Dry eyes is one of the commoner problems affecting our eyes. A thin film of tears normally coats the eyes. This film goes a long way towards keeping them comfortable. Dry eyes syndrome is particularly common after the age of 40 and is estimated to affect about 1 per cent of the population.
As is the case with all the body's processes a lot of engineering has gone into providing the very effective film of tears which is much more complex than one would assume. The film coating the eyes is made up of three layers. The inner layer is the mucus layer. It is produced by cells in the conjunctiva and this secretion is quite sticky. The aqueous layer made up of watery fluid covers this layer. This watery fluid is the salty tears produced by lachrymal glands which are part of the eye. This layer is in turn covered by a very thin superficial layer made up of fats. The inner mucus layer adheres to the eye and also has a hydrophilic property which promotes an even spread of the watery fluid making up the middle layer. The middle layer of tears is what prevents the eyes from becoming dry. Flow of this fluid also clears foreign particles such as dust and debris from the surface of the eye. Incidentally, this is the secretion that we associate with tearing from the eyes during emotional outbursts. The outer layer of fats (lipids), which is produced by glands in the eyelids called the Meibomian glands prevents undue drying of eyes by retarding the evaporation of tears from the eye.
Dry eyes result from a deficiency of tears and this deficiency may affect any of the three layers of this fluid mucin, aqueous tears or the lipid component. The aqueous layer maybe deficient as a result of inadequate production or excessive loss by evaporation. When the problem is of deficiency of production of aqueous tears, the condition is called Kerato-Conjunctivitis Sicca (KCS). If the dryness of eyes occurs due to excessive evaporation of tears, it is usually because the superficial lipid layer is deficient and in this case the condition is considered to be of Meibomian Gland Dysfunction (MGD).
Deficient aqueous production may occur as part of the Sjogren syndrome in which the dryness also extends to other parts of the body such as the nose and mouth. This syndrome often occurs in the setting of auto-immune disorders such as rheumatoid arthritis, systemic lupus (SLE) and scleroderma. The immune system of the body normally mounts an attack only against foreign particles or substances and so protects it from infections. However, the immune system sometimes gets confused enough to attack the cells of the body itself. This immune attack on own cells constitutes an auto-immune disease. Rheumatoid arthritis, SLE and scleroderma are all examples of autoimmune diseases and in these as well as other autoimmune disorders Sjogren syndrome can be associated causing dryness of eyes as well as other areas.
Sometimes, Sjogren syndrome of dryness of multiple areas can occur in the absence of any other underlying autoimmune disorder and then the patient is considered to suffer primary Sjogren syndrome.
Aqueous tear production is also deficient in a number of other disorders. Important in developing countries is deficiency of vitamin A. This leads to dryness of the eyes, night blindness, corneal ulcerations and so to blindness. In fact this is one of the most important preventable causes of blindness.
If the lachrymal glands that produce tears are surgically removed for any cause, this will obviously affect the capacity of the eye to produce tears resulting in dry eyes. Occasionally, lachrymal glands may never develop in a child and this will also result in a similar situation of dry eyes.
Trachoma is another very important preventable cause of blindness and is rampant in many parts of Africa and also the Middle East. This disease is caused by an infection of the eye. HIV is another infection with the capability of leading to dry eyes.
The outflow of tears from the lachrymal glands get obstructed in a variety of diseases including trachoma referred to above as well as after chemical burns or other diseases of the eyes. This again causes a deficit of aqueous tears.
Deficient sensations in the eyes can also indirectly lead to dry eyes because the stimulus to tears production is lacking. Such deficient sensations occur as a result of many diseases including diabetes, corneal infections with herpes, disorders of nerves supplying the eyes as well as very importantly from long-term contact lens use, especially if not suiting the particular individual.
Medicines given for other diseases can be a major culprit in the causation of dry eyes. Since the effect can be rapidly reversed this is a very important cause to be considered. Drugs with the potential to cause dry eyes include antihistamines often used on long-term basis for treating allergies, some blood pressure medications, some drugs given to relieve abdominal pain and even oral contraceptives.
Many patients with dry eyes may not appreciate the dryness and be without symptoms. In those with symptoms, the common ones include a feeling of dryness of the eyes with irritation in the form of burning or itching. Some people have a feeling as if there is a foreign object in the eye. Some also complain of intolerance to light and maybe blurring of vision.
Symptoms generally increase as the day progresses in persons with KCS with continued use of the eyes. The symptoms also increase in these persons if there is exposure to inclement environmental conditions. In contrast, in a person with MGD symptoms may be maximum on getting up from sleep.
In some patients with dry eyes syndrome there maybe paradoxical symptoms of excessive tearing from the eyes. This occurs because of marked irritation of the exposed portions of the eye because of dry eyes causing the eyes to water. This can be confusing and misleading unless the doctor is given the opportunity to examine the eyes carefully.
To start with, dry eyes only cause irritation and discomfort. With serious disease there can be secondary changes in the cornea including ulcerations that may leave irregularity of the surface during healing and thus interfering with vision. It is not unusual for any corneal ulcers that occur to get secondarily infected. Should this occur, the prognosis for vision is much worse as opacities of the cornea are a not unusual outcome in such cases.
In the early stages except for a decreased tear meniscus, there may not be any abnormality on examination of the eye. In more advanced cases, when dryness of the eyes has led to corneal ulcerations or irregularity these can be detected. The eyes are also often reddened.
As far as investigations are concerned, tear production is measured by the Schirmer test. In this test a special thin filter paper is placed in the eye for a predetermined period and its wetting is observed to give an idea about tear production.
In the Tear break-up test the time duration between instillation of a dye into the eye and the appearance of dry spots on the surface of the cornea is noted. The eye has to be examined by a slit lamp which is an instrument all eye doctors usually have.
Other dyes can be instilled in the eye to give an idea of small breaks in the integrity of the corneal surface. While they do not measure dryness of the eye directly, they give a good idea about the ill effects of the dryness.
Other tests such as analysis of specific constituents of tears or measuring its osmolarity are less frequently carried out.
Depending on the suspected cause of dryness, investigations may be carried out to confirm or rule out such conditions.
Treatment is directed towards the underlying cause as well as obviating the end result of dry eyes and secondary damage to the corneal surface. In mild cases artificial tears are used. These are solutions constituted to resemble tears chemically and have special preservatives added to increase their life. Usually, they require to be applied at least 4 or 5 times a day. To prevent dryness at night, an ointment is advised for application to the eyes.
For more severe cases, the openings of the ducts draining tears are blocked surgically. This allows tears to remain in the eyes for much longer times and may help prevent drying of eyes. Depending on the severity, some or all of the drainage openings maybe blocked. Really severe disease may warrant attempts at controlling the environment such as use of a humidifier.
For patients with specific problems such as MGD, particular antibiotics may help. For persons with Sjogren syndrome or other immune diseases, use of drugs to alter the immune response either locally or orally may be used. Some of these measures are promising in showing good response. However, unless dry eyes are tackled early they lead to irreversible changes in the cornea which can interfere with vision to a significant extent.
Dr Asheesh Mehta
is Specialist Physician, Family Medical Centre, Fujairah
Courtesy Gulf Today