Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Dry Eye Syndrome (DES) is a common eye disorder afflicting millions of individuals worldwide. Recent data gathered from large epidemiological studies suggest that in the United States, the prevalence of dry eye in individuals over 50 years old is nearly 8 percent in women, and nearly 5 percent in men. The prevalence increases with age. In Japan, a country with one of the highest rates of DES, the prevalence of dry eye among the elderly approaches 70 percent. In addition to age, other factors may increase the risk of DES. For example, post–menopausal estrogen therapy has been associated with a 70 percent increase in risk of dry eye. Other risk factors may include chronic diseases such as diabetes mellitus, a diet rich in high fatty acids, and use of certain medications such as steroids.
What happens in the eye to cause or contribute to DES? The normal corneal surface is composed of a superficial lipid layer (0.5 um) secreted chiefly by the meibomian glands, an aqueous layer (7 um) secreted by the lacrimal and accessory glands of Krause and Wolfring, and a mucin layer (0.1 um) secreted by conjunctival goblet cells. The mucin acts as an interface between the hydrophobic epithelium and aqueous tear film. The aqueous (composed of electrolytes, glucose, and anti-infectious agents such as lysozyme, immunoglobulins and lactoferrin) helps maintain the health of the epithelium and provide nutrition to the corneal epithelial surface. The outermost thin lipid layer helps stabilize the aqueous tear film and reduce the rate of evaporation.
Dry eye can be caused by decreased tear production, increased tear evaporation, or through instability of the corneal tear film. Dry eye caused by decreased tear production could be due to Sjogren syndrome (keratoconjunctivitis sicca with dry mouth), and more commonly due to the use of certain medications such as anticholinergic agents or antihistamines. In addition, any disease affecting the lacrimal gland (i.e., infiltrative conditions such as sarcoidosis, or tumors such as benign or malignant mixed tumor) can reduce the baseline production of aqueous. Increased tear evaporation could be caused by chronic diseases such as thyroid eye disease, or could be iatrogenic such as due to cosmetic eyelid surgery. Seventh nerve palsy may cause lagophthalmos, which could also lead to dry eye. In addition, a variety of environments may have less than normal humidity levels – airplanes and air conditioned rooms are notorious for causing dry eye. Finally, dry eye could result from a loss of stability in the corneal tear film. Meibomian gland dysfunction (MGD) is a very common condition associated with rosacea, and inspissated meibomian glands prevent secretion of the superficial lipid layer that would otherwise stabilize the aqueous tear film. Loss of goblet cells due to autoimmune or inflammatory diseases such as Stevens Johnson Syndrome (SJS) or Ocular Cicatricial Pemphigoid (OCP) is another culprit. Chronic irritation through contact lens overuse is another common de-stabilizer of the tear film.
One increasingly common cause of dry eye is staring at computer screens for prolonged periods. Blink rates of individuals using computers are significantly reduced when compared with those engaged in regular conversation or doing non-screen related activities. The decreased blink rates are likely due to centralized, autonomic signals activated during intense concentration and visual fixation. The reduced blink rates lead to dry eye symptoms probably because they cause increased evaporation of the tear film between blinks.
A myriad of therapies are available for dry eye, including tarsorraphy. However, one of the most effective, widely used and first line therapies for DES is artificial tears. The patient can choose from a wide variety of tears sold over the counter. If the patient requires symptomatic relief of dry eye up to 4-5 times a day, then any type of artificial tear can be used. However, for those that require an artificial tear more frequently (i.e. 8 or 10 times a day or even more), then the preservatives in some artificial tears may cause eye irritation; for these patients, most Ophthalmologists will recommend preservative free artificial tears. Gel formulations are also available; they cover the ocular surface for prolonged periods and provide extended relief, but often cause too much blurring to be practical therapies during the daytime. Cutting edge research continues in ocular surface research, and the next several years are sure to bring more innovative therapies for this common disorder.