Dry eye disease is a common condition. About 7% of the population have been diagnosed with the disease, but studies show that the prevalence of dry eye disease could be as high as 50%.  This condition disrupts the natural equilibrium of the tear film either due to decreased tear production or increased tear evaporation.

In order to understand dry eye disease, we must first understand the tear film.  The tear film is made up of three different layers: lipid, aqueous, and mucin.  The lipid layer is furthest away from the cornea, the front surface of the eye, and its purpose is to keep the rest of the tear film from evaporating.  The components of this layer are produced by the meibomian glands.  Next is the aqueous layer, which makes up the majority of the volume of the tear film.  It is made up of water and proteins and produced by the lacrimal gland and smaller glands in the conjunctival epithelium.  This layer is responsible for spreading the tears across the front surface of the eye.  The mucin layer is the layer that's closest to the cornea and is made up of mucin and immunoglublins.   This layer helps anchor the tear film to the cornea and also helps prevent damage to the cornea from shear force during blinking. The combination of these layers allow the tear film to keep the eye lubricated, help see better, protect against pathogens, and provide oxygen and nutrients to the cornea (which lacks blood vessels).

Dry eye can occur any time the ocular surface homeostasis is not maintained.  We will go over the causes of dry eyes, however all dry eyes can be categorized into two types: Aqueous Deficiency and Evaporative Dry Eye.  Aqueous deficient dry eye occurs when the lacrimal gland and other glands that produce the aqueous layer are not functioning properly and this causes dry eyes.  Evaporative dry eye occurs when the meibomian glands do not produce any or enough of the oil portion of the tear film and the tears evaporate.  Evaporative dry eye is common in patients that have blepharitis, which will be discussed in a later post.  One can also be diagnosed with both types and can have mixed dry eye.

There are a large number of causes for dry eye disease.  Certain systemic conditions, mostly autoimmune diseases, such as Sjogren's syndrome, rheumatoid arthritis, lupus, or scleroderma can cause it.  These autoimmune conditions cause Aqueous Deficient Dry Eye Disease.  Systemic medications can also cause dry eye disease.  Some examples of them are antihistamines, antidepressants, birth control pills, and prescription acne medication.  There are also some lifestyle and environmental causes of dry eye disease.  Smoking, use of contact lenses, and sleeping with a ceiling fan are all environmental factors that can cause dry eyes.

There are many different symptoms for dry eye disease.  Here are some of the most common symptoms. There is usually a burning or stinging sensation, and it is usually worse in the evening.  Patients can experience red eyes.  Patients also experience grittiness, like they have sand in their eyes.  They will also notice that their vision will fluctuate within minutes.  Sometimes when they blink it improves their vision, only momentarily, to have it become blurred again.  One of the most common symptoms that sometimes can be the most confusing to patients is that the eyes start to water more with dry eyes.  When the eyes are dry, there is a signal sent to the brain to create more tears, but unfortunately with evaporative dry eye this isn't the right part of the tear film and consequently the eye is still dry and this cycle continues.  We've kept the list short for the purpose of brevity, but there are many other symptoms.

The tear film plays an important role in protecting the eye and helping us see better.  When for any reason it is not produced properly it can cause complications to our eyes or vision.  We will go over these complications in the next post.