Am I a Good Candidate for LASIK?

Whether LASIK is the best option for you will ultimately depend on the judgment of your eye surgeon, who will make that determination during a preoperative evaluation. However, good candidates for LASIK have some basic conditions in common.

Ideal Age
A good candidate is at least eighteen years old, because the vision of people younger than eighteen years often continues to change. Myopia may continue to
increase in some patients until their mid to late twenties.

Stable Prescription
No matter what your age, to be considered a good candidate for LASIK, your vision prescription should be stable. In practical terms, your prescription is stable if your
glasses or contacts are at least a year old and you still see well with them.

Treatable Eyesight Parameters

A good candidate for LASIK has an eyeglass prescription that falls within certain parameters. If you are nearsighted, you may have myopia of up to –12.00 diopters. If
you are farsighted, your hyperopia may be up to +6.00 diopters. Your level of astigmatism may be as high as 6.00 diopters. These are normal parameters, but they can vary
from patient to patient and from doctor to doctor.

Surgically Ideal Eyes

You will not know until the preoperative examination whether your eyes meet the standards required for LASIK.

These would include:

• A cornea of the right thickness (not too thin)
• A cornea that is structurally normal (not irregularly shaped)
• Generally healthy eyes (no eye diseases or injuries that could interfere with the outcome)

Conditions That May Prevent Surgery

Any number of factors could make you a poor candidate for LASIK. Some patients fear their eyesight is too poor, yet later discover, after meeting with their eye doctor,
that it falls within treatable parameters that yield successful outcomes. So, do not assume you are a poor candidate until you have consulted with your eye doctor and he or she confirms it.

Contemplating Laser Eye Surgery

Severe Refractive Error
If your refractive error is so severe that it falls outside normal treatable parameters, you may not be a candidate for LASIK. To correct extreme nearsightedness or farsightedness
requires too much deep sculpting and corneal reshaping. Other vision correction procedures, such as the implantable contact lens (see chapter 11), may be preferable. But to be certain, get your eye doctor’s opinion.

Other Health Conditions

You may be a poor candidate for LASIK if you have any of the following conditions:

• Thin cornea. LASIK will not weaken a normal cornea, but if your cornea is unusually thin, LASIK could weaken it, causing distortion in your vision. If you have a thin cornea, PRK may be a better option for you, because it won’t weaken a thin cornea.
• Abnormally structured cornea. This condition is not treatable with LASIK.
• Pregnancy. Vision can be unstable during pregnancy, especially during the third trimester. As a result, the measured refraction may be inaccurate. On the other hand, if a woman is early in her pregnancy and her vision hasn’t changed, it may be possible in special cases to do LASIK.
• Cataract. Cataract is a clouding of the lens within the eye that causes blurry vision. If you have a cataract, LASIK can accentuate the blurring of vision caused by the cataract.
• Corneal dystrophies. These are inherited conditions in which one or more parts of the cornea lose normal clarity due to a buildup of cloudy material.
• A history of ocular herpetic keratitis. The same herpes virus that causes cold sores on the lips can cause recurring infection in the eye, resulting in scarring and blurred vision. This is not a sexually transmitted disease.
• Diabetic retinopathy. This is a potentially blinding complication of diabetes that damages the eye’s retina. Patients with diabetes who have normal corneal sensation and do not have retinopathy are usually candidates for LASIK, however.
•  Severe dry eye. Patients with severe dry eye can have healing problems after LASIK. On the other hand, mild to moderate dry eye that is treated before surgery does not cause healing problems, so this condition does not automatically mean LASIK is not an option. Learn more about LipiFlow for chronic dry eye.

Although the conditions listed above are generally contraindications to LASIK, none are absolute contraindications. If you have one of these conditions, consultation with an
experienced LASIK surgeon will help you determine whether LASIK is still a possibility for you.

Unrealistic Expectations

If you have unrealistic expectations, you may be disappointed with the outcome of laser eye surgery. As a patient, you are responsible for understanding exactly what the procedure can and cannot do. For example, you might still need eyeglasses for performing certain activities, such

Contemplating Laser Eye Surgery

“Some people aren’t good candidates for LASIK. When I recommend against the surgery, my concern is that these patients will go somewhere else and have the surgery done when they
shouldn’t.” Dr. Robert Maloney

It is best to think of LASIK as reducing your dependence on eyeglasses and contact lenses and improving your natural vision.

Issues to Discuss with Your Doctor

The following factors represent areas of controversy in terms of whether a patient is a good candidate for LASIK. Some eye doctors recommend that people with these conditions avoid LASIK surgery, while others believe that decisions need to be made on a case-by-case basis. If you have any of these conditions, discuss them with your eye surgeon.

• Unusually large pupils. One of the potential side effects of LASIK is glare or seeing halos around lights at night. If you have unusually large pupils, more light enters your eyes at night. Some doctors believe that this extra light causes more glare and halos. A number of major studies have now shown that this is not the case, that there is little if any correlation between pupil size and night vision. More and more doctors are coming to agree with us that pupil size is relatively unimportant to your candidacy for LASIK.
• Nursing mothers. Some doctors are concerned that vision may change while a mother is nursing. This is not our experience. If more than two months have passed since delivery, you are a candidate for LASIK. However, if you are nursing and do have the surgery, we recommend that you do not take any oral sedatives, like Valium, that your doctor routinely offers, because they will get into your breast milk.
• Pacemakers. Some pacemakers that are more than twenty years old can be affected by electromagnetic fields emitted by equipment like the excimer laser. Patients with older pacemakers can still have LASIK,but they may require that a technician be present to oversee the pacemaker. Newer pacemaker models are unaffected by the laser.
• Autoimmune diseases. These diseases are caused by an abnormal attack by your immune system on the natural, healthy cells of your body. Certain autoimmune diseases, such as rheumatoid arthritis, are rarely associated with corneal melting, or dissolving, in patients who have eye surgery. Autoimmune conditions can also cause severe dry eye. In general, we find that if you have an autoimmune disease but are under age sixty-five and don’t have severe eye dryness, you may be a candidate for LASIK.
• The use of certain prescription medicines. Some doctors believe that certain prescription medicines impair healing. These prescription drugs include Accutane, used to treat severe acne; amiodarone, prescribed to treat irregular heartbeat; and oral steroids, often used by individuals with severe allergies, with asthma, or with inflammatory diseases such as arthritis and lupus. Tell your eye doctor about any prescription and over-the-counter medications you are taking.

As explained earlier, patients who are nearsighted have corneas with too much curvature in proportion to the length of their eyes. Once the corneal flap is made and lifted back, the excimer laser reshapes the underlying stroma to achieve a flatter cornea. The surgeon’s carful, precise measurements, which are programmed into the computer, guide the excimer laser. When the reshaping is complete, the flap is replaced. The result is that light rays coming through the cornea now come to a point of focus on the retina, rather than in front of it.

LASIK and Hyperopic Correction

Farsighted patients, on the other hand, have corneas that are proportionately too flat for the length of their eyes. The excimer laser is programmed to remove tissue from just the periphery of the stroma, leaving the middle untouched; this creates more of a domed shape. The increased curvature of the cornea will allow light rays to focus on the retina, rather than behind it.

LASIK and Astigmatism

To treat astigmatism, the excimer laser removes tissue in a somewhat oval fashion, adjusting the shape of the cornea in one direction more than the other. The goal is to produce a symmetrical surface so that light rays passing through the cornea at various places will meet at a single point of focus on the retina.

LASIK: State of the Art

No LASIK surgeon can promise 20/20 vision without the use of corrective eyeglasses or contact lenses. However, more than 99 percent of typical myopic patients who receive LASIK can see well enough to legally drive without glasses, usually by the morning after surgery. With today’s technological advances, the typical patient has a greater than 95 percent chance of achieving 20/20 vision.

The procedure that originally made wide use of the excimer laser was photorefractive keratectomy (PRK), first performed in 1987. Instead of using a microkeratome to remove the corneal disc, PRK uses the laser to accurately sculpt the cornea one microscopic layer at a time. PRK has seen vast improvements since those early days.

PRK sculpts the cornea by first removing the epithelium, the outer protective layer of the cornea. The laser works its way down into the stroma, or structural part of the cornea, where the real reshaping takes place. The epithelium then grows back over the next 48 to 72 hours. PRK presents some inconveniences. First, PRK can leave your eyes sore for 24 to 48 hours after surgery while the eye heals. Second, because the corneal surface is left exposed, patients experience blurred vision for almost a week following surgery.

LASIK

In 1990, researchers conceived of ways to avoid these side effects associated with PRK; they developed a new procedure called LASIK, a type of refractive surgery in which a laser is used to reshape the cornea, improving vision. LASIK is an acronym for laser-assisted in situ keratomileusis.  The component words of LASIK are defined as:

LASer-assisted – performed with the excimer laser

In situ – performed on the cornea after a flap of corneal tissue has been lifted

Keratomileusis – a process of carving the cornea to reshape it

During a LASIK procedure, an extremely thin flap is created in the outermost layers of the cornea using a thin blade called a microkeratome or a laser. This flap is gently lifted back like a hinged lid, exposing the corneal tissue beneath. The cornea is then precisely sculpted by the excimer laser into a new shape to correct the patient’s vision. The flap is set back in place and is held in position by the eye’s natural suction.

Clinical trials on LASIK began in the United States in 1991. A broad series of clinical investigations culminated in its approval by the FDA in 1999.

How LASIK Corrects the Eye’s Focus

How can a laser beam correct vision? The excimer laser is uniquely suited to the task of refractive corneal surgery because it ablates, or vaporizes, tissue by breaking apart the molecules without creating damaging heat. The unparalleled precision of the excimer laser makes it the ultimate reshaping tool. The laser is so precise that it would take 600 pulses to break through one strand of human hair. This precision allows the surgeon to sculpt the exposed corneal bed, gently and precisely, into a more desirable shape that allows rays of light to focus properly on the retina. The result is improved vision.

Learn more Boston LASIK Surgery, make your consultation with Dr. Kornmehl today!

History of Vision Correction Surgery

Although many pioneering contributions led to the development of modern refractive surgery- that is any surgical procedure to help the eye focus light correctly- a key breakthrough occurred in the middle of the last century.  In 1949, Dr. Jose Barraquer of Bogota, Colombia, developed the idea of lamellar corneal surgery (“lamellar” means “layered”).  He discovered that lamellar surgery could reshape the cornea, enhancing the eye’s ability to focus.  To do so, Barraquer removed a disc of the front portion of the cornea with an instrument called a microkeratome.  The instrument was affixed to the eye through use of a vacuum ring; then the microkeratome shaved a small amount of the cornea at a predetermined depth.  Dr. Barraquer froze the disc and then ground it into a new shape with a small lathe.  He placed the newly shaped disc back on the cornea.  The procedure of carving the cornea is called keratomileusis.

Two important refinements followed.  In 1985, Dr. Casimir Swinger developed a method of reshaping the disc without freezing it (nonfreeze keratomileusis).  Then, in 1987, Dr. Luis Ruiz, a protégé of Barraquer, used an automated microkeratome to reshape the cornea directly on the eye.  This procedure, automated lamellar keratoplasty (ALK), was used to correct high levels of myopia and hyperopia.  It is important to note that patients who have undergone these two procedures, precursors to today’s laser vision correction, have not experienced long-term complications from the corneal reshaping.

The Arrival of the Excimer Laser

The excimer laser was first used on human eyes in the late 1980s.  The laser technology marked a significant advancement in the science of vision correction; the excimer laser uses a cool ultraviolet beam of light to vaporize tissue- that is, break up the molecules- with exacting precision and without harming adjacent tissue.  Each pulse of the excimer laser removes a mere 1/100,000 inch of tissue.

 Learn more about Laser Vision Correction, make your appointment with Dr. Ernest Kornmehl today.

So, numbers such as 20/20 or 20/40 describe your visual acuity but do not measure your refractive error- how accurately your eye bends, or refracts, light.  When an eye doctor measures your refractive error, what you end up with is your eyeglass prescription.  Finding an eye doctor whose measurements are impeccable is crucial, not just for your eyeglass prescription but also, as you will learn later, for laser vision correction.

Your eyeglass prescription is written in numbers.  The type and degree of refractive error is quantified in units of measure called diopters.  If you have ever wondered what those numbers mean, here is how to read and understand your prescription.

To arrive at your prescription, your doctor takes three measurements during the eye exam: sphere, cylinder, and axis.  Your prescription for glasses may look something like this:
OD  -1.25   —-    —-
OS  -1.25  -.25  x 170
OD and OS refer to the right and left eyes, respectively.  The first number next to OD or OS represents the sphere.  The sphere measure tells the eye doctor where your eye focuses light: on the retina (normal vision), in front of the retina (myopia), or behind the retina (hyperopia).  In other words, the sphere measure reveals whether you are nearsighted or farsighted.  A negative diopter indicates myopia, or nearsightedness.  A positive diopter indicates hyperopia, or farsightedness.  The higher the number, the stronger the prescription.  In the example above, the person has mild myopia (-1.25 diopters) in both eyes.

The number in the second column represents the cylinder.  The cylinder measure indicates whether or not the patient has astigmatism.  If the cylinder column is not blank, you have some degree of astigmatism.  The larger the number, the more astigmatism you have.  The example above reveals that this person has no astigmatism in the right eye, and a small amount (-.25 diopter) in the left eye.

If astigmatism is present, your eye doctor takes an axis measurement.  The axis measure indicates where irregularity lies on the eyeball.  In the prescription above, the astigmatism in the left eye is positioned at the 170-degree axis.

Eyeglasses have been around for hundreds of years.  As early as the thirteenth century, inventors in China and Europe inserted magnifiers into frames, making the first prototype for our modern-day eyeglasses.  Like the early versions, today’s eyeglasses work like magnifying glasses that enhance the eye’s ability to focus sharply, whether near or far.  The amount of curvature in the spectacle lens determines how light bends before it reaches your cornea.  Vision is corrected, depending on the angle of refraction, to compensate for your focusing error.

Eyeglasses have a number of advantages.  They are usually affordable, are easy to maintain, and can be adapted for a number of different uses, such as reading, active sports, and driving.  They also have disadvantages.  Eyeglasses may restrict peripheral vision, the outer part of your field of vision; prove difficult in certain weather conditions, such as rain or snow; and make images appear smaller or larger than they really are.  They may cause a number of visual aberrations, including halos around lights, and the lenses usually need to be replaced as your vision changes.  Eyeglasses may interfere with certain occupations and recreational activities- swimming, for example.  And some people just don’t like the way they look in glasses.

Contact lenses offer another option for correcting vision.  Like eyeglasses, they make up the difference between the amount of refraction your eye can accomplish on its own and what is needed for sharp focus.  Because they are extremely thin and are custom-shaped for your cornea, contact lenses float on the surface of your eye; they are held in place by natural suction and are constantly lubricated by the eye’s own moisture.

Contact lenses have some advantages over eyeglasses.  For example, contacts enable the wearer to have more natural vision (including better peripheral vision), cause little noticeable change in cosmetic appearance, and allow more freedom in recreational activities.  On the other hand, contacts may require maintenance- continuous, frequent cleaning.  Users must buy cleaning and storage solutions.  The lenses may tear easily.  They may be inconvenient for traveling, and also are easily lost.  Contacts may be uncomfortable for patients with dry eyes or for those who live and work in polluted city air.  They may cause visual aberrations (including halos and uneven vision) and always carry an increased risk of infection and possible corneal scarring.  Individuals who live in higher altitudes may become intolerant of contact lenses over time because of the air’s lower oxygen and humidity content.

The variety of contact lenses available today is dazzling.  Costs for contacts vary widely, depending on the type you need.

Orthokeratology, is a technique for treating myopia, or nearsightedness.  It uses a series of rigid contact lenses that apply pressure to the cornea to flatten it.  The effects are not permanent and require continued dependence on daily-wear maintenance lenses to retain the reshaping.  Orthokeratology is generally only effective, even temporarily, for low levels of nearsightedness.  The technique is expensive and high maintenance and requires continuous follow-up visits.  Long-term effects can include permanently warped corneas.  The risk of infection may also be greater than that from normal contact lens wear.

Interested in LASIK in Boston, visit Kornmehl Laser Eye Associates for a consultation.

Your Boston eye doctor may refer to your vision problem as your refractive error, or focusing problem.  How well you see is determined, for the most part, by how accurately your eyes are able to bend, or refract, light.  In a normal eye, the focus comes to a point on the retina.  But sometimes this does not occur.  The result?  Various forms of vision impairment, or aberrations.  Vision problems fall into one of two basic groups: low-order aberrations and higher-order aberrations.

Low order-aberrations, also known as nearsightedness, myopia is a condition in which you can see nearby objects well, but objects at a distance appear blurred.  This happens when light bouncing off a faraway image enters the eye through the cornea and comes to a point of focus too soon, before it reaches the retina.  Myopia may be due to a cornea that has too much curvature, which causes the light to “overbend” and focus in front of the retina.  Myopia also occurs when the eyeball is too long- the retinal wall is too far back for the combined focusing power of the cornea and lens.

People with hyperopia, or farsightedness, see distant objects more clearly than nearby objects when they are young but may have difficulty with both as they get older.  In hyperopia, the light rays coming into the cornea are not bent sharply enough and are focused behind, rather than on, the retina.  The result is a blurred image.  This usually happens in people whose eyeballs are too short from front to back or whose focusing muscles around the lens are too weak.  Another cause of hyperopia, though rare, is a cornea that is not curved enough.

Because muscles are more elastic in youth, younger people who are mildly hyperopic can actually compensate for it by using the focusing muscles around the lens to fine-tune the focus by bending light more steeply.  This action brings the point of focus forward toward the retina, allowing them to see more clearly.  However, because the muscles weaken and the lens becomes less pliable as we age, the individuals eventually lose the ability and may no longer wee well at a distance or close up.  After age forty, they may be completely dependent on eyeglasses or contact lenses for both close and distant vision.

Many individuals with myopia or hyperopia also have some degree of astigmatism.  People with significant astigmatism experience blurred or distorted vision with all objects, whether near or far.  Astigmatism means that your cornea, instead of being spherical like the side of a basketball, is slightly oval, shaped more like the side of a football.  Your cornea is more curved in one direction that the other.  As a result, light rays entering the eye from different points on the cornea’s surface are bent irregularly and are focused at several different points, rather than meeting at just one focal point.  Almost everyone has a small degree of astigmatism.

Farsightedness is often confused with presbyopia, which literally means “old eyes”.  Presbyopia is the age-dependent need for reading glasses or bifocals.  After age forty, and in most people by age forty-five, the ability to focus on an object close up, such as a restaurant menu, becomes more difficult.  This happens to everyone.  It is due to a loss of flexibility in the lens and a weakening in the muscles that enable the lens to flex and fin-tune the focus.  Presbyopia  typically continues to worsen until age sixty-five.  When this occurs, people who already wear eyeglasses may need bifocals, and those who have never worn eyeglasses may require reading glasses.

Higher-order aberrations are focusing problems that are not correctable with glasses or contact lenses.  Higher-order aberrations, which are a result of subtle irregularities in the focusing mechanism of the eye, cause a loss of crispness, clarity, and contrast.  If you have significant higher-order aberrations, you may have trouble distinguishing between shades of gray.  Higher-order aberrations may also affect one’s night vision; people with these problems may see glare or halos around lights.  Approximately 17 percent of visual errors are considered higher-order aberrations.

However, changes within the eyeball may occur, resulting in impaired vision.  Objects that we once viewed with crystal clarity may become blurred or distorted.  To better understand how vision may change, let’s first examine the anatomy of the eye.

You may have heard the comparison between a camera and the human eye.  Just as a camera takes in light and transforms it into an image on film, your eye does virtually the same thing, only the “film” is your retina and your brain “develops” the image.  We see objects when light, which is reflected by the objects, passes through the eyeball lens and strikes the retina, at the back of the eye.  Our brains then interpret the shapes, colors, and dimensions of the objects we see.  A clearly focused object is the result of normal vision.  However, just as an improper amount of light entering a camera lens will distort a photo, if light entering the eyeball does not strike the retina, the result may be distorted vision.

The outer layer surrounding the eyeball is made up of two parts: the sclera and the cornea.  The sclera- the white, opaque part of the eye- makes up the back five-sixths of the eye’s outer layer and provides protection for the eyeball.  The cornea, about the size of a dime and as thick as a credit card, makes up the remaining sixth of the eye’s outer layer.  It is the transparent dome, similar to the crystal of a wrist watch, at the front of the eyeball.  The cornea provides most of the eye’s focusing power, so small changes in its curvature can make an enormous difference in how clearly you see objects.

The cornea has three main layers.  The epithelium is the thin outer protective layer of cells; it is made up of the same kind of tissue that covers most of your body, and is continually regenerating, or renewing itself.  The stroma is the strong, fibrous layer that makes up 90 percent of the cornea’s thickness and provides the cornea with its structure and shape.  The endothelium is the single cell layer that lines the inside of the cornea and helps regulate the cornea’s fluid content.

The iris, which determines one’s eye color, is located behind the cornea.  It is composed of connective tissue and smooth muscle fibers.  The muscles of the iris control how much light passes through to the retina.

The black circle in the middle of the iris is the pupil.  It can be likened to the aperture, or shutter, of a camera.  When it is very bright, as on a sunny day, the iris muscles make the pupil constrict, or become small, so only a small amount of light will pass into the eye.  In darkness the opposite happens, and the pupil dilates, or enlarges, to let in more light.

The lens is a circular structure located directly behind the pupil and held in place by slender, strong ligaments.  Although most of the bending of light is accomplished by the cornea, the curved lens fine-tunes the angle of light passing through it, focusing the light onto the retina.  When the ligaments tighten, the lens becomes flatter, or less convex, allowing you to see objects at a distance.  When the ligaments relax, the elastic lens becomes rounder, or more convex, like a magnifying glass, so you can see objects that are close.  The ability of the lens to refine the focus through flexing is called accommodation.

The vitreous humor is a jellylike substance that fills the space between the lens and the retina on the inner back wall of the eye. It is about 99 percent water.  Light passes through the vitreous humor before striking the retina.

The retina is a complex layer of nerve tissue that lines the inside back wall of the eyeball.  Similar to film in a camera, the retina “captures” the image through an electrochemical reaction to light.  Electrical impulses are then transmitted through the optic nerve to the brain, which interprets, or “develops,” the image.