Dry Eye Syndrome
What is dry eye?
The dry eye syndrome is caused by decreased tear secretion or increased evaporation. The condition causes discomfort and, in severe cases, damage to the cornea.
The tear film
The tears cover the eye as a thin layer - the tear film - which keeps the eye moist, ensures a smooth corneal surface, protects the epithelium (outer layer of the cornea) and acts as a barrier to infection. It is made up of 3 layers.
The Mucin layer is the inner layer in contact with the cornea. The mucin is made up of proteins, sugars, enzymes and material from the immune system. It is secreted by the goblet cells scattered throughout the conjunctiva. The secretion is partly controlled by the nervous system. Damage to these cells will cause a decrease in the amount of the mucin in the tears.
The aqueous layer is the middle layer. It consists of water, salts, proteins, vitamins, anti-bacterial material and hormones. The salts maintain a steady PH (acidity) of the tears and the proteins act as a barrier to infection. The aqueous is secreted constantly by small glands in the conjunctiva and by the main tear gland which is situated behind the outer part of the upper lid. The activity of this gland is under nervous control and increases when crying and when the eye is irritated such as by a grain of sand.
The lipid (fatty) layer is the outer layer in contact with the air. The lipid is secreted from the Meibomian glands which open along the upper and lower lid margins. The secretion is stimulated by blinking. The lipid stabilises the tear film, gives it a smooth surface and prevents evaporation.
Symptoms of dry eye
Patients with dry eyes complain of burning, itching, redness and a foreign body sensation. Usually symptoms are more severe during the day, when they may be accompanied by sensitivity to light. There may be a mucus discharge and, paradoxically, excessive tearing from the main tear gland, as if in reaction to a foreign body. These tears do not relieve the symptoms because the other components of the tear film are missing. The symptoms increase when exposed to irritants such as dust and smoke, in the dry environment of an air-conditioned office and when staring at a computer screen.
Tests used for the diagnosis of dry eye
The Schirmer test. A strip of absorbent paper is placed inside the lower lid and the amount of tears absorbed in it is measured.
Tear meniscus. The height of the tear film between the lower lid margin and the eye is measured.
TBUT – Tear Break Up Time. The tears are stained with fluorescein which normally spreads evenly over the cornea. The eye is examined under a special light and the time for the first cracks to appear in the tear film is measured.
Damage to the corneal surface is demonstrated by staining the cornea with fluorescein or other dyes. Severe corneal damage is visible even without staining.
The causes of dry eye
The most frequent cause of dry eyes is underproduction of the aqueous component, usually in people over the age of 40 years. It is occasionally due to congenital abnormalities of the lachrymal gland. Underproduction of the tears may occur in patients with diabetes or autoimmune and collagen diseases such as rheumatic arthritis, or may be caused by medications such as anti-histamines, beta blockers, contraceptives and anti-Parkinson’s disease drugs. Wearing contact lenses, the prolonged use of eye drops, damage to the glands due to inflammation and nerve damage following infections may also reduce tear production.
Increased evaporation of the tears may be due to a defective lipid layer, sometimes associated with chronic inflammation of the Meibomian glands (blepharitis), particularly in older people. Decreased concentration of antibacterial agents in the tears increases the risk of blepharitis. Rarely, a congenital defect of the Meibomian glands occurs. Anti-acne treatment may affect the lipid layer.
Reduced blinking may be due to drugs or Parkinson's disease, and may occur when concentrating in front of a computer screen. Contact lenses may affect tear film stability and decrease corneal sensation.
Avoid irritants such as dust and smoke.
If possible, avoid dry air caused by air conditioning.
Consult your doctor regarding medications which may cause dry eye.
Acquire the habit of blinking while working at the computer
Rinse the eyes with cold water
Use lubricating drops:
Traditional lubricating drops are efficient but with frequent use they may damage the cornea due to the preservatives they contain. The preservatives are absorbed by contact lenses so these drops must not be used by contact lens users.
Newer drops, packed in small ampules with no preservatives. Each ampule must be discarded at the end of each day. Alternatively, some bottled drops may be kept for 1-3 months even without preservatives and other bottled drops contain special preservatives that evaporate before the drop hits the eye.
In severe cases a viscous gel or ointment can be used, usually before bed time.
Blepharitis is treated with hot compresses twice daily. A clean cloth soaked in hot water is placed over the eyelids for 10-15 minutes. The heat melts the lipid and causes its secretion from the glands. After the compress, the lid margin (were the eyelashes are) is cleaned with special wipes or with cotton wool dipped in diluted baby shampoo.
Spectacles are worn in order to prevent evaporation of tears and to keep dust and dry air from the eyes. Normally the tears drain to the nose through small ducts in the inner part of the upper and lower lids. These ducts can be blocked with tiny plugs which cause no discomfort and can be removed at any time. With the plugs in place, less lubricating drops are needed to keep the eye humid.
In cases associated with inflammation, steroid eye drops may be helpful, but side effects limit their use.
When the goblet cells are damaged, Cyclosporin-A drops (Restasis) may improve their function: A three months trial is necessary to assess response to the treatment.
Omega 3 tablets or eye drops have been found to decrease the inflammation.
Systemtic antibiotics of the tetracycline group are prescribed in severe cases of blepharitis
and in very severe cases it is possible to suture the eyelids and partially close the eye in order to decrease the area of exposed cornea.
Dry eye is usually a chronic condition. Treatment relieves the symptoms but unfortunately cannot completely eliminate the condition, especially in severe cases.