Biology of Dry Eye
What is the biological background to dry eye syndrome?
Dry eye, which is one of the most common diseases of the eye, develops when a reduction in the quantity or quality of tears occurs. Any condition that reduces the production, alters the composition or impedes the distribution of the tear film on the cornea can lead to instability of this protective shield. Instability causes the tear film to break up earlier than normal, leading to the formation of dry spots on the cornea. If not rectified, this can lead to dry eye.
The most common cause (aetiology) of dry eyes is an abnormality in the function of the lacrymal gland where most of the tear film is produced. However, it can also occur when the lacrymal gland is functioning normally. Sometimes dry eyes can develop if there is a deficiency in one of the components of the tear film.
Tear FilmThe tear film on the cornea contains three layers.
The underlying mucin layer:
- consists of mucus that helps the tears to spread evenly and adhere to the epithelial surface of the cornea
- allows the eye to remain wet
- aids lubrication of the eye
The middle aqueous layer:
- makes up the majority of the tear film
- contains mostly water, ions and nutrients that are important to the health of the eye
- keeps the eye moist and clear
The outermost lipid layer:
- prevents the evaporation of tears from the surface of the eye and maintains surface tension
It has been established that most forms of dry eye exhibit similar features:
- Symptoms - such as burning, dryness, redness, itching or grittiness in the eyes
- Damage to the ocular surface which can be diagnosed using dyes such as fluorescein, rose bengal and lissamine green
- Tear film instability - established by measurement of the tear break-up time
- Tear film hyperosmolarity (hypertonicity or surface tension) which is thought to be the common denominator between all forms of dry eye
Like most eye conditions, dry eyes are often related to other disorders in the body. For example, it is commonly associated with dryness of other mucous membranes. It can also be a sign of autoimmune diseases such as rheumatoid arthritis, lupus erythematosus or Sjögren's syndrome *.
There are many other causes of pathological dry eyes, such as age or medication. In many patients, dry eyes are not associated with systemic diseases. In fact, they are often the result of aggressive environmental factors such as smoke, exhaust fumes, dust, air conditioning, prolonged use of a computer screen and contact lens wear. Refractive surgery that permanently changes the shape of the cornea can also result in dry eyes.
* Sjögren's syndrome:
For further information on this condition please visit the website of the British Sjögren's Syndrome Association.