Astigmatism is a common vision condition characterized by an irregularly shaped cornea or, less commonly, the lens inside the eye. This irregularity causes light entering the eye to bend unevenly, leading to blurred or distorted vision at all distances. Instead of a perfectly round shape like a basketball, an astigmatic eye might resemble a football or the back of a spoon. This condition is a type of refractive error, meaning the eye does not focus light correctly onto the retina.
Understanding Astigmatism
The irregular curvature in an astigmatic eye means that light rays entering the eye do not converge to a single focal point on the retina. Instead, they create multiple focal points, either in front of or behind the retina, or both.
Astigmatism can be broadly categorized into two main types based on the affected part of the eye. Corneal astigmatism, the most common type, occurs when the cornea itself has an irregular shape. Lenticular astigmatism, on the other hand, results from an irregularly shaped lens inside the eye.
Eye care professionals diagnose astigmatism through a comprehensive eye examination, including a visual acuity test where you read letters on an eye chart. A refraction test uses a phoropter or automated refractor to determine the precise lens power needed for clear vision. Keratometry, another diagnostic tool, measures corneal curvature by analyzing reflected light.
How Glasses Provide Vision Correction
Prescription glasses are a primary method for correcting astigmatism, working by refocusing light onto the retina to achieve clearer vision. They do not alter the physical shape of the eye but rather compensate for the existing irregular curvature. This correction is achieved through the use of specialized lenses known as cylindrical or toric lenses. Unlike standard spherical lenses that bend light uniformly, cylindrical lenses have different curvatures in various meridians, allowing them to bend light more in one specific direction to counteract the unevenness of the astigmatic eye.
An eyeglass prescription for astigmatism includes specific values that customize this correction. The “sphere” (SPH) value addresses general nearsightedness or farsightedness, while the “cylinder” (CYL) value indicates the amount of lens power needed to correct the astigmatism itself. The “axis” value, measured in degrees from 1 to 180, specifies the exact orientation or angle at which the cylindrical power should be applied to compensate for the eye’s unique curvature. These measurements are determined during an eye exam and are crucial for crafting lenses that provide optimal visual clarity and comfort.
When Glasses May Not Be Enough
While glasses are highly effective for correcting most cases of astigmatism, there are situations where they might not provide optimal vision or comfort. For individuals with a high degree of astigmatism, particularly those with 1.5 diopters or more, glasses may still leave some residual blur or cause noticeable distortion, especially in the peripheral vision. This can happen because the compensatory bending of light by the lenses can introduce its own visual effects that some individuals find bothersome.
Irregular astigmatism, a less common form where the cornea’s curvature is uneven across its surface rather than in a consistent oval shape, often presents a greater challenge for standard eyeglasses. This irregularity can be caused by corneal scarring from injury, disease like keratoconus where the cornea thins and bulges, or even previous eye surgeries. In such cases, the complex and unpredictable nature of the corneal surface makes it difficult for a fixed-power cylindrical lens to provide a uniform and clear focus. Furthermore, some individuals simply prefer not to wear glasses due to lifestyle preferences, professional requirements, or cosmetic reasons, prompting them to explore other vision correction alternatives.
Other Approaches to Vision Correction
For individuals who find glasses insufficient or undesirable for their astigmatism correction, several other methods are available. Contact lenses, particularly toric contact lenses, are a common alternative. These specialized soft lenses are designed with distinct curvatures in different meridians to provide varying refractive powers, similar to cylindrical eyeglass lenses, ensuring stable and consistent vision even with eye movement. Rigid gas-permeable (RGP) lenses are another option, particularly effective for moderate to severe astigmatism, including some cases of irregular astigmatism, as their firm structure creates a smooth, uniform refractive surface over the eye.
Refractive surgery offers a more permanent solution by physically reshaping the cornea. Procedures like LASIK (Laser-Assisted In Situ Keratomileusis) and PRK (Photorefractive Keratectomy) use an excimer laser to precisely remove microscopic amounts of corneal tissue, thereby altering its curvature to allow light to focus correctly on the retina. While LASIK involves creating a thin flap in the cornea before reshaping, PRK reshapes the outer surface directly after removing the outermost layer. These surgical options are typically considered based on the severity of astigmatism, corneal thickness, and individual lifestyle, following a thorough consultation with an eye care professional.