What Is Dry Eye?

Dry eye is a multifactorial disease of the ocular surface characterized by a loss of balance of the tear film which results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by higher osmolarity (salt concentration) of the tear film and inflammation of the ocular surface. Dry eyes occur when either the eye cannot produce enough tears to lubricate the ocular surface or the tears that are produced evaporate too quickly.
When either of these two mechanisms occurs, it results in eventual breakdown of the ocular surface which in turn causes an inflammatory response. Symptoms are varied and can range from minor irritation and vision fluctuation to moderate to severe debilitating disease and vision loss. Dry eye is also characterized by a disconnect between the symptoms of discomfort and the severity of the condition.
The tear film, which is not the same as our tears, is a rather complex structure. We can break it down schematically, for purposes of discussion, into two layers. The aqueous or water layer which is formed by tiny glands in the mucous membranes inside our eyelids. This water layer contains salt molecules, and when it’s in balance our eyes feel comfortable. Overlying the water layer is an oil layer which comes from the oil glands inside of the eyelids. The oil coats the water layer which protects it from evaporation. If the oils are not formed properly or the glands get clogged, the oil layer breaks down too quickly thus allowing the water layer to evaporate (evaporative dry eye). This reduced water layer then becomes more salty which causes ocular surface damage and inflammation. This in turn causes the oil glands to get even more clogged up and a vicious cycle begins.