A 'Dry eye' is a catch-all term used to cover all disorders where the precorneal tear film of the eye is deficient. This thin layer of fluid covers the cornea and the corners (cul-de-sac) of the eye (where the tear ducts lie), and the conjunctiva, the thin mucous membrane that lines the inner surface of the eyelids. The job of this fluid is to nourish the cornea, remove any foreign entities like bacteria and lubricate the eyelids. This helps the eyes to blink which, in turn, helps to spread the tear film over the surface of the eye.
When it isn't due to surgery, dry eye can also result from a problem with the meibomian glands, which secrete the fatty component of tears, or a simple deficiency of the tear film itself. People who don't blink often enough or whose eyes don't spread the tear film efficiently can also suffer from dry eye. It tends to mostly affect women after the menopause, but it can be seen in men or women of any age, and be linked to psoriasis, rheumatoid arthritis or psoriatic arthritis. Unfortunately, it is also a common adverse effect of laser eye surgery.
To combat the problem, medicine has come up with 'ocular' lubricants - artificial tears. These work by bulking up the volume of the tear film. However, they can only do this in contact with the eye surface.
The first generation of these agents were made of cellulose ethers such as methylcellulose, known to be highly viscous. They were of variable effectiveness, so medicine moved on to polymers such as polyvinyl alcohol and polyvinylpyrrolidone. This generation of artificial tears work, but needs to be reapplied too often for comfort.
Consequently, the pharmaceutical companies have now turned to longer-acting gels containing polymers, such as polyacrylic acid. These swell up in water and retain moisture, and relieve the condition for longer than the earlier polymers - without reapplication (Acta Ophthalmol Scand, 1997; 75: 457-61; Eur J Ophthalmol, 1998; 8: 81-9).
But at what cost? These preparations use preservatives like benzalkonium chloride, toxic to the cornea, which keeps the eye moist. Thus, using artificial tears containing this preservative for any length of time is likely to make the problem worse and impair vision over the long term (Am J Ophthalmol, 1988; 105; 670-3).
Although the newer preservatives sodium perboate and polyquaternium are thought to be less dangerous (Curr Eye Res, 1991; 10: 645-56), using any chemical in your eye for any length of time can create further problems. The conjunctiva, which comes in contact with the tear film, is highly permeable - some two to 30 times more permeable to drugs than the cornea (Pharmaceut Res, 1991; 8: 1039-43).
In a sense, when you use artificial tears or any chemical eye solution, you are mainlining chemicals or plastic directly into your eye. And this is exacerbated by artificial-tear solutions. When patients treated with polyvinylpyrrolidone without preservative were compared with those using the same preparation with preservative, the preservative-treated group showed an increased permeability of their eye surface (Arch Ophthalmol, 1992; 99: 873-8).
Another possible treatment is a lubricating lotion containing white paraffin, which melts on contact with a hot object like the eye. However, these tend to make blurred vision worse.
Aside from the dangers, artificial tears are a poor substitute for the real thing. Tears are a complex mix of water, electrolytes and proteins, and contain both antibodies and enzymes to fight off bacteria and infection.