Crossed versus conventional pseudophakic monovision for high myopic eyes: a prospective, randomized pilot study

This study was a prospective randomized comparative study. Patients with myopic diopter greater than − 6.0D (including − 6.0D), axial length greater than 26.00 mm (including 26.00 mm), and lens opacification that affect life quality, who had strong desire of spectacle independence have been included in this study. All the subjects underwent bilateral cataract surgery at the First Affiliated Hospital of Chongqing Medical University from November 2018 to September 2019. This research adhered to the tenets of the Declaration of Helsinki and Uniform Requirements for manuscripts submitted to Biomedical journals.

The exclusion criteria included any other ocular pathological conditions that would affect the visual acuity after surgery, such as dysfunction of optic nerve, macula, retina or cornea; severe ocular opaque other than cataract; history of non-cataract ocular surgery or inflammation; eyes planned for extracapsular cataract extraction; intractable synechia of iris; strabismus; individuals who cannot identify the dominant eye before surgery; patients refusing to be treated or having difficulty for follow-up examination.

Severe anisometropia (greater than 3.0D) may decrease stereoscopic function, thus insufficient unaided reading capacity also leads to dissatisfaction [18]. We assigned all the subjects randomly to one of the two groups before surgery. Based on the results of previous studies [21, 22, 24] and clinical experience, the design used in this study was about 1.5D anisometropia between both eyes to provide relatively sufficient near visual ability while avoiding losing the tolerance of stereopsis [20, 24]: the crossed monovision group was assigned with patients whose refraction was targeted to − 2.00D in the dominant eye and − 0.50D in the non-dominant eye, and the conventional monovision group with patients whose refraction was targeted to − 0.50D in the dominant eye and − 2.00D in the non-dominant eye.

The dominant eye was determined by the hole-in-card test in which each patient was asked to look at a fixed target at 2 m through a 3 cm diameter hole in a center of a cardboard held in their hands. The eyes were then covered one at a time and the eye that kept the alignment was recorded as the dominant eye. The test was repeated three times to confirm the dominant eye.

This was a pilot prospective, randomized clinical trial. Randomization was carried out according to simple randomization with randomized table. For each group, 25 participants were planned to randomly assigned, received intended treatment, and were analyzed for the primary outcome. The coordinate staff assigned each patient randomly to one of the two groups. The grouping information was passed by the coordinate staff to a member of the operating room to prepare the IOL. The surgeon performing the surgery was not informed to which group the patients belonged. Neither the patients, examiners, nor data analysts knew the group assignment.

Proceeded from the principle of double blindness, neither the surgeons nor the patients knew the specific grouping before and after operation. Before the surgery, the assistant staff had informed and helped the patient understand the design method of monovision and its possible advantages and disadvantages.

Before operation, patients had a complete preoperative ophthalmic examination including binocular uncorrected distance visual acuity (BUDVA), binocular uncorrected near visual acuity (BUNVA), binocular best corrected distance visual acuity (BCDVA), binocular best corrected near visual acuity (BCNVA), subjective and objective refraction, biomicroscopy of the anterior and posterior eye segments, intraocular pressure (IOP), macular optical coherence tomography (OCT), keratometry, optical biometry. The IOLMaster 700 (Carl Zeiss Meditech AG, Jena, Germany) was used to measure preoperative axial length. Corneal curvature was measured using PENTACAM (PENTACAM®OCULUS, Germany), and the mean value of these meridians were used for the IOL power calculation. The IOL power was calculated using Haigis formula [25, 26].

Standard phacoemulsification with IOL implantation was performed by one surgeon (KH M.D.). Under topical anesthesia, the corneoscleral incision was made at 135 degrees. After continuous circular capsulorhexis, cataract phacoemulsification and cortex aspiration were performed. The width of the main incision was 2.8 mm and the diameter of capsulorhexis was 5.5 mm. Then a monofocal aspheric posterior chamber IOL (Tesnis 1-piece ZCB00, Abbott Medical OPTICS, Inc) was implanted in the capsular bag.

Antibiotic, steriod and non-steroid anti-inflammatory eye drops were used 2 weeks after surgery.

Examinations of visual function including BUDVA, BUNVA, BCDVA, BCNVA, and the manifest spherical equivalent value (MRSE) were conducted 2 weeks, 1 month and 3 months after binocular consecutive surgery. MRSEs were calculated using spherical and cylindrical powers (MRSE = sph + cyl/2). An application system (Zengshineng, Galen RS20YY-JT2005) of visual bio-informatics stimulation technology was used for assistant examination of the stereoscopic function [27, 28]. The stereoacuity at 0.8 m (near) and 1.5 m (distant) was categorized into three levels respectively as normal (100 arcsec), reduced (200–400 arcsec) and lack of stereopsis. The contrast visual acuity (contrast VA) was examined at high to low contrast levels using the contrast sensitivity tester (Vista Vision™ Clinica De Oftalmologie, Romania). A questionnaire based on the Visual Function Questionnaire 25 [29] was requested to complete 3 months after bilateral cataract surgery. On the questionnaire, patients were asked to rate their need for glasses or contact lenses after bilateral surgery from three levels (never need glasses or contact lenses, sometimes need glasses or contact lenses, always dependent on glasses or contact lenses). Patients were also asked other questions: rating their satisfaction from four levels (very happy, happy, neutral and unhappy); rating their eye-hand and eye-feet coordination without glasses (or contact lenses) from three levels (no problem, almost no and sometimes); rating their sports-related activities without glasses (or contact lenses) coordination from three levels (no problem, almost no and sometimes); rating their frequency of using glasses (or contact lenses) for near (within 0.4 m) / intermediate (1-3 m) / far (> 3 m) distance activities from three levels (never, sometimes and always). The results of the questionnaire were collected anonymously and recorded by face-to-face inquiry.

Nonparametric tests were used for some data that do not follow the normal distribution. All the visual acuity was expressed as the logarithm of the minimum angle of resolution (logMAR). Continuous variables such as binocular uncorrected or corrected logMAR visual acuity (BUDVA, BUNVA, BCDVA, BCNVA), MRSE, and contrast visual acuity were compared between the conventional and crossed monovision groups, using the Mann-Whitney U test. Categorical variables such as the ratio of men to women and the grades of stereopsis were compared between the two groups using the χ² test and Fisher exact test. Statistical analyses were performed using SPSS statistical software (version 22.0; SPSS Inc., Chicago, IL). The sample size was calculate based on 80% statistical test power, 0.05 significance level. A P value of less than 0.05 was considered statistically significant.

In the crossed monovision group, mean MRSE 3 months post-operation was − 1.76 ± 0.54D in the dominant eyes and − 0.31 ± 0.94D in the non-dominant eyes, while in the conventional monovision group mean refraction was − 0.39 ± 0.82D in the dominant eyes and − 1.72 ± 0.85D in the non-dominant eyes. The mean MRSE in the conventional monovision group was not statistically different as compared with that of the crossed monovision group on 2 weeks,1 month and 3 months follow-up after surgery (P > 0.05) (Table 3). However, the mean MRSE in eyes of near vision of the two groups did not reach the set level on 1 month or 3 month (Table 3), but not statistically different from that of 2 weeks post-operation (P > 0.05).

Binocular uncorrected and corrected visual acuity for both near (0.4 m) and far (5 m) distance (BUDVA, BUNVA, BCDVA, BCNVA) didn’t show statistical difference between the two groups at postoperative 2 weeks (P = 0.761 for BUDVA, P = 0.829 for BUNVA, P = 0.831 for BCDVA, and P = 0.283 for BCNVA), 1 months (P = 0.854 for BUDVA, P = 0.434 for BUNVA, P = 0.251 for BCDVA, and P = 0.538 for BCNVA) or 3 months (P = 0.573 for BUDVA, P = 0.576 for BUNVA, P = 0.792 for BCDVA and P = 0.263 for BCNVA). (Table 2).

The bilateral contrast visual acuity, which was converted into the logarithm of the minimum angle of resolution (logMAR) was better in conventional monovision group than that of crossed monovision group, however it had no statistical difference between the two groups (P > 0.05) at the seven level of contrast percentages of visual targets of postoperative 2 weeks, 1 month or 3 months (Fig. 1).

The differences of binocular near stereopsis between the two groups were not statistically significant among the three checkpoints (P > 0.05, Fig. 2). Meanwhile, the results of distant stereopsis also showed no significant differences between the two groups (P > 0.05, Fig. 2).

A total of 20 (83.3%) patients of the conventional group and 18 (81.8.0%) in the crossed group didn’t wear spectacles after cataract surgeries, which has no statistically significant difference between two groups (P = 1.000). Patients’ satisfaction, eye-hand and eye-feet coordination, sports-related coordination, difficulties for near (0.4 m) distance tasks without glasses (or contact lenses), difficulties for intermediate distance activities without glasses (or contact lenses) and difficulties for far (> 3 m) distance tasks without glasses (or contact lenses) didn’t differ significantly between the crossed and the conventional group 3 months after surgery (P > 0.05, Fig. 3).

After binocular surgery, we measured the eye dominance of all patients again at postoperative 2 weeks, 1 month and 3 month. No change of eye dominance from one to the other was found.