What is dry eye?
Other than what the name implies, a report from the 2007 Dry Eye Workshop defined dry eye as,
“Dry is a multifactorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and inflammation of the ocular surface”.
This definition is a little bit more complex than what was original thought, and indicates a number of different factors are at play. Understanding each of the separate factors is important, and makes accurately diagnosing and treating the condition a complex process. Therefore a thorough comprehensive eye exam is an integral part of properly diagnosing and managing dry eye.
Tear Film Basics
To better understand dry eye we must first understand the physiological properties of the tear film. Most people think of tears as a simple fluid, with a consistency similar to water. Although part of this statement is true, there are a few marked inaccuracies. For simplicity sake we will discuss the tears as a three zone system. The first zone is directly adjacent to the ocular surface. It is comprised of a mucin rich layer that interacts with tiny protrusions (microvilli) from the ocular surface. This interaction enhances wettability and allows better tear coverage over the ocular surface.
The second layer, known as the aqueous layer, is the largest component of the tear film and is sandwiched between the innermost mucin layer and the outermost lipid layer. It is produced by three glands, two of which provide baseline production and the third kicks into overdrive during time of emotional stress (think Old Yeller), photic stimulation (bright lights), and inflammation (dry eye or trauma).
The third and final layer is a lipid rich layer produced by tiny glands on the margins of your eyelids. This layer is extremely important and its dysfunction is the most common cause for dry eye. It sits on the front part of the tear film and serves as a barrier between the middle acqueous layer and the environment. This barrier serves to prevent evaporation of the tears and resultant dehydration of the front of the eye. The condition is termed evaporative dry eye and is considered to include the largest segment of dry eye sufferers.
The next segment will expand on the cause and potential treatment options for evaporative dry eye.