In this first comprehensive national survey on the prevalence of CVS in computer office workers from a South Asian country, the 1 year prevalence of CVS was 67.4 %. Previous studies on CVS from Malaysia (68.1 %) and Nigeria (74.0 %) have demonstrated similar results [13, 17]. In contrast, another study among medical and engineering students in Chennai has found a higher prevalence of CVS (80.3 %) , whereas a study among keyboard users in Mauritius has found a lower prevalence of CVS (59.9 %) . The higher prevalence observed in the study from Chennai (80.3 %) is possibly due to the involvement of neck and shoulder pain as a symptom of CVS by the study team, whereas our definition of CVS consisted only of eye/visual symptoms apart from headache. Also in our study only the symptoms which lasted at least 1 week were considered as symptoms of CVS whereas they had no specification on duration of symptoms and therefore included even transient symptoms .
The most common symptom reported in the present cohort was headache (45.7 %), followed by dry eyes (31.1 %) and pain in and around the eyes (28.7 %). Megwas and Daguboshim reported that headache (41.8 %), pain (31.6 %) and eye strain (26.7 %) were the most prevalent visual symptoms among VDT users . Headache was the most commonly reported symptom in computer users in several other similar studies [13, 20, 21]. Headaches is often accompanied by other symptoms of CVS, though many patients do not consider it to be a directly vision-related problem . Human eyes need to adjust themselves in order to see objects from different distances, such as by changing the size of pupil, lengthening or shortening the lens to change eye focus, and contracting extra-ocular muscles to coordinate the two eyes. If computer user needs to view computer screen while looking at a paper on the table from time to time, the eyes have to adjust constantly. In addition, the words and images on a computer screen are difficult for the eyes to focus on due to their poor edge resolution. The eyes tend to change the focus to a resting point and then refocus on the screen. For these reasons, constant focusing and refocusing is required. These constant changes take place thousands of times a day when a computer user stares at a computer screen for hours, which then stresses the eye muscles leading to eye fatigue and discomfort causing headaches .
According to the results of the binary logistic regression analysis, the most significant risk factor for development of CVS was pre-existing eye disease (OR: 4.49) followed by use of contact lenses (OR: 3.21). Supporting this finding, a study done in Malaysia has revealed that use of correction spectacle/lenses were significantly associated with CVS (OR: 1.91) in multivariate logistic regression analysis, even after adjustment for other confounding variables . Furthermore, university students who were wearing spectacles experienced symptoms of CVS significantly more often than those who were not wearing spectacles . A study by Logaraj et al. also revealed that medical and engineering students wearing corrective lens (spectacle or contact lens) showed a significantly higher risk of developing headache (OR: 1.80) and blurred vision (OR: 2.10) . Possible explanations for the increased risk of CVS among those using correction spectacles/lenses is because the computer tasks are types of near work where letters on the screen are formed by tiny dots called pixels, rather than a solid image, it causes the eyes which already have some corrective problem to work a bit harder to keep the images in focus .
Female gender was also significantly associated with the risk of developing CVS (OR: 1.28). Many studies have reported a significant association between female gender and prevalence of CVS [17, 18, 24]. However, when considering individual symptoms, Logaraj et al. reported redness, burning sensation, blurred vision and dry eyes were comparatively more in males than in females . Our study revealed prevalence of red eyes, changes in visualizing colours and excessive tearing were significantly higher among males. Daily computer usage (OR: 1.10) and duration of occupation (OR: 1.07) also significantly predicted the risk of CVS. Evidence from many other studies supports these findings [13, 20, 24]. Rahman and Sanip, in their study reported that spending more than 7 h per day on computer at work was a significant predictor for CVS (OR: 2.01) . Mutti and Zandic reported more pronounced visual symptoms in people spending 6–9 h daily at a computer , while Stella et al. observed the same in people using computer more than 8 h daily . However present study did not reveal a significant association of daily computer usage with presence of severe CVS.
Present study demonstrated ergonomics practices knowledge (OR: 1.24) was associated significantly with increased risk of developing CVS. This may be because ergonomics practices knowledge is higher among frequent computer users with long duration of occupation and higher daily computer usage than among infrequent computer users. Therefore prevalence of CVS could be increased among those with higher ergonomics practices knowledge as increased duration of occupation and daily computer usage are significant risk factors for CVS. Although ergonomics practices knowledge is present, lack of implementation of ergonomic practices knowledge at their work place also may be reason for higher prevalence of CVS. Strengthening the fact, present study demonstrated that in those who had heard of the term ‘Ergonomics’, only 44.6 % implemented them at work place. Results from a study by Khan et al. are as follows; as far as the distance from the computer screen was concerned, 42 % respondents were aware, while only 32 % always maintained it. In the same study, although 55 % knew, only 35 % kept the top line of print at their eye level .
Lack of a VDT filters significantly predicted (OR: 1.02) the risk of CVS. Use of antiglare filters over VDT screens has been associated with shorter, less frequent and less intense eye complaints in some studies . Significantly lower prevalence of visual complaints in the subjects who used antiglare screen was also observed by Saurabh et al. . However in contrast to this, study by Reddy et al. reported that the use of VDT filters did not help in reducing the symptoms of CVS . According to the binary logistic regression analysis, increasing age was a significant risk factor for CVS only in females. In contrast to this, Rahman and Sanip reported age group of less than 27 years old was a significant predictor for CVS (OR: 2.89). The explanation for this finding as given by them was a significant negative correlation between age of the respondents and duration of computer usage at work (r = 0.213, p < 0.001) . In our study population although there was no significant correlation between age and daily computer usage, a significant positive correlation was found between age of respondents and duration of occupation in all adults (r: +0.69, p < 0.001) and in females (r: +0.70, p < 0.001). This explains higher prevalence of CVS in older age.
Adjusting the brightness of screen and angle of gaze were not significantly associated with CVS in regression analysis. However Stella et al. reported that respondents employing a gaze angle of less than 15° recorded the lowest visual complaints whereas visual complaints were more pronounced with viewing angles of 30–50° . Improper viewing angle has been identified as a factor contributing to CVS in a review article on CVS . It is recommended that the screen should be placed 10–20° below the eye level . Higher viewing angles expose a greater area of conjunctiva and cornea to air and increase the chances of irritant-like symptoms . Although not significant in regression, our study also reported mean angle of gaze to the monitor was significantly higher in those with CVS than in those without CVS. Significantly higher risk of developing visual symptoms was reported among students viewing computer at a distance of less than 20 inches by a previous study . Another study supported this finding by reporting more pronounced visual symptoms when computer is viewed at distance less than 10 inches . However present study revealed there is no significant association between distance from face to monitor and CVS. This may be due to inaccurate measurements reported by respondents. Taking a break was not significantly associated with prevalence of CVS as demonstrated by present data. Reddy et al. reported a similar finding . In contrast to these, several studies reported a significant risk of getting visual symptoms when not taking frequent breaks [14, 17, 33].
The main limitations of this study were that it was a cross-sectional study and it did not include ophthalmic examinations and the symptoms reported were self reported. There are several limitations to our study; the cross sectional design limits the inference of causality and can only demonstrates an association between CVS and identified risk factors. Hence, prospective follow up studies among computer office workers without CVS is required to identify risk factors for CVS during subsequent follow up. Not including neck and shoulder pain as a symptom of CVS was also a limitation. It has been considered as an extra-ocular symptom of CVS in many studies and reviews on CVS [14, 20, 21, 23, 30]. Since the study did not involve examination of their practices while they were actually working on their computers self reported measurements like viewing distance and length of time they work may be less accurate.