Prevalence of convergence insufficiency among secondary school students in Khartoum, Sudan.

BACKGROUND: Few studies have examined the prevalence of convergence insufficiency (CI) in a school-based population in Sudan. This study sought to determine the prevalence of CI and its related clinical characteristics among Sudanese secondary school students.
MATERIALS AND METHODS: A descriptive cross-sectional survey was conducted in Central Khartoum North, Sudan, in the academic year 2013/2014. A total of 4211 secondary school students, with a mean age of 15.5 ± 2.5 years, underwent complete eye examination, and were screened for symptoms associated with near work. Near and distance heterophoria was measured with the alternate cover test using a prism bar; near point of convergence (NPC) and positive fusional vergence (PFV) at near were determined.
RESULTS: Of the 4211 students screened, 329 (7.8%) were diagnosed with CI. Of these, 173 (52.6%) students were male and 156 (47.4%) were female; there was no significant relationship between sex and CI (P > 0.05). Standard schools had a higher prevalence of CI (43%) than geographic schools (36%) and there was a significant association between CI and the type of school (P < 0.05). In most of the students (78.42%), CI was due to both remote NPC and decreased PFV; in 20.36% of the students, CI was due to remote NPC only, and in very few students (1.22%), it was due to decreased PFV only.
CONCLUSIONS: These findings suggest that CI is prevalent in the secondary school population in Central Khartoum North, Sudan.

Introduction

Convergence insufficiency (CI),first described by Von Graefe [12] in 1855 and later elaborated by Duane,[3] is a common binocular vision disorder; it causes muscular asthenopia and ocular discomfort, and is therefore of considerable clinical significance. Evans defined CI as a condition involving an inability to obtain or maintain sufficient convergence for comfortable binocular vision at near distance.[4] It is characterized by exophoria that is greater at near than at distance, a remote near point of convergence (NPC), or decreased positive fusional vergence (PFV) at near.[2345678]

CI may have a significant negative effect on quality of life, reducing visual performance and comfort, and interfering with school learning.[8910111213141516] The academic demands of reading and writing tasks increase in higher grades, and thus the presence of CI becomes manifest when near visual tasks are carried out for extended periods. CI typically affects students in the secondary school grades, when students read smaller print and face increased reading demands.[17] Von Noorden and Campos stated that “CI seldom becomes a clinical problem until a patient reaches the teenage years. Increased schoolwork and prolonged periods of reading may then exacerbate the characteristic symptoms.”[17] These symptoms include visual discomfort, eye fatigue, headache, blurred vision, diplopia, lack of concentration, and words appearing to move or jump. Symptoms are aggravated before examinations when special demands are made on the near vision complex for extended periods, sleep and general health is reduced, and anxiety is increased.[8912181920] Vision therapy is used to improve binocular skills in students with symptomatic CI, to increase their ability to perform tasks more comfortably.[812]

CI prevalence varies across the globe. The prevalence among children and adults in the US ranges from 2.25% to 8.30%.[8121314212223] A British survey found that nearly 1 in 300 children suffered from CI,[24] while a study conducted in Spain found that nearly 1 in 100 (0.8%) symptomatic patients in an optometric clinic had CI.[25] A study from Romania showed that three in five (60.4%) adolescents complaining of blurred vision during near work suffered from CI.[26] The prevalence of CI in the Iranian population was 5.46%,[27] while it was 16.5% and 17.6% in Indian urban and rural arms, respectively.[28]

In Sudan, the secondary education consists of 3 years. Public secondary schools are of two types: standard schools, which accept students with high success rates in the final examinations of the primary education, and geographic schools, which accept students with any success rates and admit students according to their geographic distribution.

Although CI is common globally, research on CI prevalence in school-based populations in Sudan is lacking. This study was conducted to assess the prevalence of CI among Sudanese secondary school population in Central Khartoum North.

Materials and Methods

This cross-sectional facility-based study involved a school screening survey aimed at identifying students with CI and determining its prevalence, to obtain baseline data for the main clinical trial “Efficacy of Vision Therapy for Convergence Insufficiency in Secondary School Students.”[29] The study used a convenience sample of ten public secondary schools located in Central Khartoum North in Sudan, in the academic year 2013/2014. All procedures, field work, and data collection were performed from August 2013 until January 2014. A total of 4211 students matched the inclusion criteria and were enrolled in the study. The primary inclusion criteria were that they were in the first, second, or third grades in the selected secondary school at the time of the study and that they were willing to participate in the study, after written informed consent was given by their parents. Other eligibility criteria included being symptomatic, best vision, or best-corrected visual acuity of ≥6/9 in each eye at distance. Exclusion criteria included being asymptomatic, vision ≤6/9 in each eye at distance, uncorrected refractive errors, and a history of strabismus, hyperopia, or amblyopia. In addition, students with systemic diseases, such as Parkinson's disease, or those who used medications that might influence accommodation, vergence, and ocular motility, were excluded. Three main diagnostic criteria for CI were exophoria at near at least 4Δ greater than at far, receded NPC of ≥8 cm break, and insufficient PFV ≤15 base-out prism.

The data collectors were graduates of the Faculty of Optometry and Visual Sciences, who received training to complete the survey forms used in the study, which included demographic variables of the students as well as the history of ocular and systemic disease, trauma to the eye, eye surgery, and the use of ocular or systemic medication.

To diagnose CI, near and distance heterophoria were measured with the alternate cover test using a prism bar at 40 cm and 6 m, respectively. The target used in the cover test was a single character one line above the visual acuity on the Snellen charts. Then, the NPC was measured using the RAF binocular gauge (Haag-Streit, Harlow, UK), a rod calibrated in centimeters on which a card-holder slides backward or forward. In this holder, a card carrying a black vertical line is inserted. The proximal end of the rod is placed over the upper lip of the student while the eyes remain fixed on the vertical line. The student was asked to try to keep the vertical line single while it was slowly moved toward him/her, until the line appeared double, or the examiner objectively observed a loss of binocularity. At this point, the distance from the spectacle plane or the lateral canthus was read on the scale and recorded as NPC in centimeters.

The PFV was measured using a horizontal prism bar (Haag-Streit) at 40 cm (target: N5 text). The base-out prism bar was placed in front of the right eye using the step method at near (40 cm) distance. The prism bar was placed in front of the right eye while wearing best correction. The student was asked to keep the target single while the prism bar was moved in front of his/her eye and to report when they experienced sustained blur or diplopia. The amount of prism was increased until the student reported sustained diplopia. The break and recovery points were determined subjectively and objectively and recorded in prism diopter (PD).

To increase the sensitivity of the test, PFV and NPC measurements were repeated five times as explained above, waiting 30 s between measurements. The average of five measurements was recorded for each student.

The prevalence of CI was summarized as percentage and was statistically analyzed using t-tests and Chi-square tests with 95% confidence intervals in the total sample.

Ethical approval was obtained for the study from the Ethics Committee of the Ministry of Higher Education and Scientific Research. Permission was obtained from the participating schools' administrators to conduct the research in their schools, after obtaining the written approval letter. A parent or a legal representative of each student signed an agreement for each student participating in the study.

Results

The demographic characteristics of the students with CI are shown in Table 1. A total of 329 of 4211 (7.8%) students had CI. Their ages ranged between 13 and 18 years. The distribution of students with CI among participating schools is shown in Figure 1.

Table 1

Demographics of the students with convergence insufficiency (n=329)

Figure 1

Prevalence by school

Sex

Of the 329 students diagnosed with CI, 173 (52.6%) were male and 156 (47.4%) were female. Chi-square tests showed no significant association of either sex with CI (P > 0.05) [Table 2]. The sex distribution was approximately equal between males and females, among both the screened student population (male:female 2347:1864), and the students diagnosed with CI (173:156).

Table 2

Significance of demographics and clinical characteristics of the students with convergence insufficiency

School type

Three types of schools were involved in this study: standard, geographic, and mixed schools; 142 (43%) students went to five standard schools, 119 (36%) students went to four geographic schools, and 68 (21%) students went to one mixed school (standard and geographic schools together in one school building). The association with the type of school is illustrated in Table 2. Chi-square tests showed that there was a significant association between the type of school and CI status (P < 0.05); standard schools had a greater percentage of students with CI (43%) than the other types of schools [Table 1].

Grade

The percentage of students with CI in the first, second, and third grades of secondary school was 45.2%, 29.1%, and 25.5%, respectively. There was thus a high prevalence of CI among the students in the first grade, which decreased with age [Figure 2].

Figure 2

Prevalence by grade

Clinical characteristics according to sex

There were two main clinical characteristics of CI: PFV at near distance and NPC. The t-test showed a significant relationship between sex and PFV (P < 0.05). The PFV was greater in females than in males by 2.35 ± 0.701 PDs. No significant correlation was noted between sex and NPC (P > 0.05) [Table 2].

Causes of CI

Most students (78.42%) with CI in this study presented with a reduction in both NPC and PFV, while 20.36% of students had reduction in NPC only, and very few students (1.22%) had only reduced PFV [Table 3].

Table 3

Distributions of causes of convergence insufficiency

Discussion

In this screening survey, we determined the prevalence of CI among a secondary school population in Central Khartoum North, Sudan, and found it to be 7.8%. The prevalence of CI in the world is not clear, because no general population-based studies are available,[3031] although several studies have reported the prevalence of CI in specific clinical populations, which varies widely, ranging between 1.75% and 33%.[62232333435] Hence, the median prevalence of CI in the populace is 7%, which is similar to the findings of Duane [3] and White and Brown,[6] which reported a prevalence of 7.5%. It has also been reported that there is no significant difference in the prevalence of CI between adults and children.[14]

The discrepancies in the reported CI prevalence may be due to differences in the definition of CI, the samples studied (clinical vs. general population samples), different methods of analysis, and differences in testing protocols (some studies measure NPC with a pencil, whereas others use an accommodative target). The best population estimates are obtained from three studies involving North American school-age children who were assessed in their respective elementary [132223] and ranged from 2.25% to 8.3%, which agreed with our finding (7.8%). Nevertheless, the definition of CI was not uniform among these studies, and the prevalence of CI varied among ethnic/racial groups.

Dragomir, Sapkota et al., and Aziz et al. showed that CI was more frequent in females than in males.[263637] However, in a study performed in Korea, a higher prevalence of CI was found in males.[38] The present study showed no significant difference in CI between males and females.

Furthermore, standard schools had a higher prevalence of students with CI (43%) than geographic schools (36%). This variation may be due to the greater academic burden in the former than the latter type of school. Other studies have conveyed that symptoms of CI likely occur when students use their eyes in a two-dimensional reading environment for prolonged periods of time.[3940]

The largest proportion of students with CI was in their first grade of secondary school, in the age group of 14–15 years; the prevalence of CI decreased with age. The reason for this is not clear and requires further research.

This study analyzed the relationship between sex and PFV and NPC. PFV at near was significantly greater in females than in males, while there was no significant sex-based difference in NPC. There is no available literature on sex-based differences in PFV and NPC, and further study is necessary to verify this finding.

This study demonstrated that more CI patients displayed reduced NPC and PFV than only reduced NPC or PFV. NPC was the most important investigation for the evaluation of CI because 98.78% of students with CI demonstrated reduced NPC. This contrasted with the findings of a previous study that reported that only 5% of students with CI had reduced NPC.[41]

Conclusions

These findings suggest that CI is prevalent in the secondary school population in Central Khartoum North, Sudan; thus, CI investigations should be incorporated in the school screening program.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.