An Introduction to Laser Vision Correction

Nearsightedness, farsightedness and astigmatism are visual problems frequently corrected by glasses or contact lenses. However, there are a variety of surgical procedures available that may reduce or eliminate dependence on these devices.

Understanding 20/20

Your visual acuity is medically described as the number 20 over another number. 20/20 is used as the standard for normal vision. Normal vision is not perfect vision, for it allows for some imperfections.

Your visual acuity is measured with an eye chart. Each line of the chart has a number next to it to indicate the distance in feet at which a person with normal vision can read that line. Larger letters have higher numbers next to them, because they can be read from farther away. For example, in the chart above, the "E" is on the 400 line, while the "F" and "P" are on the 200 line. A person with normal vision can read the "E" at 400 feet, while the "F" and "P" are only  visible from 200 feet.

The first number in the description of your visual acuity indicates the distance from which you read the chart. The standard is 20 feet, so the first number is usually 20.

The second number represents the smallest line on the chart that can be read. For example, if you can only read down to the 60 line, then your vision would be 20/60. In other words, you can see at 20 feet what a person with normal vision can see from 60 feet.

Many people don't realize that 20/15 vision is even better than 20/20. With 20/15 vision, you can see at 20 feet what person with normal vision would have to move to within 15 feet of the chart to see.

Refractive surgery can significantly improve a person's uncorrected vision. It is even possible to provide the ability to see with 20/20 vision. People with low to moderate degrees of refractive vision problems have predictably good results. Severe refractive vision problems may still require the use of glasses or contacts, even after the most successful refractive surgery.

General Restrictions and Limitations

All patients considering laser vision correction must be aware of certain factors. Patients should be at least 18 years old, and in some cases 21, and show stability in their refractive prescription. There is controversy regarding the treatment of children due to both the legal and medical situations involved. The FDA currently limits the lower age of laser treatments to between 18 and 21 years of age depending on the treatment and laser type. It's best that patients do not have any significant diseases of the eye, like keratoconus or herpetic keratitis, nor any systemic conditions such as lupus, scleraderma, dermatomyositis, and depending on the severity, rheumatoid arthritis. Women are not considered as candidates during or immediately following pregnancy.

It is also important that patient expectations are realistic. While we have moved significantly closer to the "ideal" refractive surgical procedure, we have not yet fully achieved this reality. While the vast majority of patients will be very pleased with their surgical results, the body's natural responses are uniquely individual and will sometimes deviate from the expected response.

How Lasers Correct Vision

Myopia (Nearsightedness)

With myopia, the cornea is too steeply curved, and too much focusing power is produced. As a consequence, light is focused in front of the retina. 

When the excimer laser is used to correct myopia, the laser gently flattens the contour of the cornea, which lessens its focusing power and allows light to focus farther back, directly on the retina.

Hyperopia (Farsightedness)

With hyperopia, the corneal contour is too flat, and not enough focusing power is produced. As a consequence, light is focused behind the retina.

When the excimer laser is used to correct hyperopia, the laser gently steepens the contour of the the cornea, which increases its focusing power and causes the light to come into focus directly on the retina.

Astigmatism

Astigmatism causes light entering the eye at different axes to be focused different amounts. For example, light entering vertically (from 12 o'clock to 6 o'clock) may be focused more than light entering horizontally (from 9 o'clock to 3 o'clock). In an eye without astigmatism, light is focused the same amount in each axis.

The net result of astigmatism is blurred vision. Often letters appear slanted or with "tails" coming off of them. sometimes the affected eye sees double.

When the laser is used to correct astigmatism, the cornea is reshaped by the laser to allow proper focusing, regardless of the axis light enters the eye. In practical terms, astigmatism is treated similar to myopia and hyperopia, but with different amounts of treatment to each axis.