Dry Eye Syndrome
Dry eye syndrome is one of the most common problems treated by eye specialists in the UK. Despite its name, it often causes excessive watering of the eyes, but it is usually caused by a problem with the quality rather than the quantity of the tear film that lubricates the eyes.
The tear film can be simply thought of as being comprised of 3 layers. The innermost layer is described as a mucin layer which coats the cornea(the eye’s outer clear window) and forms a foundation to keep the tear film in place across the surface of the eye. The middle layer is called theaqueous* layer and this provides moisture, nutrients and oxygen to the cornea, together with antibodies to fight infection. Thisaqueous layer consists of 98% water and is made increasingly viscous (‘sticky) in the deeper parts by the presence of soluble mucins. The outermost layer is known as the lipid layer and is an oily film that seals the tear film onto the eye and helps to prevent evaporation of the underlying aqueous layer.
The three layers of tears are made in a number of different glands around the eye. The aqueous (watery) layer is produced in the lacrimal gland, located under the upper eyelid. Several smaller glands in the lids make thelipid and mucus layers. With each blink, the eyelids spread the tears over the eye. Excess tears flow into two tiny drainage ducts in the corner of the eye by the nose (the tear ducts). These ducts lead to tiny canals that connect to the nasal passage. The connection between the tear ducts and the nasal passage is the reason that crying causes a runny nose.
In addition to lubricating the eye, tears can also be produced as a response to outside stimulus such as an injury or emotion. However, these ‘reflex tears’ do little to soothe a dry eye, which is why someone with watery eyes may still complain of irritation. Like so many things, dry eye syndrome can have many causes. One of the most common reasons for the syndrome is simply the normal ageing process. As we grow older, our bodies produce less oil, about 60% less at age 65 then at age 18. This is more pronounced in women, who tend to have drier skin then men. The oil deficiency also affects the lipid layer of the tear film. With less oil available within the overall bodily system there is less oil to make the lipid layer which seals in the watery layer, the tear film therefore evaporates much faster, leaving dry areas on the cornea, which then become uncomfortable leading to dry eye syndrome.
Many other factors, such as hot, dry or windy climates, high altitudes, air-conditioning and cigarette smoke can add to the problem and some people also find their eyes become irritated when reading or working on a computer. Stopping periodically to rest and blink helps to keep the eyes more comfortable. Contact lens wearers may suffer from dryness, because the contact lenses absorb the tear film, causing proteins to deposit on their surface.
Dryness can also be caused by medications such as antihistamines, nasal decongestants, birth control pills, some medications for high blood pressure, antidepressants and isotretinoin tablets for acne. Thyroid disorders, vitamin A deficiency and Parkinson’s disease (due to decreased blinking) may also cause dryness. Dry eye syndrome can also be a symptom of inflammatory disorders such as rheumatoid arthritis and systemic lupus erythematosus. In these cases referral to an ophthalmologist must be sought, as it can be associated with dryness of the mouth and other mucous membranes; this manifestation is known as Sjogren’s syndrome. Women also frequently experience problems with dry eyes due to hormonal changes as they enter the menopause.