Comparison of pars plana with anterior chamber glaucoma drainage device implantation for glaucoma: a meta-analysis

Glaucoma drainage device (GDD) was invented by Molteno in 1969 [1]. Since then, it has been widely used in the management of refractory glaucoma for more than 4 decades. A high success rate with an excellent intraocular pressure (IOP) control is achieved. In the last few years, some surgeons even recommended it as a first-line surgery for glaucoma.

In a standard procedure, the GDD is applied in the anterior chamber (AC) and serves as a shunter to draw aqueous humor through a tube to a subconjunctival end plate. However, it has been reported that many serious complications occurred during the follow up, especially in the anterior segment, such as corneal failure, flat anterior chamber, hyphema etc. [2‐7]. Lots of attempts have been made to address these problems. This, in turn, usually aggravates the burden of patients both physically and mentally. Besides, for patients with inadequate anatomical space in the AC or those with previously compromised corneas, it is not feasible.

As an alternative surgery, pars plana (PP) insertion of a GDD into the vitreous cavity was first described in 1991 [8]. Nevertheless, it has its own operative risks as well, including vitreous incarceration of the tube, vitreous hemorrhage, and retinal detachment [9].

Until recently, only a few comparative studies have compared these two kinds of surgeries in the treatment of glaucoma [10‐16]. To the best of our knowledge, comparisons of the efficiency and safety between these two methods have not been systematically reviewed and published. Therefore, we systematically analyzed the available literature to evaluate the efficiency and safety of PP GDD implantation versus AC GDD implantation for glaucoma.

In our present study, four retrospective clinical studies were reviewed. After pooling the results of these studies, we found that both procedures shared similar efficacy of reduction in the IOP and number of glaucoma medications. For safety, both procedures resulted in similar complications in terms of corneal failure and overall complications.

In a long time, glaucoma drainage devices have been reserved for patients diagnosed with refractory glaucoma. However, with its favorable advantages, it has been introduced to routine surgical management of glaucoma patients as well. In the traditional surgery, GDD is inserted into the anterior chamber. And it shunts aqueous outflow from the anterior chamber into the subconjunctival space, which could lower IOP to normal values. In spite of all these advantages, the routine placement of GDD has been associated with serious anterior segment complications [19]. To avoid this, pars plana insertion of a GDD into the vitreous cavity was first described in 1991 [8]. Theoretically, the posterior location of GDD could reduce the risk of anterior segment complications. And in recent years, several clinical studies were designed to compare the therapeutic effects between the two methods [10‐12, 14]. According to what we know, this is the first meta analysis to evaluate clinical effects and safety between PP GDD and AC GDD for glaucoma.

In our analysis, we found that the reductions of IOP and glaucoma medications in both groups were both remarkable. According to the four included studies, there was no statistically significant difference in efficiency between two procedures.

On the other hand, the incidence of corneal failure and overall complications were compared and analyzed respectively. Since PP GDD avoided the interference of anterior chamber, it was conjectured that it might come out with a lower corneal failure incidence. However, based on our analysis, we did not find any obviously difference. Of these, only in one study, Seo et al., 2015 compared the changes in corneal endothelial cells after PP GDD with those after AC GDD and its average follow-up was 18 months [12]. It showed significant difference between the 2 groups, which illustrated that endothelial cell damage in the PP GDD group appeared to be lower than that in the AC GDD group. Other clinical studies did not show specific details on endothelial cell number. Some further studies may be needed to confirm the effect on endothelial cell. So far, the two procedures had similar corneal failure incidence. This may be explained by the reason that the effect on corneal endothelial cells may not be so obviously different that the incidence of corneal failure was unable to detect this change in both groups. With regard to overall complications, the two groups did not show any significant advantages over the other one. This may be explained by the fact that clinicians have more effective means to intervene in case of anterior segment complications and the outcomes are better. However, when it comes to the posterior segment complications, the surgical intervention and treatment are limited with poor outcomes generally. Several studies analyzed this and our results were similar to theris [10‐16].

However, several limitations should be taken into account when considering the results of this meta analysis. First, in reviewing the literature, the studies included are all retrospective studies because of the absence of randomized studies in the database, which may have potential sources of selection bias. There is still the possibility of underlying bias where there are different clinical indications for placing AC and PP GDD. As the PP is often reserved for cases where AC is not feasible or the cornea is at a high risk, we cannot exclude this bias. This needs to be confirmed with further RCTs, which are still rare in the publishing studies. Typically for meta-analysis research, publication bias cannot be excluded. Additionally, the pooled data were only from the mean follow up of various studies of different durations, introducing a potential heterogeneity. And we speculated that outcomes were stabilized after 1.5 years. Later, the state of IOP, number of glaucoma medications and the incidence of corneal failure and overall complications became stable. The changes were subtle if the differences existed. On the other hand, different studies adopted different criteria for participants. It might be another source of heterogeneity in the results. Also, the antiglaucoma therapy difference among the studies served as another point of heterogeneity, which should not be neglected. Finally, because of the limited number of studies available in the analysis, we did not perform subgroup analysis.

Our results showed that both the PP GDD and AC GDD procedures had similar efficacy of reduction in the IOP and number of medications. They are also both comparable on the safety with similar incidence of corneal failure and overall complications. However, there is still an urgent need for pragmatic RCT with long duration and a large sample size to further determine the efficacy and safety (especially, the endothelial cell number change) of PP GDD in the treatment of glaucoma.