For more than a half a century since Sir Harold Ridley implanted the first intraocular lens (IOL), many have worked hard to improve and perfect the calculation of the correct power of the implant. In 1974, when Hoffer began interest in this subject, he speculated on the use of IOLs in infants and children, and this endeavor was taken up by David Hiles in Pittsburgh. Over these past 40 years, IOL power formula authors and equipment manufacturers have pretty much left pediatric ophthalmologists to their own devices in obtaining the proper biometry and calculations for these children.
The problems encountered in selecting the proper IOL power for each child are 2-fold. The first is the collection of biometry data and choice of formula and the second is deciding whether to emmetropize the eye at this age or to place a power more appropriate for adulthood and correct the eye to emmetropia using spectacles or contact lenses. Hoffer has proposed the initial implantation of multiple IOLs or phakic IOLs to obtain emmetropia, with sequential removal as the child ages to maintain emmetropia.
There is a long list of difficulties in reporting the results of the accuracy of formulas in predicting pediatric IOL power. Obtaining a large series of cases is difficult, and therefore collaborative studies are used to increase the numbers. This leads to combining eyes that were measured with different biometry instruments and biometry techniques, combining bilateral eyes as well as different patient ages and IOL styles, surgical techniques, and IOL placements. This all leads to a corruption in the data collected and analyzed.
Obviously, collecting the data to evaluate the performance of various formulas is different than it is for adults. To do this you need the following:
1) Axial length: Measuring the axial length (AL) often requires examination under anesthesia, and a 2009 survey reported that 85% of measurements use the contact applanation A-scan method rather than the more accurate immersion technique. We know this introduces a shortening error on average of 0.25 mm in adults, and recent excellent studies by Trevidi and Wilson showed a 0.27 mm average shortening in pediatric eyes, as well. Why does the use of the contact method continue? There is the false impression that contact is easier than immersion. This is totally untrue, especially under anesthesia; and once pediatric ophthalmologists take the little time needed to learn how to do it properly, our pediatric population will be better off.
2) Keratometry: Measuring corneal power (K) in children under anesthesia is compromised by fixation problems.
3) Estimated lens position: Estimating estimated lens position (ELP) in small children is fraught with error, since the relationships of the anterior chamber structures are different in children, especially at different ages. The A-constants and ELP estimation algorithms of the modern theoretic formulas were based on adult eyes and do not take this into account. There is also the added factor that these relationships may be different after a posterior capsulotomy and anterior vitrectomy are performed, which is common in pediatric eyes.
4) Postoperative refraction: Since it is impossible to perform a normal subjective refraction on most of these children, the refractive result is determined by retinoscopy, usually under general anesthesia. This is fine for clinical evaluation of the status of the patient but less than optimal in reporting a scientific study of IOL formula prediction accuracy. Also, the timing of the refraction is important because the eye is growing all the time, thus changing the postoperative refraction.
The article in this issue of the Journal by Kekunnaya and associates used “standard applanation technique,” and “the SRK II formula was used to decide the IOL power.” They implanted “either rigid PMMA nonfoldable lenses or acrylic hydrophobic foldable lenses” and a posterior capsulotomy and anterior vitrectomy was performed in most cases. Forty-seven of the 135 eyes in the study (35%) were bilateral, compounding the data. It is clearly stated that the prediction errors were calculated from target refractions. This should have been determined a priori and written in the patient record.
In eyes with short AL and steep Ks, as in this study, the Hoffer Q and all theoretic formulas predict a higher IOL power than the SRK II regression; thus, the shorter the eye, the higher the difference with respect to SRK II. The prediction error should have been of opposite sign. When comparing the MAE of the formulas, the lens factor (A-constant, pACD) for each formula must be adjusted (using the Excel Query function) until the ME is equal to zero. Only when the ME of all the formulas tested is zero may the MAEs be compared. The authors made no mention in their Methods section whether this was done properly. Since the absolute errors are not a Gaussian distribution, it is the median absolute error (MedAE) that should be compared, not the MAEs.
It is disturbing to read a study, fraught with all these errors, report that the outdated SRK II regression formula yields superior prediction results compared to that of the modern theoretic formulas and recommend its use. Nihalani and VanderVeen recently reported that the Hoffer Q formula was superior in pediatric IOL power calculation. The Infant Aphakia Treatment Study (IATS) is a laudable effort to help sort out some of these issues; however, it is surprising to see that VanderVeen and associates chose to use not the Hoffer Q in the IATS collaborative study but rather a formula that has been shown not to be as accurate when the axial length is short.
Another issue is that the Hoffer Q formula is notorious for difficulties in programming it correctly. In 2001, Oshika and associates reported that errors in programming the formula led them to first publish that the Hoffer Q was the worst (by a large margin) in microphthalmic eyes and later that it was actually the best when the formula was programmed correctly. The authors did not report how it was programmed.
This April, the IOL Power Club devoted a significant portion of its meeting to evaluating ways to solve some of these issues for children requiring IOL implantation. We hope that in the future manufacturers will develop laser interferometry instruments that are able to be used on small children under anesthesia. Finally, we make a plea that the use of regression IOL power formulas be ended once and for all worldwide, and that scientific studies confirm the correct programming of formulas and report median absolute errors.