3.1 Baseline
None of our data of age, TMH-t0, and Ave.BUT-t0 shows any statistical significance (p > 0.05) between the computer group and the control group, indicating the baselines of the two groups are consistent and comparable (table 1-2). Table 3 shows the meibomian gland scores of the computer group and the control group. No statistical difference was found between the computer group and the control group, indicating the conditions of meibomian gland in the two groups have consistent baseline.
3.2 Computer Uses Reduced Tear Meniscus Height
The use of computer negatively affected the tear meniscus height. Figure 1 shows the changes of TMH (ΔTMH) for both t=60 min and t=120 min group, and the results and the relevant statistical parameters were summarized in Table 4. ΔTMH was defined as the TMH at respective time point subtracted by the TMH-t0. The ΔTMH at both time point were statistically more negative than their controls (Fig. 1), meaning the tear films of the computer group were getting thin at both t60 and t120 (Fig. 1). The control group shows a just positive ΔTMH at t60 while the computer group shows a negative (Fig. 1a) one, which further evidenced its thinning of tear film (Fig. 1a). In addition, the change of TMH was also dependent on use time of computer. Figure 2 shows the time-dependent TMH for the computer group and the control group. The results and the relevant statistical parameters were summarized in Table 5. The computer groups presented a trend of decreasing TMH from t0 to t120, with statistical difference between t0 and t60, and t0 and t120 (Fig. 2). In contrast, the control group did not show any trend and no statistical difference was found (Fig 2). Collectively, the evidences in Fig. 1 and 2 show the computer has caused negative influences on eye surfaces and reduced the TMH after 60-min working using computers. The TMH reflects the amount of tear, and the reduction of it will compromise its function of wetting and nutrient supplying. This reduction could lead to the damage on the microenvironments and epithelial cells on eye surface over long term and may eventually progress into dry eye. Interestingly, from t60 to t120, the change of TMH is not statistically significant (Fig. 2). This means the negative effects of computer use on TMH were most severe in the first 60-min use of computer but made no further reduction from 60 min to 120 min. The negative effects on the 60min-120min period could have been smaller than 0min-60min period and it could have a peak effect at 60 min.
There are several explanations for the abovementioned negative effects on TMH. 1) The reduced eyeblink. Eyeblink is a simple action but it wets the eye surface, and it also increases the secretion of lipid and tear from the meibomian gland and their spreading on eye surface. The eyeblink was reduced from 10-15 times per minute to 5-6 times per minute after using VDT7. With a reduced eyeblink, the exposure of eye surface to the surroundings may have increased and so did the evaporation of tear, which possibly resulted in a lower TMH. 2) Increased exposure area of eye surface. The user of computer mostly looks at the monitor horizontally, which exposes a bigger area of eyeball than tilted view angle. 3) Dry environment. The computer uses often happened in a dry indoor environment. Liang et al.8 suggested the environmental factors tend to cause the evaporation-type dry eye. When humidity and temperature both increased, the tear evaporation lessened9, 10, while a in dry and cold conditions the tear evaporation increased. The trend was also supported by a study11 in animal model that the controlled dry environment can induce insufficient tear secretion of rat. The absence of statistical significance of TMH among the control group (Fig. 2) indicates a healthy subject can resist the influence of dry environment under normal uses of computer. Lastly, the slower decrease of TMH in 60-120 min period than 0-60 min period (Fig. 2). The long-time uses at 60-120 min could have caused discomfort to the eye, and hence the users blinked more frequently as a response to the discomfort, leading to a more replenished tear film.
3.3 The Effect of Computer Use on BUT
Tear break up time (BUT) was also measured in this study. Figure 3 shows representative dynamic change diagrams of tear film obtained using Keratograph D analyzer. The instrument separated cornea into 24 sections, and 8 squares for each section. The color indicated the position and time of the tear film breakage, green for longer BUT and red for shorter BUT. From Fig. 3a to 3d, the increased computer usage apparently shortened BUT. An averaged BUT for each square was defined as Ave.BUT; each eye has one Ave.BUT reflecting the overall time of tear breakup. The change of average BUT (Δ Ave.BUT), defined as Ave.BUT at specific time subtracted by Ave.BUT at t0. Figure 4 shows the Δ Ave.BUT of computer group and control group at different time points. The results and the relevant statistical parameters were summarized in Table 6. Our observation did not find negative impact of computer use on Ave.BUT until t=120min. Specifically, the Δ Ave.BUT at t60 of computer group is not statistically different from the control group, whereas at t120, the computer group is statistically more negative than the control group, indicating a significantly reduced Ave.BUT caused by the 120-min use of computer. Time-dependent Ave.BUT also reflects the time-dependent change of tear film health. Figure 5 shows the Δ Ave.BUT at different time points for the computer and control group. It reveals the trend of Ave.BUT change over time. The results and the relevant statistical parameters were summarized in Table 7. In the computer group, a decreasing trend of Ave.BUT can be seen from t0 to t120 (Fig. 5). The Ave.BUT at t60 and t120 are both statistically lower than t0. In contrast, the control group does not show any trend, and no statistical difference was found among any time point. Apparently, working using computer will start to decrease the Ave.BUT at 60 min of working (or earlier) and can last till at least 120 min of working.
The Ave.BUT is an indicator of the stability of the tear film. Our data in Fig 3-5, collected using noninvasive Keratograph D analyzer instrument, suggests that the use of computer adversely affected the stability of the tear film. This adversary effects of computer use on Ave.BUT is similar to that on TMH (Fig. 3.2-3.3). The use of computer could have led to the reduction of the frequency of blink and the secretion of tears, resulting in a deterred replenishment of tears and hence decreased both TMH and Ave.BUT. The deterioration of TMH and Ave.BUT came in close association because they are both the result of the insufficient replenishment of tears. Another possible reason is the reduction of mucoprotein. A study12 shows the users of VDT suffered from a reduced mucoprotein, a signal of secretion disorder and the break of dynamic balance in the tear film. It possibly caused the decrease of Ave.BUT as well. The third possible explanation is the duration of VDT use. The Ave.BUT of 230 children was found to statistically reduce with an increasing VDT use. Lastly, the reduction of TMH may have contributed to the decrease of Ave.BUT. The tear film contains a middle layer of aqueous tear; the reduction of overall height of TMH will lead to a lower tear storage and thus result in a shorter Ave.BUT.
3.4 Limitations
The subject sample size of this study is not sufficiently large to reflect the general disciplines. It should only be used to preliminarily estimate the effects of computer uses on the eye surfaces of the nurses working in our hospital. The size should be increased to minimize the error. Secondly, the present study investigated subjects among nurses, an occupation undertaken by more female than male, and hence lacked male subjects. Also, the time points in this study are not enough. More time points should be investigated to reflect the trend before the 60-min working and after the 120-min working using computer.