We appreciate the interest of Carifi and associates in our recent article. They have raised some excellent points regarding the limitations of our study, some of which we acknowledged in the article. Since our study was retrospective and included patient data from as far back as 15 years ago, some of the tools and instruments used in the study have subsequently been replaced in practice with more robust and accurate techniques. We have performed additional analysis of our data in hopes of addressing some of these concerns.
Intraocular lens (IOL) calculations have certainly improved in the past 15 years. In our original study, the older SRK II formula was used to calculate the IOL power in 24 of the 77 study eyes. Analyzing the remaining 53 cases that used the newer SRK-T formula, we still found a significant correlation between intraocular pressure (IOP) changes (postoperative IOP – preoperative IOP) and the extent of refractive surprise (postoperative spherical equivalent refraction – preoperative predicted spherical equivalent refraction) (r = −0.32, r 2 = 0.10, P = .02). Thus, our findings cannot be attributed to the use of the outdated SRK II formula. We recognize, however, that other IOL calculation formulas may have resulted in fewer refractive surprises. We did not collect data on corneal astigmatism as part of our study; however, neither preoperative nor postoperative refractive astigmatism greater than 1.00 cylinder power was associated with refractive surprise (preoperative astigmatism >1.00: r = −0.05, r 2 = 0.003, P = .81; postoperative astigmatism >1.00: r = −0.33, r 2 = 0.11, P = .08).
Dr Carifi and his colleagues are skeptical, perhaps rightly so, that a “marginal rise” in IOP could have led to a change in axial length. In our study, we found that a mean IOP rise of 2 mm Hg resulted in a mean −0.36 diopter shift between predicted and actual refraction, which we hypothesized was attributable to a change in axial length. However, we want to emphasize that the 2 mm Hg and -0.36 diopter shifts represent summary data of the 77 eyes in the study group, and we would not normally expect a 2 mm Hg rise in IOP to cause a significant change in axial length in an individual eye. As illustrated in Figure 2 of the original article, the most myopic refractive surprise occurred in eyes that had the most postoperative increase in IOP, particularly those with preoperative IOPs below 8 mm Hg. Indeed, analyzing only the 38 cases in which the preoperative IOP was greater than 8 mm Hg, we found no significant correlation between IOP changes and extent of refractive surprise (r = −0.16, r 2 = 0.027, P = .32). In contrast, of the 39 cases in which the preoperative IOP was 8 mm Hg or less, the correlation between IOP changes and extent of refractive surprise was statistically significant (r = −0.47, r 2 = 0.22, P = .003).
We agree with Carifi and associates that future studies with optical axial length measurements before trabeculectomy, before cataract surgery, and after cataract surgery would provide more definitive support of our hypothesis. We thank them for their insights.