What Is Astigmatism and How Can It Be Corrected?
Imagine seeing the world like you are viewing it through an out-of-focus camera. That is what people with astigmatism see. This common refractive error — when the shape of your eye prevents light from focusing correctly on your retina — occurs in 33% of Americans, both young and old.
Below, we explain what causes astigmatism, how your eye doctor can test for it, and the three ways in which it can be treated.
Types of Astigmatism
If you think of a normal, healthy eye as a basketball, then an astigmatism is like having an eye the shape of a football. The front surface of the eye, known as the cornea, is not curved equally in all directions, which affects how light rays bend and the clarity of your vision.
In a normal eye, a smooth cornea (the outside surface of the eye) and lens (the clear part of the eye behind the colored iris) help focus light rays onto your retina (the thin layer of light-sensitive tissue that lines the back of the eye) to allow for clear vision. However, astigmatism causes light to focus on different parts of the eye. This results in a person seeing fuzzy or blurry images. It can also cause you to see halos, glare, or streaking around objects.
When there is a distortion of the cornea that affects vision, this is called a corneal astigmatism. It is the most common type of astigmatism. A lenticular astigmatism is a distortion of the lens. Trauma is often a cause of lenticular astigmatism.
Although astigmatism is common, eye doctors are unsure why cornea or lens shape varies by person. However, they do know it tends to be related to our genetics, so you are more likely to inherit it if it runs in your family. Astigmatism can also be acquired after an eye injury, disease, or surgery.
There are two types of astigmatism — regular or irregular — classified by their meridians, or sections of the eye formed by imaginary lines drawn over the eye that intersect at the pupil. There are also several other types of astigmatism you can develop with a refractive error that we’ll explore below.
Regular Astigmatism
A healthy eye has a spherical cornea or lens, meaning the curves are the same. However, an eye with astigmatism has a steep curve and a flat curve, also known as the two principal meridians. To visualize this, imagine the eye is a clock with a line running vertically from 12 o’clock to six o’clock (the 90-degree axis) and another line running horizontally from three o’clock to nine o’clock (the 180-degree axis).
Regular astigmatism is when the two principal meridians are 90 degrees apart, with one meridian being steeper than the other. This type of astigmatism is most common and can be corrected by glasses, contact lenses, or laser surgery.
Regular astigmatism can also be classified as “with-the-rule” or “against-the-rule” to describe its shape. In with-the-rule astigmatism, your cornea is shaped like a football lying on the ground with the steepest curves running vertically. This type of astigmatism is most common in younger people. With against-the-rule astigmatism, your cornea is shaped like a football on a tee with the steepest curves running horizontally. This type of astigmatism is more common in older adults.
Irregular Astigmatism
Whereas regular astigmatism occurs when the two principal medians are always at right angles, irregular astigmatism occurs when the two principal medians are not symmetric. In other words, the curvature is uneven, and this makes it harder to correct than regular astigmatism.
This type of astigmatism is typically caused by disease, such as keratoconus (gradual thinning of the cornea), corneal degeneration, corneal scarring, or ocular surface disease.
Astigmatism Combined with Refractive Errors
You can also have astigmatism combined with other refractive errors. Each of the five types is defined by the location of where the cornea focuses light on the retina, also known as the focal point.
In a normal eye, the meridians focus light at a single point on the retina. When you have astigmatism, the eye cannot focus light on a single point and instead creates two focal points.
For reference, myopic is another term for nearsightedness — when objects farther away are blurry — and hyperopic is another term of farsightedness — when objects close to you are blurry.
- Simple Myopic: This is a combination of astigmatism and nearsightedness in which one focal point comes before the retina, while the other is on the retina.
- Compound Myopic: This is a combination of astigmatism and nearsightedness in which both focal points come before the retina.
- Mixed: This is a combination of astigmatism and both nearsightedness and farsightedness in which one focal point is before the retina and the other focal point is behind the retina.
- Simple Hyperopic: This is a combination of astigmatism and farsightedness in which one focal point is on the retina while the other is behind the retina.
- Compound Hyperopic: This is a combination of astigmatism and farsightedness in which both focal points are behind the retina.
How to Test for Astigmatism
A comprehensive eye exam is the most accurate way to test for astigmatism. Your eye doctor can use a number of devices to test visual acuity and the curvature of your cornea.
A phoropter, an instrument with lenses and dials to test refractive errors, helps determine which type of lens correction you need. Your eye doctor will place lenses with varying strengths in front of your eyes while you view a Snellen Chart.
They may also measure how well light focuses on your retina by using an autorefractor. This device shines light into your eye and when the light bounces off the back of your eye it records any distortions in the return image to calculate the proper correction needed.
Your eye doctor can also use a keratometer to measure the axis or the curvature of the cornea, although a corneal topographer is becoming increasingly popular to map and detail your cornea.
Astigmatism is measured in diopters, a unit of measurement of the refractive power of a lens. On a prescription, your eye doctor lists this measurement under “cylinder.” Typically, most people have an astigmatism between 0.5 and 0.75 diopters. A measurement of more than 1.5 diopters usually requires contacts or glasses to correct the astigmatism.
Cylinder is one of three measurements included in a prescription. The other two are “axis” and “sphere”. Measuring the axis helps identify an astigmatism by determining where on the cornea it is located. Your reading will be between 0 and 180 degrees, with the 90-degree axis running vertically across the eye and the 180-degree axis running horizontally across the eye.
Finally, the sphere reading on your prescription includes either a plus sign to indicate farsightedness or a negative sign to indicate nearsightedness. A higher number requires a stronger prescription.
How to Fix Astigmatism
There are three main ways to treat astigmatism: glasses, contacts, or refractive surgery.
Glasses
One of the most common ways to fix an astigmatism, glasses contain a cylindrical lens that compensates for the uneven curves in your cornea or lens. People with a severe astigmatism may be better suited for glasses instead of contacts.
Contacts
Like glasses, contacts can offer astigmatism correction, except they sit directly on your eye. There are several types of lenses available to correct astigmatism, including soft lenses, gas permeable contact lenses, and hybrid lenses.
- Soft contact lens: Toric lenses are a specific type of soft contacts used to treat astigmatism. The lenses are weighted to ensure they remain aligned properly. They also have varying powers in different meridians (the imaginary perpendicular lines in your eye) to help correct your vision.
- Gas permeable contact lens: Also known as RGP or GP contact lenses, this type of lens is more rigid to help maintain its shape on the eye. Although rigid lenses can lead to sharper vision, the fit and feeling is not for everyone.
- Hybrid contact lenses: These lenses use rigid gas permeable lens material for the center of the contact surrounded by a soft material.
Refractive Surgery
Laser surgery reshapes the cornea to correct your refractive error. There are two surgical options: LASIK and photorefractive keratectomy (PRK). The main difference between LASIK and PRK is how eye surgeons access the cornea.
- LASIK surgery features an ultrafast femtosecond laser that uses tiny pulses of light to create a flap that is folded back during the surgery so your ophthalmologist can access the cornea. An excimer laser then reshapes the cornea and smooths out any distortions to eliminate your refractive error.
- PRK surgery involves removing the outer surface of the cornea, known as the epithelium, before reshaping the inner cornea with a laser. The epithelium regrows after surgery. PRK is an alternative to LASIK for people who have dry eyes or thin corneas.
Premium Lenses for Cataract Surgery
Although unrelated, people who have astigmatism and cataracts can receive corrective surgery to treat both conditions. In a typical cataract surgery, an eye surgeon removes the cloudy lens caused by cataracts and replaces it with a clear, artificial intraocular lens (IOL). However, people who also have astigmatism can receive premium lens implants to correct it.
