This article is for informational purposes only and does not diagnose any conditions
Astigmatism is a very common eye condition that can caused blurred or distorted vision. Astigmatism is is not an eye disease, but rather a term used to describe a type of refractive error related to a variation in the shape of the cornea or lens.
Astigmatism is a very common eye condition that can caused blurred or distorted vision. Astigmatism is not an eye disease, but rather a term used to describe a type of refractive error related to a variation in the shape of the cornea or lens. The cornea is the clear protective layer at the front of the eye, located over the iris (the colored part of the eye) and the lens. The lens is the clear structure at the front of the eye that refracts and focuses rays of light to go through to a specific point on the retina at the back of the eye, creating vision. Normally, the cornea and lens are smooth, clear, and evenly curved in all directions, precisely focusing the light that passes through.
Astigmatism occurs when the cornea or lens are not perfectly round, and are slightly irregular, oval or cylindrical. This difference in shape causes the rays of light to be less focused on a specific point on the retina. This causes vision to be blurred or distorted. Corneal astigmatism refers to astigmatism caused by the cornea. Lenticular astigmatism refers to astigmatism caused by the lens.
Most people have some irregularity to their lens or cornea resulting in some degree of astigmatism. The good news is that even though these small amounts of astigmatism do affect vision, it is not very noticeable or so bothersome that it requires treatment.
Are there noticeable symptoms of astigmatism?
Small degrees of astigmatism may cause no visual symptoms or disturbances and most people are unaware they have it. Larger degrees of irregularity and resulting astigmatism can cause distorted vision or blurred vision at all distances, squinting, eye discomfort or irritation, headache or fatigue. The distortion occurs because the eye is unable to focus light rays to a single point on the retina.
People with astigmatism can describe their vision as if they are looking into a distorted, wavy mirror.
These symptoms do not necessarily mean that an astigmatism is present but suggest that a complete eye examination by an eyecare provider is needed.
An adult with significant astigmatism will likely notice that their vision is distorted or not as good as it could be, but children are less likely to be aware or complain about blurred or distorted vision. Significant astigmatism, when present in children, can cause amblyopia, which can result in permanent vision loss.
How is astigmatism diagnosed?
Astigmatism can be generally be detected during a regular eye examination with refraction. The examination usually begins with testing visual acuity by reading letters on an eye chart to determine how clear vision is at different distances. The optometrist or ophthalmologist will then perform a series of tests that assess overall eye health and a specific test, called refraction to measure refractive error, or the way the eye bends light before it reaches the retina. The same tests to determine near (myopia) or farsightedness (hyperopia), also measures the degree of astigmatism.
There are two main types of refraction, autorefraction or subjective refraction. An autorefraction is done by an electronic device known as an autorefractor. The autorefractor shines light into the eye and measures how the light changes as it enters the eye and bounces off the retina at the back of the eye.
A subjective refraction (subjective because it is based on the patient response) involves a bright light that is directed at the eye and the patient is asked to look through a series of different lenses (phoropter). These lenses help to determine what type of lens correction (glasses or contacts) will provide the individual with the clearest vision.
The amount of corneal astigmatism can be measured by a diagnostic instrument that measures the curvature of the cornea called a keratometer. A more detailed map of the shape of the surface of the cornea can be done with an imaging device called corneal topography.
What is refractive error?
Refractive errors are the different conditions that can alter the way that the structures of the eye, primarily the cornea and lens, bend or refract the light that reaches the retina at the back of the eye, creating vision.
Astigmatism is a component of refractive error and often occurs in combination with another refractive error, myopia (nearsightedness, or better vision when looking at objects closely) or hyperopia (farsightedness, or better vision when looking at objects that are far away). Presbyopia is another type of refractive error that refers to the loss of near focusing ability that occurs with age, leading people to get reading glasses to help with near activities.
How do I read my prescription?
Once your optometrist or ophthalmologist has completed your eye examination and refraction, they will give you a prescription, based on all the measurement they performed. OD (from latin, oculus dexter) refers to the prescription for the right eye and OS (from latin, oculus sinister) refers to the left eye. If something pertains to both eyes, OU (from latin, oculus uterque) is used.
There are three numbers on your prescription, the Sphere, the Cylinder and the Axis. The first number describes the amount of near or farsightedness, the last two numbers describe elements of astigmatism.
If someone requires reading glasses to correct presbyopia, then the doctor may write “add +2.00D” to the prescription to indicate the need for additional correction, like a bifocal.
Sphere – the first number on a prescription, measured in diopters (unit of measurement of optical power), indicates whether someone is nearsighted (sees things better at closer distances) or farsighted (sees things in the distance better than things up close). A “+” (plus) sign means that the correction is for people who are farsighted (hyperopic), and a “-” (minus) sign means someone is nearsighted (myopic). The higher the number, the stronger the lens power is.
Cylinder – the second number, describes the degree of astigmatism, also measured in diopters. A ‘0’ zero indicates that there is no astigmatism. The majority of people have an astigmatism of between 0.5 to 0.75 D (diopters), but measurements of 1.5D or more tend to cause more notable symptoms that require correction.
Axis – the third number is measured in degrees, from 0 to 180°. This number indicates the location on the cornea where the astigmatism (irregularity) is located.
Example of a prescription:
OD -4.0 + 0.75 X120°
OS -2.75 + 1.25 x 80°
The prescription above indicates that in the right eye, this person is myopic/nearsighted (-4.0 diopters) with 0.75 diopters of astigmatism, located at 120°. In the left eye, the person is less nearsighted (-2.75 diopters) with a higher degree of astigmatism (1.25 diopters) located at 80°.
What causes astigmatism?
Astigmatism and the irregularities of the cornea and lens can occur during normal eye growth. Despite normal growth processes, some misalignment of the different structures of the eye may happen. Astigmatism can usually be detected in childhood, so it is recommended that children be examined by an optometrist or ophthalmologist before they begin school to correct and avoid any potential problems with vision before they negatively affect learning. In the United States, 28.4% of children between the ages of five and 17 have astigmatism. 
Genetics can also influence the development of refractive error, including astigmatism. Additionally, some eyes can develop astigmatism if pressure is applied to the eye lids or cornea. New or a change in astigmatism has also been observed following an eye injury or eye surgery.
In very rare cases, a condition called keratoconus may occur in which the cornea becomes progressively thinner and develops a cone shape. Individuals with keratoconus have high degrees of astigmatism due to the misshapen cornea, which results in poor vision that cannot be corrected with prescription glasses. Although some people can achieve improved vision with contact lenses, more drastic measures, including corneal transplants may be recommended.
Astigmatism cannot develop or worsen from excessive reading in low light or sitting very close to the screen of a computer or television.
How can astigmatism be corrected?
Prescription Glasses - For most people, nearly all degrees of astigmatism can be easily corrected with prescription glasses or contact lenses, made to their specific measurements. If the individual does not have any bothersome symptoms, the astigmatism is small and there is no correction needed to adjust for near or farsightedness, then corrective lenses are usually not prescribed.
The prescription lenses prescribed to correct astigmatism are called toric lenses. Toric lenses have the correction for near or far sightedness and an added component called cylinder. A cylinder correction provides improved ability to bend light in a particular axis, so it better focuses at a specific point on the retina, allowing for clearer vision. Astigmatism can change over time, and usually increases. Regular eye examinations with refraction will assess any changes to astigmatism measurements and determine if changes to the prescription lenses are needed.
Contact Lenses - Astigmatism can also generally be corrected with contact lenses. Previously, people with astigmatism were limited to the types (rigid gas permeable) of contact lenses they were able to wear. Now, there are numerous toric soft contact lens options available to correct astigmatism. An eye care professional will be able to discuss which lens options might be best to meet each individual’s needs.
Corneal refractive therapy or Orthokeratology (OrthoK) has been studied as a treatment for astigmatism, but with mixed results. Orthokeratology involves wearing specially designed, gas permeable contact lenses overnight with the intention of temporarily reshaping the cornea and reducing astigmatism. The changes in the shape of the cornea are not permanent, and the eye returns to its original state if contact lens wear is discontinued. Although some improvement has been noted in people with lower levels of astigmatism, there have also been safety concerns raised, related to the risk of potentially blinding microbial keratitis (severe cornea infection) from contact lens wear. 
Laser Surgery - Some people choose to have their astigmatism corrected during laser or other refractive procedures, where the surgeon can use a laser to change the shape of the cornea.
Intraocular surgery - Intraocular surgery can also be used to correct astigmatism. If the astigmatism is caused by an existing cataract (lenticular astigmatism), then cataract surgery should generally be able to correct the astigmatism by removing the cataract and implanting an artificial lens (known as an intraocular lens or IOL). Sometimes patients can be implanted with a toric IOL where the artificial lens is shaped to counteract the astigmatism.
 Kleinstein, R. N.; Jones, LA; Hullett, S; et al. (2003). Refractive Error and Ethnicity in Children. Archives of Ophthalmology. 121 (8): 1141–7. https://www.ncbi.nlm.nih.gov/pubmed/12912692
 VanderVeen DK, Kraker RT, Pineles SL, Hutchinson AK, Wilson LB, Galvin JA, Lambert SR. Use of Orthokeratology for the Prevention of Myopic Progression in Children: A Report by the American Academy of Ophthalmology. Ophthalmology. 2019 Apr;126(4):623-636. https://www.ncbi.nlm.nih.gov/pubmed/30476518