Astigmatism significantly affects night vision, often making tasks like night driving difficult and uncomfortable. This common refractive error is an imperfection in the way the eye focuses light, and its effects become noticeably worse when ambient light levels drop. The primary reason for this visual challenge is the irregular shape of the eye’s front surface, which prevents light from converging clearly onto the retina.
Understanding Astigmatism
Astigmatism is a widespread vision condition where the cornea or the lens has an uneven curvature. Instead of being perfectly spherical, the surface is shaped more like an American football, with one curve being steeper than the other. This irregularity prevents incoming light rays from refracting properly as they enter the eye.
In an eye without astigmatism, light focuses sharply onto a single point on the retina, creating a clear image. With astigmatism, light is scattered and focused on multiple points instead of a single plane. This results in blurry or distorted vision at both far and near distances, and often occurs alongside other refractive errors.
How Astigmatism Distorts Vision at Night
The irregular focusing of light caused by astigmatism leads to pronounced visual distortions around bright light sources in dark environments. Instead of seeing a sharp point of light, people with uncorrected astigmatism frequently experience glare, streaking, halos, and starbursts. The irregular corneal shape causes light from headlights or streetlights to be stretched and scattered across the retina.
These distortions significantly impact safety, particularly while driving. A single, bright headlight may appear to have multiple streaks radiating from it, or be surrounded by a bright, distracting halo. This reduced visual clarity and contrast makes it difficult to distinguish objects, judge distances, and read road signs after dark. The effort required to overcome these effects can also lead to eye strain and headaches.
The Role of Pupil Dilation in Low Light
Astigmatism is far more noticeable at night due to a natural biological response called mydriasis, or pupil dilation. In bright daylight, the pupil constricts to a small aperture, acting like a pinhole camera. This small pupil forces incoming light to pass only through the relatively flatter, central portion of the cornea and lens.
When the pupil dilates in the dark to maximize light intake, it allows light to enter through the entire corneal surface, including the more highly curved periphery. This peripheral area of the eye often has greater irregularities and optical aberrations than the center. Consequently, the dilated pupil admits more scattered light through the uneven surface, magnifying the refractive error. The stark contrast between a bright light source and the surrounding darkness further makes the scattered light more prominent and distracting.
Options for Correction and Management
Several options exist to correct astigmatism and significantly improve night vision. The most common approach involves prescription eyewear, specifically eyeglasses with lenses designed to counteract the eye’s irregular curvature. Adding an anti-reflective coating to eyeglass lenses minimizes glare and reflections from the lens surface, enhancing nighttime clarity and contrast.
For contact lens wearers, specialized Toric contact lenses are shaped to address the asymmetrical curvature of the astigmatic eye. When properly fitted, these lenses provide a consistently clearer image by stabilizing to maintain the correct orientation. Some modern Toric lenses also incorporate features like blue-violet light filtration to enhance night driving comfort.
Surgical correction is a permanent option, with procedures like LASIK and PRK working to reshape the cornea to a more spherical form. These refractive surgeries correct the underlying cause of the astigmatism, leading to reduced reliance on glasses or contacts. Determining the best treatment requires a comprehensive eye examination and consultation with an eye care professional.