Cataract surgery has the advantage of quick visual recovery and relatively low complications [27–29]. Since cataract surgery alone has an effect of reducing IOP in patients with angle closure glaucoma (ACG) [16, 17, 30] and open angle glaucoma (OAG) [31–33], phacoemulsification plays an important role in the treatment of glaucoma. In patients with ACG, phacoemulsification significantly deepens the ACD and resolves angle crowding. Because these changes are small in patients with OAG, the exact mechanism why IOP decreases after cataract surgery in patients with OAG remains controversial [29]. Some studies reported that both changes of angle configuration and trabecular meshwork or extracellular matrix remodeling are involved [27, 28]. In the current study, the fact that IOP significantly decreased postoperatively in the phaco-only group also supports previous studies.
Preoperatively, the ACD of the combined group was significantly shallower than that of the phaco-only group. This finding can be explained by the prevalence of ACG (34 eyes) in the combined group, whereas the phaco-only group had twelve patients with ACG. Previous studies reported that the refractive outcome of patients with ACG was difficult to predict [34, 35], but our result showed relatively stable refractive outcome despite the large number of patients with ACG in the combined group.
Recently, the indications of phacotrabeculectomy are as follows: i) medically uncontrolled glaucoma, ii) tolerance of glaucoma medications, iii) postoperative IOP spikes may worsen visual field damage, iv) suspected compliance of glaucoma medications [15–17, 36]. Several studies reported that the IOP lowering effect of phacotrabeculectomy was superior than that of phacoemulsification alone in patients with glaucoma [36–38]. In the present study, the mean preoperative IOP of combined group was 29.60 ± 10.04 mmHg. It was significantly reduced to average of 13.35 ± 3.12 mmHg at 12 months postoperatively, and the postoperative need of glaucoma medications was significantly decreased. As in the previous studies, phacotrabeculectomy was found to be an effective treatment for lowering IOP.
In this study, we aimed to investigate the long-term refractive outcomes after phacotrabeculectomy, and we found that there was no significant difference in refractive outcomes compared to phacoemulsification alone until 24 months after surgery. It means that phacotrabeculectomy was effective not only for IOP control but also for stable refractive outcomes. Previous studies reported that myopic shift occurs after phacotrabeculectomy compared to phacoemulsification alone [11, 23–25]. A decreased ACD after trabeculectomy causes a myopic shift, and a decreased AL after trabeculectomy causes a hyperopic shift, conversely. Some authors of these studies estimated that a decrease in ACD had a greater effect on refractive errors than a decrease in AL, leading to myopic shift. In the current study, since the refractive errors were compared using the absolute value of the difference of refractive errors, we could not analyze whether myopic shift or hyperopic shift occurred after phacotrabeculectomy.
Law et al. [24] reported that K-value was increased after phacotrabeculectomy. Changes of not only ACD and AL but also K-value affect refractive errors, and if K-value increases, hyperopic shift may occur. Since this study did not measure postoperative K-value, ACD, and AL, it was not possible to analyze which factors had more significant effect on refractive outcomes. However, we hypothesized that there was no significant difference in refractive outcomes between combined group and phaco-only group because the changes of ACD, AL, and K-value after phacotrabeculectomy had a global effect.
Most of the studies that focused the refractive outcomes of phacotrabeculectomy analyzed only short-term refractive outcomes less than 6 months [11, 18, 22–25], and one study reported by Chung et al. [12] had a limitation that the follow-up period of the control group was average of 4.81 months. Therefore, the present study is clinically significant because we analyzed long-term refractive outcomes of phacotrabeculectomy up to 24 months postoperatively.
Tzu et al. [39] reported that the risk factor for refractive errors in combined cataract and glaucoma surgery included old age. Though the follow-up time was less than 6 months and glaucoma drainage device surgery was included in the combined group, it remains a meaningful result. Old age has been associated with structural changes of scleral collagen fiber and changes of ACD [40, 41], therefore, some studies reported that age of patients may affect the refractive outcomes after cataract surgery [42, 43]. Our study also supports this result, as old age appeared to be risk factor up to 12 months after phacotrabeculectomy. However, old age did not appear as a risk factor after 24 months of phacotrabeculectomy, which may be because of the relatively small number of patients.
Our group previously reported that one risk factor causing unpredictable refractive errors after cataract surgery in patients with glaucoma was large LV [44]. We speculated that large LV predispose to larger displacement of IOL position, resulting in unstable refractive outcomes. In agreement with the previous report, in the current study, large LV was a risk factor that could cause refractive errors up to 12 and 24 months after phacotrabeculectomy. Therefore, we suggest that LV plays an important role in predicting refractive errors after combined phacotrabeculectomy surgery in glaucoma patients as well as cataract surgery.
Ozaki et al. [45] reported that the LV of primary angle closure (PAC) patients was 1.034 mm on average, and that of normal people was 0.419 mm, and Hsia YC et al. [46] reported that LV of OAG patients was 0.55 mm on average. It has already been found that the increased LV is a risk factor of PAC and a predictive factor for refractive outcomes after cataract surgery in patients with glaucoma [44, 45], but there have been no studies analyzing the cut-off value of LV that can cause unpredictable refractive errors. We analyzed the long-term data after phacotrabeculectomy and found that a LV thickness of 0.855 or more was a risk factor for unstable refractive outcomes. Therefore, the surgeon should be very careful when operating patients with LV greater than 0.855 mm because unpredictable refractive errors can be obtained.
The current study has several limitations. First, we could not analyze which ocular parameters had a significant effect on refractive outcomes because some ocular parameters were not measured after surgery. Second, both patients with ACG and patients with OAG were included in the combined group, so the effect of angle status was not considered. Third, we did not consider digital massage or releasable suture removal that could affect refractive errors after phacotrabeculectomy. Finally, we did not analyze the complications that could occur after phacotrabeculectomy because only patients without complication were included. In the future, larger and more long-term studies that consider these factors will be needed.