Astigmatism and nearsightedness (myopia) are two distinct types of refractive errors, though they frequently exist together. Both conditions represent a problem with how the eye focuses light, preventing a clear image from forming precisely on the retina. Myopia and astigmatism originate from different structural abnormalities within the eye’s components. These differences dictate the symptoms a person experiences and the specific corrective measures required.
The Mechanics of Nearsightedness (Myopia)
Nearsightedness occurs when the physical structure of the eye causes incoming light to focus incorrectly, resulting in distant objects appearing blurry while nearby objects remain clear. This focusing error happens because the eyeball is slightly too long from front to back, or because the cornea has a curve that is too steep. With myopia, light converges at a point in front of the retina, rather than directly onto it.
This premature focusing causes the light to diverge before reaching the retina, resulting in the perception of blurriness in the distance. Myopia often develops and progresses during childhood as the eye grows. The visual consequence is a lack of clarity when looking far away.
The Mechanics of Astigmatism
Astigmatism involves an irregularity in the shape of the cornea or the lens inside the eye. Instead of a uniformly spherical curvature, like a basketball, the cornea is shaped more like a football. This uneven curvature means the eye bends light differently along various meridians, or axes, as it enters the eye.
Because light is not refracted equally in all directions, it cannot converge to a single, sharp focal point on the retina. The irregular shape causes light to focus at two or more distinct points, which leads to vision that is distorted, stretched, or shadowed at all distances. This distortion is different from the simple blurriness caused by myopia.
Correction and Co-occurrence
Astigmatism and myopia are often diagnosed simultaneously, as it is common for a person to have both an elongated eyeball and an irregularly curved cornea. When they co-occur, the person experiences the blurred distance vision of myopia along with the distortion at all ranges caused by astigmatism. This combination requires a specialized approach to vision correction.
The primary difference in treatment lies in the specific type of corrective lens used. Myopia alone is corrected using a standard spherical lens, which pushes the single focal point backward onto the retina. Astigmatism requires a toric or cylindrical lens, which possesses different powers in different meridians to compensate for the eye’s irregular shape.
Toric lenses must be precisely oriented on the eye to align with the specific axes of the corneal irregularity. Refractive surgeries, such as LASIK, can also address both conditions by precisely reshaping the cornea.