Astigmatism is when the front window of the eye, the cornea, is shaped slightly more like a football than perfectly round like a basketball. As a result, the eye has multiple focal points, with the steeper axis of the cornea having a stronger refractive power and the flatter axis of the cornea having a weaker refractive power. The result is a blurred image, with some of the image focused in front of the retina and some focused behind the retina. Glasses or contact lenses can neutralize astigmatism by having “cylindrical” power, or more refractive power in one axis of the lens than the other axis. As a result a clear image is focused onto the retina.
Laser vision correction focuses light onto the retina by reshaping the astigmatic cornea to make it rounder. The following is a “topography” or map of a cornea with significant astigmatism (about 5 diopters). The “hotter” colors represent steeper cornea while the “cooler” colors represent flatter cornea. Think of this shape as a football laying on its side with the laces parallel to the ground.
Laser vision correction steepens the flat part of the cornea and flattens the steep part, so that the cornea is round, or “spherical”. In this way, light is focused onto the retina clearly without glasses or contact lenses. The following cornea map is the same cornea after treatment with laser vision correction. Note there is not a dramatic difference between a steep “hot” axis and flat “cool” axis. The cornea is round like a basketball.
The vision without glasses in this eye improved from 20/800 to 20/20 after LASIK.
Laser vision correction for astigmatism has improved greatly over the last 10 to 15 years. This improvement is from a few factors. The size or diameter of the treatment covers a larger area and neutralizes more of the astigmatism toward the peripheral or side cornea. Eye trackers have improved to allow for continuous treatment on the very correct location on the cornea, if as the eye makes small, normal, involuntary eye movements. The natural shape of the cornea is maintained so that glare and halo symptoms are not induced.
Modern laser vision correction can treat mild, moderate, and even some marked amounts of cornea astigmatism, allowing for great visual acuity and quality of vision, without glasses or contact lenses.
For more information about laser vision correction at Nashua Eye, call Mary at (603) 689-9323.
