A Novel Method of Measuring Tear Evaporation Rates using Infrared Thermography

A Novel Method of Measuring Tear Evaporation Rates using Infrared Thermography

Andrea Petznick¹, Samantha Sze Yee Lee¹, Jen Hong Tan², U. Rajendra Acharya^2,3,4, Eddie Ng⁵, Louis Tong^1,6,7,8

¹Singapore Eye Research Institute, Singapore

²School of Engineering, Ngee Ann Polytechnic, Singapore

³SIM University, Singapore

⁴Singapore Institute of Technology – University of Glasgow, Singapore

⁵School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore

⁶National Eye Centre, Singapore

⁷Duke-NUS Graduate Medical School, Singapore

⁸Yoon Loo Lin School of Medicine, National University of Singapore

PURPOSE: Dry eye is a very common medical problem and meibomian gland dysfunction (MGD) may be an aggravating factor. There is no single reliable diagnostic tool to determine the presence and severity of dry eye and MGD. The measurement of dynamic tear evaporation rates would allow for a more objective diagnosis. In this study, we present a novel non-invasive method of measuring tear evaporation rates using infrared thermography that is reliable and able to detect changes after heat therapy.

METHODS: Infrared thermographic sequential images of ocular surface temperature were recorded from healthy participants (mean age ± SD: 24±7 years) while remaining in a consulting room with a temperature of 22.08±0.77°C and humidity of 63.82±0.02%. 1) Repeatability of the equipment was tested by taking 2 measurements 20 min apart (n=16). 2) Measurements were taken before and 60 min after 5-min heated eye mask wear (Eyegiene^®, US) (n=10) to test for changes in ocular surface temperature and tear evaporation. Tear evaporation rates were calculated based on ocular surface temperature profiles and controlled for body and room temperature as well as room humidity using principles of thermodynamics. Statistical comparisons were performed using paired t-test with p<0.05.

RESULTS: 1) There were significant correlations between the first and second measurements of ocular surface temperature (r=0.94) and tear evaporation rates (r=0.98) (p<0.01). The coefficients of repeatability were 0.42˚C for ocular surface temperature and 6.78 Wm^-2 for tear evaporation rates. 2) Ocular surface temperature before heat therapy was 33.66±0.53˚C which significantly decreased to 32.50±0.97˚C after heat application (p<0.01). Tear evaporation rates significantly increased from 65.64±12.51 to 78.72±21.53 Wm^-2 following heat treatment (p<0.01). 

CONCLUSIONS: We established that ocular thermography has a good repeatability in measuring ocular surface temperatures and tear evaporation rates under constant conditions and may therefore present a valuable tool for diagnosing dry eye and MGD. Findings also suggest a drop in ocular surface temperature and an increase in tear evaporation 60 min after heating, but it is possible that vasodilation in ocular tissues may have affected calculations of tear evaporation rates after heating.