Microinvasive Glaucoma Surgeries

iSTENT (Generation 1, G1)

The first generation iStent is L-shaped, with a 1-mm-long body and an angled pointed tip designed to pierce the TM tangentially ( Fig. 10.26.1 ). There are retention rings on the outer aspect of the hollow body to keep the stent in place. It is composed of nonferromagnetic titanium with heparin coating and received FDA approval for placement in conjunction with cataract surgery in patients with mild-to-moderate open-angle glaucoma in June 2012.

Technique

This procedure can be done before or after phaco. The anterior chamber is filled with viscoelastic, particularly along the nasal angle. The patient’s head is tilted 30°–45° away from the surgeon and the operating microscope tilted to a similar degree toward the surgeon. Along with a coupling agent (most commonly a viscoelastic), a direct gonioscopy lens (Koeppe, Hoskins-Barkan, Swan-Jacob, or Richardson lens) is gently placed on the ocular surface to view an upright direct image of the angle.

The inserter is introduced through the main wound. The tip of the device should pierce the TM at a 30° angle. Then, pivoting within the temporal wound, the surgeon gently slides or sweeps the device into the canal. Reflux of blood from the snorkel end of the device is expected. If the blood causes loss of visualization, viscoelastic should be added locally to tamponade the bleeding and push the refluxed blood away from the area of interest. The inserter can be used to push the device and ensure it is in the correct position. During implantation, movement of the eye indicates that the iStent is hitting the back wall of Schlemm’s canal and a flatter approach should be taken.

Results

Bahler et al. demonstrated that insertion of one stent nearly doubles outflow facility, and insertion of two stents more than quadruples it. Other strong evidence comes from a randomized, parallel trial by Samuelson et al. investigating 240 eyes randomized to phaco plus iStent or phaco alone. They found a 22% difference in the proportion of patients in the iStent group with an unmedicated IOP ≤21 mm Hg compared with the control group at 12 months (72% vs. 50%; p < 0.001). In addition, there was a significant difference in the proportion of patients who achieved at least a 20% IOP decrease without ocular hypotensive medication (66% phaco plus iStent vs. 48% phaco alone). There were no vision-threatening complications observed during the study period. Reported iStent-related complications included hyphema (0%–70%), stent malposition (2.6%–3%), and stent obstruction (4%–30%). At 2 years, the IOP decrease from baseline was 8% in the phaco plus iStent group versus 1% in the phaco group alone.

Technique

This procedure can also be done before or after phaco. The anterior chamber is filled with viscoelastic. The microscope is tilted as previously described. A direct gonioscopy lens is gently placed on the ocular surface to view an upright direct image of the angle. The preloaded handpiece is introduced into the eye, and the protective sleeve is retracted. The device delivery tip, or trocar, is used to pierce the TM to create a slight dimpling before the stent is deployed. Pushing too hard will result in a misfire, and the stent may not deploy or be “overinserted” too deep. Not applying enough pressure to create a dimple may lead to superficial deployment. Once the TM is punctured by the trocar, the surgeon must then ensure that the trocar is centered within the shaft of the injector. The most common error is to have a slight bend or bias in the trocar that can cause stents to misfire or damage the injector, preventing the deployment of additional stents. The two stents are generally placed several clock hours apart depending on the view and comfort level of the surgeon.

Results

The US Investigational Device Exemption pivotal trial randomized patients with mild to moderate glaucoma to iStent plus phaco ( n = 387) or phaco alone ( n = 118). The investigators found the iStent inject group achieved a statistically significant reduction in unmedicated IOP, with 75.3% of the iStent inject plus phaco cohort achieving a 20% reduction in IOP compared with 61.9% in the phaco-only cohort at 24 months. The most common postoperative issues with the iStent inject were inability to visualize both stents, stent obstruction, and IOP spikes.

Technique

This procedure is performed in a similar manner to the procedures previously described. The eye is filled with viscoelastic and the head/scope properly positioned to obtain a direct view of the angle. Similar to iStent, this device is preloaded in an injector. However, the technique with Hydrus is different compared with TM stenting. An incision should be made lateral to the traditional temporal incision for phaco, in what would be directly in line with the intended area of placement. The injector must carefully pierce the TM, then as the slider is advanced forward, pressure must be released (avoid indenting the TM) to safely deploy the device. When complete, blood can be seen refluxing from the tip of the stent. After successful implantation in the angle, the windows of the device should be visualized through the translucent TM for 90°, and the 1-mm inlet portion should remain in the anterior chamber.

Results

The first peer-reviewed study was a randomized, parallel trial completed in 2015 that compared phaco with Hydrus versus standalone phaco in patients with primary open-angle glaucoma (POAG), pseudoexfoliation, or pigmentary glaucoma. Seventy-three percent of combined patients were off medications at the end of the study, while only 38% of patients in the phaco alone group were medication free. The HORIZON study (2019) randomized 369 eyes to phaco with Hydrus Microstent and 187 eyes to phaco alone and followed the patients for 24 months. Unmedicated IOP was reduced by ≥20% in 77.3% of Hydrus Microstent group compared with 57.8% of the phaco alone group (difference = 19.5%, 95% confidence interval [CI]: 11.2%–27.8%; p < 0.001). Further, the number of medications was reduced by 1.4 in the Hydrus Microstent group compared with 1.0 in the phaco alone group ( p < 0.001). The COMPARE trial (2020), which randomized eyes to receive standalone treatment in phakic patients with the Hydrus Microstent or two first-generation iStents, found that the Hydrus outperformed two iStents in reducing medication requirement at 12 months (46.6% of Hydrus patients were medication free compared with 24% of iStent patients).

The most common complication after Hydrus Microstent implantation is peripheral anterior synechiae (PAS) formation (18% of patients with focal 1- to 2-mm PAS in the Hydrus II study). Other potential postoperative complications include intraoperative or postoperative hyphema, macular edema, and transient IOP spikes. If iris adhesions to the implant develop and obstruct flow, argon laser can be used to attempt to relieve microstent obstruction.

Technique

This procedure can be done before or after phaco. First, a clear corneal incision at least 1.6–1.8 mm in size is made. Some suggest lateral enlargement of this wound to improve range and eliminate possible striae. The microscope is then tilted 30°–45° toward the surgeon and the patient’s head away from the surgeon. To deepen the chamber, the irrigation bottle should be raised to maximum height with continuous irrigation, because viscoelastic is not used during ablation to avoid optical interfaces and trapping of ablation bubbles. Under direct gonioscopic view, the handpiece is used to engage and ablate the nasal TM. It is important to confirm that the pigmented TM has been properly ablated by visualizing the white of the outer wall of Schlemm’s canal (SC). Blackening, in contrast, indicates coagulative heat damage that may result in poor wound healing. An “outward push” during ablation should be avoided to prevent damage to distal collector channels. The handpiece can be rotated to ablate up to 180° of the TM using both forehand and backhand techniques. Postoperatively, surgeons may prescribe pilocarpine 1%–2% for up to 8 weeks to help prevent PAS formation.