Astigmatism is a common refractive error that causes blurred or distorted vision. This condition arises when the eye’s shape prevents light from bending correctly to focus on the retina, the light-sensitive tissue at the back of the eye. Instead of creating a single, sharp image, light rays are dispersed, leading to a focus issue within the eye. This optical imperfection can affect vision at various distances, making both near and far objects appear unclear.
The Cornea’s Role in Vision and Astigmatism
The cornea, the transparent, dome-shaped outer layer at the front of the eye, is the eye’s primary focusing surface. A typical cornea has an evenly rounded, spherical shape, like a basketball. This uniform curve ensures light rays bend evenly and focus precisely on the retina, producing clear images.
With corneal astigmatism, the cornea’s surface is irregularly shaped, resembling a football or spoon. This asymmetrical curve causes light to refract unevenly, focusing at multiple points instead of one. The brain then receives multiple, overlapping images, resulting in blurry, stretched, or distorted vision at any distance. While astigmatism can also originate from an irregular shape of the eye’s internal lens (lenticular astigmatism), most cases are due to the cornea’s irregular curvature.
Identifying Corneal Astigmatism
Recognizing corneal astigmatism often begins with noticing specific visual symptoms. These include blurred or distorted vision, whether objects are near or far. Individuals might also experience persistent eye strain, particularly after reading or prolonged screen use, along with recurrent headaches. Squinting to bring objects into clearer focus is another frequent sign, as is difficulty seeing clearly in low-light conditions or at night.
An eye doctor diagnoses corneal astigmatism during a comprehensive eye examination. An optometrist or ophthalmologist uses instruments to assess how the eyes bend light. A phoropter, a device with multiple lenses, determines the precise lens power needed for clearest vision by allowing the patient to compare combinations. Specialized tools like a keratometer measure corneal curvature, while a corneal topographer creates a detailed, computerized map of the cornea’s surface, revealing irregularities and the extent of astigmatism.
Corrective Measures for Corneal Astigmatism
Corneal astigmatism is corrected through two main approaches: corrective lenses and refractive surgery.
Corrective Lenses
Eyeglasses use cylindrical lenses to compensate for the cornea’s irregular shape. These lenses redirect light to focus properly on the retina, improving visual clarity. Contact lenses also offer effective correction, with specialized “toric” soft lenses widely available. Toric lenses have different powers in various meridians and a unique design to ensure they remain properly oriented on the eye, aligning with the cornea’s irregular curves. Rigid gas permeable (RGP) contact lenses can also correct astigmatism, particularly for more severe or irregular cases, by providing a smooth, uniform surface over the cornea.
Refractive Surgery
For a more permanent correction, refractive surgery reshapes the cornea. LASIK involves creating a thin corneal flap, using an excimer laser to sculpt underlying tissue, then repositioning the flap. This allows the cornea to heal with a more uniform curvature. PRK is another laser option where the outer corneal layer (epithelium) is removed before the excimer laser reshapes the tissue. The epithelium regrows, conforming to the new shape and correcting astigmatism.