An Overview of Keratoconus

Keratoconus is a medical condition that causes the eye's cornea to bulge outward into a cone shape, becoming steeper over time. The cornea is the clear, dome-like structure on the front part of the eye. "Keratoconus" is a Greek word meaning "cone-shaped cornea." With the condition, vision becomes extremely distorted and blurred.

Symptoms

Keratoconus may start appearing in the teenage years and level off after the age of 40. In the early stages, it may go unnoticed. Although keratoconus always seems to be worse in one eye, it is usually a condition that occurs in both eyes.

When keratoconus progresses, vision becomes very blurry and distorted. Vision worsens because, as the cornea bulges forward, irregular astigmatism (imperfection of the curvature of the eye) and nearsightedness (seeing near objects clearly, while objects farther away are blurry) develop.

As the condition progresses, corneal scarring may occur, causing further vision loss. Some patients with keratoconus notice frequent vision fluctuations, while others only see changes over a period of years.

People with keratoconus often complain that vision doesn't improve much with corrected eyeglasses. In some cases, the cornea can bulge forward and become so thin that scarring develops, further impeding vision. In rare cases, the cornea will decompensate, causing severely reduced vision or even blindness.

Causes

The exact cause of keratoconus is a bit of a mystery. However, scientists believe that genetics, the environment, and hormones may influence the development of keratoconus. Allergies may also be involved, and people who constantly rub their eyes may mechanically cause keratoconus.

Genetics

It is thought that some people have a genetic defect that causes certain protein fibers in the cornea to become weak. These fibers act to hold the cornea together, maintaining its clear, dome-like structure. When these fibers become weak, the cornea begins to bulge forward.

Some scientists believe that genetics plays a strong role in keratoconus because, occasionally, relatives will each develop keratoconus.

Environment

People with keratoconus tend to have allergies, specifically atopic allergic diseases such as hay fever, asthma, eczema, and food allergies. Many patients who develop keratoconus have a history of vigorous eye rubbing.

Some of these people have allergies and some don’t, but they all tend to rub their eyes. This vigorous eye rubbing can cause damage to the cornea, causing keratoconus to develop.

Another popular theory about what causes keratoconus involves oxidative stress. For some reason, people who develop keratoconus have a decrease in antioxidants within the cornea. When the cornea does not have enough antioxidants, the collagen within the cornea becomes weak, and the cornea begins to bulge forward.

Mechanical factors, such as eye rubbing or, in some cases, excessive ultraviolet exposure, may cause oxidative stress.

Hormonal Causes

Because of the age of onset of keratoconus, it is thought that hormones may play a large role in its development. It is common for keratoconus to develop after puberty. Keratoconus can also develop or worsen in pregnant people.

Diagnosis

Often, people with early keratoconus first develop astigmatism. Astigmatism occurs when either the cornea or the lens inside the eye has an oblong shape, like a football, instead of a spherical shape, like a basketball.

A cornea with astigmatism has two curves, one flat curve and one that is steep. This causes images to appear distorted in addition to appearing blurry. However, with keratoconus, patients with astigmatism tend to come back into their optometrist’s office a little more frequently, complaining that their vision seems to have changed.

Because the cornea gradually becomes steeper with keratoconus, nearsightedness is also frequently diagnosed. Nearsightedness causes objects to become blurry at a distance.

Eye doctors investigating keratoconus will measure the steepness of the cornea with a keratometer. They may notice a gradual steepening over time, and corneal topography testing will be ordered.

Corneal topography is a computerized method of mapping the shape and steepness of the cornea. A corneal topographer produces a color map that shows steeper areas in hotter, red colors and flatter areas in cooler, blue colors.

Topography will typically show an inferior steepening of the cornea. Sometimes topography will also show an asymmetry in shape between the top half of the cornea and the bottom half of the cornea.

Along with a comprehensive eye examination, eye doctors will also perform a slit lamp examination using a special upright bio-microscope to examine the cornea. Often, keratoconus patients will have fine lines in their cornea called Vogt’s striae. In addition, a circle of iron deposition around the cornea—called a Fleischer ring—may be visible.

Treatment

There are several ways to treat keratoconus, depending on the severity of the condition.

Soft Astigmatism Contact Lenses

In the early stages of keratoconus, a soft toric contact lens may be worn. A toric lens is a lens that corrects astigmatism. The lens is soft, but it contains two powers: one set power range and also a different power range of 90 degrees away.

Rigid Gas Permeable Contact Lenses

In moderate stages of keratoconus, a rigid gas permeable lens is worn. A rigid gas permeable lens provides a hard surface, so that any corneal distortion may be covered up.

As keratoconus advances, it may become more difficult to wear a rigid gas permeable lens because of excessive lens movement and decentration of the lens. Rigid gas permeable lenses are small lenses, usually around 8–10 millimeters in diameter, and move slightly with each eyelid blink.

Hybrid Contact Lenses

Hybrid contact lenses have a central lens made of a rigid gas permeable material with a surrounding soft skirt. This provides more comfort for the person wearing the lens. Because the center is rigid, it still delivers the same vision correction as a regular rigid gas permeable lens.

Scleral Contact Lenses

Scleral contact lenses are lenses that are made of a material similar to what rigid gas permeable lenses are made of. However, scleral lenses are very large, covering the cornea and overlapping onto the sclera, the white part of the eye.

A scleral lens completely vaults the steepest part of the cornea, increasing comfort and reducing the chances of scarring.

Corneal Cross-Linking

Corneal cross-linking is a relatively new procedure that acts to strengthen the bonds in the cornea to help retain its normal shape. The procedure involves applying riboflavin (vitamin B2) to the eye in a liquid form. An ultraviolet light is then applied to the eye to solidify the process.

Corneal cross-linking typically does not cure keratoconus or reduce the steepening of the cornea, but it prevents it from worsening.

Penetrating Keratoplasty

Rarely, keratoconus may worsen to the point where a corneal transplant is needed. During a penetrating keratoplasty procedure, donor cornea is grafted onto the peripheral part of the recipient’s cornea.

Newer laser procedures have increased the success of a corneal transplant. Typically, corneal transplants are successful. However, rejection is always a concern.

It is also difficult to predict the outcome of the procedure on a patient’s vision. Although the transplant may be successful, the patient may still end up with a fairly high prescription and a need to wear glasses.