What Causes Dry Eye?

In order to understand what causes dry eyes, you need to know a few important things about the structure of the eye and about the composition of tear film.

Tear Composition

There are 2 important tear secretion glands in your eyes, lacrimal glands and meibomian glands, and they each produce different components of the tear film. There are 3 layers to the tear film that coats the eye: oil, water, and mucus. A problem with any of these layers of the tear film can be what causes dry eyes for an individual.

Oil

The outer layer of the tear film is produced by small meibomian glands on the upper and lower inside corners of your eyelids, which contain fatty oils called lipids. The oils coat the tear surface and slow down evaporation of the middle, watery layer. If your oil glands don’t produce enough oil, the watery layer evaporates too quickly, which is what causes dry eyes. Dry eyes are common in people whose meibomian glands are clogged. Meibomian Gland Dysfunction is more common in people with inflammation along the edges of their eyelids (known as blepharitis), with rosacea, and with other skin disorders.

Water

The middle layer of the tear film is mostly water, with a little bit of salt mixed in. This layer, produced by the lacrimal tear glands at the upper outer corner of the eyes, cleanses your eyes and washes away foreign particles and irritants. What causes dry eyes is when the eye produces too little of the watery layer or when the watery portion of the tears evaporates too quickly. Furthermore, if your eye produces inadequate amounts of water, the oil and mucus layers can touch each other and cause a stringy discharge.

Mucus

The inner layer of mucus helps spread tears evenly over the surface of your eyes. If you don’t have enough mucus to cover your eyes, dry spots can form on the front surface of the eye (the cornea). Doctors can see uneven patches using a fluorescent dye.

Causes of Dry Eyes Based on Tear Film Composition

Decreased Tear Production

Inadequate tear production is usually what causes dry eyes. The aqueous tear layer is affected, resulting in aqueous tear deficiency or lacrimal hyposecretion. The lacrimal gland does not produce sufficient tears to keep the entire conjunctiva and cornea covered by a complete layer. This usually occurs in people who are otherwise healthy.

Increased age is associated with decreased tearing; this is the most common type found in postmenopausal women.

Sjogren’s syndrome and other autoimmune diseases are often associated with decreased tear production. This includes rheumatoid arthritis (RA), lupus (SLE), Graves Disorder, graft versus host (GVH) and more.

Drugs such as sedatives, diuretics, antidepressants, anti-hypertensives, oral contraceptives, antihistamines, nasal decongestants, beta-blockers, and pain relievers can cause decrease tear production. Infiltration of the lacrimal glands by tumors, or inflammation of the lacrimal glands can also cause this condition.

Abnormal Tear Composition

Dry Eyes can also be caused by abnormal tear composition, which results in rapid evaporation or premature destruction of the tears. When caused by rapid evaporation, it is termed evaporative dry eyes. In this condition, although the tear gland produces a sufficient amount of tears, the rate of evaporation of the tears is too rapid. The watery portion of the tears evaporates, leaving small amounts of very salty (hypertonic) liquid behind. As a result, the eye cannot keep the conjunctiva and cornea entirely covered with a complete layer of tears, particularly during certain activities and in certain environments.

Additional Causes of Dry Eyes

Dry eyes can also be caused by one’s environment, life style factors, certain physical abnormalities, and infections of the eye. We hope that consulting these individual pages will help you understand more about what causes dry eyes. These categories include:
Dry Eyes and Aging
Dry Eyes and Allergies
Dry Eyes and Computer Vision Syndrome (CVS)
Dry Eyes with Contacts
Dry Eyes and High Altitudes
Dry Eyes and LASIK
Dry Eyes and Menopause
Dry Eyes at Night
Dry Eyes and Stress
Dry Eyes in Winter
Blepharitis
Meibomian Gland Dysfunction
Watery Eyes

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