Astigmatism is a common refractive error caused by an irregularly shaped cornea or, less frequently, the lens inside the eye. This imperfection affects how the eye focuses light. Eyeglasses are the standard, highly effective way to correct astigmatism by precisely compensating for the eye’s uneven focusing power.
What Astigmatism Is
Astigmatism occurs when the cornea or the lens is not perfectly spherical. The surface is curved more like a football, having different curvatures in different directions. This irregularity causes light entering the eye to focus at multiple points instead of converging onto a single, sharp point on the retina.
This uneven focus results in vision that is blurry or distorted at all distances. Astigmatism often exists alongside other refractive errors, such as nearsightedness or farsightedness. The condition is frequently present from birth and is generally inherited.
The Mechanism of Correction with Glasses
Glasses correct astigmatism using specialized cylindrical or toric lenses. Unlike standard spherical lenses, these lenses have different corrective powers in different meridians. This unique design counteracts the uneven bending of light caused by the eye’s irregular shape.
The prescription involves two primary measurements: cylinder power and axis. The cylinder power, measured in diopters, specifies the strength of the correction needed to compensate for the difference in curvature. This power is applied along one plane of the lens, where the eye’s irregularity is greatest.
The axis is a number between 1 and 180 degrees that indicates the precise orientation of the astigmatism on the eye. The corrective lens must be precisely fitted into the frame so the cylinder power is aligned perfectly with this axis. This ensures the lens adds the necessary power exactly where needed, causing light rays to converge into a single focal point on the retina.
When Glasses Are Not Enough
Glasses are highly effective, but they have limitations with severe or complex cases of astigmatism. For individuals with a high degree of astigmatism (above 3 or 4 diopters), the thickness of the corrective lens can become a practical issue. Thicker lenses may also cause visual distortions, requiring a period of adaptation.
The effectiveness of glasses relies heavily on the precise alignment of the lens axis. A slight rotation on the face can misalign the cylindrical correction, leading to residual blur or discomfort. Some forms of irregular astigmatism, such as that caused by keratoconus, cannot be fully corrected with standard glasses or soft lenses.
In these situations, glasses may improve vision significantly but still not achieve perfect 20/20 clarity. The residual blur occurs because glasses cannot fully compensate for the complex, non-uniform surface of an irregularly shaped cornea.
Other Options for Correcting Astigmatism
Beyond standard eyeglasses, alternatives for correcting astigmatism include contact lenses and refractive surgery.
Contact Lenses
Contact lenses designed for astigmatism are called toric contact lenses, and they employ the same cylindrical correction principle as glasses. These lenses are engineered with special features, such as prism ballasting or thin zones, to help them stabilize and remain aligned on the eye’s surface. For more severe or irregular cases, Rigid Gas Permeable (RGP) lenses are often recommended. Their firm structure helps mask the irregular corneal shape, offering a smoother optical surface.
Refractive Surgery
Refractive surgery, such as LASIK (Laser-Assisted in Situ Keratomileusis) or PRK (Photorefractive Keratectomy), is a more permanent solution. These procedures use a laser to physically reshape the cornea, permanently altering its curvature to correct the astigmatism. Toric intraocular lenses are also an option for people undergoing cataract surgery or a refractive lens exchange. These implanted lenses contain the cylindrical correction and can address moderate to high astigmatism from within the eye. The choice among these options depends on the severity of the astigmatism, individual lifestyle, and visual goals.