Astigmatism is a common type of refractive error where the eye does not bend or refract light correctly, resulting in blurred vision. This condition affects how the eye focuses light onto the retina, causing objects to appear blurry or distorted at any distance. Since light is not focused to a single point, people often experience symptoms like eyestrain and difficulty with night vision. Corrective measures are routinely used to manage this condition, and this article will explore how common vision correction tools, particularly glasses, are utilized to achieve clearer sight.
What Astigmatism Actually Is
Astigmatism occurs because of an imperfect curvature in the eye’s main focusing structures, either the cornea or the lens. In a normally shaped eye, the cornea and lens have a spherical curvature, similar to a basketball, which bends light evenly to focus it precisely on the retina. With astigmatism, the curvature is irregular, often resembling the shape of a football.
This asymmetrical shape means that light entering the eye is bent more steeply in one direction, or meridian, than another. Instead of focusing light onto a single point on the retina, the light is focused on multiple points or along focal lines. This uneven focus is what makes an object look blurry or wavy, affecting vision both up close and far away.
The Role of Glasses in Correction
Glasses are the most common method for managing astigmatism, correcting the vision while worn without altering the physical shape of the eye. The correction is achieved through specialized cylindrical lenses, also called toric lenses. These lenses are designed with different refractive powers along different meridians to compensate for the eye’s irregular curvature.
The cylindrical lens works by adding the precise amount of power needed to bend the light rays that are incorrectly focused by the astigmatic eye. The lens introduces a corrective curve opposite to the eye’s irregular curve, neutralizing the uneven bending of light. This directs all incoming light to converge sharply at a single focal point on the retina, restoring clear vision.
An eyeglass prescription for astigmatism includes specific measurements to ensure this precise correction is achieved. The prescription contains a “cylinder” value, which indicates the amount of lens power required to correct the astigmatism, measured in diopters. Equally important is the “axis” measurement, a number between 0 and 180 degrees, which pinpoints the exact orientation of the astigmatism on the eye. Without the correct axis, the cylindrical correction would be misaligned.
Other Methods for Vision Correction
Contact Lenses
Beyond glasses, contact lenses offer an alternative for managing astigmatism. Soft toric contact lenses are shaped like a slice of a donut, creating different focusing powers on the vertical and horizontal orientations, similar to cylindrical eyeglass lenses. These lenses are specifically designed with features like thin-thick zones or ballasting to help keep them in the correct rotational alignment on the eye.
Rigid gas permeable (RGP) contact lenses provide another option, especially for higher or more irregular forms of astigmatism. The stiff, non-flexible nature of the RGP lens allows it to sit on the cornea and create a smooth, spherical front surface. The space between the lens and the cornea fills with tears, forming a “tear lens” that effectively masks the underlying corneal irregularity, providing exceptionally sharp vision.
Refractive Surgery
The only methods that physically change the structure of the eye to correct the condition are refractive surgeries, such as LASIK (Laser-Assisted In Situ Keratomileusis) and PRK (Photorefractive Keratectomy). Both procedures use a laser to precisely reshape the cornea, altering its curvature to correct the refractive error. This reshaping allows light to focus correctly on the retina without the need for an external corrective lens.
LASIK involves creating a thin flap in the outer corneal tissue, lifting it, and then using the laser to reshape the tissue underneath before repositioning the flap. PRK involves removing the outermost layer of the cornea before using the laser to reshape the underlying tissue. While both surgeries aim for the same visual outcome, they differ in the method of accessing the cornea and the subsequent recovery time.