Astigmatism is a common refractive error that occurs when the eye’s focusing surfaces, either the cornea or the lens, are not perfectly spherical, leading to blurred or distorted vision. While astigmatism affects visual clarity throughout the day, its effects become significantly more pronounced in low-light environments. The unique visual disturbances experienced at night result from how the eye attempts to compensate for the darkness, which ultimately magnifies the underlying refractive error.
How Astigmatism Distorts Light
In a person without astigmatism, the cornea is shaped like a basketball, focusing incoming light onto a single sharp point on the retina. With astigmatism, the curvature is irregular, often resembling the shape of an American football. This asymmetrical shape is the fundamental cause of the vision problem. The irregular surface causes light rays to be refracted unevenly along different axes as they pass through the eye, preventing convergence at a single focal point. Instead, the light focuses into lines, which the brain perceives as an image that is stretched, blurred, or shadowed, resulting in distorted vision.
Why Nighttime Conditions Magnify Vision Issues
The increase in visual disturbance at night is directly linked to pupil dilation. In dim conditions, the pupil widens to let in as much light as possible. During the day, the small pupil size acts like a pinhole, forcing light to pass through the central part of the cornea and lens. When the pupil is dilated, light enters through the peripheral edges, which frequently possess a greater degree of refractive error. As more distorted light reaches the retina, the visual blur and light scattering become far more noticeable than they are in bright daylight.
The Specific Visual Symptoms of Night Astigmatism
People with astigmatism describe their nighttime vision by referring to the appearance of point sources of light, such as headlights or streetlights. Instead of appearing crisp, these light sources are often surrounded by distinct visual phenomena:
- Starbursts, where bright rays radiate outward from the center, often corresponding to the axis of the astigmatism.
- Halos, which manifest as bright, circular rings of light surrounding the source due to scattered light from the irregular cornea.
- Ghosting or double vision, where an image is overlapped or shadowed, particularly noticeable around high-contrast edges.
These distortions can make activities like night driving challenging, as glare from oncoming traffic reduces contrast sensitivity and makes identifying hazards difficult.
Options for Correcting Nighttime Vision Problems
Several options exist to manage the effects of astigmatism and improve nighttime clarity, most commonly involving prescription eyewear designed to counteract the irregular shape of the eye.
- Glasses incorporate a cylindrical power into the lens to compensate for the varying curvature. Applying an anti-reflective coating can also significantly reduce glare and reflections.
- Soft toric contact lenses are weighted and shaped to remain stable on the eye and provide necessary corrective power.
- Refractive surgery, such as LASIK or PRK, reshapes the cornea for permanent correction, though some patients may still experience mild residual glare.
- Orthokeratology (Ortho-K) involves wearing specialized rigid contact lenses overnight to temporarily reshape the cornea, providing clear vision during the day and night.