The dry eye: A diagnostic dilemma

The dry eye is an extremely symptomatic condition that occurs world-wide an d can develop under many circumstances. Although it usually is termed kerat oconjunctivitis sicca or keratitis sicca, other terms may be used to refer to specific dry-eye conditions. Xerophthalmia is a term usually used to des cribe ocular dryness secondary to vitamin A deficiency, whereas xerosis ref ers to the totally dry keratinized ocular surface seen after Stevens-Johnso n syndrome and other severe forms of cicatrizing conjunctivitis. The terms sicca syndrome and sicca complex also are used frequently to describe the d ry eye associated with Sjogrens syndrome. To avoid confusion, the generic t erm keratoconjunctivitis sicca (KCS) should be used to indicate the presenc e of a dry eye, regardless of the associated syndrome or disease process. E ven though KCS is caused by a number of different diseases, the patient's s ymptoms are remarkably consistent. Complaints of ocular foreign-body sensat ion, redness, itching, and inability to produce tears (even under stress) a re universal. Clinical findings may vary, depending on the underlying disea se process producing the dryness but, in every case, the damage to the corn ea 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 aq ueous tear layer, which comprises the bulk of the tear film, is secreted by the lacrimal gland proper and the accessory lacrimal glands of Wolfring an d 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 hydropho bic corneal epithelial cells after each blink of the eyelids. Floating on t he surface of the aqueous layer, literally like oil on water, is the superf icial lipid layer composed of oily aliphatic and cholesterol esters secrete d by the meibomian glands along the margin of the eyelid. Even though the l ipid layer usually is less than 100 nm thick, it spreads over the aqueous t ear film, forming a complete thin cover that retards evaporation.(2,3) This complex mixture bathes the surface of the cornea and conjunctiva and provi des a moist milieu for the epithelial cells. If any of the components is la cking as a result of disease, a typical dry-eye syndrome may develop.