Dry eye is marked by symptoms of ocular irritation, fatigue, photophobia and other symptoms resulting from ocular surface damage. The most prevalent form of dry eye is evaporative dry eye (EDE), which is primarily due to a decreased quality and quantity of the tear lipid layer. The tear lipid layer is an important component to the tear film, acting as a barrier against tear evaporation, inhibiting it by approximately 75-90%. Excessive tear-film evaporation leads to increased tear salinity, resulting in long-term phenotypic and pro-inflammatory changes to the cornea. Unfortunately, there has only been limited development in treatment options for EDE, due to low repeatability of clinical test and poor correlation with symptoms seen in current diagnostic tests.
This has served as an impetus to look for new ways to diagnose it, with recent research focused on using infrared thermography (IRT) to assess ocular surface cooling (OSC) as an indirect measure of tear film evaporation. This is based on the theory that when tear evaporation occurs, the phase change from liquid to gas is associated with heat transfer to the surrounding environment, and thus OSC. This was not conclusively demonstrated until our study; more importantly, the study showed that localized areas of OSC represented regions of tear lipid layer breakup, which was important as it is thought to be where the discomfort associated with EDE originates.
Once a greater understanding of what OSC represented was gained, it was important to define the repeatability of it. For the measurement to be clinically significant, a study was designed that determined the inter-day and intra-day profile repeatability, which was found to have a good inter-day repeatability in an EDE cohort. Next, it was determined that IRT appeared to overcame a major issue found with current EDE tests, which is the poor association between signs and symptoms.
Even if tear-film instability reflects the level of irritation on the ocular surface, it is important to recognize that ocular discomfort experienced by an individual is not solely defined by physical disruption to the ocular surface, but by how cognition influences its perception. One of the most important cognitive factors in influencing pain perception is pain sensitivity, as it is linked with analgesic use after surgery and risk of developing chronic pain. Research on this topic has been limited, as measuring pain sensitivity involves determining the level of induced pain that can be tolerated, making it time-intensive, expensive and requires inducing pain in healthy subjects. The Pain Sensitivity Questionnaire (PSQ) remedies this issue, as it is a self-rating instrument that asks individuals to rate the pain they feel they would experience in imagined painful situations. In our work, a higher PSQ score (i.e., greater pain sensitivity) was associated with increased dry eye symptoms and contact lens discomfort.
In conclusion, the work described in the dissertation appears to suggest the viability of a novel diagnostic test for dry eye; IRT is objective, repeatability and seems to be associated with ocular discomfort. Nevertheless, IRT cannot solely be used to understand the connection between signs and symptoms of dry eye but will rely on also recognizing that the inter-individual variability in pain sensitivity plays a significant role in defining that relationship. Significant advances in EDE were made but further understanding of how tear-film instability induces ocular discomfort is needed.
