Astigmatism is a common refractive error that causes blurred vision due to an irregularly shaped cornea. Instead of being round like a basketball, a cornea with astigmatism is shaped more like a football, which prevents light from focusing properly on the retina. To diagnose and manage this condition, eye care professionals use corneal topography. This non-invasive imaging test creates a detailed, three-dimensional map of the cornea’s surface for accurate assessment and treatment planning.
The Role of Corneal Topography in Diagnosing Astigmatism
Corneal topography is instrumental in distinguishing between the primary types of astigmatism: regular and irregular. Regular astigmatism is characterized by a cornea that curves uniformly, steeper in one direction and flatter in the other. This common form can be corrected with standard eyeglasses or toric contact lenses.
Irregular astigmatism presents a more complex challenge, as its shape is non-uniform and can result from eye diseases like keratoconus, eye trauma, or surgical complications. Unlike regular astigmatism, this type cannot be fully corrected with simple prescription lenses. Corneal topography is the definitive method for identifying these complex corneal surface irregularities, allowing ophthalmologists to differentiate between the two types. This ability to detect subtle surface changes is also used for screening candidates for refractive surgery.
The Corneal Topography Procedure
You will be seated in front of the topography machine, resting your chin on a support and focusing on a light target inside the instrument. The device projects a series of concentric, illuminated rings, known as a Placido disk, onto your cornea. A camera within the machine captures the reflection of these rings.
The procedure is painless, non-contact, and takes only a few seconds to complete for each eye. The only preparation required is for patients to stop wearing contact lenses for a designated period before the test. This is because contacts can temporarily alter the cornea’s natural shape, and removing them allows the cornea to revert to its true form, ensuring the most accurate measurement.
Interpreting Topography Maps for Astigmatism
The output of a corneal topography test is a colorful map that represents the cornea’s curvature. These maps use a color-coded system to illustrate the corneal landscape: warmer colors, such as reds and oranges, signify steeper areas of the cornea. Cooler colors, like blues and greens, represent the flatter portions of the corneal surface.
For an eye with regular astigmatism, the map displays a distinct and symmetrical “bow-tie” or “hourglass” pattern. In contrast, the map for irregular astigmatism lacks this symmetry and might show a distorted or inconsistent pattern. Specific abnormal patterns can also point to underlying conditions; for instance, a steep, cone-like protrusion on the map is a hallmark sign of keratoconus.
Guiding Astigmatism Treatment
The information from a corneal topography map directly influences the treatment strategy for astigmatism. When the map confirms regular astigmatism, treatment options are straightforward. These can include precisely prescribed toric soft contact lenses, rigid gas permeable (RGP) lenses, or refractive surgeries like LASIK or PRK, which aim to permanently reshape the cornea.
For irregular astigmatism, standard corrective methods are often insufficient, so the topography map guides the eye doctor toward more specialized solutions. These may include custom-designed scleral lenses or hybrid lenses that combine a rigid center with a soft outer skirt. In cases where astigmatism is caused by a progressive condition like keratoconus, the map is used to plan treatments such as corneal cross-linking or topography-guided laser ablation.