Astigmatism is a common refractive error causing blurry or distorted vision at all distances. This condition develops when the eye’s light-focusing structures, the cornea or the internal lens, possess an irregular, non-spherical curvature, often curved more like one side of a football than a perfect dome. While astigmatism impacts daytime clarity, its effects are especially pronounced when viewing bright point sources of light, such as car headlights, against the darkness of night. Uncorrected astigmatism makes it challenging for the eye to focus light precisely, resulting in predictable visual distortions after sundown.
The Distinct Visual Phenomena of Light Distortion
For a person with uncorrected astigmatism, a simple car headlight transforms from a clear point of light into a complex, distracting visual artifact. The most common distortion is streaking, where lights appear elongated or smeared, often extending vertically, horizontally, or diagonally across the field of view. The direction of this streak is directly related to the axis of the individual’s astigmatism.
Another frequent experience is the starburst effect, where bright lights seem to radiate outward with fine, spiky filaments. This effect is particularly noticeable with high-contrast light sources, such as streetlights and newer LED headlights. A third common phenomenon is the appearance of halos, which are rings of light surrounding the main source. These distortions intensify at night because the pupil dilates in low light, allowing more scattered light to enter the eye and exacerbate the effects of the irregular curvature.
How Astigmatism Creates Streaking and Glare
The root cause of these visual disturbances lies in the eye’s inability to converge incoming light rays to a single, sharp point on the retina. In a healthy eye, the cornea is spherical, bending light equally across all meridians. Conversely, an astigmatic cornea is toric, meaning it has two different curves, much like the surface of an egg or a spoon. This uneven shape creates two distinct refractive powers for light passing through it.
Light traveling through the flatter curvature focuses at one point, while light passing through the steeper curvature focuses at a different point, resulting in two separate focal lines instead of one unified focus. Because the light is scattered and stretched between these two focal points, the image projected onto the retina is smeared, which is perceived as streaking, starburst, or glare.
Corrective Measures for Improved Night Vision
The most effective strategy for managing the light distortion caused by astigmatism involves correcting the underlying refractive error with specialized optics. Standard contact lenses or spectacle lenses cannot fully resolve the issue because they only offer a spherical correction. Instead, toric lenses are necessary, as they are specifically engineered with two different power curves to precisely compensate for the two different curves of the astigmatic eye.
Toric contact lenses are designed with stabilization features to prevent rotation and ensure the corrective meridians align perfectly with the eye’s astigmatic axis. For those seeking an alternative to daily wear, certain refractive surgery procedures, such as LASIK or photorefractive keratectomy (PRK), can permanently reshape the cornea to a more spherical contour. Another non-surgical option is orthokeratology, which uses rigid contact lenses worn overnight to temporarily reshape the corneal surface.
If night vision is deteriorating, consulting an optometrist or ophthalmologist is the first step to obtain an accurate diagnosis and an updated prescription. Beyond optical correction, small environmental adjustments can offer supplementary relief during night driving. Maintaining a scrupulously clean windshield and minimizing internal dashboard light glare can help reduce secondary light scatter and improve overall contrast.