Refractive changes after lens implantation in childhood: Author's reply
1998; Elsevier BV; Volume: 105; Issue: 9 Linguagem: Inglês
10.1016/s0161-6420(98)99001-7
ISSN1549-4713
Autores Tópico(s)Corneal surgery and disorders
ResumoThe questions Dr. Kashani raises are extensive. In this reply I have provided a brief analysis of the basis of the theoretic model, as well as a response to each of his points. This article uses a logarithmic model to predict pseudophakic refractions in children. This theoretic model is based on three things: 1) the observed logarithmic decline in refraction in aphakic children; 2) an optical analysis of the effects of ocular growth on pseudophakic refractions; and 3) the assumption that putting an IOL in a child’s eye does not significantly affect its growth when compared to aphakia. The logarithmic model is based on the largest published long-term study of aphakic refractions in children. This study analyzed 1320 refraction observations from 156 eyes, with a median follow-up time of 8.8 years.1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google Scholar It showed that the mean refraction of aphakic eyes tends to follow a logarithmic decline from infancy through age 20 (P < 0.01, R2 = 0.97). The rate of myopic shift (as defined in the text) was found to be unaffected by any measured parameter in children over 6 months of age, except for a slightly greater rate in unilateral cataract eyes. The validity of this logarithmic model of aphakic myopic shift remains unchallenged. The mathematical analysis of the optics of a growing pseudophakic eye is plainly laid out in the appendices of the article.1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google Scholar This analysis clearly shows that if the aphakic refraction is known, a knowledge of axial length (AL) and cornea curvature (K) makes little difference in the prediction of pseudophakic refractive changes with ocular growth (Appendix 1). The accuracy and validity of these calculations remain unchallenged. The one and only significant assumption made in applying the logarithmic model of aphakic refractive change to pseudophakic refractions is that putting an IOL in a child’s eye does not affect the eye’s growth when compared to aphakia. This is the one area in which the theoretic model may stand or fall. However, there are several reasons for thinking that this assumption is correct, and I will elaborate them in my response to Dr. Kashani’s points. Dr. Kashani noted that the research was limited to documented refractions, without measurement of K or AL. As stated previously, a mathematical analysis of the optics of the growing pseudophakic eye proves that measurement of K and AL would have made only a small difference (a maximum of 0.67 D) to the calculations in the article (Appendix 1). Thus the measurement of K or AL can be considered irrelevant because the results of the study would have been virtually unchanged had these measurements been available. Indeed, aphakic refraction measurements are inherently dependent on the values of K and AL of the measured eyes, and may be a more accurate reflection of the refractive state of the eye than measurements of K and AL ever could be. He states that “as soon as an optimal IOL is implanted, the pseudophakic eye will behave differently and will grow toward emmetropia in children.” He then cites four articles to support this assertion. All of these articles show some myopic shift in pseudophakic children. The relatively old age at surgery and short follow-up of the patients in these studies would by themselves result in a smaller myopic shift than found in our study patients; thus, these articles do not contradict the predictions of the model. For instance, Hutchinson et al found a mean myopic shift of 0.99 D in their patients (average age, 6.3 years; mean follow-up, 3.2 years). The model predicts that a patient of this age and follow-up length would have approximately 1.38 D of myopic shift. This difference is too small and the expected variance in myopic shift is too great to use this result to contradict the model. He states that one of our inclusion criteria was to have the first refraction 3 months after cataract extraction. This is a simple misunderstanding: as stated in the Methods section, the first postoperative refraction had to be done within 3 months. As Table 1 shows, the median delay to first refraction was 0.05 year (18 days).1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google ScholarTable 1A Comparison of the Averaged Pseudophakic Refraction Data from Dahan and Drusedau5Dahan E. Drusedau M.U. Choice of lens and dioptric power in pediatric pseudophakia.J Cataract Refract Surg. 1997; 23: 618-623Abstract Full Text PDF PubMed Scopus (178) Google Scholar with the Predictions of the Theoretic Model1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google ScholarMeasured DataTheoretic Model CalculationsInitial Age (yrs)Initial AL (mm)Initial RefractionFinal Age (yrs)Final RefractionPredicted Refraction at Final Age0.7818.875.547.69−0.83−3.732.3721.413.635.84+0.90+0.475.0522.332.208.75−0.40+0.33Refractions are in diopters. Open table in a new tab Refractions are in diopters. In answer to another question, the aphakic correction was generally fit for distance in older children and for an intermediate zone in very young children. Dr. Kashani states that the percentage differences in Figure 1 are widely spread more toward hyperopia, and thus do not support the logarithmic model. In fact, the mean of all of these differences is nearly zero, supporting the model. The reason for the wider variability is that the variance in final myopic shift is inversely related to the log of age at surgery (as stated in the Discussion).1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google Scholar Therefore children under age 2 would be expected to show a much greater variance than older children do. Because of this large variance, the number of patients in this group is not sufficient to demonstrate a significant “pattern of spread” toward hyperopia in this figure. A larger study would clarify this concept. Dr. Kashani quotes several articles in support of the concept of emmetropization, and other articles that support various factors affecting the growth of the eye, some of which imply that the growth of pseudophakic eyes may be different than normal or aphakic eyes. Essentially, he claims that pseudophakic eyes have a rate of refractive growth that is different than aphakic eyes, and thus contests the third basis of the article. To counter this claim I will cite three lines of evidence. First, Lambert et al2Lambert S.R. Fernandes A. Drews-Botsch C. Tigges M. Pseudophakia retards axial elongation in neonatal monkey eyes.Invest Ophthalmol Vis Sci. 1996; 37: 451-458PubMed Google Scholar demonstrated that pseudophakia had nearly the same effect on ocular growth as aphakia in neonatal monkeys (see page 454; Fig. 1; and the statistics in the text). I confirmed this fact in discussion with the lead author, Scott Lambert (Charleston, April, 1997). This supports the sole assumption that the theoretic model makes. Second, Gordon and Donzis3Gordon R.A. Donzis P.B. Refractive development of the human eye.Arch Ophthalmol. 1985; 103: 785-789Crossref PubMed Scopus (526) Google Scholar studied 148 eyes of normal children, measuring axial length and cornea curvature. We took the AL and K data from this article and calculated what the refractions would have been if these children had been aphakic.4McClatchey S.K. Parks M.M. Myopic shift after cataract removal in childhood.J Pediatr Ophthalmol Strabismus. 1997; 34: 88-95PubMed Google Scholar These calculated “aphakic” refractions from normal children very closely matched the observed mean aphakic refractions of our patients (see Fig. 5 in that article4McClatchey S.K. Parks M.M. Myopic shift after cataract removal in childhood.J Pediatr Ophthalmol Strabismus. 1997; 34: 88-95PubMed Google Scholar). This implies that the myopic shift seen in aphakic eyes can be accounted for by the normal growth of the eye. Thus Dr. Kashani’s arguments about the blur of aphakia or the loss of regulation by the lens causing increased myopia in aphakic children are contradicted by the largest long-term study of aphakic refractions in children to date. Third, a recent article by Dahan and Drusedau5Dahan E. Drusedau M.U. Choice of lens and dioptric power in pediatric pseudophakia.J Cataract Refract Surg. 1997; 23: 618-623Abstract Full Text PDF PubMed Scopus (178) Google Scholar may provide the control group of pseudophakic eyes that Kashani requested. They published the long-term refractive follow-up on 156 pseudophakic children’s eyes. They reported the following data in three age groups: age at IOL implantation, initial axial length, first refraction, last refraction, and last age. The initial data from this study were put into the theoretic model of the article, with the assumptions that the IOL had an A-constant = 118.0, and the average K = 44.00 D; the IOL power was calculated to match the first refraction. The model gave the predicted refractions at the last age, and these were compared with the measured refractions (Table 1). Dahan and Drusedau’s data closely match the theoretic model predictions in the two older groups, but apparently show less myopia than predicted in the youngest age group. However, it is important to note that the predictions are based on the data that were averaged for each group by Dahan and Drusedau; this averaging would result in the greatest errors at lower ages because of the logarithmic nature of ocular growth and the greater variance in myopic shift in children less than 2 years old.1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google Scholar Thus this youngest age group is most likely to be at variance with the predictions because of the effects of averaging. In addition, some of the eyes in this younger age group were less than 6 months of age at surgery, and the model predicts less myopic shift for these patients1McClatchey S.K. Parks M.M. Theoretic refractive changes after lens implantation in childhood.Ophthalmology. 1997; 104: 1744-1751Abstract Full Text PDF PubMed Scopus (72) Google Scholar, 4McClatchey S.K. Parks M.M. Myopic shift after cataract removal in childhood.J Pediatr Ophthalmol Strabismus. 1997; 34: 88-95PubMed Google Scholar: this was not taken into account in the table. Without the data for each individual eye, it is not possible to make a statistical comparison of these data with the theoretic model predictions, but the fit appears quite good. I think that all of Dr. Kashani’s objections can be answered by a careful analysis of the existing data. He states that “the experimental and clinical data do not support the authors’ concept” and “there are inherent problems that have not been addressed.” Based on the preceding discussion of his objections, I assert that the experimental and clinical data do indeed support the application of this theoretical model to pseudophakic children, and that there are no inherent problems that have not been addressed. The beauty of the theoretic model of pediatric pseudophakic refraction is that it can be used to predict future refractions of these children at any age. Indeed, this has been done in a computer program that shows a graph of predicted future refractions with standard deviation limits (McClatchey SK. A computerized intraocular lens calculator for childhood cataracts. Presented at the meeting of the American Association for Pediatric Ophthalmology and Strabismus in Charleston, South Carolina, on April 2, 1997). If future studies show that pseudophakic eyes have a different rate constant of myopic shift than aphakic eyes, the computer program could use this new value in its calculations. How I wish Dr. Kashani were correct, and that pseudophakic children did not have as much myopic shift as the theoretic model predicts. My 7-year-old son has been pseudophakic in one eye since surgery for posterior lenticonus at the age of 3.77 years. His initial spherical equivalent refraction of +1.50 D has gradually declined to the current refraction of −2.50 D in this pseudophakic eye, whereas the cycloplegic refraction of his normal eye has remained constant at +1.00 D. His refraction has closely followed the model’s predictions, and I anticipate that he will have a refraction of approximately −8.00 D by the age of 20.
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