Chronic Dry Eye Information:

Tear Quality of Healthy Eyes

Healthy Tear Composition

Healthy eyes produce tears that contain natural nutrients and lubricants, which in turn create a film that cleanses, protects, and moisturizes the ocular surface. Healthy tears are normally composed of a lipid layer and an aqueous/mucin gel layer that contains a complex mixture of electrolytes, proteins, and mucins. The lipid layer contains a variety of lipids that protect the tear film against evaporation while mucins allow for the wetting of the ocular surface and stabilize the tear film against the stresses exerted by each blink cycle.

The general function of important aqueous components is explained below.

Electrolytes

Electrolytes, such as sodium (Na), chloride (Cl), calcium (Ca) and potassium (K), help to increase the solubility of all the proteins and mucins found in the aqueous phase of the tear film. As previously discussed, electrolytes (as well as high concentrations of proteins and mucins) help maintain a constant osmolarity (about 295-305 mOsm/L) and pH (approximately 7.4) that are important for ocular surface health.

Proteins

The complex mixture of proteins in tear film resists invasion by infectious agents, promotes growth and healing of the epithelium, and helps maintain the health of the ocular surface. In keeping with the exposed nature of the ocular surface, by far the most abundant proteins found in tears are defensive in function. Many tear proteins possess antibacterial properties, either killing bacteria outright or limiting their growth in the tear film that would otherwise serve as a very rich growth medium for any invading microorganisms. The most important proteins include

• Immunoglobulins and cytokines
• Growth and wound-healing factors

Immunoglobulins and Cytokines

Immunoglobulins (antibodies such as IgE) are defensive. They recognize foreign molecules, triggering the immune system to initiate an immune response to fight the invading organisms. Cytokines help regulate the immune response. Some cytokines augment the immune response, leading to activation of T-cells, production of more inflammatory substances, and increased recruitment of inflammatory cells that fight the invading organisms. Cytokines can also affect the neural signal to the lacrimal gland, resulting in a decrease in the quality and quantity of natural tears. On the other hand, some cytokines repress immune activation when it is inappropriate, in order to minimize the tissue damage that a full-blown immune response could promote.

Growth and Wound-Healing Factors

Cells in the corneal epithelium constantly die and are replaced. This constant turnover naturally requires a supply of appropriate growth factors to stimulate the production of replacement cells. Growth factors such as EGF (epidermal growth factor) are present in normal tears and serve to stimulate the replenishment of specialized epithelial cells from immature stem cells.

Wounding of the corneal epithelium triggers the activation of a variety of factors that help repair and replace the lost corneal tissue. EGF also promotes wound-healing enzymes called proteases that are normally latent in the tear film to become activated and assist in the tissue remodeling that must occur during epithelial recovery from wounding.

It cannot be stressed enough that since there is no blood supply to the ocular surface, natural healthy tears are essential for maintaining optimal ocular health.

Altered Tear Composition in Chronic Dry Eye

In Chronic Dry Eye, tear composition is altered, so that osmolarity of the tear film is usually increased. This affects epithelial health and impacts sensory nerve potentials.

Soluble mucins in the tear film are often markedly decreased in concentration. This causes reduced tear viscosity, consequent thin spots and tear break-up following blink cycles.