Astigmatism is a widespread visual condition classified as a refractive error, meaning the eye does not bend light correctly. This common issue arises when the shape of the cornea or the lens inside the eye is slightly irregular. A noticeable symptom is the distortion of light sources, particularly at night, where they appear blurry, streaked, or extended. This difficulty is a direct consequence of how the eye’s unique shape processes incoming light.
Understanding the Uneven Focus
The root cause of astigmatism lies in the geometry of the eye’s primary focusing structures. In an eye without this condition, the cornea and lens are uniformly curved, like a basketball. This smooth, even curve allows light rays to be refracted equally in all directions, converging precisely onto a single point on the retina for a clear image.
When astigmatism is present, the curvature is uneven, resembling the shape of an American football. This irregular shape causes light to be refracted differently along separate axes. Instead of focusing light onto one distinct spot on the retina, the rays are scattered and form multiple, staggered focal points inside the eye. This prevents the formation of a single, sharp image, resulting in blurred or distorted vision at any distance.
Describing the Common Light Distortions
The light distortions experienced by people with astigmatism result directly from this uneven light focusing. Instead of appearing as crisp points, bright light sources are broken down into distinct patterns. One common effect is the appearance of streaks, where light seems stretched outward in a line, often resembling a smear. These streaks typically align with the axis of the eye’s irregular curvature.
Another frequent distortion is the starburst effect, characterized by multiple, fine spikes radiating outward from a central light source, such as a headlight. Halos are the third major distortion, appearing as faint or blurry rings of light surrounding the source.
These visual disturbances are significantly intensified in low-light conditions, such as during night driving. In the dark, the eye’s pupil naturally dilates to maximize light entry. This larger opening allows more peripheral light rays to pass through the outer edges of the irregular cornea and lens. Since these outer rays are the most severely unfocused, their increased entry exacerbates the scattering effect, making the streaks, starbursts, and halos much more pronounced.
How Correction Restores Point Focus
Correcting astigmatism involves counteracting the uneven curvature of the eye to ensure light focuses properly on the retina. The most common method utilizes specialized toric lenses, found in both eyeglasses and contact lenses. Unlike standard spherical lenses, toric lenses possess different refractive powers along their vertical and horizontal orientations.
This unique design compensates for the eye’s asymmetrical curvature. The different powers within the lens bend incoming light unevenly, precisely canceling out the eye’s natural uneven refraction. Toric lenses must be precisely aligned with the specific axis of the astigmatism, often including stabilization features to prevent rotation. Surgical options, such as LASIK, achieve a similar result by using a laser to permanently reshape the cornea, correcting the irregular curvature.