Refractive errors are common vision problems that occur when the eye cannot correctly focus light onto the retina, the light-sensitive tissue at the back of the eye. This focusing failure causes blurred vision and is typically related to the shape or length of the eyeball. Myopia, often called nearsightedness, and astigmatism are two of the most frequently diagnosed types of these errors. While both conditions affect the clarity of sight, they arise from fundamentally different structural abnormalities within the eye.
Understanding Myopia
Myopia, or nearsightedness, is a refractive error where the eye focuses images in front of the retina instead of directly on its surface. This misfocus occurs due to a mismatch between the eye’s length and its focusing power. The most common cause is the eyeball growing too long from front to back, a condition called axial myopia.
A less frequent cause, known as refractive myopia, involves the cornea or the lens being excessively curved. In a myopic eye, parallel light rays from distant objects converge prematurely before reaching the retina. This means the eye has too much focusing power for its length, resulting in clear near vision but unclear distance vision.
Understanding Astigmatism
Astigmatism results from an irregularity in the curvature of the cornea or, less commonly, the lens within the eye. In an eye without astigmatism, the cornea is shaped like a perfect sphere, similar to a basketball, which allows light to enter and focus at a single point. Conversely, an eye with astigmatism is shaped more like an American football, featuring different curvatures in different planes.
This uneven surface causes light entering the eye to be refracted unequally along different axes, preventing a single, sharp focal point from forming. Instead, the light focuses at two or more separate points, which may be in front of, behind, or on the retina. This structural issue creates the characteristic visual distortion associated with the condition.
Recognizing Distinct Symptoms
The structural differences between the two conditions lead to noticeably different visual experiences. Myopia primarily manifests as difficulty seeing objects that are far away, such as reading street signs or seeing a whiteboard in a classroom. Objects at a normal reading or working distance, however, remain sharp and clear. A common reaction to compensate for this distant blur is frequent squinting, which temporarily changes the light path and improves focus.
Astigmatism, by contrast, causes vision to be blurry or distorted at all distances, both near and far. The multiple focal points created by the irregular curvature cause images to appear stretched, shadowed, or ghosted. People with astigmatism often report seeing streaking or halo effects around lights, especially when driving at night. The constant effort the eye makes to compensate for the uneven focus frequently leads to symptoms like eye strain and headaches. It is very common for an individual to have both myopia and astigmatism simultaneously.
Correction and Management Options
Correcting these refractive errors requires addressing the specific way each condition bends light within the eye. For simple myopia, eyeglasses or contact lenses use spherical lenses, which have a uniform curvature across the entire surface. These lenses have a concave, or minus, power that diverges the light rays, pushing the focus point back onto the retina. This simple, uniform design is effective for correcting the eye’s overall excessive focusing power.
To correct astigmatism, a more specialized lens, known as a toric lens, is required. Unlike spherical lenses, toric lenses feature different curvatures and powers along two perpendicular axes, or meridians, to counteract the irregular shape of the cornea. Toric lenses must be precisely aligned on the eye to ensure the different powers correctly compensate for the specific orientation of the astigmatism. These lenses can also incorporate spherical power to correct coexisting myopia.
Refractive surgery, such as LASIK (Laser-Assisted In Situ Keratomileusis) or PRK (Photorefractive Keratectomy), offers a permanent management option by physically reshaping the cornea. The excimer laser ablates corneal tissue to correct the curvature responsible for both the overall steepness found in myopia and the uneven shape of astigmatism. Another non-surgical method is Orthokeratology, or Ortho-K, which involves wearing rigid gas-permeable contact lenses overnight. These lenses temporarily reshape the cornea while the person sleeps, providing clear vision during the day without the need for glasses or contacts.