COMMENTARY

Keratoconus: Effect on Cataract Surgery Outcomes

Christopher J. Rapuano, MD

Disclosures

April 24, 2014

Viewpoint

Correct IOL power calculations depend primarily on accurate measurements of the corneal power and axial length. Keratoconus should not adversely affect axial length measurements. However, accurate corneal power measurements are quite challenging in eyes with irregular astigmatism, such as in keratoconus.

This inaccuracy is caused by multiple factors, including assumptions made by the keratometers for normal corneas that do not hold true for keratoconic corneas. In addition, keratometry is centered on the steepest part of the cornea -- but in keratoconus eyes, that may not be the center of the visual axis.

Of interest, the investigators were rather successful in their mean IOL power calculations for the eyes with mild to moderate keratoconus. However, the span of postoperative refractive error was quite large in these 2 groups, ranging from approximately +5 to -4 D. When the actual K values were used in the severe keratoconus eyes, the mean spherical equivalent error was almost +7 D, with a very large range of about 17 D. The results were somewhat better when a standard K value of 43.25 D was used, but there was still a big range, spanning +6 to -6 D.

At this time, we do not have an ideal method to evaluate corneal power in eyes with keratoconus. I suggest obtaining measurements with several different machines and comparing the results to select the best number. When it does not appear that keratometry measurements are anywhere near accurate, it is probably best to use a standard K reading in the range of 43-45 D.

Patients with keratoconus should be told that the accuracy of the IOL calculation is not as good as in eyes without keratoconus. Fortunately, this will not come as a surprise to most of them.

Patients with milder forms of keratoconus may be in glasses in which the spherical equivalent outcome is quite important. Patients with moderate to severe keratoconus are more likely be in soft, rigid gas-permeable, or even scleral lenses. Although the accuracy of the IOL power calculation is not as good in many of these eyes, fortunately for them, higher degrees of postoperative refractive error can usually be corrected in the contact or scleral lens.

Several practical issues are important to consider when performing cataract surgery in eyes with keratoconus. If an eye has severe keratoconus and the patient is considering a corneal transplant in the future, then it is probably better to use a standard K reading in the range of the corneal surgeon's average K reading (usually 43-46 D) instead of the patient's actual measured K reading. Furthermore, if the patient or surgeon is considering a rigid gas-permeable contact lens or a corneal transplant in the future, a toric IOL is probably not a good choice.

Finally, corneas with keratoconus act differently from normal corneas in that they tend to be thinner and floppier, leading to a higher risk for leakage from clear corneal cataract incisions. Surgeons should have a low threshold for suturing clear corneal cataract incisions in eyes with keratoconus.

Abstract

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