To compare accuracy of intraocular lens (IOL) power calculation formulae in infantile eyes with primary IOL implantation.
Comparative case series.
The Hoffer Q, Holladay 1, Holladay 2, Sanders-Retzlaff-Kraff (SRK) II, and Sanders-Retzlaff-Kraff theoretic (SRK/T) formulae were used to calculate predicted postoperative refraction for eyes that received primary IOL implantation in the Infant Aphakia Treatment Study. The protocol targeted postoperative hyperopia of +6.0 or +8.0 diopters (D). Eyes were excluded for invalid biometry, lack of refractive data at the specified postoperative visit, diagnosis of glaucoma or suspected glaucoma, or sulcus IOL placement. Actual refraction 1 month after surgery was converted to spherical equivalent and prediction error (predicted refraction − actual refraction) was calculated. Baseline characteristics were analyzed for effect on prediction error for each formula. The main outcome measure was absolute prediction error.
Forty-three eyes were studied; mean axial length was 18.1 ± 1.1 mm (in 23 eyes, it was <18.0 mm). Average age at surgery was 2.5 ± 1.5 months. Holladay 1 showed the lowest median absolute prediction error (1.2 D); a paired comparison of medians showed clinically similar results using the Holladay 1 and SRK/T formulae (median difference, 0.3 D). Comparison of the mean absolute prediction error showed the lowest values using the SRK/T formula (1.4 ± 1.1 D), followed by the Holladay 1 formula (1.7 ± 1.3 D). Calculations with an optimized constant showed the lowest values and no significant difference between the Holladay 1 and SRK/T formulae (median difference, 0.3 D). Eyes with globe AL of less than 18 mm had the largest mean and median prediction error and absolute prediction error, regardless of the formula used.
The Holladay 1 and SRK/T formulae gave equally good results and had the best predictive value for infant eyes.
Primary implantation of an intraocular lens (IOL) remains controversial for infants, and the selection of an appropriate IOL power is challenging. The Infant Aphakia Treatment Study was a multicenter, randomized, controlled clinical trial sponsored by the National Eye Institute to determine whether primary IOL implantation in infants younger than 7 months of age with a unilateral congenital cataract results in improved visual outcomes over contact lens correction of aphakia. Half of the 114 infants enrolled in this multicenter study were randomized to receive an IOL with a lens power determined using the Holladay 1 formula. The mean absolute prediction error was 1.8 ± 1.3 diopters (D), with greater error noted in eyes with shorter globe axial length (AL).
There are known challenges in obtaining accurate biometry and refractive measures in children and infants because of the lack of patient cooperation, limitations in equipment, and errors induced or exaggerated by the small size of the infant eye. Additionally, by the time the postoperative refraction is obtained, rapid growth of an infant’s eye may result in a myopic shift from the predicted postoperative refraction calculated at the time of surgery. The Haigis, Hoffer Q, and Holladay formulae have been found to give the lowest prediction error in pediatric or short eyes with a globe AL of less than 22 mm, but most studies have very few extremely short eyes (AL <20 mm) for analysis.
In this study, we analyzed the prediction error of the commonly used IOL power calculation formulae Hoffer Q, Holladay 1, Holladay 2, Sanders-Retzlaff-Kraff (SRK) II, and Sanders-Retzlaff-Kraff theoretic (SRK/T) as applied to the eyes of infants who received primary IOL implantation in the Infant Aphakia Treatment Study and looked for associations between eye characteristics and formula predictability.
The study followed the tenets of the Declaration of Helsinki, was approved by the institutional review boards of all participating centers ( Appendix 1 ), and was in compliance with the Health Insurance Portability and Accountability Act. The off-label research use of the AcrySof IOLs (Alcon Laboratories, Fort Worth, Texas, USA) was covered by United States Food and Drug Administration investigational device exemption G020021.
The design, surgical techniques, optical correction regimens, follow-up schedule, examination methods, and baseline characteristics of patients enrolled in the Infant Aphakia Treatment Study ( ClinicalTrials.gov identifier, NCT00212134 ) have been reported previously, and therefore are only summarized briefly in this report. Infants with a unilateral visually significant cataract (≥3 mm central opacity) and an age of 28 to 209 days at the time of cataract surgery were eligible for enrollment in the study. The main exclusion criteria were persistent fetal vasculature associated with stretching of ciliary processes or involvement of the optic nerve or retina, corneal diameter less than 9 mm, premature birth (<36 weeks gestational age), presence of a medical condition that may interfere with later visual acuity testing, or acquired cataract. Patients were randomized either to have an IOL placed at the time of the initial surgery (with spectacle correction) or to be left aphakic (with contact lens correction).
Data were collected prospectively. All available A-scans were reviewed by a certified echographer to assess quality as described previously; one scan was invalid, and this eye was excluded from analysis. Additional eyes were excluded from analysis for lack of refractive data at the 1 month postoperative visit, diagnosis of glaucoma or suspected glaucoma by the 3-month postoperative visit (which could result in an unpredictable and early myopic shift), or sulcus IOL implantation (which could result in alterations in the effective lens position).
Glaucoma was defined as intraocular pressure (IOP) of more than 21 mm Hg with 1 or more of the following anatomical changes: (1) corneal enlargement; (2) asymmetrical progressive myopic shift coupled with enlargement of the corneal diameter, AL, or both; (3) increased optic nerve cupping defined as an increase of 0.2 or more in the cup-to-disc ratio; or (4) the use of a surgical procedure for IOP control. A patient was designated a glaucoma suspect if he or she either (1) had 2 consecutive IOP measurements of more than 21 mm Hg on different dates after topical corticosteroids had been discontinued without any of the anatomic changes listed previously or (2) took glaucoma medications to control IOP without experiencing any of the anatomic changes listed previously.
Screening Examination Under Anesthesia
Before randomization, each infant underwent an examination under anesthesia to confirm study eligibility and to perform biometry of both eyes. Keratometry was performed with a hand-held keratometer, with an average of at least 2 readings that varied by less than 1 D. A-scan ultrasonography was performed using immersion in most cases. Measurements were obtained from the scan with the best waveforms (i.e., highest peaks with a perpendicular retinal spike) using the phakic setting. If applanation A-scan ultrasonography was used, the A-scan with the greatest anterior chamber depth was used.
Surgical Technique and Intraocular Lens Power Determination
The IOL power was determined in the operating room based on A-scan ultrasonography and keratometry readings using the Holladay 1 formula. According to the Infant Aphakia Treatment Study protocol, an IOL power was chosen that was closest to the power predicted to produce a +8.0 postoperative refraction for infants 4 to 6 weeks of age and a +6.0 D postoperative refraction for infants older than 6 weeks. Because the normal fellow eye in an infant was expected to be hyperopic (typically about +2.0 D), some degree of anisometropia was expected. In patients in whom the calculated IOL power was more than 40.0 D, a 40.0-D IOL (the maximum power available) was selected. Infants randomized to the IOL group had the lens aspirated followed by the implantation of an AcrySof SN60AT IOL (Alcon Laboratories). After IOL placement, a posterior capsulectomy and an anterior vitrectomy were performed.
Follow-up examinations were performed by an Infant Aphakia Treatment Study certified investigator as described in detail elsewhere. Retinoscopy was performed under cycloplegia to determine residual refractive error at the 1-month postoperative examination. This measure was converted to spherical equivalent (half of the cylinder added to the sphere) and was compared with the predicted refraction.
Calculation of Prediction Error
The predicted refraction was calculated for each of the formulae studied (Hoffer Q, Holladay 1, Holladay 2, SRK/T, and SRK II) using the IOL power implanted and the patient’s AL, anterior chamber depth, lens thickness, corneal diameter, and average keratometry recorded at the time of surgery. The anterior chamber depth and lens thickness values were not available for all eyes, because this information was not collected on original study forms, but instead was gathered later from available A-scans that had been performed and collected at the time of surgery. Holladay IOL consultant software was used to calculate prediction error for Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulae. Results of SRK II formula calculations were obtained using Microsoft Excel (Microsoft Corp, Redmond, Washington, USA). The prediction error and absolute prediction error were calculated as follows:
Prediction error = Predicted refraction − Actual refraction
Absolute prediction error = | Predicted refraction − Actual refraction |