What Is The Cornea?
The cornea, the eyes outermost layer, is clear and dome shaped. It covers the front of the eye and has the major task of focusing one’s vision. Because there are no blood vessels in the tissue of the cornea, nourishment comes directly from tears and the aqueous humor. There are five layers to this tissue, with each having important functions.
Epithelium (Outermost Layer)
The epithelium blocks any foreign material such as dust particles and bacteria from entering the eye as well as allowing for the absorption of oxygen and nutrients to the rest of the cornea.
This layer, behind the basement membrane of the epithelium, contains fibers called collagen. If large scars develop from injury to Bowman’s layer in the cornea, vision loss may occur.
Behind Bowman’s layer is the stroma, the thickest layer of the cornea. Much like the previous layer, the stroma contains collagen. Collagen is great for strength and elasticity in the eye as well as producing light-conducting transparency in the cornea.
The next layer of tissue, behind the stroma, is Descemet’s membrane. This tissue protects against infection and injuries for it contains collagen fibers that are unique compared to stroma fibers. This layer, once injured, can repair itself quite well.
Endothelium (Innermost Layer)
Endothelial cells are important in that they keep the cornea clear. Normally, with fluid leakage to the stroma from the inside of the eye, the endothelium pumps excess fluid out. If this did not occur, the stroma would result in over swelling, thus becoming thick and opaque. In a healthy eye, there needs to be homeostasis between fluid moving into the cornea and fluid being pumped out.
Symptoms Of Corneal Conditions
The cornea has the ability to heal quickly after most injuries or diseases, due to its ability to repair naturally. However, with natural repair to the cornea occurring, there may be some major symptoms that call for medical attention:
- Pain in the eye
- Excessive redness and tearing
- Sensitivity to light
- Scarring to the cornea
- Blurred vision
If one is experiencing these symptoms, he or she should seek immediate professional eye care.
Conditions That Affect The Cornea
Dry Eye is a condition in which the eye produces less tears but also ones that are lower in quality. This condition causes an inability for surface lubrication of the eye. Symptoms may include:
- A scratchy feeling in the eye
- A burning or stinging sensation in the eye
- Excess tearing followed by periods of dryness
- Discharge from the eye
- A feeling of heavy eyelids or blurred vision
When the weather is warm and dry, pollen related allergies affect the eye. Symptoms may be minor with no need for medical attention, but they tend to be bothersome. Antihistamine decongestant eye drops will effectively reduce any symptoms. In the event of rain and cooler weather, symptoms may reduce due to the decrease in pollen in the air.
Noninfectious Keratitis, or inflammation in the cornea, can be caused by a minor injury as well as from wearing contact lenses for too long. Viruses or bacteria can cause infectious Keratitis in the cornea, often related to improper cleaning of contact lenses, or the overuse of old contact lenses that should have otherwise been discarded. Antibacterial eye drops may treat minor corneal infections but if the problem is more severe, intensive antibiotic or antifungal treatment may be required.
Corneal diseases affect vision in many different ways. Some diseases cause episodes of pain in the eye, some cause severe visual impairment, while others cause no vision problems, with discovery only occurring during routine eye exam. Some of the most common corneal diseases include keratoconus or thinning of the cornea, Fuchs’ dystrophy or gradually worsening vision, lattice dystrophy or lattice-like patterns of deposits in the stroma, and map-dot- fingerprint dystrophy or abnormal development of the basement membrane in the cornea.
Shingles, or the reactivation of the varicel-lazoster virus, is the same virus that causes chickenpox. The virus may travel through nerve fibers and emerge in the cornea, thus causing decreased sensitivity. Once the shingles have cleared it is important to have the eyes examined several months later, for the symptoms of shingles may still be present.
Ocular Herpes, or herpes of the eye, is a persistent viral infection that is caused by the herpes simplex virus, which is known to cause cold sores. The sexually transmitted herpes simplex virus, also known to cause genital herpes, can also cause ocular herpes. This condition may present itself as sores on the eyelid or surface of the cornea with a deeper spreading into the cornea, causing inflammation. There is no cure for ocular herpes but over time it may be controlled with antiviral medication.
Iridocorneal Endothelial Syndrome (ICE)
Iridocorneal Endothelial Syndrome usually presents itself in only one eye, for any given case, with three main symptoms:
- Swelling in the cornea
- Changes in the iris, visible change in the colored part of the eye
The symptoms listed above are caused by the movement of endothelial cells from the cornea to the iris. This movement of cells also blocks fluid outflow channels of the eye, causing glaucoma. Glaucoma is treatable but the progression of ICE is unpreventable. If swelling in the cornea becomes too severe, a transplant of the cornea may be necessary for ultimate relief.
Pterygium is a tissue growth on the cornea. Some Pterygia grow slowly over a lifetime, with other Pterygia stop growing early in their development. A Pterygium rarely grows so large that it covers the eye’s pupil. Researchers believe that chronic exposure to UV light from the sun may be a determining factor for developing a Pterygium. Simple fixes like wearing protective sunglasses and hats while in the sun could drastically decrease the development of a Pterygium. Unless a Pterygium affects vision, it is recommended that one does not get it surgically removed for it may grow back over time.
If surgery for a Pterygium is the best option for a patient, our doctors will reconstruct the outer surface of the cornea with an amniotic membrane graft to correct the cornea. This technique prevents reoccurrence and additionally promotes good cosmetic appearance.
Stevens-Johnson Syndrome (SJS)
Stevens-Johnson Syndrome, called erythema multiforme major, is a disorder of the skin that affects the eyes. Symptoms include:
- Painful blisters on the skin and in the mouth, throat, genitals, and eyelids
- Flu-like symptoms
- A rash of blisters that spreads
SJS can cause severe conjunctivitis, inflammation inside the eye (iritis), corneal blisters and holes, with a possibility for significant vision loss.
Treatment For Corneal Conditions and Disease
Phototherapeutic keratectomy (PTK) is a surgical technique that uses laser technology and UV light to restore the cornea. This technique helps to postpone the need for corneal grafting and/or replacement.
Corneal Transplant Surgery
This type of surgery removes portion of the cornea that is damaged and needs to be replace. The replacement procedure uses healthy donor tissue. Penetrating keratoplasty, in the past, was the standard procedure for corneal transplant with replacing the entire cornea with donor tissue. This procedure may be the only option for advanced keratoconus and corneal scarring.
A newer procedure, called lamellar keratoplasty, is a partial thickness transplant that may be helpful for less severe corneal injury. In this type of transplant, the surgeon moves and replaces the layer(s) of the cornea that are diseased with healthy tissue. This will leave the cornea more structurally intact and prevents complications in the future.
Descemet’s Stripping with Endothelial Keratoplasty (DSEK) is also a newer cornea transplant technique that replaces damaged cell layers of the cornea instead of the entire thickness of the cornea. This technique allows for a speedy recovery and healing that is much stronger. Because this technique improves visual results with a smoother interface, it has become the preferred treatment method for Fuchs’ dystrophy.
The FDA recently approved cross-linking of the cornea. During this procedure, riboflavin drops saturate the cornea. Research has shown that increasing the amount of collagen cross-linking will increase the strength of the cornea. Although this is not a cure for keratoconus (thinning of the cornea), it will slow down the progression of keratoconus and need for a corneal transplant.
Intacs are thin plastic, semi-circular rings that are inserted into the middle layer of the cornea. When they are inserted in the keratoconus cornea they change the shape and location of the cone, flattening the cornea. The placement of Intacs reinforces the cornea and eliminates some or all of the irregularities caused by keratoconus.
A keratoprosthesis is an artificial cornea called KPro. This procedure may be the only option for those who have not had successful corneal tissues implants or have had some type of tissue rejection. The Boston type-1 KPro is made of clear plastic, consists of three parts, and is the most used keratoprosthesis. This procedure is achieved by taking donor tissue and clamping that tissue between the front and back plates of the artificial cornea.