Dry eye can also develop when lacrimal function is normal and the volume and composition of the lacrimal fluid are sufficient. The tear abnormality is created by other periocular diseases, usually leading to increased tear evaporation. These conditions are referred to as evaporative dry eye and each is independently capable of producing dry eye.
Alteration of the lipid layer
Dry eye can be caused by blepharitis, a condition in which contamination by skin lipids causes the tear film to destabilise, resulting in an increase in tear evaporation. Diseases of the Meibomian gland also increase tear evaporation as insufficient lipids are produced for resurfacing the tear film with each blink. In addition, a qualitative alteration of the Meibomian lipid takes place in such a way as to destabilise the tear film. Dystrophy of the lacrimal glands, as well as a condition called distichiasis (a double row of eyelashes on an eyelid, turned in against the eyeball) can also cause evaporative dry eye.
Lid related disorders
Blinking is important in maintaining the lipid layer because it forces the lipid from the Meibomian glands and spreads it over the surface of the cornea, sealing in the aqueous layer. Thus, infrequent blinking (which occurs, for example, in Parkinson’s disease or during excessive use of computer screens) leads to drying of the ocular surface between blinks. Disorders of the lid aperture can also lead to dry eye; for example, the increased width of the palpebral aperture that occurs with abnormal protrusion of the eyeball in thyroid disease is associated with ocular drying and tear hyperosmolarity. Lid deformity, resulting in lid–globe incongruity, leads to poor resurfacing of the ocular tears and is yet another cause of evaporative dry eye.
Contact lens wear
Contact lens wear increases tear evaporation from the ocular surface by disrupting the lipid layer of the tear film. This causes irritation, infection, protein deposits and pain. Research has shown that dry eye is the leading cause of contact lens discomfort.
Any disorder that results in the elevation of the ocular surface is associated with local drying of the cornea. For example, xerophthalmia (where vitamin A deficiency causes dryness of the conjunctiva and cornea) is a condition in which tear-deficient and evaporative dry eye can both occur. The disorder is characterised by defective surface wetting associated with abnormal surface changes, including goblet cell loss. The loss of the goblet cells increases dry eye.
Any of the disorders discussed so far, whether tear deficient or tear sufficient, may occur in conjunction with any other – and several of them commonly do. For example, lacrimal gland deficiency may be accompanied by Meibomian gland deficiency, while cicatricial conjunctival disease may cause dry eye by occlusion of the lacrimal gland ducts and by causing a lid incongruity that interferes with tear resurfacing with each blink. When the signs of more than one form of dry eye are present, it may not always be possible to differentiate their relative contribution to the dry eye condition.
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