Have you ever wondered what the numbers on your contact lens box mean?
Of the three numbers used to describe your prescription (whether for glasses or contacts), two of the three have to do with the degree of astigmatism in your eyes.
Astigmatism is a common condition in both children and adults that can cause blurriness in vision. In fact, it is so common that most people have some degree of astigmatism. Very slight degrees of astigmatism do not cause vision problems severe enough to be treated.
Astigmatism is a refractive error. “Refractive” refers to the way that the eyes bend, or refract, the light entering the eye.
Reading a chart of letters is the visual acuity test that most people associate with a visit to the optometrist. Visual acuity tests pick up any potential refractive errors. The three types of refractive errors are:
- myopia (nearsightedness),
- hyperopia (farsightedness), and
Astigmatism usually results from an irregularly shaped cornea. Less often, it is the lens underneath the cornea that has the imperfections in shape. In either case, an eye with astigmatism will stretched out in one direction, oblong like a football instead of perfectly round. The cornea is the clear layer at the front of the eye where light enters. This protective layer covers the dome created by the iris, pupil, and lens.
When this dome is not the same smooth shape in all directions, light rays are refracted in a distorted way at the back of the eye. Instead of coming into sharp focus on the retina in the back of the eyeball, the light rays focus too far ahead or too far behind the actual wall of the retina. The resulting image appears blurry. Sometimes, double vision can result.
During an eye exam, optometrists take measurements of the degree of refractive error from nearsightedness, farsightedness, and astigmatism. Three machines are typically used to take these measurements for astigmatism.
(1) The topography machine, also known as a keratometer, measures the reflection of a circle of light on the outside of the dome of the eye formed by the cornea, iris, pupil, and lens. This will indicate how curved the cornea is in each direction.
(2) The autorefractor measures how a beam of light changes as it bounces off the back of the eye. If images are focusing too far ahead or too far behind the wall of the retina, the autorefractor will be able to measure the difference from normal.
(3) Finally, your eye doctor will present a series of choices in corrective lenses. The phoropter machine has many types of lenses that the patient looks through. Telling the optometrist which one improves your vision the most gives a precise picture of how a corrective lens must be shaped in order to bring your vision to normal.
Using these three machines during the eye exam, your optometrist will end up with a measurement of your astigmatism in units called diopters. A perfectly round eye has 0 diopters, and corrective lenses usually start at 1.5 diopters. Most people have a measurement of between 0.5 and 0.75, and the American Academy of Ophthalmology does not recommend glasses or contacts for this degree of astigmatism. It is generally small enough to maintain good vision without correction.
Three numbers describe the resulting corrective lens prescription. While the first number, the spherical measurement, tells whether the patient is farsighted (plus sign) or nearsighted (minus sign), the last two numbers describe the astigmatism. The cylinder measurement indicated how irregular the cornea is, and the axis (measured in degrees from 0 to 180) tells exactly where on the cornea the astigmatism can be found.
Taken together, these measurements are a universal way of describing to eye care professionals everywhere what corrective lenses are needed to bring the patient to normal vision.