Explaining Astigmatism
Vision, William

Explaining Astigmatism

At the boys’ most recent vision appointment we discovered that Will has astigmatism; Jack does not. I have astigmatism as well, so I can understand first-hand how it can affect their vision.

With astigmatism, the cornea (outer covering of the eye) is mis-shapen. For people with normal vision their cornea is round like a baseball, where both axes (vertical and horizontal) have the same curvature. For people with astigmatism the cornea is oval like an egg or football, where one axis is more curved than the other. Therefore when the light comes through the lens it isn’t focused into a single point. Instead the light is refracted into two or more points, making the object appear blurry, wavy or distorted. Unlike near or farsightedness, the distance from an object doesn’t make it easier or harder to see.

Diagnosing astigmatism is done during normal eye exams. The doctor performs a retinoscopy by placing different lenses between a retinoscope, a hand-held lighted instrument, and the eye. The retinoscope measures the focusing power of the eye. A keratometer can also be used to measure the shape of the cornea. It focuses light on the corner and measures its reflection. This is mainly used to fit contact lenses.

There are several types of astigmatism.

  • Myopic astigmatism is where both axes are nearsighted, but at differing levels, meaning the light is focused before the retina.
  • Hyperopic astigmatism, both axes are farsighted or the light is focused beyond the retina.
  • Mixed astigmatism happens when one axis is nearsighted and the other is farsighted.

Astigmatism is also determined to be regular or irregular. The most common type is regular.

  • Regular astigmatism, the axes are perpendicular.
    With-the-rule which means the football shaped eye is sitting horizontally.
    Against-the-rule where the eye looks like a football standing vertically or on its end.
  • Irregular, they are not perpendicular.

Astigmatism is a fairly common eye condition and most people have some mild form of it. For those that are a larger astigmatism, it can be corrected with glasses, contact lenses or sometimes corrective surgery.

Vision with astigmatism can look distorted or blurred. Long straight lines can appear curved and reading some letters and numbers can be difficult to distinguish like P and F or B and 8. In with-the-rule vertical lines are the clearest, with a horizontal blur. Most children have with-the-rule astigmatism. Against-the-rule astigmatism occurs most in older eyes and shows a vertical blur, with the horizontal lines are clearest. The illustration below shows the difference.

Photo from http://en.wikipedia.org/wiki/Astigmatism_(eye)

I have with-the-rule astigmatism. I have found that it can be difficult for me to read numbers with several 1’s and 0’s in a row, since the lines blur horizontally for me. I also notice it in words with the letters “l” and “i” together (like Will’s name), the lines blur and I have to look closely to make sure I have the right number of each. I have never been able to draw a straight line, so maybe that is the reason too (or at least that is the excuse I am going to use).

For Will, his vision isn’t good enough to be affected by the blur from his astigmatism, so at this time it doesn’t do any good to fit him with glasses it. If his vision gets better then we will consider them at that time.


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