Dry eye is a common condition that involves the surface of the eye. Although it seems straight forward, there are actually several different types of dry eye. Before breaking down the different types, it is important to know a little bit about the tear film. The tear film is the layer that is involved in dry eye, and it is composed of three different components: the lipid layer, aqueous layer, and mucin layer. The lipid (or fat) layer is where the oil from the eye sits. It helps prevent evaporation of the underlying layers, and dysfunction of this layer can often cause the symptoms of dry eye. The lipid layer is produced mainly by the glands in the eyelids of the eye, and blepharitis (or inflammation of the eyelid) is often a cause for dysfunction of these glands. The next layer (middle layer) is the aqueous layer which is basically the water layer. A lot of people think that a low level of this layer is the only cause for dry eye, but it is just one of many. This is the thickest part of the tear film, and when the tear film is reduced, it is usually this layer that is the culprit. People with Sjogren's disease have a low aqueous layer. The bottom layer is the mucin layer, and this is produced in part by the clear area of the eye over the sclera called the conjunctiva. An imbalance in the mucin layer can often cause crusting, mucous, and bacterial overgrowth. The dysfunction or imbalance of any of these layers is what causes dry eye. Based on these factors, and external factors, dry eye is broken into two main types called evaporative dry eye and aqueous deficient dry eye. In my next post, I will discuss the different types of evaporative and aqueous deficient dry eye.