Astigmatism is a common condition where the eye’s shape prevents light from focusing correctly, resulting in blurred or distorted vision at any distance. This imperfection involves an unequal curvature of the eye’s light-focusing surfaces, causing light rays to scatter instead of converging at a single point on the retina. Astigmatism is categorized based on the anatomical location of the defect, the uniformity of the curvature, and how it functionally affects the final image. Understanding these classifications is essential for accurate diagnosis and selecting the most effective corrective method.
Structural Types: Corneal and Lenticular Astigmatism
Astigmatism is first classified by which part of the eye possesses the irregular shape. The most frequent form is Corneal Astigmatism, which occurs when the cornea, the clear outer dome at the front of the eye, is curved unevenly. Instead of being spherical, the astigmatic cornea is shaped more like an American football, with one meridian being significantly steeper than the other. This difference in curvature causes the incoming light to be refracted unevenly.
The second type is Lenticular Astigmatism, caused by an irregular shape or position of the eye’s internal lens, located behind the iris. While both structural types cause blurred vision, corneal astigmatism is far more prevalent. The lens may develop an irregularity due to age-related changes, trauma, or health conditions like diabetes.
Categorizing Astigmatism by Regularity
A distinction for treatment planning is whether the curvature defect is uniform, which defines the astigmatism’s regularity. Regular Astigmatism is the most common presentation, characterized by two principal meridians—the steepest and the flattest curves—that are perfectly perpendicular to each other. This predictable, uniform distortion allows for straightforward correction with standard lenses.
The principal meridians in Irregular Astigmatism are not perpendicular to one another, or the curvature changes randomly across the surface. This non-uniformity means the light rays are scattered in complex, unpredictable ways. Irregular astigmatism is often caused by eye injury, corneal scarring, or internal eye diseases. A progressive condition called keratoconus, where the cornea thins and bulges into a cone shape, is a frequent cause.
Functional Classification by Refractive Error
Astigmatism is functionally classified based on where the two focal points created by the irregular curvature fall relative to the retina. This classification determines the specific power and axis required for a corrective lens prescription. Since astigmatism causes two separate focal lines to form, the position of both lines relative to the retina defines one of five subtypes.
In Simple Myopic Astigmatism, one focal line rests directly on the retina, meaning vision along that meridian is clear, while the other focal line falls in front of the retina, causing nearsighted blur. Conversely, Simple Hyperopic Astigmatism has one focal line correctly positioned on the retina, but the second line falls behind the retina, resulting in farsighted blur. Both are considered “simple” because one meridian focuses light correctly.
When both focal lines are positioned on the same side of the retina, the condition is classified as “compound.” Compound Myopic Astigmatism occurs when both focal lines are located in front of the retina, indicating a combination of astigmatism and overall nearsightedness. Similarly, Compound Hyperopic Astigmatism is diagnosed when both focal lines fall behind the retina, combining astigmatism with overall farsightedness.
The final type is Mixed Astigmatism, where the two focal lines straddle the retina, with one line falling in front and the other behind the retina. This simultaneous presence of myopic and hyperopic focus requires a unique corrective lens design to bring both focal points together onto the retina. The precise measurement of these focal points guides the final, individualized lens prescription.
How Specific Types Influence Correction Methods
The specific type of astigmatism dictates the most appropriate method of vision correction. For Regular Astigmatism, the predictable nature of the curvature allows for excellent correction using standard eyeglasses and soft toric contact lenses. These lenses are designed with two different powers at a specific orientation (axis) to counteract the eye’s unequal curves.
Irregular Astigmatism presents a challenge because standard soft lenses mold to the uneven corneal surface, failing to create a smooth optical plane. Instead, specialized correction is required, such as Rigid Gas Permeable (RGP) lenses or scleral lenses. These lenses are made of firm material that maintains its shape, effectively creating a new, smooth refracting surface over the irregular cornea, which neutralizes the distortion.
The Functional Classification by Refractive Error directly determines the power and design of the corrective lens. A person with Mixed Astigmatism, for instance, requires a lens that simultaneously corrects for both near and farsightedness along different meridians. The refractive error measurements dictate the specific cylinder power, sphere power, and axis orientation incorporated into the glasses or contact lenses. Regular astigmatism is also the form most successfully treated with refractive surgeries, such as LASIK, which surgically reshape the cornea.