The performance of the Hydrus Microstent in reducing IOP was evaluated in the HYDRUS II study: a prospective, single-masked, randomized controlled clinical trial conducted in seven European centers . Patients were randomized to either cataract surgery combined with Hydrus Microstent implantation or cataract surgery alone and then followed up for 2 years. In order to accurately determine the efficacy of the surgery, patients underwent a washout of their hypotensive medications, similar to the protocol in the Ocular Hypertension Treatment Study  (Table 5.1).
One year after Hydrus Microstent implantation in combination with cataract surgery, the mean washed-out IOP decreased significantly by 9.7 mmHg (from 26.3 mmHg to 16.6 mmHg) compared with preoperative IOP. This effect persisted for 2 years after surgery, with the mean washed out diurnal IOP only 0.3 mmHg higher than that at 1 year at 16.9 mmHg. There was also a significant decrease in IOP at 1 year for eyes which underwent cataract surgery alone, with a mean reduction of 9.2 mmHg (from 26.6 mmHg to 17.4 mmHg). However, at 2 years, there was an increase in IOP to 19.2 mmHg in eyes which underwent cataract surgery alone, and this was significantly higher than the 2-year IOP in eyes which underwent Hydrus Microstent implantation in combination with cataract surgery.
Combined Hydrus Microstent implantation and cataract surgery were also associated with a reduction in glaucoma medications, with 77.1% and 72.3% of patients requiring no medications to achieve target IOP at 1 year and 2 years after surgery, respectively. Amongst patients who underwent cataract surgery alone, the proportion of patients who were medication-free at 1 year and 2 years after surgery was significantly lower compared with those who underwent combined Hydrus Microstent implantation and cataract surgery, at 49% and 36.4%, respectively. Moreover, patients who underwent combined Hydrus Microstent implantation and cataract surgery required only half the mean number of glaucoma medications at 2 years compared with those who underwent cataract surgery alone.
The results of the Hydrus II study are similar to that of other smaller case series. Ahmed et al. reported the 6-month outcomes of 28 eyes with mild-to-moderate primary open-angle glaucoma after combined phacoemulsification and Hydrus implantation . Baseline IOP was 17.9 ± 4.1 mmHg with 2.4 ± 1.0 glaucoma medications, and washed-out IOP was 29.9 ± 5.8 mmHg before surgery. The IOP and the mean number of glaucoma medications at 6 months were significantly reduced to 15.3 ± 2.3 mmHg and 0.1 ± 0.4, respectively. Fea et al. reported the 2-year results of 92 eyes that underwent combined cataract surgery and Hydrus Microstent implantation . This was a retrospective study with no washout of glaucoma medications. Nevertheless, a 20% reduction in the mean IOP from 19.4 mmHg preoperatively to 15.7 mmHg at 2 years after surgery was observed. In a single-center and single-surgeon observational study, Al-Mugheiry et al. reported the 2-year results of 25 eyes which underwent combined Hydrus Microstent implantation and cataract surgery. At the end of 2 years, the mean medicated IOP was reduced from 18.1 (±3.6) mmHg to 15.3 (±2.2) mmHg .
The HORIZON study was a 24-month prospective, multicenter, single-masked randomized controlled trial which compared the reduction in IOP and medication use in subjects who underwent combined cataract surgery and Hydrus Microstent implantation (HMS, n = 369) with those who underwent cataract surgery alone (NMS, n = 187). At 24 months, 77.3% of the HMS group eyes achieved ≥20% reduction in unmedicated modified diurnal IOP (MDIOP) compared with 57.8% of NMS group eyes (difference = 19.5%, 95% confidence interval [CI] 11.2%–27.8%, p < 0.001). The mean ± standard deviation decrease in unmedicated MDIOP was −7.6 ± 4.1 mmHg in the HMS group and −5.3 ± 3.9 mmHg in the NMS group at 24 months (difference = 2.3 mmHg, 95% CI −3.0 to −1.6, p < 0.001). The safety profile was similar in both groups with no serious ocular adverse events related to the microstent . These results confirmed the findings of the HYDRUS II study, that the efficacy of combined phacoemulsification and Hydrus Microstent implantation in lowering the IOP and glaucoma medications was superior to phacoemulsification alone.
Although Hydrus Microstent implantation is typically performed in combination with cataract surgery, the device can also be implanted as a solo procedure. Fea et al. reported the 1-year outcomes of Hydrus Microstent implantation as a solo procedure in 31 eyes (20 phakic, 11 pseudophakic) with primary open-angle glaucoma . One year after surgery, the mean IOP decreased by 6.6 ± 5.6 mmHg (from 23.1 mmHg to 16.5 mmHg) and 47% of eyes were medication-free. This nonrandomized prospective study also compared the results of Microstent implantation with selective laser trabeculoplasty. At 1 year, IOP reduction was similar between Hydrus Microstent implantation and selective laser trabeculoplasty. However, eyes that underwent Hydrus Microstent implantation required significantly fewer medications to achieve target IOP compared with those which underwent selective laser trabeculoplasty, even though the eyes which underwent Hydrus Microstent implantation had more severe glaucoma. Gandolfi et al. reported the 2-year results of stand-alone Hydrus Microstent implantation compared with canaloplasty . There was a significant and similar IOP decrease in both groups, from 24 ± 6 mmHg to 15 ± 3 mmHg in eyes that underwent Hydrus Microstent implantation and from 26 ± 4 mmHg to 16 ± 2 mmHg in eyes which underwent canaloplasty. In the COMPARE study, Ahmed et al. randomized 152 eyes from 152 patients with open-angle glaucoma to standalone MIGS consisting of either one Hydrus Microstent or two iStent Trabecular Micro-Bypass devices in a prospective, multicenter, randomized clinical trial. At 12 months, the Hydrus group had a higher rate of complete success (39.7% vs. 13.3%, p < 0.001) and reduced medication use (difference = −0.6 medications, p = 0.004), with more patients in the Hydrus group being medication-free (difference = 22.6%, p = 0.0057) .