Vision correction prescriptions detail the power required to focus light onto the retina. When a person has astigmatism, the prescription includes two specific values to address this irregularity: the Cylinder (CYL) and the Axis (AX) measurements. Understanding whether these values remain constant or change over time is a common concern. The stability of these numbers depends on natural physiological processes and external factors affecting the structure of the eye.
Understanding Cylinder and Axis Values
The Cylinder value quantifies the magnitude of the irregular curvature in the eye, indicating the degree of astigmatism present. Measured in diopters, the cylinder number specifies the amount of lens power needed to correct the difference between the eye’s steepest and flattest meridians. A higher cylinder number signifies a greater severity of the astigmatism.
The Axis value determines the orientation of this irregular curvature, specifying where on the corrective lens the cylinder power must be applied. This number is measured in degrees, ranging from 1 to 180, acting as a map coordinate for the correction. The Cylinder and Axis values work together, ensuring the corrective lens aligns the power in the exact location to achieve clear vision.
Natural Progression of Astigmatism Over Time
Cylinder and Axis values are not static and often undergo slow, predictable changes throughout life, especially in response to natural aging. During childhood, astigmatism may increase slightly as the eye grows, but it generally stabilizes in young adulthood. Changes in the prescribed cylinder are relatively uncommon for people younger than their mid-30s.
Significant changes tend to occur later in life, often after the mid-40s. The most common pattern involves a shift in the axis orientation, known as the transition from “with-the-rule” (WTR) to “against-the-rule” (ATR) astigmatism. WTR astigmatism, where the steepest curve is vertical, is more prevalent in younger eyes.
As the eye ages, the lens and cornea can stiffen and change shape, causing the steepest curvature to gradually shift toward the horizontal meridian, resulting in ATR astigmatism. This transition often passes through an oblique cylinder axis orientation before settling into the ATR pattern. This slow, long-term drift is a normal physiological change related to aging.
Specific Factors Driving Significant Shifts
While most changes are slow and age-related, some factors can cause rapid or substantial shifts in cylinder and axis measurements. One condition is Keratoconus, a progressive disorder where the cornea thins and bulges outward into a cone shape. This rapidly distorts the corneal shape, leading to substantial increases in cylinder power and significant shifts in the axis.
A change in cylinder of 1.00 diopter or more within a 12-month period, particularly in a young adult, can be a warning sign for Keratoconus. Significant changes can also occur following external trauma, such as corneal scarring from an injury or infection. This irregular healing alters the eye’s curvature.
Eye surgeries, including cataract procedures or refractive surgeries like LASIK, can also intentionally or unintentionally alter these values. For example, the placement or shift of an intraocular lens after cataract surgery can introduce or change astigmatism. Regular, comprehensive eye examinations are necessary to monitor any shifts and distinguish expected physiological changes from those caused by disease or injury.