The normal tear film, which not only coats the surface of the eye, but also the inner lining of the upper and lower eyelids, is a complex balanced solution of saltwater, mucous and lipid (fatty molecule). These three components not only provide lubrication to the eye, but they have been elegantly designed by nature to remain on the eye between blinking without falling off onto the cheeks.
Normal Tear Formation - 3 Different Types of Tear Glands
The large Lacrimal Glands are large and hidden below the upper eyelids, against the surface of the eye. They are responsible for "reflex tearing," including tearing when the eyes are exposed to wind, dryness and other irritants, as well as crying. The lacrimal glands put our large quantities of tears on demand.
The Meibomian glands are located in the upper and lower eyelids, and secrete a fatty "lipid" layer consisting mostly of phytosterols which act as a vapor barrier against tear evaporation.
Smaller tear glands are also located in the eyelids and are responsible for basal tear secretion, which is akin to a soaker hose in a garden. These small glands provide constant but slow wetting throughout the day and are critically important to provide all day comfort.
The "Cycling" of Tears
Tears are formed by each of the three types of tear glands. The eyelids then blink, which help to distribute the complex tear formula evenly on the eye surface. When the eyelids open and close, they not only move up and down; when they close, they also pull in toward the nose. This has the effect of "sweeping" the tears into the emptying channels, which are called the lacrimal canaliculi. Tears eventually empty into the nose, which is why we sniffle when we cry.
Thus, fresh tears are created, smoothly painted on the eye by the eyelids, and then swept by the eyelids away to make room for new tears.
What Causes Dry Eye Symptoms?
Contrary to popular belief, dry eye is not always caused by a deficiency of saltwater tears. In some cases, there is poor function of the Meibomian glands. Or the surface conjunctival tissue may produce inadequate mucin. Finally, there can be structural problems of the eyelids that allow tears to evaporate too quickly or not to spread evenly.
Dry Eye is usually found in healthy people, although diseases such as Sjogren's Syndrome, Rheumatoid arthritis and Sarcoid can be causative. Following Lasik, many patients report dry eye symptoms for longer than the typical two month post-operative period. This may be due to severing the sensory corneal nerves during the procedure.
Limitations of Standard Therapies
Many artificial tear drops are blinked away in just a few minutes and provide only very temporary care. Ointments are messy and blur vision.
Punctal plugs can cause irritation from the plugs themselves, or mucus build-up due to poor tear clearance. Prescription eyedrops frequently burn and are not effective for a large percentage of people; plus, they are very expensive. Laser therapies to the skin surface seem to offer only temporary benefit in some people, and are also expensive.