With the increasing incidence of cataracts, the pressure is rising to achieve better vision in patients who requiring bilateral cataract surgery. Multiple studies [8-12] reported on the positive correlation of binocular REs after sequential cataract surgery and effectiveness of adjusting the target RE in the second eyes by correcting 50% of the errors from the first eyes for improving visual quality. Scholars [10, 13] speculated that this finding may be attributed to the revision of predictive errors from post-operative ACD. However, in China there are few reports focused on this. Our previous observation verified that modifying the IOL power in the second eyes based on 50% of RE from the first eyes can reduce the error of the second eyes [14]. However, the fluctuation of refraction was only observed for 1 day, it is difficult to evaluate stability, and that is our motivation for this work.
In this study, 94 RE data of patients after bilateral cataract surgery with stable refraction more than 1 month after surgery were selected first. It was found that the RE values of both eyes were closely related (r=0.760, P<0.001). It means that the postoperative refractive shift of the second eye can refer to the corresponding first eye. In 2010, Landers and Goggin [11] found a statistically significant correlation between RE values in both eyes (P=0.003). Covert et al. [12] confirmed that there was a positive correlation between binocular RE values after sequential cataract surgery and the refractive status of the second eyes were improved successfully by adjusting the IOL power according to 50% of the RE of the first eye. In 2011, Olsen [10] showed that the correlation coefficients of the binocular RE for the SRKⅡ, SRK/T and Olsen formulas were 0.56, 0.38 and 0.27, respectively (P<0.001). Aristodemou et al. [15] further verified Covert’s conclusion after comparing the adjustment coefficient from 10% to 90%. At the same time, the influence of measurement errors was excluded and they insisted that the RE was mainly originated from post-operative effective lens position (ELP).
But our research differs from the above studies which employed multiple surgeons and IOL power formulas. This study used optimized formula based on AL, excluding the effects of ocular surface and intraocular diseases, performed by the same surgeon (Hong Yan). Therefore, the factors affecting the RE value can be reduced, and a high positive correlation between the binocular RE values is demonstrated. In addition, this study showed that there is a high positive correlation between the AL values of both eyes (r=0.970, P<0.001), which was consistent with Covert’s result (r=0.979) [12].
Second, our study selected the RE data of patients with large RE (greater than 0.5 D) from the original data for second eye correction. We compared the absolute RE values of bilateral eyes between adjusted and the control groups, indicating that the MARE of the adjusted second eyes is significantly lower than that of the corresponding eyes, amounting to approximately half of the MARE of the first eyes. This result was consistent with an earlier study [16]. While the difference between MARE values of both eyes without adjustment was not statistically significant. Fraser et al. [17] proposed that contrast sensitivity and stereopsis rather than vision are the key factors affecting the improvement of vision-related quality of life after cataract surgery. Jivrajka et al. [8] also reported that the substitution of half of the error from the first eyes into the calculation of IOL power of the respective second eyes can improve their outcomes. However, the difference between binocular diopters should be considered carefully to avoid visual discomfort due to monovision or anisometropia [18]. Our research showed that the binocular ALs are highly correlated when the IOL power was adjusted for the second eye, which may be a main reason for the useful adjustment.
Furthermore, there is an essential question about how to assure an adequate time interval between bilateral cataract surgeries, i.e. how to choose the operation time for the second eye. The preceding debates [19-21] do not advocate simultaneous bilateral cataract surgery, not only due to ethical constraints, but it is more important to take into account the severe consequences of post-operative endophthalmitis. Our previous study on bilateral sequential cataract surgery showed the aqueous humor of the second eye had a higher level of TGF-β2, but not of proinflammatory cytokines or chemokines compared with those in the first eye, implying a protective mechanism preventing the sympathetic immune reaction induced by the first-eye cataract surgery [22]. However, immediate sequential bilateral cataract surgery is becoming popular in recent years [23, 24]. With the increasing expectation for post-operative visual quality, the focus of the operation is not only safety, but also the best possible visual recovery. Many studies [13, 25, 26] suggested that refractive status was stable one month after operation. Thus, bilateral surgery should be performed over 4 weeks, rather than simultaneously or in a short time span [27].
Last but not the least, multiple studies reported that the IOL power in the second eyes can be calculated according to the ACD of the first eyes. Muthappan et al. [28] studied the effect of post-operative ACD of lateral eyes on the refractive outcome of the second eyes and it indicated that the refractive outcome can be improved when the RE is relatively large in the first eyes. Our follow-up study will focus on the improvement of refractive outcome in the second eyes according to the post-operative ACD in the respective first eyes. If this hypothesis can be verified, it will provide a scientific basis for clinical application and promotion.