Astigmatism is a common vision condition that affects how the eye focuses light, leading to blurred or distorted vision. This refractive error results from an imperfection in the eye’s curvature, preventing light from focusing sharply onto the retina.
What Causes Astigmatism
Astigmatism primarily arises from an irregular curvature of the cornea, the clear front surface of the eye, or less commonly, the lens inside the eye. Normally, the cornea and lens have a smooth, spherical shape, similar to a basketball, which allows light to bend evenly and focus precisely on a single point on the retina. With astigmatism, this curvature is more akin to a football or an egg, being steeper in one direction than another.
This uneven shape prevents light rays entering the eye from converging at a single focal point on the retina. Instead, light focuses at multiple points, or in front of and behind the retina, leading to blurred or distorted vision. While often hereditary and present from birth, astigmatism can also develop after eye injuries, certain eye diseases, or eye surgery.
How Lights Appear Distorted
Individuals with astigmatism often perceive lights in distinct, distorted ways, particularly in low-light conditions. One common phenomenon is streaking, where lights appear to have lines extending from them, often horizontally or vertically. This can make point sources of light, like streetlights or car headlights, appear elongated or smeared.
Another frequent visual distortion is starbursts, where lights seem to radiate outward with rays or spikes, resembling a star. This effect is especially noticeable around bright sources at night, making night driving more challenging. Halos, appearing as rings of light around bright sources, are also commonly experienced.
Increased glare sensitivity is another significant symptom, making lights appear excessively bright or causing discomfort. This heightened sensitivity can make night driving particularly difficult, as glare from oncoming headlights becomes more pronounced. These visual effects are amplified in dim lighting because the pupil dilates, allowing more unfocused light to enter the eye.
The Science Behind Light Distortion
The irregular curvature of the cornea or lens in an eye with astigmatism causes light rays to refract improperly as they enter the eye. Instead of focusing light precisely onto one point on the retina, the uneven surface causes light to scatter across multiple focal points.
Streaking occurs because the light is stretched or elongated in a particular direction corresponding to the steepest curve of the eye’s surface. Starbursts result from light scattering in multiple directions, creating the radiating lines, while halos form due to light being unfocused and spread out around the source.
Correcting Astigmatism
Several methods are available to correct astigmatism and improve vision, particularly by reducing light distortions. Eyeglasses are a common and non-invasive solution, utilizing specialized cylindrical lenses to compensate for the eye’s irregular shape. These lenses help to properly refract light, guiding it to focus correctly on the retina.
Contact lenses also offer effective correction, with toric contact lenses specifically designed for astigmatism. These lenses have different refractive powers along their vertical and horizontal axes and are weighted or shaped to remain properly oriented on the eye. Toric lenses work by creating a uniform focal point on the retina, counteracting the irregular curvature.
For those seeking a more permanent solution, refractive surgery procedures like LASIK (Laser-Assisted In Situ Keratomileusis) and PRK (Photorefractive Keratectomy) can reshape the cornea. LASIK involves creating a thin flap in the cornea and using a laser to reshape the underlying tissue, while PRK removes the outer layer of the cornea before laser reshaping. These surgical methods aim to make the cornea more spherical, allowing light to focus accurately and reducing or eliminating the distorted appearance of lights.