Astigmatism is a common imperfection in the eye’s shape, specifically involving the curvature of the cornea or the lens, which causes light to focus unevenly on the retina. This refractive error results in blurred or distorted vision, often making objects appear stretched or wavy at any distance. An eye with astigmatism is shaped more like a football than a basketball, creating multiple focal points. This condition is highly prevalent, and while many people have a minor degree of it, the severity can range dramatically, influencing the vision and the type of correction required.
Understanding Diopters and Measurement
The standard unit used to measure the degree of refractive errors, including astigmatism, is the Diopter (D). On an eye prescription, this measurement is typically indicated by the “cylinder” or “CYL” value. The Diopter measurement quantifies the difference in curvature between the steepest and flattest meridians of the eye, reflecting the amount of visual distortion caused by the irregular shape. A higher Diopter number signifies a greater difference in curvature and, consequently, a higher degree of astigmatism that needs correction. The prescription will also include an “Axis” measurement, which is a degree value from 1 to 180 that specifies the orientation of the astigmatism on the cornea, allowing for precise lens placement.
Classifying Astigmatism Severity
The severity of astigmatism is generally categorized by the Diopter measurement, moving from mild to extreme levels. Mild astigmatism is typically defined as a measurement of less than 1.0 D, a common level that often produces minimal or no noticeable symptoms. Moderate astigmatism falls within the range of 1.0 D to 2.0 D, usually requiring corrective lenses to ensure clear vision for daily activities. Astigmatism is classified as severe when the measurement exceeds 2.0 D to 3.0 D, with anything above that range often termed high or extreme astigmatism. Clinically, measurements above 4.0 D are consistently considered high, and cases exceeding 10.0 D are extremely rare. These high measurements represent the practical “highest level” encountered in clinical practice, often necessitating specialized forms of correction.
Implications of Extreme Astigmatism
Extreme levels of astigmatism result in visual disturbances that go far beyond simple blurriness. The severely irregular curvature causes pronounced distortion, making both near and distant objects appear significantly warped or shadowed. Individuals may experience a phenomenon known as ghosting or double vision, especially when looking at lights or text. The constant effort the eyes and brain make to resolve the distorted image leads to secondary symptoms. Severe eye strain, persistent headaches, and eye discomfort are common complaints due to the muscles working overtime to focus. High astigmatism often severely impairs night vision, causing increased glare and halos around light sources due to the scattering of light within the irregularly shaped eye.
Management Strategies for High Levels
Correcting high or extreme astigmatism often requires specialized solutions beyond conventional eyeglasses or standard soft contact lenses. Eyeglasses can be made with customized toric lenses, which have the necessary corrective cylinder power and axis to address the irregularity. For very high prescriptions, specialized contact lenses are frequently preferred because they can provide sharper, more stable vision. Rigid Gas Permeable (RGP) lenses and Scleral lenses are particularly effective for high astigmatism, especially when the corneal shape is irregular due to conditions like keratoconus. RGP lenses maintain their shape and are designed to mask the corneal irregularity by creating a smooth, new optical surface. Scleral lenses are larger and vault over the entire cornea, neutralizing the irregularity, offering superior comfort and clarity for the most complex cases. Surgical options, such as refractive procedures like LASIK or PRK, can also be tailored to reshape the cornea to reduce the astigmatism.