EyeNet Magazine



   
 
Clinical Update: Glaucoma
Don’t Miss Glaucoma in Post-LASIK Patients
By Miriam Karmel, Contributing Writer
 
 

In the right hands, LASIK is a great procedure," said Richard A. Lewis, MD, who had the surgery nearly a decade ago. "I believe in it." But as a glaucoma specialist and past president of the American Glaucoma Society, Dr. Lewis is keenly aware of the difficulty it presents for accurately diagnosing glaucoma. "Our ability to diagnose and observe patients with glaucoma after refractive surgery is in doubt," Dr. Lewis observed in an editorial several years ago.1

He is still sounding the alarm.

Conservatively, some 200,000 eyes are at risk of a missed glaucoma diagnosis, according to Thomas W. Samuelson, MD, who lectures with Dr. Lewis on the difficulty of diagnosing glaucoma in the refractive era. That figure equals 2 percent (the estimated percentage of the population that is genetically predisposed to get glaucoma) of the 10 million LASIK and excimer PRK procedures that have been done in the United States.

Dr. Samuelson, adjunct associate professor of ophthalmology at the University of Minnesota, and attending surgeon at Minnesota Eye Consultants, added that LASIK doesn’t cause glaucoma or make patients more susceptible to the disease.

Screening Tools Dulled

The problem is that it’s difficult to apply the normative parameters of conventional screening tools—particularly Goldmann applanation tonometry—to eyes where the corneal curvature and thickness has been surgically altered.

As Dr. Samuelson noted: "Because these eyes have had surgically thinned corneas, a lot of these patients won’t have the red flag of IOP."

Gold standard not a great standard. Refractive surgeon Ernest W. Kornmehl, MD, agrees. "The bottom line is that Goldmann [applanation] tonometry is the gold standard to measure intraocular pressure," said Dr. Kornmehl, clinical instructor at Harvard and associate clinical professor of ophthalmology at Tufts University. "Unfortunately, that’s not a good way of measuring pressure for patients who have had LASIK, because of a reduction in corneal thickness. The question is: How significant is that reduction?"

Nobody knows. It is known, however, that pressure readings tend to be overestimated in thick and more rigid corneas, and underestimated in thinner corneas. And it has been reported that IOP may decrease after LASIK, possibly due to changes in central corneal thickness.2

What’s more, ever since the Ocular Hypertension Treatment Study (OHTS), which identified central corneal thickness as a risk factor for glaucoma, doctors have been advised to measure corneal thickness along with IOP.

But so far, as Dr. Kornmehl and others noted, the existing nomograms that attempt to adjust for corneal thickness and calculate "true" IOP are unreliable.

Though he doesn’t advocate calculating an adjustment for corneal thickness, Dr. Samuelson said that clinicians might want to do a mental adjustment of IOP. "Just identifying the thin cornea," he said, will be a reminder "to monitor the patient more carefully and look at other screening parameters."

New Tonometers

Some clinicians are turning to newer devices, such as the Pascal dynamic contour tonometer and the Reichert Ocular Response Analyzer (ORA), which aren’t as dependent on central corneal thickness. The Pascal measure of IOP is relatively independent of corneal rigidity or thickness. The ORA measures a corneal compensated IOP. (A fuller discussion of the new tonometers appeared in "New Tonometry: The Search for True IOP," EyeNet, May 2005.)

One advocate of the new tonometry, particularly for refractive patients, is Jay S. Pepose, MD, PhD, director of Pepose Vision Institute. Dr. Pepose is concerned that the inverse relationship between central corneal pachymetry and the risk of developing glaucomatous damage reported in the OHTS trial may have led to some false assumptions. "[After OHTS], a lot of people said, ‘Let’s measure thickness and make a linear adjustment to the Goldmann reading. If thick or thin, add or subtract a couple of points.’ That’s not based on any hard science and is not what the study concluded," said Dr. Pepose, who is also clinical professor of ophthalmology at Washington University, St. Louis.

Uncooperative corneas. In fact, the correlation between central pachymetry and IOP is quite low and nonlinear. And in some cohorts—black patients in the Barbados study, for example—there is no apparent correlation between IOP and pachymetry, Dr. Pepose said.

In some conditions, such as Fuchs’ dystrophy, where the cornea is thick and floppy, simple attempts at linear correction of Goldmann tonometry might actually be in the wrong direction, Dr. Pepose added. "Corneal biomechanics are about more than just pachymetry alone."

Rather, it is the aggregate of viscoelasticity, hydration, regional pachymetry and other factors, which the new generation of tonometers takes into account, he said.

As a result, Dr. Pepose uses the Pascal on patients at higher risk of glaucoma, rather than relying on Goldmann. "We consider the Goldmann measurement to be unreliable in many people who have had refractive surgery where flap creation and laser ablation may have dramatically altered the biomechanical signature of the cornea," he said.

Dr. Pepose has studied various tonometers and found that the Pascal gave the lowest difference between pre- and post-LASIK measurements, and had the tightest standard deviation, though final validation of these instruments may require manometric studies, he said.

Beyond IOP

Not everybody is ready to endorse the newer tonometers. "It’s still too early to say one version is better than another," said Dr. Samuelson, explaining that we lack definitive manometric studies comparing the different modalities.

In the meantime, tonometry is only part of the diagnostic workup. "Clinicians cannot rely solely on IOP," Dr. Samuelson said. "We have to continue to drive home the point we’ve said for years: Glaucoma is a diagnosis made on careful optic disc exam, nerve fiber exam and visual fields and imaging studies."

In time, these issues may be sorted out. But for now, as Dr. Samuelson noted, "Refractive surgery is here to stay. More and more people are going to have refractive surgery, and it’s important to understand the effects on our ability to diagnose glaucoma."

Follow That Patient

Complicating this picture are the many patients who are happy with the results of refractive surgery and are lost to follow-up. Perfect vision is not to be confused with perfect eye health, but do your refractive patients know that?

Good vision is one thing. "It is the LASIK surgeon’s responsibility to make clear to these patients that this [refractive surgery] will not prevent ocular disease," said Dr. Kornmehl. "It is a functional procedure." In fact, Dr. Kornmehl seizes the time with his refractive patients to teach them about ocular health, and tells every patient with pathological myopia that they are at higher risk for retinal disease. "All we’re doing is correcting the front of your eye. This will not prevent glaucoma."

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1 Ophthalmology 2000;107(9):1621–1622.
2 Fournier, A.V. et al. J Cataract Refract Surg 1998;24(7):905–910.
3 Ophthalmology 1998;105(12):2193–2196.

Watching the Refractive Patient

Here are some tips from the experts for diagnosing and tracking glaucoma in refractive surgery patients.

DR. KORNMEHL  The refractive surgeon must ask if there’s a family history of glaucoma. Don’t dissuade the patient from the procedure, but if there’s a family history, educate and inform and be explicit regarding the monitoring issues.

Be sure the comprehensive ophthalmologist is aware of the surgery. If there’s any question regarding pressure, measure IOP with one of the newer tonometers. When using Goldmann applanation tonometry, I take the pressure peripherally. Have the patient look to the right or left and applanate the untreated cornea. This technique is based on a study by Ozlem Abbasoglu, MD, and colleagues.3

DR. LEWIS  Get a good baseline visual field test and a disc image in high-risk LASIK patients. After their vision comes back to a good level, repeat those tests to see if there’s been change. The procedure shouldn’t produce glaucoma or glaucomatous change. We want to prove that to the patient.

Make sure patients understand the risks and the need for a yearly exam.

DR. SAMUELSON  Realize the challenge. The IOP is artificially attenuated, so use other diagnostic tools and pay attention to the optic nerve.

For patients at risk for glaucoma, it is prudent to measure the IOP several times prior to LASIK, and again several times in the early postoperative period (following discontinuation of steroid). If possible, control for time of day for the measurements.



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