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What is astigmatism?
Astigmatism is a refractive disorder of the eye where the cornea
is distorted being curved more in one direction than another.
The cornea is the clear dome in front of the eye ball. In severe
cases the cornea can become cone shaped.
Normally, incident light coming from all directions are equally
refracted by the cornea. In case of astigmatism the cornea unequally
refracts this incident light from various directions. Due to unequal
refraction the affected person sees blurred images at all distances.
The blurring can range from mild to severe.
How can contact lenses help?
Contact lenses help by correcting the distortion of the corneal
shape. Considerable advances have been made in correcting astigmatism
by contact lenses. Today various types of contact lenses are available
to correct almost all types of astigmatism. Contact lenses are
better than the conventional eye glasses in correcting some astigmatism
cases.
What are the types of contact lenses and how do they function?
The types of contact lenses used to resolve astigmatism are:
- Soft contact lenses.
- Toric lenses.
- Rigid gas permeable or RGP lenses.
- Toric RGP lenses.
Soft contact lenses cannot change the corneal shape much. Therefore
they are not very good at correcting astigmatism. Still some people
prefer soft contacts as they are more comfortable.
But soft contact lenses are good at correcting small amount of
corneal astigmatism. What these soft contact lenses actually do
is more a masking of the minor astigmatism than a genuine correction.
However, it seems to suit some people fine. Best results are obtained
when the power of the soft contact is equal to that of the spherical
lens of the eye glasses.
Toric lenses: In cases where the soft contact lens cannot mask
the astigmatism, soft toric lenses are next best alternative.
Toric lenses are made from the same material as soft contacts
and can be soft or RGP type. But they are designed differently.
Toric lenses have two powers in them; one for astigmatism and
another for myopia and hyperopia. Toric lenses are designed to
specifically correct some types of astigmatism. They function
by neutralizing the effects of the distorted cornea. Their design
keeps the lenses comparatively stable when your eyes blinks or
looks around. Sharp vision is maintained as the lens cannot rotate
on your eyes.
Toric lenses cost more than other contact lenses. There fitting
is also costlier since it requires more skill and time.
RGP lenses: These are rigid lenses. Unlike soft contact lenses,
the rigidity of RGP lenses does not allow them to be distorted
by the astigmatic cornea. Rather, the RGP lenses make the cornea
conform to their shape to correct the corneal distortion.
When, in cases of mild or moderate astigmatism, RGP lenses are
placed on the astigmatic cornea the space between the back of
the RGP lens and the surface of the cornea is filled up by tears.
This layer of tears is shaped such that it becomes a tear lens.
This tear lens functions to correct the astigmatism.
The tear lens is not affected by lens rotation. As a result,
the RGP contact lens needs to have power only to correct any near
or far sightedness since the power to correct astigmatism is provided
by the tear lens.
Toric RGP lens: In more severe types of astigmatism the cornea
is too distorted. This makes it difficult for the RGP lenses to
fit or center properly on the irregularly shaped cornea. The lens
remains unstable and moves around whenever the eye blinks. Such
conditions do not allow the formation of a tear lens to correct
astigmatism. In such cases toric RGP lenses that have the power
to correct astigmatism are used.
Toric RGP lenses have a spoon shaped back surface, which fits
well with the front surface of an astigmatic cornea. This close
fitting provides more comfort and stability. The increased stability
prevents the lens from rotating on the eye allowing different
powers to be ground on different parts of the toric lens. The
variable powers of a toric RGP lens can correct astigmatism without
depending solely on the tear lens.
How do I get contacts for astigmatism?
Astigmatism should no more be a big worry to you. Today there
are contacts available that can correct most degrees of astigmatism. However,
only an authorized eye practitioner can prescribe the type of
lens
needed to correct astigmatism.
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