Astigmatism develops when the front surface of your eye, the cornea, curves more steeply in one direction than the other. Instead of being shaped like a basketball (evenly round), the cornea is shaped more like a football or the back of a spoon. This uneven curvature bends light unevenly, so it focuses on two different points instead of one, producing blurry or distorted vision. Most people are born with some degree of astigmatism, but it can also develop or worsen over time from several different causes.
What Happens Inside the Eye
Your eye has two main structures that focus light: the cornea (the clear front window) and the lens (sitting just behind the pupil). Either one can contribute to astigmatism, though the cornea is responsible for the larger share in most people. When the cornea has different curvatures along different axes, light rays entering the eye don’t converge on a single focal point on the retina. The result is that both near and distant objects can look blurry, stretched, or doubled at the edges.
The lens inside the eye adds a smaller, relatively constant amount of astigmatism throughout life. In younger people, the lens actually compensates for some of the cornea’s unevenness, partially canceling it out. As you age, this balancing act shifts, which is why your astigmatism prescription can change even if nothing else about your eye health has changed.
Born With It: Genetics and Early Development
Astigmatism is often present at birth. The cornea simply forms with an uneven curvature during fetal development, and this is the most common way people end up with the condition. It runs in families. If one or both of your parents have astigmatism, you’re more likely to have it too, though no single gene has been pinpointed as the cause. It appears to involve multiple genetic factors influencing corneal shape.
Many children have mild astigmatism that stays stable or even decreases slightly as they grow. A large study of nearly 100,000 children in China found an overall astigmatism prevalence of 59.3%, with rates climbing from about 55% in primary school to over 71% in junior high school. Boys had slightly higher rates than girls, and children in urban areas were affected more than those in suburban zones. These numbers suggest that both biology and environment play a role during childhood eye development.
How Astigmatism Changes With Age
Even if your prescription has been stable for years, astigmatism commonly shifts after your mid-40s. In younger adults, the steepest corneal curve tends to run vertically, a pattern called “with-the-rule” astigmatism. Over the decades, the cornea gradually flattens along the vertical axis, and the steepest curve rotates toward horizontal, becoming “against-the-rule” astigmatism.
About 55% of people experience this shift after their mid-40s, and 41% don’t notice a change until their mid-50s. The rate of change isn’t symmetrical either. The shift toward against-the-rule astigmatism in your 50s and beyond progresses roughly twice as fast as the earlier changes, about 0.71 diopters per decade compared to 0.34 diopters per decade in younger years. This is why someone who never needed astigmatism correction before might suddenly find that their glasses prescription includes a cylinder component for the first time in middle age.
Eye Rubbing and Corneal Damage
Chronic, vigorous eye rubbing is one of the clearest behavioral risk factors for developing or worsening astigmatism. Rubbing your eyes hard enough to see flashes of light (those bright spots or streaks behind closed lids) applies significant mechanical force to the cornea. Over time, this repeated pressure can weaken and distort the corneal tissue.
The main concern is that chronic rubbing can trigger or accelerate keratoconus, a condition where the cornea progressively thins and bulges into a cone shape. Keratoconus causes irregular astigmatism that often can’t be fully corrected with standard glasses or soft contact lenses, especially in advanced stages. People with allergies are particularly at risk because itchy eyes lead to more frequent rubbing. If you find yourself rubbing your eyes often, using allergy drops or cold compresses to relieve the itch is a much safer alternative.
Keratoconus and Progressive Thinning
Keratoconus deserves its own mention because it’s the most significant medical cause of worsening astigmatism. It typically begins in the teenage years or early twenties and affects both eyes, though usually one more than the other. The central or near-central cornea thins, its outer protective layer breaks down, and the weakened tissue bulges forward under normal eye pressure.
This bulging creates steep, irregular curves that scatter light in unpredictable ways. Unlike the regular astigmatism you’re born with, where the curvature is simply uneven but smooth, keratoconus produces an irregularly warped surface. Vision with keratoconus often deteriorates over years, requiring progressively stronger corrections or specialty rigid contact lenses that create a smooth optical surface over the irregular cornea. In severe cases, corneal transplant surgery becomes necessary.
Injuries and Eye Surgery
Any trauma that scars or reshapes the cornea can introduce astigmatism. A scratch, puncture wound, or chemical burn that heals unevenly will leave the corneal surface with irregular curvatures. Even surgical incisions on the eye can change corneal shape. Cataract surgery, for example, requires a small incision in the cornea, and the healing process can slightly alter its curvature. Surgeons plan their incision placement partly to minimize this effect, and in some cases they deliberately position incisions to reduce pre-existing astigmatism.
Corneal transplants, procedures for glaucoma, and even certain laser eye surgeries can change astigmatism levels. LASIK and similar refractive procedures intentionally reshape the cornea, and while they’re designed to correct astigmatism, they can occasionally introduce small amounts of new astigmatism if healing is uneven.
Why Your Prescription Keeps Changing
If you’ve noticed your astigmatism correction shifting every few years, it helps to understand that your cornea is not a static structure. It responds to internal pressure, eyelid tension, tear film quality, and the slow structural changes of aging. The lens inside your eye also changes shape over time, particularly as it stiffens and loses its ability to flex for close-up focus (the same process behind reading glasses after 40).
Small prescription shifts of a quarter diopter every few years are normal and expected. Rapid changes, especially increasing astigmatism in one eye during your teens or twenties, warrant closer evaluation because they can signal keratoconus or other corneal conditions that benefit from early treatment. Regular eye exams catch these shifts before they significantly affect your vision.