Dry eye syndrome is one of the most common reasons that patients schedule an appointment with their eye care professional. Symptoms of dry eyes include irritation, foreign-body sensation, burning, itching, and excess tearing. However, there are a number of disorders that may mimic these symptoms and they form a broad category of diagnoses referred to as Ocular Surface Disease. Ocular surface complaints are most commonly secondary to one of two disease states: 1) aqueous tear deficiency, and 2) evaporative dry eye. Since these two conditions are managed differently, it is important for your eye care provider to accurately diagnose your condition. Nevertheless, these two entities frequently coexist in many patients and require multiple treatment approaches.
“Aqueous tear deficiency” refers to an inadequate tear production by your eyes (i.e., “My eyes don’t make enough tears”). This form of dry eye syndrome is more common with aging (although young patients can also be affected) and in females, especially perimenopausal females due to hormonal changes that affect tear production. It is also more common in contact lens wearers and is a frequent cause of contact lens intolerance, forcing many patients to stop wearing contacts during their middle age years due to chronic irritation and dryness. Contact lens wear in the setting of dry eyes increases the risk of contact lens-related corneal infections. Systemic medications may also exacerbate dry eye complaints. Common offenders include antidepressants, antihistamines and allergy medications, oral contraceptives, and beta-blockers. Finally, aqueous tear deficiency dry eye may be associated with certain underlying autoimmune diseases to include rheumatoid arthritis, lupus, and Sjogren’s syndrome.
“Evaporative dry eye” refers to an unstable and dysfunctional tear film (i.e., “My eyes make enough tears, but the tears that I do make evaporate too quickly”). The normal tear film consists of three layers; The innermost layer that is in direct contact with the cornea is the mucin (mucous) layer, the outermost layer of the tear film is the lipid (oily) layer, between the inner mucin layer and outer lipid layer is the aqueous (watery) portion of the tear film. Patients with evaporative dry eye have an abnormal outer lipid/oil layer with insufficient coating of the tear film resulting in evaporation and drying of the underlying aqueous/water component of the tear film. This, in turn, causes the eyes to feel dry with symptoms of redness, burning, itching, tearing, and crusting on the eyelashes upon awakening. While evaporative dry eye tends to be worse in the morning, the same symptoms that occur in aqueous tear deficiency dry eye are more problematic later in the day. This fact can help to identify the underlying cause of your dry eyes based on when your symptoms are most severe. Evaporative dry eye is frequently associated with disorders of the eyelids, namely blepharitis, meibomian gland dysfunction (MGD), and ocular rosacea. The meibomian glands function to secrete the lipid/oil layer over the tear film. When these glands become clogged and poorly functioning, the tear film lacks the normal oil layer that is supposed to slow the evaporation of the water component of your natural tears. As a result of the lack of this lipid/oil layer, the normal tear film breaks up and evaporates prematurely. This causes the eyes to dry, thus patients suffer from dry eye symptoms.
“If my eyes are dry, then why do I have excess tearing?”
This is a common question among dry eye sufferers and seems to be counterintuitive. When the eye becomes dry as a result of poor tear production or evaporation of the tear film, the cornea develops tiny dry spots called “punctate epithelial erosions” (PEE). These dry spots are similar to small isolated corneal abrasions and are very irritating to the eye. Because of this eye surface irritation and drying, the tear producing lacrimal glands in the eye increase their tear production which causes the common symptom of excess tearing (known as epiphora). In essence, it is the eye’s attempt to heal those tiny dry areas on the cornea. Proper treatment of the underlying dry eye state usually improves this symptom of excessive tearing.