Tears are now known to have layers: an outer fatty layer produced by the meibomian, or tarsal, glands at the rim of the eyelids; a middle watery layer from the lacrimal gland in the upper outer corner of each eye; and an inner protein-rich lubricating layer of mucin from the goblet cells of the conjunctiva that covers the whites of the eyes and lines the eyelids. A disruption of any one of these systems can result in dry eye.

The fatty layer keeps tears from evaporating too quickly and helps them cling to the surface of the eye. The watery layer continuously moistens the eye, nourishes the cornea and flushes away toxins and foreign bodies. And the mucin layer inhibits microbial growth and binds water.

Dry eye disease also turns out to have far more possible causes — and, as a result, various specific treatments — than was once thought. As the above description suggests, it is not just a matter of insufficient tears from the lacrimal glands.

Possible causes include defects in the parts of the eye that produce each of the layers in tears; an inflammatory disease like allergy or chronic blepharitis (an inflammation of the eyelids); environmental conditions like tobacco smoke or a dry climate; a hormonal imbalance (as occurs, for example, at menopause); the use of contact lenses; a vitamin deficiency; an underlying systemic disease like diabetes or rheumatoid arthritis; prolonged use of certain medications (diuretics, antihistamines, antidepressants and cholesterol-lowering drugs, among others); and damage to nerves in the eye, as can happen in LASIK eye surgery.

One of the more common chronic causes in older adults is Sjogren’s syndrome, an autoimmune condition that affects moisture-producing tissues throughout the body, including the lacrimal glands, Dr. Bishop said.

Among current possible treatments described by researchers at the Schepens Eye Research Institute and Massachusetts Eye and Ear Infirmary are topical applications of the immunosuppressant cyclosporin A; antibacterial and anti-inflammatory derivatives of the antibiotic tetracycline (like doxycycline); and high doses of essential fatty acids (the omega-3 fatty acids DHA and EPA in fish oil and flaxseed oil, used topically and orally) that inhibit inflammation and are now being tested in a major study funded by the National Eye Institute.

When conventional remedies fail, specialty eye drops can be made using the patient’s own blood serum diluted with saline.