Astigmatism

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Astigmatism is an eye condition that means you have blur in a specific direction, or axis (technically, depending on the notation used for your prescription, the axis may indicate the angle of the eye's meridian where you have the least focusing power, or the one where you have the most). Astigmatism is compensated with cylinder lenses. A cylinder lens adds power along one particular meridian of the eye.

Astigmatism often reduces spontaneously as myopia is corrected.

Understanding astigmatism

Astigmatism is caused by an irregularly shaped cornea or lens. The first is called corneal astigmatism, which is the more common form, and the second is called lenticular astigmatism. "Regular" astigmatism is often described as a having the cornea shaped like a rugby ball, rather than spherical like a basketball. The optics of an idealised lens of such a form would cause incoming light on different planes (corresponding to the two principal axes of the lens) to be focused at different offsets beyond the lens.

Astigmatism

Most diagrams of lenses show only a single vertical plane. In the real world, of course, there is a full cone of light arriving on the lens from the source object. This diagram shows two different cross-sections through the incident cone of light, aligned with the two axes. Rays in the horizontal cross-section (which contains the stronger curve) are focused earlier - at the label 'T' (for 'Tangential'). The rays in the vertical cross-section (the 'Saggital') are focused further behind, at 'S'. The other rays around the light cone are focused at points in between the two, giving an image smeared out along the axis.

Stenopaeic slit

The Stenopaeic slit is a simple tool which can be used in diagnosis / measurement of astigmatism. It is simply a disk with a narrow slit which can be rotated to find the clearest image. By reducing light coming in the "wrong" direction, the spherical correction on each axis can be measured directly.

Analogy with Chromatic Aberration

It may be simpler to picture the effect by comparing with Chromatic Aberration. In both cases, an extra variable means different parts of the light are focused differently.

Chromatic aberration lens diagram

(Unfortunately the astigmatism diagram choose red and blue the wrong way round !)

  • If the red light is focused on the retina, the green/blue light is focused in front, and is blurred.
  • If the blue light is focused correctly, green and red are focused beyond the retina and is blurred.
  • It's not possible to get everything into focus using only spherical lenses.

As a compromise, focusing the green light on the retina causes a little bit of myopic blue blur and hyperopic red blur. This corresponds to the "circle of least confusion" in astigmatism.

On a prescription, there are two different conventions for specifying the cylinder. This corresponds to either quoting the spherical correction to focus red on the retina, and then the additional "minus" required to focus blue; or a spherical correction for blue, and then how much that can be reduced by for red. The average of the two, or the "spherical equivalence", is then the correction required to put green on the retina.

The analogue of the Stenopaeic slit is this model is a simple coloured filter : by allowing only monochromatic light into the eye, the spherical correction for each colour can be measured separately.

If the object is moved away, beyond your blur horizon, so that your eye can no longer keep the green light focused, all colours will suffer myopic blur, but blue will have the worst blur. This corresponds to the directional blur in astigmatism. Adding some spherical correction would allow you to push the green back into focus.

Chromatic aberration could be treated by adding some material which applies the opposite chromatic error - bending the blue light out a bit more than the red light, to cancel the error introduced by the eye. This corresponds to cylinder correction. (But as with all analogies, it's starting to stretch a bit thin...)

Irregular Astigmatism

A real cornea, of course, doesn't conform to expectations. Being messier, it just has a bulge, which means that the image is not only smeared out along the axis, but is rotated, resulting in multiple (blurred) images being perceived on the retina.

Astigmatism (Eye)

Childhood Astigmatism

Astigmatism in young children often changes after they reach school age.[1]

Reducing astigmatism

As with myopia, astigmatism should be tackled in small steps when selecting lenses for differential or normalized glasses. If only a small amount of cylinder correction is present, say 0.25 diopters, the cylinder correction can be dropped, with no other changes. Otherwise, cylinder should be reduced in small increments. If sphere is being reduced, cylinder should not be changed at the same time, and conversely, if cylinder is being reduced then sphere should not be changed. The only time that both sphere and cylinder should be changed, is when converting to the spherical equivalent.

The link between astigmatism strength and visual acuity is weak. How your eyes and visual cortex respond to astigmatism is a greater factor in visual acuity than the number of dipoters.[2] If the standard advice for correcting astigmatism does not work for you, you may need a more gradual reduction.

The spherical equivalent of cylinder lenses may be useful, to simplify the reduced lens path. 0.50 cylinder means the power varies from 0D on one axis to 0.5D on the perpendicular axis. This can be substituted by the spherical power with the averaged value of 0.25D. For example, a full prescription of "-1.00 Sphere -1.50 Cylinder" could be converted to "-1.75 Sphere". The resulting spherical equivalent is not intended to compensate for the asymmetry of the lens, so it will introduce some directional blur.

Regardless of how the myope reduces their lenses, the goal of each reduction is to have a small amount of "useful blur", to be cleared up with active focus and good habits.

Axis

When reducing cylinder value, you should never change the axis value if it has been consistent for some time. The axis only refers to the orientation in which the cylinder is affecting your eyes, and any reduction of cylinder will happen on this orientation.

Resources

References

  1. Dobson, V.; Fulton, A. B.; Sebris, S. L. (1984-01-01). "Cycloplegic refractions of infants and young children: the axis of astigmatism". Investigative Ophthalmology & Visual Science. 25 (1): 83–87. ISSN 1552-5783.
  2. Remón, Laura; Tornel, Marta; Furlan, Walter D. (2006-05). "Visual Acuity in Simple Myopic Astigmatism: Influence of Cylinder Axis". Optometry and Vision Science. 83 (5): 311–315. doi:10.1097/01.opx.0000216099.29968.36. ISSN 1538-9235. Check date values in: |date= (help)