Monday, August 31, 2009

Refractive disorders: Astigmatism.(5) Solutions

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Just as myopia and hyperopia, astigmatism can be corrected with eyeglasses, contact lenses and/or refractive surgery.
In any treatment, the correction is more complex because of the asymmetry of this refractive error.

  • Eyeglasses, with usually the same correction to look far away and to look up close (although there are some cases in which the astigmatism may be different in one distance or another).

    The lenses used for compensating astigmatism depend on the kind of astigmatism we have.
    So far, the compensation of a refractive error was simple, because a spherical lens modified equally every ray that crossed it at any point of it; that is, any ray that got to the lens, got to the retina. In the astigmatism, this is MORE COMPLICATED.
    Because all surface (of cornea or crystalline lens) has not got a symmetrical curvature (there will be a meridian with more curvature and other with a lesser one), the light that gets into the eye, gets to different points regarding the retina. Therefore, if we put a spherical lens in front of a astigmatic eye, it will only correct a meridian. This way, all rays, that go through the lens and later through the eye, will keep on getting to different points with respect to the retina, because some of rays will be focused on it, but others will be focused behind or in front of the retina (depending on the kid of astigmatism).

    To make it simpler. Think that an astigmatic person sees the image distorted; on the other hand, it is usually accompanied by hyperopia or myopia that causes seeing blurred images. So, for you to understand it better, “this person will see blurred through one meridian, and distorted through the other”. Consequently, we need a lens with two different powers to compensate both effects: one spherical lens to compensate the blur and other cylindrical or toric lens to compensate the distortion. Each one will focus the rays that go through by each meridian, INTO the retina.

    The eyeglasses with astigmatism may usually make you feel a little sick when you begin to use them, mainly with refractions over 1.00 or 1.50, that is why it is advisable to start using them gradually. In fact, in high degree, firstly diagnosed astigmatism, the graduation is usually prescribed gradually, increasing it little by little.

  • In the past, astigmatism could only be compensated with eyeglasses, because the cylindrical lens demands it to be worn with specific angle degrees. The contact lenses on the eye ares in constant movement because of the blinking, and in the past, this prevented that stability from happening. When said contact lenses were made, these were made with a material that lasted for a long time, because manufacturing was expensive. Latter research lead to the current situation, where there are many systems to stabilize them, and the cylindrical power in disposable contact lenses is higher and higher as time goes by.
    Therefore, nowadays, this refractive disorder may be compensated with soft contact lenses as well as with lenses; and in adults as well as in children.

    In the cases of astigmatism caused in keratoconus, they are usually compensated with rigid gas permeable contact lenses, with the purpose of holding the process of growth of the cornea a little bit. But sometimes we choose the soft contact lenses in the cases where the other lenses are impossible to wear due to the blinking; on one hand, because the eyelids may expel the lenses; and on the other, because they are not stabilized in the best position possible, therefore causing blurred image. Anyway, these patients usually achieve better Visual Acuities with contact lenses than with eyeglasses.

    Also, the astigmatism caused by a problem such as a deformation of the eyeball by palpebral disorders (as chalazion), treating the underlying cause will resolve the astigmatism. If the patient suffers a severe astigmatism, her best option is the semi-rigid permeable contact lens.

  • In the other hand, vision therapy is useful when the visual system has been compensating a little difference of graduation from between some meridians and others, and the focusing (accommodation) and team coordination (fusion) are tired.

    We use vision therapy when symptoms exist and we have to teach the visual system to use its accommodation correctly again.
    Many times simply wearing some eyeglasses is just not enough because they do not resolve completely the problem.
    Although initially you see well with them or maybe you have started to use them and everything seems to be going better, shortly you may be uncomfortable again and unable to perform at work or when studying.
    During a time you were using your visual system incorrectly and you have to re-educate it; the eyeglasses by themselves will not do it.

  • And finally, the refractive surgery is another option. It consist of changing the shape of the cornea, and therefore correcting the astigmatism in the cornea or in the crystalline lens.
Refractive disorders: Astigmatism.(1) Vision
Refractive disorders: Astigmatism. (2) Appearance
Refractive disorders: Astigmatism. (3) Different features
Refractive disorders: Astigmatism. (4) Symptoms
Refractive visual disorders. Some clarifications.
Some numbers...


Anonymous said...

Nice post, kind of drawn out though. Really good subject matter though.

Anonymous said...

Nice post, kind of drawn out though. Really good subject matter though.