Am I a candidate for LASIK?

Precautions

Age: Patients must be at least 18 years old for treatment on some laser systems and 21 on others. Surgeons with lasers that are only FDA-approved to treat those 21 and older can perform an "off-label" treatment on those 18-21, but 18 is the absolute minimum on all systems.

Stability: Laser vision correction should be avoided if the glasses prescription is unstable because if the eyes are still changing, the benefits of surgery could be short lived. Stability is typically an issue of patients in their teens and twenties whose eyes are still growing, though older patients with various medical and ocular conditions can have unstable prescriptions, too. Instability is often formally defined as a change of more than 1.00 D in either sphere or cylinder over the past year.

Corneal disease or abnormality

Prior corneal injury or surgery

Thin corneas: Laser vision correction changes the corneal contour by removing small amounts of tissue. As a result, the cornea is thinned. In order to maintain adequate thickness, surgeons must analyze three variables:

Initial corneal thickness
Type of procedure planned: flap or flapless
Ablation depth - the amount of tissue the laser treatment will remove.

The average corneal thickness is 540µm (540µm = 540 microns = 0.540 mm), and most surgeons choose a flap depth between 120µm and 180µm. The ablation depth is proportional to the amount of correction needed:

For a flap procedure, such as LASIK or IntraLASIK, the standard of care in the United States is to leave at least 250µm behind after subtracting out the flap thickness and ablation depth. For example, a patient with a 540µm cornea, 160µm flap thickness and a -6.00 D glasses prescription would calculate out as follows:

540µm initial thickness
-160µm flap thickness
-72µm ablation depth for -6.00 D treatment
=308µm residual thickness.

This patient measures out well above the 250µm minimum residual thickness requirement and meets the criteria for surgery.

However, consider a second situation in which the patient has a thinner than normal cornea (500µm) and an even higher -10.00 D glasses prescription:

500µm initial thickness
-160µm flap thickness
-120µm ablation depth for -10.00 D treatment
=220µm residual thickness.

This second patient is not a candidate for laser vision correction due to insufficient corneal thickness. This scenario is not uncommon. Thin corneas are the number one reason that patients with high prescriptions are non-candidates for laser vision correction.

Because there is no flap depth to subtract out, flapless procedures better preserve corneal thickness. The calculation is simply a matter of subtracting the ablation depth from the initial corneal thickness. For example, our patient with 540µm corneas and a -6.00 D glasses prescription would calculate as follows:

540µm initial thickness
-72µm ablation depth for -6.00 D treatment
=468µm residual thickness

With flapless procedures, the standard of care with respect to the minimum allowable residual thickness is less clearly defined. Most surgeons try to keep it between 375µm and 400µm, so it is entirely possible to have corneas that are too thin for flapless surgery, too.

Unlike LASIK and IntraLASIK, flapless procedures are also limited by concern over haze formation because haze becomes more likely as the ablation depth increases. One study from the University of Texas-Houston Medical Center concluded that an ablation depth of more than 18% of the total corneal thickness leads to a high risk of clinically significant haze.

In order to prevent haze, many surgeons who perform flapless surgery now treat high-risk corneas with a medication called Mitomycin-C. Studies have shown a dramatic decrease in haze with Mitomycin, and a number of well-known surgeons have reported an excellent safety profile over thousands of cases, but some surgeons are still waiting for more long-term data before using it themselves.

Large pupils: The relationship between pupil size and night vision glare and halo concerns is one of the most hotly debated topics in the field of laser vision correction.

The pupil is the dark, round opening in the colored iris through which light enters the eye. In daylight, the pupil remains relatively small, usually between 3mm and 4mm in diameter. At night, it dilates to let more light into the eye, typically to 6-7mm in diameter.

Conventional wisdom holds that night glare and halos are more likely when the pupils open up beyond the laser treatment diameter. Because most laser treatments are 6mm to 7mm, patients with average 6mm to 7mm nighttime pupils are considered low risk. However, in patients with 8mm or 9mm pupils, unfocused light is thought to enter the eye from peripheral, untreated regions of the cornea and cause glare and halos.

Although conventional wisdom makes intuitive sense, it has not been supported by a number of studies published in the American Academy of Ophthalmology's highly respected Ophthalmology journal.

The first study, "Pupil size and quality of vision after LASIK", published in 2003 by Steve Schallhorn, M.D., out of the United States Naval Medical Center in San Diego, concluded "Patients with large pupils had more quality of vision symptoms in the early postoperative period, but no correlation was observed 6 months after surgery."

The second, "Risk factors for night vision complaints after LASIK for myopia", published in 2004 by Michel Pop, M.D., out of Montreal, Canada, also concluded that pupil size was not a risk factor for night vision complaints within the first year after LASIK for myopia.

Glaucoma is characterized by high pressure within the eye that causes damage to the optic nerve - the cable that carries vision from the eye to the brain.

Glaucoma raises a number of unique concerns for laser vision correction. Because laser treatments thin the cornea, the most commonly used technique to measure eye pressure gives a falsely low reading after surgery. During flap creation for LASIK or IntraLASIK, the vacuum device that holds onto the eye causes a temporary but significant rise in eye pressure that could lead to optic nerve damage. Glaucoma patients who have undergone trabeculectomy surgery to lower the eye pressure are at increased risk that the procedure will stop working. Steroid eye drops, which are commonly used after surgery, could lead to further elevations in eye pressure. Finally, trapped fluid underneath a LASIK or IntraLASIK flap can cause falsely low eye pressure readings.

Uveitis refers to inflammation inside the eye. The most common form occurs inside the front chamber of the eye and is known as iritis. A long list of systemic and localized eye diseases can cause uveitis. Because laser vision correction could potentially reactivate some types of uveitis, added caution is necessary in deciding on surgery.

Blepharitis is characterized by a buildup of crusty, oily secretions at the base of the upper and/or lower lid eyelashes. Quite often, staphylococcal bacteria grow in this mix. Because blepharitis increases the likelihood of infection and inflammation, lid hygiene and treatment with antibiotic ointment are often necessary before surgery.

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