Dry eye syndrome can have many different causes, including:
- environmental factors, such as a hot or windy climate
- illnesses
- side effects of medicines
- hormonal changes
- ageing
However, in many cases of dry eye syndrome there is no single identifiable cause.
The importance of tears
You often only notice your tears when something happens that makes you laugh or cry. However, your eyes are always covered by a thin layer of liquid, known as a tear film (see box to the left).
Tears are produced and regulated by a system known as the lacrimal functional unit.
Lacrimal functional unit
The lacrimal functional unit describes the system of different parts of the body that work together to regulate the production of tears.
The lacrimal functional unit is made up of a number of different parts, which are described below.
- The lacrimal gland produces a liquid substance that makes up the majority of your tears.
- The Meibomian glands produce a fatty liquid that makes up the outer layer of the tear film.
- The eyelid spreads tears across the surface of your eye when you blink.
- The cornea is the clear layer at the front of your eye that contains smaller glands, which also produce some of the liquid that makes up your tears.
- The tear ducts are two small channels at the side of your eye that allow your tears to drain into your nose.
If any part of the lacrimal functional unit is affected, the whole system can break down, resulting in one of two outcomes, or possibly both:
- the quantity of tears is affected - the body does not produce enough.
- the quality of the tears is affected - the tears evaporate before the body has a chance to replace them.
Either outcome can cause dry eye syndrome.
Also, as the eyes are no longer adequately protected by the tear film, the immune system (the body's defence system) tries to compensate by sending special, infection-fighting cells to the eye. It is these cells that cause the inflammation (redness and swelling) of the eye, which is associated with more serious cases of dry eye syndrome.
Risk factors
A number of factors can interfere with the lacrimal functional unit. These are explained in more detail below.
Hormones and the nervous system
Hormones (powerful chemicals that are produced by the body) and the nervous system (your nerves, brain and spinal cord) play an important part in tear production.
Hormones stimulate the production of tears. The changes that occur in hormone levels during the menopause (when a woman’s periods stop) may explain why older women are more susceptible to dry eye syndrome.
The nervous system can trigger an increase in tear production, often as a way of protecting your eyes from potentially harmful substances. This is why, for example, your eyes water if you are exposed to smoke.
Environment
Environmental factors can have a drying effect on your eyes, causing your tears to evaporate. These factors include:
- sun
- wind
- dry climate
- hot blowing air
- high altitude
Regular exposure to these environmental factors can cause dry eye syndrome. People are particularly at risk when they are exposed to a combination of environmental factors. For example, the combination of a dry climate and warm air conditioning, found in many offices, can cause dry eye syndrome in some people.
Occupation
When you are carrying out an activity that requires visual concentration, such as reading, writing or working with a computer, you tend to blink less frequently. This can cause your tears to evaporate and lead to dry eye syndrome.
Medicines
Several medicines are thought to cause dry eye syndrome in some people. These include:
- antihistamines
- antidepressants
- beta-blockers
- diuretics
Beta-blockers and diuretics are often used to treat high blood pressure (hypertension).
Laser surgery
Some people who have had laser-assisted in-situ keratomileuis (LASIK) surgery find that they have dry eye syndrome in the weeks after surgery. However, the symptoms usually clear up after a few months.
Contact lenses
Sometimes, contact lenses irritate the eye and cause dry eye syndrome. Changing to a different type of lens, or limiting their use, usually help to resolve the symptoms.
Medical conditions
Most people with dry eye syndrome also have blepharitis (inflammation of the rims of the eyelids). Blepharitis is commonly associated with:
- seborrhoeic dermatitis - a common skin condition that is often referred to as seborrhoeic eczema
- rosacea - a common yet poorly understood chronic (long-term) skin condition that mainly affects the face
Other medical conditions that can cause dry eye syndrome in some people include:
- contact dermatitis (eczema) - a condition that causes inflammation of the skin
- allergic conjunctivitis - inflammation of the conjunctiva (the transparent layer of cells that covers the white part of the eyeball and the inner surfaces of the eyelids)
Less commonly, dry eye syndrome can be caused by a number of other conditions:
- Sjögren's syndrome - a condition that causes excessive dryness of the eyes, mouth and vagina
- rheumatoid arthritis - a condition that causes pain, swelling and inflammation in the joints
- lupus - a condition where your immune system attacks healthy body tissue
- scleroderma - a skin condition that can also affect the blood vessels
- previous trauma (serious injury) - such as burns or exposure to radiation
- shingles - an infection of a nerve and the area of skin around it, caused by the herpes varicella-zoster virus
- Bell's palsy - a condition that causes weakness or paralysis to the muscles of one side of the face
- HIV - a virus that attacks the body's immune system
Ageing
As you get older, you produce fewer tears. This, combined with the effects of the menopause, probably explains why dry eye syndrome is common among older women.
