Monday, January 02, 2006

General Information, Symptoms and Cause of Dry Eye

Dry eye diseases are quite common and account for significant morbidity including irritation, pain, decreased vision, predisposition to corneal ulceration, infections and in severe cases, blindness. Approximately 7-10 million Americans currently use artificial tear preparations (Lemp 1998) and the prevalence of the dry eye has been estimated to range from 8.4-35% in the elderly (Oden 1998, Moss 2000) Over $100 million are spent each year for various treatments of dry eye including the hundreds of available drops and viscous substances to cover the dry eye (Lemp 1998, Marube 1998).


Symptoms of Dry Eye

Patients with dry eye disease often complain of a gritty burning sensation in their eyes. They may experience photophobia (the fear of light due to this irritation). They may have blurred vision and even decreased visual acuity. The symptoms are often worse in dry hot climates. However any climate with low humidity may aggravate the condition (such as indoor heating in the winter). Dry eye disease has a higher incidence in elderly people and and particularly women.

The key criteria for the objective diagnosis of dry eye include:
1. decreased tear break-up time indicating an unstable tear film between blinks. Click on the link to see serial photos of the tear film breaking up at different times.
To view a color image of the fluorescein stained appearance of dry spots at the end of the tear break up time, click on this link.
2. abnormal Schirmer's test, indicating tear flow is reduced. In this test the distance tears flow on a strip of filter paper is measured (see photograph in this link).
3. fluorescein staining of the cornea indicating an epithelial defect (click on the link for an an image)
4. Rose Bengal staining of the conjunctiva and cornea(see the figure in the link).



The basic structure of tears includes oil floating near the air interface, aqueous mixed with proteins and soluble mucins beneath the oil, and a dense membrane bound mucin layer on the cornea surface (Click on figure to the left to enlarge). The aqueous layer of tears is produced by the lacrimal glands at the upper outer of the eyelid and within the eyelid. The oil layer is produced by the Meibomian glands and are secreted from the small orifices that dot the inside edge of both eyelids (Figure left, Click to Enlarge). The soluble mucins are largely produced by specialized Goblet cells that are scattered within the conjunctiva among epithelial cells. For a detailed look at conjunctival anatomy with clinical, histologic and ultrastructural photos click on this LINK. All components act in concert to provide adequate lubrication and protection to the cornea.



Conditions that May Cause Dry Eye

Dry eye disease may be caused by any condition that disrupts the components of tears or the sources of those components. For example the water and protein portion of tears is produced by the lacrimal gland. Conditions that produce inflammation in the lacrimal gland such as rheumatoid arthritis may produce dry eye disease. Endocrine function appears to be critical for the lacrimal gland function; menopausal women have a higher incidence of dry eye. Drugs such as antihistamine and psychotropic drugs may aggravate dry eye disease. Surgery may aggravate dry eye disease such as lacrimal gland removal, excessive eyelid surgery (overdone blepharoplasties), and LASIK. Conditions that affect the mucin lubrication of the cornea such as ocular infections (trachoma), and Vitamin A deficiency may produce dry eye. Burns to the mucin producing cells from chemical, thermal or radiation may cause dry eye. The lipid layer that help prevent evaporation of the tear film may be altered in chronic blepharitis and acne rosacea. Often a combination of these and many other conditions may produce or worsen dry eye disease.

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