The cornea is the clear window that makes up the front of the eye and lets light into the eye. The cornea is made up of five layers. The outermost layer, or corneal epithelium, is made up of fast-growing cells. It is important in protecting the rest of the cornea from dirt and bacteria, and in absorbing oxygen and nutrients from your tears. The innermost layer, or corneal endothelium, is a very thin layer of cells. It is responsible for pumping excess fluid out of the cornea. If too much fluid gets into the cornea, it will become cloudy.
The cornea also helps focus light so you see clearly; only when the cornea is clear and smooth can your vision be clear. If the cornea becomes cloudy or warped, your vision will be blurry or distorted.
Astigmatism is a common condition. With astigmatism, instead of being perfectly round like a basketball, the cornea is shaped more like a football - longer in one direction than in another. This produces uneven focusing of light as it enters the eye, which results in distorted vision. Astigmatism can be something you are born with or it can be the result of various eye diseases or even eye surgery. Astigmatism is often easily corrected with glasses. Until recently, people with astigmatism have not been able to wear soft contact lenses with much success. Now there are disposable soft contacts available especially for people with astigmatism. Astigmatism can also be treated with refractive surgery, like LASIK or limbal relaxing incisions.
Keratoconus is a degenerative condition of the eye. It causes thinning of the cornea, allowing the cornea to become more cone-shaped than spherical. It is typically diagnosed in the patient's teen years and reaches its most severe state in the patient's twenties or thirties. The cause of keratoconus is unknown, and its progression is often unpredictable. In most cases, vision can be corrected with rigid gas permeable contact lenses. Severe progression of the disease may lead to a need for corneal transplant surgery.
Infections of the cornea are rare, but they can be blinding. Most people who get corneal infections have identifiable risk factors, such as contact lens wear or blepharitis (inflammation of the eyelids). Viruses can infect the cornea; often these are minor and are associated with the common cold. Sometimes, serious viruses like herpes can infect the cornea. Corneal infections usually produce some scar tissue after they heal. If the scar is off to the side of the cornea, it may not affect your vision. A scar in the center part of the cornea can greatly reduce vision, and may requires a corneal transplant to recover vision.
Corneal swelling can caused blurred vision and 'halos' around lights. Severe swelling can result in pain as the nerves in the cornea are damaged. Over-wear of contact lenses or poorly fit contact lenses is a common cause of corneal swelling. If improper contact use is the cause of swelling, it can be treated with eye drops by changing contact lenses and contact lens habits.
One of the most common disease causes of corneal swelling is called Fuchs' Corneal Dystrophy. In this disorder, the cells in the endothelium start to die off, and the cornea takes on too much water. The swollen cornea loses its transparency and becomes cloudy, reducing vision. In some cases of Fuchs' Corneal Dystrophy, the swelling can be treated with eye drops, but they are not a cure. In most cases, as the swelling progresses, a corneal transplant will become necessary.
Swelling can also be caused by the older style implants that were placed in the eye after cataract surgery in the 1970s and early 1980s. These implants occasionally cause the cornea to swell up, reducing vision. In many of these cases, a corneal transplant may become necessary.