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Astigmatism. Symptoms, Causes and Treatment


Astigmatism is a type of refractive error, like myopia (nearsightedness) and hyperopia (farsightedness). Refractive error is a problem with the accuracy of light focusing on the retina, the light-sensitive tissue inside the eye. Refractive error results from an atypical eye shape, causing light rays not to refract (bend) properly.

Refractive error is the most common ocular problem affecting all age groups. The estimated worldwide prevalence of astigmatism in adults is 40.4%.

In developed countries, refractive error is correctable. In developing countries, however, uncorrected refractive error causes substantial visual impairment.

Symptoms

When astigmatism is mild, it may not impact vision very much, if at all. When it becomes more significant, however, it can affect visual clarity.

With astigmatism, you may find your vision blurry and distorted. This generally impacts both far and near vision. You may find yourself squinting to see better. You may experience eyestrain or headaches.

Astigmatism is diagnosed during an eye examination. Tests for astigmatism may include:

  • Visual acuity: This is when you're asked to read the eye chart. You may have a refractive errors such as astigmatism if you cannot read the chart well.
  • Keratometry or Topography: These are measurements of the curvature of your cornea. Keratometry simply gives numerical values. Topography, in addition to providing corneal curvature values, provides a detailed image of the corneal contour.
  • Refraction: This is the vision test behind the phoropter. A phoropter is a machine that contains various lenses to test your vision. It contains lenses to correct nearsightedness, farsightedness, and astigmatism. You may be asked which lenses make the chart looks clearer, which helps your doctor to measure your refractive error.

Causes

Astigmatism is caused by an imperfect shape of the cornea or crystalline lens. The cornea is the front clear protective structure of the eye. The crystalline lens sits behind the iris. Both structures serve to focus light on the retina. When the cornea or lens is misshapen, astigmatism results. When the problem stems from the cornea, it’s called corneal astigmatism. When the issue comes from the lens, it’s called lenticular astigmatism.

Astigmatism can be regular or irregular. Regular astigmatism is more common than irregular astigmatism.

Normally, the eye's curvature is smooth and even in all directions, like a basketball or soccer ball. With regular astigmatism, the shape is more akin to a football, with two different curvatures.

Regular astigmatism often develops early in life. Nobody knows why some people have a different cornea or lens shape. Astigmatism can also develop as a result of eye diseases, like cataracts, for example.

In irregular astigmatism, unlike regular astigmatism, the eye shape is asymmetrical. It can have a curvature of any contour, with peaks and valleys in any area.

Irregular astigmatism can result from eye disease. For example, keratoconus is a condition where the cornea thins and bulges, creating irregular astigmatism. Irregular astigmatism can also result from injury or surgery to the eye. When the cornea heals, it scars and becomes misshapen. Irregular astigmatism tends to cause more visual difficulties than regular astigmatism.

Regular astigmatism treatments

Regular astigmatism is most commonly treated with glasses and contact lenses. While wearing glasses or contacts doesn’t cure the condition, it allows you to see more clearly.

Contact lenses that are correct for astigmatism are called toric. Soft toric contact lenses are generally sufficient to correct astigmatism. Sometimes, rigid gas-permeable (RGP) lenses are used instead. These can provide crisper vision, especially in higher amounts of astigmatism.

Refractive surgery is also an option for astigmatism. Unlike glasses and contact lenses, which merely compensate for astigmatism, refractive surgery treats the underlying cause. The most common type of surgery used for this purpose is laser-assisted in situ keratomileusis or LASIK. Photorefractive keratectomy (PRK) is also often used. LASIK and PRK reshape the corneal curvature to improve vision. There are limits to how much astigmatism can safely be corrected by refractive surgery.

Irregular astigmatism treatments

Glasses and soft contact lenses are not as helpful in correcting irregular astigmatism. Custom-fitted RGP lenses are necessary. Fitting these lenses is challenging and is best done by an eye doctor experienced in this area.

Refractive surgery is sometimes used for irregular astigmatism, though it is not always advised.

Key takeaways

Astigmatism is a very common type of refractive error that can cause blurry, distorted vision, eye strain, and headaches.

Astigmatism is caused by an imperfect shape of the cornea or crystalline lens. It can stem from the cornea and/or the lens and be either regular or irregular.

The condition is most commonly treated with glasses and contact lenses, though refractive surgery may also be an option for some patients.

Astigmatism may fluctuate over one’s lifetime, so adjustments to one’s prescription may be necessary to optimize vision.

References

Boyd, K., 2022. What Is Astigmatism? Symptoms, Causes, Diagnosis, Treatment [online] American Academy of Ophthalmology. Available at: https://www.aao.org/eye-health/diseases/what-is-astigmatism

Hashemi H, Fotouhi A, Yekta A, Pakzad R, Ostadimoghaddam H, Khabazkhoob M. Global and regional estimates of the prevalence of refractive errors: Systematic review and meta-analysis. J Curr Ophthalmol. 2017;30(1):3-22. Published 2017 Sep 27. doi:10.1016/j.joco.2017.08.009

Astigmatism. [online] American Optometric Association. Available at: https://www.aoa.org/healthy-eyes/eye-and-vision-conditions/astigmatism?sso=y

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