Astigmatism Correction

IntroductionAuthor: Sherman Winston Reeves, M.D., M.P.H.

Astigmatism is a common condition where light entering the eye gets bent or focused unequally in one meridian versus another. A common analogy is a spoon (or football) with its curvatures varying, being less curved, or flatter, along the length of the spoon and more curved along the width of the spoon. A person with astigmatism sees a smear or shadow image coming off the object viewed. It can coexist with nearsightedness or farsightedness, and will cause things to appear blurry at distance and near. Uncorrected astigmatism may also cause eyestrain and headaches. Fortunately, it is usually easily corrected by glasses or contact lenses. Contact lenses that correct astigmatism fall into two main types: rigid gas permeable and soft. Both vary considerably in their advantages and disadvantages which will be detailed in this article.

What is astigmatism?
Astigmatism is a type of refractive error where light rays focus at different points in the eye. This creates a blurry image affecting both the distance and near acuity. Astigmatism can be caused by the shape or curvature of the cornea being more football- shaped versus round in a spherical eye or by the lens inside of the eye. There are two types of astigmatism: regular and irregular. Regular astigmatism, which affects the majority of people, occurs when part of the eye which needs the most amount of power exists 90 degrees away from the area in the eye needing the least amount of power. It is symmetrical, and because of this regularity, it is easily corrected with glasses or contacts. In irregular astigmatism, the cornea, which is the front curve of the eye, may not be smooth or it may be asymmetric. This causes light to be scattered throughout the eye with many different focus points. Neither glasses nor soft contacts can correct this condition and because of this, rigid gas permeable lenses are often a medical necessity to provide the bearer with crisp vision.

What causes astigmatism?
Astigmatism is not a disease. It often occurs naturally with some likelihood of a familial inheritance. It can change naturally over a person’s lifetime. Sometimes, astigmatism can be a symptom of eye diseases such as keratoconus or cataracts. It also can be a result of various ocular surgeries or injuries.

How is astigmatism detected?
Astigmatism is easily quantified by two common tests during an eye exam. The first is keratometry, a quick test measuring the two main corneal curvatures of the cornea. Your eye doctor may do this as part of a routine eye exam. It is especially important if you are being fit with contact lenses. The second is the refraction. This is the part of the exam in which you choose between a variety of views or lenses to determine your prescription for glasses and eventually contact lenses. Not as commonly used is topography, which is a mapping of the cornea that shows the astigmatism with even greater detail and may be done if you are considering surgery or if there is suspicion of irregular astigmatism.

What types of contacts for astigmatism correction?
Gas Permeable (GP)
A GP lens is a semi-hard lens which is often smaller than a soft contact lens. It corrects for astigmatism by acting to make the surface of the eye spherical. It does this by usually being spherical itself; one’s tears fill the space between the lens and the spoon-shaped cornea. If a high amount of astigmatism is coming from the natural lens in the eye versus the cornea, this spherical design doesn’t work as well, and your doctor may prescribe a more sophisticated lens such as a front-toric or bitoric design. If your eye has a lot of astigmatism, he or she may fit you with a back-toric design or other specialty lens to get a more comfortable fit.
The advantages of GPs include clear, stable astigmatic correction, and they also can be more economical over a person’s wearing lifetime. That is, the same pair of lenses may last a person several years, even though the initial cost can sometimes be a bit more than a soft lens. They are durable and not as likely to tear or develop deposits as a soft lens. They are also easy to handle and get in and out of the eye. Some are available in special materials that permit up to thirty days of continuous wear. There is even some data suggesting that GPs can slow down the progression of nearsightedness in childrens’ developing eyes.
GPs are the primary way to correct for irregular astigmatism, which may be the result of diseases like keratoconus or seen after a cornea transplant or other surgery. Again the tears fill the space between the lens and the irregular cornea making for a smoother optical surface and crisper vision.
The biggest disadvantage of a GP lens is its initial comfort. As you can imagine, a rigid piece of plastic in the eye takes more getting used to than a soft. It can take a couple of weeks of consistent wear to become accustomed to how this lens feels in the eye. A small number of patients never find them comfortable. Most patients adapt and can wear the lenses all their waking hours comfortably.

What About Correcting Astigmatism with Soft Toric Contact Lenses?
A soft toric contact lens is a hydrophilic, flexible material lens, which has different powers in opposing meridians built into the lens. In order to correct for the astigmatism appropriately, the powers have to align with the right areas of the eye. There are various designs to allow for this, but a common one is prism ballast, where there is a thicker spot on the soft lens, which naturally rests on the bottom of the lens to help hold the lens in position. Without that, or another stabilization method, the vision would be highly variable. As the patient blinks the contact lens could endlessly rotate.
The advantages of a soft toric contact lens include easy adaptability; the comfort is usually quite good even initially. They come in an extended wear option, allowing up to thirty days of continuous (overnight) wear. They can come in colors, which can change even the darkest eyes to a variety of colors. Soft toric contacts are available in a variety of replacement schedules. There are conventional lens designs, in which you keep the same lenses for a year, and more commonly those called ‘disposable lenses’ which you wear for one month, 2 weeks, or only once and throw away, with no need to ever clean them. Perhaps the most popular is a two to four week disposable contact lens.
Prices vary quite a bit depending on the brand and replacement schedule, but they are often less expensive than a pair of eyeglasses. With disposable lenses, it is much easier to change the powers, which is convenient for a person with an unstable prescription. Disposables, when replaced regularly, are also healthier for the eyes compared to yearly replacement lenses because they are less likely to have dirt and deposits that attract bacteria and can cause infections. Handling and cleaning soft contacts is usually quite simple as well.
Their biggest drawback is the potential for fluctuating vision. No matter how good the lens design and fit, all soft toric lenses rotate and move somewhat in the eye, and as explained earlier, this can be a source of frustration when you can see clearly one minute, and are blurry the next. The more astigmatism a person has, the more sensitive he/she is likely to be to lens movement. A way to test if this may be an issue for you is to take your glasses and rotate them in front of your eyes. If a little movement creates a big blur, you may be better off, vision-wise, with an RGP instead.

What Other Contact Lens Options Exist for Correction of Astigmatism?
Beyond GP and soft toric contact lenses, there are now hybrid lenses, which either have a soft lens surround (or skirt) with a rigid central portion or the lens, or are made of a stiffer soft lens material. Lens designs such as these try to combine the benefits that each lens has to offer and may be best for a difficult to fit eye, like one with irregular astigmatism.
Another type of contact lens, which really is a type of GP, is called Corneal Refractive Therapy (CRT) or Orthokeratology lenses. This, essentially, is an RGP lens worn while sleeping designed to mold the eye. The fit of the contact is such that it reshapes the cornea by pushing on the steep areas much like an oral retainer does to teeth. The flattening effect of the contact lens typically lasts all day to enable the wearer to see clearly without a lens while awake, but then the user would need to resume wear again the next night to maintain clear vision the following day. It works best for mild to moderate amounts of nearsightedness and less than one and a half diopters of astigmatism.

What Type of Complications Exist with Astigmatism Correcting Contact Lenses?
With any type of contact lens, there is an increased risk of infection. For this reason it is extremely important to be fit by an eye doctor with a proper lens for your particular eye and to follow the appropriate cleaning and wearing directions. In addition to infection, overwearing contacts can lead to hypoxia wherein the normally clear cornea can develop blood vessels from a lack of oxygen. Contacts can also induce warpage of the cornea, which can make it hard to see clearly out of glasses immediately after removing the contacts.

Who cannot wear contacts?
Your eye doctor can tell you if you are an appropriate candidate for contacts. He/she will want to make sure that you don’t have severe dry eye or blepharitis that may interfere with the comfort of contacts. Sensitivity to the lens cleaning solutions may also make contact lens use uncomfortable, but specific solutions are available to address that problem, if necessary.



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source :
http://www.improveyourvision.com/understanding-vision/problems-diseases/astigmatism.html