Microinvasive Glaucoma Surgery (MIGS)
- 1 Introduction
- 2 Indications
- 3 Contraindications
- 4 Minimally invasive glaucoma surgery approaches 16
- 5 Devices and Outcomes
- 6 Conclusion
- 7 References
Open angle glaucoma (OAG) is a disease affecting an estimated 2.8 million people in the United States as of 20101, accounting for an approximate cost of $1.5 billion per year.2 Initial therapy for OAG typically consists of medications that can aid in decreasing aqueous humor production or increasing aqueous humor outflow. Additionally, trabeculoplasty is another option for the treatment of early, mild, open angle glaucoma, though long-term efficacy has been found to be similar to medical management.3 Traditionally, pharmacologic and laser treatment have been the mainstays of therapy until the disease progresses to a stage that warrants the need for trabeculectomy. This procedure is typically reserved for those with moderate to advanced cases of glaucoma due to the invasiveness of the procedure and possible complications.4 While there had previously been limited surgical options for patients with mild-moderate glaucoma, developing micro invasive glaucoma surgery (MIGS) procedures are creating new options for those who do not meet the criteria for trabeculectomy. These procedures have a higher safety profile with fewer complications and a more rapid recovery time than other invasive techniques. They have been shown to be effective in decreasing IOP as well as a patient’s need for medications, which is important given a typically low compliance rate for medication adherence.5
Patients that are candidates for microinvasive glaucoma surgery are:
- Patients with mild-moderate glaucoma
- Primary open-angle glaucoma, pseudoexfoliation glaucoma, or pigmentary dispersion glaucoma
- Glaucoma is uncontrolled with maximum pharmacologic treatment or there are barriers preventing adequate medication dosing
- Age greater than 18
- Patients with clinically significant cataract, as surgery may be performed simultaneously.
All patients should have a pre-operative comprehensive eye exam including gonioscopy and a detailed medical history.
Relative contraindications for this procedure may include angle-closure glaucoma, secondary glaucoma moderate-advanced glaucoma, previous glaucoma surgery, or severely uncontrolled IOP. Other considerations include patients with previous refractive procedures as well as monocular patients.6
Minimally invasive glaucoma surgery approaches 16
Increasing trabecular outflow
- Hydrus stent
- Gonioscopy assisted transluminal trabeculotomy
- Excimer laser trabeculotomy
Reducing aqueous production
Devices and Outcomes
Trabectome is a surgical system developed by NeoMedix (Tustin, CA). It was introduced in 2004 that allows a trabeculotomy to be performed via an internal approach. The system works by removing a strip of trabecular meshwork and the inner wall of Schlemm’s canal in order to create a path for the drainage of aqueous humor.7 The device itself consists of a one-use, disposable hand piece that is used for electrocautery, irrigation, and aspiration. It is connected to a generator with a frequency of 550 kHz that allows adjustments in 0.1 watt increments and is controlled via a 3-stage Foot Pedal Control that initiates irrigation, aspiration and electrocautery in sequence. Continuous irrigation and aspiration allows for removal of debris and regulation of temperature. Additionally, the tip of the Trabectome is bent at a 90° angle to create a protective triangular footplate and allow for easier insertion into Schlemm’s canal as well as coated to allowed smoother movement within the canal. Ablation of 60°-120° allows for re-establishment of the drainage pathway. Maeda et al8 evaluated the outcome of surgeries using Trabectome in 80 eyes of 69 patients. A mean preoperative IOP of 26.6 ± 8.1 mmHg was found to decrease to a mean postoperative IOP of 17.4 ± 3.4 mmHg within 6 months after the surgery. Average number of medications also decreased from 4.0 ± 1.4 to 2.3±1.2 at 6 months. The study reported no serious complications, including chorodial effusion, chorodial hemorrhage, or infection. Thirteen patients required subsequent surgeries due to uncontrolled IOP, with 10 patients receiving trabeculectomy and 3 patients receiving repeat Trabectome. One patient had an IOP spike that was uncontrolled by medications at one week and needed trabeculectomy.8 In a study completed in 2008 involving 304 eyes in patients with open angle glaucoma, similar results were found. Mean IOP was found to have decreased from 20.0 mmHg ± 6.3 to 15.5 and average number of medications was found to have decreased from 2.65 ± 1.13 to 1.44 ± 1.29 at one year. Complications included iris injury in 4 patients and IOP spike in 26 patients. General advantages of this surgery are the ability to bypass the trabecular meshwork and form a direct pathway to Schlemm’s canal. The conjunctiva is able to be spared and there is no bleb formation. Importantly, the surgery is able to be combined with simultaneous cataract surgery. General disadvantages are the lack of circumferential flow, cleft closure, and that the IOP reduction is limited by episcleral venous pressure and Schlemm’s canal resistance.8
The iStent Trabecular Micro-Bypass Stent (Glaukos, Laguna Hills, CA) received FDA approval in 2012. The device is a heparin-coated, non-ferromagnetic titanium stent with a snorkel shape to facilitate implantation. The device is placed using a single-use, sterile inserter through a 1.5mm corneal incision. The applicator is inserted into the anterior chamber and across the nasal angle. The pointed tip allows penetration of the trabecular meshwork and insertion into Schlemm’s canal and three retention arches ensure that the device will be held in place.7 The iStent itself is the smallest FDA approved device, measuring at 0.3mm in height and 1mm in length. In a prospective randomized clinical trial, the effectiveness of phacoemulsification and stent placement compared to cataract surgery alone was evaluated in 239 patients, with 116 patients receiving the stent. Patients involved in the study were those with mild-moderate glaucoma who had an unmedicated IOP between 22 and 36 mmHg. Patients were followed out to 24 months and an endpoint of an IOP ≤ 21 mmHg without ocular hypotensive medications was used. At 24 months, there was a significantly increased (P=.036) proportion of patients that met the endpoint criteria in the stent group (61%) versus the control group (50%). There was also an increased proportion of patients with an IOP reduction of ≥ 20% in the stent group, though the difference was not significant. At 24 months, the overall mean IOPs of the stent group and the control group were similar at 17.1 ± 2.9 mmHg and 17.8 ± 3.8 mmHg, respectively. Of note, while the mean IOP in the stent group was stable between the 12 and 24 month follow up, the mean IOP of the control group rose from 17.0 ± 3.1 mmHg. Additionally, the number of medications in the stent group (0.2 ± 0.6) was significantly lower (P = .011) than in the control group (0.4 ± 0.7) at 12 months. While the trend was similar at 24 months, the difference was no longer significant. There was no difference between the two groups in terms of complications.6 A literature review by Augustinus9, concluded that iStent implantation with phacoemulsification resulted in a significantly lower, long-term decrease in IOP and number of medications used compared to phacoemulsification alone. No major complications were found in the review.
The CyPass Micro-Stent was created by Transcend Medical Inc. and is currently being evaluated as a treatment modality in the U.S. in the COMPASS Clinical Study. It has been studied in clinical trials in Europe since 2009. The device itself is a polymide, supraciliary device for ab-interno implantation. The goal of the device is to create a controlled cyclodialysis with stented outflow to the supraciliary space. The stent is 6.35mm long with an outer diameter of 0.51mm. During surgery, the implant is loaded onto a retractable guide wire, inserted through the initial phacoemulsification incision, and advanced toward the sclera spur. The guide wire is used to perform blunt dissection of the ciliary body in order to allow passage into the supraciliary space where the stent can be placed. In the CyCLE study, 238 patients received the CyPass Micro-Stent in addition to cataract surgery. The first cohort consisted of 90 eyes that initially had an uncontrolled IOP ≥ 21 mmHg at baseline; 40 patients had a 12 month follow up visit. At that time, the mean IOP was 16.3 mmHg and there was a 48% reduction in medication usage. The second cohort consisted of 148 eyes and had an initial controlled IOP < 21 mmHg. At the 12 month follow up, 73 eyes were examined and were found to have a stable IOP and a 75% reduction in medication usage. There were no adverse effect reported in the study among either group.10 In another multicenter study11, 184 patients were divided up into the same two cohorts, as indicated by the CyCLE study guidelines, and received cataract surgery along with stent placement. There were 91 patients in Cohort 1 (IOP ≥ 21 mmHg) and 93 patients in Cohort 2 (IOP < 21 mmHg). The average age was 73.0 years ± 8.21 with a mean baseline IOP of 21.1 ± 5.91 mmHg for both groups. The mean number of medications was 2.1 ± 1.12. At the 6 month follow up, 57 patients of Cohort 1 were available and were found to have a mean IOP of 15.6 ± 0.53 mmHg, significantly reduced from baseline (P < .001). The mean number of medications was also significantly reduced from baseline to 0.9 ± 0.15 (P < .001). In Cohort 2, 41 patients had 6 months of follow up. The mean IOP was 15.6 ± 0.68 mmHg and the mean number of medications was 0.6 ± 0.07. Therefore, IOP remained controlled at < 21 mmHg and there was a significant reduction in medications at 6 months (71%, P < .001). The most common postoperative events included transient hypotony (25/26 cases resolved by one month) and transient IOP increase (19 cases). Nine cases of elevated IOP needed additional surgical intervention, including trabeculectomy or additional stent placement. Overall, the device has shown in initial trials that there can be a significant decrease in number of medications used as well as a significant decrease in uncontrolled IOP or maintenance of a controlled IOP. Additionally, the device has been shown to have a high safety profile with minimal adverse events.
XEN Glaucoma Implant
The XEN Glaucoma Implant (AqueSys Implant) was created by AqueSys Inc and is an investigational device that is currently undergoing clinical trials. The implant itself is made out of a soft, collagen-derived, gelatin that is known to be non-inflammatory. The goal of implantation is to create an aqueous humor outflow path from the anterior chamber to the subconjunctival space. The implant is injected through a small corneal incision with the use of an inserter similar to those used for IOLs. Similar to other implants, it can be performed in conjunction with cataract surgery. While limited, there is company provided data available from international trials. Overall there have been 118 subjects that have received the implants. The mean preoperative IOP was 23 mmHg with an average of 3 medications. At the 12 and 18 month postoperative follow ups, the mean IOP had decreased to 15.4 ± 4.5 mmHg and then 14.5 ± 3.1 mmHg, respectively. At 24 months it was 14.3 ± 5.1 mmHg. At all time points, the average number of medications was one and 33% of patients were using no medications at 24 months. Further trials in the United States are currently being completed.
The Hydrus Microstent was developed by Ivantis Inc. and currently has FDA approval for phase IV clinical trials. The microstent is made from a super-elastic, biocompatible, nickel-titanium alloy (nitinol) and is 8mm in size. The “intracanalicular scaffold” is inserted into Schlemm’s canal to maintain patency and establish outflow. The procedure can be performed in conjunction with cataract surgery and uses the same corneal incision.12 In a European study involving 69 patients, 29 received the stent along with cataract surgery while 40 patients received the Hydrus stent alone. At six month follow up, the average IOP was found to have decreased from 21.1 to 15.6 in the combination surgery group. Additionally, use of glaucoma medication was reduced from an average of 2.1 to 0.4. In the group that received the Hydrus stent only, the mean IOP had decreased from 21.6 mmHg to 16.9 mmHg at the 6 month follow up. Medications decreased from an average of 1.7 to 0.6 drops. In the combination surgery group, 85% of patients were free of medication at 6 months compared to 70% of patients in the stent-only group. Further trials in the United States are currently being completed12.
Gonioscopy Assisted Transluminal Trabeculotomy (GATT)
GATT is a form of ab interno trabeculotomy which was described by Grover et al in 2014. Under the guidance of a gonioscopy lens, a goniotomy is made in the nasal trabecular meshwork which serves as the entry point for the iTrack microcatheter (iScience Interventional Corp, Menlo Park, CA), which has a 250 micron diameter13. A modified technique has also been described using 4-0 nylon suture instead of the microcatheter. Microsurgical forceps are used to advance the microcatheter into Schlemm canal circumferentially 360 degrees, tracking its progress with its illuminated distal tip. Once it has been passed through the entire canal, the catheter is externalized to create a 360-degree trabeculotomy. In their original review, Grover et al reported on the 6 and 12 month outcomes of 85 patients; 57 patients with primary open angle glaucoma showed an average IOP decrease of 11.1 ± 6.1 mm Hg and 1.1 fewer medications 13. For the 28 patients with secondary glaucoma, IOP decreased an average of 19.9 ± 10.2 mm Hg and 1.9 fewer medications 13. The most common complication was a transient hyphema reported in 30% of patients which resolved by one month. Since the original publication, Grover and colleagues have reported the successful use of the GATT technique in primary congenital glaucoma, juvenile open angle glaucoma, and even eyes with prior incisional glaucoma surgery 14,15. These early results represent a promising, conjunctival sparing technique that can be used in conjunction with, or independent of cataract surgery.
In patients with mild-moderate glaucoma with an IOP that has been unable to be controlled by medications or who have poor medication compliance, micro invasive glaucoma surgery appears to be a viable option. Clinical trials have shown there to be a significant decrease in IOP over periods of up to 24 months along with a significant decrease in medication usage. The procedure has maintained a high safety profile with minimal adverse effects. While the Trabectome and iStent are currently the only FDA approved devices, there are multiple new devices pending. Micro invasive glaucoma surgery can provide a method of treatment for glaucoma patients that decreases reliance on medications without the safety risks of more invasive procedures.
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