What Is Astigmatism? What Causes Astigmatism?

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Main Category: Eye Health / Blindness
Article Date: 26 Jul 2009 - 0:00 PDT

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Astigmatism is an eye condition with blurred vision as its main symptom. The front surface of the eye (cornea) of a person with astigmatism is not curved properly - the curve is irregular - usually one half is flatter than the other - sometimes one area is steeper than it should be.

When light rays enter the eye they do not focus correctly on the retina, resulting in a blurred image. Astigmatism may also be caused by an irregularly shaped lens, which is located behind the cornea.

Astigmatism may occur in children and adults; it is fairly common and is not contagious. It is usually congenital - is present at birth - but can develop after an eye operation or an injury to the eye. It belongs to a group of eye conditions called refractive errors. Myopia (short-sightedness), hypermetropia (long-sightedness) and presbyopia (aging of the lens in the eye) are types of refractive errors.

A refractive error means that the shape of the eye does not bend light properly, resulting in a blurred image. Light has to be bent (refracted) by the lens and the cornea correctly before it reaches the retina in order to see things clearly.

About half of all adults in the USA aged 20 and older have refraction errors in their eyes, a study carried out by researchers at the National Eye Institute revealed.

The two most common types of astigmatism are:

• Corneal astigmatism - the cornea has an irregular shape
• Lenticular astigmatism - the lens has an irregular shape

What causes astigmatism?

• Corneal astigmatism
  
  The cornea is a clear (transparent) layer of tissue that covers the front of the eye. As well as transmitting and focusing light into the eye, it protects the eye from infection and damage. The cornea needs to have a perfect curve in order to bend (refract) light properly as it goes into the eye.
  
  If the cornea does not curve perfectly - if one half is flatter or steeper than the other - the light that hits it will not refract properly and the retina at the back of the eye will receive an imperfect image. The person will have blurred vision from that eye - astigmatism.
  
  
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  People with astigmatism commonly have an oblong-shaped (oval-shaped) cornea rather than a perfect sphere shape. A ping-pong ball is a perfect sphere, while an American football or a rugby ball has an oblong shape. If the cornea's curve is like an oblong the light rays will focus on two points in the retina, rather than just one.
  
  Experts are not sure why some people are born with a cornea that does not curve properly. Some studies indicate a baby has a higher risk of having astigmatism if one or both of the parents have it.
  
  A higher percentage of babies born prematurely, as well as those born with a low bodyweight have astigmatism, compared to other babies.
  
  Certain types of surgery or eye injuries that cause scarring of the cornea may cause astigmatism.
  
  Keratoconus, a degenerative disorder of the eye where the cornea gradually thins and changes to a more conical shape, can also cause astigmatism.


• Lenticular astigmatism
  
  The problem is similar to corneal astigmatism, but exists in the lens rather than the cornea. The lens has variations in its curvature, rather than having a perfect curve, causing images to reach the back of the eye (retina) imperfectly. Most patients with lenticular astigmatism have a normally shaped cornea - the defect is only in the curvature of the lens.
  
  People with diabetes commonly develop lenticular astigmatism because high blood sugar levels can cause the lens to change shape. The process usually develops slowly and is generally detected when the patient starts receiving treatment for the diabetes. When diabetes is controlled through treatment and blood sugar returns to more normal levels the lens' shape will also return to normal - many patients will notice the return as hypermetropia (long-sightedness). It is advisable for patients who start diabetes treatment to wait at least one month for the lens to settle down before being measured for glasses.

What are the symptoms of astigmatism?

• Blurred or distorted vision at all distances.
• Photophobia - sensitivity to light.
• Headaches.
• Excessive squinting.
• The person is constantly closing his/her eyes.
• Eye strain - occurs more often when the eye has to focus for long periods, as in reading from paper or a computer monitor.

How is astigmatism diagnosed?

The ophthalmologist, optometrist, or orthoptist may use the following tools to examine the eyes:

• Visual acuity test - this involves reading letters on a chart. The letters become progressively smaller on each line.


• Astigmatic dial - this is a chart showing a series of lines which make up a semi-circle. People with perfect vision will see the lines clearly, while those with astigmatism will see some more clearly than others.


• Keratometer - also known as an ophthalmometer. This device measures the reflected light from the surface of the cornea. It measures the radius of the curvature of the cornea and can assess the degree of abnormal curvature of the cornea.


• Keratoscope - also known as Placido's disk - this device is marked with lines or circles and observes corneal reflex. It uses light to project rings on the cornea. By measuring the spacing between the rings it calculates the variations of curvature, which in turn calculates the degree of astigmatism. A videokeratoscope is a keratoscope fitted with a video.


• The importance of regular eye tests
  
  Astigmatism is very common - experts say most of us are born with a degree of astigmatism. Most children who are born with astigmatism will not realize they have it until they have an eye test. Reading and concentrating at school may be affected if a child has undiagnosed astigmatism. Therefore, regular eye tests are important.
  
  In the UK babies have an eye test soon after they are born. They have a follow-up test about six weeks later. Doctors recommend that children receive a comprehensive eye test when they are four years old; and then every year up to the age of 16. Adults should have an eye test every couple of years.

What are the treatments for astigmatism?

If the astigmatism is very mild the health care professional will suggest no treatment at all.

Corrective lenses

Corrective lenses bend the income light rays in a way that compensates for the error caused by faulty refraction so that images are properly received onto the retina. Whether the corrective lenses are in glasses or contact lenses is up to the patient - they are equally effective.

Experts say children can wear contact lenses as long as they are careful about using them properly. This includes not wearing them for too long and cleaning them properly. This may be difficult to achieve if the child is under the age of twelve.

People of any age who use contact lenses need to be aware of good lens hygiene. Otherwise there is a significant risk of eye infection.

There are three types of contact lenses. 1. Rigid contact lenses. 2. Gas permeable contact lenses. 3. Soft contact lenses.

• Rigid contact lenses - these are usually made of a combination of glass and plastic. The eye is more likely to produce new blood vessels which may eventually affect vision. This happens because rigid contact lenses stop oxygen from getting into the eye, so it produces new blood vessels which feed oxygen into those areas which had a drop in oxygen supply caused by the lens.


• Gas permeable contact lenses (oxygen permeable contact lenses) - these use polymers; a type of plastic. Polymers are permeable - they do not stop oxygen from getting into the eye - and prevent the risk of new blood vessels appearing in the eye and obscuring vision.


• Soft contact lenses - these are made of hydrogel, a combination of water and polymer. Soft lenses also allow oxygen to move through the lens and into the eye. The water in them lets the oxygen through, not the plastic. As the water soon evaporates soft lenses can only be worn for one day, and then discarded. Soft lenses made of silicone gel may be worn for longer.

Laser eye surgery

• Photorefractive keratectomy (PRK) - some of the outer protective layer of the cornea is removed. An excimer laser changes the shape of the cornea by removing tissue. When the cornea heals it usually has a more even and spherical curve. This procedure can be moderately to very painful. It can take up to one month for vision to recover. An excimer laser is a laser device that does not produce heat.


• LASEK (Laser epithelial keratomileusis) - alcohol is used to loosen the surface of the cornea, which is then removed - a much thinner layer is affected, making the eye less vulnerable to damage or injury, compared to PRK. A laser is then used to change the shape of the cornea. The cornea is then placed back. LASEK may be a better option for a patient whose cornea is thin. This procedure is usually less painful than PRK, but slightly more painful than LASIK, and can take up to one week for vision to recover.


• Laser in situ keratectomy (LASIK) - the doctor uses a device called a keratome to make a thin, round hinged cut into the cornea. This can also be done using a special cutting laser. The flap is then lifted and an excimer laser sculpts the shape of the cornea under the flap. LASIK causes less pain than the other procedures, and the patient will recover his/her vision within a few days. Hence, LASIK is usually the preferred laser treatment option. Even so, vision won't completely stabilize for about one month.
  
  A review of LASIK carried out by the American Society of Cataract and Refractive Surgery found an overall patient satisfaction rate of 95.4%.
  
  LASEK and PRK are better procedures than LASIK if the cornea is thin.

Laser eye surgery is not suitable if:

• The patient is under the age of 21 - The structures of the eyes of people under 21 are still changing and should not be altered. The minimum age in different countries varies from 21 to 18.


• Vision is still changing - the vision of some older people may still be changing. Experts say that a person's vision should be stable for at least three years before undergoing laser surgery.


• Patients with diabetes - in some cases laser surgery may worsen abnormalities in the eye caused by diabetes.


• Pregnant or breastfeeding mothers - during pregnancy and breastfeeding hormone fluctuations still exist within the eye, making it more difficult to carry out surgery accurately.


• People with some immune conditions - people with rheumatoid arthritis, lupus, or HIV, for example, may find it harder to recover after surgery.


• People with other existing eye conditions - people other eye conditions will need to have those treated first, before becoming eligible candidates for laser eye surgery. Examples are cataracts and glaucoma.


• People taking certain medications - if the patient is taking medications, such as Accutane or oral prednisone, he/she should not undergo laser eye surgery.

What are the risks of laser surgery?

• Correction error - the surgeon may have taken out the wrong amount of tissue and the patient's vision worsens.


• Epithelial in-growth - the surface of the cornea starts growing into the cornea itself. This can cause vision problems and may need further surgery.


• Ectasia - the cornea becomes too thin. This can result in worse vision. Sometimes the deterioration of vision may be severe.


• Keratitis - the cornea becomes infected.

In most countries the risk of complications from laser eye surgery is very small. It depends on how experienced the surgeon is, and how accurately the patient was assessed beforehand. According to the National Institute of Health and Clinical Excellence, UK, the risks of complications in the UK for LASIK surgery are as follows:

Correction error - 0.6% chance
Epithelial in-growth - 1.3% chance
Ectasia - 0.2% chance
Keratitis - 0.16% chance

Patients should ask the surgeon about his/her experience with the procedure, success rates, what equipment is going to be used, and what follow-up program there is.

Sources: National Health Service (NHS), UK, The Mayo Clinic, Wikipedia, HHS (Department of Health and Human Services USA), NIH (National Institutes of Health, USA).

Written by Christian Nordqvist
Copyright: Medical News Today
Not to be reproduced without permission of Medical News Today

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