With the incidence of age-related cataract rising gradually, the pressure is rising to achieve better vision in patients who requiring bilateral simultaneous or sequential cataract surgery, especially when the first operated eye does not achieve emmetropia. Bilateral eyes are relevant in AL, ACD and corneal curvature[8]. Multiple studies[8-12] reported on the positive correlation of binocular RE 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. The authors[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 on the calculation methods of IOL power for the second eyes of patients who underwent bilateral sequential cataract surgery. Our previous observation verified the finding 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, since the fluctuation of refraction of the second eyes was only continuously observed by us to 1 day after surgery, it is difficult to evaluate stability of the APSE objectively.
In this study, 94 RE-values of patients after bilateral cataract surgery with stable refraction more than 1 month after surgery were selected. It was found that the RE values of both eyes were closely related (r=0.760, P<0.001). In 2010, Landers and Goggin[11] found a statistically significant correlation between RE values in both eyes (P=0.003), although APSE data in each eye of individual patients were considered independently. The same year, 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). Aristidemou et al.[15] further confirmed Covert’s conclusion after comparing the adjustment coefficient from 10 to 90%. Their results showed that the MARE is reduced from 0.32 D to 0.28 D, which verifies that the RE values of the first eye can be used to improve the RE values of the corresponding second eye. At the same time, the influence of measurement errors was excluded and they insist that the RE is 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. Our study used optimized formula based on AL to calculate IOL power, excluding the effects of ocular surface and intraocular diseases, all procedures were performed by the same surgeon (Hong Yan). By these measures, the factors affecting the RE value can be reduced, and a high positive correlation between the binocular RE values is demonstrated. In addition, our 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].
Our study also analyzed and compared the absolute RE values of bilateral eyes between adjusted and unadjusted groups. We found 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 of patients without adjustment was not statistically significant, MARE value of the second eyes was slightly lower than that of the contralateral eyes. This may be caused by the vision-related quality of life improvement after the first-eye cataract surgery. 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 or depressive symptoms after first-eye cataract surgery. This indicates 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 refractive outcomes. Jivrajka et al.[9] conducted a prospective study of 250 patients, in which every surgery was completed by the same surgeon, using the same type of IOL and calculation formula (Haigis formula). They improved the post-operative visual acuity of the second eyes obviously by dividing the error data of the first eyes exceeding 0.50 D into 4 groups (±0.50 D to ±1.00 D, and exceeding ±1.00 D), and then modifying 50% of the error of the data to correct the IOL power in the second procedure. 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 sequential cataract surgeries, i.e. how to choose the operation time for the second eye. The preceding debates[19-21] surrounding the time interval do not advocate simultaneous bilateral cataract surgery, not only due to ethical constraints, but more importantly in order to take into account the severe consequences of post-operative endophthalmitis. However, immediate sequential bilateral cataract surgery is becoming popular in recent years[22, 23]. 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, 24, 25] suggested that refractive status was stable one month after operation, and refractive status shorter than this term should not be used as the basis for correcting the corresponding eye. Based on these clinical observations, bilateral surgery should be performed over 4 weeks, rather than simultaneously or in a short time span[26].
Additionally, multiple studies reported that the IOL power in the second eyes can be calculated according to the ACD in the corresponding first eyes. In 2015, Muthappan et al.[27] studied the effect of IOL diopter calculation by post-operative ACD of lateral eyes on the post-operative refractive outcome and evaluated the stability of post-operative ACD. The study used two different methods to predict the post-operative refractive outcome, one to predict the post-operative ACD and refraction using the Olsen formula, and the other based on the value of ACD in the fellow eyes 1 day and 1 month after operation, after which the authors compared the RE of the two methods. The results indicated that the refractive outcome in the second eyes can be improved by using the post-operative ACD value when the error is relatively large in the first eyes, whereby the post-operative ACD value 1 month after surgery was better for prediction than the one obtained 1 day after the operation. Our follow-up study will focus on the improvement of refractive outcomes 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 promotion and application.