Cataract Surgery in Eyes with Keratoconus

Using several devices for obtaining preoperative measurements and several keratoconus-specific formulas for IOL power calculation will help optimise refractive outcomes after cataract surgery in the challenging group of eyes with keratoconus, according to Adi Abulafia MD.

“Nevertheless, patient counselling is critical,” he said. “Importantly, therefore, underpromise and overdeliver.”

Prediction errors common

Whereas at least 80% of normal eyes generally achieve refractive outcome ±0.5 D of target, the prediction accuracy is dramatically lower when standard formulas calculate IOL power for eyes with keratoconus.

“When standard IOL power calculation formulas are used, eyes with keratoconus typically end up with a hyperopic error that increases with worsening of keratoconus stage,” Dr Abulafia said.

The challenges to achieving predictable refractive outcomes in eyes with keratoconus include difficulty obtaining accurate corneal refractive power due to corneal curvature asymmetry. In addition, tear film irregularities in eyes with keratoconus limit measurement repeatability. Furthermore, a change in the ratio between the anterior and posterior corneal surfaces in eyes with keratoconus invalidates the use of the standard keratometric index.

“There are technologies for measuring the posterior cornea, but they are not perfect,” Dr Abulafia said. “Difficulty in predicting the effective lens position and obtaining a reliable subjective refraction in the multifocal corneal are other challenges.”

Achieving better accuracy

Currently, three keratoconus-adjusted formulas are available to calculate IOL power for monofocal lenses—the Holladay consulting software, Kane keratoconus, and Barrett True K for keratoconus, which is unique in its use of posterior cornea values. In cases of a planned implantation of a toric IOL, surgeons should use an IOL calculator developed specifically for toric IOL calculations in eyes with keratoconus; these options include the Kane and the Barrett True-K keratoconus toric calculators.

He noted while controversy remains about using toric IOLs to correct astigmatism in eyes with keratoconus, they can be suitable for select patients.

“The best candidates might be patients with mild to moderate stable keratoconus, no central corneal scarring, satisfactory vision with glasses prior to cataract development, and are contact lens intolerant or want not to wear hard or scleral contact lenses after surgery,” Dr Abulafia said.

Outlining his approach to toric IOL calculation in eyes with keratoconus, Dr Abulafia said he inputs the astigmatism data obtained from several measuring devices into the Barrett True K toric calculator to generate an integrated K value. Cylinder correction and axis are determined by entering the integrated K and measured posterior cornea data into the keratoconus-specific Barrett True K toric calculator.

Then he decides on IOL power by considering the spherical equivalent prediction range generated by the Barrett True K with the measured or predicted posterior cornea, the SRK/T, and Kane keratoconus formulas.

Dr Abulafia spoke during Cornea Day of the 2023 ESCRS Congress in Vienna.

Adi Abulafia MD is the director of cataract surgery at Shaare Zedek Medical Center, Jerusalem, Israel. adi.abulafia@gmail.com