Sequential bilateral cataract surgeries can be contraindicated for some patients due to poor general health conditions, cognitive impairments, mood disorders, or socio-economic factors. In these cases, unilateral cataract surgery is preferred for those patients. Additional research on the effects of unilateral cataract surgery on the fellow eye would guide doctors’ decision making and counseling of patients and their guardians.
In this study, the authors investigate objective changes in the untreated fellow eye after unilateral cataract surgery including visual function, anterior segment parameters, and IOP.
Discussion
Changes in ocular parameters of the treated eyes after cataract surgery have been previously reported [
4-
7]. In this study, we focused on the responses of the untreated fellow eye in the first month after unilateral cataract surgery. The data revealed significant reductions in pupil size, deepening of ACD, and improvement of UDVA in the fellow eyes of our cohort. AOD and TISA showed increasing tendencies.
It is important to understand the settings in which the patients were examined. For all routine ocular examinations, we maintained photopic conditions. For monocular examinations of visual acuity, CS, and AS-OCT, we used a translucent occluder positioned 1.5 cm in front of the eye not being assessed to allow the light to enter both eyes equally, so that when the untreated eye was assessed, an enhanced light reflex remained in the treated eye to maintain a contralateral pupil reflex [
8]. In this way, we were able to assess the fellow eye’s ocular parameters in a more real-world physiologic condition.
Numerous studies have reported objective changes in treated eyes after cataract surgery. During surgery, an average thickness of 4.4 mm of crystalline lens is replaced with an artificial lens that is typically <1 mm thick [
13]; therefore, ACD and anterior chamber volume increase, giving the iris more freedom of movement, which decreases pupil size [
5,
6]. In addition, removal of opaque media increases light stimulus and strengthens the pupillary reflex in the treated eye [
8]. Extraction of the crystalline lens also affects the peripheral angle, and studies report increases in AOD [
7] and peripheral angle degree [
4] in non-glaucomatous open-angle eyes after cataract surgery. IOP becomes lower in treated eyes after cataract surgery due to less crowding at the peripheral angle and increased aqueous humor outflow [
7,
14]. Our results are consistent with previous studies: treated eyes had decreases in pupil size and IOP and increases in ACD, AOD 500, AOD 750, TISA 500, and TISA 750 at postoperative week 1 and month 1.
More light stimulus in the treated eye affects pupil size in the fellow eye through indirect pupillary reflex. Light stimulus entering the optic nerve (cranial nerve II, CN II) is passed to the pretectal nucleus, and crossed and uncrossed nerve fibers are passed to the ipsilateral and contralateral Edinger-Westphal nuclei of the midbrain. Signals are then transmitted to postganglionic nerve fibers that exit with CN III and constrict the ipsilateral and contralateral iris muscles [
8]. Because light signal input increases in the treated eye after cataract extraction, the indirect pupil reflex of the fellow eye also increases. In our study, pupil size in the fellow eye was significantly smaller at postoperative week 1 and month 1. Narrower light entry improves visual acuity by deepening the depth of focus [
15] and reducing peripheral aberration [
16]. On the other hand, smaller pupils can decrease light stimuli to the retina, causing vision to deteriorate. However, in our study, the UDVA of the fellow eye improved significantly at postoperative week 1 and month 1. In a previous study that analyzes the effects of pinhole glasses in patients with no ocular disease (mean age of 35.5 years), UDVA significantly improved from 0.44 ± 0.46 to 0.19 ± 0.25 (
p < 0.001) after pinhole glass application [
15]. Likewise, if media opacities are minimal, pinholes can improve distance visual acuity. In our cohort, cataract grading of the fellow eye was mild (mean nuclear sclerosis, 2.24; cortical opacity, 1.44; posterior supcapsular opacity, 0.16) with a favorable baseline CDVA of 0.14 logMAR. In patients with mild media opacity, smaller pupil size positively affects visual function in the fellow eye by enhancing the depth of focus. On the other hand, for near targets, the near accommodative reflex, which passes through a different midbrain nucleus than the light reflex, controls iris constriction [
17], so the effect of indirect light reflex might be less obvious when aiming at a near target.
No significant changes in the CS of fellow eyes were observed. Smaller pupils may reduce CS by diffraction and decreasing the degree of light stimulus [
18]. On the other hand, larger pupil size can hinder CS by larger aberration [
18]. One study reported that aperture sizes larger than 2.0 mm caused no diffraction-related problems of CS [
19]. In our study, the mean postoperative pupil diameters of fellow eyes were 3.21 ± 0.74 mm at postoperative week 1 and 3.14 ± 0.71 mm at postoperative month 1, which is likely not small enough to cause CS impairment. These data represent an affirmative result that there was no change in CS, even when pupil size decreases.
We hypothesized that stimulating iris contraction in the fellow eye might affect structures around the peripheral angle in the same manner as topical miotics work. Our results revealed that ACD, AOD, and TISA tended to increase, but the increases failed to reach statistical significance, probably hindered by bulky crystalline lens of the fellow eye. It has been reported that 2% pilocarpine, a cholinergic agent used for angle closure glaucoma, reduced pupil diameters from 2.96 ± 1.04 to 2.38 ± 1.11 mm (a 19.6% reduction) in normal populations [
20]. In our cohort, pupil diameter decreased from 3.59 ± 0.86 to 3.14 ± 0.71 mm (a 12.5% reduction) at postoperative month 1. Therefore, although the pupil constriction effect on the fellow eye was not significant, it had a positive effect of widening the surround angle.
In
Table 4, we tried to identify factors that affect visual acuity improvements in untreated fellow eyes. The change of UDVA of fellow eyes at postoperative month 1 was positively correlated with the change of UDVA of treated eyes. Although not fully understood, there is a possibility that improved visual function of the treated eye might affect cognitive ability by enhancing visual stimulus. Cognitive improvements in the elderly after cataract surgery have been reported in previous studies [
21-
23]. One study performed magnetic resonance imaging with glaucoma patients and reported that long-term visual deprivation due to retinal damage reduced grey matter volume in the visual cortex [
24]. In contrast, grey matter volume in the visual cortex increased after unilateral cataract surgery in middle-aged and elderly patients, suggesting brain plasticity [
25]. The authors suggested that improved visual parity between bilateral eyes after unilateral cataract surgery may facilitate stereopsis and binocular fusion, thereby increasing the amount of grey matter in the visual cortex. Although the evidence is indirect, brain plasticity after cataract surgery might affect visual function in the fellow eye after cataract surgery.
For patients whose physical or mental conditions contraindicate sequential bilateral cataract surgery, our study suggests two ideas. First, unilateral cataract surgery can favorably impact the visual acuity of an untreated eye with a mild cataract and similar SE from pinhole effects and brain plasticity. Second, when both eyes have narrow angles, the peripheral angle in the untreated eye may widen, and reduce the risk of intermittent angle closure of the fellow eye. This study suggests reasons to not rush a second eye operation in patients with poor conditions. This suggestion is consistent with a previous study on the impact of bilateral or unilateral cataract surgery on subjective visual functioning. The authors concluded that patients benefit more from bilateral cataract surgery than unilateral cataract surgery only when the fellow eye has a severe cataract or poor visual acuity [
26].
To our knowledge, this is the first study to focus on changes of the untreated eye after monocular cataract surgery. It is necessary to understand that positive visual changes from structural differences in the contralateral eye could occur after unilateral cataract surgery. This information is expected to helpful when unilateral cataract surgery is planned for various reasons.