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What is the Cornea?
The cornea is the eye’s outermost layer. It is the clear, dome-shaped surface that covers the front of the eye and plays an important role in focusing your vision. The cornea acts as a barrier against dirt, germs, and other particles that can harm the eye. The cornea shares this protective task with the eyelids and eye sockets, tears, and the sclera (white part of the eye). The cornea also plays a key role in vision by helping focus the light that comes into the eye. The cornea is responsible for 65-75 percent of the eye’s total focusing power. The cornea consists of a highly ordered group of cells and proteins with essential nerve endings. The human cornea has five layers, listed here outermost to innermost.
The Cornea
Corneal Epithelium
The cornea’s outermost region makes up approximately 10 percent of the tissue’s thickness.
Bowman’s Layer
This sturdy layer of collagen protein fibers is beneath the membrane of the epithelium. If this layer is damaged, it can develop a scar when it heals. If the scar is towards the middle of the eye, it often causes some vision loss.
Corneal Stroma
The stroma layer comprises 90 percent of the cornea’s tissue. The unique shape of this layer and lack of blood vessels produce the cornea’s transparency.
Descemet’s Membrane
This layer is a thin piece of tissue. Its main function is to protect against injuries and infection. The Descemet’s membrane consists of collagen fibers.
Corneal Endothelium
An extremely thin layer, this is the innermost layer of the cornea. These cells are necessary in order to keep the cornea transparent.
Why are Tears Important to the Cornea?
Every time we blink, tears are distributed across the cornea to keep the eye moist, help wounds heal, and protect against infection. Tears form in three layers...
The Cornea
Lipid Layer
An outer, oily (lipid) layer that keeps tears from evaporating too quickly and helps tears remain on the eye.
Aqueous Layer
A middle (aqueous) layer that nourishes the cornea and the conjunctiva – the mucous membrane that covers the front of the eye and the inside of the eyelids.
Mucin Layer
A bottom (mucin) layer that helps spread the aqueous layer across the eye to ensure that the eye remains wet.
Common Conditions That Affect the Cornea
Minor injuries or scratches to the cornea can usually heal on their own. However, deeper injuries can cause corneal scarring, resulting in a haze on the cornea that impairs vision. If you have a deep injury, or a corneal disease or disorder, you could experience:
  • Pain in the eye
  • Sensitivity to light
  • Reduced vision or blurry vision
  • Redness or inflammation in the eye
  • Headache, nausea, fatigue
If you experience any of these symptoms, seek help from an eye care professional.
The most common allergies that affect the eye are those related to pollen, particularly in the spring and summer months when the weather is warm and dry. Allergy Symptoms include redness, itching, tearing, burning, stinging, and watery discharge, although usually not severe enough to require medical attention. Antihistamine decongestant eye drops effectively reduce these symptoms. Rain and cooler weather, which decreases the amount of pollen in the air, can also provide relief.
Keratitis is an inflammation of the cornea. Noninfectious keratitis is usually caused by a minor injury or from wearing contact lenses too long. Infectious keratitis is usually caused by bacteria, viruses, fungi or parasites. Often, these infections are also related to contact lens wear, improper cleaning of contact lenses and overuse of old contact lenses. Minor corneal infections are usually treated with antibacterial eye drops. If the problem is severe, it may require more intensive antibiotic or antifungal treatment to eliminate the infection, as well as steroid eye drops to reduce inflammation.
Dry Eye Syndrome
Dry eye syndrome is a condition in which the eye produces fewer or lower quality tears and is unable to keep its surface lubricated. The main symptom of dry eye is usually a scratchy feeling or as if something is in your eye. Other symptoms include stinging or burning in the eye, episodes of excess tearing that follow periods of dryness, discharge from the eye, and pain and redness in the eye.
What are Corneal Dystrophies?
A corneal dystrophy is a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of material that clouds the cornea. These diseases are usually inherited, affect both eyes, progress gradually, happen in otherwise healthy people, don’t affect other parts of the body and aren’t related to diseases affecting other parts of the body. Corneal dystrophies affect vision in different ways. Some cause severe visual impairment, while a few cause no vision problems and are only discovered during a routine eye exam. Other dystrophies may cause repeated episodes of pain without leading to permanent vision loss.
Kerataconus is a progressive thinning of the cornea. It is the most common corneal dystrophy in the U.S., affecting one in every 2,000 Americans. It is most prevalent in teenagers and adults in their 20s.
Fuchs’ Dystrophy
Fuchs’ dystrophy is a slowly progressing disease that usually affects both eyes and is slightly more common in women than in men. It can cause your vision to gradually worsen over many years, but most people with Fuchs’ dystrophy won’t notice vision problems until they reach their 50s or 60s.
Lattice Dystrophy
Lattice dystrophy gets its name from a characteristic lattice-like pattern of deposits in the stroma layer of the cornea. The deposits are made of amyloid, an abnormal protein fiber. Over time, the deposits increase and the lattice lines grow opaque, take over more of the stroma, and gradually converge to impair vision.
Map-Dot-Fingerprint Dystrophy
Map-Dot-Fingerprint dystrophy, also known as epithelial basement membrane dystrophy, occurs when the basement membrane develops abnormally and forms folds in the tissue. The folds create gray shapes that look like continents on a map. There also may be clusters of opaque dots underneath or close to the map-like patches. Less frequently, the folds form concentric lines in the central cornea that resemble small fingerprints.
What other diseases can affect the cornea?
Herpes Zoster (Shingles)
Shingles is a reactivation of the varicella-zoster virus, the same virus that causes chickenpox. If you have had chickenpox, the virus can live on within your nerve cells for years after the sores have gone away. In some people, the varicella-zoster virus reactivates later in life, travels through the nerve fibers, and emerges in the cornea. If this happens, your eye care professional may prescribe oral anti¬-viral treatment to reduce the risk of inflammation and scarring in the cornea. Shingles can also cause decreased sensitivity in the cornea.
Ocular Herpes
Herpes of the eye, or ocular herpes, is a recurrent viral infection that is caused by the herpes simplex virus (HSV-1). This is the same virus that causes cold sores. Ocular herpes can also be caused by the sexually transmitted herpes simplex virus (HSV-2) that causes genital herpes.
Iridocorneal Endothelial Syndrome (ICE)
Iridocorneal endothelial syndrome (ICE) is more common in women and usually develops between ages 30¬-50. ICE has three main symptoms; visible changes in the iris, the colored part of the eye, swelling of the cornea and the development of Glaucoma.
A pterygium is a pinkish, triangular tissue growth on the cornea. Some pterygia (plural for pyterygium) grow slowly throughout a lifetime, while others stop growing. A pterygium rarely grows so large that it covers the pupil of the eye.
Stevens-Johnson Syndrome
Stevens-Johnson Syndrome (SJS), also called erythema multiforme major, is a disorder of the skin that also affects the eyes. SJS is characterized by painful blisters on the skin and the mucous membranes of the mouth, throat, genitals, and eyelids.
What treatments are there for advanced corneal disease?
Laser Surgery
Phototherapeutic keratectomy (PTK) is a surgical technique that uses UV light and laser technology to reshape and restore the cornea. PTK has been used to treat recurrent erosions and corneal dystrophies, such as map-dot-fingerprint dystrophy and basal membrane dystrophy. PTK helps delay or postpone corneal grafting or replacement.
Corneal Transplant Surgery
With full thickness transplants, the damaged cornea is removed and replaced with a donor cornea. Tiny stitches secure the transplant. Partial thickness transplants use fewer stitches. Either type of surgery usually takes 30 minutes and is performed as an outpatient procedure. Corneal transplant surgery removes the damaged portion of the cornea and replaces it with healthy donor tissue. Corneas are the most commonly transplanted tissue worldwide. More than 47,000 corneal transplants are performed in the U.S. each year. In the past, the standard approach to corneal transplants was to surgically replace the entire cornea with donor tissue, a technique known as penetrating keratoplasty. This is called a full thickness transplant, and may still be the only option for people with advanced keratoconus and scarring, severe herpetic scarring, or traumatic injury that affects the whole cornea. However, most people who require a cornea transplant undergo a newer procedure called lamellar keratoplasty. This is called a partial thickness transplant. In this procedure, the surgeon selectively removes and replaces the diseased layer(s) of the cornea and leaves the healthy tissue in place. Replacing only diseased layers with a donor graft leaves the cornea more structurally intact and leads to a lower rate of complications and better visual improvement. Corneal transplants are generally done under local anesthetic, take approximately 30 minutes and are performed as an outpatient procedure.
Anterior lamellar keratoplasty (ALK)
Anterior lamellar keratoplasty or ALK is an alternative treatment that selectively replaces the front part of the cornea when it is scarred or distorted. In ALK, the surgeon dissects the cornea into two thin pieces and removes the front, scarred part. A matching area of healthy tissue from a donor cornea is then used to replace the area that was removed. Your eye will be stronger after surgery and you will be able to resume normal activities sooner. Depending on the type of ALK, sutures may or may not be needed.
Endothelial lamellar keratoplasty (ELK)
Endothelial lamellar keratoplasty (ELK) is a cornea transplant technique that is the preferred way to restore vision when the inner cell layer of the cornea stops working properly. EK selectively replaces only the diseased layer of the cornea, leaving healthy areas intact.
Artificial Cornea
A keratoprosthesis (KPro) is an artificial cornea. A KPro may be the only option available for people who have not had success with corneal tissue implants or who have a high risk of tissue rejection (such as those with Stevens-Johnson syndrome or severe chemical burns). The Boston type-1 KPro is the most used keratoprosthesis. It is made of clear plastic and consists of three parts, with donor cornea tissue clamped between front and back plates. When fully assembled it has the shape of a collar button. The procedure to insert a KPro is performed by an ophthalmologist, usually on an outpatient basis.