Reducing the Risk of Refractive Surprise in Hyperopic Patients

Dermot McGrath reports

Prevention is better than cure when it comes to reducing the risk of a refractive surprise after IOL implantation in hyperopic patients, according to Nino Hirnschall MD, PhD, FEBO.

“The reality is that short eyes requiring a higher IOL power are at high risk of a refractive surprise. We need to explain this to the patient before surgery, and especially when we are implanting a multifocal or toric lens, as they will not be happy afterwards if the refractive target is not met,” he advised.

Although data from the EUREQUO database in 2019 of 171,930 cataract extractions showed an average spherical equivalent error of -0.04 D with 74% of patients within 0.5 D of target refraction, Dr Hirnschall said such refractive accuracy does not typically extend to short eyes (axial length less than 22 mm).

“If we look at the study from Aristodemou [et al]i , it turns out that of the subset of patients with an axial length of 20 mm to 22 mm, not even half were within 0.5 D of target refraction. For very short eyes, less than 20 mm, maybe 30% are within 0.50 D. So, we are not very good in these cases, and there is clearly scope for improvement,” he said.

While there are multiple potential sources of error in IOL power calculations, Dr Hirnschall cited a study by Norrby et alii that showed preoperative estimation of postoperative intraocular lens (IOL) position, postoperative refraction determination, and preoperative axial length (AL) measurement are the largest contributors of error (35%, 27%, and 17%, respectively) with a mean absolute error (MAE) of 0.6 D for an eye of average dimensions.

“Although axial eye length measurements are quite good nowadays, we need to keep in mind that an error in axial length measurement of 1.0 mm in normal eyes means a refractive surprise of 2.5 D, whereas a short eye more than 1.0 mm equates to a 4.0 D to 6.0 D refractive error,” he said.

A key part of the problem lies in the false assumption that the anterior chamber depth in short eyes is usually flat, Dr Hirnschall explained.

“Studies by Rafik [et al] and Erb-Eigner [et al] have shown short eyes can in some cases have a regular anterior chamber depth. This does not fit our IOL power calculations because all the formulae used include some ratio between axial eye length and anterior chamber depth. The second big problem is that with short eyes, we are using a higher-powered IOL, and this has a greater impact,” he said.

In terms of the best IOL power calculation formulae to use, Dr Hirnschall said that while popular formulae such as Hoffer Q, Barrett Universal, and Hill-RBF Calculator are acceptable for normal eyes, other formulae showed a better prediction for short eyes.

“Studies have shown modern formulae such as Castrop-Rauxel, Kane, OKULIX (ray tracing), and EVO 2.0 perform better in short eyes,” he said.

Other tips to improve outcome accuracy include optimising IOL constants with online tools such as Ocusoftiii and IOL Coniv and learning from the first eye surgery to correct for the second eye, Dr Hirnschall concluded.

“The reality is that short eyes requiring a higher IOL power are at high risk of a refractive surprise.”

This presentation was made at the 39th Congress of the ESCRS in Amsterdam.

i Journal of Cataract & Refractive Surgery, 2011; 37: 63–71. ii Journal of Cataract & Refractive Surgery, 2008 Mar; 34(3): 368–76. iii ocusoft.de/ulib iv https://iolcon.org

Dr Hirnschall is an ophthalmologist at the Kepler University Clinic in Linz, Austria. nino.hirnschall@gmail.com