The Ex-PRESS surgery and the trabeculectomy both significantly reduced IOP even in cases of low preoperative IOP. The trabeculectomy reduced IOP more compared to the EXP. There was no significant between-group difference in the surgeries' success rates.
There have been several reports that both EXP and Trab are useful for treating low-IOP glaucoma. Naito et al. reported that Trab lowered the IOP from 13.9 to 8.1 mmHg at 2 years , and Schultz et al. stated that Trab lowered the IOP from 13.1 to 8.5 mmHg . The present surgical outcomes are equivalent to these previous reports, which also indicated that Trab could suppress the progression of visual field deterioration [4, 9].
In a study by Aihara et al., the use of EXP lowered the IOP from 14.8 to 10.0 mmHg in one year, achieving a 31.1% reduction of IOP . In our present study, the IOP reduction provided by EXP was equivalent to 28.0% at 1 year post-surgery. The Collaborative NTG study reported that a 30% reduction was recommended for normal-tension glaucoma (NTG) . In our present patient series, the reduction in the EXP group at 2 years was 22.8%, whereas that in the Trab group was 33.4%. The reason for this difference is that in the EXP procedure, a small amount of aqueous humor can flow out, and as a result, the IOP might be higher than that achieved with a Trab. We reported that the volume of filtered blebs after EXP declined by 26% per year . It is expected that a large bleb will be maintained with a low IOP for a long period. The trabeculectomy has the advantage of lowering the IOP to a greater degree compared to EXP. Trab had a double layer scleral flap, which could decrease IOP more.
There are several ways to define success after glaucoma surgery . Since the present patients' preoperative IOP was low, we did not use a cut-off IOP value for the definition of surgical success. There was no significant difference between the Trab and EXP surgeries when the <20% or <30% reductions in IOP were successful. Considering that a 30% reduction is recommended, Trab seems to be slightly better. This study was a retrospective analysis and did not compare preoperative and postoperative visual field results; it is thus unclear whether a 20% reduction in the IOP is sufficiently effective. Oie et al. reported that there was a correlation between the IOP reduction ratio and the speed of deterioration in the visual field .
We observe that the postoperative VA was less deteriorated in the EXP group. Since our study included cases of simultaneous cataract surgery, we could not investigate the changes in VA affected by glaucoma surgery without this factor. In the Trab group, the VA was more likely to deteriorate despite the inclusion of more patients with simultaneous cataract surgeries in this group. This might be have contributed to the decrease in the IOP being too low. Naito et al. reported that an IOP <7 mmHg posed a risk of a decline in the VA . An investigation by Beltran et al. revealed that patients who underwent a Trab were more likely to lose ≥2 Snellen lines compared to those who underwent an EXP . Astigmatism is likely to occur when the IOP is low , and astigmatism might affect VA. Several studies reported that VA recovery after EXP was more rapid than that after Trab [18, 23, 25]. EXP might therefore have an advantage concerning postoperative VA.
In the present study, there was no significant between-group difference in the reduction ratio of ECD at 2 years post-surgery, but the reduction of ECD was slightly greater in the Trab group. Several studies reported that Trab or EXP reduced the ECD by 2.2–23.0% in a 2-year period [13, 17, 26, 27]; however, in one of the studies the EXP surgery reduced the ECD rapidly , whereas others observed that EXP could not reduce ECD rapidly [17, 28]. We reported that postoperative ECD varies depending on the insertion position of the EXP . If the EXP is inserted in the correct position (the trabecular meshwork), it might prevent ECD loss more effectively. The EXP procedure does not require the resection of the iris and trabecular meshwork, and it results in less inflammation compared to a Trab. Strong inflammation might decrease the ECD more rapidly [30, 31].
There are many reports that EXP has fewer complications than Trab [11, 17, 18]. In the present study as well, the vitreous hemorrhage and vitreous prolapse that are characteristic of a Trab did not occur in the EXP group. For these reasons, EXP might pose a lower risk of complications. The EXP also has a characteristic complication of tube obstruction , but this did not occur in the present series of patients.
The preoperative IOPs of our patients were very low, and surgeries that require further IOP reduction could pose a high risk of complications associated with low IOP such as shallow anterior chamber, choroidal detachment, and hypotony maculopathy. Notably, the Trab results in more outflow to outside of the eye, and we thus suspected that the Trab might result in more complications associated with low IOP. However, there was no between-group difference in complications associated with low IOP. Arimura et al. reported that EXP caused choroidal detachment in 18% of their patients whereas Trab caused it in 12.5%, which was not a significant difference . Our present results are similar. Appropriate laser suture-lysis could lower the risk of complications.
In terms of medical economy, Patel et al. reported that EXP was associated with greater surgical cost compared with Trab .
There are some study limitations to address. This was a retrospective analysis. There is a risk that the results would vary greatly depending on the preoperative IOP values. We did not consider IOP fluctuations. Our patient population included cases with simultaneous cataract surgery, and it has been reported that simultaneous cataract surgery has poorer surgical results and is more likely to reduce the ECD rapidly [34, 35]. We did not define the indications for glaucoma surgery, cataract surgery, and additional glaucoma medications. The number of patients was small, and the follow-up period was short.
Even in an era when many glaucoma devices are available, the number of surgical methods for patients with low preoperative IOP might be limited. Since both EXP and Trab could adjust the postoperative IOP to some extent, these surgeries were considered first for our patients. Trab has a greater ability to lower the IOP. Since the surgical results of glaucoma focus on the postoperative IOP, it seems that Trab is the better surgical method. However, we cannot recommend surgery that has a high possibility of postoperative complications, even if the IOP decreases more. Both surgeries were useful for patients with low preoperative IOP, and both have advantages and disadvantages. It is necessary to judge these surgeries by the outcomes over a long-term follow-up.