Why Astigmatism Causes Blurry Vision, Especially at Night
Benjamin Bert, MD •
"The cornea is not perfectly round. It's actually slightly oval. It's a very subtle difference, but it's important from a visual perspective," says Manhattan-based ophthalmologist Robert Friedman.
Astigmatism is fairly common — it affects about one in three Americans, according to the American Academy of Ophthalmology. And it occurs more frequently as you grow older. For example, an estimated 23% of 20- to 39-year-olds, 28% of 40 to 59-year-olds, and 50% of those 60 and older have astigmatism.
The condition can occur with nearsightedness (myopia) or farsightedness (hyperopia), and there are several methods to treat it. Learn more about astigmatism, how light affects the condition, and ways to help alleviate the symptoms.
What is astigmatism?
Astigmatism is one of four kinds of, what are called, refractive errors, in which the shape of the eye doesn't bend light correctly, causing blurred vision. The other types of refractive errors are:
- Nearsightedness: Difficulty seeing unless objects are close to the eyes.
- Farsightedness: The ability to see clearly if objects are far away, but close-up vision is blurry.
- Presbyopia, or when you have trouble focusing on objects up close and need reading glasses.
Patients can have more than one of these refractive conditions simultaneously.
Corneal vs. lenticular astigmatism
Most people are born with astigmatism. Usually, it's either because their cornea — the clear window in front of the eye — is more oval than round or their lenticular lens — is overly curved.
Both kinds of astigmatism can occur in one or both eyes, but corneal astigmatism is most common, says Friedman.
How astigmatism affects light perception
Ophthalmologists say blurry vision from astigmatism is the same no matter what time of day it is. However, blurry vision can become especially noticeable at night because of the contrast between light and dark.
"People will say, 'I have trouble driving at night, but not in the day.' And it's not that in the daytime, they don't have the refractive error, it's just that they're able to overcome it a lot easier than at night when there's less light," says Anna Park, an ophthalmologist in the Martin P. Kolsky Group Chartered in Washington, DC, and an attending physician at the Washington Hospital Center.
The reason vision may not be as blurry during the day is because there's less light for your eye to focus. To compensate, the pupil dilates to let in more light. A larger pupil means more of the cornea and lens are being used to focus the light. Using more of these surfaces increases the symptoms of astigmatism or other refractive errors.
"Night vision is the same as day vision, it's just that the light rays aren't focused on your retina as they should be, so the picture you get is blurred and distorted. In general, in lower light, refractive errors come out more often," says Park.
Most astigmatisms are mild, Chan says. "So people may not notice the problem during the day. But at night, lights — headlights, stop signs — will seem to come at you."
As an aside, Park says that it's important to note that screens and devices don't cause astigmatism or have any effect on them since it's a physical condition of the eye's shape and not based on behavior.
Symptoms of astigmatism
Experts agree the main symptom of astigmatism is not seeing clearly. That may then cause other symptoms, including:
- Blurred vision at all distances: People with astigmatism may complain of blurred vision for objects both up close and far away, says Park.
- Eyestrain and headaches: "Eyes muscles are trying to focus, and if things are not clear, you can get eyestrain," Park says. This may lead to headaches.
- Streaky vision: Astigmatism may cause lights to seem streaky, says Vicki Chan, an ophthalmologist at Kaiser Permanente in Los Angeles. While viewing an eye chart in a doctor's office, the "E" may appear offset, or the chart letters may look like one is on top of the other, as opposed to the overall blurriness of other conditions.
- Circular lights appear as ovals: "If you look at a circular light from far away and it [has] an ovoid distortion to it, that would be astigmatism," Friedman tells Insider. But ovoid, or oval, distortions are often difficult to perceive, he says.
- Difficulty functioning at certain distances: Just because a person has astigmatism, doesn't mean all tasks will be impacted similarly. Some people may be able to read if their astigmatism aligns in a certain direction, Friedman says, but have problems with other tasks that require seeing from far away.
How to treat astigmatism
If you have a history of vision problems, it may be time to consult a professional.
"When we prescribe glasses or contacts, what we're doing is providing a prescription that'll help focus the light coming into the eye and to the retina so that the picture is clear," says Park.
- Glasses: Prescription glasses refocus the images and lights to correct astigmatism, says Chan. Tinted glasses reduce some glare but won't help with astigmatism.
- Contact lenses: If it's strong enough to require contacts, toric lenses have two light-bending powers that cross perpendicularly to treat astigmatism.
- Laser surgery: Lasik laser surgery can be used to correct astigmatism. If a patient has corneas deemed too thin for Lasik, photorefractive keratectomy (PRK) is an option.
Blurred vision at all distances, eye strain that causes headaches, and streaky vision are a few key signs of astigmatism. Most people are born with astigmatism but in rarer cases, astigmatism can be caused by trauma.
In general, in lower light, refractive errors come out more often, which is why people with astigmatism may have trouble seeing or driving at night. The condition exists in the daytime, too, it's just exacerbated by the dark.
Moreover, astigmatism may change — for the better or worse — with age, so it's important that if you have blurred vision, it's best to see a healthcare professional for a diagnosis.