Definition of dry eyeDry eye, 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 precorneal tear film leads 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 of dry eye is an abnormality in lacrimal gland function. However, it can also occur when the lacrimal gland is functioning normally. Sometimes dry eye can develop if there is a deficiency in the mucin component of the precorneal tear film. The various aetiologies, which can act independently or interact to cause dry eye, have features in common that may be included within a single definition:
“Dry eye is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles.”
The Ocular Surface Journal TFOS International Dry Eye WorkShop (DEWS II)
The term ‘dry eye’ encompasses many different ocular disorders that can lead to the dry eye condition. Thus, global criteria are required for the diagnosis of dry eye which do not necessarily identify a particular aetiology. 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 – 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) – thought to be the common denominator between all forms of dry eye
Pathological and non-pathological dry eye
Both internal and external factors can cause dry eye; thus, the condition may also be classified into pathological or non-pathological dry eye.
Pathological dry eye
Like most eye conditions, dry eye is 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 syndrome (in which the entire lacrimal gland, which is responsible of 90% of tear production, may be destroyed by the invasion of inflammatory cells). There are many other aetiologies of pathological dry eye, such as age or medication.
Non-pathological dry eye
In many patients, dry eye is not associated with systemic diseases. In fact, it is 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 can also result in dry eye and includes procedures such as laser-assisted in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK), types of refractive surgery that permanently change the shape of the cornea.
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