Astigmatism is a type of refractive error caused by the irregularities in the shape of a person’s cornea. In this condition, the eye fails to focus the light equally on the retina leading to blurred or distorted vision. It can be present at the time of birth, or can develop gradually in life. It is a common eye condition which usually occurs with myopia (nearsightedness) or hyperopia (farsightedness) and can be easily diagnosed with a simple eye exam. It is a refractive error and is not an eye disease or eye health issue. It is simply a problem with how the eye focuses light.
Astigmatism usually causes vision to be blurred or distorted to some degree at all distances. Some of its symptoms are eye strain, headaches, squinting and eye irritation.
Astigmatism is usually caused by an irregularly shaped cornea. Instead of the cornea having a symmetrically round shape (like a ball), it is shaped more like an egg, with one meridian being significantly more curved than the meridian perpendicular to it. (To understand what meridians are, think of the front of the eye like the face of a clock. A line connecting the 12 and 6 is one meridian; a line connecting the 3 and 9 is another.)
The steepest and flattest meridians of an eye with it are called the principal meridians. In some cases, it is caused by the distortion of shape of the lens inside the eye. This is called lenticular astigmatism, to differentiate it from the more common corneal astigmatism. It’s important to schedule an eye exam for your child to avoid vision problems in school from uncorrected astigmatism.
There are three primary types:
- Myopic . One or both principal meridians of the eye are nearsighted. (If both meridians are nearsighted, they are myopic in differing degree.)
- Hyperopic . One or both principal meridians are farsighted. (If both are farsighted, they are hyperopic in differing degree.)
- Mixed . One prinicipal meridian is nearsighted, and the other is farsighted.
Astigmatism is also classified as regular or irregular. In regular, the principal meridians are 90 degrees apart (perpendicular to each other). In irregularsm, the principal meridians are not perpendicular. Most astigmatism is regular corneal astigmatism, which gives the front surface of the eye an oval shape. Irregular can result from an eye injury that has caused scarring on the cornea, from certain types of eye surgery or from keratoconus, a disease that causes a gradual thinning of the cornea.
Astigmatism is detected during a routine eye exam with the same instruments and techniques used for the detection of nearsightedness and farsightedness. Your eye doctor can estimate the amount you have by shining a light into your eye while manually introducing a series of lenses between the light and your eye. This test is called retinoscopy.
Astigmatism can usually be corrected with eyeglasses, contact lenses or refractive surgery. Refractive surgery is one of the less common correction options, however, since it is a laser procedure that changes the shape of your eyes, it comes with risks associated with most surgeries. It should be treated as soon as possible. Once diagnosed, regular visits to an eye doctor are required as it can fluctuate over time, making it necessary for prescriptions to be modified.