This study examined the ability of the objective and automated precorneal and prelens NIBUT measurements to determine tear film instability in individuals using CLs. In this study, the previously internationally accepted FBUT cut-off value (less than 10 seconds as tear instability) was taken into consideration in identifying individuals with tear film instability [11, 14]. We determined that the precorneal first and average NIBUT values automatically and objectively measured by the Sirius multifunctional topography device, were able to successfully detect tear instability. However, we found that the reliability and diagnostic ability of prelens first and average NIBUT measurements over CL was very low. As suggested in the literature, the reason for this may be that the device cannot distinguish between the thin areas and break-up areas in the prelens tear film since the prelens tear layer becomes very thin after CL wear . Minor distortions caused by CL may also have limited the device's ability to reliably identify prelens tear film instability.
Bhandari et al. found a good correlation between FBUT and precorneal NIBUT in patients with dry eye and healthy individuals. They determined the precorneal NIBUT cut-off value as 6.2 seconds with 86.1% sensitivity and 81.1% specificity in the diagnosis of dry eye . Best et al. found a significant correlation between FBUT and precorneal NIBUT before CL wear. They also stated that the basal NIBUT value could be used to predict CL drop-out, and those with a NIBUT value below 10 sec should be followed up carefully . Similarly, we found a significant correlation between FBUT and precorneal NIBUT in our study. In addition, we determined the optimum cut-off values for the precorneal first and average NIBUT measurements as 8 seconds and 12.65 seconds, respectively in determining tear instability with high specificity and sensitivity. We consider that studies with larger series are needed to provide more reliable cut-off values. We believe that future technological and software-related developments will reduce variations in the results of conducted studies.
In our study, we found no significant correlation between the prelens NIBUT values and the FBUT or precorneal NIBUT values, and the ROC analysis did not reveal a sufficiently reliable cut-off point for the prelens NIBUT values to predict tear instability. Similar to our study, Llorens-Quintana et al. reported that there was no significant correlation between precorneal NIBUT and prelens NIBUT . Pult et al. and Chui et al. similarly noted that prelens NIBUT was not a reliable predictive method for identifying symptomatic and asymptomatic CL users [21, 22]. Glasson et al. found that the precorneal NIBUT value measured before CL wear showed a significant difference between the patients with and without CL intolerance, while the prelens NIBUT measured six hours after CL did not significantly differ between the two groups . These results show that prelens NIBUT is not sufficient to determine CL intolerance. These studies support the results of our study in terms of demonstrating the low reliability of prelens NIBUT. Interestingly, we detected no correlation between precorneal and prelens NIBUT, which were both measured by the same automated method. We think that the identification of prelens tear kinetics by NIBUT is not as practical and easy as precorneal NIBUT due to the formation of different tear kinetics in the presence of lenses.
It has been reported in the literature that there is a relationship between the distribution and stability of tear film visual acuity . Since NIBUT determines tear instability by evaluating topographic regularity indices, it also measures the anatomical function of the tear, as well as its physiological function. Since irregularities in the mires cause a decrease in image quality in the macula, it directly affects visual function [1, 24]. This feature can help simultaneously evaluate both tear instability and its effect on vision quality using NIBUT in CL wearers with complaints about vision [1, 25]. In our study, we determined that precorneal NIBUT was able to reliably detect tear instability. For this reason, we believe that this technique is useful in evaluating CL candidates with its potential to reveal more detail about the precorneal tear film that traditional clinical methods cannot detect.
To the best our knowledge, this is the first study to evaluate the ability of precorneal and prelens NIBUT to show tear stability with respect to FBUT using the Sirius multifunctional topography device. However, our results need to be confirmed by further studies with a larger sample size.