Aphakic glaucoma in children

Letters to the Editor APHAKIC GLAUCOMA IN CHILDREN To the Editor: We were intrigued by the recent report of Asrani et al1 suggesting that pseudophakia protects against “aphakic” glaucoma in children. We were concerned, however, that the method in which they reported their data may have introduced a bias against detecting glaucoma in pseudophakic children. The authors reported follow-up data from 377 eyes with primary pseudophakia (Table 3 in their article) and compared them to eyes with aphakia with regard to the development of glaucoma. The aphakic children, however, had much longer follow-up than pseudophakic children. Although survival analysis can be used to compare 2 populations with differing follow-up provided certain requirements are met, it is important to recognize that fewer than 25% of the pseudophakic patients in this study had more than 5 years of follow-up, and the 2 curves in the survival analysis do not begin to diverge until after 5 years. Simon et al2 reported a mean interval of 6.8 years from the time of cataract surgery until the diagnosis of glaucoma. The number of children in the Asrani et al series with greater than 6.8 years of follow-up is not provided but must be substantially less than 10%. Hence, we believe it is too early for glaucoma to have developed for most of these patients, and they therefore should have been excluded from the analysis. There are several requirements that must be met to use survival analysis as a statistical tool, and we do not believe these requirements were met in the study.3 First, patient accrual must be uniform between the groups for the follow-up interval. The aphakic children had much longer follow-up than the pseudophakic children, and most pseudophakic children were not examined after 5 years, so accrual was not similar between the groups. Second, the number of patients with censored (incomplete) data are not provided, and we believe most pseudophakic children should actually have been censored, since they had not been observed for an adequate period. Finally, the number of patients at risk should have been provided at regular intervals throughout the time axis. In addition to the above problems, the authors did not use any statistical methods to compare the 2 curves to see if they were significantly different. They mentioned that

the χ2 test gave a P value of .02; however, they should have used a log-rank test. They mention the log-rank test in the methods, but it is unclear if the .02 P value was based on this test. A Cox proportional-hazards regression analysis test would be more appropriate because it could measure the effect of confounding variables, including microphthalmia, on the development of glaucoma. Alternatively, a tabular summary of the Kaplan-Meier survival estimates could have been presented. When the proper requirements are not met, survival curves can often appear quite misleading, and we believe this may have occurred in the Asrani et al study. We would like to see the data presented again in one of the aforementioned formats. We hope that the authors are correct that pseudophakia somehow protects an eye from becoming glaucomatous. We also have had a low incidence of pseudophakic glaucoma in our patients but believe it is probably due to selection bias, since we typically did not implant intraocular lenses into microphthalmic eyes 5 to 10 years ago. Until better data are available to determine the true incidence of pseudophakic glaucoma, however, we believe that rigorous attention to intraocular pressure in these patients is of utmost importance and that lowering ones’ level of suspicion on the basis of the report by Asrani et al would be cavalier. Sean P. Donahue, MD, PhD Daniel Byrne, MS Departments of Ophthalmology, Neurology, and Pediatrics Vanderbilt University School of Medicine Nashville, Tennessee M. Edward Wilson, MD Debajyoti Sinha, PhD Storm Eye Institute Charleston, South Carolina References 1. Asrani S, Freedman S, Hasselblad V, Buckley EG, Egbert J, et al. Does primary intraocular lens implantation prevent “aphakic” glaucoma in children? J AAPOS 2000;4:33-9. 2. Simon JW, Mehta N, Simmons ST, Catalano RA, Lininger L. Glaucoma after pediatric lensectomy/vitrectomy. Ophthalmology 1991;98:670-4. 3. Lang TA, Secic M. How to report statistics in medicine: annotated guidelines for authors, editors, and reviewers. Philadelphia (PA): American College of Physicians; 1997. 75/8/111133 doi:10.1067/mpa.2000.111133

J AAPOS 2000;4:389-90. Copyright © 2000 by the American Association for Pediatric Ophthalmology and Strabismus. 1091-8531/2000/$12.00 + 0

Journal of AAPOS

December 2000

389