How To Treat Astigmatism

Many nearsighted or farsighted patients also have what is called “astigmatism”, which is a refractive error whereby light is not focused to a single point. Vision is indistinct at every distance because the cornea - which should be dome-shaped - is "out-of-round" or shaped rather like the back of a spoon. Such a cornea is more steeply curved in one direction than the other, having two "axes" that are usually perpendicular to each other. Depending on the person, 0.5 diopters or more of astigmatism starts to cause noticeably blurred vision.

Laser surgery can smooth out the astigmatic cornea's curvature, changing the shape of the central cornea (mainly the central 6 or 7 millimeters) to look more like a symmetrical soup bowl than the back of a lopsided spoon. To correct astigmatism surgically, which is more difficult than treating nearsightedness, physicians selectively remove tissue to make the curvature of the steepest and flattest "meridians" (the corneal curves of greatest and smallest refractive power) more alike. For example, if you have simple myopic astigmatism, the curvature of the meridians can be evened out by lasering tissue from the steepest meridian to make it flatter - thereby moving the light-bending effect of this forward axis back toward the retina. Less tissue would be removed from the other, flatter meridian.

Some refractive surgeons target astigmatism with a specially "masked" broad-beam laser that is driven by customized computer software. Since the surface topography of the cornea is digitized, or put in numerical form, the computer knows which areas are elevated. During a more modern surgical technique using a "flying-spot" laser, a fast, small-beam laser dances around the cornea, chipping away at the higher spots and avoiding the flatter areas.

There are other causes of astigmatism besides irregularities on the front surface of your cornea. The back of your cornea also can be malformed. Furthermore, since your eye is a two-lens system, problems with the front and back curvature of your crystalline lens inside your eye can cause astigmatism. In fact, any misalignment of the internal components of your eye can blur images placed on your retina. A few people have astigmatism because of a disparity between their line-of-sight (the visual axis which goes from what they are looking at to the light-sensing photoreceptors on the retinal fovea) and their optic axis (the way the light strikes the retina). Densely packed with receptors, the fovea - a tiny depression in the retina - is the area of clearest vision. In these patients, the rays of light fail to strike the fovea correctly. Laser surgery is unable to treat these problems.

Currently, refractive surgeons can only correct the part of your astigmatic error caused by irregularities in the curvature of the front surface of your cornea. Consequently, the more disparity between treatable surface corneal astigmatism and total astigmatism, the less predictable the surgical result. To even out the cornea's front surface, the laser can be programmed to make the cornea more bowl-shaped. Unfortunately, some patients have noticeable astigmatism caused by problems inside their eyes. These patients will still have residual astigmatism after surgery, even if the front of the cornea is treated. In fact, many eyes that manifest no refractive astigmatism (as measured during an eye exam) actually have astigmatic defects inside the eye, but they are counterbalanced by astigmatism on the surface of the cornea. This is one instance in which two wrongs do make a right, because the error in one lens offsets that in the other to provide clear focus.