In the current study, we found that lower tear meniscus dimensions significantly decreased and blink frequency significantly increased in SBUT dry eye after exposure to airflow of 1.5
m/s. In normal eyes, there was an increase in lower tear meniscus dimensions and little change in blink frequency.
Assuming that airflow exposure would make the tear film more prone to evaporation, a decrease in lower tear meniscus dimensions with the airflow exposure was expected in both groups. However, in the current study, it was found only in SBUT dry eyes, and no such pattern was found in normal eyes. According to the literature [19
], in the initial stages of dry eye, it is considered that ocular surface damage results in reflex stimulation of the lacrimal gland. Reflex trigeminal activity is responsible for increased blink rate and compensatory lacrimal secretion. SBUT dry eye is a type of evaporative dry eye in which normal lacrimal tear secretion maintains normal tear volume, but tear stability is impaired. We speculate that, in normal eyes, the increase in tear meniscus volume caused by tear secretion through the reflex sensory loop [20
] compensates adequately for the changes in tear film induced by airflow exposure, resulting in a net increase in tear meniscus dimensions. In contrast, a decrease in tear meniscus dimensions was seen in 8 out of the 9 SBUT dry eyes. In SBUT dry eye, we speculate that greater evaporation occurs in the setting of an unstable tear film by overriding the normal reflex tearing response, which results in a net reduction of tear volume in SBUT dry eyes compared with normal eyes, even though evaporation was not directly measured. Although we did not specifically evaluate the relationship between subjective symptoms and airflow, most of the SBUT dry eye subjects reported ocular discomfort with airflow exposure. Based on the results in the current study, some practical lifestyle modification advice for the SBUT dry eye patients are possible. Airflow control such as redirecting vents when using the air conditioner or using moisture chamber glasses [21
] would be helpful in terms of keeping in moisture and reducing evaporation of tears by limiting airflow over the eyes.
We are assuming that normal eyes and SBUT dry eyes have nonpathologic reflex lacrimal production, based on the normal Schirmer I test conducted at screening. However, there may be a difference in the afferent arm of the reflex tearing response between the normal eyes and SBUT dry eyes. Normal eyes may be more efficient than SBUT dry eyes in producing reflex tearing to balance the increased evaporation observed in the setting of airflow exposure. It would be helpful to clarify the reflex tearing response in SBUT dry eye and aqueous tear-deficient dry eye in future studies.
Airflow exposure and evaporation of the tear film may stimulate the corneal blink reflex, resulting in greater blink frequency in SBUT dry eyes than in normal eyes. With regard to blink frequency in normal subjects, our findings are inconsistent with a previous study [6
] that reported an increase in blink frequency with airflow exposure in healthy eyes. The disagreement between the results in the previous study [6
] and the present study can be explained partly by the use of different measurement conditions. In the current study, the subjects were instructed to gaze at the built-in target of the OCT and blink freely, whereas, in the previous study, subjects were not asked to gaze at a target.
Previously, we used simultaneous measurements of ocular aberrations and lower tear meniscus dimensions to demonstrate that baseline tear meniscus just before the blink correlates with the initial postblink optical quality, especially in SBUT dry eye [14
]. Although the blink rate used in the current study (involuntary blinking) was different from the blink rate used in the previous study (voluntary blinking every 6 seconds) [14
], we can hypothesize that some SBUT dry eyes after airflow exposure would show greater degradation in optical quality during the initial postblink period because the tear meniscus dimensions are smaller. In the setting of an office environment, it is important to understand the relationship between airflow and ocular symptoms. The controlled-environment chamber allows independent modification of temperature, humidity, and airflow, facilitating the investigation of tear dynamics under many variations of everyday environmental conditions. Further studies that have greater sample size and include aqueous tear-deficient dry eye patients, using a controlled-environment chamber, will help to investigate the effect of different environmental conditions and further characterize the response of the tear film to different environmental stressors.
There are some limitations to this study. The images captured by our commercial time domain anterior segment OCT had limited resolution, so there was difficulty in detecting the tear film boundary in some images where the cross-points of the tear film, eyelid, and cornea were faint [14
]. It will be important to use higher resolution OCT to make precise measurements of the tear film thickness. In the current study, we did not examine the repeatability of tear meniscus measurement with OCT. Although previous papers have showed the good repeatability in measuring the dimension of the tear meniscus [22
], we may need to know the repeatability with the OCT used in this study. The enrolled SBUT dry eyes were different from “typical dry eye” of aqueous tear-deficient dry eye, in a precise sense. [15
] There are many borderline cases that fall between evaporative dry eyes and healthy eyes, in which short TBUT and dry eye symptoms are found without ocular surface damage and tear deficiency. Moreover, the correlation between subjective symptoms and airflow-induced tear meniscus change would have been of help.
In conclusion, lower tear meniscus dimensions were observed to decrease and blink frequency was observed to increase in SBUT dry eye after exposure of 1.5
m/s airflow, while, in normal eyes, lower tear meniscus dimensions were observed to increase. Airflow exposure results in a decrease of tear meniscus dimensions in SBUT dry eyes, most likely due to the greater susceptibility of the tear film to evaporate in SBUT dry eye. Measurement of the tear meniscus with anterior segment OCT is useful as a noninvasive and objective method for evaluating tear film dynamics.