A properly fit contact lens should ride or float on the eye on a thin film of fluid (tear film). With each blink, this film of fluid between the contact lens and the eye is replaced with fresh fluid, allowing debris to be washed away and fresh oxygen that is dissolved in the fluid to reach the cornea. If this film is compromised in quantity or quality, the contact lens may become uncomfortable or even unhealthy to wear.
At each examination, the eye doctor will evaluate the fit of the contact lens and look for signs of early damage to the cornea that may be a result of a compromised tear film. If the contact lens itself is the problem, changing the lens curvature, diameter, or material may correct the problem. Sometimes the chemicals in multipurpose storage solutions for contact lenses can cause changes to the eye's surface that mimic chronic dry eye, and this can be eliminated by switching to a hydrogen-peroxide based cleansing system. Other causes of compromise include exposure to chronic wind and dust, and incomplete or infrequent blinking, such as when staring at a computer for long periods without resting the eyes.
If the doctor suspects that a patient may be suffering from dry eyes, the next step is to determine the cause so treatment can be tailored appropriately. The eye's tear film is actually composed of three separate layers, and a deficiency in any one of the three layers will result in an inadequate tear film.
- The mucin layer. This layer lies closest to the eye and is produced by the cells on the eye's surface. These cells can be damaged by exposure to certain chemicals and medications, heat, or autoimmune disorders. The doctor will ask the patient detailed questions and will place dyes on the eye to assess the level and location of the damaged cells. Treatment may involve eliminating exposure to certain chemicals or prescribing anti-inflammatory eye drops such as Restasis or steroids.
- The aqueous (watery) layer. This layer is produced by the lacrimal glands. Function of the lacrimal glands can diminish with age or certain diseases. Again, the doctor will ask several questions and evaluate the severity of the dryness by placing dyes on the eye. Additional tests such as the Schirmer's test (in which small strips of paper are placed in the corners of the eyes to measure the amount of tears that are being produced) or measurement of the concentrations of substances within the tears may be performed. An inadequate watery layer can be supplemented with artificial tear drops. The watery layer can be made to stay on the eye longer by placing tiny plugs in the eyelid's punctae, the small openings on the eyelids through which tears from the eye drain into the nasolacrimal duct and then into the nose.
- The lipid (oily) layer. The eyelids have a special row of oil glands called meibomian glands that open onto the surface of the eye. With each blink, the oils secreted from these glands coat the surface of the watery layer, preventing the watery layer from evaporating too quickly between blinks. The doctor will examine the openings of these glands and assess whether the oils are flowing freely or are backed up in the glands. Also the tear break-up time (the amount of time it takes for the tears to evaporate from the eye's surface) will be assessed. Thickened or blocked oil can be corrected with warm compresses, gentle massaging of the lids, and daily cleansing of the eyelid margins and eyelashes with mild soaps or baby shampoo. Thickened oils also can be thinned with dietary changes such as consuming more omega-3 fatty acids (found in fatty fish) and medications such as low doses of tetracyclines. Tetracyclines are particularly effective in people who also suffer from rosacea; however, they may not be used in women who are or might become pregnant.
Once the dry eye problem is treated and the quality of the tear film is improved, wearing contact lenses becomes both safer and more comfortable.
Cavanagh, H. Dwight, et al. "Castroviejo Lecture 2009: 40 years in search of the perfect contact lens." Cornea 29.10 (2010): 1075-1085.
Pflugfelder, Stephen C. "Tear Dysfunction and the Cornea: LXVIII Edward Jackson Memorial Lecture." American Journal of Ophthalmology 152.6 (2011): 900-909.e1.