Glaucoma is not infrequent in children undergoing early cataract surgery, with long-term rates of up to approximately 30 % [6, 25]. Elevated intraocular pressure in infantile eyes commonly results in anatomic changes that may affect corneal curvature or result in increased axial elongation and myopic shift; many studies of ocular growth thus exclude eyes with recognized ocular hypertension or glaucoma.

Finally, some studies have demonstrated associations between axial elongation and deprivation amblyopia or vision. In patients with unilateral cataract, a smaller preoperative interocular axial length difference at surgery (suggesting less stimulus deprivation induced axial elongation) may be predictive of good post-operative visual acuity [26]. A correlation between higher rates of refractive growth and poor final visual acuity, particularly for unilateral cases, has been demonstrated in children with cataract surgery before 1 year of age [27]. Finally, for children undergoing unilateral cataract surgery between 2 and 6 years of age, no difference in final visual acuity or myopic shift was found by aiming for emmetropia vs. leaving residual hyperopia (≥2 diopters) [27].

Limited cooperation and density of the cataract in young infants and children usually prevent optical coherence measurements of keratometry and axial length. Ultrasound biometry is typically performed with the child under general anesthesia, at the time of cataract surgery, so portable equipment for keratometry and axial length measures must be available. While contact and immersion ultrasound technique for axial length measurements often give similar accuracy with regard to expected post-operative refraction [28, 29], when there is a significant discrepancy, immersion ultrasound is more predictable [30].

Understanding typical performance of IOL calculation formulas in the pediatric population is helpful. Prediction error, which represents the difference between the predicted and actual post-operative refraction (obtained within a few weeks after surgery), is used to compare accuracy of formulas and early refractive outcomes. Commonly available formulas include the Sanders-Retzlaff-Kraff (SRK) II regression formula, the SRK Theoretic (SRK/T), Holladay 1, Hoffer Q, and Haigis formulas. While IOL calculation formulas are highly predictable within the range of typical keratometry and axial length measures found in older children and adults, the wide range of keratometry and axial length combinations found in infantile eyes as well as the anatomic variances that may alter effective lens position can impact the ability of a formula to perform optimally [31, 32]. While some studies comparing formula performance have found that the SRK II formula works at least as well as theoretic formulas for pediatric eyes [29, 33–36], others found that theoretic formulas perform significantly better [37, 38]. Formulas specifically designed for shorter eyes seem to perform inconsistently in infantile eyes, so there is no clear evidence that they are superior [38, 39].

Overall, prediction error using any of the formulas is higher in pediatric cohorts compared with that in adult studies, with mean absolute prediction errors typically ranging from 0.7 to 1.5 diopters. [29, 30, 34, 35, 37, 40]. Higher prediction error has been associated with steep keratometry measures [22, 29], younger age at surgery [29], and short axial length [37, 39, 40]. Errors in axial length measurement can result in large prediction errors in pediatric eyes using all formulas [41]. Two large series involving children under 2 years of age have shown that SRK II formula may be the least variable [34] and that predictability is not influenced by axial length [36]. The SRK/T formula was used for infant eyes with reasonable mean prediction error (1.63 diopters), but a wide range in error was noted [20]. The Infant Aphakia Treatment Study utilized the Holladay 1 formula, with a mean prediction error of 1.7 ± 1.3 diopters (median, 1.2 diopters) [3], but comparison of mean and median prediction error for various formulas showed that the SRK/T and T2 formulas would have performed equally well or better [38, 42]. An association of higher prediction error with shorter axial length <18.0 mm at surgery was found in these infantile eyes. In another cohort of eyes with axial length <20 mm, SRK II performed the best, with a mean absolute prediction error of 1.84 ± 3.55 diopters [43].

Given that ocular growth and myopic shift is expected for every infant eye that has IOL implantation, various strategies have been proposed for finding a target refraction in the early postoperative period that will provide reasonable amount of correction early, not induce significant anisometropia in the short or long term for unilateral cases, and not result in significant refractive error after eye growth is complete.

Several guidelines for IOL power selection have been published (Table 8.2). The majority recommend selection of a desired postoperative refraction based on age at surgery; more recent publications tend to recommend more residual hyperopia. Others recommend reducing the IOL power by a percentage of the IOL predicted to result in emmetropia, or by axial length at the time of surgery. All of these strategies will leave an infant eye significantly hyperopic immediately after the surgery, in anticipation of at least six diopters of myopic shift in the early years and a predicted refractive result of low to moderate myopia by adulthood. In addition to these general guidelines, Hoevenaars and colleagues advocated for residual hyperopia of as much as 7–8 diopters when operating at 3 months of age, with slightly higher amounts for unilateral cases. The Infant Aphakia Treatment Study aimed for postoperative hyperopia of +8.0 diopters for children operated on at 4–6 weeks of age and +6.0 diopters for children over 6 weeks of age [4].

Another tool that is useful when selecting an IOL power is McClatchey’s Pediatric IOL Calculator [46]. McClatchey and Parks [47] studied aphakic refractions from a large series of patients operated on before age 10 years with at least 7 years follow up to calculate the range of final refractions for those eyes if primary IOL implantation had occurred instead, aiming for emmetropia at the time of surgery. These calculations predicted a median pseudophakic refraction at last follow of −6.6 diopters with a variance (−36.3 to +2.9 diopters), especially for children operated on in the first 2 years of life. From this data, McClatchey developed the program to predict the range of possible final refractive outcomes and standard deviation for an eye based on baseline biometry measures, age at surgery, and IOL implanted. Others have also used the program to confirm early predictability compared with standard IOL calculation formulas [43].