Minimally invasive glaucoma surgery: A comprehensive review

TM Bypass

The primary site of resistance to aqueous outflow is juxtacanalicular TM. MIGS techniques address this by ablating or excising the TM or bypassing it with stents, facilitating direct aqueous humour drainage into Schlemm’s canal.^[11]

This approach is suitable for patients POAG, PXFG, ocular hypertension and pigmentary glaucoma, aiming for a target pressure of mid-teens. However, such procedures are contraindicated in cases of primary angle-closure glaucoma, NVG, corneal opacity, angle dysgenesis and high episcleral venous pressure glaucoma.^[12]

Complications of TM bypass can include IOP spike hyphema, obstruction, malposition and, displacement of the stent and PAS formation.^[13]

iStent (Glaukos, Laguna Hills, CA)

The iStent, the first Food and Drug Administration-approved MIGS device, was introduced to the market in 2012. As Shown in Figure 2 this tiny stent is made of heparin-coated titanium, measuring 1 mm in length and 0.3 mm in width, with a 0.25 mm high snorkel having a central lumen of 120 mm that projects into the AC. Designed with a ridge shaped like a snorkel and featuring three arches for retention, the iStent ensures adequate placement in the nasal angle of SC.^[14]

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Figure 2:

Istent Glaukos (image courtesy: The cleveland clinic).

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An ab interno Implantation of iStent is performed through a clear corneal incision with a pre-loaded inserter under direct gonioscopy. Spiegel et al. observed a reduction of approximately 25% IOP in about 70% of patients, thereby significantly decreasing medication burden. Placement of multiple iStents can lead to an IOP reduction of 40–44%, while a single stent can achieve a 20–27% reduction in IOP within 1 year.^[15]

iStent inject (Glaukos, Laguna Hills, CA)

Approved by the Food and drug administration (FDA) in 2018, this second-generation TM bypass stent, like its predecessor the iStent, is a small medical implant made of heparin-coated, non-ferromagnetic titanium. As shown in Figure 3, the stent measures 360 mm in height and 230 micrometres in diameter, with a lumen of 80 mm. The stent has a tapered end called the head, which sits in SC, while the thorax extends into the trabecular meshwork and AC.^[16]

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Figure 3:

Istent inject (image courtesy: Dr. John Behrdal).

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The iStent inject has two stents and comes with a preloaded injector system (G2-M-IS injector). It is implanted using an ab interno approach under direct gonioscopy. It is placed nasally into the TM and Schlemm’s canal, spaced 30–60° apart.^[17]

According to an analysis by Lavia et al., the iStent inject results in a 35–39% reduction in IOP and decreases the need for medications, with 65–75% of patients being medication-free after 1 year.^[6] In addition, studies have shown that the reduction in IOP achieved with the implantation of two stents is comparable to the reduction achieved with two medications after 1 year.^[18]

iStent inject^® W

iStent Inject^® W is a third-generation device developed by Glaukos to achieve a better positioning of iStent. Figure 4a shows the basic structure of istent Inject w and Figure 4b shows intraoperative image of istent Inject W implantation. This device has the same height as the second-generation iStent with a broader base diameter of 360 µm for better drainage.^[14]

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Figure 4:

(a) iStent inject^® W (Courtesy: Glaukos) and (b) intraoperative gonioscopic image of iStent inject^® W (Courtesy: Dr SD, SCEH).

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Hydrus stent (Ivantis Inc, Irvine, CA, USA)

Hydrus implant, as shown in Figure 5 is a small, crescent-shaped stent measuring 290 µm in diameter and 8 mm in length. It is made of nitinol, known best for its shape memory; it features an inlet with three openings that rest in the AC. With a preloaded injector, it is usually implanted in the inferonasal/inferotemporal quadrant through a clear corneal incision.^[19]

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Figure 5:

Hydrus Microstent (image courtesy: Vince Giuseffi).

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The stent has been designed with intracanalicular scaffolds and occupies nearly 120-150 degrees of SC sparing the posterior portion of the collector channel. It operates through a unique trimodal mechanism: Bypassing the TM to allow aqueous flow, dilating SC by 4–5 times to facilitate outflow through collector channels and enhancing overall aqueous humour drainage.^[20]

The HORIZON study found that 80% of cases achieved more than 20% IOP reduction. About 73% of eyes were drug-free at 24 months.^[20] The COMPARE study, which compared the Hydrus stent to the iStent, concluded that 39.7% of eyes Hydrus experienced more than a 20% IOP decrease, compared to 13.3% in iStent. Additionally, approximately 46.6% of patients with the Hydrus stent were AGM-free, compared to 24% with iStent.^[21]

Kahook dual blade(KDB)

It was launched in 2015. It can be performed stand-alone or with phacoemulsification. It utilises a single-use disposable blade with a sharp tip. A clear corneal incision is made, and the KDB is inserted. Its pointed tip pierces TM and enters SC. The ramp lifts and stretches TM, and during circumferential movement in the canal, parallel blades excise a strip of the TM, exposing the outer wall of Schlemm’s canal. This strip of TM can be removed using aspiration or intraocular forceps.

This technique facilitates minimal TM tissue damage.

Figure 6a shows the basic structure and Figure 6b shows the intraoperative image of KDB. Dorairaj and Tam reported significant reductions in IOP and the number of medications needed in patients with angle-closure glaucoma.^[22]

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Figure 6:

(a) Kahook dual blade (KDB) (Courtesy: AAO) and (b) intraoperative gonioscopy of KDB (Courtesy: DR.SD, SCEH).

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Trabectome

Introduced by Baervelt, Chuck and Irvine, Trabectome got FDA certified in 2004 For the management of paediatric and adult glaucoma. It utilises a plasma-mediated ablation system with continuous irrigation and aspiration and a bipolar 550 Hz electrode. It creates an opening between the anterior chamber, Schlemm’s canal and the collector channels. Figure 7a shows the basic structure and Figure 7b shows the intraoperative image of Trabectome. According to a study by Dubey et al., the overall success rate with the trabectome was 76% in Indian patients.^[23]

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Figure 7:

(a) Trabectome (Courtesy: Prof Guss Guzzard, Moorfields Eye Hospital). (b) Intra operative gonioscopy image of Trabectome (Courtesy: Dr SD, SCEH).

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Laser Trabeculostomy

First described by Berlin in 1987 and utilised by Vogel and Lauritzen in 1997, this technique employs an excimer laser with a fibre optic delivery system. It uses an 80-nanosecond pulse of 308 nm xenon chloride laser to target TM. Each energy pulse delivers 1.2 mJ for 10–60 nanoseconds at a 20 Hz frequency. This procedure effectively removes the tissue obstructing the aqueous drainage with minimal damage to the surrounding.

10 microchannels spaced 500 µm apart are created over a 90-° area. This process is also referred to as pneumatic cranioplasty. The procedure typically results in a reduction of IOP between 20 and 40%.^[24]

Gonioscopy-assisted transluminal trabeculotomy (GATT)

In 2014, Grover et al.^[25] described this technique which involves microsurgical forceps and an iTrack microcatheter. It has a shaft diameter of 200 µm, featuring a distal tip with an illuminating catheter for location monitoring. Under direct gonioscopy, the suture or microcatheter is guided toward the nasal quadrant into SC and is then advanced 360° with the help of microforceps. Traction is applied to break the TM leading to 360° trabeculotomy.^[26]

GATT has demonstrated effectiveness in approximately 70–90% of cases, with an IOP reduction ranging from 30 to 40%, compared to the ab externo approach.^[27] Figure 8 shows intraoperative image of 360 degrees Suture GATT.

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Figure 8:

Suture gonioscopy assisted transluminal trabeculotomy (Courtesy: Dr. SD, SCEH).

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Tanito ab interno micro-hook trabeculotomy

The tip of a modified sinskey-like instrument called Tanito microhook is introduced into the SC and swept circumferentially over the required clock hours (varies from 180 to 270°) to incise the inner wall of SC and TM. Figure 9 depicts the intraoperative image of tanito microhook trabeculotomy.

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Figure 9:

Ab interno trabeculotomy with Tanito Microhook (Courtesy: intraoperative gonio image on Tanito Microhook, Courtesy : Dr Masaki Tanito, MD).

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Maheshwari et al. study showed a complete surgical success of 89.1% at the end of 1 year in the phaco-trabeculotomy group.^[28]

AbiC

Interno Canaloplasty (AbiC) is facilitated by the iTrack microcatheter, which boasts a 200-µm thickness, featuring a bulbous 250-µm tip and a lubricating coating for smooth passage. The device is equipped with a visco-injector and fibre optic enabled illumination which aids in navigating through the SC. Following catheter insertion and positioning, it is withdrawn, allowing for the insertion of a Visco surgical device. The Visco surgical device is instrumental in viscodilating areas of resistance, including the Schlemm canal and collector channels. Through viscodilation, perforations within the TM are created, thereby enhancing aqueous outflow. This technique typically results in a 30–40% reduction in IOP over 12 months, often necessitating fewer than two anti-glaucoma medications for effective management.^[29]

Cypass stent

Figure 10 shows the cypass stent. FDA approved in 2016, the Cypass Stent is a flexible, fenestrated polyamide stent measuring 6.35 mm × 510 µm, with a lumen of 300 µm. It is a pre-loaded device with a guidewire to assist in shaping the sclera for dissection and insertion between the AC and suprachoroidal space.^[30] However, in 2018, it was withdrawn from the market due to concerns regarding increased loss of endothelial cells.^[31]

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Figure 10:

Cypass (Courtesy: San Jose Eye institute).

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iStent supra

Made of biocompatible polyethersulfone, this curved device has a heparin-coated tube with a lumen of 0.16 mm. It features a titanium sleeve and retention ridge for stabilisation. Mounted with a guide for easy insertion into both the AC and suprachoroidal space via an ab interno approach under gonioscopic guidance. When used in conjunction with Travoprost, iStent Supra has been demonstrated to reduce IOP by 20% in advanced glaucoma patients.^[32] Figure 11 depicts the structure of iStent supra.

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Figure 11:

iS’tent Supra (Courtesy: Glaukos).

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Xen implant (Xen Gel Stent)

This FDA-approved pre-loaded device comes with a disposable 27G injector composed of porcine collagen-derived gelatine cross-linked with glutaraldehyde and is hydrophilic in nature.^[33] In the dry state, it is rigid and straight and becomes flexible when hydrated. It measures 6 mm in length with an internal lumen diameter of 45 µm and an external diameter of 150 µm. Available in three variations: XEN 140, XEN 63 and XEN 45. Typically implanted in the superonasal or superior quadrant.^[34]

InnFocus MicroShunt (PRESERFLO MicroShunt)

It comprises two segments: a longer proximal part measuring 4.5 mm and a shorter distal part measuring 3 mm. It is Constructed from poly (styrene-block- isobutylene-block-styrene) or SIBS. Features an external lumen diameter of 350 µm and an internal lumen diameter of 75 µm. According to Batlle et al., patients receiving the shunt with mitomycin C (MMC) exhibited controlled IOP within the low teens for up to 3 years post-implantation. The Preserflo MicroShunt is implanted through an ab externo approach and performed under topical anaesthesia. The procedure involves creating a small conjunctival peritomy, followed by the preparation of a sub-tenon pocket treated with MMC. Subsequently, irrigation is performed, and a scleral tunnel is created, into which the device is inserted parallel to the iris plane. The Tenon layer and conjunctiva are then closed to facilitate bleb formation without leaks.^[35]

Recommendations

MIGS procedures have significantly reduced surgical times by 50% compared to most traditional filtering procedures or GDD implantations. These procedures typically take 15–30 min. The subconjunctival approach seems more effective than other MIGS techniques to achieve pressures in low-teens.

Current evidence confirms that MIGS are effective in reducing IOP and the burden of medications compared to standalone phacoemulsification. Complication rates for MIGS are also less as compared to traditional surgeries. However, for a procedure to be considered clinically advisable, it must be not only safe but also at least as effective as established alternatives. This review reveals a scarcity of studies directly comparing different MIGS techniques or assessing MIGS against pharmacotherapy. More comparative data, especially including standard treatment, could greatly aid ophthalmologists and healthcare authorities in determining the best therapeutic options, potentially reducing medication burdens and associated costs. Significantly more evidence is required to reach this level of certainty. Future research should standardize key outcome features, include comparative treatments and long-term assessments of treatment sustainability and late complications, and preferably be randomised. Reporting washed-out IOPs and incorporating functional and structural markers of glaucoma progression in long-term success definitions are also recommended. In addition, larger cohorts are needed to ensure statistical power and to identify individual biomarkers for truly personalised therapy.

Ultimately, these comprehensive data will equip clinicians with the important details to make evidence-based decisions and select the best option for the management of each glaucoma patient.