Essentially, Dry Eye Syndrome is caused by three things :
1. Reduced tear production
2. Increased tear evaporation
3. An abnormality in the production of mucus or lipids
Reduced Tear Production
Age, hormonal changes and some medical conditions (Sjogren Syndrome, rheumatoid arthritis or lupus) can cause a decrease in the production of the aqueous layer. Some medications (antihistamines, antidepressants and beta-blockers) may also cause a reduction in tear production.
Increased Tear Evaporation
Tear evaporation is often caused by a reduction in the lipid layer (which protects the aqueaous layer). If you cannot or do not blink often enough, evaporation will also increase. Notably, computer use is a cause of dry eyes because we do not blink enough when staring at the screen. This is also true when watching television or reading. Some medical conditions (Bell's Palsy) may affect the ability to blink and, hence, result in dry eyes.
Abnormalities In Mucus Or Lipid Production
One cause of increased tear evaporation is a reduction in mucus or lipid production. This is an issue in its own right as it can be caused by Blepharitis and other related conditions.
Abnormal production of mucus can result from damage to the eyes (chemical burns, etc) and will stop the tear film from being spread over the eyes. Poor lipid production is often assosicated with Blepharitis and Rosacea.
Dry eye produces discomfort and reduces vision when the tear film becomes chronically table or unavailable thereby leading to dryness of the conjunctiva and cornea. Sometimes, a person with a dry eye will have excess tears running down the cheeks, which may seem confusing. People who have Chronic Dry Eyes are diagnosed by a doctor and usually have a decrease in the amount of tear production. Many people also find their eyes become irritated when reading or working on a computer. Stopping periodically to rest and blink keeps the eyes more comfortable In addition, because these emergency tears tend to arrive too late, the eye needs to regenerate and treatment is necessary.
Dry eyes are caused by a lack of tears. A sad movie or a wedding can make your tears flow. But tears aren't produced only on those occasions. It is usually caused by a problem with the quality of the tear film that lubricates the eyes. Tears are a combination of water, for moisture; oils, for lubrication; mucus, for even spreading; and antibodies and special proteins, for resistance to infection. People with dry eyes often feel discomfort in a variety of forms. Sometimes even if the eyes are dry they may feel as though they 'water'; this is due to poorly spreading tears. When your eyes become irritated by dust or are bothered by wind, smoke or fumes, extra tears form to help wash away the foreign material. In response, the eye is flooded with tears to try to compensate for the underlying dryness. However, these tears are mostly water and do not have the lubricating qualities or the rich composition of normal tears. The eye depends on the flow of tears to provide constant moisture and lubrication to maintain vision and comfort. Dry eye syndrome is a chronic lack of sufficient lubrication and moisture in the eye.