Astigmatism is a condition of the eye which causes either blurred vision or a sense that each eye is seeing objects slightly differently. A structural problem — usually from either an unevenly curved cornea or lens — causes light coming into the eye to not focus correctly. This disorder is quite common, affecting about 30% of people to some degree. Treatment is not always needed, but when the problem is severe, prescription lenses or surgery can help improve the person's vision.
Anatomy
The human eye is usually a perfectly round sphere. Light that comes into the eye is focused by the cornea and the lens onto a point on the retina, where the image is detected and communicated to the brain. In someone with astigmatism, the cornea or the lens is not perfectly round; often it's shaped more like a football. When the eye is not curved properly, the light that comes into the eye cannot be focused on one single point, causing the image to be blurry.
Astigmatism is a type of refractive error, meaning that the eye does not bend light coming into it to focus on the correct place on the retina. Nearsightedness (myopia) and farsightedness (hyperopia) are also refractive errors, caused by the cornea being too curved or too flat, respectively. The eyes of people who are nearsighted focus the image in front of the retina, while in those who are farsighted focus behind it. People can have both an astigmatism and another refractive error; it is estimated that about 2/3 of people who are nearsighted have the other condition as well.
Types
There are different types of astigmatism, depending on which part of the eye is irregularly shaped, where the light is focused, and whether or not the principal meridians of the eye are perpendicular. This condition can be caused by an defect in either the cornea or the lens, although the corneal type is more common.
A normal eye focuses an image on a single point; optometrists draw an imaginary plus-sign (+) at the center of the pupil where this focus point should be. These are referred to as the vertical and horizontal or principal meridians. In someone with astigmatism, the two meridians do not focus at the same point. If one focuses on the retina while the other does not, it's known as simple, while if both focus either in front or behind the retina, it's known as compound. One point in front and one behind is called mixed.
As with nearsightedness and farsightedness, the focus in front of the retina is called myopic and behind is hyperopic. Therefore, if one meridian focuses in front of the retina while the other is on it, it would be called simple myopic astigmatism.
When the principal meridians line up perpendicularly — meaning that they meet at a 90° angle — it is known as "regular" astigmatism. If the angle is off, it's called "irregular," and is a bit more difficult to treat. The irregular type is often caused by an injury to the eye or a disease called keratoconus, in which the cornea takes on a cone-like shape.
Symptoms
Minor astigmatisms often go largely unnoticed, but severe cases may cause headaches, squinting, and tired eyes in addition to blurry vision. Most people with cornea irregularities are born with them, but may not notice any problem until they get older. Even those who have few symptoms can be diagnosed with this condition in the course of a routine eye exam. Since many of the signs are not obviously or directly related to vision, people may find that treatment improves headaches they were not even really aware of.
Detection
There are several ophthalmological tests to determine the presence and level of astigmatism. A keratometer and a corneal topographer are instruments that can be used measure the curvature of the cornea. An autorefractor can give an estimate of the eye's ability to focus light properly. These instruments are non-invasive and most ophthalmologists and optometrists are able to detect even slight curvature problems during a simple eye exam.
Sometimes astigmatism can be detected at home by covering one eye to look at an object, and then changing to cover the other eye. By switching back and forth while looking at a single object or in one direction, the person may notice that the object seems to move, as though each eye is seeing it in a slightly different location. This usually indicates the presence of corneal curvature.
Treatment
Glasses or contact lenses can be prescribed to treat most forms of astigmatism, but they generally do not correct the condition. Often, two different lenses will help the eyes focus together, thus offsetting the uneven focusing. Eyeglasses cannot be used to treat people with the irregular form, although certain types of contact lenses typically can help. One type of contact lens actually helps to reshape the eye; this treatment is called orthokeratology or Ortho-K.
Surgical correction is an option for severe cases. A number of different procedures can be used to reshape the cornea of the eye and make it more spherical. Laser in situ keratomileusis (LASIK) surgery uses a laser to sculpt the underside of the cornea after the top part is lifted up. Photorefractive keratectomy (PRK) and a similar technique called laser-assisted subepithelial keratomileusis (LASEK) remove or fold back the very outer layer of the cornea and reshape the top surface. An optometrist can help a patient decide which method is a better option if surgery is recommended.
