Astigmatism: Definition, Types and Correction Options

Last updated on April 4th, 2017 at 05:39 pm

Astigmatism is probably the most misunderstood vision problem. For starters, it’s called “astigmatism,” not “stigmatism.” (You don’t have “a stigmatism”– you have astigmatism.)

Like nearsightedness and farsightedness, astigmatism is a refractive error, suggesting it is not an eye disease or eye health problem; it’s merely an issue with how the eye focuses light.

In an eye with astigmatism, light fails to come to a single concentrate on the retina to produce clear vision. Instead, numerous focus points occur, either in front of the retina or behind it (or both).

In Europe and Asia astigmatism affects between 30 and 60% of adults. Individuals of all ages can be affected. Astigmatism was first reported by Thomas Young in the early 1800s.

Astigmatism Symptoms

Astigmatism generally causes vision to be blurred or misshaped to some degree at all ranges.

Symptoms of uncorrected astigmatism are eye strain and headaches, especially after reading or other prolonged visual jobs.

Squinting likewise is a typical symptom.

What Causes Astigmatism?

Astigmatism generally is triggered by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being substantially more curved than the meridian perpendicular to it.

To understand what meridians are, think about the front of the eye like the face of a clock. A line linking the 12 and 6 is one meridian; a line linking the 3 and 9 is another.

The steepest and flattest meridians of an eye with astigmatism are called the primary meridians.

In many cases, astigmatism is caused by the shape of the lens inside the eye. This is called lenticular astigmatism, to separate it from the more common corneal astigmatism.

Types of Astigmatism

There are 3 primary types of astigmatism:

  • Myopic astigmatism. One or both principal meridians of the eye are nearsighted. (If both meridians are nearsighted, they are myopic in differing degree.)
  • Hyperopic astigmatism. One or both primary meridians are farsighted. (If both are farsighted, they are hyperopic in varying degree.)
  • Combined astigmatism. One prinicipal meridian is nearsighted, and the other is farsighted.

Astigmatism also is classified as regular or irregular. In routine astigmatism, the primary meridians are 90 degrees apart (perpendicular to each other). In irregular astigmatism, the primary meridians are not perpendicular. A lot of astigmatism is regular corneal astigmatism, which provides the front surface area of the eye a football shape.

Irregular astigmatism can result from an eye injury that has actually caused scarring on the cornea, from specific types of eye surgery or from keratoconus, a disease that causes a progressive thinning of the cornea.

Astigmatism Definition, Types and Correction Options

How Common Is Astigmatism?

Astigmatism often happens early in life, so it is important to set up an eye examination for your child to avoid vision issues in school from uncorrected astigmatism.

In a recent study of 2,523 American children ages 5 to 17 years, more than 28 percent had astigmatism of 1.0 diopter (D) or higher.

Likewise, there were significant distinctions in astigmatism prevalence based on ethnic culture. Asian and Hispanic children had the greatest occurrences (33.6 and 36.9 percent, respectively), followed by whites (26.4 percent) and African-Americans (20.0 percent).

In another study of more than 11,000 glasses wearers in the UK (both children and adults), 47.4 percent had astigmatism of 0.75 D or higher in a minimum of one eye, and 24.1 percent had this quantity of astigmatism in both eyes. The prevalence of myopic astigmatism (31.7 percent) was roughly double that of hyperopic astigmatism (15.7 percent).

Astigmatism Test

Astigmatism is found during a regular eye exam with the same instruments and strategies used for the detection of nearsightedness and farsightedness.

Your optometrist can estimate the quantity of astigmatism you have by shining a light into your eye while manually presenting a series of lenses between the light and your eye. This test is called retinoscopy.

Though numerous optometrist continue to perform retinoscopy, this manual procedure has been changed or supplemented in many eye care practices with automated instruments that offer a much faster initial test for astigmatism and other refractive errors.

Whether your eye examination consists of retinoscopy, an automated refraction, or both, your optometrist or ophthalmologist will carry out another test called a manual refraction to fine-tune the outcomes of these initial astigmatism tests.

In a manual refraction (also called a manifest refraction or subjective refraction), your eye doctor positions an instrument called a phoropter in front of your eyes. The phoropter contains lots of lenses that can be introduced in front of your eyes one at a time so you can compare them.

As you look through the phoropter at an eye chart at the end of the test space, your eye doctor will show you various lenses and ask you questions along the lines of, “Which of these two lenses makes the letters on the chart look clearer, lens A or lens B?” Your answers to these questions assist identify your spectacles prescription.

Astigmatism Correction Options

Astigmatism, like nearsightedness and farsightedness, normally can be corrected with spectacles, contact lenses or refractive surgery.

In addition to the round lens power used to remedy nearsightedness or farsightedness, astigmatism needs an additional “cylinder” lens power to correct the distinction in between the powers of the two principal meridians of the eye.

So a glasses prescription for the correction of myopic astigmatism, for instance, could look like this: -2.50 -1.00 x 90.

  • The first number (-2.50) is the sphere power (in diopters) for the correction of myopia in the flatter (less nearsighted) principal meridian of the eye.
  • The second number (-1.00) is the cylinder power for the additional myopia correction required for the more curved principal meridian. In this case, the total correction required for this meridian is -3.50 D (-2.50 + -1.00 = -3.50 D).
  • The 3rd number (90) is called the axis of astigmatism. This is the place (in degrees) of the flatter principal meridian, on a 180-degree rotary scale where 90 degrees designates the vertical meridian of the eye, and 180 degrees designates the horizontal meridian.

If you wear soft toric contact lenses for astigmatism correction, your contact lens prescription will likewise consist of a sphere power, cylinder power and axis classification.

Gas permeable contact lenses are also a choice. Because these lenses are rigid and optically replace the cornea as the refracting surface of the eye, a cylinder power and axis may or may not be needed, depending upon the type and severity of astigmatism correction required. The very same holds true for hybrid contact lenses.

Refractive surgery such as LASIK likewise can correct most types of astigmatism. Go over with your eye doctor which procedure is best for you.

Alik Muradov (Eyexan Team Member) / author of the article
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