Cornea/external disease (including blepharitis, dry eye syndrome)
What is cornea?
The cornea is the clear, protective outer layer of the eye. Along with the sclera (white of the eye), it serves as a barrier against dirt, germs, and other particles that can harm the eye's delicate components. The cornea is also capable of filtering out some amounts of the sun's ultraviolet light but enough to replace the need for wearing wraparound sunglasses outdoors. If the cornea of your eye becomes damaged through disease, infection, or injury, the resulting scars can interfere with vision by blocking or distorting light as it enters the eye.
What is blepharitis?
Blepharitis is a common and ongoing condition where the eyelids become inflamed (swollen), with oily particles and bacteria coating the eyelid margin near the base of the eyelashes. This annoying condition causes irritation, itchiness, redness, and stinging or burning of the eyes. While the underlying causes of blepharitis aren't completely understood, it can be associated with a bacterial eye infection, symptoms of dry eyes or certain types of skin conditions such as acne rosacea.
What is dry eye syndrome?
Keratoconjunctivitis sicca (KCS), also called dry eye syndrome (DES) or keratitis sicca, is an eye disease caused by eyedryness, which, in turn, is caused by either decreased tear production or increased tear film evaporation. It is found in humans and some animals. KCS is the most common eye disease, affecting 5 - 6% of the population. Prevalence rises to 6 - 9.8% in postmenopausal women, and as high as 34% in the elderly. The phrase "keratoconjunctivitis sicca" is Latin, and its translation is "dry [inflammation] of the cornea and conjunctiva".