The prevalence of Amblyopia in a young Population

Introduction

Amblyopia is one of the most common causes of unilateral visual impairment in children, young adults, and adults (Attebo, Mitchell, & Smith, 1996; Klein, Wang, Klein, Moss, & Meuer, 1995; Robaei et al., 2006). The prevalence of amblyopia is estimated at 1%–3% in the general population. There are different reports of the prevalence of amblyopia worldwide, depending on the demographic characteristics of the study population, and different measurement methods (Barrett, Bradley, & Candy, 2013). The prevalence of amblyopia in children under 15 years varies from 0.4% in a study by Lim et al. (2004) to 3.6% in a study by Williams et al. (2008). In adults, the prevalence of amblyopia ranges from 0.73% in a study by Quah, Tay, Chew, and Lee (1991) to 5.6% as reported by Elflien et al. (2015). Most conducted studies have reported the sum of unilateral and bilateral amblyopia while unilateral amblyopia is far more prevalent (Barrett et al., 2013). Although the pathophysiology of amblyopia is not clear, strabismus, anisometropia, combination of anisometropia and strabismus, and uncorrected visual acuity have been reported as the most important risk factors (Koo, Gilbert, & Vander Veen, 2017). Unless there is overt strabismus or organic defects like ptosis, amblyopia is not usually detected in childhood; therefore, amblyopia developed in childhood will remain to be a potential risk factor for visual impairment in adolescence. A population-based study in the Netherlands showed that the cumulative risk of unilateral visual impairment is higher by 8% in amblyopic people compared to people without amblyopia (10% and 18%; van Leeuwen et al., 2007). University students are an important class of the society and their ocular health is very important in their quality of life and educational progression. Therefore, the presence of amblyopia in these individuals and its detection in this age group indicates lack of early diagnosis and treatment in early years of life. The results of this study and similar studies in adults show lack of proper implementation of screening programs in children. On the other hand, considering some medical interventions for amblyopia in adults in recent years, it seems that awareness of the burden of amblyopia in adolescents and adults is also useful. With regard to the importance of amblyopia, the aim of this study was to determine the prevalence of amblyopia and its determinants in Iranian university students.

Materials and methods

In the present university-based cross-sectional study that was conducted in Kazerun, south of Iran, multi-stage sampling was used to select the participants. There are four universities in Kazerun. Each university was considered a stratum. Then, the list of all majors offered in each university was prepared and each major was considered a cluster. A total of 27 majors were selected. Then, after coordination with Education Deputy of each university, the list of all students in each major was prepared and from each major, a number of students were randomly selected proportional to the size of the major.

Results

Prevalence of amblyopia

A total of 32 students (2.19%, 95% CI: [1.43, 2.93]) had amblyopia. The prevalence of amblyopia was higher in boys (2.83%, 95% CI: [1.17, 4.47]) than girls (1.96%, 95% CI: [1.11, 2.78]). No significant difference was found in the prevalence of amblyopia between boys and girls (p-value = 0.315). The highest prevalence of amblyopia was seen in the age group 18–20 years (3.93%) while no amblyopic cases were detected in students above 30 years (p-value = 0.071; Table 1). Amblyopia was strabismic, refractive (isometropia and anisometropia), and mix (strabismus and anisometropia) in 6.2%, 71.8%, and 25.0% of the cases, respectively. Table 1 presents different types of amblyopia according to age group, sex, and cause. According to Table 2, amblyopic students had a significantly higher mean SE and cylindrical power and a significantly lower mean uncorrected visual acuity compared to non-amblyopic subjects (p value ≤ 0.001).

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Table 1.

Prevalence of amblyopia according to age, sex, and its types in 1462 students above 18 years.

Age group	Sex	Refractive errorN (%)(anisometropia/isoametropia)	StrabismusN (%)	MixN (%)(anisometropia/strabismus)	Total (%)
18–20	Male	3 (75.0)	0	1 (25.0)	9 (3.93)
	Female	4 (80.0)	0	1 (20.0)
21–25	Male	5 (83.3)	0	1 (16.7)	18 (1.71)
	Female	8 (66.6)	1 (8.4)	3 (25.0)
26–30	Male	1 (100)	0	0	5 (3.79)
	Female	2 (50.0)	1 (25.0)	1 (25.0)
>30	Male	0	0	0	0 (0.00)
	Female	0	0	0
Total		23 (71.8)	2 (6.2)	7 (22.0)

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Table 2.

Comparison of mean uncorrected visual acuity (UCVA), sphere, and cylindrical power between amblyopic and non-amblyopic students.

	Non-amblyopicMean (95% CI)	AmblyopicMean (95% CI)	p-Value
UCVA (decimal)	0.81 [0.79, 0.82]	0.37 [0.25, 0.48]	<0.0001
Sphere (diopter)	−0.75 [–0.68, –0.81]	−2.44 [0.97, 3.90]	<0.0001
Cylinder (diopter)	−0.50 [–0.46, 0.55]	−2.78 [–1.92, 3.63]	<0.0001

Discussion

This study is the first study of the prevalence of amblyopia in an Iranian population above 18 years. On the other hand, the study subjects and the target population are unique considering the importance of ocular health in university students. The prevalence of amblyopia was 2.19% in this study, which is high when compared with the results of similar studies in age groups above 18 years. Nowak et al. reported that the 12-year prevalence of amblyopia (1993–2004) was 0.8% in an 18- to 34-year-old Polish population. Also, according to another study in Singapore, the prevalence of amblyopia in adult males was 0.35% (Nowak, Gos, Jurowski, & Smigielski, 2009; Rosman, Wong, Koh, & Tan, 2005). The results of other studies in age groups above 18 years are presented in Table 3.

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Table 3.

Prevalence rates of amblyopia in different studies.

Name (region, country)	Population and size and age group	Criteria	Prevalence of amblyopia	Common cause
Elflein et al. (2015) (Mainz-Bingen, Germany)	Gutenberg Health StudyN = 322735–44 years old	Best corrected vision in worse eye ≤ 0.63 with a two-line difference or ≤0.5 without such a difference	5.6%	Anisometropia: 49%Strabismus: 23%Mix (anisometropia/strabismus): 17%
Hoeg et al. (2015) (Næstved, Denmark)	Danish General Suburban Population Study (GESUS)N = 3826Age group = 20 years and older	Two-line difference in the BCVA values between the two eyes, with <20/40 in the worse-seeing eye	1.44%	Anisometropia: 45.5%Strabismus: 21.8%Mix (anisometropia/strabismus): 218%
Flom and Neumaier (1966) (California, USA)	Population-basedN = 7017AgeGroup = 10–50 years old	Criterion of 20/40 or worse acuity with more than one line difference between the eyes	1.7%	–
Faghihi, Ostadimoghaddam, and Yekta (2011) (Mashhad, Iran)	Cross-sectional studyN = 2150Age group = 6–21 years old	BCVA to 20/30 or less in one eye or two-line interocular optotype acuity differences in the absence of pathological causes	1.9%	Anisometropia: 65.9%Strabismus: 24.4%Isoametropia: 9.8%
Rosman et al. (2005) (Singapore)	Population-based, cross-sectionalN = 122,956Age group = 18–19 years old	BCVA of 6/12 or worse not attributable directly to any underlying structural abnormality of the eye or visual pathways	0.35%	Anisometropia: 37.1%Strabismus: 5.7%
Wang et al. (2011) (Yongnian, China)	Rural Adult Population of ChineseThe Handan Eye StudyN = 6830Age group = 30 and more	BCVA was 20/32 or worse in the amblyopic eye and was not attributable directly to any underlying structural abnormality of the eye or visual pathway	2.8%	Anisometropia: 67.3Strabismus: 5.4Mixed (strabismus/anisometropia): 4.4%
Nowak et al. (2009) (Poland)	Military malesN = 969Age group = 18–34 years old	BCVA of <20/40 in the affected eye, not attributable to an underlying structural ocular or visual pathway abnormality	0.8%	–
Quah et al. (1991) (Singapore)	Screening of National Service pre-enlisteesN = 6556Age group = 18–19 years old	Visual acuity of 6/12 or less in one or both eyes in the absence of ocular pathology	0.73%	Anisometropia: 50%Strabismus: 18.7%High astigmatism: 14.5%

BCVA: best-corrected visual acuity.

According to Table 3, the highest prevalence of amblyopia in adults was reported to be 5.6% in more than 15,000 German people aged 33–45 years (Elflein et al., 2015). Moreover, our estimated prevalence was higher in comparison with a number of studies conducted in Iranian children and adolescents (below 18 years). The prevalence of amblyopia was less than 2% in 7-year-old Iranian children in a study by Hashemi et al. (2014). A range of 1.9%–2.7% is reported in other studies conducted in Iranian children under 18 years of age (Faghihi et al., 2011; Rajavi et al., 2015; Yekta et al., 2010; Yekta et al., 2016). Since amblyopia is developed in childhood (Simons, 2005), its presence in older age groups indicates lack of timely detection and treatment due to the absence of proper screening programs at a national level or insensitivity of the families.

Due to lack of awareness of the importance of early diagnosis and rapid treatment of children with amblyopia in a certain time period, some studies have clearly shown a higher prevalence of amblyopia in adults who did not undergo vision screening in childhood in comparison with adults who underwent vision screening. Høeg et al. (2015) reported a significant difference in the prevalence of amblyopia between adults above 20 years who were offered vision screening in childhood versus those who were not (0.44%, n = 2, 95% CI: [0.12, 1.60] vs 1.78%, n = 41, 95% CI: [1.24, 2.33]).

Therefore, one of the reasons for the difference in the prevalence of amblyopia in adults in different countries may be the presence or absence of preschool vision screening programs and health policies in previous years, which has an important role in timely detection and treatment of amblyopia in childhood and its decreased prevalence in adulthood. Unfortunately, since the data of preschool vision screening were not available, we were unable to compare the prevalence of amblyopia between the students who were offered preschool vision screening and those who were not. However, since preschool vision screening was started less than two decades ago in Iran, it seems that a great proportion of the participants did not receive vision screening or were screened in early years of implementation. Another reason for the difference in the prevalence of amblyopia is the use of different diagnostic criteria. Attebo et al. (1998) reported a prevalence of 3.2% using visual acuity 20/30 or worse and 2.9% using visual acuity 20/40 or worse in adults above 49 years. Moreover, the prevalence of amblyopia was 1.19% in Chinese children aged 6–72 months using visual acuity 20/30 or worse while Chai et al. (2010) believed that the prevalence would increase to up to 2.7% if the criteria of American Association of Pediatric Ophthalmology and Strabismus (20/40 or worse) were used (American Academy of Pediatrics, 1996).

In addition to the role of the above factors in the difference in the prevalence of amblyopia in adults, ethnic and racial differences also play an important role. Since some effective factors in amblyopia like refractive errors and strabismus are affected by racial and ethnic factors (Maconachie, Gottlob, & McLean, 2013; Wojciechowski, 2011), the prevalence of amblyopia will differ in different ethnic and racial groups. Xiao, Morgan, Ellwein, and He (2015) found a much lower prevalence of amblyopia in African children than other ethnic groups, including Mongoloid and Hispanic children.

Another important point about the population in this study and the difference between the prevalence of amblyopia in this population with other studies is that amblyopic students are more likely to be diagnosed and treated at childhood if they are raised in families whose parents have paid more attention to all aspects of their children, such as their education and health. Therefore, it is likely that the prevalence of amblyopia in the population of this study (students) will be underestimated in relation to non-student peers. Thus, there will be more prevalence of amblyopia in adults.

We found no significant difference in the prevalence of amblyopia between different age groups. The results of many studies in children and adults have shown no increase or decrease in the prevalence of amblyopia with age (Flom & Neumaier, 1966; Nowak, Jurowski, Gos, & Smigielski, 2010; Wilson & Welch, 2013). For example, Chia et al. (2010; 6- to 72-month children) and Huynh et al. (2006; 6- to 12-year-old children), as studies conducted in children, and the Handan Eye Study (Chinese adults above 30 years; Wilson & Welch, 2013) and the Visual Impairment Study (Australian adults above 40 years; Brown et al., 2000), as studies conducted in adults, showed no difference in the prevalence of amblyopia between their age groups. The results of a systematic review by Barrette et al. (2013) concluded that lack of difference in the prevalence of amblyopia in adult age groups is not unexpected considering the fact that amblyopia is developed in childhood, although the reasons for the lack of difference in childhood are also not clear. In spite of the results of above studies regarding lack of difference in the prevalence of amblyopia between different adult age groups and the consistency of the results of the majority of the studies, few studies have reported differences in the prevalence of amblyopia between their age groups. For example, a population-based study in a Danish population showed a higher prevalence of amblyopia in middle-aged and elderly people as compared to younger individuals. The authors concluded that the results clearly showed the effect of implementation and development of the vision screening program in recent years versus more previous years since the prevalence was 0% in individuals aged 20–29 years and 1.5% in subjects above 50 years. Moreover, increased adherence to treatment secondary to improvements in treatment methods was another reason for the lower prevalence of amblyopia in younger age cohorts (Hoeg et al., 2015).

Our results showed no significant difference in the prevalence of amblyopia between boys and girls, which is consistent with the results of some previous studies conducted in children, adolescents, and adults (Huynh et al., 2006; Wilson & Welch, 2013; Xiao et al., 2015).

According to our findings, refractive errors were the leading cause of amblyopia in students (71.8%). Studies conducted in older age groups have also introduced anisometropia as the most common cause of amblyopia. Anisometropia accounted for more than 40% of the cases in amblyopia in studies performed by Elflein et al. (2015); Hoeg et al. (2015), Rosman et al. (2005), and Wang et al. (2011). Some recent studies in children have emphasized the role of strabismus in amblyopia. We believe that vision screening programs in children have led to identification and treatment of refractive errors in recent years while the frequency of undetected refractive errors is higher in older studies or studies conducted on the elderly since vision screening programs were not available in their childhood, resulting in higher prevalence of refractive amblyopia.

Conclusion

Our results showed that with regard to the history of preschool vision screening in Iran, it is very probable that amblyopic individuals in this study did not have their vision screened in the past. The high prevalence of amblyopia in Iranian university students (more than 2%) signifies the importance of early detection in childhood. Our study showed that refractive errors are the leading cause of amblyopia in the study population as a result of lack of routine screening programs in previous years. Therefore, it is expected that development of preschool screening programs for amblyopia in recent years and improved knowledge and sensitivity about these programs lead to a marked decrease in the frequency of amblyopia in age cohorts in the future.