Glaucoma is an optic neuropathy with characteristic changes in the optic nerve head and visual field. It is a leading cause of irreversible blindness worldwide. The number of people with glaucoma in Asia is estimated to increase to 59.51 million in 2020, and 80.87 million in 2040.¹

From the Singapore Chinese Eye Study, the age-standardised prevalence of glaucoma in Singapore is 3.2%.² Currently the Intraocular Pressure (IOP) is the only modifiable risk factor, and lowering it is the only proven treatment to reduce disease progression. Treatment is life-long, and can be achieved with topical medications, laser or surgery.

Poor adherence to glaucoma medications and multiple side effects, including ocular surface toxicity, are major barriers to effective medical therapy. Non-compliant rates of at least 25% are commonly reported in glaucoma patients.³

While the conventional trabeculectomy and tube shunt surgeries are effective in lowering the IOP, they carry significant surgical risks and are usually offered to patients with more severe disease or uncontrolled IOP on maximum tolerated medical therapy.⁴

NEW SURGICAL PROCEDURES

In recent years, a new group of surgical procedures known as Minimally Invasive Glaucoma Surgery (MIGS) have emerged. They lower the IOP by increasing drainage of the intraocular fluid (aqueous humour) with minimal dissection and tissue disruption.

They have shown promising results in the short-to-medium term, and have a higher safety profile and faster recovery time than conventional glaucoma surgeries.

As there is less IOP reduction compared to conventional glaucoma surgeries, these procedures are targeted at patients with mild to moderate glaucoma. They can be performed together with cataract surgery, or as a stand-alone procedure.

1. Schlemm’s Canal Devices
iStent
The iStent (Glaukos Corporation, Laguna Hills, CA, USA) is a 1mm snorkel-shaped titanium device that increases drainage of the intraocular fluid. The device is injected into the Schlemm’s canal, a part of the eye’s drainage system, from the inside of the eye using a special viewing contact lens. It was FDA-approved in 2012.

The largest multicentre trial studying the device found that more eyes that underwent phacoemulsification with a single iStent implantation had controlled IOP without glaucoma medications, compared to phacoemulsification alone at 1 year (72% versus 50%, P<0.001), and 2 years (61% versus 50%, P=0.036).^5,6

Two meta-analyses concluded that iStent implantation is effective in reducing the IOP and the number of glaucoma medications, either combined with phacoemulsification, or as a solo procedure.^{7, 8}

For iStent as a solo procedure, the IOP reduction was 22% at 18 months after 1 implant, 30% at 6 months after 2 implants, and 40% at 6 months after 3 implants.⁸

iStent Inject
Currently there is a newer, second generation implant, called the iStent Inject (Glaukos, Laguna Hills, CA, USA). The new device has a cone-shaped design for easier implantation, and a modified injector that can be simultaneously loaded with 2 stents (Refer to Figure 1).

A prospective multicentre study has found that following implantation of 2 iStent Injects, majority of the patients (86.9%) achieved a reduction in the number of glaucoma medications.⁹

There were few to no adverse events reported following implantation of the iStent and iStent Inject.

Hydrus Microstent
The Hydrus Microstent (Ivantis Inc, Irvine, California, USA) is an 8mm nitinol device. Like the iStent, it is also injected into the Schlemm’s canal, a part of the eye’s drainage system, from the inside of the eye. It acts as a scaffold, and dilates 3 clock hours of the canal, promoting circumferential flow of the intraocular fluid into the drainage system (Refer to Figure 2).

The multicentre HYDRUS II study randomised 100 patients with open angle glaucoma and cataracts to undergo either cataract surgery with Hydrus Microstent or cataract surgery alone.¹⁰

At 24 months, there was a significantly lower diurnal IOP in the eyes with the Hydrus Microstent (16.9 +/- 3.3mmHg versus 19.2 +/- 4.7mmHg, P=0.0093), and a greater proportion of eyes with the Hydrus Microstent not on glaucoma medications (73% versus 38%, P=0.008), compared to eyes with cataract surgery alone. Adverse events were similar in the 2 groups.

In another multicentre study, there was a significant IOP lowering, following cataract surgery with Hydrus Microstent implant in eyes with primary open angle glaucoma, with an average drop in IOP of 4mmHg from baseline at 2 years post implantation (P<0.001), and 64% of the patients were medication-free.¹¹

2. Subconjunctival Devices
XEN Gel Implant
The XEN Gel Implant (Allergan, Dublin, Ireland) is a biocompatible 6mm porcine collagen-derived gelatin tube, that is inserted from the inside of the eye into the subconjunctival space. That is the space between the outer membrane of the eye (conjunctival) and the white wall of the eye (sclera).

Conjunctival dissection is avoided, unlike in conventional glaucoma surgery. The inner lumen size of 45microns and the tube length provide flow restriction to minimise excessive lowering of the IOP. Aqueous drains into the subconjunctival space and a bleb is created. It was FDA-approved in 2016.

A multicentre cohort study of 354 eyes compared the efficacy and safety of the XEN Implant to trabeculectomy in various types of uncontrolled glaucoma (primary and secondary open angle glaucoma, and primary angle closure glaucoma).¹²

There was comparable IOP reduction, overall success rates and safety for both procedures up to 30 months of follow-up. The preoperative median IOP was 24mmHg, and that was reduced to 13mmHg postoperatively in both groups. Bleb needling was higher in the XEN group (43% versus 31%) (Refer to Figure 3).

3. Suprachoroidal Devices
CyPass Micro-Stent
The CyPass (Alcon, Fort Worth, TX, USA) is a fenestrated 6.35mm long polyamide implant, that is inserted from the inside of the eye into the suprachoroidal space between the white wall of the eye (sclera) and the thin layer of tissue behind the retina (choroid). The CyPass Micro-Stent was FDA-approved in 2016.

The CyPass device demonstrated a sustained 2-year reduction in IOP and glaucoma medications in patients with mild to moderate primary open angle glaucoma in the multicentre COMPASS trial.13 There were 505 patients who underwent either phacoemulsification with CyPass or phacoemulsification alone.

At 2 years, the mean IOP reduction was greater in the Cy- Pass group compared to the control group (7.4mmHg versus 5.4mmHg, P<0.001), and more patients in the CyPass group were medication-free (85% versus 59%, P<0.001). Reported adverse effects were usually transient and did not affect the visual outcomes (Refer to Figure 4).

CONCLUSION

Current data suggest that MIGS can benefit patients with mild to moderate open angle glaucoma in the short-to-medium term, with a good safety profile. While they provide more surgical options to patients with less severe disease, further studies are needed to determine their long-term safety and efficacy.

The iStent Inject, Hydrus Microstent, and XEN Gel Implant are currently available at the Singapore National Eye Centre (SNEC). The CyPass Micro-Stent is still pending HSA approval.

Dr Annabel Chew is a Consultant at the Department of Glaucoma at the Singapore National Eye Centre (SNEC). She graduated from the Yong Loo Lin School of Medicine at the National University of Singapore, and completed her Ophthalmology residency and Glaucoma fellowship at SNEC.

GPs can call for appointments through the GP Appointment Hotline at 6322 9399 for more information.

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