Study design, setting, and participants
This study utilized data from SANHANES-1 [17], which enrolled participants of all ages. In brief, SANHANES-1 applied a stratified, multi-stage disproportionate cluster sample approach to select a total of 10,000 households within the Enumeration Areas (EAs) stratified by province and locality type. A total of 27,580 eligible individuals of all ages occupied 8166 valid households out of the 10,000 households, of which 25,532 (92.6%) participated in the interview. In addition, 12,025 (43.6%) individuals consented to undergo a clinical examination. Questions on vision difficulty and psychological distress were administered to participants aged ≥ 15 years. The final analytic sample comprised 6859 participants aged ≥ 15 years who underwent a physical examination (including vision assessments) and responded to the questions on vision difficulty, psychological distress, health status and sociodemographic characteristics. The demographic characteristics of the sample who volunteered to undergo a clinical examination differs slightly from the full interview sample. Therefore, the analytic sample comprises a higher proportion of females (60.8% vs. 54%) and rural participants (44% vs. 36%) and a lower proportion of participants aged 15–44 (66.3% vs. 71.1%) than the full sample. Details of SANHANES-1 eligibility, methodology, examination procedures and derivation of the analytic sample are summarized in Fig. 1.
Ethical approval
The Research Ethics Committee of the South African Human Sciences Research Council (HSRC) approved the study (REC number: 6/16/11/11). All procedures employed in the study were in adherence with the tenets of the Declaration of Helsinki. Written informed consent/assent was obtained from all the survey participants. In addition, written informed consent/assent of parents/guardians was also obtained for children aged ≤ 17 years.
Measures
Sociodemographic variables in our analysis included sex, age (15–44, 45–54, 55–64, or ≥ 65 years), and population group (African, White, Coloured, Indian/Asian). Of note, the term Coloured refers to individuals with mixed race (i.e., Mixed European and African or Asian ancestry) and is used in all national statistical reporting [18]. Socioeconomic characteristics included the highest education level (no formal schooling/grades 0–7, grade 8–12, and higher education), wealth index [1 (lowest wealth)-5 (highest wealth), and residence (rural/urban)].
Assessment of visual function
Well trained and equipped survey and clinical teams comprising of interviewers, a medical doctor, a registered nurse and a clinic assistant were involved in this assessment. Survey staffs conducted interviews and clinical examinations were performed by the clinical team. The primary independent variables were self-reported difficulty in seeing objects close-up (hyperopia) and self-reported difficulty seeing objects at a distance (myopia), and clinician assessed vision loss. Using the Snellen chart, the medical doctor assessed the subjects’ visual acuity to ascertain whether the participant had vision loss and if so, the type of vision loss. The type of vision loss was categorized into blurred vision, a need for more light, difficulty reading, loss of peripheral vision, difficulty driving at night, double vision, difficulty in distinguishing colours, straight lines looking wavy, and sensitivity to glare. The categories were not mutually exclusive, that is, a participant could experience multiple types of vision difficulties. Clinician assessed vision loss was defined as presenting visual acuity (PVA) worse than Snellen 6/12 in the better eye. For self-reported visual difficulties, self-reported myopia was assessed by the question: “In the last 30 days, how much difficulty did you have in seeing and recognizing an object or a person you know across the road (from a distance of about 20 m)”, where the participant was asked to answer including times when wearing glasses/contact lenses if used; with options for none, mild, moderate, and severe and extreme/cannot do. Similarly, self-reported hyperopia was measured by the question: “In the last 30 days, how much difficulty did you have in seeing and recognizing an object at arm’s length (for example, reading) where the participant was asked to include when wearing glasses/contact lenses if used.” In addition, self-reported use of eyeglasses or contact lenses to see things close up or far away such as when reading newsprint or identifying someone far away was obtained. The interview questions on self-reported vision difficulties were from the World Health Organization’s (WHO) Study on global ageing and adult health (SAGE), which was conducted in several countries, including in South Africa in 2007 [19]. Participants were also asked when last they had their eyes examined. All doctors recruited in the study were trained in standardized procedures of measuring visual acuity.
Psychological measures
The primary dependent variable was psychological distress, which was measured using the Kessler-10 psychological distress scale (K-10) [20]. The scale consists of 10 items (e.g., ‘In the past 4 weeks, about how often did you feel nervous that nothing could calm you down?’) where each item has five-level response scale: ‘all of the time’ (5), ‘most of the time’ (4), ‘some of the time’ (3), ‘a little of the time’ (2), and ‘none of the time’ [17]. The total score of the scale ranges between 10 and 50 where a score < 20 indicates low/minimal distress, a score from 20–24 indicates mild distress, a score from 25 to 29 indicates moderate distress, and a score ≥ 30 signifies severe distress [20]. Prior research indicates that the Kessler-10 scale relates with the Composite International Diagnostic Interview (CIDI) questionnaire which is now the standard tool for the assessment of mental disorders. This makes Kessler 10 scale a good tool for the assessment of psychological distress [21]. The scale has been validated in the South African context [22].
Hazardous alcohol use
Hazardous drinking was assessed using a three-item alcohol screening tool, the Alcohol Use Disorders Identification Test-Consumption (AUDIT-C) [23].
Tobacco use
Tobacco smoking status of participants (current smoker, ex-smoker, never smoker) was measured by self-reported current and past tobacco smoking.
Experience of traumatic event
For the assessment of the experience of any traumatic event, participants responded either yes or no to fourteen (14) listed events with a preamble ‘have you ever experienced any of the following events’ (for instance, ‘severe automobile accidents’ and ‘learned about the sudden, unexpected death of a family member or a close friend?’) [17].
Assessment of physical ill-health conditions
History of cardiac disease was assessed by participants’ self-report of whether a medical officer or health worker had ever told them that they have any of these conditions: heart (cardiac) disease, heart failure, stroke, rheumatic heart disease, a heart attack or chest pain (angina) [17] Diabetes was assessed by self-report of previous diagnosis of high blood sugar or diabetes by a health professional. Blood pressure was measured during the clinical examination. Hypertension was defined as having systolic blood pressure ≥ 140 mmHg, diastolic blood pressure ≥ 90 mmHg or current use of hypertensive medication.
Data analysis
We analyzed data using Stata 15.0. (StataCorp, Texas, USA, 2016). The analyses utilized sample weights to adjust for unequal probabilities of selection and nonresponse as well as for the complex survey design using the ‘svy’ commands in Stata. Descriptive statistics were used to summarize the demographic, socioeconomic, health status and eye care characteristics. Chi-square tests were used to test the difference between estimates of psychological distress and level of vision difficulty and vision loss. A series of multiple logistic regression models were used to investigate the association of self-reported vision difficulties (both myopia and hyperopia) and clinician assessed vision loss with the binary outcome; mild to severe psychological distress. The binary outcome was coded by dichotomizing the Kessler-10 scale into two categories with a total score < 20 for no or minimal psychological distress (coded 0) and ≥ 20 for mild to severe psychological distress (coded 1) [24]. The following variables were added to each model: Model (1) adjusted for age, sex and population group; Model (2) Model 1 plus the socioeconomic variables (education, wealth quintile and urban/rural residence), Model (3) Model 2 plus the health status variables (tobacco smoking status, hazardous alcohol drinking, BMI, diabetes, hypertension, cardiac disease and lifetime experience of traumatic event(s) and Model (4) Model 3 plus use of a visual aid and years since last eye examination. The selected variables were included in the models as possible confounders based on a review of the literature, as they have been shown to have associations with both psychological distress and vision loss. Odds ratios (OR) with 95% confidence intervals were calculated. All estimates were considered statistically significant at p < 0.05.
