Dry eye syndrome

Dry eyes is the common name for corneal and conjunctival dryness due to deficient tear production (the cornea is the clear, dome-shaped outer surface of the eye over the pupil; the conjunctiva is the mucous membrane covering the entire front of the eye and inside of the eyelids). It may feel as if there is something gritty in the eye, or that the eye lids are heavy and it is difficult to blink, or there is blurred vision. There are two main types of chronic dry eye:

• Deficiency: triggered by an inadequate production of tear volume
• Evaporative: more common, this condition is triggered by an acceleration of tear evaporation due to poor quality tear production

When insufficient tears are produced, or the composition of the tears changes, it can affect both the health of the eye and vision -- tears on the surface of your eye affect how your eye focuses light and color. Depriving the eye of moisture may lead to significant complications, notably a greater number of infections that result in inflammation of the conjunctiva. Left untreated, dry eye may increase the number of corneal abrasions and ulcerations to the cornea (keratist) that may ultimately lead to deterioration of vision.

Tears are a complex mix of mucus, water, fatty oils and different proteins designed to keep the surface of the eye smooth and protected from environmental pathogens and irritants. Tears are made of three different layers of fluid secreted from three different glands around the eyes:

• Meibomian glands produce an outer, oily layer that reduces evaporation;
• Lacrimal glands produce the middle layer of watery tears and water-soluble proteins that help to nourish the cornea and conjunctiva;
• Goblet cells produce the innermost mucin layer of tears. Glycoprotein binds the water from the middle aqueous layer to ensure the eye remains wet.

A healthy eye produces a continuous bath of basal tears, which nourish the cells of the cornea. Approximately 1.2 ml of tears are produced each day; half a liter each year. A second type of tear is called reflex tears. These are produced in reflex to an irritant in the eye, or exposure to external irritants such as wind, smoke or onions. The third type of tear is emotional tears. These are produced after a powerful emotional stimulation and may have higher amounts of proteins than other tears.

The risk of developing dry eye increases with age and women appear to suffer with the condition more than men.