Refractive error refers to the measure of a person’s shortsightedness (myopia), farsightedness (hyperopia), and/or astigmatism (corneal curvature), which are caused by an incongruity between the axial length and the refractive power of the optical elements (mainly cornea and lens) of the eye.1 Globally, refractive errors are one of the most common causes of visual impairment, the fourth leading cause of blindness, and the second cause of curable blindness.2 How refractive error affects vision depends on the type (myopia, hyperopia, or astigmatism), the category (axial or refractive), and the form of correction used (spectacles, contact lenses, intraocular lenses, or refractive surgery).2
In 2003, it was estimated that more than 2.3 billion people in the world suffered from poor vision because of corrected and uncorrected refractive error.3 More recent estimates suggest that more than 640 million people are visually impaired because they do not have access to corrective treatment such as glasses, contact lenses, or refractive surgery.4,5 Estimates in 2013 indicated that uncorrected refractive error accounted for 52.9% of visual impairment worldwide,4 suggesting that it is a global visual health challenge. Epidemiological studies have reported differences in the prevalence of refractive error. Reasons for differing estimates include differences in age, race, and ethnicity of the populations studied. Other reasons for differences might include measurement technique, the use of cycloplegics, and the criterion levels for the refractive error categories.
Many people in developing countries, including South Africa, have limited access to eye care services, which can be caused by the unavailability of human resources (particularly optometrists) and services not always being provided, accessible, or used, leading to a high prevalence of visual impairment.6 The resulting uncorrected refractive errors can lead to lost economic gain and missed employment and educational opportunities, resulting in a generally reduced quality of life.7
Despite the fact that refractive error is one of the most common causes of visual impairment and the second leading cause of blindness,8 few scientifically valid population-based studies of refractive error in adults have been conducted in South Africa. Naidoo et al.9 conducted a population-based study to assess the prevalence of refractive error and visual impairment in school-aged African children (aged 5 to 15 years) in South Africa in 2003 and found that the prevalence of uncorrected, presenting, and best-corrected visual acuity (VA) of 20/40 or worse in the better eye was 1.4%, 1.2%, and 0.32%, respectively. Refractive error was the cause in 63.6% of the 191 eyes with reduced vision.9 A population-based refractive error study of young adults aged 15 to 35 years has also recently been concluded. The current study looks at the refractive error component of a larger presbyopia study10 that was conducted in the Inanda, Ntuzuma, and KwaMashu (INK) areas and therefore completed the picture of refractive error in the missing age group (35 years and older). This information is necessary to determine the regions, population groups, and age cohorts most in need of intervention and services required and to provide the basis for effective service delivery, monitoring, and evaluation.
The study area was composed of INK, historically disadvantaged communities of the eThekwini municipality of the province of KwaZulu-Natal, South Africa, during apartheid. The total population in this area is approximately 500,000 people and consists of a mixture of urban, periurban, and semirural areas. The area varies in the provision of and access to services and resources compared with more advantaged areas but is representative of the reality of most health districts in KwaZulu-Natal and South Africa. As in the rest of South Africa, the delivery of eye care follows the district health model and, currently, eye care services are provided by four community health centers, one district hospital, one private hospital, and several private optometry practices.
The research followed the tenets of the Declaration of Helsinki, and all participants gave consent to participate in the study after the nature of the study had been explained to them. The research was approved by the Biomedical Research and Ethics Committee of the University of KwaZulu-Natal. Refractive error data were collected during a multicountry population-based presbyopia study that has been described elsewhere.10 The key methods are summarized below for purposes of this article.
The INK areas are on the outskirts of the coastal city of Durban. A cluster sampling method was used to select the study population using the enumerator areas (EAs) created during the 2001 census in a geographic information system. A total of 265 EAs were listed, and 20 EAs were randomly selected. All occupants of eligible households 35 years or older who had lived in the INK area for 6 months or more and signed the consent form to partake in the study were enrolled.
An enumeration team was responsible for visiting households within the selected clusters in a door-to-door manner. All persons 35 years or older were enumerated by age, sex, education, and spectacle use.
Enumerated subjects meeting the selection criteria were requested to attend designated community sites for VA measurement, refraction, and near-vision assessment. For those disabled or nonambulatory participants, transport was provided to take them to and from the testing sites (schools and community halls). The clinical examination process was as follows:
Autorefraction was used to define refractive error. All participants underwent autorefraction using a handheld auto-refractor (Retinomax K-Plus; Nikon, Tokyo, Japan). This auto-refractor was calibrated once a day before commencement of the clinical examination process. Three or more readings were taken for the right and left eyes of each participant. Auto-refractor confidence readings less than 7 were discarded and repeated until acceptable confidence levels were obtained.
Presenting distance VAs in ambient lighting conditions were measured using the LogMAR “E” chart at 4 m (Precision Vision, Villa Park, IL). Visual acuity was measured monocularly then binocularly and recorded as the smallest line at which at least four of the five optotypes were identified correctly. Participants with spectacles were asked to remove them before testing and then to wear them for repeat testing. Pinhole VAs were obtained for those participants who failed to reach 20/20 aided (with own spectacles) or unaided acuity. Retinoscopy was performed on all participants, followed by subjective refraction to confirm findings. All these other measures were only for clinical care.
Near VA was measured in ambient lighting conditions with and without near spectacles if used using a LogMAR near-vision “E” chart (Precision Vision). A string was attached to the near-vision chart to ensure a measurement distance of 40 cm from the eyes. Visual acuity was measured monocularly and binocularly and recorded as the smallest line at which at least four of the five optotypes were identified correctly.
After presenting near VA measurement, those with VA of less than 20/40 were progressively tested with plus sphere spectacles to obtain best corrected binocular vision. The spherical diopter correction was recorded, along with the corresponding best corrected near VA.
Ocular Health Assessment
Ophthalmoscopy was conducted using direct ophthalmoscopes by an optometrist, and patients with poor views of the ocular fundus, suspicious retinal signs, and/or potentially glaucomatous cupping were referred for further dilated-pupil fundus investigation at their local hospital. Anterior segment examinations consisted of direct observation of the conjunctiva, sclera, and cornea by a diagnostically certified optometrist for any ocular abnormalities.
A pilot study was conducted in a cluster that was not part of the 20 EAs selected for the study. This study provided the opportunity to monitor and observe personnel in carrying out all aspects of the study in a representative field setting, from enumeration through data entry. Findings from the pilot study were fed back and discussed with clinical and nonclinical staff to address areas of inconsistencies and methods with which to correct these inconsistencies. The pilot also indicated areas of improvement in the logistical and operational aspects of the study.
In this study, myopia was defined as a spherical power of less than −0.5D in both eyes or in one eye (if the other eye was emmetropic). Hyperopia was defined as a spherical power of greater than +0.5D in both eyes or in one eye (if the other eye was emmetropic). A cylindrical power of less or equal to −0.5D in both eyes or in one eye (if the other eye was emmetropic) was considered as astigmatism. Astigmatism was classified as with-the-rule (WTR) if the axis meridian lay 15 degrees on either side of the horizontal and against-the-rule (ATR) if the axis meridian lay 15 degrees on either side of the vertical, and oblique if the axis meridian lay between 15 and 75 degrees or 105 and 165 degrees.11 The same definitions were used in other similar population-based studies.11,12 These definitions have emerged from both the World Health Organization and the National Eye Institute recommendations and have allowed comparisons across countries and regions.13 Where the refractive error in each eye was different (one eye myopic and the other hyperopic), it was recorded as antimetropia. Emmetropia was defined as spherical power between −0.25 and +0.5D. For this study, presenting and aided VAs worse than and including 20/40 that showed improvement with pinhole were considered for distance refraction. If a participant’s near VA was worse than N8 (20/40) at 40 cm, near add testing was conducted until best near vision was achieved. Spectacles for near were dispensed at the testing site, whereas participants needing spectacles for distance were referred to an optometrist at a state hospital where they could receive spectacles free of charge. Participants with ocular pathologies were provided with referral letters to their local hospitals for an ophthalmological examination.
Data Management and Analysis
The data forms were checked for accuracy and completeness in the field before data entry at the Brien Holden Vision Institute Durban offices. The participants’ refractive error data were analyzed using STATA (version 11.0). The analysis included the generation of descriptive statistics to report the means, standard deviation, ranges, proportions, and their 95% confidence intervals (CIs). The comparisons of mean participant ages and the ages of those with myopia, hyperopia, and astigmatism were performed using t-tests. A 5% level of significance was used for all statistical analyses.
Because of the large number of participants needing to be seen at the testing sites, quality assurance was done on days when the numbers permitted because this was done by one of the designated clinicians who was authorized to perform this function throughout the study to ensure continuity. Findings by the original clinicians were not made known to the designated clinician. The final results from the designated clinician were compared with the results from the original clinicians.
Of the 2764 persons aged 35 years and older who were enumerated, 1939 (70%) were examined and considered to be the study participants. Their ages ranged from 35 to 90 years, with a mean of 53.05 ± 11.4 years, and consisted of 483 (25%) men and 1456 (75%) women. The mean age for women was 53.6 ± 10.7 years (95% CI, 52.6 to 54.5), whereas the mean age of the men was 52.9 ± 11.6 years (95% CI, 52.3 to 53.5). The majority (80%) of participants were aged between 38 and 69 years. A total of 111 (5.7%) had no education, 645 (33.3%) had less than primary school education, 513 (26.5%) had completed primary school, 251 (12.9%) had secondary school education, 144 (7.4 %) had at least high school education, and 275 (14%) did not report their education level.
Presenting VA of 20/40 or better in at least one eye was found in 1421 (73.3%) of the participants, whereas VA worse than 20/40 in the better eye was present in 518 (26.7%) of the participants. Presenting VA was with the habitual correction and therefore represented presumably correctable refractive error.
Of the 1939 participants, 1111 (57.3%) were found to have refractive errors. With respect to the study population, 221 (11.4%) had myopia, 731 (37.7%) had hyperopia, 18 (0.9%) had antimetropia, whereas 498 (25.7%) has astigmatism. Table 1 shows the distribution of the refractive errors by sex, age, and education.
For the right eye, myopia ranged between −0.625 and −18.5D, hyperopia ranged between +0.625 and +13D, and astigmatism ranged between −0.75 and −7.75D. The mean spherical equivalent (SE) was 0.32 ± 1.5 SD (95% CI, −0.25 to +0.38D). For the left eye, myopia ranged between −0.625 and −17.5D, hyperopia ranged between +0.625 and +13.375D, and astigmatism ranged between −0.75 and −5.25D. The mean SE was +0.31 ± 1.5 SD (95% CI, +0.24 to +0.38D).
Of those examined, 221 (11.4%; 95% CI, 9.1 to 13.7%) had myopia. The mean age among the myopes was 57.2 ± 14.9 years, higher than the mean age of the study population. One hundred forty-one (16.6%) men and 80 (9.7%) women presented with myopia; this higher prevalence in men being reversed among the 80+-year age group. There was a statistically significant relationship between myopia and sex (p < 0.01). Myopia showed no significant relationship with education (p = 0.09). Eighteen (16.2%) participants examined who had no education had myopia, and 17 (11.8%) of those with at least high school education had myopia.
Of the 1939 participants, 731 (37.7%) (95% CI, 35.5 to 39.9%) had hyperopia, their mean age being 56.8 ± 9.8 years, higher than the mean age of the study population (53.0 ± 11.4 years). One hundred forty-two (29.4%) men and 589 (40.5%) women had hyperopia, the prevalence being significantly higher in women than in men (p < 0.01). Forty-four (39.6%) of the participants examined who reported having no education presented with hyperopia, whereas 40 (27.8%) with at least high school education had hyperopia, this condition showing a significant relationship with educational levels (p < 0.01).
A total of 498 (25.7 %) participants had astigmatism, which included those with only astigmatism and those with myopia or hyperopia concurrent with astigmatism. Mean age was 58.3 ± 12.0 years. This was significantly different from that of the study population (p < 0.01). A total of 146 (30.2%) men and 352 (24.2%) women had astigmatism, presenting as a significant difference in the presence of astigmatism between the sexes (p < 0.01). Thirty-four (30.6%) participants examined who reported no education had astigmatism, whereas 85 (25%) of those with at least high school education presented with astigmatism. However, astigmatism was not associated with levels of education (p = 0.15). There was a significant association between age and the type of astigmatism for both eyes (p < 0.01). With-the-rule astigmatism was more prevalent in the younger age group (aged 35 to 49 years) than the older age group (aged 50+ years), whereas ATR astigmatism showed the opposite trend. The distribution of the types of astigmatism by age for the right and left eyes is illustrated in Table 2. Oblique astigmatism increased sharply from SE = −5 to SE = 0. With-the-rule astigmatism increased from SE = −18 to SE = −1, where it sharply increased to SE = +1. Against-the-rule astigmatism was lower than oblique and WTR astigmatism and showed an increase from SE = −9 to SE = +2. The average cumulative frequency graph for SE by type of astigmatism for the right and left eyes is shown in Fig. 1.
An analysis of the impact of refractive error on VA was performed to provide context to the burden of refractive errors. In the right eyes, 45.21% of participants with myopia had moderate visual impairment (<20/63 to >=20/200), whereas in the left eye, 46.45% had this level of visual impairment (Table 3). Hyperopia constituted most of the refractive errors where participants had no visual impairment (>=20/40), 57.26% and 57.95% in the right and left eyes, respectively (Table 3). The impact of astigmatism on VA showed a similar trend as that of hyperopia (Table 3).
Approximately 98 million of the 259 million people in the world who suffer from visual impairment do so because of uncorrected refractive errors.6 There are insufficient data on the prevalence and types of refractive errors in different populations and age groups to allow for effective planning and intervention programs in South Africa. There is therefore a need to conduct prevalence studies in different parts of the country to determine the regional magnitude of refractive error, which will assist health authorities to formulate appropriate strategies to address refractive error.
Table 4 provides an overview of prevalence of refractive errors in selected surveys among adult populations of different ages.
Our estimated prevalence of refractive error is higher than the 30.8% reported in a previous South African study conducted in Cape Town14 but different from those reported in Nigeria15 and Kenya.16 It is however not possible to make general overall comparisons among these results because of differences in ethnic backgrounds and age groups used in these studies. For example, in the Cape Town study, participants’ ages ranged from 16 to 74 years and the race distribution included black, white, and colored (mixed race). In addition, the study defined myopia as the SE in the better eye of −1D or worse, hyperopia as the SE value in the better eye of greater than or equal to +1D, and astigmatism as −0.5 cylinder or worse in the better eye (Table 4).
The prevalence of myopia in this study is lower than that reported in Ghana17 and comparable to those reported in South Africa,14 Australia,18 India,19 and Taiwan20 but differ substantially from those reported in Nigeria,15 Western Europe,18 United States,18 Singapore,21 and China.22 This may suggest that different regions may have different predisposition to myopia because of various factors. However, the variability in the definition of myopia, in age cohorts, and in the selection of participants prevents a direct comparison. For example, the Eye Disease Prevalence Research Group (EDPRG)18 included people 40 years and older and defined myopia as an SE value of less than or equal to −1D, whereas the present study included adults aged 35 years and older and defined myopia as an SE value of less than −0.5D (Table 4). The current study found an increase in myopia with an increase in age (except in the 80+-year age range), possibly reflecting increasing prevalence of age-related cataracts (nuclear sclerosis).15 The prevalence of myopia was lower in women than in men, except for the 80+-year age group, which differs from previous studies23–25 that showed that myopia was higher in women than in men. There was no statistically significant difference between myopia and educational status (p = 0.09). In this sample, myopia contributed a significant degree of visual impairment (Table 3). This burden of visual impairment caused by myopia can be reversed as the condition is fully correctable by spectacles or contact lenses. These suggest the need for intervention programs for individuals with these refractive errors.
The prevalence of hyperopia in the current study was similar to that reported in Ghana,17 higher than that reported in South Africa,15 Nigeria,16 Australia,18 Western Europe,18 United States,18 Singapore,21 and China22 but lower than that reported in Taiwan.20 However, the prevalence of emmetropia in the present study was much higher (42.7%). The upper limit of hyperopia was high because there may have been some aphakia. This wide variation could in part be caused by the variations in definitions of hyperopia and/or age groups and ethnicities used in the various studies. For example, the EDPRG18 defined hyperopia as an SE value of greater than or equal to +3D, whereas Dandona et al.,19 Cheng et al.,20 Wong et al.,21 Liang et al.,22 and the current study defined hyperopia as an SE value of greater than or equal to +0.5D (Table 4). Hyperopia was found to be significantly associated with age, this being similar to reports of earlier studies.16,26 The increase in hyperopia with age may be caused by a loss of residual accommodation or a decrease in the power of the aging lens. The prevalence of hyperopia was higher in women than in men, this being similar to other studies.21,26 This may be because, on average, women’s eyes have a shorter axial length and shallower anterior chamber depth than those of men, resulting in a higher probability of being hyperopic.27 The condition was significantly associated with educational status, a finding similar to that of Otutu et al.14 in three communities of Cape Town, South Africa. More than one-half of hyperopes had VAs of 20/40 or better. However, 42.74 and 42.05% of hyperopia in the right and left eyes, respectively, contributed to some form of visual impairment (<20/40 to <20/200). Correction by spectacles or contact lenses could significantly impact visual impairment because of hyperopia in this group.
Astigmatism included those with only astigmatism as well as those with myopia or hyperopia concurrent with astigmatism. The prevalence of astigmatism in this study is comparable to that reported in rural China22 but very different to those reported in other countries (Table 4). There is therefore a wide variation in the prevalence of astigmatism among the studies. It is again difficult to compare them because definitions and methods varied. The EDPRG18 defined astigmatism as a cylinder of 1D or more in the eye with higher astigmatism, whereas Dandona et al.19 and Liang et al.22 excluded participants who were wearing corrective spectacles and analyzed results of the right eyes only. Otutu et al.14 defined astigmatism as −0.5D cylinder or worse in the better eye (Table 4).
The prevalence of astigmatism increased with age. This is consistent with the results of other studies,26,28 which found that mean total astigmatism increased with age. Nuclear sclerosis cataract and the change in refractive index of the crystalline lens at older ages may contribute to the increase in astigmatism.29 This sample had ATR as the most common form. The proportion of different types of astigmatism was related to the presence and magnitude of ametropia, WTR being more commonly detected in myopes and ATR in moderate ametropia (Fig. 1). Other studies30,31 have reported similar findings. There was a significantly higher prevalence of astigmatism in men than in women (p < 0.01), this being in contrast to the South African (Cape Town) study,14 which reported no significant difference in the presence of astigmatism in both sexes (p = 0.09). However, astigmatism showed no significant association with levels of education (p = 0.15). About one-half of astigmats had VAs of 20/40 or better, with an almost similar proportion contributing to some form of visual impairment (<20/40 to <20/200). It is therefore important to inform people with astigmatism of the benefits of proper correction.
Several limitations of our study must be acknowledged. First, the relatively low response rate (70%) may have affected the prevalence of refractive errors. Second, the number of men and women were disproportionately distributed and could have influenced the results of the study. This may be caused by a number of reasons including high levels of unemployment among women and the mobility and migration of men to other areas in search of employment. Third, there was no way to compare participants and nonparticipants and, therefore, the degree of bias is unknown. An additional limitation is that measures were noncycloplegic.
The results indicate that refractive error affects approximately two-thirds (57.3%) of the population (aged 35 years or older) in the INK area of KwaZulu-Natal, South Africa. The frequency for myopia was 11.4%, hyperopia 37.7%, astigmatism 25.7%, antimetropia 0.9%, and emmetropia 42.7%. Myopia and astigmatism were found to increase with increased age, and hyperopia increased with increased age except for the 80+-year age group. Myopia and astigmatism were more common in men than in women, whereas hyperopia was more common in women. In addition, hyperopia was associated with the level of education, whereas myopia and astigmatism were not associated with educational levels. These data on the prevalence of refractive errors can be useful for planning refractive services. The study also highlights the need to develop standardized protocols to facilitate comparisons between studies.
Khathutshelo P. Mashige
Department of Optometry School of Health Sciences
University of KwaZulu-Natal
Private Bag X54001
The authors thank the Department of Health and local councillors, eThekwini Municipality, the Brien Holden Vision Institute, and all individuals who participated in the project.
Financial support was received from the Brien Holden Vision Institute–Implementation of the study in Durban. The authors alone are responsible for the content and writing of this article.
Received July 4, 2014; accepted May 1, 2015.
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Keywords:© 2016 American Academy of Optometry
refractive error; myopia; hyperopia; astigmatism; visual impairment; prevalence