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Posterior chamber lens implantation is increasingly employed in the management of paediatric cataract, sometimes in infants within a few weeks of birth. The need to implant a lens of fixed power into an eye that is still growing creates a problem—what power lens should be selected? This is especially a dilemma in infants under 18 months of age, the period of most rapid postnatal ocular growth, during which there can be a substantial myopic shift.
Using hand held instruments, reliable keratometry and axial length measurements can be obtained in children, and implant power calculations using adult formulas are accurate in the paediatric eye.1 However, implanting a lens at the calculated power risks significant myopia at ocular maturity. The paediatric cataract surgeon therefore now requires an additional “formula”, one that will predict the final refraction for any selected lens on the basis of the patient’s age.
An initial suggestion was to utilise a cross sectional study of the refractive data in a normal population.2 The objection to this approach is that the pseudophakic eye is not normal in a variety of ways and may therefore grow differently. For example, the cataract may have been one manifestation of a wider ocular abnormality affecting growth. Given its important prenatal role in ocular development, loss of the crystalline lens might also have an effect, and there is evidence that both amblyopia3 and the physical properties of the implant4 may alter growth in axial length. As happened for adult intraocular lens power calculations, the most accurate formula is likely to derive from a study of outcome, in this case of children followed up until growth of the eye is complete. This requires many years of follow up and adequate data are not yet available.
Long term follow up data are, however, already available for aphakic paediatric eyes and have been used to derive a theoretical model that surgeons could use to predict future refraction at any age, and to develop a computer program to make the required calculations.5 This is an important step and in the future we can expect such a program will become an integral part of the software in standard equipment for calculating implant power, but it will be based on pseudophakic not aphakic follow up data.
For the present, the requirement is for the accumulation of data on the refractive changes in pseudophakic eyes and a study by Flitcroft and colleagues in this month’s BJO (p 265) makes a useful contribution to that process. The authors carried out a prospective observational study of changes in refractive power, keratometry, and axial length in 35 pseudophakic eyes.
An admitted problem is that follow up for many patients was relatively short, a feature of most of the studies in this field. It does cover the period when the eye is making its most rapid growth, and during this time the pseudophakic eyes studied showed axial elongation similar to that found in normal eyes. A further report when these eyes have reached maturity will be required to confirm the authors’ initial conclusions and advice on implant power selection. In the interim it should be noted that the predictions of final refraction in the younger children are based on the presumption of continuing normal growth rather than actual follow up.
Unlike some previous studies, Flitcroft et al do provide detailed information for each patient and this is important. As the numbers in individual series will never be large, the most reliable formula is likely to be derived by meta-analysis. In addition, other factors affecting myopic shift may be isolated—for example, amblyopia and the initial postoperative refraction.3 Publication of full data for all studies should enable earlier assessment of the impact of such variables.
Information from animal and human studies confirms the existence of visually directed control of growth in the axial length of the eye, but its mechanism remains unknown.6 7 With an implant of fixed power and good vision it is possible to speculate that the control system might reduce the normal increase in axial length, and so lessen the problem of myopic overcorrection.8 This would be a serendipitous phenomenon to enjoy, and it may have occurred in some cases.3-11 In the longer term, there is the possibility that when we understand the control of postnatal growth of the eye we may learn to manipulate it, to reduce the refractive problems inherent in implants of fixed power.
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