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Cornea & External Eye Diseases |
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 The cornea is the eye's outermost layer. It is the clear, dome-shaped surface that covers the front of the eye. It is like the watch glass of a wrist watch.
Cornea is a powerful refracting surface and provides about 2/3rd of the eye's focusing power. It provides us a clear window to look through and its clarity is critical for good vision. Diseases of the cornea lead to loss of its transparency, thus reducing the vision. Injury, infection, genetic diseases and malnutrition can affect the functioning of the Cornea. |
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Cornea is a highly organized group of cells and proteins. Unlike most tissues in the body, the cornea contains no blood vessels to nourish or protect it against infection. Instead, the cornea receives its nourishment from the tears and aqueous humor that fills the chamber behind it. The cornea must remain transparent to refract light properly, and the presence of even the tiniest blood vessels can interfere with this process. To see well, all layers of the cornea must be free of any cloudy or opaque areas.
The corneal tissue is arranged in five basic layers, each having an important function. These five layers are: |
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| The epithelium is the cornea's outermost region, comprising about 10 percent of the tissue's thickness. The epithelium functions primarily to: (1) Block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea; and (2) Provide a smooth surface that absorbs oxygen and cell nutrients from tears, then distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane. |
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| Lying directly below the basement membrane of the epithelium is a transparent sheet of tissue known as Bowman's layer. It is composed of strong layered protein fibers called collagen. Once injured, Bowman's layer can form a scar as it heals. If these scars are large and centrally located, some vision loss can occur. |
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| Beneath Bowman's layer is the stroma, which comprises about 90 percent of the cornea's thickness. It consists primarily of water (78 percent) and collagen (16 percent), and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency. |
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| Under the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. Descemet's membrane is composed of collagen fibers (different from those of the stroma) and is made by the endothelial cells that lie below it. Descemet's membrane is regenerated readily after injury. |
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| The endothelium is the extremely thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear. Normally, fluid leaks slowly from inside the eye into the middle corneal layer (stroma). The endothelium's primary task is to pump this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water, become hazy, and ultimately opaque. In a healthy eye, a perfect balance is maintained between the fluid moving into the cornea and fluid being pumped out of the cornea. Once endothelium cells are destroyed by disease or trauma, they are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy. |
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How does the cornea respond to injury? |
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| The cornea copes very well with minor injuries or abrasions. If the highly sensitive cornea is scratched, healthy cells slide over quickly and patch the injury before infection occurs and vision is affected. If the scratch penetrates the cornea more deeply, however, the healing process will take longer, at times resulting in greater pain, blurred vision, tearing, redness, and extreme sensitivity to light. These symptoms require professional treatment. Deeper scratches can also cause corneal scarring, resulting in a haze on the cornea that can greatly impair vision. In this case, a corneal transplant may be needed. |
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What does cornea service include? |
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The cornea service is dedicated to the medical and surgical treatment of problems involving the cornea and other anterior structures of the eye like eyelids, conjunctiva, sclera, anterior chamber, iris and lens.
The clinic caters to the following Cornea & External Eye Diseases:–
- Corneal infections
- Eye allergies corneal complications arising from other forms of ocular surgery
- Ocular surface trauma
- Ocular surface disorders like blepharitis, meibomitis, conjunctival and scleral diseases
- Refractive errors (nearsightedness, farsightedness and astigmatism)
- Conjunctivitis (red eye)
- Dry eye syndrome
- Ectatic corneal diseases like keratoconus, keratoglobus and pellucid marginal degeneration
- Pterygium
- Corneal dystrophies and degenerations
Surgical procedures:-
- Corneal transplantation
- Keratoprosthesis (Artificial cornea)
- Corneal Collagen Cross-Linking with Riboflavin (C3R)
- Scleral grafting
- Ocular Surface Reconstruction- corneal, limbal, and amniotic membrane transplants allow ocular surface reconstruction in severe surface disease
- Tissue adhesive application to seal corneal perforation
- Anterior stromal puncture
Other facilities:-
- Specular Microscopy- Study of the corneal endothelium (cells that maintain corneal clarity)
- Pachymetry- Measurement of corneal thickness
- Corneal Topography (by Orbscan)- Mapping of the cornea
- Aberrometry- Measurement of the aberrations in the eye
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What are some diseases and disorders affecting the cornea and ocular surface? |
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| Some diseases and disorders of the cornea are: |
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This term describes a group of diseases that cause swelling, itching, burning, and redness of the conjunctiva, the protective membrane that lines the eyelids and covers exposed areas of the sclera, or white of the eye. The patient may also have discharge and watering. It is essentially inflammation of the conjunctiva.
Acute infective conjunctivitis may be due to various microorganisms. The commonest organisms are various bacteria and viruses. Bacterial conjunctivitis usually requires treatment in the form of antibiotic eyedrops. Viral conjunctivitis usually does not require medication and the body's defence mechanisms will clear the infection. Cold compresses will provide relief. |
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Sometimes the cornea is damaged after a foreign object has penetrated the tissue, such as from a poke in the eye. At other times, bacteria or fungi from a contaminated contact lens can pass into the cornea. Situations like these can cause painful inflammation and corneal infections called keratitis. These infections can reduce visual clarity, produce corneal discharges, and perhaps erode the cornea. Corneal infections can also lead to corneal scarring, which can impair vision and may require a corneal transplant.
As a general rule, the deeper the corneal infection, the more severe the symptoms and complications. We should bear in mind that corneal infections, although relatively infrequent, are the most serious complication of contact lens wear.
Depending on the cause corneal infections are treated with antibacterial, antifungal, antiviral or antiprotozoal eye drops. If the problem is very severe, it may require more intensive treatment to eliminate the infection. Frequent visits to the eye doctor may be necessary for several months to eliminate the problem. |
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"Dry eye" is a condition characterized by an unstable tear film causing irritation and discomfort in the eye. This is because of either decreased quantity of tears or their increased evaporation from the eye. The symptoms are non-specific and can range from a tired or itching eye to diminished vision in severe cases.
The tear film consists of three layers--an outer, oily (lipid) layer that keeps tears from evaporating too quickly and helps tears remain on the eye; a middle (aqueous) layer that nourishes the cornea and conjunctiva; and a bottom (mucin) layer that helps to spread the aqueous layer across the eye to ensure that the eye remains wet. As we age, the eyes usually produce fewer tears. Also, in some cases, the lipid and mucin layers produced by the eye are of such poor quality that tears cannot remain in the eye long enough to keep the eye sufficiently lubricated.
The main symptom of dry eye is usually a scratchy or sandy feeling as if something is in the eye. Other symptoms may include stinging or burning of the eye; episodes of excess tearing that follow periods of very dry sensation; a stringy discharge from the eye; and pain and redness of the eye. Sometimes people with dry eye experience heaviness of the eyelids or blurred, changing, or decreased vision, although loss of vision is uncommon.
Dry eye is more common in women, especially after menopause. Surprisingly, some people with dry eye may have tears that run down their cheeks. This is because the eye may be producing less of the lipid and mucin layers of the tear film, which help keep tears in the eye. When this happens, tears do not stay in the eye long enough to thoroughly moisten it.
Dry eye can occur in climates with dry air, as well as with the use of some drugs, including antihistamines, nasal decongestants, tranquilizers, and anti-depressant drugs. People with dry eye should let their doctor know all the medications they are taking, since some of them may intensify dry eye symptoms.
People with connective tissue diseases, such as rheumatoid arthritis, can also develop dry eye. It is important to note that dry eye is sometimes a symptom of Sjögren's syndrome, a disease that attacks the body's lubricating glands, such as the tear and salivary glands. A complete physical examination may diagnose any underlying diseases.
Artificial tears, which lubricate the eye, are the principal treatment for dry eye. They are available over-the-counter as eye drops. Sterile ointments are sometimes used at night to help prevent the eye from drying. Using humidifiers, wearing wrap-around glasses when outside, and avoiding outside windy and dry conditions may bring relief. For people with severe cases of dry eye, temporary or permanent closure of the tear drain (small openings at the inner corner of the eyelids where tears drain from the eye) may be helpful. |
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A corneal dystrophy is a condition in which one or more parts of the cornea lose their normal clarity due to a buildup of cloudy material. There are over 20 corneal dystrophies that affect all parts of the cornea. These diseases share many traits:
- They are usually inherited.
- They affect the right and left eyes equally.
- They are not caused by outside factors, such as injury or diet.
- Most progress gradually.
- Most usually begin in one of the five corneal layers and may later spread to nearby layers.
- Most do not affect other parts of the body, nor are they related to diseases affecting other parts of the eye or body.
- Most can occur in otherwise totally healthy people, male or female.
Corneal dystrophies affect vision in widely differing ways. Some cause severe visual impairment, while a few cause no vision problems and are discovered during a routine eye examination. Other dystrophies may cause repeated episodes of pain without leading to permanent loss of vision.
Some of the most common corneal dystrophies include Fuchs' dystrophy, Keratoconus, Lattice dystrophy, Map-Dot-Fingerprint dystrophy and Macular dystrophy |
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Keratoconus is a condition in which the middle of the cornea thins and gradually bulges outward, forming a conical shape. This abnormal curvature changes the cornea's refractive power, producing moderate to severe distortion (astigmatism) and blurriness (nearsightedness) of vision. Keratoconus may also cause swelling and a sight-impairing scarring of the tissue.
Studies indicate that keratoconus stems from one of several possible causes:
- An inherited corneal abnormality. About seven percent of those with the condition have a family history of keratoconus.
- An eye injury, i.e., excessive eye rubbing or wearing hard contact lenses for many years.
- Certain eye diseases, such as retinitis pigmentosa, retinopathy of prematurity, and vernal keratoconjunctivitis.
- Systemic diseases, such as Leber's congenital amaurosis, Ehlers-Danlos syndrome, Down syndrome, and osteogenesis imperfecta.
Keratoconus usually affects both eyes. At first, people can correct their vision with eyeglasses. But as the astigmatism worsens, they must rely on specially fitted contact lenses to reduce the distortion and provide better vision.
If the keratoconus progresses to a stage where contact lens cannot be stabilized over the bulging cornea or the cornea has become too scarred, a corneal transplant may be needed. This operation is successful in more than 90 percent of those with advanced keratoconus.
By nature. Keratoconus is a progressive disease. For decades the mainstay of treatment for keratoconus involved visual rehabilitation by spectacles, contact lenses (semi-soft) or cornea transplantation. There was nothing available to arrest or slow the progression of the disease. We now have a surgery called Corneal Collagen Cross-Linking with Riboflavin (C3R) which does this. It strengthens and stabilizes the cornea by promoting formation of bonds between the collagen sheets comprising the cornea. This surgery is showing very promising results. |
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A pterygium is a pinkish, triangular-shaped tissue growth on the cornea. Some pterygia grow slowly throughout a person's life, while others stop growing after a certain point. A pterygium rarely grows so large that it begins to cover the pupil of the eye.
Pterygia are more common in sunny climates and in the 20-40 age group. The causes for the development of pterygia are still not well understood. However, since people who have pterygia usually have spent a significant time outdoors, many doctors believe ultraviolet (UV) light from the sun may be a factor. In areas where sunlight is strong, wearing protective eyeglasses, sunglasses, and/or hats with brims are suggested. While some studies report a higher prevalence of pterygia in men than in women, this may reflect different rates of exposure to UV light.
Because a pterygium is visible, many people want to have it removed for cosmetic reasons. Surgery can remove the pterygia. However, it should be noted that, pterygia are notorious for recurrences and the recurrences are usually even more aggressive. Lubricants can reduce the redness and provide relief from the chronic irritation. |
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What is a corneal transplant? |
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A corneal transplant involves replacing a diseased or scarred cornea with a new one. When the cornea becomes cloudy, light cannot penetrate the eye to reach the light-sensitive retina. Poor vision or blindness may result.
In corneal transplant surgery, the surgeon removes the central portion of the cloudy cornea and replaces it with a clear cornea, usually donated through an eye bank. Following surgery, eye drops to help promote healing will be needed for several months.
The chances of success of this operation have risen dramatically because of technological advances, such as less irritating sutures, or threads, which are often finer than a human hair; and the surgical microscope. Corneal transplantation has restored sight to many, who a generation ago would have been blinded permanently by corneal injury, infection, or inherited corneal disease or degeneration. |
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What is the success rate of cornea transplant? |
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Corneal transplants are one of the most common and most successful of all transplants.
The procedure may be successful in upto 90 percent of some cases. However, the success depends on a number of factors- recepient's eye condition and corneal disease, donor cornea quality and the surgical technique and skill.
The first year after a transplant is the most crucial, because most complications, if they occur, will occur within the first year of the transplant. The more common complications include increased eye pressure which can cause damage to the eye nerve (glaucoma) and corneal graft rejection.
In the event of graft failure due to rejection, corneal transplantation may be repeated several times if previous transplants have failed. However, the success rate of repeat transplants is lower than the first time. In such repeat grafts, immunosuppressive medication may be used to prevent rejection.
Warning signs for rejection are decreased vision, increased redness of the eye, increased pain, and increased sensitivity to light. If any of these last for more than six hours, you should immediately call your eye doctor. Rejection can be successfully treated if medication is administered at the first sign of symptoms. |
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Where does the donor cornea come from? |
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| The healthy donor cornea used for transplantation is supplied by an Eye Bank. Eye Banks collect, evaluate, store and distribute donated corneas. The corneas are collected from human donors within a few hours of death. Stringent tests are performed to ensure the safety of the person receiving the cornea. The Eye Bank verifies the donor's medical history and cause of death, and performs blood tests to ensure that the deceased person did not have contagious diseases like AIDS, hepatitis B or C. |
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