Astigmatism is a common condition where the eye does not focus light evenly, leading to blurred or distorted vision. This refractive error is caused by an irregular curvature of the cornea or the lens inside the eye, which prevents light from bending properly onto the retina. While astigmatism often affects both eyes, it is entirely possible to have astigmatism in only one eye or to have a significantly different degree of astigmatism between the two eyes.
The Mechanics of Astigmatism
Astigmatism occurs because the front surface of the eye (the cornea) or the internal lens is not perfectly spherical. A normal eye surface is shaped like a basketball, allowing light to focus at a single point on the retina. An eye with astigmatism is shaped more like an American football, having a steeper curve in one direction and a flatter curve perpendicular to it.
When light enters this irregularly shaped structure, it is refracted unevenly, creating multiple focal points instead of one sharp point on the retina. This focusing error results in vision that appears stretched, fuzzy, or shadowed at any distance. Astigmatism is classified as corneal (caused by the cornea’s shape) or lenticular (caused by an irregularity in the internal lens).
Why Astigmatism Affects Only One Eye
The development of astigmatism is frequently attributed to genetic factors, meaning it is often inherited and present from birth. Even when inherited, eye growth is not perfectly symmetrical, and one eye may develop a different degree of curvature than the other. This imbalance, where the refractive error differs significantly between the two eyes, is medically termed anisometropia.
Astigmatism can also be acquired later in life, often affecting a single eye asymmetrically. Conditions like keratoconus, where the cornea progressively thins and bulges into a cone shape, frequently begin or progress more severely in one eye. Physical trauma, such as an eye injury or an infection leading to corneal scarring, can permanently alter the curve of one cornea, causing an irregular astigmatism. Even temporary pressure from a large stye or blocked oil gland can distort the corneal shape of one eye.
Identifying Unilateral Symptoms
When astigmatism is present in only one eye or is much worse in one eye than the other, the symptoms can be subtle and easily overlooked. The most common signs include blurred or distorted vision, eye strain, and headaches, particularly after visual tasks. A person may also notice difficulty with night vision, where lights appear to have halos or excessive glare.
The brain naturally attempts to compensate for the difference in image quality between the eyes. It tends to favor the better-seeing eye and suppress the image from the eye with the greater refractive error. This compensation can mask symptoms, leading to chronic discomfort or, in children, a risk of developing amblyopia (lazy eye).
Testing Procedures and Correction Options
Diagnosing and measuring astigmatism requires a comprehensive eye examination by a specialist. Several tools are used to precisely map the eye’s irregular curvature. A keratometer measures the curvature of the cornea, while a corneal topographer provides a detailed map of the corneal surface, revealing imperfections. The refraction test, often performed using a phoropter, helps determine the exact lens prescription needed for the clearest vision.
Correction Methods
Correction for astigmatism focuses on neutralizing the uneven light refraction. Eyeglasses with a cylindrical prescription are a common solution designed to compensate for the mismatched curves. Toric contact lenses are a specialized option, featuring different powers in different meridians and a mechanism to prevent rotation on the eye. For a more permanent solution, refractive surgeries such as LASIK or photorefractive keratectomy (PRK) use a laser to precisely reshape the cornea.