The dry eye: A diagnostic dilemma

The dry eye is an extremely symptomatic condition that occurs world-wide and can develop under many circumstances. Although it usually is termed keratoconjunctivitis sicca or keratitis sicca, other terms may be used to refer to specific dry-eye conditions. Xerophthalmia is a term usually used to describe ocular dryness secondary to vitamin A deficiency, whereas xerosis refers to the totally dry keratinized ocular surface seen after Stevens-Johnson syndrome and other severe forms of cicatrizing conjunctivitis. The terms sicca syndrome and sicca complex also are used frequently to describe the dry eye associated with Sjogrens syndrome. To avoid confusion, the generic term keratoconjunctivitis sicca (KCS) should be used to indicate the presence of a dry eye, regardless of the associated syndrome or disease process. Even though KCS is caused by a number of different diseases, the patient's symptoms are remarkably consistent. Complaints of ocular foreign-body sensation, redness, itching, and inability to produce tears (even under stress) are universal. Clinical findings may vary, depending on the underlying disease process producing the dryness but, in every case, the damage to the cornea and conjunctiva responsible for the typical signs and symptoms of KCS is mediated by an alteration in the tear film.(1) The tears are a complex mixture of the secretions of several glands. The aqueous tear layer, which comprises the bulk of the tear film, is secreted by the lacrimal gland proper and the accessory lacrimal glands of Wolfring and Krause. The aqueous layer floats on a mucin layer secreted mainly by the conjunctival goblet cells. The mucin layer acts as a surface-wetting agent, allowing the aqueous layer to spread out smoothly and to coat the hydrophobic corneal epithelial cells after each blink of the eyelids. Floating on the surface of the aqueous layer, literally like oil on water, is the superficial lipid layer composed of oily aliphatic and cholesterol esters secreted by the meibomian glands along the margin of the eyelid. Even though the lipid layer usually is less than 100 nm thick, it spreads over the aqueous tear film, forming a complete thin cover that retards evaporation.(2,3) This complex mixture bathes the surface of the cornea and conjunctiva and provides a moist milieu for the epithelial cells. If any of the components is lacking as a result of disease, a typical dry-eye syndrome may develop.