Eye Anatomy
Light enters the eye through the cornea, a transparent region of the otherwise white sclera. The cornea provides focusing power for the eye and is the surface that contact lenses are placed upon. Because the cornea is transparent, the colored iris is visible behind it.
The pupil is the dark, round opening in the center of the iris. It is an aperture that regulates the amount of light that enters the eye by constricting in bright light and dilating in dim light. Though the scientific data is conflicting, large pupils at night are generally considered a risk factor for glare and halo problems after laser vision correction.
Directly behind the pupil is the lens, which also provides some of the eye's focusing power.
During reading, muscles in the ciliary body contract, which changes the shape of the lens and brings near objects into focus. Around age 40, the lens becomes rigid and the muscles are no longer able to change its shape, so reading glasses or bifocals become necessary.
The retina is made of photosensitive cells that convert light images into electrical impulses that travel via the optic nerve to the brain. The macula is a specialized area in the center of the retina that is responsible for fine detail.
The focusing power of the cornea, the focusing power of the lens and the length of the eye determine the clarity of the vision. If these elements are matched together perfectly, light rays are properly focused on the retina and the vision is clear.
Refractive Errors - Myopia (Nearsightedness)
Often referred to as nearsightedness, myopia is the most common focusing problem. Myopic individuals can see near objects clearly but have difficulty seeing far away.
Myopia results when light rays focus in front of the retina. The cause is often a cornea with too much curvature (i.e., too much focusing power) or an eye that is too long.
When a laser is used to treat myopia, it gently flattens the contour of the cornea, which lessens its focusing power and allows light to focus farther back, directly on the retina.
Refractive Errors - Hyperopia (Farsightedness)
Hyperopia is more commonly referred to as farsightedness and is the opposite of myopia. Those who are farsighted have difficulty focusing on distant objects and are even blurrier up close.
A cornea that is too flat or an eye that is too short results in farsightedness. These conditions cause the light rays to focus at a point behind the retina. When a laser is used to correct hyperopia, it gently steepens the contour of the cornea, which increases its focusing power and causes the light to come into focus directly on the retina.
Refractive Errors - Astigmatism
Astigmatism often occurs along with nearsightedness or farsightedness.
With astigmatism, light entering the eye from different directions is 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 have tails. Sometimes, the affected eye sees double. In practical terms, astigmatism is treated similarly to nearsightedness and farsightedness, but with different amounts of treatment on each axis.
Presbyopia
Presbyopia is an unfamiliar word, but it is one of the most important concepts to understand when considering laser vision correction.
Presbyopia affects the near vision. In order for a person with clear distance vision to see up close, the ciliary muscles inside the eye must contract and cause the lens to change shape. In younger individuals, the lens is flexible and changes shape easily, so near objects come into focus. However, around 40 years of age, the lens hardens to the point that it no longer changes shape, and reading glasses are needed to provide a boost to the near vision.
Here is a summary of how presbyopia affects various individuals:
Most people over 40 who Need help to see up close with
Wear glasses Bifocals (some can just remove their glasses)
Wear contact lenses Reading glasses or bifocal contacts
Were born with and still have clear distance vision Reading glasses
Have both eyes laser corrected for distance Reading glasses
Monovision
After learning about presbyopia, the first question many over 40 ask is if there is any way to reduce the need for reading glasses after laser vision correction.
The answer is yes. There is an option known as monovision.
Monovision has been used for years in the fitting of contact lenses. The most common technique is to correct the dominant eye for distance vision and the leave the non-dominant eye somewhat blurry at distance, but better up close.
Monovision is a compromise. The distance vision isn’t quite as good as it could be, because one eye is doing most of the work, and the near vision isn’t perfect for the same reason. However, for many, the brain is able to fill-in the gaps and everything seems clear.
For patients over 40 who are interested in monovision, most surgeons will recommend a first-hand experience of what it is like, most commonly through in-office trial lenses or take-home contact lenses, prior to laser vision correction. As much as the benefits may sound appealing, there is no substitute for actually looking through monovision lenses, as some patients don’t tolerate the inherent imbalance as well as might be hoped.
Presbyopia and monovision are extremely important topics to understand. With current laser technology circa 2006, patients over 40 have only two realistic options. The first is to have both eyes fully corrected for distance and wear reading glasses for up close. The other is to have monovision.
Visual Acuity Testing - What 20/20 Means
Visual acuity is described as the number 20 over another number. 20/20 is considered normal, though not perfect vision, as it allows for some imperfections.
Each line of the eye chart has a number next to it that indicates the distance in feet at which a person with normal vision can read it. Larger letters have higher numbers, because they can be read from greater distances. For example, in the chart above, the "Big E" is on the 200 line, while the "H" and "N" are on the 100 line. A person with normal vision can read the "E" at 200 feet, while the "H" and "N" are only visible from 100 feet.
The first number in the description of visual acuity indicates the distance from the patient to the chart. The standard is 20 feet, so the first number is usually 20.
The second number represents the smallest line on the chart the patient can read. For example, if the patient can only read down to the 50 line, then the vision is 20/50. In other words, the patient can see at 20 feet what a person with normal vision can see from 50 feet.
Many people don't realize that 20/15 vision is even better than 20/20. With 20/15, the patient can see at 20 feet what a person with normal vision would have to move to within 15 feet of the chart to see.
Laser vision correction can significantly improve a person's uncorrected vision. With today's technology, most patients are able to see 20/20 or better. Patients with low to moderate degrees of correction have predictably good results. However, some patients with extremely high prescriptions may still require the use of glasses or contacts for the best possible vision, even after successful surgery.
Understanding Prescriptions
In order to understand the relative ease of a particular correction and the likelihood that a patient will qualify for surgery, it is helpful to learn about eyeglass and contact lens prescriptions.
The basic format for a prescription is: +/– sphere +/– cylinder x axis.
Here are some examples:
-3.25 -1.75 x 176
+1.25 -0.50 x 90
+2.00 +1.50 x 87
Plano +0.50 x 66
-6.50 sph.
Sphere refers to the amount of nearsightedness (myopia) or farsightedness (hyperopia). If the sphere number is negative, the patient is nearsighted. If it is positive, he or she is farsighted. Sphere numbers of zero are called "plano" or abbreviated "PL".
Cylinder refers to the amount of astigmatism. If no astigmatism is present, then the prescription will only have a number for sphere. Cylinder can be expressed as either a positive or negative number. Most surgeons use positive numbers, known as "plus cylinder format", while most non-surgeons use negative numbers, know as “minus cylinder format”.
Prescriptions in minus cylinder need to be converted into plus cylinder in order to accurately read the following tables. To convert a prescription, simply add the sphere and cylinder to obtain the converted sphere and change the cylinder to a positive number. If the axis is =>90, then subtract 90 for the new axis, otherwise add 90.
Prescription Conversion Example
original prescription in minus cylinder = -3.25 -1.75 x 176
-3.25 + (-1.75) = -5.00 = new sphere
+1.75 = new cylinder
176 - 90 = 86 = new axis
-5.00 +1.75 x 86 = converted prescription in plus cylinder
Axis refers to the direction on the clock hour of the astigmatism. It is not necessary to understand anything about the axis number when contemplating laser vision correction, nor is it necessary to understand any of the following terms that are occasionally found on a prescription: "Add", "Prism", "Base Curve", "Diameter" and "Pupillary Distance (PD)". However, if a prescription contains the word “Prism”, this finding should be mentioned to the doctor, as it may indicate an eye alignment problem that makes laser vision correction inadvisable.
Here are some prescription examples:
Sample Prescription #1: –3.75 –0.50 x 180
-3.75 -0.50 180
sphere cylinder axis
Because the sphere is negative, this individual is nearsighted. Notice, however, that the cylinder number is in minus format (i.e., a negative number). When converted to plus cylinder format, the resultant prescription is –4.25 +0.50 x 90.
Sample Prescription #2: +2.25 sph, add +1.25
+2.25 none none
sphere cylinder axis
Because the sphere is positive, this individual is farsighted. The "add" number is used when a bifocal lens is needed to help with reading. The "add" number is not relevant for our purposes. Notice there is no number for cylinder, so this individual doesn't have astigmatism.
Sample #3: –2.25–1.50x110, BC=8.8 Dia=14.0
-2.25 -1.50 110
sphere cylinder axis
This is a contact lens prescription. The terms BC (base curve) and Dia (diameter) refer to the size of the lens. It is not necessary to understand these terms for our purposes. When converted to plus cylinder format, the prescription reads –3.75 +1.50 x 20.
With the basics in mind, let’s take a look at what prescription numbers mean with respect to the relative ease of a particular correction and the likelihood of qualifying for surgery.
The basic concept is as expected. Eyes with lower prescription numbers are easier to correct and less likely to run into issues that make surgery inadvisable.
But, what is a low prescription?
Though each surgeon has his or her own criteria, here are some general guidelines. Please note that prescriptions with a negative number for astigmatism (cylinder) must be converted to plus cylinder format before using the tables below.
Myopia (Nearsightedness)
0.00 to -3.00 low
-3.01 to -6.00 moderate
-6.01 to -9.00 high
-9.01 to -12.00 very high
<-12.01 extreme
Hyperopia (Farsightedness)
0.00 to +2.00 low to moderate
+2.01 to +4.00 moderate to very high
+4.01 to +6.00 very high to extreme
Astigmatism
0.00 to +1.50 low to moderate
+1.51 to +3.00 moderate to high
+3.01 to +4.50 high to very high
+4.51 to +6.00 very high to extreme
