Astigmatism is a frequent cause of visual phenomena like halos, glare, and starbursts that often appear around light sources, particularly in dark environments. This condition is classified as a refractive error, meaning it affects how the eye correctly bends, or refracts, incoming light to form a clear image. When people report seeing bright rings around streetlights or headlights at night, astigmatism is one of the most likely culprits behind this visual distortion. The experience is most noticeable when the eye attempts to focus on bright points of light against a high-contrast, dim background.
The Mechanism of Astigmatism
Astigmatism occurs because the cornea, the clear front surface of the eye, or sometimes the lens inside the eye, possesses an irregular curvature. Instead of having a smooth, spherical shape like a baseball, the surface is shaped more like the side of an American football, with one curve being steeper than the curve perpendicular to it. This imperfect shape prevents light from bending uniformly as it enters the eye.
In an eye without this error, all light rays converge precisely onto a single focal point on the retina, which creates a sharp image. However, the uneven curvature of an astigmatic eye causes incoming light rays to scatter, leading to multiple focal points either in front of or behind the retina, or both. This scattering of light is the physical root of the visual distortions. The result is that the eye receives an image that is stretched, blurred, or distorted because the light cannot be focused accurately onto the retina.
Visual Disturbances Associated with Astigmatism
The irregular light refraction caused by astigmatism results in specific visual symptoms. Halos manifest as luminous, distinct rings of light surrounding a central light source, like a circular aura. Glare is characterized by an uncomfortable, excessive brightness or a washed-out effect that spreads across the field of vision, making it difficult to see surrounding objects. Starbursts appear as pointed rays or spikes of light radiating outward from a central source.
These three effects are typically magnified in low-light conditions, such as during night driving, because the pupil dilates in the dark to let in more light. When the pupil opens wider, it allows more light to pass through the irregular edges of the cornea and lens, increasing the amount of scattered, unfocused light that reaches the retina. This light scatter also contributes to general blurring and distorted vision, which can lead to eye strain and headaches.
Correction and Management Options
Astigmatism is a manageable refractive error, and its associated visual disturbances can often be eliminated or significantly reduced with appropriate correction. The most common treatment involves using corrective lenses, such as eyeglasses. These lenses are ground to compensate for the irregular curvature, ensuring light is bent correctly to create a single focal point on the retina.
For contact lens wearers, specialized soft contact lenses called toric lenses are designed to correct astigmatism. Toric lenses have different powers in different meridians of the lens to match the specific irregular shape of the eye.
Refractive surgery options, such as LASIK (Laser-Assisted In Situ Keratomileusis) or PRK (Photorefractive Keratectomy), offer a more permanent solution by using an excimer laser to physically reshape the cornea. By restoring the cornea to a more spherical shape, these methods allow light to focus properly, managing the halos, glare, and starbursts that compromise clear vision.