387339 A Flow Evaporimeter for Measuring Human-Tear Evaporation Rates
Tear evaporation is critical to healthy functioning of the human eye. As tear aqueous evaporates, aqueous salinity increases. At elevated evaporation rates, increased salinity stimulates afferent cornea epithelial nerves signalling dry-eye symptoms. Fortunately, the covering tear-film lipid layer (TFLL) retards evaporation and thus serves a vital role for a healthy tear system. With an insufficient or inadequate TFLL, however, evaporation can be enough to irritate and even damage epithelial cells. Continuing salt irritation and damage is the consensus origin of evaporative dry eye — an unsolved malady that currently affects 30% of the world population.
Although advances in the understanding of dry eye have been made over the past 15 years in epidemiology, pathogenesis, clinical manifestation, and ocular therapy, one fundamental challenge remains to untangle the hidden link between tear-film evaporation and dry eye: a quantitative and reliable in-vivo measurement of tear-film evaporation on human subjects. Lack of a precise, inexpensive, and easy-to-use diagnostic tool prevents clinicians from employing tear-film evaporation rate as simple diagnostic test for dry eye. Current devices for measuring tear-evaporation rate incompletely account for air flow and relative humidity in the surrounding environment, yielding suspect and inconclusive results.
We developed a modified flow evaporimeter with controlled temperature, relative humidity, and air flow that provides precise and physically meaningful tear-evaporation rates measurement both in vitro and in vivo. The need for air flow with controlled local flow profile in front of the eye is twofold. First, well-defined flow is essential for evaporation-rate measurement because establishing completely stagnant conditions in a chamber directly in front of eye is not possible due to blinking. Second, tear evaporation into the environment exposes the eye to air flow varied with environmental conditions and activities, as in walking and running, to air conditioning, to forced-air heating, etc. Even during sitting, the human eye is exposed to air currents. To understand the relationship between tear evaporation and environmental factors, air flow is requisite.
The prototype Berkeley Flow Evaporimeter is designed with two concentric Plexiglas® cylinders attached perpendicularly to a sealed goggle. The air flow system provides impinging-jet flow (within laminar range) onto the eye at a controlled flow rate, relative humidity, and temperature. Air moistened by tear evaporation then exits through the outer cylinder where its temperature and relative humidity are measured capacitively. To validate the instrument, an in-vitro tear evaporation model was established by fitting the evaporimeter to an artificial eye model made of 99% water agar gel from an anatomically correct, female mannequin (MD-HelenF3, Amazon). The measured water-evaporation rate was first examined by both evaporimetric and gravimetric methods. Results demonstrate the precision of the evaporimeter at 2%, a significant improvement over current evaporimeters. Pure-water evaporation rates measured at various flow rates and relative humidities are in agreement with model-predicted values. We also conducted preliminary in-vivo study on human subjects. Results validate the potential of reliable measurement on human subjects. The measured in-vitro evaporation rate is consistent with those reported in all currently available literature.
Our study indicates considerable promise for the Berkeley Flow Evaporimeter to serve as a reliable and quantitative clinical tool that is comfortable, safe, and simple to use. Since an inexpensive, easy-to-use flow evaporimeter capable of conveniently measuring precise tear-evaporation rates will be attractive to clinicians, we are confident that our flow evaporimeter will be widely adopted to investigate, monitor, and diagnose dry-eye disease. Further, because the physical underpinnings bolstering our flow evaporimeter are compelling, we anticipate revealing how tear-film evaporation rate is affected by the physical properties of the TFLL and how it relates to the other clinical parameters that are commonly obtained during dry-eye diagnosis.