Did you know that most people who wear contact lenses have astigmatism? In which case, why do so many people believe that they can’t wear contacts due to their astigmatism? This post explains why.
A fancy word for a (mostly) normal variation on eye shape
Astigmatism is one of those words that sound much worse than they really are. Perhaps the fact that ‘stigma’ in contained in this term is part of the reason that many people assume it must mean something bad.
In fact, astigmatism simply means that your cornea (the front surface of your eye) is not perfectly spherical (or evenly round) in shape. If you have astigmatism, one contour of your cornea is more curved than the other. This is often illustrated by the comparison of the shape of a soccer ball (perfectly round) to a rugby ball (more egg-shaped) although in the eye the effect is more subtle.
How rare is astigmatism?
Not rare at all! In fact, it is far more common to have a cornea with a little astigmatism (.25 to .50 D) than it is to have a perfectly spherical cornea. It’s usually only incorporated into your glasses prescription if you need glasses for other reasons – such as distance or reading vision.
Uncorrected astigmatism of more than about 0.75 dioptres (the unit of measurement of spectacle lens power) can cause symptoms, such as:
- Distorted or blurred vision at all distances
- Difficulty driving at night
Can I still wear contact lenses?
Yes, of course. Contact lenses often provide clearer vision, a wider field of vision and greater comfort than glasses. Which contact lenses will suit you best will depend on how much astigmatism you have:
If you, like many people, have a small degree of astigmatism (up to 0.50 D) in combination with your short-sightedness or long-sightedness, you can usually wear regular soft, disposable contact lenses which don’t have any correction for astigmatism. This is because you can still achieve 20-20 vision even with a small amount of uncorrected astigmatism. (In fact, most people who don’t need glasses at all will still have a small degree of astigmatism when they have their eyes tested).
Moderate astigmatism is fairly common. If you have 0.75 to 2.5 dioptres of astigmatism you fall into this category and you will need a specially shaped contact lens called a toric lens. There are many toric contact lens designs, including daily disposable lenses available to correct this level of astigmatism.
More than 2.5 dioptres of astigmatism is considered high. If this is you, you may need a custom-made toric contact lens to correct your vision. Sometimes a hard contact lens (called a rigid gas permeable or RGP) is required for best success. These lenses provide excellent vision when fitted by an expert.
Sometimes astigmatism is due to a disease of the cornea, such as keratoconus. The cornea becomes thinner over time and bulges out. This type requires much greater expertise to successfully fit with contact lenses. Other eye conditions, such as corneal graft or injury, can also cause irregular astigmatism.
In extreme cases, the best kind of contact lens is a scleral lens, which is large enough to vault the entire cornea.
With an experienced contact lens practitioner, there will almost always be a contact lens for you, no matter what your level of astigmatism.