Abstract
Background
Despite extensive global research on convergence insufficiency (CI), school-based epidemiological data in Taiwan remain scarce. This study provides the first school-based estimate of CI prevalence and diagnostic characteristics among elementary students in Kaohsiung, Taiwan.
Methods
A cross-sectional school-based screening was conducted from September 2024 to January 2025 among students in grades 3–6 from public elementary schools across four administrative districts of Kaohsiung City—Gushan, Lingya, Sanmin, and Fengshan. Among the 656 initially enrolled children, 219 were included in the binocular vision assessment after excluding those with CISS scores < 16 or substandard visual acuity (n = 169). All underwent CISS evaluation and standardized binocular testing based on the Convergence Insufficiency Treatment Trial (CITT) framework. According to diagnostic criteria, CI was categorized as Low-CI (one sign), High-CI (two signs), or Definite-CI (three signs).
Results
Among 219 children, the overall prevalence of CI (≥ 1 diagnostic sign) was 35.2%, including 17.9% with Low-CI, 14.2% with High-CI, and 3.1% with Definite-CI. Girls more frequently exhibited a single CI sign—most often deficient positive fusional vergence (PFV < 15 Δ) or failing Sheard’s criterion—whereas boys showed more multiple signs. No significant grade-level differences were observed. Mean CISS scores were similar across subgroups (P = 0.985).
Conclusion
This study provides the first school-based baseline data on convergence insufficiency (CI) among 9–12-year-old children in southern Taiwan, revealing that approximately one-third exhibited at least one CI sign with notable gender-related differences and supporting the integration of binocular vision screening into school-based vision health programs for early detection and intervention.
Keywords: Convergence insufficiency, Schoolchildren, Binocular vision, Prevalence, Convergence Insufficiency Symptom Survey (CISS), Convergence Insufficiency Treatment Trial (CITT), Kaohsiung, Taiwan
Background
Convergence insufficiency (CI) is a binocular vision dysfunction characterized by reduced ability to sustain eye convergence during prolonged near viewing. Children with CI may complain of eyestrain, headache, blurred or double vision, and poor reading endurance, which can interfere with academic performance and daily comfort [1–3]. These complaints are particularly concerning in modern educational contexts, where prolonged near activities, such as textbook reading and digital device use, are integral to daily learning and may negatively affect academic performance [4–6].
The reported prevalence of CI varies substantially across studies, reflecting differences in diagnostic criteria, methodology, and population demographics. Previous investigations have documented prevalence estimates ranging from approximately 2% (95% confidence interval [CI]: 1.4–2.6%; n = 2,262) to more than 30%, depending on diagnostic criteria and study population [7–9]. School-based studies have reported CI prevalence between 4 and 10% in U.S. children (n ≈ 500) [10, 11], 6–7% in Portuguese samples (n = 586) [12], and about 5% in Spain (n = 1,006; 95% CI: 4.1–6.5%) using standardized diagnostic criteria [13]. In Asia, Meng et al. observed that CI-type intermittent exotropia was more common than the basic type among Chinese children with myopia, underscoring the broader clinical significance of CI in high-myopia populations [14].
CI frequently coexists with accommodative insufficiency (AI), which exacerbates visual fatigue and reduces near-task efficiency in students. Children presenting both conditions report greater reading difficulty and symptom severity than those with CI alone [12, 15, 16]. This comorbidity exacerbates the functional burden on students, as it compounds difficulties in sustaining clear and comfortable near vision.
Clinical trials, such as the CITT, have confirmed that structured office-based vergence and accommodative therapy is more effective than home-based or placebo exercises in alleviating symptoms and improving CI signs [17]. These findings established vision therapy as the most effective intervention for symptomatic CI. Nevertheless, most school vision screenings worldwide continue to emphasize monocular visual acuity while neglecting binocular vision anomalies, meaning that many children with CI remain undiagnosed [18, 19].
Although multiple epidemiological studies of CI have been reported in the United States and Europe, relatively few have been conducted in Asian populations. Importantly, no school-based epidemiological studies have been conducted in southern Taiwan. Given Taiwan’s strong emphasis on early education and extensive screen use, identifying visual stressors such as CI has implications for national school health policy. This represents a critical gap, given Kaohsiung City’s distinctive educational environment marked by heavy academic workloads, early and intensive use of digital devices, and a high prevalence of myopia. These factors are likely to influence binocular vision status and could contribute to an increased risk of CI among school-aged children in this region.
The present study conducted a school-based investigation of CI among Kaohsiung elementary students (grades 3–6) across multiple districts with the following objectives: (1) Estimate the prevalence of CI using CITT-based diagnostic standards. (2) Evaluate symptom severity using the Convergence Insufficiency Symptom Survey (CISS). (3) Compare the findings with international data to inform future school vision-health initiatives. This research establishes baseline data on CI prevalence and symptoms to support the inclusion of binocular vision screening in school health programs, enabling earlier detection and reducing the academic and visual burdens associated with CI. These findings provide a reference for developing region-specific visual health policies and incorporating binocular vision screening into school health programs in southern Taiwan.
Methods
Study design and participants
This cross-sectional, school-based study was conducted from September 2024 to January 2025 among elementary schoolchildren in grades 3 to 6 in Kaohsiung City, Taiwan. A total of 656 students and their parents were recruited after obtaining written informed consent. Schools were selected using a convenience sampling approach in collaboration with the Kaohsiung City Department of Health, including four administrative districts (Gushan, Lingya, Sanmin, and Fengshan). The study protocol was reviewed and approved by the Institutional Review Board of Kaohsiung Municipal Kai-Syuan Psychiatric Hospital (KSPH-2024-20) and adhered to the tenets of the Declaration of Helsinki.
Sampling procedure and sample size calculation
The required sample size for prevalence estimation was calculated using the formula n = Z² × p × (1 − p) / d², where Z = 1.96* for a 95% confidence level, p = 0.30* (expected prevalence based on previous literature [7–13]), and d = 0.05* (precision). The minimum required sample size was n = 323. Considering an expected response rate of approximately 70%, at least 462 students were targeted.
Screening and inclusion procedure
All 656 participants first completed the Convergence Insufficiency Symptom Survey (CISS), a previously published and validated questionnaire, as a screening assessment [17]. Based on the questionnaire results, students with CISS scores ≥ 16 (N = 388) were invited for further visual assessments, whereas those scoring below 16 (N = 268) were excluded. Distance visual acuity was subsequently evaluated using a standardized logMAR chart. Only students with monocular visual acuity better than 0.1 logMAR in each eye and an interocular difference of no more than two lines were included in the binocular vision assessment (N = 219). The experimental procedure is illustrated in Fig. 1.
Fig. 1.
Flowchart of participant recruitment and examination procedures in the school-based convergence insufficiency screening program
Baseline evaluation of visual function
Exclusion criteria
Participants were excluded from further binocular vision assessment if they met any of the following criteria: (1) a Convergence Insufficiency Symptom Survey (CISS) score < 16; (2) substandard distance visual acuity, defined as monocular visual acuity worse than 0.1 logMAR in either eye or an interocular difference greater than two lines; or (3) the presence of manifest strabismus detected during preliminary ocular alignment screening.
Ocular alignment was assessed using the cover test at distance (6 m) and near (40 cm) to ensure that all participants met the inclusion criteria. All eligible participants (N = 219) underwent a comprehensive baseline assessment of binocular vision function to identify convergence insufficiency (CI) and related anomalies. All tests were administered in a standardized sequence by the same certified optometrist to ensure measurement consistency.
The examination protocol included the following components: (1) Habitual visual acuity (HVA) in each eye, recorded in logMAR units under standardized illumination. (2) Ocular alignment was evaluated by the Von Graefe method to determine distance (6 m) and near (40 cm) lateral phoria. (3) Near point of convergence (NPC) was measured using a ruler with a fixation target, and both break and recovery points were recorded. (4) Fusional vergence amplitudes were measured at near using automatic dual rotational Risley prisms (ADRRPs; OrthoV Co., Ltd., Taiwan), including both negative fusional vergence (NFV) and positive fusional vergence (PFV), with break and recovery points recorded. Negative fusional vergence (NFV) at near was also measured to document overall vergence ability; however, only positive fusional vergence (PFV) was applied as a diagnostic sign for CI. All assessments followed the diagnostic framework established by the Convergence Insufficiency Treatment Trial (CITT) to ensure methodological consistency with previous clinical studies.
Based on these measurements, convergence insufficiency (CI) was defined by the presence of three diagnostic signs: Sign A, exophoria at near at least 4 Δ greater than at distance; Sign B, near point of convergence (NPC) break point exceeding 6 cm; and Sign C, positive fusional vergence (PFV) break less than 15 Δ or failure to meet Sheard’s criterion. Participants were further categorized according to the number of signs present: (1) Low-CI: presence of one diagnostic sign; (2) High-CI: presence of two diagnostic signs; (3) Definite-CI: presence of all three diagnostic signs; and (4) Normal binocular vision (NBV): children without any of these signs.
Data analysis
All assessment data were stratified by gender and grade (middle and senior elementary grades). The continuous variables were presented with mean and standard deviation (SD), while the categorical variables were described with frequency and percentage. Normality of continuous variables was assessed using the Shapiro–Wilk test before performing two-way ANOVA. Two-way analysis of variance (ANOVA) was used to examine the main effect of gender and grade for continuous assessments. Fisher’s exact test was applied to categorical comparisons. Boxplots were used to compare CISS score distributions among groups classified by the number of CI signs. The level of significance was set at 0.05. For multiple pairwise comparisons, Bonferroni correction was applied to control for type I error inflation. All statistical analyses were performed using SPSS Version 19.0 for Windows (IBM Corp., Armonk, NY).
Results
Baseline characteristics
As illustrated in Fig. 1, a total of 656 elementary schoolchildren (grades 3–6) were initially enrolled in the school-based screening. Of the 656 initially enrolled children, 268 (40.9%) with CISS scores < 16 and 169 (25.8%) with substandard visual acuity were excluded, leaving 219 (33.4%) who met the inclusion criteria for binocular-vision assessment. Their baseline characteristics were analyzed according to gender (male and female) and grade level (middle-grade and upper-grade groups), as summarized in Table 1.
Table 1.
Baseline characteristics of school-aged participants included in the binocular-vision assessment, stratified by gender and grade level (N = 219)
| Grade level | Total | Middle grade | Upper grade | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Gender | Male | Female | Total | Male | Female | Total | Male | Female | Total | |||||||||
| Mean±SD | Mean±SD | Mean±SD | Mean±SD | Mean±SD | Mean±SD | Mean±SD | Mean±SD | Mean±SD | ||||||||||
| Age (years) | 11.7±0.9 | 11.6±0.9 | 11.6±0.9 | 10.9±0.6 | 10.8±0.5 | 10.9±0.6 | 12.2±0.7 | 12.2±0.7 | 12.2±0.7 | |||||||||
| Habitual visual acuity, HVA (LogMAR)) | 0.09±0.15 | 0.07±0.16 | 0.08±0.15 | 0.09±0.12 | 0.08±0.14 | 0.08±0.13 | 0.09±0.16 | 0.07±0.17 | 0.08±0.17 | |||||||||
| Heterophoria (Δ) | ||||||||||||||||||
| Distance Lateral phoria (DLP) | -0.9±4.8 | -0.8±5.1 | -0.8±5.0 | -0.9±5.3 | -0.9±5.1 | -0.9±5.1 | -0.9±4.5 | -0.7±5.2 | -0.8±4.9 | |||||||||
| Near Lateral phoria (NLP) | -1.8±6.9 | -0.8±6.7 | -1.4±6.8 | -0.9±6.3 | -0.7±6.0 | -0.8±6.1 | -2.5±7.3 | -0.9±7.2 | -1.8±7.3 | |||||||||
| Near Point of Convergence (cm) | ||||||||||||||||||
| Break | 7.8±4.4 | 6.8±4.2 | 7.4±4.3 | 7.5±4.3 | 6.4±4.3 | 7.0±4.3 | 8.1±4.5 | 7.1±4.2 | 7.6±4.4 | |||||||||
| Recovery | 9.5±4.8 | 8.3±4.4 | 9.0±4.7 | 9.4±4.7 | 8.2±4.5 | 8.9±4.6 | 9.6±4.9 | 8.4±4.4 | 9.1±4.7 | |||||||||
| Fusional vergence (Δ) | ||||||||||||||||||
| Negative Fusional Vergence, NFV | ||||||||||||||||||
| Break | 17.7±5.8 | 18.7±5.3 | 18.2±5.6 | 18.0±6.0 | 18.6±5.6 | 18.3±5.8 | 17.5±5.8 | 18.7±5.2 | 18.1±5.5 | |||||||||
| Recovery | 7.6±6.6 | 8.2±5.5 | 7.9±6.1 | 7.2±6.3 | 7.5±5.0 | 7.3±5.7 | 7.9±6.9 | 8.7±5.7 | 8.3±6.3 | |||||||||
| Positive Fusional Vergence, PFV | ||||||||||||||||||
| Break | 27.5±9.7 | 28.8±8.3 | 28.1±9.1 | 26.6±9.3 | 28.5±7.8 | 27.4±8.6 | 28.2±10.0 | 29.0±8.7 | 28.6±9.3 | |||||||||
| Recovery | 16.2±10.6 | 17.3±9.9 | 16.7±10.3 | 14.9±9.8 | 17.7±10.0 | 16.1±10.0 | 17.2±11.2 | 17.1±9.8 | 17.1±10.5 | |||||||||
| CISS | 23.3±6.3 | 24.7±8.1 | 23.9±7.2 | 23.2±6.2 | 24.6±8.0 | 23.8±7.1 | 23.3±6.4 | 24.7±8.3 | 23.9±7.3 | |||||||||
The results of the two-way analysis of variance (2-way ANOVA) showed no significant effects of gender or grade level on any of the measurements, and no interaction was observed between the two factors
The overall mean age was 11.6 ± 0.9 years, with participants in the middle-grade group averaging 10.9 ± 0.6 years and those in the upper-grade group averaging 12.2 ± 0.7 years. Habitual visual acuity (HVA), recorded in LogMAR units, was 0.08 ± 0.15 for the entire sample, with no clinically meaningful difference across subgroups. For ocular alignment, distance lateral phoria (DLP) had a mean value of − 0.9 ± 5.0 prism diopters (Δ), and near lateral phoria (NLP) was − 1.4 ± 6.8 Δ, indicating a slight exophoric tendency at both distances. The mean near point of convergence (NPC) break point was 7.4 ± 4.3 cm, and the recovery point was 9.0 ± 4.7 cm. In terms of binocular fusion ability, negative fusional vergence (NFV) at near showed a break value of 18.3 ± 5.8 Δ and a recovery value of 7.5 ± 6.1 Δ. Positive fusional vergence (PFV) at near was higher, with break and recovery values of 28.1 ± 9.1 Δ and 17.3 ± 10.9 Δ, respectively. Symptoms of visual discomfort were assessed using the Convergence Insufficiency Symptom Survey (CISS), with a mean score of 23.9 ± 7.5 across all participants.
The assumptions of normality and homogeneity of variance were verified before performing two-way ANOVA. Statistical analysis using two-way ANOVA revealed no significant main effects of gender or grade level on any of the measured visual parameters. Specifically, the main effects of gender (F₁,₂₁₅ = 0.87, p = 0.35) and grade (F₁,₂₁₅ = 1.02, p = 0.31) were nonsignificant, and there were no significant interaction effects between gender and grade level (all p > 0.05), indicating that the distribution of visual function measures was consistent across subgroups.
Gender differences in binocular vision subgroups and CI diagnostic signs
Table 2 presents the distribution of binocular vision subgroups and diagnostic signs of convergence insufficiency (CI) by gender, with results further stratified by grade level. A total of 219 school-aged participants (116 boys and 103 girls) were included in the binocular vision assessment and further categorized into middle-grade (N = 91) and upper-grade (N = 128) groups. The distribution of binocular vision subgroups and the number of convergence insufficiency (CI) signs were analyzed by gender using Fisher’s exact test.
Table 2.
Distribution of binocular vision subgroups and diagnostic signs of convergence insufficiency (CI) by gender, stratified by grade level
| Total | Middle grade | Upper grade | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Male (N = 116) | Female (N = 103) | Total (N = 219) | Male (N = 50) | Female (N = 41) | Total (N = 91) | Male (N = 66) | Female (N = 62) | Total (N = 128) | |||||||||||||
| N | % | N | % | N | % | P | N | % | N | % | N | % | P | N | % | N | % | N | % | P | |
| Diagnostic signs | 0.007 | 0.034 | 0.194 | ||||||||||||||||||
| NBV | 26 | 22.4 | 22 | 21.4 | 48 | 21.9 | 14 | 28.0 | 10 | 24.4 | 24 | 26.4 | 12 | 18.2 | 12 | 19.4 | 24 | 18.8 | |||
| Only A | 2 | 1.7 | 5 | 4.9 | 7 | 3.2 | 0 | 0.0 | 4 | 9.8 | 4 | 4.4 | 2 | 3.0 | 1 | 1.6 | 3 | 2.3 | |||
| Only B | 18 | 15.5 | 10 | 9.7 | 28 | 12.8 | 7 | 14.0 | 4 | 9.8 | 11 | 12.1 | 11 | 16.7 | 6 | 9.7 | 17 | 13.3 | |||
| Only C | 17 | 14.7 | 35 | 34.0 | 52 | 23.7 | 6 | 12.0 | 13 | 31.7 | 19 | 20.9 | 11 | 16.7 | 22 | 35.5 | 33 | 25.8 | |||
| Only A + B | 1 | 0.9 | 0 | 0.0 | 1 | 0.5 | 1 | 2.0 | 0 | 0.0 | 1 | 1.1 | 0 | 0.0 | 0 | 0.0 | 0 | 0.0 | |||
| Only A + C | 8 | 6.9 | 2 | 1.9 | 10 | 4.6 | 4 | 8.0 | 1 | 2.4 | 5 | 5.5 | 4 | 6.1 | 1 | 1.6 | 5 | 3.9 | |||
| Only B + C | 33 | 28.4 | 25 | 24.3 | 58 | 26.5 | 15 | 30.0 | 9 | 22.0 | 24 | 26.4 | 18 | 27.3 | 16 | 25.8 | 34 | 26.6 | |||
| A + B + C | 11 | 9.5 | 4 | 3.9 | 15 | 6.8 | 3 | 6.0 | 0 | 0.0 | 3 | 3.3 | 8 | 12.1 | 4 | 6.5 | 12 | 9.4 | |||
| Number of signs | 0.047 | 0.042 | 0.526 | ||||||||||||||||||
| NBV (zero sign) | 26 | 22.4 | 22 | 21.4 | 48 | 21.9 | 14 | 28.0 | 10 | 24.4 | 24 | 26.4 | 12 | 18.2 | 12 | 19.4 | 24 | 18.8 | |||
| Low-CI (1 sign) | 37 | 31.9 | 50 | 48.5 | 87 | 39.7 | 13 | 26.0 | 21 | 51.2 | 34 | 37.4 | 24 | 36.4 | 29 | 46.8 | 53 | 41.4 | |||
| High-CI (2 signs) | 42 | 36.2 | 27 | 26.2 | 69 | 31.5 | 20 | 40.0 | 10 | 24.4 | 30 | 33.0 | 22 | 33.3 | 17 | 27.4 | 39 | 30.5 | |||
| Definite-CI (3signs) | 11 | 9.5 | 4 | 3.9 | 15 | 6.8 | 3 | 6.0 | 0 | 0.0 | 3 | 3.3 | 8 | 12.1 | 4 | 6.5 | 12 | 9.4 | |||
P-value by Fisher’s exact test to compare the distribution of the number of signs between genders
NBV: normal binocular vision; Diagnostic signs of convergence insufficiency (CI): Sign A, exophoria at near ≥ 4Δ greater than at distance; Sign B, near point of convergence (NPC) break > 6 cm; Sign C, positive fusional vergence (PFV) break < 15Δ or failure to meet Sheard’s criterion
In the overall sample, significant gender differences were observed in both the distribution of CI subgroups (P = 0.007) and the number of CI signs (P = 0.047). Girls were more likely to exhibit a single diagnostic sign (Low-CI, 48.5%), most commonly Only C (deficient positive fusional vergence, PFV < 15 Δ or failure to meet Sheard’s criterion). In contrast, boys showed higher proportions of multiple diagnostic signs, including High-CI (2 signs) and Definite-CI (all three signs). The most frequent sign combination among both genders was B + C (NPC > 6 cm and PFV < 15 Δ).
When stratified by grade level, these gender-related differences were mainly evident among middle-grade students (P = 0.034 for subgroup distribution; P = 0.042 for number of signs). In this group, girls predominantly presented with a single diagnostic sign—particularly Only C—whereas boys showed higher proportions of High-CI and Definite-CI. In contrast, no significant gender difference was observed among upper-grade students (P = 0.194 for subgroup distribution; P = 0.526 for number of signs).
Overall, these findings indicate that convergence insufficiency patterns differ by both gender and grade level. Girls in the middle grades were more likely to present with mild CI features (Low-CI, mainly Only C), whereas boys tended to exhibit more pronounced forms (High-CI and Definite-CI).
Grade-level differences in binocular vision subgroups and CI diagnostic signs
Table 3 presents the distribution of binocular vision subgroups and diagnostic signs of convergence insufficiency (CI) by grade level for the total sample and stratified by gender. A total of 219 school-aged participants were analyzed and categorized into middle-grade (N = 91) and upper-grade (N = 128) groups to compare the distribution of binocular vision subgroups and the number of CI diagnostic signs. Fisher’s exact test was used to assess grade-related differences in the overall sample and within each gender group.
Table 3.
Distribution of binocular vision subgroups and diagnostic signs of convergence insufficiency (CI) by grade level, stratified by gender
| Total | Male | Female | |||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Middle grade | Upper grade | Total | Middle grade | Upper grade | Total | Middle grade | Upper grade | Total | |||||||||||||
| N | % | N | % | N | % | P | N | % | N | % | N | % | P | N | % | N | % | N | % | P | |
| Total | 91 | 128 | 219 | 50 | 66 | 116 | 41 | 62 | 103 | ||||||||||||
| Diagnostic signs | 0.390 | 0.621 | 0.369 | ||||||||||||||||||
| NBV | 24 | 26.4 | 24 | 18.8 | 48 | 21.9 | 14 | 28.0 | 12 | 18.2 | 26 | 22.4 | 10 | 24.4 | 12 | 19.4 | 22 | 21.4 | |||
| Only A | 4 | 4.4 | 3 | 2.3 | 7 | 3.2 | 0 | 0.0 | 2 | 3.0 | 2 | 1.7 | 4 | 9.8 | 1 | 1.6 | 5 | 4.9 | |||
| Only B | 11 | 12.1 | 17 | 13.3 | 28 | 12.8 | 7 | 14.0 | 11 | 16.7 | 18 | 15.5 | 4 | 9.8 | 6 | 9.7 | 10 | 9.7 | |||
| Only C | 19 | 20.9 | 33 | 25.8 | 52 | 23.7 | 6 | 12.0 | 11 | 16.7 | 17 | 14.7 | 13 | 31.7 | 22 | 35.5 | 35 | 34.0 | |||
| Only A + B | 1 | 1.1 | 0 | 0.0 | 1 | 0.5 | 1 | 2.0 | 0 | 0.0 | 1 | 0.9 | 0.0 | 0.0 | 0.0 | ||||||
| Only A + C | 5 | 5.5 | 5 | 3.9 | 10 | 4.6 | 4 | 8.0 | 4 | 6.1 | 8 | 6.9 | 1 | 2.4 | 1 | 1.6 | 2 | 1.9 | |||
| Only B + C | 24 | 26.4 | 34 | 26.6 | 58 | 26.5 | 15 | 30.0 | 18 | 27.3 | 33 | 28.4 | 9 | 22.0 | 16 | 25.8 | 25 | 24.3 | |||
| A + B + C | 3 | 3.3 | 12 | 9.4 | 15 | 6.8 | 3 | 6.0 | 8 | 12.1 | 11 | 9.5 | 0 | 0.0 | 4 | 6.5 | 4 | 3.9 | |||
| Number of signs | 0.204 | 0.312 | 0.416 | ||||||||||||||||||
| NBV (zero sign) | 24 | 26.4 | 24 | 18.8 | 48 | 21.9 | 14 | 28.0 | 12 | 18.2 | 26 | 22.4 | 10 | 24.4 | 12 | 19.4 | 22 | 21.4 | |||
| Low-CI (1 sign) | 34 | 37.4 | 53 | 41.4 | 87 | 39.7 | 13 | 26.0 | 24 | 36.4 | 37 | 31.9 | 21 | 51.2 | 29 | 46.8 | 50 | 48.5 | |||
| High-CI (2 signs) | 30 | 33.0 | 39 | 30.5 | 69 | 31.5 | 20 | 40.0 | 22 | 33.3 | 42 | 36.2 | 10 | 24.4 | 17 | 27.4 | 27 | 26.2 | |||
| Definite-CI (3signs) | 3 | 3.3 | 12 | 9.4 | 15 | 6.8 | 3 | 6.0 | 8 | 12.1 | 11 | 9.5 | 0 | 0.0 | 4 | 6.5 | 4 | 3.9 | |||
P-value by Fisher’s exact test to compare the distribution of the number of signs between grades
NBV: normal binocular vision; Diagnostic signs of convergence insufficiency (CI): Sign A, exophoria at near ≥ 4Δ greater than at distance; Sign B, near point of convergence (NPC) break > 6 cm; Sign C, positive fusional vergence (PFV) break < 15Δ or failure to meet Sheard’s criterion
For the overall population, no significant differences were found between middle- and upper-grade students in either the distribution of CI subgroups (P = 0.390) or the number of CI diagnostic signs (P = 0.204). The proportions of participants with normal binocular vision (NBV), Low-CI (one sign), High-CI (two signs), and Definite-CI (three signs) were 21.9%, 39.7%, 31.5%, and 6.8%, respectively. The most prevalent single sign across both grade levels was Only C (deficient positive fusional vergence, PFV < 15 Δ or failure to meet Sheard’s criterion), whereas the most common sign combination was B + C (NPC > 6 cm and PFV < 15 Δ).
When analyzed by gender, similar trends were observed. Among boys, no significant grade-related differences were detected (P = 0.621 for subgroup distribution; P = 0.312 for number of signs). Most boys presented with Low-CI (31.9%) or High-CI (36.2%), with B + C being the most frequent sign combination. Among girls, likewise, no significant differences were found between middle- and upper-grade groups (P = 0.369 for subgroup distribution; P = 0.416 for number of signs). The majority of girls exhibited Low-CI (48.5%), primarily represented by Only C, whereas the proportions of High-CI (26.2%) and Definite-CI (3.9%) were comparatively lower.
Overall, the distribution of binocular vision subgroups and the number of CI diagnostic signs did not differ significantly between middle- and upper-grade students, regardless of gender. These findings suggest that the prevalence and pattern of convergence insufficiency characteristics remain relatively consistent across grade levels in this age range.
Comparison of CISS scores among binocular vision subgroups with different numbers of CI diagnostic signs
Figure 2 illustrates the distribution of Convergence Insufficiency Symptom Survey (CISS) scores among the four binocular vision subgroups: normal binocular vision (NBV), Low-CI (one sign), High-CI (two signs), and Definite-CI (three signs). The respective sample sizes were 48, 87, 69, and 15 participants. The mean CISS scores were 23.04 ± 7.04 for NBV, 23.48 ± 7.86 for Low-CI, 23.67 ± 6.43 for High-CI, and 23.47 ± 6.38 for Definite-CI.
Fig. 2.
Distribution of Convergence Insufficiency Symptom Survey (CISS) scores among binocular-vision subgroups: NBV (n = 48), Low-CI (n = 87), High-CI (n = 69), and Definite-CI (n = 15). Boxes = interquartile ranges; lines = medians; whiskers = 1.5 × IQR. Individual data points represent each participant’s measurement within the corresponding subgroup, and the error bars denote ± 1 standard deviation (SD) from the mean
Boxplot analysis revealed substantial overlap of interquartile ranges among the four groups, indicating no clear trend of increasing symptom severity with the number of CI diagnostic signs. A one-way analysis of variance (ANOVA) confirmed that the mean CISS scores did not significantly differ among the subgroups (F(3,215) = 0.05, P = 0.985). These results suggest that symptom severity, as measured by the CISS, was not significantly associated with the number of clinical CI signs. In summary, approximately one-third (35.2%) of students exhibited at least one CI diagnostic sign, with notable gender-related differences but no significant grade-level effect.
Discussion
This study investigated the prevalence and diagnostic characteristics of CI in school-aged children in Kaohsiung, Taiwan, using standardized CITT-based criteria. The overall prevalence of CI (≥ 1 diagnostic sign) was 35.2%, indicating that more than one-third of children demonstrated at least one binocular vision anomaly relevant to school performance. The prevalence observed in the current study falls within the higher range of previously reported values, which vary widely from 2% to over 30% depending on diagnostic criteria, population, and methodology [1–3, 7, 13, 16].
In Spain, Sánchez-Cuadrado et al. reported that 5.3% of schoolchildren aged 6–14 years met al.l three CI criteria, whereas 12.2% exhibited two signs and 23.8% displayed one sign [13]. Similarly, Nunes et al. found a prevalence of 6.8% for definite CI and 15.3% for probable CI among Portuguese children aged 10–14 years [12]. In contrast, Rouse et al. and Menjivar et al., using the same CITT criteria, reported lower rates of 4–6% among U.S. fifth and sixth graders [10, 11, 18]. The notably higher proportion of Taiwanese children with two or more CI signs (17.3%) in the present study suggests that local educational and lifestyle factors—particularly intensive near work, early academic exposure, and extensive digital device use—may contribute to increased vergence stress.
This regional difference supports the notion that visual demands and educational practices influence binocular function. Asian data similarly point to higher CI-related dysfunctions. Wajuihian and Hansraj observed vergence anomalies in 16–18% of South African students [8], while Hussaindeen et al. reported non-strabismic binocular anomalies in 16.5–17.6% of Indian children [16]. Meng et al. recently demonstrated that the convergence-insufficiency subtype accounted for the majority of intermittent exotropia cases in Chinese myopic adolescents, underscoring the link between excessive near-work demands, myopia, and convergence weakness [14]. Beyond school-age populations, research across different age groups and clinical settings also demonstrates the broader relevance of binocular vision anomalies. Abdi and Rydberg reported frequent asthenopia and binocular coordination problems among schoolchildren, associated with near-work demands [20]. Lara et al. observed a high prevalence of binocular dysfunctions in a clinical population, indicating that such anomalies are not limited to children and may persist across broader patient groups [21]. Among university students, Porcar and Martínez-Palomera identified substantial rates of binocular dysfunction, suggesting that vergence and accommodative problems remain functionally significant in young adulthood [22]. Taken together, these findings highlight the continuity of binocular vision disorders across the lifespan and reinforce the importance of CI-related assessments in both pediatric and non-pediatric populations.
The prevalence of CI in the present study may have been influenced by the exclusion of 169 children (25.8%) who did not meet the visual acuity criteria (worse than 0.1 logMAR or an interocular difference greater than two lines). As the binocular vision status of these excluded children was not assessed, the direction of this potential bias cannot be determined, and the true prevalence of CI may therefore have been either under- or overestimated.Conversely, because the analysis was restricted to children with adequate visual acuity, the findings more accurately represent CI occurrence among functionally normal, school-ready children—consistent with the methodological rigor adopted by Sánchez-Cuadrado et al. [13].
Differences in diagnostic cut-offs may also explain inter-study variation. The present study applied CITT-based diagnostic standards to ensure comparability with previous clinical investigations [17, 18, 23]. However, variations in examiner expertise, lighting conditions, target type, and measurement sequence (e.g., variations in testing order and examiner technique [23]) can affect vergence results and prevalence estimation.
Our gender-based analysis revealed behavioral and physiological differences influencing binocular performance. Gender-related differences were statistically significant only among middle-grade students, whereas no significant effect was observed among upper-grade groups. Palomo-Álvarez and Puell similarly found that girls often reported near-work fatigue despite fewer measurable anomalies, while boys exhibited stronger associations between clinical findings and performance deficits [6]. Grade-level comparison revealed no significant differences, consistent with reports by Rouse et al. [10] and Menjivar et al. [18], suggesting that CI prevalence stabilizes by mid-childhood once visual demands and reading skills are established.
Interestingly, the CISS scores did not significantly differ among subgroups. This lack of association between subjective symptoms and objective findings echoes the results of Borsting et al. [2, 11] and Nunes et al. [12], who observed that some children with multiple CI signs reported minimal visual discomfort. This weak correlation may reflect children’s limited symptom awareness or adaptive suppression of visual discomfort during near work, leading to under-reporting of symptoms. Hence, school screenings should not rely solely on symptom questionnaires but should include objective binocular vision testing (NPC, PFV, phoria) for accurate detection [18, 19, 24].
The finding that over one-third of schoolchildren exhibited at least one CI sign highlights an unmet need for binocular vision screening in schools in Kaohsiung and other regions of Taiwan. Current national screening programs emphasize monocular visual acuity and refractive status while overlooking binocular coordination problems. Practical screening measures, such as near-point-of-convergence (NPC) ruler testing or simple fusional-range checks conducted by trained school nurses, could be integrated into existing vision-screening programs. Integrating simple tests such as NPC and PFV into existing programs could enable early identification and referral for vision therapy, as supported by clinical trial evidence showing that structured vergence/accommodative training effectively improves CI [17, 25]. Moreover, undiagnosed CI has been linked to reduced reading fluency, attention difficulties, and academic underperformance [4, 5], reinforcing the educational importance of functional vision care. Collectively, these findings reinforce the need to integrate binocular-vision screening and evidence-based vergence/accommodative therapy into national school health policies to address functional vision deficits early.
Limitations
This study has several limitations. First, the exclusion of participants with subnormal visual acuity may have resulted in a conservative estimation of CI prevalence, as children with minor refractive errors or borderline acuity were not assessed. This criterion may also have introduced a degree of selection bias toward functionally normal children. Moreover, because the school-based screening was performed under habitual viewing conditions rather than after full refractive correction, binocular vision measures could have been influenced by uncorrected refractive errors. Although visual acuity thresholds were implemented to reduce this effect, future studies incorporating refraction to maximum plus for best visual acuity (MPBVA) before binocular testing are recommended.
Second, because the study population was limited to schools within Kaohsiung City, the findings may not be generalizable to other regions of Taiwan with different educational environments or visual demands. Third, all binocular vision tests were administered by a designated optometrist following standardized procedures; however, potential observer effects cannot be entirely ruled out.
Finally, the cross-sectional design precludes conclusions regarding causal relationships between near-work exposure and the development of CI. Nevertheless, these limitations do not diminish the importance of the present findings, which provide the first population-based estimates of CI prevalence in southern Taiwan and highlight practical directions for integrating binocular-vision screening into school health programs.
Conclusion
This study provides the first school-based evidence on the prevalence and diagnostic characteristics of convergence insufficiency among 9–12-year-old children in Kaohsiung, southern Taiwan. Approximately one-third of the participants exhibited at least one sign of CI, with notable gender-related differences in diagnostic patterns. These findings establish essential baseline data for southern Taiwan and support the potential value of incorporating binocular vision screening into school-based vision health programs to facilitate earlier identification and intervention.
Acknowledgements
We thank president Chih-Wei Hsu and the members of the Kaohsiung city optometrists’ association for their assistance in conducting the binocular vision assessments.
Abbreviations
- ADRRPs
Automatic dual rotational Risley prisms
- AI
Accommodative insufficiency
- CI
Convergence insufficiency
- CISS
Convergence Insufficiency Symptom Survey
- CITT
Convergence Insufficiency Treatment Trial
- DLP
Distance lateral phoria
- HVA
Habitual visual acuity
- NBV
Normal binocular vision
- NFV
Negative fusional vergence
- NLP
Near lateral phoria
- NPC
Near point of convergence
- PFV
Positive fusional vergence
- VA
Visual acuity
Author contributions
SYH and JJH conceived and designed the study, supervised the research process, and drafted the manuscript. YCC contributed to data collection. SMY was responsible for participant recruitment and the administration of visual assessments. YWC, CHL, CTK, and CYP performed statistical analyses and assisted in data interpretation. MWH and CHL provided clinical supervision and methodological consultation. JJH critically reviewed and revised the manuscript. All authors read and approved the final manuscript.
Funding
This study was supported by the Department of Health, Kaohsiung City Government, Taiwan (Grant No. 11432863800).
Data availability
The datasets used during the current study are available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
This study was reviewed and approved by the Institutional Review Board of Kaohsiung Municipal Kai-Syuan Psychiatric Hospital (approval no. KSPH-2024-20). The research was conducted in accordance with the ethical standards of the Declaration of Helsinki. Written informed consent was obtained from the parents or legal guardians of all participating students before the commencement of data collection. All children voluntarily participated in the vision screening and were free to withdraw at any stage without consequence.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The datasets used during the current study are available from the corresponding author on reasonable request.