Definition of Dry Eyes
Before jumping into cause and treatments of dry eyes, you have to know some useful information about tears or tear film.
Healthy eyes are covered by fluid or tears all the time. Tears have many important functions which will help to protect your eyes against infections, maintain clear vision and lubrication to facilitate eyelid blink.
Tears are produced by special glands that surround the eye globe. Most of people think that tear film consist of water only, actually it consists of three layers, and each of these layers is important to keep the tears stable. Instability of tears will cause dry eyes syndrome.
These layers are lipid or oil layer, aqueous or water layer and lastly the mucous layer.
The outer layer which faces the atmosphere is the lipid or oil layer. This layer is produced by sebaceous glands that present on the eyelid margin which is called meibomian glands.
Other small sebaceous glands can play a role in lipid secretion called Zeis glands which present around the eyelashes follicles.
This layer acts as a barrier that prevents aqueous layer from evaporation. Also it helps in blinking by maintain lubrication.
In case of meibomian gland dysfunction there will be deficiency in lipid layer which makes the watery layer to evaporate quickly and cause dry syndrome
The next layer is called water or aqueous layer which is the largest component of tear film. It is secreted by one main gland called lacrimal gland and another small accessory glands called Krause and Wolfring glands that distributed around the eye.
This layer consists mainly of water and also many electrolytes and antibacterial factors with enzymes to prevent against bacterial infection. Also this layer provides oxygen supply to cornea.
Body tissues have its oxygen supply through blood vessels but cornea has no blood supply so it has its oxygen supply through tears and also from aqueous humor from inside the eye.
The last layer which comes in direct contact with the cornea called mucous or mucin layer. It acts as a lubricant to allow distribution of tears all over the cornea and to facilitate blinking without causing erosions on corneal surface. It is secreted by glands called goblet cells or glands that are found in conjunctiva.
Any deficiency in one of these layers will cause tear instability and lead to dry syndrome.