Wound construction, IOL calculations, lens selection among variables in special population
Reviewed by Divya Srikumaran, MD
Cataract surgery in patients with keratoconus requires special considerations with respect to wound construction, IOL calculations, and the choice of implant.
Baltimore-Good outcomes are possible with cataract surgery in patients with keratoconus-even without corneal transplantation.
Keratoconus is a bilateral non-inflammatory ectasia characterized by progressive thinning and steepening of the cornea and development of high irregular corneal astigmatism. Treatment depends on disease severity and ranges from glasses and contact lenses to corneal transplantation, according to Divya Srikumaran, MD.
Patients with keratoconus typically have the disease stabilize later in life. Yet, many of these patients subsequently develop visual disability from age-related cataracts.
“Cataract surgery in these patients requires special considerations with respect to wound construction, IOL calculations, and the choice of the implant you will place in these eyes,” said Dr. Srikumaran, assistant professor of ophthalmology in the division of cornea, cataract, and external diseases, Wilmer Eye Institute, Johns Hopkins School of Medicine, Baltimore, and medical director, Wilmer at Odenton.
Wound construction is another variable that should be carefully considered and chosen in patients with keratoconus, Dr. Srikumaran said.
“These corneas are very steep and thin,” Dr. Srikumaran said. “Wound healing may not be the same, and the ability to make a self-sealing, clear corneal incision may not be the same as in a patient without keratoconus so it is safer to suture these wounds.”
Alternatively, some surgeons advocate using a modified scleral tunnel incision in these patients, where entry is made through the sclera with a small limited conjunctival peritomy, to ensure a more stable and water-tight incision than may be created in a thin corneal incision, she explained.
Lens power calculations are more challenging in patients with keratoconus, Dr. Srikumaran continued.
“Most of the formulas we have are based on ideal eyes,” she said. “The steep corneas and the high astigmatism make it difficult to obtain accurate and consistent keratometry readings on these patients.”
In addition, eyes with keratoconus tend to have higher axial lengths and deeper anterior chambers, making the effective lens position (ELP) different compared with a normal eye, she added.
The first-generation IOL power calculation formula, the SRK, is reasonably accurate for average axial lengths and normal corneal curvature.
“The second-generation formulas were developed to try to modify the formula with a fudge factor for longer and shorter eyes,” Dr. Srikumaran said.
Third-generation formulas use axial length and keratometry to try to predict ELP and improve refractive outcomes. The fourth-generation formulas use the measured anterior chamber depth and additional variables to predict even more accurately the ELP postoperatively and thus improve refractive outcomes.
In 2007, researchers from Wills Eye Institute compared SRK, SRK II, and SRKT with first-, second- and third-generation formulas. They found that the SRK II was the most accurate in achieving the desired postoperative refraction.
“We were surprised to see that SRKT, a more advanced formula and one that has been shown to be better in eyes with myopia, would not perform better than the SRK II,” Dr. Srikumaran said. This may be affected by a small sample size and variability in the stages of keratoconus.
Monofocal lenses should be used in all these patients, Dr. Srikumaran continued.
“These patients will already have high astigmatism resulting in higher-order aberrations,” she said. “If you place a multifocal lens you may worsen the quality of vision by inducing even more aberrations or reducing contract sensitivity.”
The new toric lenses have not been FDA approved in these patients, but there has been a lot of interest in this patient population. Some recent limited case studies that suggest that improved uncorrected visual acuity may result from these lenses in these patients. Additional studies would need to determine the ideal candidates for this.
“But in a patient in whom contact lenses will be required postoperatively, or in whom there is any consideration of needing contact lenses, a toric lens should not be placed,” Dr. Srikumaran concluded.
Divya Srikumaran, MD
Dr. Srikumaran did not indicate any financial interest in the subject matter. This article was adapted from Dr. Srikumaran’s presentation at the 25th annual Current Concepts in Ophthalmology meeting, held in association with Wilmer Eye Institute and Ophthalmology Times.