Dry Eye is not just a symptom, but a chronic disease of the ocular surface that affects the delicate layer of tears (known as the tear film) that covers the front surface of the eye. The tear film is critical to the eye for both comfort and clear vision because it seals, lubricates, and protects the ocular surface, and because it is the first element that light hits before being directed to the back of the eye to create an image.
The normal tear film is made up of three layers, each with a very important function. The outermost oil (lipid) layer keeps the water layer below from evaporating. The water (aqueous) layer forms the bulk of the tears and provides oxygen and nutrients to the tissues, and flushes away toxins and debris. The thin innermost mucin layer stabilizes the tear film and anchors it to the front surface of the eye. Dry eye disease occurs when a problem arises with one or more of the layers of the tear film causing the delicate ecosystem of tears to lose homeostasis, or become unbalanced.
What does this look like?
Think for a moment about a glass of ocean water. If it sits out in the open air, we know the water will evaporate leaving a film of thickened salt sludge on the bottom of the glass. If we were to pour vegetable oil into the glass, it would coat the top of the water layer and the water would not evaporate. Our tear film works in a similar manner. The tear film, composed of mostly water and salts, spreads across the front surface of the eye with each blink. If there is not enough water in the tear film (aqueous deficiency or low tear volume), or poor quality or low volume of oil in the oil layer, the tears will evaporate quickly causing the salts in the tears to concentrate on the front surface of the eye.
This leads to direct damage to the tissues, inflammation, and even changes to the nerves (neurosensory (nerve) abnormalities), all of which begin to cause the signs and symptoms patients with dry eye experience.
"Dry eye is a multifactorial disease of the ocular surface characterized by a loss of homeostasis of the tear film, and accompanied by ocular symptoms, in which tear film instability and hyperosmolarity, ocular surface inflammation and damage, and neurosensory abnormalities play etiological roles.”
-Tear Film Ocular Society Dry Eye Workshop II (TFOS DEWS II),
Definition and Classification Report, 2017