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Latest generation formulas yield more accurate refractive outcomes in small adult eyes
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Different IOL power calculation formulas yielded varying refractive outcomes after cataract surgery in small adult eyes, according to a study.
“Eyes with short axial length achieve worse refractive outcomes than ‘normal’ eyes,” Gianluca Carifi, MD, the corresponding author, told Ocular Surgery News. “Only 43% and 64% of eyes achieved a final refractive result within 0.5 D and 1 D of target, respectively.”
Existing IOL power prediction formulas are unreliable in determining effective lens position, Carifi said.
“We can say that the available formulas still struggle to predict with accuracy the effective lens position, and it is also to be highlighted that the IOL tolerance from the nominal lens power is 1 D for highly powered lenses,” he said. “This means that an IOL labeled as 35 D could have an effective dioptric power ranging anywhere from 34.01 D to 35.99 D. This might certainly create problems for surgeons in trying to optimize the formula used and researchers who try to develop formulas that work accurately in these eyes.”
The fourth-generation Holladay 2 formula and the third-generation Hoffer Q formula are recommended in eyes with an axial length shorter than 22 mm, the authors said in the study, which was published in the American Journal of Ophthalmology.
The retrospective study included 28 small eyes of adults who underwent phacoemulsification with implantation of an AcrySof SA60AT IOL (Alcon) in the capsular bag. IOL power ranged from 35 D to 40 D.
Mean preoperative spherical equivalent was +8.53 D, mean axial length was 19.86 mm, and mean anterior chamber depth was 2.56 mm. The mean refractive target was –0.44 D.
IOLMaster software (Carl Zeiss Meditec) was used to calculate required IOL power with the Hoffer Q, Holladay 1, Holladay 2, Haigis, SRK-T and SRK-II formulas.
Results and conclusions
All formulas generated mean absolute errors of about 1 D, but the Holladay 2 formula yielded the most accurate mean numerical error and mean absolute error.
“The Holladay 2 formula was the most accurate in our case series,” Carifi said. “However, three latest generation formulas — Holladay 2, Haigis and Hoffer Q formulas — all showed similar results: no statistically significant difference.”
The SRK formulas were the least accurate.
“The old SRK-II formula was by far the worst, as expected, and was investigated as a control,” Carifi said. “In addition, as recommended by the vast majority of colleagues, our study confirmed that the SRK-T formula should not be used for IOL prediction in small eyes.”
With the Hoffer Q formula, mean numerical error was 0.22 D and mean absolute error was 0.95 D.
Also with the Hoffer Q formula, mean postoperative refraction was within 0.5 D of target in 11 eyes (39%), within 1 D of target in 17 eyes (61%) and within 2 D of target in 25 eyes (89%).
Mean postoperative refraction was within 0.5 D of emmetropia in 12 eyes (43%) and within 1 D of emmetropia in 18 eyes (64%).
With the SRK-T formula, final refraction was within 0.5 D of target in six eyes (21%).
Until more data are available, surgeons should calculate required IOL power with the Hoffer Q, Haigis and Holladay 2 formulas and choose the most suitable result, the study authors said. – by Matt Hasson
Reference:
Carifi G, et al. Am J Ophthalmol. 2015;doi:10.1016/j.ajo.2014.11.036.For more information:
Gianluca Carifi, MD, can be reached at Moorfields Eye Hospital, 162 City Road, London EC1V, United Kingdom; email: gianlucacarifi@gmail.com.Disclosure: Carifi reports no relevant financial disclosures.