What Is Astigmatism?

Patients often hear "you have astigmatism" at an exam and picture a serious eye condition, when in fact it is one of the most ordinary reasons for blur. Others notice that letters look smeared or doubled, that lights streak at night, or that vision is never quite crisp even in glasses, and they want to know why. Understanding that astigmatism is simply the eye's shape, and highly correctable, settles most of that concern.

A normally shaped cornea is round like a basketball, so it bends incoming light to a single focus on the retina. In astigmatism the cornea is shaped more like a football, curved more steeply in one direction than the other. Light passing through the steeper and flatter meridians focuses at different points — some in front of the retina, some behind — so no single sharp image forms.

The practical result is blur or distortion at every distance, not just near or just far. People with mild astigmatism may notice only slight softening of detail, while those with significant astigmatism can find it interferes with reading and driving until it is corrected.

Astigmatism is a refractive error, in the same family as nearsightedness and farsightedness. It occurs when the front surface of the eye — or occasionally the internal lens — is not symmetrically curved, so light comes to focus at multiple points relative to the retina. Most astigmatism is "regular," meaning the two main curvatures are at right angles and can be fully corrected with glasses or standard contact lenses.

Many people are born with some astigmatism, and it tends to stay relatively stable through adult life. It frequently pairs with myopia or hyperopia, which is why a prescription often lists all three. "Irregular" astigmatism, where the corneal surface is distorted unevenly, can result from keratoconus, corneal scarring, or eye surgery, and it does not correct fully with ordinary glasses. That distinction guides treatment.

Ordinary astigmatism is benign and stable. It deserves closer evaluation when it is increasing rapidly, when the amount or axis changes markedly between exams, when vision is not correctable to sharp with glasses, or when it appears with heavy eye rubbing, a family history of keratoconus, or worsening distortion. These features raise the possibility of keratoconus or another corneal problem, which is diagnosed with corneal topography and managed differently from routine astigmatism.

The core test is refraction, which measures both the amount and the orientation (axis) of the astigmatism. The doctor also examines the cornea at the slit lamp and, when astigmatism is high, irregular, or changing, obtains corneal topography — a map of the corneal surface — to look for keratoconus or other irregularity. Measuring the cornea's curvature is part of the workup, especially before contact lens fitting or any refractive or cataract surgery.

Most astigmatism is corrected with glasses using cylindrical (toric) lenses that compensate for the uneven curvature. Toric contact lenses do the same; rigid gas-permeable lenses are especially helpful for irregular astigmatism because they create a smooth optical surface. Laser vision correction such as LASIK or PRK can reshape the cornea to reduce or eliminate regular astigmatism in suitable candidates. During cataract surgery, toric lens implants or corneal incisions can correct astigmatism at the same time. Irregular astigmatism from keratoconus may need specialty contact lenses or corneal crosslinking.

• Do not assume astigmatism means a serious eye disease; in most people it is simply the eye's shape.
• Do not rub your eyes hard or habitually, as vigorous rubbing is linked to worsening keratoconus in susceptible people.
• Do not ignore astigmatism that is climbing quickly or vision that will not sharpen with glasses — get corneal imaging.
• Do not buy generic non-prescription lenses expecting them to correct astigmatism; the correction must match your axis.

Schedule an exam if your vision is blurry or distorted at all distances, if lights streak or ghost at night, or if glasses no longer make things sharp. For patients in the Hanover area, an exam pins down the exact amount and axis of astigmatism and checks the cornea for any irregularity. Book sooner if your astigmatism has been rising quickly or there is a family history of keratoconus.

Astigmatism is a common, correctable refractive error caused by an unevenly curved cornea or lens that blurs vision at all distances; glasses, contacts, or laser correction handle it well, and only rapidly changing or irregular astigmatism needs a closer look.