Excess digital device use is reported to aggravate binocular vision disorders and asthenopic symptoms [1]. Viewing distances and font size parameters in these devices will determine the demand (i.e. accommodation and vergence) on the binocular vision system. Closer the viewing distance, higher is the demand. One could hypothesize, more asthenopic symptoms to be present for closer viewing distance and/or smaller font size. We undertook a study to test this hypothesis.
A prospective cross-sectional study recruited 114 participants (age: 19–50 years, mean age: 26 ± 6, females = 72), employees of our institute, with written informed consent. The study protocol adhered to the tenets of Declaration of Helsinki and was approved by the institutional review board. Participants completed a survey, that was modified from the convergence insufficiency symptoms survey [2]. The sum of the scores on this survey could range from 0 to 39. A total score of 0 indicated no symptoms and a score of 39 is highly symptomatic. Viewing distance and font size (through photodocumentation and image analysis) was measured from the participant’s workstation and their own smartphone at their habitual viewing distance, to evaluate in real-life work settings. Two-step cluster analysis of the symptom score was used to classify the participants as symptomatic and asymptomatic.
Participants with symptom score >8 were identified as symptomatic (n = 30). A significant (independent t test, p < 0.01) difference was observed between asymptomatic and symptomatic participants only for the viewing distance and not for the font size (p > 0.12), in both the devices. Symptomatic participants had a closer viewing distance (Fig. 1) for both computers (56 ± 8 cm vs 62 ± 10 cm) and smartphones (30 ± 6 cm vs 35 ± 7 cm). A good correlation (r = 0.52, p < 0.001) was observed between the two digital devices’ viewing distances (i.e. those who held the smartphone closer, also viewed the computer at a closer distance). While there was a trend (p < 0.001) of higher symptoms score having closer viewing distances (Fig. 2) for both smartphone (Spearman r = −0.34) and computer (Spearman r = −0.35), this correlation was weak. Caution needs to be exercised, since a correlation does not mean causation, conversely, a weak correlation may not rule out a causation either.
Fig. 1. Viewing distance of digital devices.
Boxplots showing the comparison of viewing distance (cm) for smartphone and computer devices between the asymptomatic and symptomatic participants.
Fig. 2. Viewing distance and symptoms score.
Correlation between symptoms score and viewing distance of (a) smartphone and (b) computers. Note the y-axis scale is different between the two devices.
In agreement to the hypothesis, symptomatic participants had a closer viewing distance, however there was no correlation to the font size viewed. Greater eyestrain has been reported when viewing distance was reduced for visual display units [3]. Preference for a longer viewing distance of 60 cm or beyond has been reported earlier when using visual display units [4]. Viewing distance is found to reduce and eyestrain to increase upon prolonged smartphone use [5].
Beyond checking for dry eye symptoms, and advising on 20-20-20 rule, a recommendation to increase the viewing distance can also be made to reduce digital eye strain. It is particularly easy to measure the smartphone viewing distance of a patient and if found closer (<35 cm), a recommendation can be made to increase the distance for both smartphone and computer (>62 cm), since they are correlated measures.
Acknowledgements
Ms. Anshita Sodhi for her help with data collection. Ms. Shakthi Pradheepa for formatting help.
Author contributions
PNS conceived the study question, design, data analysis and writing. SR contributed to study design, data collection, analysis and reviewing the manuscript.
Funding
Hyderabad Eye Research Foundation for support to author PNS.
Data availability
Data will be provided upon request.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Data Availability Statement
Data will be provided upon request.