September 2013

 

COVER FEATURE

 

Refractive challenges and innovations Diagnostic curve balls


by Michelle Dalton EyeWorld Contributing Writer

   

Pentacam flag: The overall yellow D flag suggests that PRK may be the best option despite thick corneas with high astigmatism but no obvious inferior steepening.

Source: Lou Probst, MD

An example of inferior steepening of the left eye Source: Uday Devgan, MD

In comparison, an example of a normal cornea in the right eye Source: Uday Devgan, MD

When topography or tomography show a warning sign, what should you do?

Diagnostics are an invaluable tool for surgeonsespecially when patients are choosing an elective procedure such as laser vision correction, where outcome expectations are heightened. Technology provides critical information, but there are times when technology may not match up completely with what a clinician sees. "In my mind, I am always trying to match the evidence with the clinical situation," said Y. Ralph Chu, MD, Chu Vision Institute, Bloomington, Minn.

When those situations do occur, "repeat the test," said Carlos Buznego, MD, Center for Excellence in Eye Care, Miami, and voluntary assistant professor of ophthalmology, University of Miami's Bascom Palmer Eye Institute. "Whenever I get a bizarre reading on a topography, the first thing I do is repeat the test."

Vance Thompson, MD, director of refractive surgery, Vance Thompson Vision, Sioux Falls, S.D., is a "big believer in being sensitive to your diagnostic devices. This is elective surgeryand it's OK to approach it conservatively."

Always thinking about the standard caveat to "Do no harm," while LASIK has a success rate of "more than 99%, there is always a risk, same as with any surgical procedure," said Uday Devgan, MD, in private practice, Los Angeles, and chief of ophthalmology, Olive View UCLA Medical Center. "So if your tomography comes back suspicious, you need to pay attention to that."

Some centers will only use tomography, while others pay closer attention to topography. "While tomography offers a very detailed analysis, one of the clear messages from earlier studies is that the most important screening tool for patients who might develop ectasia is topography. It can be very confusing," said Lou Probst, MD, chief laser surgeon and senior medical advisor, TLC The Laser Eye Centers (locations in Chicago; Madison, Wis.; and Tampa, Fla.). "I think in the effort to make a great diagnostic tool, Oculus [Wetzlar, Germany] created a great diagnosis tool in the Pentacam." The Pentacam analyzes five subcategories of a patient's eye and if it detects an anomaly will produce a yellow or red flag, Dr. Probst said. "The machine has become so analytical and provides so much information it is actually altering the outcome of clinical practice." He said it's not unusual to see a patient with good topography and no clinical issues, yet the Pentacam will show a yellow flag or two.

Dr. Chu said he takes a more conservative approach as well, by either suggesting PRK or recommending against LASIK when those flags are raised. He'll even recommend some patients wait altogether "because there may be a time when diagnostic technologies will improve to where we can get a definitive answer" about the anomalies. Dr. Thompson added a patient's level of refractive error will help sway his preference as well; a 1 or 2 D "may not alter" his comfort level with LASIK, but for refractive errors and diagnostics that push a patient into a borderline scenario, "why not just do what leaves the cornea stronger? I'll recommend PRK, since it also minimizes complications like ectasia." Other casesa moderate myope with suspicious looking, against-the-rule cylinder "and you start to wonder if either treatment makes sense," he said, especially if the surgeon is comfortable implanting a phakic IOL. As Dr. Buznego quoted a colleague of his: "Patients are always their disease until you operate on them. Once you've operated, it's now your disease to manage and control."

Potential topographic red flag causes

Topographically, atypical patterns suggestive of corneal instability generally lead clinicians to look for asymmetrical astigmatism, or subclinical keratoconus, or forme fruste keratoconus (FFKC). "Those will also show up on an Orbscan [Bausch + Lomb, Rochester, N.Y.] because that device also has Placido rings," Dr. Probst said, and the Pentacam is likely to discern those same abnormalities. "For every good topographic image you see, there might be 15 in the garbage that were unreadable if the patient has a very poor tear surface," Dr. Buznego said. If the technician has to work really hard to get a good topographic image but doesn't convey that, it's possible the surgeon will believe he/she has a straightforward case. Many still believe topography is the "the most valuable tool in screening for keratoconus," Dr. Probst said, but others will swear the Pentacam is the only device necessary. While no one would argue topography is helpful in diagnosing ocular surface disorders, geography may play a part in a poor reading, Dr. Devgan said. For instance, some patients with dry eye changes will have worse topography readings in the dry, arid summer months in Los Angeles than in other months. Dr. Devgan will often postpone those patients' surgeries until fall to prevent exacerbating the dry eye situation during those hot months, he said. Dr. Chu educates the patient about the diagnostic dilemma to jointly determine how to proceed. "Technology is not an absolute answer," he said. "There is still some part that has to apply the science, and that's a personal choice for each surgeon and patient to make together if there is any kind of refractive dilemma."

Especially when it comes to dry eye, Dr. Buznego said when he's in doubt, "it's OK to give patients a pass and tell them that right now there may be procedures that are better given the significant dry eye or ocular surface issues present."

Dr. Probst said he believes both topography and tomography need to be usedtopography for the anterior surface and tomography to determine changes and progression of anything deemed abnormal.

Potential tomographic red flag causes

Although tomography "is certainly a very detailed analysis," it's not concentrating on the anterior curvature of the cornea, Dr. Probst said. Tomography can help identify inferior steepening or mild FFKC, Dr. Devgan said, and those scenarios elevate a patient's risk for complications. "PRK may be safer than LASIK, but there are still grey areas. I'd rather err on the side of caution and not operate on a patient with suspicious corneas."

Dr. Chu said the technology is still relatively new, and even 15 years after the Orbscan was first introduced clinicians are still trying to understand it. "People debated its worth years ago, and now we're debating the level of accuracy we need to measure the posterior corneal surface," he said. "Everyone is trying to avoid ectasia, and we're all trying to find the one magic diagnostic tool that will do that for us." When all the various tomographers and topographers match, it's an ideal situation; a mild discrepancy needs to be discussed with the patient before moving forward; a substantial discrepancy or abnormal test "will make us lean more toward surface ablation," he said. "If there are critical measurements that are abnormal, we will advise against surgery altogether."

With the Pentacam, any of the five readings that is more than two standard deviations away from the normal population is flagged yellow; three standard deviations away is flagged red. Virtually "no one is going to be comfortable proceeding with surgery" if the final reading is red flagged, Dr. Probst said. Since the Pentacam was introduced a few years ago, particularly with the Belin/Ambrosio Display (BAD), Dr. Probst noticed a "substantial increase" in the number of PRK procedures performed industry-wide and a "measurable increase in the non-candidacy rate of patients for LASIK," both of which he attributes to a more conservative approach based on the Pentacam results. While PRK is a "fine procedure," it can be more technically difficult for the surgeon and tougher on the patient, Dr. Probst added. "It's not a nice, slick procedure like LASIK is. There has been a subset of patients who have undergone PRK simply because of a sub-D Pentacam flag who would have been just fine with LASIK." Dr. Probst noted the latest software version of the Pentacam (BAD III) eliminates the various flags on the sub-D readings to provide clinicians with a flagging of the final D value in hopes of eradicating the uncertainty the various sub-readings may have caused; this is, in fact, how the BAD analysis was originally intended to be used.

Eye rubbing may be another reason for varied readings from the diagnostic exams, Dr. Thompson said. "Our understanding of the significance of eye rubbing has increased tremendously over the past 20 years. I ask all patients if they rub their eyes and if they do, a subtle diagnostic sign that may have not bothered me becomes a reason to recommend against surgery."

Anterior base membrane dystrophy can also cause abnormal-looking topographies but result in a normal tomography, Dr. Chu said. "We're still learning so much about the posterior corneal surface."

Editors' note: Dr. Buznego has financial interests with Allergan (Irvine, Calif.) and Bausch + Lomb. The other physicians have no financial interests related to this article.

Contact information

Buznego: cbuznego19@gmail.com
Chu: yrchu@chuvision.com
Devgan: devgan@gmail.com
Probst: 708-562-4682
Thompson: vance.thompson@vancethompsonvision.com