Astigmatism is a common refractive error where the eye’s focusing surfaces are curved imperfectly, causing light to scatter and vision to blur. The shape of the eye is a dynamic structure that responds to growth, internal pressures, and age-related tissue changes. These changes mean that an astigmatism prescription is not fixed but evolves significantly throughout a person’s life.
The Nature of Astigmatism
Astigmatism occurs when the eye is shaped more like a football than a perfectly round baseball, causing light to focus unevenly on the retina. This irregular shape leads to two distinct focal points instead of a single, precise one, resulting in blurred or distorted vision. The condition is a type of refractive error, often co-occurring with nearsightedness or farsightedness.
The irregularity can stem from two primary structures: the cornea or the internal lens. Corneal astigmatism, the more common form, involves an uneven curvature of the transparent front surface of the eye. Lenticular astigmatism is caused by an irregular shape or tilt of the lens located just behind the iris. Both forms disrupt the smooth bending of light required for clear sight.
Progression During Childhood and Young Adulthood
Astigmatism is highly prevalent in infancy, often due to the steepness of newborns’ corneas. This early astigmatism often corrects itself naturally as the eye develops and the cornea flattens during the first few years of life. By school age, the prevalence of astigmatism decreases considerably as the visual system matures.
During childhood and adolescence, the astigmatic error does not remain perfectly stable, though the overall prevalence in the population may not change. Some children will lose their astigmatism, while others will develop it, highlighting the dynamic nature of eye growth. For most individuals, astigmatism generally stabilizes throughout young and middle adulthood, typically between the ages of 20 and 45. This period is marked by relative stability before the predictable structural changes of later life begin.
The Typical Shift After Middle Age
The most dramatic and predictable change in astigmatism begins to occur in middle age, generally starting around age 50 or 60. This shift is characterized by a rotation in the axis of the astigmatism, moving from a “with-the-rule” (WTR) pattern to an “against-the-rule” (ATR) pattern. WTR astigmatism, common in youth, means the steepest curve of the eye is vertical.
Against-the-rule astigmatism, which dominates in older age, means the steepest curve becomes horizontal. This predictable shift is driven by two main mechanisms related to aging. The first is a gradual flattening of the cornea along the vertical meridian, potentially due to decades of pressure from the upper eyelid. A more significant factor is internal change within the eye’s crystalline lens.
As the lens ages, it becomes less flexible and denser, a process known as presbyopia, which eventually leads to cataract formation. These lenticular changes, particularly the development of a cataract, alter the internal astigmatism of the eye. In young eyes, the slight internal astigmatism of the lens often partially cancels out the corneal astigmatism, but this compensatory balance is lost with age. The total refractive astigmatism often increases in magnitude and shifts toward the ATR orientation, leading to more pronounced blur.
Corrective Measures for Age-Related Changes
Managing age-related astigmatism involves addressing the underlying structural changes, especially those involving the lens. Standard methods like specialized eyeglasses and toric contact lenses can correct the refractive error by compensating for the eye’s irregular curvature. These are effective non-surgical options for mild to moderate astigmatism.
Since the aging lens is a major contributor to the late-life shift toward against-the-rule astigmatism, cataract surgery offers a unique opportunity for correction. During a routine cataract procedure, the cloudy natural lens is removed and replaced with an artificial intraocular lens (IOL). For patients with significant astigmatism, a specialized option is a toric IOL, which has corrective power built into the lens itself.
Implanting a toric IOL during cataract surgery can correct the astigmatism and the clouding simultaneously, resulting in improved vision and reduced dependence on glasses. Other surgical methods, such as limbal relaxing incisions (LRIs), involve making small cuts on the corneal periphery to relax the steep curvature. LRIs are often used in conjunction with IOLs or for mild to moderate astigmatism corrections.