Astigmatism happens when the front surface of your eye (the cornea) or the lens inside your eye curves more steeply in one direction than the other. Instead of being shaped like a basketball, with equal curvature in every direction, the eye is shaped more like a football. This uneven curvature bends light unevenly, so it focuses on multiple points instead of one, producing blurry or distorted vision at any distance. Most people are born with some degree of it, but astigmatism can also develop later in life from injuries, eye conditions, or surgery.
Genetics Play the Biggest Role
The most common way you “get” astigmatism is simply by inheriting it. Research points to a major genetic component, with evidence for a single dominant gene that influences corneal curvature. If one or both of your parents have astigmatism, you’re significantly more likely to have it too. Many people are born with a slight mismatch in corneal curvature that either stays stable or shifts during childhood and adolescence.
This inherited form is called regular astigmatism, meaning the two curves of the cornea are consistent and predictable, just unequal. It’s extremely common. A large study of nearly 100,000 children found that 59.3% had some measurable astigmatism, with rates climbing from about 55% in elementary school to over 71% in junior high. Most of these cases are mild and may not even require correction, but the numbers show just how normal this variation in eye shape really is.
Corneal vs. Lenticular Astigmatism
There are two places the uneven curvature can occur. Corneal astigmatism, the more common type, comes from the cornea itself having different curvatures along different axes. Lenticular astigmatism comes from an unevenly curved lens sitting behind the cornea. Both types blur your vision the same way, and both are corrected with lenses designed to compensate for the specific curves measured during an eye exam. Some people have a combination of both.
Eye Rubbing and Corneal Changes
Chronic, vigorous eye rubbing can physically reshape your cornea over time. The mechanical pressure compresses corneal cells, reduces cell growth, triggers cell death, and degrades the structural proteins that keep the cornea firm. Research published in Frontiers in Bioengineering and Biotechnology found that rubbing temporarily softens the cornea and makes it more pliable, with measurable thinning of the central cornea in nearsighted patients after rubbing.
This matters because a softer, thinner cornea is more vulnerable to developing an irregular shape. The link between habitual rubbing and keratoconus (a condition where the cornea progressively thins and bulges) is well established, and keratoconus is one of the leading causes of irregular astigmatism. If you have allergies or dry eyes that make you rub frequently and forcefully, the cumulative micro-trauma can contribute to corneal shape changes that worsen astigmatism over years.
Keratoconus and Progressive Thinning
Keratoconus deserves its own mention because it produces a type of astigmatism that’s harder to correct. In this condition, the central or near-central cornea progressively thins and steepens into a cone-like shape, creating irregular astigmatism that distorts vision in unpredictable ways. Standard glasses often can’t fully correct it because the corneal surface is no longer smooth and symmetrical.
Keratoconus typically appears in the teens or twenties and worsens over a decade or two before stabilizing. It affects both eyes, though usually one more than the other. The exact trigger isn’t fully understood, but it involves a breakdown of the cornea’s structural layers combined with genetic susceptibility and mechanical stress from things like eye rubbing. Rigid contact lenses or specialized procedures can help manage the vision problems it causes.
Eye Injuries and Corneal Scarring
A direct injury to the eye can cause astigmatism if it damages the cornea deeply enough to leave a scar. Minor surface scratches typically heal cleanly, but deeper injuries from sharp objects or blunt force (like a ball hitting the eye) can damage the cornea’s inner layers permanently. The resulting scar tissue distorts the corneal surface, bending light irregularly and producing blurry, uneven vision.
This type of astigmatism is irregular, meaning the distortion doesn’t follow a neat, predictable pattern. It’s harder to correct with standard glasses, though specialty contact lenses that create a smooth optical surface over the scarred cornea can help considerably.
Astigmatism After Eye Surgery
Eye surgery, particularly cataract surgery, can introduce small amounts of astigmatism. Any incision in the cornea changes its shape slightly as it heals. The size of the incision matters: studies show a 3.0 mm incision induces roughly three times more astigmatism than a 2.2 mm incision. Modern surgical techniques use the smallest incisions possible to minimize this effect, but some degree of corneal shape change is unavoidable.
Surgeons sometimes use this to their advantage, positioning incisions strategically to reduce pre-existing astigmatism during the procedure. For people who already have significant astigmatism before cataract surgery, specialized lens implants or additional corrective steps can address both issues at once.
How Severity Is Measured
Eye doctors measure astigmatism in diopters, which reflect how much the uneven curvature bends light off course. The scale is straightforward:
- Mild: less than 1.00 diopter, often unnoticeable without an eye exam
- Moderate: 1.00 to 2.00 diopters, typically causing noticeable blur
- Severe: 2.00 to 3.00 diopters
- Extreme: over 3.00 diopters
Most people fall in the mild range. Mild astigmatism may not need correction at all if it doesn’t bother you, though it can contribute to eye strain and headaches, especially during long periods of reading or screen use. Moderate and higher levels almost always benefit from glasses, contact lenses, or refractive surgery. Your prescription will include a cylinder value (the amount of astigmatism) and an axis number (the angle of the uneven curvature), which together tell a lens maker exactly how to shape your correction.
Why It Changes Over Time
Astigmatism isn’t always static. Children’s corneas are still developing, which is why prevalence shifts significantly between age groups. The jump from 55% in elementary school to 71% in junior high likely reflects both natural growth changes and increasing near-work demands during those years. In adults, astigmatism often shifts gradually with age as the cornea and lens slowly change shape. It’s common for the axis of astigmatism to rotate over decades, which is why a prescription from five years ago may no longer feel right even if the overall amount hasn’t changed much.
The takeaway is that astigmatism is overwhelmingly a normal variation in eye shape, most often inherited, and present to some degree in the majority of people. The less common causes, including keratoconus, injury, chronic eye rubbing, and surgical changes, tend to produce more irregular and harder-to-correct forms. Regular eye exams catch shifts in your astigmatism early, keeping your correction accurate and your vision sharp.