In this study, PPR and recommended IOL power calculated by three different generation formulas, Barrett Universal II, Haigis and SRK/T were influenced by pupil dilation in different ways. Barrett Universal II was the most sensitive to pupil dilation, followed by Haigis. SRK/T was not influenced by pupil dilation. The change in ACD and LT before and after pupil dilation were more closely involved in influencing Barrett Universal II and Haigis. The change in WTW before and after pupil dilation also influenced only Barrett Universal II.
The improvement of the accuracy of the biomechanical measurement and PPR has gained attention as they are important in choosing the most suitable IOL. Therefore, we have to consider all factors that influence the biomechanical variables and PPR.
Regarding the biomechanical parameters, different generation IOL calculation formulas include different parameters to estimate the effective lens position (ELP), an important factor for PPR. Although the detailed components of the formulae are complex, the vital part for its comprehension is as follows. SRK/T uses corneal curvature radius and axial length (AL), which was published by Retzlaff et al. in 1990.1) ELP is estimated based on the ACD and AL in Haigis.2) and Barrett Universal II (new generation formula) uses AL, corneal curvature radius, ACD, LT, and WTW. 3) Many studies have investigated the influence of pupil dilation on these biometric measurements. In a clinical setting, pupil dilation is a vital process of preoperative examination. Therefore, it is important to analyze the possible influence of pupil dilation on PPR and recommended IOL power in third, fourth, and new generation IOL power calculation formulas, and to investigate the correlation between variables.
In many studies, AL and corneal curvature radius are not affected by pupil dilation.10-12) However, ACD was reported to be influenced by pupil dilation.10-14) Compared to ACD, few researches have dealt with the influence of pupil dilation on LT and WTW. Wang X et al.8) demonstrated that LT was significantly affected by pupil dilation. The results on ACD and LT are logical because the ciliary muscles relax and dilator muscles contract through pupil dilation, and as a result, the lens becomes thinner and ACD becomes deeper. It is controversial whether pupil dilation influences WTW. While Huang et al.10) and Arriola-Villalobos et al.15) insisted that WTW was affected by pupil dilation, the opposite result was reported by Wang et al.8) Although the researchers attributed the discrepancy in the influence of pupil dilation on WTW to the error of examinations and imaging artifact, the real mechanism remains unknown.
In our research, while ACD significantly increased after dilation, LT significantly decreased, which are consistent with other studies.16) WTW did not significantly change. When it comes to the influence of pupil dilation on PPR and recommended IOL power, the outcomes of past research vary from formula to formula and from research to research. Rodriguez-Raton et al. showed that PPR in SRK/T was not affected by pupil dilation, but PPR in Haigis was.12) Adler et al. also indicated similar results. The results were reasonable since SRK/T does not include ACD as a biometric parameter, which is significantly affected by pupil dilation, whereas, Haigis does. Our research also showed while PPR in SRK/T did not change after pupil dilation, while PPR in Haigis significantly changed. Concerning Barrett Universal II, although many studies demonstrated the superiority of the accuracy of PPR in Barrett Universal II compared to other formulas4,5), the research which dealt with pupil dilation influence on PPR and recommended IOL power in Barrett Universal II has not been published. Our research indicated that MAC in PPR calculated by Barrett Universal II was the largest, followed by Haigis and SRK/T. This suggests that Barrett Universal II was the most sensitive to pupil dilation, followed by Haigis, and then SRK/T. The difference in the sensitivity to pupil dilation among the formulas was significant. This tendency was also seen in the coincidence of recommended IOL power in each formula before and after dilation. The recommended IOL power calculated by Barrett Universal II changed most frequently among the formulas, although it was not statistically significant between Barrett Universal II and Haigis. Although some studies demonstrated that the recommended IOL power in Haigis was significantly affected by pupil dilation but not in SRK/T10),12),16), our research was the first to show that Barrett Universal II may be even more sensitive to pupil dilation than Haigis, considering PPR and recommended IOL power.
The analysis of correlation between the change in PPR and the biometric variables indicated that the newer generation formula is more sensitive to pupil dilation. The change in PPR in Barrett Universal II and Haigis showed a significant positive correlation with the change in ACD and a significant negative correlation with the change in LT, but not in SRK/T. This tendency was more remarkable in Barrett Universal II. This result indicated that the change in ACD and LT significantly influenced the change in PPR in the formulas, which included ACD as a biometric parameter, and it was even more influential on the formula that included both ACD and LT as biometric variables. Additionally, the change in PPR in Barrett Universal II indicated a significant positive correlation with the change in WTW, but not in Haigis and SRK/T. This outcome was persuasive since Barrett Universal II was the only formula that included WTW as a biometric factor. Given the fact that all biometric factors, ACD, LT, and WTW, could be significantly influenced by pupil dilation, it is convincing that the more biometric parameters IOL calculation formula includes, the more influential pupil dilation is on the formula. As a result, recommended IOL power calculated by Barrett Universal II changed in many more cases after pupil dilation compared to Haigis and SRK/T.
Thus, there are biometric factors in the IOL calculation formula that are influenced by pupil dilation. In general, the more modern generation formula is, the more biometric parameters are included. Barrett Universal II is said to be one of the most reliable IOL calculation formulas. However, this study demonstrated that, since it includes many more such biometric variables compared to previous generation formulas; eye specialists have to be familiar with these phenomena to improve the accuracy of IOL calculation.
One of the limitations of this study is that the influence of pupil dilation on prediction error in refraction was not analyzed. This additional research can enable optimization of the constant for measurement with or without pupil dilation. This idea may be more useful to improve the accuracy of IOL power calculation. We plan to analyze this investigation in our future research.