Astigmatism commonly changes over time, and a gradual increase is a normal part of aging for most adults. Refractive astigmatism shifts at roughly 0.25 diopters per decade during adulthood, which means your prescription will drift slowly over the years. But faster changes, or changes paired with other visual symptoms, can signal something that needs closer attention.
Why Astigmatism Changes With Age
The cornea, the clear front surface of your eye, doesn’t hold the same shape forever. As you age, the vertical curve of the cornea gradually flattens while the horizontal curve steepens. This shift is why most adults over 40 develop what’s called “against-the-rule” astigmatism, where the steepest curve runs side to side rather than top to bottom. Data from the Blue Mountains Eye Study found that average astigmatism roughly doubled from 0.6 diopters in people aged 49 to 59 up to 1.2 diopters in those 80 and older.
Your cornea isn’t the only part of the eye involved. The natural lens inside the eye also contributes. At birth, this lens is perfectly clear, but over decades proteins and metabolites accumulate, making it cloudier and eventually forming cataracts. As the lens changes density and shape, it can introduce its own astigmatism on top of whatever your cornea is doing. This lens-based astigmatism becomes increasingly common after age 50 and can explain why your prescription seems to shift more noticeably in middle age even if your corneal shape hasn’t changed dramatically.
Everyday Habits That Reshape Your Cornea
Chronic eye rubbing is one of the most well-documented mechanical causes of worsening astigmatism. When you rub your eyes repeatedly, you’re doing more than relieving an itch. The pressure disrupts collagen fibers in the cornea, weakens its structural resistance, and temporarily spikes the pressure inside the eye. Research in people with allergic conjunctivitis (a common cause of itchy eyes) has shown that frequent rubbing damages both the surface layer and deeper structural fibers of the cornea, ultimately compromising its biomechanical stability. If you have seasonal allergies or dry eyes that make you rub habitually, treating the underlying itch is one of the most practical things you can do to protect your corneal shape.
Even prolonged downward gaze, like hours of reading or looking at a phone, can subtly alter corneal curvature through eyelid pressure. The effect is small in most people, but it’s a real, measurable phenomenon.
When Worsening Astigmatism Signals Keratoconus
Not all progression is harmless. Keratoconus is a condition where the cornea progressively thins and bulges into a cone-like shape, producing irregular astigmatism that standard glasses can’t fully correct. It typically starts in the teens or twenties and can worsen into the thirties or beyond. The key warning signs include rapidly changing prescriptions (especially if your glasses or contacts stop working well within months of getting them), increasing ghosting or streaking around lights, and vision that doesn’t sharpen fully even with updated lenses.
Eye care providers use corneal topography, a painless imaging scan, to detect keratoconus. The scan maps the curvature and thickness of your cornea and flags suspicious patterns like inferior steepening paired with thinning, astigmatism above 2.5 diopters, or posterior elevation greater than 18 to 20 microns. In borderline cases, additional measurements of corneal thickness and higher-order optical irregularities help confirm the diagnosis. If you’re under 40 and your astigmatism is climbing noticeably between annual exams, asking for a topography scan is reasonable.
The good news is that corneal cross-linking, a procedure that strengthens the collagen bonds in the cornea, can halt keratoconus progression. A 10-year follow-up published by the American Academy of Ophthalmology found that treated eyes remained stable long term, with the average steepest corneal reading dropping by over 6 diopters after the procedure. Only 2 out of the eyes studied needed retreatment over the full decade.
Blood Sugar and Temporary Prescription Swings
If you have diabetes, your astigmatism may seem to fluctuate rather than steadily worsen. Changes in blood sugar alter the osmotic pressure inside the lens of the eye, causing it to swell or dehydrate. That shifts its thickness, curvature, and refractive index, sometimes producing sudden myopic or hyperopic changes that look like worsening astigmatism on a refraction test. These swings can go in either direction and may change from week to week depending on glucose control. Because different layers of the lens have different protein concentrations and water content, the refractive shift can be unpredictable. If your prescription seems unstable and you have diabetes (or undiagnosed blood sugar issues), the lens may be the culprit rather than a true change in your cornea.
Growths on the Eye Surface
A pterygium, a fleshy wedge of tissue that grows from the white of the eye onto the cornea, can physically distort the corneal surface and increase astigmatism. These growths are more common in people with high sun or wind exposure. As the tissue extends further onto the cornea, it reduces surface regularity and increases asymmetry, making your prescription harder to correct with standard lenses. Surgical removal can restore a more regular corneal shape, though recurrence is possible.
Astigmatism Returning After LASIK or PRK
If you had laser vision correction and your astigmatism seems to be creeping back, you’re experiencing regression, which is a known possibility. A 20-year analysis of 785 eyes treated for mixed astigmatism found an overall retreatment rate of 5.2%, with most enhancements performed within the first three years. People who had high astigmatism before surgery were nearly four times more likely to need a touch-up procedure. Regression doesn’t mean the original surgery failed. It reflects the eye’s natural healing response and ongoing age-related changes in the cornea and lens.
What to Do About It
A slow, modest increase in astigmatism over years is normal and simply means updating your glasses or contact lens prescription periodically. The practical threshold most people notice is around 0.50 to 0.75 diopters of change, which at the typical aging rate might take two to three decades to accumulate.
Faster changes warrant a closer look. If your prescription is shifting noticeably between annual exams, if your corrected vision feels blurry sooner than expected, or if you’re seeing increasing glare and halos, a corneal topography scan can distinguish routine aging from progressive conditions like keratoconus. Addressing modifiable factors, especially chronic eye rubbing, allergy management, and blood sugar stability, can also slow or eliminate changes that aren’t purely age-related.