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Patients with keratoconus pose unique challenges with IOL calculations.
Reviewed by Douglas D. Koch, MD
Houston-Eyes with keratoconus present numerous challenges for cataract surgeons. Among these are stability, the potential need for crosslinking, and the long-term visual needs of patients with respect to spectacle or contact lens use, said Douglas D. Koch, MD.
“Determining the IOL power and then managing the excessive postoperative refractive error of keratoconic eyes is a major challenge,” said Dr. Koch, professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston. “If the eye is unstable, it may be best to crosslink and wait 3 to 6 months before proceeding with surgery.”
It is critical to try to determine if the patient’s cornea is stable or still changing and if corneal transplantation may be needed in the future, Dr. Koch said.
“If patients are wearing glasses currently and are expected to wear glasses, there is no anticipated need for contact lenses or a corneal transplant,” he said. “We want to be as accurate as possible in calculating IOL powers.”
In one case example, a 59-year-old male presented for cataract surgery. (Figure 1A)
Dr. Koch used the Holladay formula on two different devices (IOLMaster and Lenstar), and aimed for a bit of myopia. (Figure 1B)
This particular patient ended up hyperopic (0.91 with one device, and 1.12 with the other), which, is unfortunately very common, he said.
“I have found that these eyes end up much more hyperopic than calculations would suggest, sometimes by over 2 D," he said.
Dr. Koch believes this is due to:
1) an inability to determine the true refractive power of the anterior cornea due to its irregularity, and
2) the posterior cornea is steeper than anticipated, reducing effective corneal refractive power.
Preliminary data indicate that the steeper the cornea, the greater the hyperopic prediction error.
“As a result, I typically aim for -0.75 for mild keratoconus and -1.75 or more for advanced cases,” Dr. Koch said.
If surgeons are going to consider a toric IOL in this patient population, Dr. Koch suggests several criteria: 1) stability documented by topography or spectacle history, 2) fairly uniform and symmetrical topography in the central 3 mm, 3) close alignment with topographic and refractive astigmatism, and 4) desire for good uncorrected vision and will not be wearing contact lenses postoperatively.
“It is especially important to verify that the patient accepts or has actually has been wearing essentially the same astigmatic correction in glasses as is seen in topography,” he said.
As an example, Dr. Koch discussed a male patient who only wore glasses and liked the astigmatic correction. (Figure 2)
The cornea was stable, although readings with the IOLMaster showed increasing astigmatism along the same meridian. IOL power calculations suggested a 6.5 or 7 D, but Dr. Koch aimed for about -1 D, he said.
Because the patient had a long eye, Dr. Koch used the Wang Koch axial length modification to reduce the risk of a “hyperopic surprise.”
The majority of the astigmatism was along the 112 to 115 meridian, and it was consistent with biometry, topography, and refraction.
“You need to try to determine how much astigmatism you’re going to correct,” he said. "I deliberately try to undercorrect the astigmatism in these eys due to measurement uncertainty and the risk of inducing aberrations with misalignments."
Dr. Koch ultimately chose a 7 D T5 lens with a targeted -1 D. Postoperatively, the patient was plano +2.25 at 103, “so he still has 2.25 D of astigmatism left,” Dr. Koch said, but “the patient was ecstatic.”
What the case shows, however, is that Dr. Koch “didn’t aim for enough myopia and that it would have required a T9 to correct all of the cylinder.”
Patients with very high K values are likely to be in contact lenses, and might be future candidates for penetrating keratoplasty or lamellar keratoplasty, he said.
For patients who might need keratoplasty in the future, Dr. Koch said he will use an arbitrary K reading that would be anticipated after a keratoplasty, knowing the patient will be wearing a contact lens to correct for the residual refractive error.
Most keratoconic eyes have high negative asphericity due to the central steepness, he said.
“Consider an IOL with zero asphericity or even positive asphericity,” Dr. Koch said. “An IOL with zero asphericity may be preferable, because it’s not going to induce disparity between the alignment of the asphericity of the IOL and the cornea.”
Intraoperative aberrometry remains a wild card, he said.
It may be premature to definitively determine its optimal use, but the technology holds promise as an aid in confirming toricity and spherical power choices, Dr. Koch noted.
Down the road, using such technologies as the Light Adjustable Lens (Calhoun Vision) or the femtosecond laser to customize the IOL power l“might be a wonderful option for these very complex eyes, he said.
However, accurately delivering laser energy through these highly distorted corneas could prove difficult, Dr. Koch noted.
“Sometimes these eyes are even tough to YAG,” he said. “These are complex eyes that will continue to raise interesting challenges in our practice.”
Douglas D. Koch, MD
This article was adapted from Dr. Koch’s presentation at Cornea Subspecialty Day during the 2015 meeting of the American Academy of Ophthalmology. Dr. Koch declared financial interests with i-Optics, PerfectLens, and Ziemer.