EXP surgery could significantly reduce IOP even in cases of low preoperative IOP. It is difficult to compare surgical outcomes between patients with low and high preoperative IOP values. When the cut-off value for success is lowered, the Low IOP Group tends to show better surgical results, and when the IOP reduction rate for success is set higher, the High IOP Group tends to show better surgical results. There are many definitions of success in glaucoma surgery [16]. The definition, therefore, might affect the interpretation of surgical outcomes.
There have been few reports on the usefulness of EXP surgery for low IOP glaucoma. Aihara et al. report that EXP surgery could lower IOP from 14.8 to 10.0 mmHg in one year, achieving a 31.1% reduction in IOP [15]. A collaborative normal-tension glaucoma (NTG) study reports that a 30% reduction is recommended for NTG [17]. Oie et al. found a correlation between the IOP reduction ratio and the speed of visual field progression [18]. Nevertheless, in the present study, in patients with a very low preoperative IOP (the mean was 13.2 mmHg), the IOP reduction ratio was 31.0% 3 years after surgery, while in the high IOP group, the reduction rate was 44.4%.
In the present study, the success rate at 3 years was 65.4% in the Low IOP Group and 77.4% in the High IOP Group. Previous studies have reported successful surgical outcomes of 53.0-73.1% at 3 years after EXP surgery [14, 19–23]. Although definitions of success vary, our surgical outcomes are comparable to those of previous studies.
There are some limitations to this study. This is a retrospective and single-facility study. There is a risk that the results would vary greatly depending on the preoperative IOP value. We did not consider IOP fluctuations. Our study included cases of simultaneous cataract surgery. It has been reported that simultaneous cataract surgery yields poorer surgical results [20]. We included a case who had previously undergone trabeculotomy. Mariotti et al. report that identifiable risk factors for failure of EXP surger include diabetes, non-Caucasian race, and previous glaucoma surgery [24]. We did not define the indications for glaucoma surgery, cataract surgery or additional glaucoma medications. And finally, the number of patients was small, and the follow-up period was short.
Even in an era when many glaucoma devices have been developed, surgical methods for patients with low preoperative IOP might be limited. EXP surgery is effective for low preoperative IOP patients.