Astigmatism is a common vision condition caused by an imperfection in the curvature of the eye’s cornea or lens, resulting in blurred vision at any distance. This refractive error is not fixed; it often changes throughout a person’s life. While shifts are typically slow and subtle, occurring over decades, a sudden, significant change can signal an underlying health issue requiring immediate attention.
Understanding How Astigmatism Forms
Astigmatism is fundamentally a problem with the eye’s shape that prevents light from focusing correctly onto a single point on the retina. In a perfectly formed eye, the cornea and lens are spherical, allowing light rays to refract uniformly. With astigmatism, the eye’s shape is elongated, resembling a football, creating two different curves and two distinct focal points.
The condition is categorized based on the location of the irregular shape. Corneal astigmatism, the most common form, occurs when the clear, dome-shaped front surface of the eye is irregularly curved. Lenticular astigmatism occurs when the internal lens, located behind the iris, has an irregular shape or is slightly tilted. A combination of both corneal and lenticular factors ultimately determines the corrective lens prescription.
Natural and Age-Related Shifts in Astigmatism
Astigmatism is often present from birth but rarely remains static throughout life due to natural development and aging. In infants, a high prevalence of astigmatism is common due to a steeply curved cornea. This often decreases or resolves as the eye grows and the cornea flattens, a process called emmetropization. For many people, the astigmatic measurement stabilizes during young adulthood before beginning a slow, predictable shift later in life.
The most notable change occurs after age 40, driven by structural alterations in the cornea and the internal lens. This period commonly features a shift from “with-the-rule” (WTR) astigmatism to “against-the-rule” (ATR) astigmatism. WTR astigmatism, where the steepest curve is vertical, is most common in young adults. With age, the cornea’s vertical meridian gradually flattens while the horizontal meridian steepens, leading to ATR astigmatism. This change may be related to decreased tension from the eyelids.
This age-related change in corneal curvature typically progresses at a rate of approximately 0.25 diopters (D) of ATR change per decade after age 50. Simultaneously, the eye’s internal lens stiffens and loses flexibility, a condition known as presbyopia. This stiffening alters the lenticular component of astigmatism, contributing to the overall change in prescription. These gradual shifts are a normal part of the aging process.
Medical Conditions and Injuries That Cause Change
While slow, age-related changes are common, certain medical conditions and physical injuries can cause a more rapid or significant shift in astigmatism. One concerning cause is keratoconus, a progressive eye disease where the cornea thins and bulges outward into a conical shape. This condition typically develops in the late teens or twenties, resulting in irregular, non-uniform astigmatism that changes quickly and causes severe vision distortion.
Changes can also be induced by external factors, such as trauma or eye surgery. Physical injuries that lead to scarring or deformation of the corneal tissue can permanently alter its shape, resulting in new or increased astigmatism. Similarly, surgical procedures, including cataract surgery, involve making incisions in the cornea that can alter its curvature, leading to temporary or permanent induced astigmatism.
Fluctuations in blood sugar levels, particularly in people with diabetes, can cause a temporary change in astigmatism measurements. High blood glucose levels cause the internal lens to swell and change shape, temporarily altering the lenticular astigmatism. These changes are often reversible; once blood sugar is stabilized, the lens typically returns to its previous shape, and the astigmatism measurement stabilizes.
How Changing Astigmatism is Measured and Managed
When astigmatism changes, eye care professionals rely on specific diagnostic tools to precisely measure the magnitude and orientation of the shift.
Measurement Tools
Traditional keratometry measures the curvature of the cornea at a few central points to determine the steepest and flattest meridians. For a more detailed assessment, especially in cases of irregular or rapidly changing astigmatism, corneal topography is used to create a comprehensive, color-coded map of the entire corneal surface.
Management Strategies
These measurements determine the appropriate management strategy, starting with updating corrective lenses. Prescription glasses or specialized toric contact lenses are designed with different powers in different meridians to counteract the irregular focus caused by astigmatism. For permanent correction, refractive surgeries like LASIK or PRK use a laser to reshape the cornea, smoothing out uneven curvatures.
For conditions like keratoconus, treatment may involve specialized procedures such as corneal cross-linking, which uses riboflavin drops and ultraviolet light to stiffen the cornea and prevent further progressive bulging. During cataract surgery, the surgeon can implant a toric intraocular lens (IOL), a specially designed lens that corrects for pre-existing astigmatism. The management of changing astigmatism is highly individualized, depending on the cause, magnitude, and specific components of the refractive error.