discovering dry eye
How Does a Contact Lens Affect the Tear Film?
BY BARBARA CAFFERY, OD, MS, FAAO
Imagine the beauty of a well-made tear film covering the surface of a healthy eye. The epithelial cells have their busy surfaces filled with microvillae and microplicae. The glycocalyx is being secreted. The goblet cells are discharging the requisite amount of mucous to the surface, smoothing the way for the aqueous. The lacrimal gland is secreting its complex addition to the tear film. Finally, with each blink, the meibomian glands secrete a drop of lipid onto the surface that spreads out to cover and protect the underlying layers.
Then Apply a Contact Lens
What really happens when you introduce a contact lens to the eye? For certain, some of the tear film is trapped under the soft contact lens. How much may be based on the design of the lens, but the literature tells us that very little of the tear film becomes trapped under the contact lens. On application there must be a portion of each layer under the lens. How much lipid, aqueous and mucus is present and in what proportion must be important, as this layer rests closest to the cornea and could influence the sensory nerves and epithelial cell health.
How does this under layer change over time? Researchers have told us that very little tear exchange occurs under a soft contact lens. Does the lipid settle in and remain or slowly leak out from under the lens? Is there exchange of the aqueous layer under the lens, and does the amount diminish or increase over the course of the day? Can the mucus move and exchange? If not, how does this affect the apical cell layers of the covered cornea and conjunctiva?
What lies on top of the lens? It is unlikely that mucus could make its way to the lens surface in the first moments of lens application. Most of it is under the lens. Certainly the bulk of the aqueous layer must be present on the lens surface. Is the lens coated with mucus immediately, or for a time does the aqueous layer lie directly on the lens? Does the lipid layer lie on top of the aqueous, or is it thinned or absent as some researchers suggest?
Effects of Adaptation
As the eye adapts to the presence of the foreign body, is there change? All foreign objects that are introduced into our bodies become coated with a biofilm that is made in part of mucus. Researchers have told us that goblet cells boost mucus production when a contact lens is placed on the eye. This activity is no doubt intended to coat the contact lens and make it more like self. The blink rate of lens wearers changes after lens application. Most novice wearers blink more frequently because of the presence of the contact lens. They usually tear more freely as well, but this response seems to subside with adaptation. The lipid layer must be affected by the blink. Perhaps the increased blink of the novice wearer results in more lipid, and this too would return to normal after adaptation.
The complexity of tear film adaptation to contact lens wear should give us all pause. Successful contact lens wear relies on the ocular surface adapting to the presence of a rather large foreign body. Thank goodness this usually works well.
Dr. Caffery has practiced optometry in Toronto, Canada, in a group setting dedicated to contact lens and tear film research since 1977.