Original article
The Ocular Surface and Tear Film and Their Dysfunction in Dry Eye Disease

https://doi.org/10.1016/S0039-6257(00)00203-4Get rights and content

Abstract

The ocular surface, tear film, lacrimal glands, and eyelids act as a functional unit to preserve the quality of the refractive surface of the eye and to resist injury and protect the eye against changing bodily and environmental conditions. Events that disturb the homeostasis of this functional unit can result in a vicious cycle of ocular surface disease. The tear film is the most dynamic structure of the functional unit, and its production and turnover is essential to maintaining the health of the ocular surface. Classically, the tear film is reported to be composed of three layers: the mucin, aqueous, and lipid layers. The boundaries and real thickness of such layers is still under discussion. A dysfunction of any of these layers can result 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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