Astigmatism is a common condition where the eye does not focus light evenly onto the retina, causing vision to be blurred or distorted at any distance. Unlike a perfectly spherical eye, which focuses light to a single point, an astigmatic eye is shaped more like an American football. This uneven curvature prevents light rays from converging to a single focal point, instead creating two distinct focal lines. Astigmatism is not a single condition, but a classification of types based on the pattern of the eye’s curvature, the anatomical source of the error, and how the eye attempts to focus light.
The Primary Distinction: Regular Versus Irregular Astigmatism
The most fundamental way eye care specialists categorize astigmatism is by the symmetry of the curvature, dividing it into regular or irregular types. Regular astigmatism is the most common type, occurring when the steepest and flattest curves of the eye’s front surface, called the principal meridians, are perfectly perpendicular, or 90 degrees apart. This symmetrical curvature is similar to a football cut lengthwise, where the shape is consistent along each meridian. Because the error is uniform and predictable, regular astigmatism is corrected easily with standard cylindrical lenses in eyeglasses or soft toric contact lenses.
Regular astigmatism itself is further categorized based on the orientation of the principal meridians: with-the-rule, against-the-rule, and oblique. With-the-rule astigmatism means the vertical meridian is steepest, like a football lying on its side, and is frequently seen in children. Conversely, against-the-rule astigmatism has the horizontal meridian as the steepest curve, which is more common in older adults. Oblique astigmatism occurs when the steepest curve is angled between 30 and 60 degrees or between 120 and 150 degrees.
In contrast, irregular astigmatism occurs when the principal meridians are not perpendicular, or the curvature varies significantly within a single meridian. This asymmetrical curvature is like a dented surface, making the light-bending pattern unpredictable. Causes include corneal scarring from injury or infection, post-surgical changes, or diseases like keratoconus, which causes the cornea to thin and bulge. Correcting irregular astigmatism is challenging and often requires specialized vision correction, as standard glasses or soft lenses cannot effectively compensate for the uneven surface.
Anatomical Origin: Corneal Versus Lenticular Types
Astigmatism can also be classified based on the anatomical structure where the curvature error originates. The eye’s focusing power is divided between the cornea, the clear front surface, and the lens, which sits behind the iris. Corneal astigmatism, the more prevalent type, is caused by an irregular curvature of the cornea itself, which provides the majority of the eye’s refractive power. Lenticular astigmatism is caused by an irregularity in the shape or position of the natural lens inside the eye.
The lens contributes to the eye’s ability to fine-tune focus, and its shape can change due to factors such as aging or cataract development. Differentiating between corneal and lenticular types is important for planning treatment. A person can have both simultaneously, requiring advanced diagnostic tools to determine the total astigmatism and its specific source.
Refractive Classification: How Astigmatism Affects Focus
Refractive classification describes the precise relationship between the two focal lines created by the astigmatic eye and the retina. This classification is crucial for determining the exact power and axis needed for corrective lenses. The error is defined as simple, compound, or mixed astigmatism, often combined with nearsightedness (myopia) or farsightedness (hyperopia).
Simple astigmatism occurs when one of the two focal lines falls directly on the retina, while the other falls either in front of or behind it. Simple myopic astigmatism has the second focal line in front of the retina, and simple hyperopic astigmatism has the second focal line behind the retina. This means that one meridian of the eye is focused correctly, but the other has a refractive error.
Compound astigmatism occurs when both focal lines fall on the same side of the retina. In compound myopic astigmatism, both focal lines fall in front of the retina, indicating two different degrees of nearsightedness. Conversely, compound hyperopic astigmatism has both focal lines positioned behind the retina, representing two degrees of farsightedness. Mixed astigmatism is a condition where the two focal lines straddle the retina, with one falling in front and the other falling behind, combining both myopic and hyperopic errors.
Identifying and Managing Different Types
Identifying the specific type of astigmatism relies on a comprehensive eye examination utilizing specialized instruments that map the eye’s surface and measure its refractive power. A keratometer measures the curvature of the steepest and flattest corneal meridians, helping to diagnose regular astigmatism. To identify irregular astigmatism, a technique called corneal topography is employed to create a detailed map of the entire corneal surface. This mapping process is particularly valuable for detecting conditions like keratoconus.
The type of astigmatism dictates the management strategy. Regular astigmatism is corrected with standard cylindrical lenses in eyeglasses or soft toric contact lenses, which provide different optical powers across different meridians. Irregular astigmatism often requires specialty contact lenses, such as rigid gas permeable (RGP) or scleral lenses, which create a smooth, artificial refracting surface over the irregular cornea. Severe lenticular astigmatism may be managed with cataract surgery, where the irregular natural lens is replaced with an artificial lens, sometimes a toric intraocular lens that corrects the astigmatism simultaneously.