10 Feb Never Enough of a Dry Eye
Intermittent blurry vision from dry eye is common, and although ultimately a corneal problem, is a condition often faced by a retina specialist. Many times, patients with retinal disease, such as diabetic retinopathy or macular degeneration, are referred to me with unexplained, fluctuating, decreased vision. It is always up to me to figure out if the change in vision is due to the dry eye, or the retinal disease.
The cornea is the transparent tissue in the front of the eye. It is the tissue covered by a contact lens. The cornea is responsible for about 2/3 of the focusing power of the eye. Not even our natural lens, which rests inside the eye, has this much focusing power.
Tear Film – When we blink, the eyelids spread tears across the surface of the eye. The tears keep the cornea hydrated. A well-hydrated cornea has a very uniform and smooth surface. The air-water interface, where the air hits the watery tears on the cornea, gives the cornea so much focusing power.
(Actually, it is the radius of curvature of the cornea and the index of refraction of the air-water interface that yields the final focusing power of the eye. This is also why laser vision correction works; it changes the radius of curvature, or roundness, of the cornea.)
In dry eye conditions, the surface of the cornea becomes, uh…dry. On a microscopic level, it is no longer smooth. A “dry” cornea is irregular and rough. It does not focus light accurately or consistently as the tear film is some what patchy. The result is a very blurry image that reaches the retina. We complain of blurry vision.
Treatment for dry eye usually consists of artificial tears and lubricants. Other treatments are available, but are beyond my scope of practice. It is important to remember, yet hard to implement, that the use of artificial tears should be regular and not used only when symptoms develop. The idea is to prevent dry eye from developing.
Symptoms of dry eye include blurry vision, burning, sensitivity to light and tearing. The blurry vision may only be at certain times of the day, during certain activities, such as reading, or may be more prevalent at certain seasons (winter is generally drier along the East coast). The cornea is a highly sensitive tissue, the dryness causes the burning and sensitivity (try not blinking for 30 seconds). The tearing develops in response to the burning (as if to wash something away).
What Does This Mean? Due to the enormous focusing power of the cornea, any small perturbation of the corneal surface can translate into rather dramatic change in vision. To a patient with retinal disease, these changes in vision are worrisome, scary and can be depressing. A key difference, however, between loss of vision from dry eye versus progressing retinal disease is that the symptoms from retinal disease should not fluctuate, especially on a daily basis.
I am always happy to “blame” the dry cornea for changes in vision and we all breathe a sigh of relief when I find the retinal disease is stable and unchanged.
It really should be called “dry cornea.”