The cornea is the transparent, outer layer of the human eye. It functions as a barrier against dirt, germs, and any other thing that could damage your eye. It also plays a vital role in vision. When light enters your eye, it is bent by the cornea’s curved edge and this determines how well your eye can focus on objects.
Unlike the majority of the tissues in the human body, the cornea is made up of proteins and cells. Blood vessels have the potential to cloud the cornea, which could prevent it from bending light properly, leading to vision problems.
Epithelium: The epithelium has two main functions. It serves as a barrier against foreign material, such as dust, water, and bacteria; and absorbs oxygen and nutrients before it distributes them to the rest of the cornea.
Bowman’s Membrane: The Bowman’s Membrane is a transparent sheet of tissue that is made of protein fibrescalled collagen. The Membrane can form scars after healing, and these scars can lead to partial vision loss.
Corneal Stroma: Also known as the substantia propria, the stroma is the thickest part of the cornea and lies between the Bowman’s Membrane and the Descemet’s Membrane.
Descemet’s Membrane: This thin sheet of tissue is a protective barrier against infection and injuries. The membrane is made up of endothelial cells that lie below it as well as collagen fibres which are different from those of the stroma. Descemet’s Membrane regenerates promptly after injury.
Corneal Endothelium: Fluid often leaks slowly from inside the eye into the stroma. The endothelium pumps the excess fluid out of the stroma. If this process did not occur, the stroma would swell with water and become blurry.
Regardless of their different structures and functions, the 5 layers of the cornea must be free of any cloudy patches. Any opaque patches on the cornea have the potential to cause vision problems.
The cornea serves two main functions. It shields the human eye from germs, dust, and other harmful matter. It also functions like a lens that controls the amount of light that enters the eye. Besides these functions, the cornea also filters out powerful ultraviolet light. Without this protection, your eye lens and retina would be vulnerable to injury from UV light.
As mentioned previously, the cornea is responsible for bending the light that goes into your eye. This process is similar to the way a camera takes pictures. Think of the cornea as a camera lens and the retina as the film. If the image is out of focus, the film (or retina) captures a blurry image. For clear sight, your cornea and eye lens focus light so that it falls precisely on the retina. The retina then converts the rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.
A number of conditions or events can hinder the cornea’s ability to function correctly. These include:• Keratitis: Inflammation of the cornea can lead to corneal scarring and even blindness. • Keratoconus: A disease that thins the cornea, causing it to change shape.• Injuries: Physical trauma to the cornea can cause corneal abrasions (scratches).
Depending on the severity of the damage caused by injuries or infection, vision may be severely compromised by cornea problems. For instance, a deep corneal abrasion may lead to complications such as infection, piercing of the cornea, displacement of the iris, and tissue destruction in the eye socket. Keratitis, on the other hand, can lead to the formation of an open sore on the cornea, known as a corneal ulcer.
If your cornea problem is caused by an abrasion, you will have to see an ophthalmologist for diagnosis and treatment. Some corneal abrasion treatment options include:• Wearing an eye patch over your injured eye to prevent you from blinking and making the injury worse.• Using moisturizing eye drops or ointment that soothes the cornea.• Making use of antibiotic eye drops or ointment to prevent an eye infection from taking place. You may also receive eye drops to dilate your pupils to relieve pain.• Use special contact lenses for pain relief.
Corneal abrasion healing time varies depending on the extent of the injury. Small abrasions usually heal within 1 to 2 days, while larger abrasions may take about a week to heal.
Corneal disease is a blanket term that refers to the many conditions that affect this part of your eye. The conditions include infections, tissue breakdown, and genetic disorders. The common symptoms of these cornea problems include pain, blurred vision, tearing, redness, and extreme sensitivity to light.
While the symptoms of common cornea problems may seem non-specific, there are some that present particular symptoms that narrow down the possible diagnoses. For instance, keratoconus symptoms include the formation of a Fleischer ring (a ring of discolouration around the front of the eye).
Corneal dystrophies are genetic eye disorders in which cause abnormal material to accumulate in the cornea. Most corneal dystrophies affect both eyes, progress over an extended period of time, do not affect other body parts, and tend to run in families.
There are over 20 different types of corneal dystrophies, and each type presents with different symptoms. Dystrophies are grouped according to which part of the cornea they affect. The three major categories are:
• Anterior Corneal Dystrophies: These dystrophies affect the epithelium and the Bowman’s Membrane and include Epithelial basement membrane dystrophy, Lisch corneal dystrophy, Meesmann corneal dystrophy, Reis-Bucklers corneal dystrophy and Thiel-Behnke corneal dystrophy.
• Stromal Corneal Dystrophies: This category of dystrophies affects the stroma but can also progress to other layers. Examples include Gelatinous drop-like corneal dystrophy, Granular corneal dystrophy, Lattice corneal dystrophy, Macular corneal dystrophy, Schnyder crystalline corneal dystrophy.
• Posterior Corneal Dystrophies: These dystrophies affect the Descemet Membrane as well as the endothelium. The dystrophies included in this group are Congenital hereditary endothelial dystrophy, Fuchs’ endothelial corneal dystrophy, Posterior polymorphous corneal dystrophy.
Symptoms of corneal dystrophies include:• Watery or dry eyes• Glare• Sensitivity to light• Foreign body sensation (the feel of something in your eye).
The cornea can heal from minor injuries on its own. In the case of an abrasion, healthy cells slide patch the injury before it causes infection or causes vision problem. However, deeper scratches can cause corneal scarring. When this is the case, a cornea transplant may be needed.
A cornea transplant, sometimes referred to as keratoplasty, is a surgical procedure that involves replacing the damaged part of your cornea with corneal tissue from a donor. Cornea transplant surgery can restore vision, reduce pain and improve a damaged cornea’s appearance.
During the procedure, a surgeon cuts through the diseased cornea to remove corneal tissue into the shape of a small disk. The donor cornea, which is cut to fit the opening, is then placed where the diseased one was removed. The new cornea is then set in place with stitches that are removed at a later visit. If a person is not eligible for a transplant, an artificial cornea is inserted. This procedure is known as keratoprosthesis.
With certain cornea problems, a cornea transplant isn’t always the way to go. In these instances, only the damaged or diseased tissue is removed and replaced. These types of procedures are:
• Endothelial keratoplasty: Diseased tissue is removed from the endothelium and Descemet Membrane before donor tissue is implanted.
• Anterior lamellar keratoplasty: This procedure involves removing diseased tissue from the epithelium and the stroma while leaves the back endothelial layer in place.
Although nothing can be done to prevent corneal diseases such as dystrophies, steps can be taken to prevent infections. Bacteria and fungi away must be kept away from the eye. Additionally, it is advised to wash your hands before and after handling your contacts or glasses.
Simple measures can also be taken to prevent corneal abrasions, which have the potential to cause corneal disease. Always wear protective eyewear in work environments that have airborne debris. Protective eyewear should also be worn when using power tools or playing sports.
If you wear contact lenses, be sure to always carefully following your eye doctor’s instructions regarding how long to wear them, when to stop using them, and which contact lens care solutions are best for you. If you have reason to believe your corneal abrasions are caused by dry eyes, visit an optometrist or ophthalmologist. If the diagnosis is confirmed, follow the treatment protocol recommended by the doctor.
Since some corneal problems are asymptomatic, it’s a good idea to regularly see an eye doctor for checkups. As is the case with many medical conditions, an ounce of prevention is better than a pound of care. Cornea transplant recipients should be especially vigilant about their eye health. Although the donated tissue usually lasts a lifetime, patients are advised to see a doctor once or two a year after the procedure.
About Dr. Sachin Bawa
Sachin Bawa is a founder of Dr. Sachin Bawa Cataract and Vision Clinic. He is a qualified ophthalmic surgeon for more than 9 years. He holds a medical degree from the University of Witswatersrand which he completed in 2004.