Abstract
Digital device usage has increased substantially in recent years across all age groups, so that extensive daily use for both social and professional purposes is now normal. Digital eye strain (DES), also known as computer vision syndrome, encompasses a range of ocular and visual symptoms, and estimates suggest its prevalence may be 50% or more among computer users. Symptoms fall into two main categories: those linked to accommodative or binocular vision stress, and external symptoms linked to dry eye. Although symptoms are typically transient, they may be frequent and persistent, and have an economic impact when vocational computer users are affected. DES may be identified and measured using one of several available questionnaires, or objective evaluations of parameters such as critical flicker–fusion frequency, blink rate and completeness, accommodative function and pupil characteristics may be used to provide indices of visual fatigue. Correlations between objective and subjective measures are not always apparent. A range of management approaches exist for DES including correction of refractive error and/or presbyopia, management of dry eye, incorporating regular screen breaks and consideration of vergence and accommodative problems. Recently, several authors have explored the putative role of blue light-filtering spectacle lenses on treating DES, with mixed results. Given the high prevalence of DES and near-universal use of digital devices, it is essential that eye care practitioners are able to provide advice and management options based on quality research evidence.
Introduction
Computer vision syndrome (CVS) is characterised by a range of eye and vision-related symptoms and has been a recognised health problem for over 20 yearsThe terms visual fatigue (VF) and digital eye strain (DES) are also used for the condition, reflecting the variety of digital devices linked to potential problems. These expressions may be more appropriate for communication with patients and the public, who may not consider devices such as tablets and smartphones to be computers. Given the massive growth in digital device usage in recent years, many millions of individuals of all ages are at risk of DES. While symptoms are usually transient, the condition can cause significant, frequent, discomfort for sufferers and may have substantial economic consequences when vocational computer users are affected through increased errors and more frequent breaks.
The following review considers new findings in this active field. Recent data and information on usage of digital devices, assessment techniques and management of the condition are presented. Furthermore, in addition to the more commonly described subjective techniques (ie, questionnaires) that are used in this area of research, objectively determined indicators of DES are considered.
Use of digital devices
Across all age groups in developed nations, engagement with digital devices has increased substantially in recent years, particularly in the field of mobile media. A multination European study including England, reported that by 3 years of age, 68% of children regularly use a computer and 54% undertake online activities. In 2016, it was estimated that UK adults typically spend 4 hours 45 min per day using digital media, with a similar pattern in the USA, where approximately two-thirds of adults aged 30–49 years spend five or more hours on digital devices.
In older age groups, use of technology is growing rapidly (figure 1) between 2011 and 2017, the proportion of the population classed as ‘recent internet users’ (within the last 3 months) more than doubled in the 75 years and over age group, and increased from 52.0% to 77.5% in those aged 65–74 years. Recent US data indicate that 37% of adults aged 60 years and over spend five or more hours per day using digital devices, and this age group prefers using laptops and desktops for browsing the internet, whereas younger adults are more likely to use smartphones for this purpose. Use of social media and multitasking is particularly prominent among younger adults with 87% of individuals aged 20–29 years reporting use of two or more digital devices simultaneously...
Symptoms and prevalence
According to the American Optometric Association, the most common symptoms associated with DES are eyestrain, headaches, blurred vision, dry eyes and pain in the neck and shoulders. Asthenopia is the formal term for eye strain, for which two distinct mechanisms and sets of symptoms were described by Sheedy et al. External symptoms of burning, irritation and tearing and dryness were noted to be closely related to dry eye, while internal symptoms of strain, ache and headache behind the eyes were linked to accommodative and/or binocular vision stress. Similarly, Portello et al also identified a clear split of computer-related symptoms into two categories: those associated with accommodation (namely, blurred vision at near, blurred distance vision after computer use and difficulty refocusing from one distance to another) and those that seemed linked to dry eye (irritated/burning eyes, dry eyes, eyestrain, headache, tired eyes, sensitivity to bright lights and eye discomfort).
The prevalence of DES has received attention in the scientific literature for over 20 years. Gauging prevalence is challenging due to the wide variety of usage conditions (both vocationally and socially) and substantial changes in these over time, along with the range of methodologies that have been applied to identify sufferers. Recent data, representative of contemporary devices and usage patterns, indicate that DES is a very common problem affecting many millions of individuals. The 2016 Digital Eye Strain report, which included survey responses from over 10 000 US adults, identified an overall self-reported symptom prevalence of 65%, with females more commonly affected than males (69% vs 60% prevalence). DES was reported more frequently by individuals who used two or more devices simultaneously, compared with those using just one device at a time, with prevalences of 75% and 53%, respectively. The finding of greater computer-related symptoms in females was in agreement with 2012 findings among a cohort of 520 office workers in New York City and may be linked to gender differences in dry eye prevalence. For 10 defined symptoms, Portello et al documented that each symptom had been experienced by over 50% of respondents at least ‘some of the time’ during computer use within the last week, each symptom was reported half of the time or more during computer use over the last week in 17.3%–39.8% of respondents, highlighting the potentially persistent and frequent nature of DES complaints...
Measuring DES
Subjective methods
A variety of instruments have been used to identify DES sufferers and grade the severity of complaints. A 10-item questionnaire created by Hayes et al has been deployed in several subsequent studies and relates to the level of ocular discomfort experienced from the symptoms listed in table 1, enabling calculation of a total symptom score. The six-item Visual Fatigue Scale requires users to respond using a Likert scale to difficulties in seeing, strange feeling around the eyes, eyes feeling tired, feeling numb, having a headache and feeling dizzy looking at the screen. It has been applied to study symptoms following the use of e-readers, indicating that reading from liquid crystal display (LCD) screens (eg, tablet devices) triggers more subjectively reported VF than reading from paper copies or e-ink displays.
The Rasch-based Computer-Vision Symptom Scale (CVSS17) was developed in Spanish to measure visual and ocular symptoms in computer users. An English version of the questionnaire may be obtained, although currently, normal CVSS values dependent on race and other factors are unknown. Seguí and colleagues developed the first validated questionnaire in English to evaluate DES in the workplace. The self-administered Computer Vision Syndrome Questionnaire (CVS-Q) requires users to indicate the frequency and intensity of 16 symptoms experienced during computer use, allowing a single symptom severity score (CVS score) to be deduced, where six points or more is considered diagnostic of the condition. The CVS-Q was used to analyse CVS in contact lens wearers. Questionnaires with verified validity and reliability are useful tools to incorporate into regular patient care and clinical trials linked to ocular and visual health of workers engaged in computer use. Subjective questionnaires have also been used to provide additional validation for objective measures of VF, which are considered in the following section.
Objective evaluation
Although DES affects a huge number of individuals, its precise physiological basis remains unclear. An array of measures of visual function have been used to provide indices of VF: accommodation parameters have received a significant amount of research attention given the accommodative nature of several DES symptoms, while critical flicker–fusion frequency (CFF) and blinking characteristics have been used regularly in recent DES research.
Critical flicker–fusion frequency
CFF is a recognised metric indicative of fatigue and mental workload and is the frequency at which a flickering light is indistinguishable from a steady, non-flickering light. A decline in this parameter has been attributed to reduced activity of the retina and/or optic nerve. Negative changes in CFF following a prolonged computer task were observed to correspond with certain subjective ocular complaints (pain in/around the eyes, eyes feeling heavy and itchy eyes) in a recent study on the effects of short wavelength-blocking spectacles, although not all studies using the measure have established a correlation between worsening of CFF and increased symptoms. CFF may be influenced by task time, with Chi and Lin reporting a significant increase in the sensitivity of this parameter to load differences when task duration was extended. For a 0.4 Hz tracking task, mean post-task CFF reductions were 1.2±1.5 Hz and 2.2±0.7 Hz for 20 min and 60 min durations, respectively.
Blinking and squinting
Blinking aids maintenance of a normal ocular surface, with most blinks instigating a cycle of secretion, dispersal, evaporation and drainage of tears. Reduced blink rate with computer use has been observed in numerous studies and may be relevant to dry eye symptoms that frequently occur with DES. Reductions in blink rate may be substantial, for example, Patel et al reported a mean rate of 18.4 blinks/min before computer use, decreasing to 3.6 blinks/min during operation, while Tsubota and Nakamori observed a mean rate of 22 blinks/min among office workers under relaxed conditions, reducing to seven blinks/min when viewing an electronic display.
Sheedy et al hypothesised that reduced blink rate may be a consequence of involuntary squinting under symptom-producing conditions, with squinting contributing to asthenopia. Two potential benefits arise from squinting: improvement in visual acuity with refractive error and decreasing retinal illumination in the presence of a glare source in the superior visual field. The voluntary squint response, measured using electromyography (EMG) of the orbicularis oculi, influences blink rate significantly, with greater squint levels causing more substantial reduction of blink rate. A later study, exposing participants to various asthenopic conditions during computer use including small font size, low contrast, induced refractive error and glare revealed that conditions that may be improved by squinting (refractive error and glare) showed an increased EMG response from the orbicularis oculi along with an increase in blink rate, while those that would not benefit from squinting (small font size and low contrast) showed no significant EMG response but a reduction in blink rate. Blink inhibition may arise from high cognitive demand or low-legibility conditions necessitating a lengthening of fixation duration and allowing increased time to acquire visual information.
Increased cognitive demand (eg, reading more challenging material) exacerbates the effect of visual stressors such as low contrast or refractive error. Rosenfield et al exposed 16 teenage subjects to texts of two distinct levels of cognitive demand, both on a modern tablet computer and hard copy printed versions. Changing the cognitive demand had a greater impact on blink rate than presentation format. Mean blink rates for the low demand task were 8.34 and 9.06 blinks/min for the tablet and paper presentations, respectively, reducing significantly to 7.43 and 6.67 blinks/min, respectively, for the high demand task. It is possible that technological improvements in digital displays mean that they are more similar to printed materials, so the substantial reductions in blink rate with computer use reported in older studies may not be indicative of modern effects.
Although reduced blink rate may be a less pertinent issue now, many individuals continue to experience signs and symptoms of dry eye associated with digital device usage. Incomplete blinking, where the upper eyelid does not cover the entire corneal surface, may be more relevant to dry eye than blink rate as tear film stability can be maintained with a reduced blink rate, providing that most blinks are complete. Incomplete blinking can result in increased evaporation and tear film break up due to reduced tear film thickness in the inferior corneal region. Argilés et al observed a reduction in blink rate during reading tasks on tablet and computer displays (table 2), as well as hard copy text. However, reading from a hard copy was associated with a significantly lower proportion of incomplete blinks (0%–5%) compared with reading from a tablet (14.5%), expanded computer display (13.5%) or electronic reading (9%; see table 2). The specific influence of digital devices on incomplete blinks is unclear, and further research is needed to address this issue, along with the possible benefits of blink training...
Conclusions
Usage of digital devices for work and social purposes, often for many hours each day, is now normal among individuals of all ages. While there are challenges in determining the prevalence of DES, levels of 50% or more have been reported in numerous published studies, indicating that a large proportion of the population are at risk, and may seek advice and/or treatment linked to the condition. Eye care professionals are well placed to advise on DES, with professional bodies such as the College of Optometrists (UK) and the American Optometric Association providing some guidance for practitioners. To provide optimum patient care related to the condition, it is essential that eye care practitioners are well informed regarding DES and the growing evidence base in this field.
DES symptoms may be split into those linked to dry eye (external symptoms), and internal symptoms related to ametropia, accommodation or vergence problems. Accommodative and/or vergence issues with computer use seem to be a consequence of demanding near work, rather than screen use per se. Nevertheless, such symptoms still require management to promote comfortable viewing. Treatment of dry eye, even mild cases, may have a substantial influence on comfort with screen use.
Questionnaires are of value in evaluating DES as they are simple to implement and capture patient experiences, although validated instruments are rare. In evaluating putative DES treatments, questionnaires have frequently been used alongside objective measures linked to VF, although correlation between objective and subjective assessments has not always been apparent. Regardless of the type of intervention proposed (eg, management of dry eye, correction of refractive error, use of computer spectacles or blue-blocking lenses), persistent effects of treatment should be explored to indicate the true value of DES management options. With the high prevalence of DES and near-universal use of digital devices, it is paramount that the condition is considered by eye care practitioners, and treatment options are supported by available research evidence.