Dry eye syndrome is a term used to describe a variety of dry eye conditions. These range from occasional dry/gritty eye sensations caused by modern living (such as tear evaporation caused by PC use in air conditioned environments) to the more severe and chronic condition of dry eye disease.
Main causes of Dry Eye
Dry eye symptoms have 2 fundamental causes - a reduction in either the quantity or quality of the tear film or both; in other words dry eyes are caused:
- By deficiencies in the production in the watery components of the tears - known as tear deficient dry eye (quantity)
- Where enough tears are produced, but tear evaporation causes dry eye signs and symptoms – known as tear sufficient or evaporative dry eye (quality)
The Tears
The tears form a film (the tear film) that is composed of a layer of water, oils and nutrients that flow over the exposed surfaces of the eye. Tears serve several purposes:
- Lubricate the cornea and eyelids, preventing dehydration while enabling the eyelid to slide smoothly over the eyes surface.
- The tear film provides nourishment in the form of oxygen, glucose, salts and minerals to the cornea.
- Provide a smooth optical surface for light rays entering the eye.
- Foreign particles are suspended in the tear film and are flushed from the eye.
- Harmful bacteria on the eye are attacked by anti-bacterial enzymes in the tears.
Up to 25% of the total tear volume is lost via evaporation from the surface of the eye. Large volumes of tears are only produced when required, such as when a foreign body is caught in the tear film and must be removed quickly, or when a person cries.
When the eye is irritated, the rate of tear production can be increased by as much as 300% and will generally cause the tear film to spill over the lower lid. The production of tears is affected by many factors such as hormonal changes, old age and medicines (first generation antihistamines, arthritic drugs and some antidepressants).
The tear film is composed of three layers that combine to allow full coverage of the ocular surfaces and for clear vision and eye comfort. The 3 layers are:
Lipid (Oily) layer - composed of an oily film, which lubricates the outer surface of the eye. This oily film helps prevent evaporation of the underlying aqueous layer while providing a smooth optical surface. It is secreted by the meibomian glands which are located along the edges of the eyelid.
Aqueous layer – consists of about 98% water, provides the cornea with oxygen and nutrients, and makes up about 90% of the tear film’s thickness.
Mucin layer (third layer, very thin) - covers the surface of the cornea and the conjunctiva. It is secreted by specific cells of the conjunctiva called goblet cells and provides stability to the tear film by helping to ‘stick’ the watery tears to naturally hydrophobic (water-hating) ocular surface.