Professor Roger Beuerman, senior scientific director of the Singapore Eye Research Institute, tells Samantha Boh the science behind how contact lenses interact with the eye
Q Why the interest in contact lenses?
A There are somewhere between 250 million and 300 million people wearing them. Most people wear contact lenses quite comfortably. But a very small 0.0001 per cent, which is still a number of people, run into problems.
One problem we are particularly interested in is that some people get feelings of discomfort, as though they have another condition called dry eyes.
Q How do you know if contact lenses affect the eyes?
A We feel that tears can shed light on this. Across the surface of your cornea is a layer of fluid that contains different types of proteins, and which is only 15 microns thick.
When you look at my eyes or anyone’s eyes, you are looking right through the tears. In a way, it is not even the cornea that influences the light, it is the tear layer. So having a proper tear layer on top of the cornea ends up focusing light very accurately onto your retina.
Some of the problems that you can imagine happening with the tear layer can then happen with the cornea.
We have done studies looking at contact lenses and know oxygen passes pretty well through most contact lenses nowadays. But do the cells of your eye feel anything? And the fact is, they do, they really seem to.
Q How do you study tears?
A We start with two methods of collection. One is the standard clinical procedure called the Type-1 Schirmer’s test using a standardised strip of filter paper. You hook it over the eyelid and you close your eyes and the tears get absorbed onto the piece of filter paper.
The other way we do it is by using a small micro-capillary tube. We collect anywhere from one to five micro- litres of tears through this method. But for people with dry eyes, this is really too much, so we use the filter paper because it is easier for them.
We then dilute the tears a bit and remove any debris or dead cells in the tears. The proteins in the sample are then identified using machines from Sciex – a life science and analytical technologies firm – which are hooked up to different protein databases.
Q How do contact lenses affect the tear layer?
A Contact lenses change the constituents in the tear layer. In a normal eye, there are different constituents coming from different sources.
In people who wear contact lenses, we find there is a bit more of an inflammatory nature to the molecules floating around in the tears. So (this causes) a little bit of irritation. This occurs after wearing the lenses for only about three hours. It varies from person to person but the best way to avoid all this is to use daily disposable lenses.
Q What causes the irritation?
A We can tell that the cells in the cornea showed signs that they were not getting as much oxygen as they were getting before. It is not a change that makes them die, but changes their physiology. What’s the long-term outcome of that? We don’t really know. But we know cells will adapt to conditions. Probably what’s happening is the cells are getting used to the conditions in your eye. They don’t die but they change how they function a little bit to adapt to the new conditions.
Q Can this change eventually cause damage?
A In most people, no, but in a few people it might. It might temporarily change the thickness of the cornea; the cornea might absorb a bit more water but, after a while, it will pump that out. In most people, the cells adapt, and they do fine. Once in a while, there might be somebody whose cells don’t adapt as well. If the cornea gets too thick, it changes the way light bends and focuses on your retina and affects vision.
Q Is there a difference between wearing daily contact lenses and extended-wear contact lenses?
A If you have something in your eye like a contact lens, which is a very thin piece of hydrogel, and you are walking around wearing that same lens every day, you don’t really know what it comes into contact with and what it picks up. There are lots of things floating around in the air, like viruses, bacteria, fungi and spores. Especially here in Southeast Asia, where the weather is wonderful for all types of microorganisms to live. On the other hand, if you change your lenses, you have thrown out all the bad stuff and you have nice clean lenses to wear.
Q Do cleaning solutions for contact lenses help?
A They work pretty well but at the end of the day, you can say we are all human and we are sloppy with that. When we do studies on contact- lens cases, we find a lot of them have bugs growing in them.
Human tears contain water, electrolytes, proteins, lipids and other small molecules.
They act as both a delivery and an excretory route for nutrients and metabolic products of the cornea.
During waking hours, the delivery of tears is continuous, but fresh fluid remains in parts of the eye until after the next blink, when fluid is drawn to form what is called the tear film or tear layer.
The stability of the tear layer is important for maintaining the health of the eye.
With age, the volume of tears and the levels of the major tear proteins in them tend to decline, which result in drier eyes.
Scientists have studied tears, using them as reference points to measure the health of the eye. They have also studied the chemicals in them.
One study, for instance, found that a woman’s tears of sadness may temporarily lower a man’s testosterone level. The researchers from Israel found that men who sniffed the tears became less sexually aroused than when they sniffed saline solution.