The study evaluated the effect of extended corneal exposure, with resulting corneal drying, on the quality of OCT scans and the quantitative measurements obtained by those scans. Prolonged corneal exposure led to significant reduction both in scan quality and NFL thickness measurements (; , ). The reduction in scan quality led to a substantial decrease in the percentage of scans that qualified for analysis ().
The SNR sharply declined at the early stages of the drying phase, reaching a plateau at 80 seconds (drying time 4) and an incline at 140 seconds (drying time 7; ). However, eyes with qualified scans beyond 130 seconds of corneal dryness showed a continuous reduction in scan quality throughout the drying period (). It is apparent that the upswing in the scan quality in the entire group was to the result of the switch in the composition of the studied population. A near constant number of eyes were qualified until drying time 4, and thereafter a gradual reduction was noted. At drying time 7, a substantial number of scans were not qualified because of the poor quality. Thus, a selection of a subset of participants with tendency of slow drying was analyzed in drying times 7 and 8 with consequent relative improvement in scan quality.
A similar feature was noted for NFL measurements. A rapid deterioration was noted initially until drying time 3, when an increase in thickness appeared (). After drying time 6, a second upswing was noted. In eyes with scans beyond 130 seconds, the first upswing was not apparent, thus demonstrating the same phenomenon as described in the quality parameters. The cause of the late upswing was unclear. Our hypothesis is that, despite the fact that the upper eyelid was taped, we could not prevent the eye movement that might allow for some tear film distribution. Many of the participants reported a blurring of vision toward the end of the drying sessions. It is possible that to minimize this blurring and to best fixate on the target, some participants moved their eye, resulting in a redistribution of the tear film.
Because of the upswing pattern of the graphs, the overall estimate of the deterioration rate as defined by using liner regression underestimates the true slope. Using a model to evaluate the changes at the early drying phase, it seemed that at drying time of 15 seconds the NFL thickness changes are beyond the 95% confidence interval of the reproducibility error of StratusOCT and thus may significantly affect the accuracy of the measurements. Moreover, this rate was calculated based on data obtained from a healthy and relatively young cohort of participants. It can be expected that in elderly patients and with the use of some of the commonly used topical medications, this rate may be accelerated further.
It should be noted that in the presence of dry cornea, OCT analysis tends to report thinner NFL values, which may be perceived as abnormal measurements in glaucomatous eyes even in eyes without actual damage.
When blinking was permitted, a gradual improvement appeared, although a significant difference was still observed in the first 2 scans while participants were allowed to blink freely. Therefore, participants who experience corneal dryness during scanning should be allowed appropriate time for blinking before an accurate scan can be acquired.
There were several limitations to this study. Topical anesthetics may affect the properties of the tear film layer. However, it would have been intolerable and unethical to conduct this experiment without their use. Because there were no significant differences in the outcome parameters between the 2 preanesthetic baseline scans and the postanesthetic scan, it seems that they did not impose a short‐term effect on the optical properties of the tear film layer. Moreover, in clinical practice, many patients are scanned after their intraocular pressure is measured, which includes the use of topical anesthetics. Another limitation of our study is that several eyes did not have measurements from the late drying scans because of the prominent deterioration in scan quality. However, if these censored eyes had been continued for the full drying time, it is likely that our findings would have been stronger.
The duration of the corneal exposure was purposely exaggerated by our experimental design to prove that OCT images could be affected in this manner. The substantial changes that appear after the very short corneal drying time emphasize the importance of this process in the routine clinical practice. Because blinking was able to restore both the image quality parameters and the thickness measurements to values that were close to baseline values, we suggest that interventions aimed to ensure a smooth optical surface, such as instructing patients to blink before scanning or instilling artificial tear drops, could lead to higher quality scans and more accurate clinical measurements. Additionally, because the drying effects were observed to be relatively long lasting, one may consider conducting imaging studies before conducting automated perimetry, during which patients are likely to be exposed to reduced blinking. Although this study was conducted using one specific imaging technology, we believe that these results and conclusions may be applicable to other imaging methods that depend on light passing through the cornea.