A prospective experimental study was conducted on 76 right eyes of 76 volunteers (31 men and 45 women). The exclusion criteria included age younger than 13 years or older than 69 years, ocular disease other than dry-eye syndrome, history of intraocular surgeries including refractive surgeries, refractive error over -6.00 to +6.00 diopters (D), strabismus, nystagmus, and best-corrected visual acuity (BCVA) less than 0.63 on the Snellen chart. No patient was excluded during the screening and testing processes. This study adhered to the Declaration of Helsinki, and the protocol was approved by the institutional review board of Seoul National University Bundang Hospital (B-1510/320-305). Written informed consent to participate in the research protocol was obtained from all subjects.

A total of 76 subjects were divided into two groups according to the degree of TBUT (dry eye <8 seconds, nondry eye ≥8 seconds), and age (older age >50 years old, younger age ≤50 years), respectively. There was no significant difference in the demographics or baseline ocular indices between the dry-eye group and the non-dry-eye group. However, the proportions of female subjects and baseline ocular symptom scores were higher in the older group than in the younger group. Demographics and baseline ocular indices are summarized in Table 1.

In this study, there were changes in mean TBUT and corneal staining scores after watching a 10-minute clip on UHD TV. First, mean TBUT decreased in the non-dry-eye group (9.01 ± 1.10 to 6.63 ± 1.91, p < 0.001), but not in the dry-eye group. After watching UHD TV, mean TBUT was shorter than 8 seconds, which met the criteria for dry-eye syndrome, which suggests that the dryness level on the ocular surface in the non-dry-eye subjects became similar to that of dry-eye subjects after watching UHD TVs. Second, corneal staining scores increased in both old and young subjects, but the differences were larger in the older group (p = 0.038). There was also a greater increase among older subjects in subjective discomfort after viewing compared with younger subjects (p = 0.016). Thus, we suggest that ocular fatigue induced by watching UHD TV, accompanied by surface epithelial defects, could be more severe in older subjects. Third, the decrease in TBUT correlated with an increase in subjective discomfort after watching UHD TV, according to participant responses on the ocular symptom questionnaire, indicating that people with dry-eye syndrome may suffer more subjective ocular fatigue and an increase in corneal epithelial defects after watching UHD TV.

Kwon et al. [8] performed an experimental study on 14 volunteers comparing ocular fatigue after watching 2D and 3D displays. They showed that subjective ocular fatigue (self-reported via questionnaire; score range, 0 to 60) increased by 3.57 after viewing 2D displays and by 7.65 after viewing 3D displays. Moreover, they found that TBUT decreased by 0.43 seconds after viewing both 2D and 3D displays and that conjunctival hyperemia (range, 1 to 10) increased by 0.29 and 0.21, respectively. Suh et al. [12] evaluated ocular fatigue after one hour of computer work in 15 volunteers. They found that ocular fatigue (self-reported via questionnaire; range, 0 to 60) significantly increased by 9.46 after computer work and TBUT decreased by 1.39 seconds; both results were statistically significant. They also found no difference in conjunctival hyperemia. Alhaag and Ramadan [16] reported that watching a 3D display at a 1.95-m distance caused significant visual fatigue, including eye symptoms, blurring, and headache. However, they did not measure objective parameters of the ocular surface such as TBUT or corneal staining. Chen et al. [17] reported that exposure to visual oscillation can cause nausea as well as motion sickness depending on the amplitude and velocity of visual motion.

It is difficult to directly compare these studies with our results due to the differences in viewing duration and in the type of displays employed in the study protocols. Nevertheless, previously reported results are consistent with our findings in that there was an increase in subjective ocular fatigue and a decrease in TBUT after viewing these UHD display systems. A particular strength of our study is the large sample size and our subgroup analyses by dry-eye syndrome and age, which revealed differences between these subgroups.

Mean tear meniscus height and area measured by anterior segment OCT were within normal range at baseline examination [18,19] and there was a positive correlation between tear meniscus height and area (r = 0.766, p <0.001). However, these variables did not change significantly after watching UHD TV in either the total or subgroup analyses. There was also no correlation between ocular indices at baseline measurements. Fukuda et al. [14] reported that tear meniscus height significantly correlated with the Schirmer test score, but not with TBUT, which is consistent with our results. This could be explained by the fact that TBUT reflects tear film stability, rather than tear fluid quantity. We did not perform the Schirmer test because it can cause irritation and reflex tearing, which could affect the measurements. Nevertheless, a possible correlation between tear meniscus height with subjective ocular symptom score cannot be excluded because tear meniscus height significantly correlates with the Schirmer test score [14], which, to date, has been widely used as a main ocular surface indicator (e.g., it is a diagnostic criterium for Sjögren’s syndrome). More reliable measurements could be achieved by measuring multiple points at close time intervals [20]. A comparison with the Schirmer score should be conducted in a future study.

Our study has a few limitations. First, the ocular symptom scores did not significantly change after watching UHD TV in the overall analyses. We used a relatively complex ocular symptom score [9], consisting of 10 questions with answers on a seven-point scale (0 to 6), to maximize the differences between before and after viewing UHD TV . However, several questions may have overlapped (i.e., “sore/aching eyes” and “irritated eyes,” “vision discomfort” and “difficulty focusing”), yielding duplicate responses. Second, 10 minutes of viewing may be too short to induce a measurable ocular discomfort. Seo et al. [11] reported that subjects started to feel definite ocular fatigue after 55 minutes of viewing 2D images. However, we did not identify a significant change in ocular discomfort in this study, despite significant changes in TBUT and corneal staining. This may indicate that objective changes, such as TBUT or corneal staining, may be antecedents to subjective discomfort. Conversely, subgroup analyses revealed that the older group and the dry-eye group showed greater tendencies toward subjective discomfort compared with the comparison groups. Third, we did not exclude systemic conditions that can affect ocular fatigue and dry eye, such as menopause [21]. Ocular fatigue is a complex and vague condition, and numerous factors may affect fatigue indices besides baseline characteristics and systemic conditions. Lastly, the percentage of female subjects and ocular symptom scores were significantly higher in the older group than the younger group. However, because there was no significant correlation between sex and baseline ocular indices, including TBUT and ocular symptom scores, even in the older group, a higher proportion of women may not have biased the results.

In summary, our study showed that objective ocular surface scores were not correlated with subjective ocular symptoms in a clear manner, but that tear break-up time and corneal staining scores changed even after a short period of UHD TV viewing. Hence, we presume that these indices reflect some portion of ocular fatigue after watching UHD TV. Moreover, we showed that the dryness of the ocular surface in non-dry-eye subjects may decline and become equal to that of dry-eye subjects after UHD TV viewing. Subjective ocular discomfort could increase in subjects over 50 years old or in people with dry-eye syndrome. Future studies are needed to further evaluate the criteria for viewing UHD TV, not only in healthy subjects, but also in children and people with specific eye diseases. Moreover, a comparative study is needed to evaluate whether ocular fatigue and surface changes after watching UHD TV differ between 2D or 3D images.

In conclusion, tear break-up time significantly decreases in non-dry eye subjects after watching UHD TV. Subjective ocular discomfort particularly increases in subjects over 50 or with dry-eye syndrome.