4.4 Dry eye

The following is played over and over in many offices around the world:

Doctor, at the slit-lamp: "Mrs Smith, the reason why your eyes are sore is because you have very dry eyes."
Mrs Smith, moves back, in disbelief: "How can that be, I tear very easily, all the time, too..."

They are both right, of course. The doctor is summarizing what he/she gleaned from symptoms and signs. The patient, on the other hand, is describing reflex-tearing caused by irritation from dry eyes.

The tear fluid is actually composed of more than just water. It has three layers, a very thin top layer of lipids, a much thicker layer of water with all sorts of solutes, and a thinner layer of mucus that interacts directly with the corneal epithelium.

Imagine a very shallow lake, with in- and out-flows, sitting on top of your cornea. If the water source slows to a trickle, then the lake slowly dries up. You can partially plug up the drain and maintain a certain water level. This is an over-simplification of course; although in clinical practice, the plugs are known as punctal plugs. And indeed if the lacrimal gland is inflammed/damaged, then the production of tears is reduced.

When the eye is chronically dry, the surface structures tend to weaken. Often you'll see subconjunctival hemorrhage. A patch of bright red spot next to the cornea (see iamge below).
In other words, the blood vessels have become so fragile, and with a little stress, e.g., contact lens insertion, sneezing, strained bowel movement, one of the blood vessels will break open and the blood leaks into the adjacent conjunctival tissue. It usually resolves on its own in a week or so. Of course there are other underlying causes for this type of hemorrhage such as blood diseases, blood-thinning medication, etc. Differentiation is needed.

So what causes dry eye? Other than a reduction in tear production, if the top lipid layer becomes so thin as to be broken, then water evaporates very quickly. This happens when there is a malfunction in lipid-generation from glands such as the Meibomian glands. This is part of the inflammation of the lid margin (aka blepharitis). This condition is further aggravated if you work in a dry environment and stare at the computer all day long.

Race seems a big factor as well as most Asians suffer from dry eye. Their tear break-up time (around 4-5 sec) is much shorter than that of the Caucasians (around 20 sec).

Some medicine such as birth control pills, levoxyl, high blood pressure pills etc can also cause dry eyes. The major ones are from (1) contact lens wear and (2) Sjogren's syndrome. Contact lens disrupts the integrity of the tear film. Sjogren's is an auto-immune disease (confirmation through blood tests for ANA and rhrumatoid factor), seen mostly in menopausal women, with dry eyes, dry mouth, and often arthritis.

The diagnosis of dry eye can be simply from patient history, supplemented with fluorescein and/or lissamine green staining - for signs of epithelial defects. In severe cases, measurements of tear break-up time, tear film integrity, and tear production rates are also done.

The treatment of dry eye has made quite a bit of progress, especially the type that responds to anti-inflammatory therapy. In this type, a pulse dosing of topical steroid is tested first. If effective, then a long-term treatment with topical soft steroids, oral omega-3 fatty acids, or cyclosporine eyedrops should follow. And when needed, artificial tears are also used.