1 IOL Power Calculation for Children Rupal H. Trivedi, MD MSCR M. Edward Wilson, MD The authors have no financial interest in the subject matter of this presentation. Intraocular lens (IOL) implantation in eyes of select children has become a common practice during pediatric cataract surgery. In the past few years, there have been several improvements in surgical management of pediatric cataract. However, IOL power calculation accuracy has not improved at the same pace. Selection of an IOL power is one of the major challenges for the long-term care of children undergoing cataract surgery. We present key steps to guide you in calculating and selecting an IOL power for a child (figure 1 & 2). After pediatric cataract surgery, the postoperative refraction commonly is different from what had been predicted or aimed for by the surgeon. Although many of the late refractive surprises are attributed to a myopic shift in refraction from axial eye growth, early refractive surprises can be attributed to inaccuracy in IOL power calculation. Below are the steps one should take to minimize early refractive surprises. Figure 1 Take measurements: Measurements on children can be very difficult or unattainable in the office. Most children need an examination under anesthesia. - Keratometry. For the K measurement, handheld keratometer is frequently used. Although the reliability of the handheld keratometer may be suboptimal for axis measurements, available
2 literature suggest that it is a practical way to obtain an acceptable K value in sedated children. - The measurements should be taken without the use of an eye-lid speculum. - Measurements should be taken as soon as possible after the induction of anesthesia (following the IOP measurement) to avoid the problems associated with corneal dryness. Balanced salt solution should be instilled to maintain a smooth corneal surface. - At least two keratometry measurements should be taken in both eyes to ensure that the results are accurate: the average K reading should be within 1 D of each other. If the two average K readings are more than 1 D different, then make a third measurement and find the average of the two closest K reading. - Biometry: Errors in axial length (AL) measurement affect the IOL power calculation the most, with an average error of 2.5 D per millimeter of AL in adults. However, this error jumps to 3.75 D per mm in short eyes (i.e., AL 20 mm or less). The ultrasound AL of the eye commonly is measured using either contact or immersion techniques. In the contact method, the probe touches the cornea and may result in corneal compression and a shorter AL. Immersion A-scan measurement eliminates corneal compression and has been shown to be superior to contact biometry in adults. It is accepted as a gold standard technique for AL measurement in adult eyes. However, pediatric cataract surgeons use the contact technique more frequently when measuring the AL of pediatric eyes at the time of cataract surgery. This statement is based on an informal, unpublished, survey send to all listserve members of The AAPOS, in which 82% surgeons reported using contact A-scan. The risk of AL measurement errors resulting from corneal compression when using contact A-scan ultrasound is well known. However, for those surgeons using contact A-scan techniques, is there a compelling reason to change to immersion A-scan with the increased technical expertise that the switch would require? This is the question we address below. We compared the contact technique with the immersion technique in terms of the magnitude of AL measurements in pediatric cataractous eyes and compared prediction errors. AL was measured by both contact and immersion techniques for all eyes, randomized as to which to perform first to avoid measurement bias. AL measurements using the contact technique were on an average 0.27 mm shorter than those obtained using the immersion technique (P<0.001). Intraocular lens power needed for emmetropia was significantly different (28.68 D vs. 27.63 D; P<0.001). During IOL power calculation, if AL measured by contact technique is used, it would have been resulted in the use of an average 1-D stronger IOL power than is actually required. This can lead to induced myopia in the postoperative refraction. Data from this prospective study of eyes that had in-the-bag implantation of an AcrySof SN60WF IOL and had refraction results available from 14 days to 3 months postoperatively were retrospectively analyzed to calculate prediction error. The mean prediction error was +0.4 D in the contact group and -0.4 D in the immersion group (P <.001) and the mean absolute prediction error, 0.7 D and 0.7 D, respectively (P=.694). The mean postoperative spherical equivalent was +2.9 D, which was significantly different from the mean predicted refraction for contact A-scan (3.3 D; P=.010) but not immersion A-scan (2.5 D; P=.065).
3 To summarize, if possible attempt to do immersion A-scan. Like keratometry, A-scan biometry should also be performed for both eyes. Take the measurements from the scan with the best wave forms (i.e. highest peaks with a perpendicular retinal spike) or, if applanation biometry is used, the A-scan with the greatest anterior chamber depth. Calculate IOL Power: - Personalized A-constant: Personalizing the lens constant for a given formula is one way to make adjustments for a variety of variables such as different styles of keratometers, ultrasound machine calibration inaccuracies, and surgical technique and to minimize prediction error. However, most physicians perform few pediatric cataract surgeries resulting in insufficient data to calculate personalized IOL constant. - IOL power calculation formula: We evaluated PE in pediatric eyes when using the Holladay II formula in the absence of preoperative refraction. Using Holladay II, Holladay I, Hoffer-Q and SRK-T formula, mean PE was 0.02, -0.21, 0.07 and -0.47 D while mean absolute PE (APE) was 0.68, 0.71, 0.72, and 0.84 D, respectively. Holladay II had the least PE for shorter eyes. - Determine the Target Postoperative Undercorrection: This is a straightforward decision in adult eyes, as visual needs assist in the determination of those calculations. For children, another factor to consider is the projected growth of the eye. The ideal IOL power should give the best help for fighting amblyopia in childhood while inducing the least refractive error in adulthood. This power can be calculated by anticipating the expected myopic shift and undercorrecting the eye that needs IOL implantation. Figure 2
4 - Age at cataract surgery. When an IOL is implanted in a child, a marked myopic shift occurs. Therefore, IOLs implanted in children are usually selected to produce undercorrection. The closer to birth the implantation is performed, the more marked the undercorrection will need to be (table). How much should one undercorrect and at what age? These recommendations must be balanced with the following additional factors discussed below. Expected postoperative residual refraction based on patient age at cataract surgery.* Age at surgery Residual refraction to minimize late myopia Median residual refraction in our series 1 st month + 12 + 8.3 2 3 months + 9 + 8.5 4 6 months + 8 + 6.0 6-12 months + 7 + 4.5 1 2 years + 6 + 3.0 2 4 years + 5 + 0.9 4 5 years + 4 + 0.5 5 6 year + 3 + 0.5 6 7 year + 2 + 0.1 7 8 year +1.5 + 0.2 8 10 year + 1 + 0.1 10 14 years + 0.5 0 >14 years Plano -0.1 *We do not recommend the use of any published table alone for deciding IOL power. These tables are only meant to help as a starting point toward appropriate IOL power selection, which is a multifactorial decision customized for each child based on many variables [especially, age, laterality, amblyopia status, likely compliance with glasses, and family history of myopia]. - Status of the fellow eye. When surgery will be done in both eyes, a larger amount of hyperopia may be acceptable, since aniseikonia can be avoided by targeting a nearly equivalent refraction in both eyes. - Visual acuity. Current advances suggest that it will probably be easier in the future to manage myopia than amblyopia. Dense amblyopia may prompt a decision to leave less hyperopia (or even to aim for emmetropia) to facilitate compliance with amblyopia therapy. In such a case, late myopia is acceptable if it helps the child recover vision during the amblyopia treatment years.
5 - Expected compliance. It is better to leave less refractive error if the child and/or family is expected to comply poorly with glasses, contact lenses or occlusion therapy. - Parents refractive error. It is also important to ask about high refractive error (especially myopia) in the parents. Because children with myopic parents can be expected to undergo more eye growth, these eyes may be left with more hyperopia than stated in the table. - IOL power. In general, the higher the IOL power, the more undercorrection is needed. For example, if at 1 month of age, one child has an emmetropic power need of 50 D and another child needs 40 D, the first child will require a higher early residual refraction. We may use an approximate expected postoperative refraction in the first child of +12 D, while in the second child the expected refraction would be +10 D. - Microphthalmia. In microphthalmic eyes, the target refraction may not be reached even with the highest available IOL power. In such eyes, the highest available IOL power should be chosen. Conclusion: Despite our best efforts, it may be that some children may eventually need an IOL exchange or refractive surgery. Surgeons who implant IOLs in young children must be prepared for a wide variance in the long-term myopic shift. Both the magnitude and the variability in this shift are likely to be greatest in children having surgery in the first few years of life. Nevertheless, we must assume that an IOL implanted in a child s eye will remain there for many years. The longterm refractive error outcome of the operated eyes of today will help us develop formulas suited for IOL power calculation for the pediatric patient of tomorrow.