Differential diagnosis of dry eye conditions.

The pre-ocular tear film is a complex biochemical structure produced by the lacrimal glands and epithelial cells on the ocular surface. Clinical syndromes of ocular irritation may result from deficiencies in one or more of these layers. At a recent dry eye workshop at the National Eye Institute, dry eye conditions were classified into those with adequate aqueous tear production and those with aqueous tear deficiency. The majority of patients with aqueous adequate dry eye suffer from meibomian gland dysfunction that results in lipid tear deficiency. Aqueous tear deficiency can be subclassified into non-Sjögren's syndrome and Sjögren's syndrome (SS) groups. Patients with non-Sjögren's aqueous tear deficiency have less-severe symptoms and ocular surface disease than those with SS. The etiology of non-Sjögren's aqueous tear deficiency has not been established, but it appears to be multifactorial. In SS, immune-mediated destruction of the lacrimal gland results in severe aqueous tear deficiency. Aqueous tear deficiencies lead to ocular surface disease, termed keratoconjunctivitis sicca (KCS). KCS results from abnormal terminal differentiation of the ocular surface epithelia and is associated with marked reduction in mucin production by these cells. Clinical features helpful in differentiating the various dry eye syndromes are reviewed.