This article in the March issue of Archives of Ophthalmology examined the accuracy of IOL power calculations and early refractive status in pseudophakic eyes of infants in the Infant Aphakia Treatment Study (IATS). The results in 56 eyes randomized to undergo primary IOL implantation indicate that a relatively large prediction error (PE) is common when performing IOL implantation in infant eyes, especially in those with axial length less than 18 mm, even when using a formula designed for short eyes. Implantation of an IOL with a power of 30.0 D or greater resulted in less residual hyperopia than expected in 88 percent of eyes, as did implantation in eyes with axial lengths of less than 18 mm.
The authors say that quality A-scans are essential in infants undergoing IOL implantation. However, they conclude that in these growing eyes, in cases with less baseline hyperopia than planned and expected axial elongation, significant myopia may result in the long term.
The IATS is a multicenter, randomized, controlled clinical trial sponsored by the National Eye Institute designed to determine whether primary IOL implantation in infants between one and six months of age with unilateral cataract improves visual outcomes compared with contact lens correction of aphakia. Half of the 114 infants enrolled in the study were randomized to receive an IOL and spectacle correction for residual refractive error, with the remainder not receiving this treatment.
IOL power was chosen based on the Holladay 1 formula. PE was defined as the calculated refraction minus the actual refraction. Infants 28 to 48 days old at IOL surgery were targeted for postoperative refraction of +8.0 D and those up to seven months at surgery for +6.0 D.
One month after surgery, overall mean (SD) absolute PE was 1.8 (1.3) D and mean PE was +1.0 (2.0) D. Absolute PE was less than 1.0 D in 41 percent of eyes but greater than 2.0 D in 41 percent of eyes. Mean IOL power implanted was 29.9 D (range, 11.5-40.0 D). Multivariate analysis revealed that only short axial length (less than 18 mm) was significant for higher PE.
The authors say that because keratometry and A-scans in infants are usually performed with the patient under anesthesia, lack of fixation may induce measurement error. Instruments are calibrated for adult eyes, and the proportional differences in the infant globe also may cause measurement errors.
However, eye growth occurs rapidly in the first six months of life; increases in axial length during the first month after surgery could result in a reduction in the amount of hyperopia measured. When eyes were compared by age group at surgery, less overcorrection was seen in the older group compared with the younger group.