Blurred vision and visual fluctuations are significant complaints in DED patients. Previous studies have found that the visual disturbances in DED are related to the unstable tear film [3-5]. As a component of the refractive system of the eye, the tear film covers the ocular surface and corrects for irregularities in the corneal epithelium [2]. The optical quality of the tear film has been considered as a parameter for evaluation of the stability of the tear film and the severity of DED [15-17]. Similarly to previous studies, our study showed that optical quality parameters (MTF cut-off, mean OSI and SD-OSI) were changed in DED patients and these changes are correlated to DED severity.
When considering the effects of ATs on DED patients, the results of previous studies evaluating quality of vision have been controversial [18-24]. In 1993, Rieger et al. [22] suggested that ATs could improve contrast sensitivity for severe DED patients. Misaki et al. [23] detected changes in visual acuity with different concentrations of ATs and found no significant difference in mild patients’ visual acuity 5 minutes after AT application. However, these two studies only focused on the changes in visual quality a few minutes after AT instillation. Tung et al. [24] presented differing results, with a decrease in visual quality 5 minutes after AT instillation, but they did not analyze different DED severity groups. In the present study, 5 minutes after instillation, the severe and mild dry eye and normal subjects showed differing results. An increase in optical quality was observed in the severe group, there was no change in the mild group, and significant degradation of optical quality was shown in normal subjects. The improvement in optical quality may be related to the severity of tear volume deficiency. In the severe group, in whom the Schirmer I test was less than 5 mm, artificial tears can quickly restore the aqueous layer and the optical role of the tear film on the ocular surface. In the mild group, a trend toward improvement in the SD OSI was observed, but these changes were not significant.
In our study, the OSI mean and SD showed no significant difference at 30 minutes after AT instillation in any of the groups. These findings are consistent with previous studies showing a rapid return of tear film biomarkers to baseline after AT instillation. In a study evaluating the tear meniscus depth and height by optical coherence tomography (OCT), Carracedo et al.[29] found no statistically significant difference in tear volume and stability between baseline and 20 minutes after AT instillation. The same results were observed by Napoli et al.[30]. In their study, 20 minutes after instillation of ATs, the tear film thickness and tear meniscus volume returned to baseline values in all groups. Moreover, in the study by Koh, there was a correlation between wavefront error and tear meniscus height [31].
In our study, the OSI mean and SD values increased after AT application in the control group, which is consistent with previous studies [24, 32]. Our first hypothesis is that the volume of ATs might dilute the original tear film in normal patients and change the biochemical properties such as viscosity or refractive index. This may lead to tear film instability and subsequently to an increased tear meniscus, as revealed by Shizuka et al.[32]. In DED patients, the key optical problem is tear deficiency on the ocular surface. Consequently, it might be replenished by ATs, with resultant improvement in visual quality. Nevertheless, in the present study, after AT instillation, the improved mean OSI in severe DED patients was more obvious than in normal subjects at baseline. Consequently, the hypothesis of the lubricant-only role of ATs might not completely explain this result. Another hypothesis could be the role of osmolarity. The cell membrane of corneal epithelial cells is a semi-permeable membrane allowing fluid transport within the intercellular space depending on particle concentration. With increased evaporation, the tear volume decreases, and the osmolarity of the tear film increases. Previous studies have shown that tear osmolarity in mild and severe DED patients is significantly increased (approximately 315 mOsm/L and 336 mOsm/L, respectively) compared to normal controls (302 mOsm/L) [33]. Most types of ATs for DED treatment have been hypo-osmolar [34]. Similarly, the AT used in the present study (Refresh Plus) has an osmolarity of 294±2 mOsm/L, lower than that of the tear film [34]. Therefore, five minutes after instillation, the ocular surface osmolarity may be equilibrated to a new lower level. In DED patients, the tear volume was replenished and tear film osmolarity decreased to the normal level, leading to an increase in tear film stability and visual quality. The effect of ATs on normal subjects might be different. The osmolarity of the tear film might decrease due to dilution by hypotonic ATs. Following the osmotic pressure gradient, the corneal epithelial cells might become edematous, characterized by swelling of the epithelial cells and distension of the intercellular spaces [35], causing light to be scattered and visual quality to be decreased.
Although there are several limitations to the present study, including the use of only one type of ATs and the lack of characterization of the type of DED (i.e. meibomian gland dysfunction vs. aqueous deficiency), we showed that the effect of ocular lubricants on visual quality may vary according to the severity of DED. Another limitation is interference of the examinations, especially Schirmer’s testing, with the tear film. To minimize the impact of prior testing, we added a control group and examined the effect of artificial tears on the following day.
The objective evaluation of optical quality with the OQAS seems to be an interesting clinical parameter that can be easily associated with and compared to the classic clinical signs used in DED. It may be a promising tool in evaluating the effects of various ATs and possibly individualizing treatment in DED patients, avoiding those preparations which cause the most blurred vision and favoring those which improve quality of vision over a longer period.