Deciphering the IOL Surprise
What to do when you miss your target refraction
James Gills, M.D. and Myra N. Cherchio, C.O.M.T.

Unfortunately, even with today's advanced techniques and technology, we haven't reached the point at which all IOL outcomes are ideal. When an unfortunate outcome does occur, it's a disappointment for everybody involved -- and managing it can be the most difficult part of cataract surgery.

In these situations it's crucial to know exactly what went wrong so that you can correct the problem and take steps to avoid it in future cases. Here, we'd like to share some of what our practice has learned over the years about tracking down the source of the problem, and the steps we take to correct it.

Tracking down the cause

In the past, when a result was unsatisfactory, we often assumed that the problem was caused by inaccurate ultrasound measurements. And we may have been right; the ultrasound equipment we used in the past had many limitations. But today, blaming unsatisfactory outcomes on faulty ultrasound measurements may only serve to cover up other problems that need to be addressed in order to correct the discrepancy.

In our practice, we're finding more and more that corneal issues are responsible for IOL surprises. Knowing that this may be the case is crucial. For one thing, corneal changes caused by the surgery often resolve if left alone. If this happens, any corrections you've made shortly after surgery will backfire.

Locating the real cause of the imperfect outcome is essential for other reasons as well. If this is the patient's first eye and the problem is caused by biological variability, the same problem could happen with the fellow eye; you need to adjust your treatment of the fellow eye accordingly. And of course, if the problem really is the result of an error or oversight, knowing this will help you avoid the same problem in the future.

Start with the simple things

When evaluating a disappointing outcome, we begin by looking at the simple things. The first steps we take include:

• Verifying the postoperative refraction. (Our optometrist or a trusted technician does this.)
• Verifying the data entry. We use Holladay II Consultant software to calculate the IOL power. This software is easy to use, but it requires a lot of data entry; it's easy to make mistakes.
• Making sure the correct lens was selected.
• Checking pre-op measurements. Was there a big discrepancy between the keratometry and topography readings? If you used ultrasound, check the validity of the original A-scan. Were the gates positioned correctly? Was the correct velocity used? Are the echoes straight and sharply rising?

If it's clear that none of these issues caused the problem, we examine the cornea and look for unexpected changes.

Evaluate the cornea

Unexpected changes in corneal curvature, or the K average, are fairly common after surgery, and this can contribute to the refraction puzzle. For this reason, we evaluate each post-op surprise with corneal topography. (Note: We use manual K-readings in this situation because the IOLMaster can have the same limitations as all automated keratometry instruments -- they're more likely to make a mistake than a careful individual.) Whether or not topography was done prior to surgery, this may reveal changes in the corneal curvature as a result of surgery, or an undiscovered condition. (This sometimes solves the mystery before we even begin the process of remeasuring.)

We also check the keratometry measurements again and compare to our preoperative ones. If the cornea is slightly flatter (the most common scenario following surgery), that may explain a hyperopic shift. If the cornea is steeper, that may account for a myopic shift.

Changes in corneal curvature can be the result of a number of different factors, including:

Relaxing incisions during surgery. This is a common cause of unexpected corneal change. If you perform relaxing incisions during cataract surgery you should know the coupling ratio (the ratio of corneal steepening to flattening caused by the incision). If it's not 1:1, you may need to adjust the lens power slightly to account for it.

In our experience, limbal relaxing incisions usually cause the cornea to flatten slightly after surgery. Before toric lenses were available, when limbal relaxing incisions were our primary means of correcting astigmatism, we added power to the lens based on the amount of astigmatism we were correcting.

Post-refractive surgery patients. These patients can be challenging -- especially those who've had radial keratotomy (RK). Even if you've done your calculations correctly, temporary corneal flattening from microedema around the incisions is common. This typically re-solves within a few weeks with the help of sodium chloride ointment or drops.

Irregular astigmatism and keratoconus. Keratoconus patients often experience a post-op hyperopic shift; their corneas are thin and fragile and tend to flatten after surgery. Left alone this often resolves, just as it often does for RK patients. These patients can be especially challenging to evaluate postoperatively because a slight head tilt can alter the refraction by effectively changing the refractive power of the cornea. If the axial length is correct, the best medicine is usually time; the refraction may be entirely different after 1 or 2 weeks.

Corneal edema. If you perform next-day cataract surgery (as we do) you may find corneal edema on the second day. This can alter corneal curvature and affect the post-op refraction. If you're planning surgery on the fellow eye, use keratometry or topography to measure how much impact the edema is having. The point is that the edema will resolve; don't factor any refractive shift when selecting the lens for the fellow eye.

Contact lens wear prior to surgery. This can lead to error if the patient didn't stop wearing the lenses long enough to allow the cornea to return to its natural shape. Check to find out how long he had them out before the pre-op measurements were taken.

Some practices take serial pre-op topographies after the patient stops wearing contacts until the scans show that the cornea has stabilized. Our policy is to have the patient go without hard lenses for 2 weeks, or soft lenses for 1 week. (Some patients find this difficult to do, but it's very important.)

Check the measurements

After evaluating the cornea, we remeasure the patient's eyes with both our IOLMaster and immersion to determine whether measurement inaccuracies contributed to missing our refractive target:

The IOLMaster is an excellent starting point for comparing the axial lengths pre- and postoperatively, but it can't be used to measure anterior chamber depth postoperatively. Also, we've found that IOLMaster readings following surgery are typically 0.1 mm short, even when corrected for the lens material. Be sure to take this into consideration when comparing measurements to pre-op.

Note: When using the IOLMaster, don't just look at the numbers -- pay close attention to the peaks! The IOLMaster occasionally gives erroneous axial length measurements when it detects a second peak. This is most common when measuring eyes with acrylic lenses. You'll need to use ultrasound if this appears to be an issue.

It's important for your staff to understand how to use immersion to measure pseudophakic patients. To get the most accurate results, we use the aphakic mode and correct the tissue velocity afterward. This is especially important when measuring a patient with a silicone lens, because not all silicone lenses are made equal. Subtracting 0.8 mm from the result (the standard adjustment that most instruments are programmed to make) may not yield the correct result; the correction factor of silicone lenses can vary from 0.3 to 0.8.

If you don't know the correction factor of the lens you're working with, call your lens company. The correction factor can also be calculated if you know the center thickness of the lens, using the formula (center thickness)(1 - 1532/1052).

In the aphakic mode, you'll have two calipers or gates to measure from the cornea to the retina. After measuring the axial length, move the retinal gate to the first lens echo to measure the effective lens position.

Note: The IOLMaster is an integral part of our biometry regime, but we have not found it to be a stand-alone instrument. It doesn't replace ultrasound. Acquiring these measurements is a complex process, and we haven't yet reached the point at which human judgment can be completely removed from the equation. Of course, because the IOLMaster is a non-contact form of biometry, it's more accurate than applanation ultrasound -- it can't compress the cornea and give a falsely short measurement. However, we've found that immersion provides greater accuracy with regard to pre-op anterior chamber depth. Also, the IOLMaster doesn't measure lens thickness, while ultrasound does.

The IOLMaster is most useful in eyes with staphyloma, which can be difficult to measure using ultrasound. However, because these eyes are so unusual, we prefer to document our measurements with B-scan as well. With the B-scan, we can actually image the staphyloma and "see" what we're measuring.

Check lens position

If the patient's refraction deviates from the target by only -0.5 D to -1.0 D, the lens power may not be the problem. In this situation, dilate the patient and make sure the lens is positioned correctly:

• Is the lens completely in the bag? A haptic out of the bag can alter lens position enough to cause a small myopic error.
• Effective lens position can also be affected by the way the bag heals around the new lens. As the bag fibroses around the lens, it may bow forward or back. (Measuring the lens position should reveal this.) If the change in position occurs gradually over a period of months, this may be the problem.

Once we've checked these factors, we also consider the possibility that the outcome is the result of multiple small errors whose effects are cumulative.

No matter what we decide, we always document our conclusions about the cause of the surprise (i.e., lens shift, incorrect measurement, corneal edema, or no obvious explanation) in the patient record.

Correcting a small discrepancy

Because many IOL problems are potentially cornea-related, a small error is often best treated by waiting a week or two to see if it will resolve. For example:

• A slight hyperopic error caused by a flatter cornea after surgery isn't unusual. The patient typically does well with a little time and sodium chloride drops or ointment.
• Edema problems and refractive shifts associated with surgery on a keratoconus patient usually resolve by themselves over time. However, if you're working with a keratoconus patient and you still have a problem after allowing time for the cornea to return to its original shape, you may need to exchange the IOL or fit the patient with a hard contact lens.
• If you can't find any explanation for a patient's outcome, it's a good idea to wait a week or two before taking action.

If we know the cause of the poor outcome, a specific remedy may make the most sense:

• If the IOL isn't positioned correctly because a haptic is out of the bag, we reposition the lens as an in-office procedure.
• If the healing process causes the lens to bow forward or back, we either laser the capsule to release tension or simply leave it alone. (If the patient is significantly bothered by the gradual change in vision, and lasering the capsule doesn't improve matters, we add a piggyback lens.)

Correcting a large discrepancy

Regardless of the reason for the outcome, if we're left with a large error that must be addressed, we have to decide whether to exchange the lens or piggyback another lens over the first. Each option has advantages and drawbacks:

Lens exchange. We usually resort to lens exchange in three circumstances (assuming that the discrepancy hasn't been caused by a biological factor that will resolve on its own):

1. The patient is still less than 1 month post-op and the capsule hasn't cemented itself around the implanted lens yet. (A new lens is preferable to having two lenses inside the eye.)

2. We find marks on the lens or damage that warrants replacement.

3. The lens is positioned incorrectly. In this situation, adding a piggyback lens won't solve the problem. Repositioning or exchange are the only workable options.

Secondary piggyback implantation. With the current availability of low plus- and minus-power lenses in silicone, a small pseudophakic refractive error can now be corrected with a secondary piggyback lens through the original incision.

Secondary piggybacks have the advantage that they can be more predictable than lens exchange, for several reasons:

• First, you can never be 100% sure of the power of the original lens. The lens might have been mislabeled, or the actual power may be outside of tolerance.
• Second, you can't be 100% confident that the exchanged lens will end up in the same position as the old lens. A small change in lens position can induce its own error.
• Third, the guesswork of hitting the target refraction is virtually eliminated with a secondary piggyback lens because the power is primarily based on the patient's refraction. In other words, with piggyback implantation, the cause of the error is irrelevant (unless incorrect lens position was the problem).

The road to ideal outcomes

The potential for "perfect" outcomes following cataract surgery isn't an illusion. For the first time ever, thanks to continuing improvements in measurement technology, IOL materials and design, and surgical technique, we've reached a level of accuracy at which we could produce better outcomes if we had lenses available in quarter-diopter steps.

Still, no matter how advanced IOL technology becomes or how great our expertise, we'll always have to deal with the occasional unsatisfactory outcome. But as we become more adept at determining causes, we'll be able to reduce the number of problems that occur and increase the likelihood that they can easily be corrected. That will make our job easier -- and our patients happier.

Dr. Gills is the founder and director of St. Luke's Cataract & Laser Institute and clinical professor of ophthalmology at the University of South Florida. Dr. Gills was ASCRS' Innovator of the Year in 1996. He sits on the Duke Board of Visitors and the Johns Hopkins Wilmer Advisory Board, and is the author of seven medical books. Myra Cherchio is director of clinical research and director of the ultrasound department at St. Luke's. She's co-authored numerous scientific papers and medical books, and has produced several educational videos.

Discussion: This case makes two valuable points: First, when re-evaluating a patient's measurements postoperatively, corneal topography can provide critical information, even if it wasn't measured before surgery. Second, when a corneal problem is identified as the cause, it should either be left alone to resolve or treated as necessary before addressing the postoperative refractive error.

Post-op corneal topography (top) uncovered a flat central region of the cornea that had failed to show up in the K-reading. This flat area gradually steepened during the following weeks (bottom).

 

Case History 3: A Corneal Mystery

This patient had a very bizarre outcome that we couldn't explain at first, even when we rechecked all our initial measurements. Those measurements were: Refraction (OD): +3.00 - 0.75 x 135 Axial length (OD): 23.69 Keratometry (OD): 40.00/40.37 x 180 The target refraction was -0.50. We performed uneventful phaco and inserted a 25.0 D PMMA lens through a 6.0 mm scleral incision. The following day, the patient's refraction was +8.50 -1.0 x 040. Axial length and keratometry matched pre-op values. We decided not to attempt to correct the refraction until we uncovered the cause of this strange outcome, and we proceeded to perform corneal topography. This turned up a flat, central region in the cornea that hadn't showed up in the initial K reading, which accounted for the hyperopia. (See images, right.) We had not done topography preoperatively, so we didn't have a map for comparison. Because the problem was corneal, we elected to wait and see if the problem would resolve by itself (as corneal problems often do, even when we don't fully understand the genesis of the problem). By the following day, the patient's refraction had shifted dramatically, to +2.50 -2.00 x 110. Even though serial measurement of the patient's corneal topography showed minimal change in the corneal curvature during the following weeks, the central flat area gradually steepened, causing a gradual myopic shift. At her final visit, she was at her target refraction.