Everything you need to know about MIGS
Minimally invasive glaucoma surgery comes with major advantages for surgeon and patient alike. Soosan Jacob, MS, FRCS, DNB reports
Various minimally invasive glaucoma surgery (MIGS) devices are seen – Trabecular bypass devices: iStent (a) and Hydrus (b); Subconjunctival filtration devices: Xen Gel stent (c), InnFocus microshunt (d); Suprachoroidal shunts: iStent Supra (e)
The management of glaucoma has taken a subtle shift in the recent past with the advent of minimally invasive glaucoma surgery (MIGS). MIGS has become an option for mild-to-moderate glaucoma when more invasive surgeries such as trabeculectomies and tube shunts carrying a higher risk of complications are viewed with some hesitancy. The minimal dissection required, relatively easy learning curve, ability to be easily incorporated by cataract surgeons into their practice, easy and beneficial combination with cataract surgery and relative lack of major complications are significant advantages.
MIGS may be indicated for different purposes – for IOP control, to decrease dependence on medications or to avoid side-effects associated with glaucoma medications. MIGS surgeries such as trabecular bypass devices have high safety and modest efficacy whereas those using subconjunctival filtration have high efficacy but can be associated with higher risk of complications.
The ab interno approach of most MIGS leaves the conjunctiva intact, thus allowing future glaucoma surgeries that may be required. Acceptance of many of these procedures by insurance providers has also helped make this a popular method for treating glaucoma, especially when coexistent with cataract. MIGS is however, generally not used in patients with advanced disease, previous filtering surgeries, angle closure glaucoma etc. MIGS may be performed using specific implantable devices or by using special techniques and instruments/ machines.
The various MIGS procedures use one of different mechanisms for IOP lowering – increasing trabecular outflow or bypassing trabecular meshwork, suprachoroidal drainage, subconjunctival filtration or decreased aqueous production.
Trabecular bypass devices are especially commonly used in combination with cataract surgery for patients with co-existing cataract and early-to-moderate glaucoma as ease of surgery, quick recovery, safety and efficacy make it an attractive solution despite being less effective than traditional glaucoma surgeries. They are, however, not effective if the episcleral venous pressure is raised.
i-Stent (Glaukos Corp): This is an FDA-approved trabecular bypass device that is placed ab interno through a clear corneal incision into the Schlemm’s canal. It is a heparin-coated, nonferromagnetic, surgical grade titanium stent less than 1mm in length with a pointed tip that is self-retaining once implanted through the trabecular meshwork into the Schlemm’s canal. Two stents are reported to give greater IOP reduction than one. The i-Stent Inject via a single entry, delivers two pre-loaded trabecular micro-bypass stents to be implanted two-to-three clock hours apart. These devices are preferred to be placed in areas with the highest density of collector channels thereby targeting a large aqueous vein, generally in the infero-nasal quadrant of the eye.
Adequate skills in intraoperative gonioscopy and visualisation of the angle are important in successful placement of the iStent. Placement is easier in patients with wide open angles, pigmented trabecular meshwork and in those without systemic or local conditions that preclude proper positioning of the eye and head. It is easier to place after removing the cataract as the anterior chamber is deeper.
Hydrus microstent (Ivantis Inc): This is also a trabecular bypass device with an 8mm long curved nitinol body that has windows for aqueous outflow and is implanted via a pre-loaded injector. It acts by providing intra-canalicular scaffolding to a quadrant of the Schlemm’s canal. It dilates the canal four-to-five times and prevents collapse of Schlemm’s canal secondary to elevated IOP. As it is placed over a larger area of Schlemm’s canal, targeted placement is less crucial.
Suprachoroidal Shunts: The iStent Supra (Glaukos Corp) is a 4mm tube made of polyethersulfone and titanium that is placed ab interno and drains into the suprachoroidal space. The Gold Shunt (SOLX Inc) and the STARflo ((iSTAR Medical) are ab externo supraciliary implants placed under scleral flaps.
Sub-conjunctival filtration devices: The XEN gel stent (AqueSys Implant) and the InnFocus Microshunt (InnFocus) use Newtonian fluid dynamics and the Hagen–Poiseuille equation to eliminate clinically significant postoperative hypotony. An area of virgin conjunctiva should be chosen. Though biocompatible and resistant to neovascularisation and fibrosis, they are generally used together with anti-metabolites like 5-FU or Mitomycin-C. The XEN gel stent is 6mm long and available with different internal lumen diameters. It is made of porcine collagen-derived gelatin cross-linked with glutaraldehyde and comes pre-loaded for implantion ab interno or via conjunctival dissection. The InnFocus Microshunt is made of SIBS (synthetic polymer of poly(styrene-block-isobutylene-blockstyrene)). It is an 8.5mm long, flexible tube with 70 micron lumen that is inserted into the AC under a scleral flap. Tiny fins on either side prevent migration. It may be used for moderate-to-advanced glaucoma and is as effective as trabeculectomy.
Trabectome [NeoMedix]: This uses a handpiece with irrigation, aspiration and electrocautery modes to perform about 60-to-120 degrees of ab interno trabeculotomy and removal of a strip of trabecular meshwork (TBM) and inner wall of Schlemm’s canal. Inadequate IOP lowering may occur since flow is not established all around. In addition, the trabeculotomy may close or may be limited by inherent episcleral venous pressure and Schlemm’s canal resistance.
Gonioscopy-assisted transluminal trabeculotomy (GATT)/ ab interno canaloplasty (ABiC): GATT creates a 360-degree trabeculotomy/viscodilatation using an ab interno approach thereby avoiding some of the disadvantages of the ab externo technique. The iTrack microcatheter (Ellex) or a 5-0 nylon/prolene suture is passed through the Schlemm’s canal 360 degrees circumferentially via an internal goniotomy incision. GATT utilises a 360-degree trabeculotomy by tightening the suture/ microcatheter while ABiC relies on 360 degrees viscodilatation. To avoid false passages, the iTrack has an illuminated tip to guide the catheter. If prolene suture is used, its leading edge is rounded out with low-temp cautery.
Excimer laser trabeculostomy: Using a goniolens or endoscope, a fibre- optic delivered 308-nm xenon chloride excimer laser is used to create between four and 10 small ostia through the trabecular meshwork, juxtacanalicular trabecular meshwork and the inner wall of Schlemm’s canal. The non-thermal approach prevents scarring and resultant closure of the ostia.
Endocyclophotocoagulation decreases aqueous production and has been combined with phacoemulsification since 1995. Micropulse cyclodiode laser (Iridex) uses ultra-short energy bursts, allowing tissue to cool between pulses thus minimising damage. High-intensity focused ultrasound (Eye Tech Care) is also available.
Dr Soosan Jacob is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India and can be reached at firstname.lastname@example.org