Original articleThe Ocular Surface and Tear Film and Their Dysfunction in Dry Eye Disease
Section snippets
Regulation of Tear Production
The production of tears is regulated through a reflex loop (Fig. 1). The ocular surface (conjunctiva, cornea, accessory lacrimal glands, and meibomian glands) and main lacrimal glands act with this neuronal loop to regulate the production of tears necessary for ocular surface homeostasis and repair.64 Stimulation of nerves at the ocular surface or in the nasal mucosa sends impulses to the brain via the fifth cranial nerve, which generate a reflex response via nerves passing to the lacrimal
Composition of the Tear Film
Tears are a complex solution composed of water, enzymes, proteins, immunoglobulins, lipids, various metabolites, and exfoliated epithelial and polymorphonuclear cells. Because of the highly dynamic characteristics of the tear film, defining its exact composition at a particular point in time is impossible. Its specific content will vary depending upon the challenges with which the ocular surface has to deal.
The tear film is composed of three main components (Fig. 2), each of which has been
The Conjunctiva and Cornea
Although the tear film provides the primary source of protection for the eye against chemical, mechanical, bacterial, and viral attack, the corneo-conjunctival surface represents the last barrier preventing penetration of the deeper ocular structures. The epithelium of the conjunctiva and cornea is, therefore, a key component of the defense mechanisms of the eye. Although the cornea and conjunctiva are exposed in a similar manner to the outside environment, the defensive mechanisms of the two
Dysfunction of the Ocular Surface and Tear Film
Deficiencies in any of the tear film layers, defective spreading of the tear film, systemic diseases, and some systemic and topical medications can disturb the ocular surface or tear film and cause dry eye disease.
Collagen Vascular Disorders (Collagenoses)
This category contains various autoimmune disorders, including the following.
Rheumatoid arthritis
Most patients with rheumatoid arthritis have some degree of dry eye disease. It is thought that lymphocytic infiltration of the lacrimal gland results in reduced tear secretion.
Scleroderma
Sicca syndrome has been described in 70% of patients with scleroderma. Patients with the limited form of the disease (CREST syndrome [calcinosis, Raynaud's phenomenon, esophageal hypomotility, sclerodactyly, and telangiectasia]) tend to have
Allergies
Dry eye is a common consequence of chronic allergic conjunctivitis due to continuing activation of the local immune system. A vicious cycle of irritation and ocular surface damage may cause dry eye symptoms to persist for months after the allergic episode.5
Mucocutaneous syndromes
One of the most severe forms of dry eye is seen in the course of Stevens–Johnson syndrome. Chronic mucocutaneous syndromes, such as cicatricial pemphigoid, also often lead to severe loss of goblet cells and dry eye in late stages of the
The Vicious Cycle of Increased Evaporation and Ocular Surface Damage
The causes of increased evaporation of water from the tear film can be found in any of the quantitative or qualitative changes occurring on the ocular surface or in the tear film itself that affects the formation and the spreading of a normal oily layer. This feature does not indicate that a primary abnormality of the oily layer is the only cause of tear film disruption.
As indicated, many other factors that are able to destroy the delicate architecture of the tear film will increase
Acknowledgements
The authors have no proprietary or commercial interest in any product or concept discussed in this article.
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