Simply put, Dry Eye Disease is a lack of tears and/or poor quality tears and is often a multifactorial disease. To understand Dry Eye Disease, it is best to understand the components and effects of tear film as both the quality and quantity of tear film can affect the health and vision of the eyes. Each component of the tear film serves an important purpose.
Our tear film is composed of three layers:
Oily layer- produced by the Meibomian glands, this layer helps keep the surface smooth and prevents evaporation.
Watery (or aqueous) layer- produced by the lacrimal glands, this water layer keeps the eye’s surface clean and washes away particles, dust and debris.
Mucus layer- produced by the conjunctiva, this is the innermost layer of tear film which helps spread the watery layer of film over the eye keeping it moist.
The different categories of DED depend upon which component of tear film is affected. The leading cause of DED is Meibomian gland dysfunction and is associated with 86% of DED cases. In this instance, the Meibomian glands do not secrete enough meibum (oil) to prevent the tear film from evaporating too quickly resulting in “evaporative dry eye”. In other cases, the cause of DED is related to the lacrimal glands not producing sufficient watery fluid to keep the eyes moisturized. This is considered “aqueous deficiency dry eye”. It is possible to have a “mixed” form of Dry Eye Disease, which would include both evaporative and aqueous deficiency components.