Understanding the Tear Film
Dry eye syndrome is a disorder that is characterized by a lack of tear production or an unstable tear film. Symptoms can vary in severity and can be debilitating in severe cases. In order to better understand your problem, it is important for you to understand the normal physiology of the eye first. It is difficult to know what abnormal is without knowing what normal is.
The eye that you can see directly consists of a clear window called the cornea. Peripheral to this is a clear vascular membrane called the conjunctiva. Underneath each of these respective layers are the iris (which gives the appearance of blue eyes or brown eyes) and the sclera (the white wall of the eyeball which gives the eye some rigidity). The upper and lower eyelids consist of your eyelashes as well as connections to your lacrimal glands, which secrete the watery component of your tears. You have two general sets of lacrimal glands. One secretes every time you blink your eyes. The other is an emergency supply that only works if your eye is sufficiently irritated and you need reinforcements. If you have ever been poked in the eye before, you understand this principle very clearly.
Each eyelid also has approximately 20-30 meibomian (oil) glands, which sit directly behind the eyelashes. The eyelids have muscle fibers within them that exert pressure of approximately 0.3 PSI every time you blink. In a normal eyelid, this should be sufficient pressure to squeeze the oil out of the meibomian glands.
The tear film is a complex that sits on top of the cornea and conjunctiva and provides oxygen and lubrication to the superficial layers of the eye. This serves a few purposes—not only does it prevent irritation that is associated with dryness, but it is also prevents fluctuations in vision. The reason for the latter is because of the basic optics of vision. Images are coming into the eye and require a smooth surface to help focus clear images onto our retina. When that surface is irregular, image quality suffers—similar to taking a photo with a fingerprint smudge on the lens. The tear film also contains antibacterial enzymes called lysozymes, which help kill bacteria and prevent infection.
Every time you blink a tear is made. Ideally enough of the tear hangs around until the next blink when the tear film is replenished. Some of the tear evaporates, but the rest of it drains through the puncta (hole that drains) of the upper and lower eyelids into the nasolacrimal system. This is the plumbing of the eyelid. The tears drain into the nose and the back of the throat and become absorbed. This is why when you cry, you get a runny nose! A majority of the tears follow gravity and drain though the lower puncta, though some still drains through the upper puncta.
The Tear Film
Now that you have some basic knowledge of the ocular surface anatomy, let’s go into greater detail on the tear film. There are three major layers to the tear film—the lipid layer, the aqueous layer, and the mucin layer. It was originally thought these were distinctly separate layers like the layers of an onion, but now we think they are probably interwoven to some extent.
The lipid layer generally consists of the outer layer of the tear film. Lipids are secreted by the meibomian glands found in each eyelid. More specifically, they are behind where the eyelashes originate. The major function of the lipid layer is to prevent evaporation of the tears by increasing tear film surface tension. Disorders of the eyelids can cause disruptions in the lipid layer.
The aqueous layer is the middle layer and provides water and moisture to the tear film. It is secreted by the lacrimal glands as well as the accessory Krause and Wolfring glands.
The mucin layer is the bottom layer of the tear film. It helps spread the tear film across the cornea and prevents bacteria and debris from sticking to the eye.