Quality Eye Care, P.C. - Glossary & FAQs

The following are some of the most commonly asked questions about LASIK and the answers we provide at Quality Eye Care, P.C.

ABOUT LASIK
• What conditions can LASIK treat?
• Is there any pain during the LASIK procedure?
• Is there any pain after the procedure?
• How much time will I need to take off work?
• How soon after LASIK can I drive?
• What kind of restrictions will I have to observe after LASIK?
• How long does the procedure take, and will I need someone to drive me home?
• Will my eye(s) be patched after LASIK? Will I need medications?
• How many follow-up visits are routinely scheduled?
• Will the vision I get from LASIK be long-lasting?
• Why is corneal thickness important?
• Why is pupil size important?

ABOUT YOUR EYESIGHT
• What does it mean to be myopic or nearsighted?
• What does it mean to be hyperopic or farsighted?
• What does it mean to have astigmatism?
• What is "emmetropia"?
• What is accommodation?

ABOUT YOUR PRESCRIPTION
• When the doctor tells me what my vision is, what do the numbers mean?
• When my doctor measures my prescription (by what is called the "refraction"), what do the numbers on my glasses prescription mean?

ABOUT LASIK

What conditions can LASIK treat?
LASIK can treat myopia (nearsightedness), hyperopia (farsightedness), and astigmatism.

Is there any pain during the LASIK procedure?
Your eyes will be anesthetized with eyedrops so that you will not feel any pain. Occasionally, patients will feel some pressure-sensation during surgery.

Is there any pain after the procedure?
Some patients will have dryness or a foreign-body sensation for the first few days, but nothing that would generally be described as painful.

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How much time will I need to take off work?
Most patients see well enough the first day after LASIK to resume work. If it is possible, patients are encouraged to avoid prolonged reading, television, or computer work for the the first 24-48 hours to encourage quick healing.

Most patients see well enough to drive the day after LASIK. Visual recovery is usually rapid, but vision sufficient for safe driving can take a variable amount of time to develop.

Protective goggles will need to be worn while you sleep for the first week, and you will need to avoid activities which could potentially cause infection or trauma.

The LASIK procedure itself only takes a few minutes per eye; however, you will need to be at the surgery center about 1 hour prior to your procedure, and stay about 15-20 minutes afterward. Your vision may be blurry in the first few hours after the LASIK procedure, and because you will be given a mild oral sedative prior to surgery, you will need someone to drive you home.

No eye patching is required. You will be given protective eye goggles to wear home after the procedure. You will be on eyedrops for the first week after LASIK.

At a minimum, visits are scheduled one day, one week, one month, and 3 months after LASIK.

As long as your eyes are not changing (this usually stops by the late 20s), and as long as you do not develop any vision- decreasing eye disease such as cataract, macular degeneration, or glaucoma, you can expect to enjoy the vision you get from LASIK well into the future.

Why is corneal thickness important when laser vision correction is being considered?
When lasik is being performed, a corneal flap is first created. After the flap is lifted out of the way, much like opening the cover of a book, the underlying corneal tissue is exposed. The laser then precisely removes the amount of tissue necessary to correct your prescription. The corneal flap is then replaced, much like closing the cover of a book. Surgeons who perform lasik want to make sure that the strength of the cornea is not weakened too much after the lasik procedure. This means making sure there is enough corneal tissue left after the laser treatment is finished.

The average cornea is about 550 microns thick (that’s about 1/2 millimeter). To some extent, the thickness of the corneal flap created by the flap maker (microkeratome) can be predicted. Dr. Yu typically uses a flap maker which creates a flap 160 microns in thickness. Using the example of the average cornea, if you subtract 160 from 550, you’re left with 390. This is the thickness of the cornea that’s exposed or remaining after the flap is lifted out of the way. The laser can now remove the amount of tissue necessary to correct your prescription; this can be calculated before the procedure.

For a given treatment size, the stronger the prescription, the more tissue that needs to be removed (and the deeper you go). Let’s say that 100 microns of tissue needs to be removed to make you see clearly. Subtracting 100 from 390 leaves you with 290 microns. This 290 microns represents what is called the residual bed. The standard of care, of sorts, has been to leave at least 250 microns in the residual bed. Some surgeons like to leave a 300 micron residual bed for more safety. When your surgeon counsels you during your lasik screening, he or she should be doing the kind of calculations you did above, but using numbers from your own eyes.

Why is pupil size important?
The pupil of the eye is the round hole or aperture located in the middle of the iris or colored part of the eye. The pupil is generally round, and changes size depending on the amount of light to which the eye is exposed; when lighting is bright, the pupil constricts or becomes smaller; when lighting is poor, the pupil dilates or becomes larger. In general, it is desirable to have the size of the treatment zone (made by the laser on the cornea) at least as large as the pupil becomes under dim lighting.The cornea is the clear dome-like structure that arches over and covers the front of the eye. The pupil and iris are behind the cornea, “under glass”, so to speak. The human cornea is about 11-12 mm in diameter. However, the corneal flap is typically smaller than the size of the cornea, as large as 9.5 mm.

Infrared pupillometer more accurately measures pupil size

To some extent, the diameter or size of the corneal flap can be specified before the procedure. A given flap maker is labeled as to how thick and large a flap it can make. Once the flap is lifted out of the way, the underlying corneal tissue has the same diameter as that of the flap, in this case, 9.5 mm. Theoretically, a surgeon could plan a laser treatment which corrects your prescription, in any diameter size he or she wishes. The treatment zone could be smaller or as large as the 9.5 mm which is exposed. If the treatment size is too small relative to the size of the pupil, it would be as if you’re looking through a tunnel. Essentially, you would be looking through part of the cornea that is now corrected for your prescription, but around this treated area, there would be a ring of your “old” cornea which has been untreated. This is why it is desirable to have a treatment zone which is larger than the size of your pupil, especially the pupil size under dim lighting. The physics of laser vision correction dictate that the larger the treatment zone, the more corneal tissue you have to use (and the deeper you go). The converse is true, in that the smaller the treatment zone, the less corneal tissue that needs to be removed.

pachymeter

Your corneal thickness (and the strength of your prescription) places a limit on how large the treatment can be and how much tissue can be safely removed (read about corneal thickness in previous question). Your doctor will measure your corneal thickness using an ultrasound device called a pachymeter. This instrument uses sound waves to determine the thickness of your corneas, in a painless fashion.

In practice, the treatment zone size does not always have to be as large or larger than the pupil size under dim lighting conditions. There are a number of factors which influence the desired treatment zone size. One is the strength of the prescription. If the prescription is not too strong, a treatment size smaller than the pupil size at night may not cause any difficulty. If the prescription is very strong, and the treatment zone size is smaller than the pupil size at night, there may be increased risk of night vision difficulty (glare, ghosting, and halos). There are natural optical effects that counteract the need for a treatment zone size as large or larger than the pupil size at night. Your ophthalmologist is the one best suited to tell you how large a treatment zone size you need. The Visx Star S4 laser with Variable Spot Scanning technology, using CustomVue (see Custom LASIK) allows the surgeon to expand the treatment zone, within the limits imposed by your corneal thickness and prescription.

ABOUT YOUR EYESIGHT

What does it mean to be myopic or nearsighted?

Myopia - Nearsightedness

Very simply, without glasses or contacts, nearsighted people can see things up-close clearly, but things in the distance are fuzzy. The more nearsighted you are, the closer you have to hold things to see clearly. In this diagram of an eye with myopia or nearsightedness, the rays of light from the object are being brought into sharp focus in front of the retina, so the image on the retina is fuzzy. In essence, the cornea's focusing power is too strong for the length of the eye.

Hyperopia - Farsightedness

The definition of farsightedness is a little more difficult to grasp. Someone who is farsighted may be able to see things in the distance clearly, and read without difficulty, if the amount of farsightedness is mild and the person is less than 40 years of age. Farsighted people can "self-correct" some or all of their farsightedness, without effort, when they are younger, but the ability to do this deteriorates as the 40s approach; first, near vision goes, and then even the distance gets fuzzy. Individuals who have a significant amount of farsightedness that cannot be "self-corrected" will complain of not being to see clearly up-close and even far away.

eye with hyperopia

In this diagram of an eye with hyperopia or farsightedness, the rays of light from the object are being brought into sharp focus behind the retina, so the image on the retina is fuzzy. In essence, the cornea's focusing power is too weak for the length of the eye.

What does it mean to have astigmatism?

In simplistic terms, if there is no astigmatism, the cornea of the eye (see Tour of the Eye) is shaped like a baseball cut in half. The curvature or steepness of the half-dome is the same all the way around. Compare this to a cornea which is similar to a football cut in half lengthwise (in the long direction, through both pointy ends). The curvature of the cornea in the long direction (along the seams) is not as steep as along the short direction. Such a cornea focuses light, not at a single point, but at 2 points. Someone who has uncorrected astigmatism may see images that are fuzzy and doubled. A cornea shaped like a football, cut lengthwise, has astigmatism.

eye with astigmatism

In this diagram of an eye with astigmatism, the rays of light from the object are being brought into sharp focus at two different points, one on the retina and the other, behind the retina. This is an eye with a cornea that has astigmatism, a non-uniform curvature like the football cut lengthwise. The two different curvatures results in two different focal points.

There are several different combinations of astigmatism, depending on where the focal points are located.

• Simple myopic astigmatism: One point in front of retina, other on the retina
• Compound myopic astigmatism: Both points of focus in front of the retina
• Simple hyperopic astigmatism: One point behind the retina, other on the retina
• Compound hyperopic astigmatism: Both points of focus behind the retina
• Mixed astigmatism: One point in front of the retina, the other behind the retina

What is "emmetropia"?

In this diagram of an eye with perfect vision, the rays of light from the object are being brought into sharp focus on the retina. The eye has no prescription and is called "emmetropic". This is what you want with laser or other vision correction procedures.

What is accommodation?

A person with healthy eyes can be made to see clearly at distance, with glasses, contacts, or even refractive surgery, if necessary. When that same person wants to see something up close, say reading a book, extra focusing power is needed to bring the focal point closer. This ability to generate extra focusing power is called accommodation. Young people have a great deal of accommodative reserve from which they can draw, so they can hold things very close to their eyes and still see clearly. After about 40 years of age, the accommodative reserve decreases, so reading up close becomes more and more difficult. The over-40 crowd finds themselves holding reading material further away from their eyes to see clearly. This is the time when patients find they need extra focusing power in their glasses for reading, so bifocals, lined or non-lined (progressives) are necessary.

ABOUT YOUR PRESCRIPTION

Patient at Phoropter

When the doctor tells me what my vision is, what do the numbers mean?

First, the level of your vision is not the same as a glasses or contact lens prescription which can be taken to an optical shop to be filled. Vision is usually given in the Snellen format, for example, 20/40. If your vision is 20/40, it means an object you can see 20 feet away, can be seen from 40 feet away by someone who has perfect vision. Therefore, someone with 20/400 vision has even worse vision; the larger the denominator or the second number, the poorer the vision. In the extreme, if the vision is even worse, such that a person cannot see the biggest letter on the eyechart, "E", the number of fingers that can be counted is a way of measuring vision. If someone has "counting fingers at 3 feet", it means the eye in question has worse than 20/400 vision, and can only identify the number of fingers held 3 feet away. The gold standard of perfect vision has been 20/20 vision, though there are patients capable of seeing better than "perfect". While most patients use both eyes together, vision is tested in each eye separately, as is the measurement of a person's prescription.

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When my doctor measures my prescription (by what is called the "refraction"), what do the numbers on my glasses prescription mean?

If your eye is just nearsighted, there will be a single negative number. The minus sign indicates nearsightedness or myopia. The number that comes after the minus sign tells you the amount or "severity" of the nearsightedness.

Examples: -1.00 means one diopter of nearsightedness(a diopter is a unit of measure of focusing power, like an inch is a unit of measure of length) -5.25 means 5.25 or 5 and 1/4 diopters of near- sightedness. This is more nearsighted than -1.00, and so thicker glasses are needed.

If your eye is just farsighted, there will be a single positive number. The plus sign indicates farsightedness or hyperopia. The number that comes after the plus sign tells you the amount or "severity" of the farsightedness.

Examples: +1.00 means one diopter of farsightedness +5.75 means 5.75 or 5 and 3/4 diopters of farsightedness. This is more farsighted than +1.00, and so thicker glasses are needed.

If your eye has astigmatism, the numbers are harder to follow. There are actually 3 numbers in a prescription for an eye that has astigmatism. The general form is S x C x Axis.

Both S and C can be either positive or negative numbers. S refers to what is called the "sphere" or spherical portion of the prescription. The C refers to the amount of astigmatism. The Axis is a number anywhere between 0 and 180 degrees; this axis number tells where the difference in corneal curvature occurs or how the astigmatism is oriented or aligned. It is not enough to specify how much astigmatism there is, you have to know where the difference in curvature is taking place, by giving coordinates. So, if you see three numbers in your prescription, you have astigmatism of some kind and severity. The bigger the second number, C, the more the astigmatism you have. There are several categories of astigmatism, and by analyzing the 3-numbered prescription, you can tell the exact type of astigmatism you have.

Examples:
-2.00+1.50x180
-3.50+3.00x45
+4.00+1.00x89
-1.50+2.50x76
+1.00-2.50x66

Interestingly, the last 2 examples are identical, they are just in 2 different formats which depend on the signs of the first two numbers in each prescription. The signs are very important.