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Corneal dystrophy

Corneal dystrophies are a group of genetic, often progressive, eye disorders in which abnormal material accumulates in the clear (transparent) outer layer of the eye (cornea). Corneal dystrophies may not cause symptoms (asymptomatic) in some individuals; in others they may cause significant vision impairment. The age of onset and specific symptoms vary among the different forms of corneal dystrophy. The disorders have some similar characteristics – most forms of corneal dystrophy affect both eyes (bilateral), progress slowly, do not affect other areas of the body, and tend to run in families. Most forms are inherited as autosomal dominant traits; a few are inherited as autosomal recessive traits.

Structure of the cornea

The cornea is unusual in that it has no blood vessels to provide its nourishment and oxygen. Instead, it receives its nourishment from the tears and the fluid (the aqueous humour) filling the space behind it. The cornea must remain completely transparent if it is to carry out its role as the eye’s outermost lens and the presence of even the smallest imperfection can interfere with this process.

The cornea consists of five distinct layers, each of which plays an important role.

Epithelium

The cornea’s outermost layer. It acts as a barrier to protect the rest of the cornea from foreign bodies and infections. It is filled with tiny nerve endings which means the cornea is very sensitive when touched. It also provides an extremely smooth surface, which is essential for good vision.

Bowman’s Layer

Lying directly beneath the epithelium, this is a thin, transparent sheet of tissue. If damaged, Bowman’s layer can form a scar when it heals.

Stroma

The stroma accounts for roughly 90 per cent of the cornea’s thickness. It consists mainly of water (75 per cent) and fibers of a material called collagen, which is very tough and elastic. The regular size and arrangement of the collagen in this layer gives the cornea its strength and elasticity, and ensures it remains transparent.

Descemet’s Membrane

Descemet’s Membrane is a thin, strong layer of tissue which acts as another protective barrier.

Endothelium

This is the innermost layer of the cornea and is extremely thin. It is vital to the transparency of the cornea because it acts as a pump and maintains the exact level of water in the stroma. Without this pump, the cornea would swell with water and would become hazy. Damage to this layer cannot be repaired and so can, therefore, cause swelling of the cornea which may lead to serious sight loss.

Effect of corneal dystrophies on the eye

Corneal dystrophies are rare conditions in which the cornea is altered without the presence of any inflammation, infection or other eye disease. The clearness (transparency) of the cornea is affected and vision may or may not be disturbed. Corneal dystrophies tend to run in families. They have been described in many different ways but because each dystrophy will start in a particular layer of the cornea, they are classified as epithelial dystrophies, stromal dystrophies or endothelial dystrophies.

The Different Types are:

KERATOCONUS OR CONE EYE

Keratoconus is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve. Keratoconus can cause substantial distortion of vision, with multiple images, streaking and sensitivity to light all often reported by the patient. It is typically diagnosed in the patient’s adolescent years and attains its most severe state between the ages of 20 and 40. If afflicting both eyes, the deterioration in vision can affect the patient’s ability to drive a car or read normal print.

Symptoms and Signs

As the cornea becomes more irregular in shape, it causes progressive nearsightedness and irregular astigmatism to develop, creating additional problems with distorted and blurred vision. Glare and light sensitivity also may occur.

Causes

New research suggests the weakening of the corneal tissue may be due to an imbalance of enzymes within the cornea. This imbalance makes the cornea more susceptible to oxidative damage from compounds called free radicals, causing it to weaken and bulge forward.

Risk factors for oxidative damage and weakening of the cornea include a genetic predisposition, explaining why keratoconus often affects more than one member of the same family. Keratoconus also is associated with over exposure to ultraviolet rays from the sun, excessive eye rubbing, a history of poorly fitted contact lenses and chronic eye irritation.

Treatment

Keratoconus treatment often depends on the severity of the kerataconus symptoms. During early stages, vision can be corrected with eyeglasses. As the condition progresses, rigid contacts may need to be worn so that light entering the eye is refracted evenly and vision is not distorted. You should also refrain from rubbing your eyes, as this can aggravate the thin corneal tissue and make symptoms worse.

Keratoconus can also be treated with Intacs, which are small curved implantable corneal devices that can reshape the cornea. Intacs are FDA approved and can help flatten the steep cornea found in keratoconus.

When good vision is no longer possible with other treatments, a corneal transplant may be recommended. This surgery is only necessary in about 10 percent to 20 percent of patients with keratoconus. In a corneal transplant, your eye specialist removes the diseased cornea from your eye and replaces it with a healthy donor cornea. A transplanted cornea heals slowly. It can take up to a year or more to recover good vision after corneal transplantation. While a corneal transplant will relieve the symptoms of keratoconus, it may not provide you with flawless vision; however, of all conditions requiring corneal transplants, keratoconus has the best prognosis for clear vision.

Treatments for moderate and advanced keratoconus include:

In the mildest form of keratoconus, eyeglasses or soft contact lenses may help. But as the disease progresses and the cornea thins and becomes increasingly more irregular in shape, glasses and regular soft contact lens designs no longer provide adequate vision correction.

Custom soft contact lenses.

Contact lens manufacturers have introduced custom soft contact lenses specially designed to correct mild-to-moderate keratoconus. These lenses are made-to-order based on detailed measurements of the person’s keratoconic eye(s) and may be more comfortable than gas permeable (GP) or hybrid contact lenses for some wearers.

KeraSoft is a range of soft and silicone hydrogel contact lenses designed to manage the condition of irregular corneas including keratoconus. They are marketed as an alternative to rigid gas-permeable lenses, offering improved comfort and longer wearing times. Ophthalmologists are using the lenses both pre and post-operatively, following cross-linking surgery and as a safer and longer term solution than corneal transplants.

Gas permeable contact lenses.

If eyeglasses or soft contact lenses cannot control keratoconus, then rigid gas permeable (RGP or GP) contact lenses are usually the preferred treatment. Their rigid lens material enables GP lenses to vault over the cornea, replacing its irregular shape with a smooth, uniform refracting surface to improve vision.

But RGP contact lenses can be less comfortable to wear than soft contacts.

Also, fitting contact lenses on a keratoconic cornea is challenging and time-consuming. You can expect frequent return visits to fine-tune the fit and the prescription, especially if the keratoconus continues to progress.

“Piggybacking” contact lenses. Because fitting a gas permeable contact lens over a cone-shaped cornea can sometimes be uncomfortable for a person with keratoconus, some eye care practitioners advocate “piggybacking” two different types of contact lenses on the same eye.

For keratoconus, this method involves placing a soft contact lens, such as one made of silicone hydrogel, over the eye and then fitting a GP lens over the soft lens. This approach increases wearer comfort because the soft lens acts like a cushioning pad under the rigid GP lens. However, most modern contacts both GP and soft typically have adequate oxygen permeability for a safe “piggyback” fit.

ClearKone hybrid contact lenses. (SynergEyes Inc.) These hybrid contact lenses combine a highly oxygen-permeable rigid center with a soft peripheral “skirt.” The ClearKone version was designed specifically for keratoconus and vaults above the eye’s cone shape for increased comfort.

The manufacturer says hybrid contacts provide the crisp optics of a GP lens and wearing comfort that rivals that of soft contact lenses.

ClearKone hybrid lenses are available in a wide variety of parameters to provide a fit that conforms well to the irregular shape of a keratoconic eye.

Scleral and semi-scleral lenses.

Larger diameters found in these gas permeable (GP) lenses enable edges to rest on the eye’s white sclera. Scleral lenses cover a larger portion of the sclera, whereas semi-scleral lenses cover a smaller area.

Because the center of scleral and semi-scleral lenses vaults over the irregularly shaped cornea, these lenses don’t apply pressure to the eye’s cone-shaped surface for a more comfortable fit. These larger lenses also are more stable than conventional gas permeable contact lenses, which move with each blink because they cover only a portion of the cornea.

One example of a scleral lens is the Boston Scleral Lens Prosthetic Device (BSLPD). In addition to correcting vision problems associated with keratoconus, this device is used to treat severe dry eyes that may be caused by Sjogren’s syndrome and other systemic diseases and to manage complications associated with a corneal transplant.

To qualify for the BSLPD, you must have a severe, unusual or otherwise not treatable condition.

Another treatment option for keratoconus that is not FDA approved is collagen cross-linking. Collagen cross-linking is a new treatment that uses a special laser and eye drops to promote “cross-linking for keratoconus” or strengthening of the collagen fibers that make up the cornea. This treatment may flatten or stiffen the cornea, preventing further protrusion.