What Is an Irregular Cornea?

The cornea is the clear tissue in front of the iris, or colored part of the eye. Normally, it has a smooth arc shape like a rainbow. The smooth curve allows light to focus in the same spot in the back of the eye to give clear vision. An uneven cornea develops bumps and dips in the surface. Light rays get bent in different directions with each change in the shape. Since the light rays focus in different places, it causes irregular astigmatism that makes images blurry. The greater the irregularity, the worse the blur becomes.

Since glasses can only bend light into one place, only part of the light rays are focused into the correct spot. The rest of the image stays blurry. If the irregularity becomes too severe, glasses may no longer work. At this point, specialists who treat corneal disorders can diagnose the cause of the problem and determine appropriate treatments.

Symptoms of Irregular Astigmatism

Some people have minor vision changes, while others may experience more severe problems. Most people will tell the doctor that vision cannot be corrected to normal with glasses. Others may notice severe glare at night or haziness around lights. If you have these symptoms, make sure to tell your eye doctor.

Causes of Irregular Corneas

There are a variety of different causes of irregular corneas. Here are some of the conditions that cause the cornea to change shape:

Congenital defects

Eye injuries

Burns

Scarring after bacterial, viral, or fungal infections

Corneal ulcers

Prior eye surgery

Severe dry eye

Pterygium

Pellucid marginal degeneration

Keratoconus

Some of these problems, such as congenital defects or scarring after injuries, burns, or infections, tend to stay stable over time. The area of highest irregularity is located at and right around the scarred area. If this is in the center of the cornea, it can decrease visual acuity dramatically.

Corneal ulcers are areas of localized inflammation and sometimes infection that cause the tissue to break down. The inflammation causes the area at and around the ulcer to thicken. Scars that develop from ulcers tend to be thinner than the rest of the cornea. Those thinner or thicker parts cause a misshapen cornea.

Prior eye surgeries can create an unusual corneal shape. This can be due to the nature of the procedure, complications, or how the individual heals. As long as the cornea is not too thin, the shape tends to stay the same. If the cornea is too thin, it will bulge out, causing the profile to change radically. This condition rarely progresses unless the tissue becomes extremely thin.
Severe dry eye may case irregularity that leads to focus problems. It tends to be worse in the part of the cornea with the greatest exposure to air where the eyelids meet.

A pterygium is a thickening on the white of the eye that can sometimes start to grow onto the cornea. Pterygia, the plural of pterygium, tend to start near the cornea on the side of the eye closest to the nose. Since they cause thickening in only one portion of the cornea, it can lead to tremendous shape change and blur.

Pellucid marginal degeneration causes the cornea to thin out towards the bottom of the cornea. The rest of the cornea tends to maintain the normal thickness, so the astigmatism changes abruptly where the thinning starts.

Keratoconus also causes asymmetrical corneal thinning. The cornea takes on a cone shape in the area of greatest thinning. The dramatic shifts in shape cause a tremendous amount of astigmatism.

Testing for Corneal Irregularity

When an eye doctor detects the shape changes in your cornea, he or she may send you for more specialized testing to confirm the diagnosis. Doctors who specialize in contact lenses will typically double check the refraction to make sure the prescription is as accurate as possible. This not only helps you see your best through glasses, but it also helps the doctor start making choices on which contact lenses would be the most appropriate for you.

After checking the vision prescription, the eye doctor will measure the shape and elevation of the front of the eye. These tests shine light or patterns onto the front of the eye. The reflections of the patterns are measured for the location and degree of distortion. These tests are painless. Some of the instruments used for this include the following:

Keratometer

Topographer

AstraMax 3-D topography

Pentacam

The instruments produce measurements and topographical maps. They help your doctor see exactly where the irregularities are in order to diagnose the problem.

Some other instruments measure corneal thickness. This helps detect what section of the cornea is thinning and helps with diagnosis. Instruments that measure the thickness of the eye use ultrasound technology. The test takes very little time. Numbing eye drops are used just before the test to prevent any potential discomfort. You may hear these instruments mentioned:

Orbscan

Pachymeter

Pentacam (this also measures corneal shape)

OCT, or optical coherence tomography

Another instrument, called the spectrometer, is a special microscope used to view the back layer of the cornea that faces into the eye. This section of the cornea, called the endothelium, is only one cell layer thick. The doctor uses this instrument to look for changes in the cell shape and enlarged cell size. He or she can also see if some of the cells have died off, leaving gaps in this layer. Abnormal or missing cells in the endothelium can lead to cloudy vision and corneal swelling.

Treatment: What Happens when Glasses no longer Help

If your vision is no longer correctable to normal, your doctor may recommend contact lenses. Many contacts work by creating a new, smooth, regular surface on the front of the eye. The tear layer fills in any irregularities between the lens and the eye, improving the optics and vision clarity. Soft lenses may be used for mild to moderate distortion. Rigid gas permeable lenses are often used when soft lenses don’t correct vision fully. More advanced designs like scleral and hybrid soft/rigid lenses might also be utilized.

Soft Contact Lenses

Soft contact lenses conform to the shape of the front of the eye. They are very comfortable and can correct low to high amounts of regular astigmatism. Since they drape over the cornea, however, these lenses cannot correct high amounts of irregular astigmatism. Contact lens specialists often turn to other options when soft contact lenses no longer work.

Rigid Gas Permeable Lenses Create a Smooth Surface

Rigid gas permeable (RGP) contacts are custom-fit lenses that can provide significant improvement in vision. The tear layer fills in the gaps between the cornea and the contact lens. This creates a smooth, regular surface that allows light to focus in one spot, creating a clear picture. These lenses are commonly used for treating keratoconus when glasses cannot correct vision and surgery is not desired or appropriate. When fit correctly, RGP lenses are extremely comfortable.

Hybrid Lenses: The Best of Both Worlds

Hybrid lenses are an option for those who need more comfort than an RGP design can provide. Lenses like Synergeyes and Duettes have a rigid center to provide crisp optics, but they also have an outer ring made from the same flexible material of a soft contact lens. This offers good vision with additional comfort for people who have very sensitive eyes.

A Scleral Lens May Be the Best Choice

Some people have too much scarring on the cornea to achieve good comfort with a more traditional contact lens. Others have so much irregularity that the contact lens slides off the cornea. In these cases, scleral lenses may be the best options. A scleral lens vaults over the top of the cornea. It has a ring, called a flange, which rests on the white of the eye, called the sclera. The tears fill in any gaps between the lens and the eye. Since a scleral lens does not rest on the cornea, it can be used in cases of severe keratoconus, scarring, or pellucid degeneration where someone can no longer tolerate a rigid or hybrid design.

Many people are concerned that a scleral lens will be uncomfortable. However, the sclera is a much less sensitive tissue than the cornea. When this type of lens is fit properly, it is very comfortable and can be worn daily.

Contact Lenses After Corneal Surgeries

People who have had corneal transplant surgery due to injury or diseases like keratoconus often find their vision is substantially improved but not 100 percent normal. Even under the perfect conditions, some corneal irregularity can remain. There are RGP, hybrid, and scleral lens designs tailored specifically to post-surgical patients. The goal is to give crisp, clear vision while protecting the eye from any damage to the graft.