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The term comes from the Greek word pterygion meaning "wing".
A pterygium grows in response to exposure to ultraviolet light. They are more common in light-skinned people living nearer the equator, particularly where outdoor activities are popular.
As the pterygium grows, it replaces the corneal epithelium, and erodes the superficial layers beneath it. These layers are Bowman's membrane, and the anterior corneal stromal lamellae. The pterygium pulls and distorts the cornea as it enlarges.
A pterygium reduces the vision in several ways:
- Distortion of the corneal optics. This begins usually when the pterygium is greater than 2mm from the corneal edge (limbus)
- Disruption of the tear film. The tear film is the first lens in the eye. Pterygia are associated with eyelid inflammation, called Blepharitis.
- Growth over the corneal centre, which leads to dramatic reduction of vision.
- Induced anterior corneal scarring, which often remains after surgical removal.
Surgical removal 
Indications for surgery, in order of decreasing importance:
- Growth over the corneal centre.
- Reduced vision due to corneal distortion.
- Documented growth.
- Symptoms of discomfort.
Surgery is usually performed under local anaesthetic with light sedation as day surgery. The pterygium is stripped carefully off the surface of the eye. If this is all that is done, the pterygium regrows frequently. The technique with the lowest recurrence rate uses a graft of conjunctiva from under the eyelid. This is placed over the defect remaining from the removed pterygium. The graft can be stitched in place, which is time consuming, and painful for the patient afterwards. An alternative is the use of tissue adhesive glue, called Tisseel, which allows faster surgery and less pain post-operatively.
Natural history 
A pterygium grows very slowly. Usually it takes several years or decades to progress, but occasionally growth may be rapid in the presence of inflammation such as Blepharitis.