Dry eye syndrome can have many different underlying causes. However, in many cases there is no single identifiable cause.
Dry eye syndrome can be caused by:
- the environment, particularly dry heat or a windy climate
- disease (see below)
- side effects of medicines
- hormonal changes
The importance of tears
You probably only notice your tears when you laugh or cry. However, the surface of your eye is always covered by a thin layer of liquid known as the tear film.
Your tears have several important functions. They lubricate your eyes, keeping them clean and free of dust, protect your eyes against infection, and aid sight by helping to stabilise your vision.
Where tears come from
Tears are produced and regulated by a system known as the lacrimal functional unit.
The lacrimal functional unit is made up of a number of different parts that work together. This is described below:
- lacrimal gland – found in the upper corner of the eye socket behind the bone and produces the watery liquid that makes up the majority of your tears
- goblet cells – found in the lining of the eye (conjunctiva) and produce a sticky mucin which allows the watery liquid part of the tear film to stick to the surface of the eye
- meibomian glands – found along the entire length of the upper and lower eyelids and produce a specialised oil that forms the outer layer of the tear film, preventing evaporation
- eyelid – spreads tears across the surface of your eye when you blink
- cornea – the clear window at the front of your eye which is vital for sight
- conjunctiva – a clear membrane that lines the back surface of the eyelid and forms a natural gutter between the eyelid and the eyeball
- tear ducts – two small drainage channels at the inner ends of the eyelids (next to the nose) that allow tears to drain into the nose through the tear duct openings
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 – either the lacrimal gland does not produce enough tears due to diseases damaging the glands, or the tears evaporate before the body has a chance to replace them
- the quality of the tears is affected – the tears contain abnormal proteins and other molecules that irritate or damage the surface of the eye
Either outcome can cause dry eye syndrome.
As the eyes are no longer adequately protected by the tear film, special signals are sent to the immune system (the body's defence system) to try to compensate and correct this deficiency. It is this process that causes the inflammation (redness and swelling) of the eye, which is frequently associated with more serious forms of dry eye syndrome.
Blepharitis and meibomian gland dysfunction (MGD)
Blepharitis or meibomian gland dysfunction (MGD) is a common condition which affects many people with dry eye syndrome. It can occur at any age and in otherwise healthy people.
Blepharitis is an inflammation of the eyelids which blocks meibomian (oil-secreting) gland openings, sometimes resulting in a cyst.
As the oils cannot freely drain onto the tear film, they begin to stagnate in the glands. This causes the oil to solidify and change in the gland. The new oils cause irritation and inflammation when they drain onto the surface of the eye.
People with existing dry eye syndrome who get blepharitis will find their symptoms get worse and the severity of the condition increases.
The symptoms are burning and stinging along with crustiness of the eyelid and lashes. Blepharitis is not usually a serious problem, but if not treated properly it can cause severe inflammation of the cornea (transparent window of the eye), particularly when associated with medical conditions. This can cause permanent visual impairment.
Read more information about how blepharitis-related dry eye syndrome is treated.
A number of things can interfere with the lacrimal functional unit. These are explained in more detail below.
Hormones and the nervous system
Hormones (powerful chemicals 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 in hormone levels that occur during the menopause (when a woman's periods stop), pregnancy or while using the contraceptive pill 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. If you have a medical problem that causes changes in nerve function, this can also cause dry eye syndrome.
Environmental factors can have a drying effect on your eyes, causing your tears to evaporate. These include:
- dry climate
- hot blowing air
- high altitude
When you carry 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 symptoms of dry eye syndrome.
Several medicines are thought to cause dry eye syndrome in some people. These include:
Laser refractive surgery
Some people who have had laser-assisted in-situ keratomileusis (LASIK) surgery find that they have dry eye syndrome in the weeks after surgery. The symptoms usually clear up after a few months, but in some cases may continue.
Sometimes, contact lenses irritate the eye and cause dry eye syndrome. Changing to a different type of lens, or limiting their use, usually helps resolve the symptoms.
Most people with dry eye syndrome also have blepharitis or meibomian gland dysfunction (MGD) (see above). Blepharitis can occur at any age, but can be associated with the following conditions:
- seborrhoeic dermatitis – a common skin condition that is often referred to as seborrhoeic eczema
- acne 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 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) caused by an allergy
- 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 and can affect any organ in the body
- lupus – a condition where the immune system attacks healthy body tissue, particularly blood vessels
- scleroderma – a skin condition that can also affect the blood vessels
- previous trauma (serious injury) – such as burns or exposure to radiation
- shingles – a reactivation of the chickenpox virus, 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
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.