Developing tools to better understand dry eye

By Karen Kawawada

If your eyes have ever watered in the wind or felt itchy after too long staring at a screen, you have some idea of what it’s like to have dry eye. For about 30 per cent of Canadians, dry eye is a chronic condition in which the eyes often feel dry, gritty, itchy, stinging – or worse. For some patients, severe dry eye can be so debilitating they’re in constant pain and they cannot see well.

Despite the prevalence of dry eye disease, scientists don’t adequately understand many aspects of it, such as how fast tears evaporate, how to accurately measure the volume of tears a patient has, or how people perceive sensations on the surface of the eye.

Dr. Paul Murphy, a professor at the University of Waterloo School of Optometry and Vision Science, has researched the ocular surface for years. Now, with a five-year Discovery Horizons grant from the Natural Sciences and Engineering Research Council of Canada, he and several colleagues are working on an ambitious set of interdisciplinary studies they hope will lead to answers to these fundamental questions.

Tears aren’t just water. The tear film is composed of three layers – an inner mucous layer that helps the tear film stick to the surface of the eye, a middle watery layer, and a top lipid layer that helps keeps tears from evaporating too quickly. Dry eye is often caused by the lipid layer not functioning well, resulting in too high a rate of tear evaporation.

“Whenever the eyes are open, evaporation is occurring,” says Murphy. “If you can’t control that evaporation well, you start losing tears and you experience discomfort because the evaporation is stimulating the nerves.”

With Dr. Alexander Wong, a professor and systems design engineer, Murphy is working on a device mounted to swim goggles that measures evaporation from both eyes at once. Not only does this allow scientists to compare evaporation in each eye, the system uses artificial intelligence processing to automate the measurement of tear evaporation rate and lipid layer thickness.

With Wong and postdoctoral fellow Dr. Ehsan Zare Bidaki, Murphy has developed a system to measure the surface temperature of the cornea using a novel two-camera system – a visual camera and a thermal camera.

“When you’re sweating, you lose temperature, because evaporation cools you down,” says Murphy. “This is based on the same idea – if you can look at changes in the temperature of the eye, you can better understand tear evaporation and ultimately the health of the eye.”

When we blink, our eyelids help form a smooth tear film over the eye surface. Between blinks, the tear film breaks down, but until now, it's been hard to measure tear break-up without using a dye that negatively affects tear stability.

With their new imaging system, the researchers aim to measure localized changes in eye surface temperature to assess tear film thinning. They’ve also spun out a company, ThermOcular AI, that aims to use their imaging system to screen patients for dry eye disease.

As anyone who’s seen water curve up a glass knows, water forms a meniscus due to surface tension. This also applies to tears. Where the tear film meets the eyelids, a curved meniscus forms.

“You have this curved surface at the junction of the eyelids, and if you have lots of tears, the meniscus is fattened up and the curvature of the surface of the meniscus is different than if you have fewer tears,” says Murphy.

Traditional methods of measuring tear volume involve placing a thread or paper on the eye to see how much fluid they soak up, but these invasive methods provoke tearing. An existing non-invasive method measures tear meniscus height – but accurately locating the top of the meniscus is difficult and small errors can produce significant differences in the estimated volume of tears.

With Wong, Murphy is working on a new method to measure tear fluid volume by examining the curvature of the meniscus.

“The idea is to use reflection,” says Murphy. “We’ll have a target that we know the shape and size of and we’ll reflect that off the curved surface of the meniscus like a mirror. Just as funhouse mirrors distort your shape, the curvature of the meniscus will distort the shape of the reflection. We can measure the size of the new, distorted shape, compare it with the original one, and work out the curvature of the meniscus.”

The front surface of the eye contains a dense network of nerves, which is why our eyes are so sensitive to pain. Understanding how the nerves on the ocular surface process information together is essential to gaining a better understanding of dry eye disease.

With professors Wong, Dr. Ben Thompson, a psychologist and vision scientist, and Dr. Sean Peterson, an engineer and fluid flow expert, Murphy is developing a device that can use up to three non-invasive stimuli at once to investigate the way people experience them.

The stimuli in question are puffs of air delivered to the eyes, whether multiple puffs at once from different directions or single puffs delivered in quick succession.  

“If I present two stimuli at the same time, is that twice as strong? Or if I present two stimuli separated by a period of time, do you feel them as two separate stimuli or do you feel them as one? That’s what we’re investigating as a way of understanding the way the nerves on your cornea process sensation,” says Murphy.

The devices Murphy and his colleagues are developing and the studies on human participants they’re planning are designed to answer important, fundamental questions they hope will lead to treatments for dry eye.

“We’re trying to understand how the tear film volume and tear film evaporation influence dry eye,” says Murphy. “If we can understand those better, we’ll be better able to treat the problem.”