]. Neveu et al.’s research [53] shows that age-dependent changes in CA/C are mainly due to increased tonic components and reduced effective convergence time and demand.

Li et al. [51] showed that stimulant AC/A measurements increase with age due to the decline in accommodative function of the crystalline lens. This decline reduces peripheral accommodation, increasing central nervous system accommodative effort and thus raising AC and stimulant AC/A. However, Mutti et al. [50] found no significant correlation between stimulated AC/A and CA/C ratios, indicating limited interrelation in routine clinical testing. Analysis of stimulated AC/A and stimulated CA/C ratio data revealed no significant negative correlation between the two, suggesting that there may be no substantial interrelation between these two sets of results in routine clinical testing.

Eye movement and age

Leat et al. [9] conducted a retrospective study on elderly individuals and found a significant correlation between age and binocular vision or eye movement disorders. Chavant et al. [54] reported that with increasing age, the latency of eye movements increases while average speed and convergence amplitude decrease. Seferlis et al. [55] tested smooth pursuit, saccade, and optokinetic eye movements in healthy presbyopic subjects, showing declines in speed and accuracy.

The negative correlation between eye movement speed and amplitude with age is likely due to progressive degenerations in the central nervous system. Factors such as reduced cerebral blood flow, neural atrophy, and metabolic decline are linked to aging and can impair neural information transmission to eye muscles, reducing speed, amplitude, and precision. Additionally, ocular structure and extraocular muscle degeneration may cause conditions like vertical strabismus and cyclodeviation. Age-related effects on near exophoria, NPC, and accommodative convergence further contribute to abnormal eye movements, especially in convergence [1, 9].

Risk factors for NSBVAs

The risk factors for NSBVAs have not been systematically studied, but multiple factors acting alone or in combination are known to contribute. Identifying these factors would aid in prevention and management. Prolonged use of electronic devices like computers, tablets, and smartphones is a common cause, affecting parameters such as heterophoria, NPC, near point of accommodation (NPA), fusional vergence, and binocular accommodative facility (BAF). Convergence excess occurs more frequently than accommodative excess [56–60]. Rueff's study [61] showed that contact lenses may induce binocular vision issues in dry eye patients. Other associated factors include caffeine intake [62], diabetes [63], and racial differences [64].

Clinical diagnosis of NSBVAs in middle-aged and older adults

The diagnosis of NSBVAs primarily relies on symptoms and clinical signs. Accurate identification of symptom types is essential for developing targeted treatment strategies to enhance visual function and improve quality of life. Common symptoms include blurred vision, diplopia, eye strain, headaches, reading difficulties, and comprehension issues [65–68]. These may worsen after prolonged near work or brief periods of reading [4, 69]. Binocular diplopia, a prevalent symptom, is not age-correlated in some studies but peaks at age 70 due to microvascular stroke or tumors [70]. Strabismus decompensation in the elderly involves increased near exophoria, reduced convergence fusion range, and anatomical degeneration, leading to vertical, horizontal, or cyclostrabismus and binocular diplopia [1, 71]. Symptoms of presbyopia overlap with those of NSBVAs, making clinical differentiation challenging [33].

The Convergence Insufficiency Symptom Survey (CISS) questionnaire measures treatment efficacy for symptomatic convergence insufficiency (CI). It uses cut-off scores of 16 for children [72] and 21 for adults [73] to differentiate CI from normal binocular vision. A score of 15 or higher is recommended for identifying CI-related symptoms in presbyopic individuals [74]. However, its diagnostic accuracy for screening CI varies across studies. For example, Horan et al. [75] found the CISS lacks specificity in adolescents, while Horwood et al. [76] suggested it is unsuitable for screening young adults. Horan et al. [75] noted that the CISS was not designed specifically for assessing CI and has been used in studies [77, 78] to identify symptoms of various binocular vision problems. Discrepancies exist between CISS scores and clinical findings in CI patients [79–81], as symptom severity does not always correlate significantly with scores [82]. Thus, relying solely on the CISS for diagnosing or evaluating CI outcomes may not be advisable. Menjivar et al. [46] emphasized that the NPC break is most effective for identifying CI in children, suggesting the need to revise the CISS by incorporating specific clinical tests.

Most diagnostic indicators for NSBVAs come from Scheiman and Wick [18]. Their validity and accuracy have been confirmed by clinical and scientific research. Researchers have also explored binocular vision function using Sheard's and Percival's criteria, refining Scheiman & Wick's diagnostic criteria further. These criteria are essential for evaluating and diagnosing such anomalies [11, 49, 83].

While this systematic review provides valuable insights into NSBVAs in aging populations, several important limitations must be acknowledged. The current evidence base remains limited by heterogeneous study designs and a predominant focus on specific measurements (phoria and vergence function) while excluding accommodative aspects relevant to presbyopia. Most critically, existing studies demonstrate significant gaps in comprehensive binocular vision assessment for middle-aged and older population, particularly regarding standardized diagnostic criteria, age-stratified prevalence data, and longitudinal functional changes. These limitations highlight the need for future multicenter studies employing unified protocols, diverse population sampling, complete binocular vision test batteries, and longitudinal designs to better characterize NSBVAs in middle-aged and older individuals aged ≥ 40 years.

Conclusions

Previous studies have demonstrated significant age-related trends in various binocular vision parameters, including heterophoria, near point of convergence, accommodative convergence to accommodation (AC/A) ratio, convergence accommodation to convergence (CA/C) ratio, fusional vergence ranges, vergence facility, and eye movement. Non-strabismic binocular vision anomalies are prevalent among middle-aged and older adults. However, current studies have significant limitations in the field of binocular vision anomalies in middle-aged and older adults. On the one hand, most existing literatures focus on the statistical analysis of abnormal incidence rates of individual binocular vision examination indices, but lack prevalence assessment based on comprehensive diagnostic criteria. Such fragmented research perspectives make it difficult to truly outline the epidemiological panorama of binocular vision abnormalities in this population, leading to cognitive biases in understanding the actual disease burden. On the other hand, the unique visual physiological characteristics of the elderly have been confirmed to differ essentially from those of children and young adults, while current diagnostic criteria still follow the evaluation system applicable to non-presbyopic populations. This mismatch in standard applicability is highly likely to cause misjudgments of prevalence rates. Therefore, establishing a normal reference range and diagnostic criteria system for binocular vision parameters specifically tailored to middle-aged and older adults has become an urgent priority. This breakthrough can not only accurately anchor the real prevalence status of binocular vision anomalies in this group, providing scientific and quantitative basis for clinical diagnosis and treatment, but also fundamentally enhance the quality of visual function and life happiness of the elderly by promoting the establishment of personalized visual health management systems. It will open up a new research paradigm for standardization in the field of geriatric eye health.

Electronic supplementary material

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Abbreviations

NSBVAs

Non-strabismic binocular vision anomalies

PRISMA

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

CASP

Critical Appraisal Skills Programme

FVD

Fusional vergence dysfunction

NPC

Near point of convergence

AC/A

Accommodative convergence to accommodation ratio

CA/C

Convergence accommodation to convergence ratio

NFV

Negative fusional vergence

PFV

Positive fusional vergence

BO

Base-out prism

BI

Base-in prism

VF

Vergence facility

NPA

Near point of accommodation

CISS

Convergence Insufficiency Symptom Survey

CI

Convergence insufficiency

Author contributions

Conceptualization: QQT; Data Curation: CYL, CKL; Formal Analysis: CYL, CKL, QQT; Investigation: CYL, CKL; Methodology: CYL, CKL, WZ, JHT, WF, HR, BWZ; Project Administration: QQT; Writing– Original Draft: CYL, CKL, QQT; Writing– Review & Editing: CYL, CKL, QQT, WZ, JHT, WF, HR, BWZ.

Funding

This work was supported by the China Eye Valley Visual Function Diagnosis and Treatment Technology and Transformation Application Laboratory "Visual Function Medical Innovation Award" Project (20220417, QQT), and the Doctoral Start-up Fund of North Sichuan Medical College (CBY20-QD05, QQT).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.