Avoiding cataract surprises after refractive surgery

By Ana Hidalgo-Simón MD, PhD

ALICANTE - It is complicated enough to calculate IOL power reliably and accurately in unoperated eyes, but what about eyes which underwent refractive surgery years ago?
Speaking at this year's closing lecture of the International Course on Refractive Surgery, Jack Holladay MD offered his advice on complicated IOL calculations and highlighted the difficulty of accurate measurement in patients who have previously undergone refractive surgery.

"Our problem is that the methods and instruments we use in regular patients are no longer valid. We are forced to use alternative methods and to use the instruments in different ways to achieve accurate measurements in these corneas," explained Dr Holladay, Clinical Professor of Ophthalmology at Baylor College of Medicine, Houston, USA.

When assessing patients who had refractive surgery in the past, Dr Holladay said the first thing to do is to get information on the viability and integrity of the cornea by performing an endothelial cell (EC) count. LASIK and other refractive surgery methods do not alter EC count, but radial keratotomy does.

"When you make an incision, the EC on the posterior surface of the cornea literally explodes, and around 10% of them are destroyed, even if you don't have microperforations. If you do, the EC loss increases to 15% or 20%, depending on the number of microperforations," he said.

Measuring aberrations
The direct ophthalmoscope can be used to measure corneal aberrations. Dr Holladay described it as a very inexpensive wavefront analyser, accurate to one eighth of a dioptre, and presented his step-by-step directions:
• Locate the ophthalmoscope at about 16 inches (40 cm) away from the eye
• With a dilated pupil, look at the red reflex. In a person without corneal aberrations the red reflex is uniform
• The aberrations can be seen as points or small areas with different brightness and shadows in different locations
• Now, move a few inches laterally and if the aberrations are located in the crystalline lens the irregularities will stay centred in the pupil. If they are located on the cornea, they would move to one side as you move laterally.

"With this simple method you can qualitatively assess aberrations up to an eighth of a dioptre, which is our present clinical accuracy, and differentiate if they are lenticular or corneal," he explained.

More conventional topography is extremely valuable to pick up irregularities in these patients. Many of them have aberrations, frequently related to their refractive surgery. When a cataract develops, the aberrations are at their worst. It is not unusual for patients to expect that when their cataracts are removed, their aberrations dissipate as well.

"Before removing cataracts in patients who had refractive surgery, it is essential to ask if they had halos, glare, or any kind of night-vision problems after refractive surgery. They frequently acknowledge it. You have to make it extremely clear that those are not going to disappear after cataract surgery, because they are not related to their cataracts," Dr Holladay warned.

In normal patients, calculations of corneal refractive power are performed knowing the anterior curvature of the cornea and assuming the index of refraction and back curvature. The problem is that the keratometers and topographers don't measure the back curvature of the cornea.

They rely on a mathematical assumption: the back curvature is 87% of the front curvature in a normal individual. That front-to-back relationship is altered after refractive surgery.

An additional problem is that keratometers generally measure a 3.2 mm in diameter from the centre of the cornea. Refractive surgery 'flattens' the cornea, which becomes steeper at the periphery. The result is that keratometers measure the peripheral points which are steeper after refractive surgery.

Sampling peripherally produces a stronger measurement than the true value after myopic surgery. It is just the opposite for hyperopic surgery, he stressed.
"Doug Koch MD and I have performed some experiments to assess the margin of error of those two problems. The assumption made about the back curvature constitutes a 12% error of underestimation of corneal power.

"The sampling error caused by measuring points at different angles following refractive surgery adds another 10% to 12%. Using traditional measurements in these patients, we underestimate the change in power versus the refractive change of the cornea by 22% to 25%," Dr Holladay explained.

He added that the solution to bypass these problems is to use alternative methods.
"You can calculate corneal power after refractive surgery using any of the following methods, in order of reliability: prior data method; hard contact lens (CL) trial method; corneal topography; automated keratometry; and manual keratometry," he said.

Prior refractive data
When corneal measurements before the patient had refractive surgery are available, you can generally assume they are accurate up to a quarter of dioptre.
It should be possible to subtract the original from the present values to obtain the refractive changes of that cornea. However, there are a couple of points to watch.

"We almost always overestimate the power of the cornea following myopic refractive surgery. With time, refractive power of the eye may change due to cataract progression or other reasons, so we need to measure post-refractive surgery power soon after the operation," Dr Holladay said.

The ideal time to take post-refractive surgery corneal measurements is between three and six months after the operation, he emphasised.
It is also worth asking the patient if they have kept the spectacles they used before refractive surgery was performed. If they have, and you can confirm that they didn't need spectacles after surgery, the change the refractive surgery effected can be measured.

Contact lens method
Very often, it is not possible to get hold of prior data. Patients may have moved or records may have become lost. The hard contact lens trial method can be very useful in these cases.

"If you have a cornea that is 44 D of power and I put a 44 D contact lens into your eye, you will have no change in your refraction. In other words, if your cornea's curvature is 7.5 mm and I put on a 7.5 mm contact lens, you should have the same refraction.
"On the other hand, if your cornea is 44 D and I put a 45 D contact lens in, you change from say 0 D to -1 D because the curvature of the lens is 1 D steeper than your cornea," he explained.

Knowing the visual refraction without the contact lens, and trying lenses of different powers, allows the clinician to deduce the power of the cornea. He recommended having four or five hard contact lenses made for this purpose, ranging from 30 D to 50 D.

But all these measurements can be questioned. Even with the more traditional methods, variation is likely. For example, corneal topography and automated and manual keratometry almost always exceed true power, resulting in hyperopic error.
"You are very likely to obtain different measurements with every technique you use. My recommendation is to measure with the keratometer, topographer, do the historical method and do the contact lens method. And then use the flattest result.

"After myopic refractive surgery, the error will almost always be on the hyperopic side. Use the lowest reliable value," Dr Holladay advised.
If the surgery performed was RK, Dr Holladay recommends a careful exploration of the incision lines. If they are filled with fibroblast and the scars appear white and well formed, you can relax. These lines will not change substantially after the cataract operation.

If, on the other hand, the incision lines are filled with endothelial cells, look pristine and clear without white and scarred lines, you should expect changes.
"With the swelling of the cornea during cataract surgery, these wounds created during RK are likely to re-open, and just like the day after an RK, the patient will have a hyperopic response. It will resolve in a week or 10 days," he warned.

The cornea flattens and moves toward against-the-rule astigmatism as we get older, so many corneas will continue to do so after the RK operation. RK weakens the structural integrity of the cornea, which accelerates the normal physiological changes.

The result is a flatter cornea and against-the-rule astigmatism. Both develop at a much faster rate and to a greater magnitude than normal ageing rates, he explained.
Dr Holladay suggested that one should always operate on these patients on the horizontal meridian, either temporal or nasal to minimise the against-the-rule astigmatism drift.

He also recommended aiming for a mild myopic result. The RK patient needs to be warned of the potential postoperative changes. In patients who had LASIK or PRK, this problem would not occur because these corneas are very stable.

Dr Holladay warned of patients with myopic staphyloma. Axial length is a common source of mistakes. In long eyes (> 26.5 mm), the difference in the "anatomic axial length" (anterior pole or corneal vertex to posterior pole) and the "optical axial length" (corneal vertex to macula) can be substantial. The optical axial length should measure the corneal vertex to the macula, as opposed to the anatomical axial length, which goes from the corneal vortex to deepest pole of the eye.

In patients with a staphyloma, the macula is usually not in the deepest part. You need to measure from the corneal vortex to the fovea; the patient needs to fix on a target while the measurement is being taken. Dr Holladay recommended the Zeiss Humphrey IOL Master, especially useful in extremely short or long eyes, where normal measuring devices are more likely to be inaccurate.

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