Tears and Dry Eye
Human tears are produced by the lacrimal glands and consist of aqueous, mucous, and lipid components. The lacrimal glands produce the aqueous portion, which is enriched with a complex mixture of electrolytes, enzymes, antibodies, vitamins, antimicrobial proteins, and other substances. The lipids are produced by the meibomian glands, which are modified sebaceous glands along the eyelid margin. Mucins are produced by conjunctival goblet cells.
Dry eye has many causes, which often overlap and interact. A deficiency may occur due to increased evaporative loss or due to decreased production. Autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus, can cause dry eye. Methotrexate and cyclophosphamide, used to treat autoimmune disorders, can also cause or exacerbate dry eye. Sjögren’s syndrome is characterized by dry eye and dry mouth. Sjögren syndrome is an autoimmune condition, where there is an inflammatory infiltrate in the lacrimal gland which leads to cell death and a reduction in tear production. Disorders such as conjunctival chalasis and eyelid laxity (i.e., the floppy eyelid syndrome), can lead to dry eye. Conditions that affect muscular control of the face, such as stroke, injury, or Bell’s palsy, can impair eyelid closure, resulting in lagophthalmos and exposure keratitis. Similarly, any condition (e.g., Parkinson’s disease) or situation (e.g., prolonged screen viewing [on a computer, cell-phone, or television, for example]) that reduces the blink rate can increase the risk of dry eye by promoting tear evaporation. Obstructive meibomian gland dysfunction alters the lipid constitution of the tears and is the most common cause of evaporative dry eye. Dry eye affects about half of patients with chronic graft-versus-host disease. Many systemic drugs have been reported to trigger dry eye.
A frequent component of dry eye is ocular pain, which is often accompanied by light sensitivity, foreign-body sensation, dryness, and irritation. Patients often report pain evoked by exposure to wind, light, and temperature extremes. Corneal neuropathic pain can be severe and can be characterized as a burning or stinging sensation, sharp pain, or a dull ache. These perceptions most likely result from dysfunctional nerves in the richly innervated cornea. Visual symptoms, notably fluctuating or blurry vision can be another consequence of dry eye.
Artificial tears are a pharmaceutical product that includes cellulose, a spreading agent and a preservative, and are a first-line therapy in treating the condition. Alternatively, in some countries preparation of autologous serum for tears is available. Here, the patient donates a unit of blood in a blood pack without anticoagulant. The supernatant derived from the manufactured product is then processed and aliquotted for the patient's use. The biochemical characteristics are similar to human tears and contain fibronectin, vitamin A, epidermal growth factor, and transforming growth factor –β. Serum for tears have been in clinical use since 1984, however there is limited data to support use, with 6 randomised controlled trials to date. Such trials have been difficult to conduct, as the color of the tears cannot be masked. Furthermore, in the control arm, artificial tears are frequently used, which also have a beneficial effect on dry eyes.(Soni and Jeng 2016)
In July 2016, the Food and Drug Administration (FDA) approved 0.5% lifitegrast ophthalmic solution (Xiidra) for treating signs and symptoms of dry eye disease. Applied topically as one drop twice daily, this medication is the first in a new class of drugs, called lymphocyte function–associated antigen 1 (LFA-1) antagonists. Unlike topical lubricants, the two FDA-approved therapies for dry eye (Restasis and Xiidra) must be administered for a period of up to several months to achieve therapeutic effects.
References
Clayton JA. Dry Eye. New Engl J Med. 2018;378:2212-2223.
Soni, N. G. and B. H. Jeng (2016). "Blood-derived topical therapy for ocular surface diseases." British Journal of Ophthalmology 100(1): 22-27.
