Glaucoma is the leading global cause of irreversible blindness. In 2013, the number of people with glaucoma worldwide was estimated to be 64.3 million and this is projected to increase to 111.8 million by 2040.1 The only evidence-based treatment for glaucoma is reduction of intraocular pressure (IOP) with the aim of preventing damage to the optic nerve,2 but topical ocular hypertensive medications are problematic in terms of patient adherence and toxicity, and their therapeutic effect wears off in time.3, 4 Incisional surgery is effective but can cause a range of ocular complications and is typically only used in moderate to advanced cases of glaucoma.5 Selective laser trabeculoplasty (SLT) is a noninvasive option but its effectiveness decreases over time.6
Until recently, there have been limited surgical options for patients with mild-to-moderate glaucoma. Minimally invasive glaucoma surgery (MIGS) is one of the most exciting recent developments in the treatment of glaucoma and is also a new option for those who do not meet the criteria for trabeculectomy. It is less invasive than trabeculectomy and can be combined with cataract surgery. MIGS uses an ab-interno approach that leaves the conjunctiva intact for potential later trabeculectomy or non-penetrating surgery.
Several different MIGS approaches are currently in clinical use, each of which presents its own challenges. MIGS involves a significant learning curve, as techniques and visualization are considerably different from traditional glaucoma and cataract surgery. Key steps in MIGS procedures include identifying the trabecular meshwork, avoiding excessive outward pressure, and correct placement of devices or ablation instruments.
The most established MIGS techniques and devices aim to remove or bypass the inner wall of Schlemm’s canal and allow aqueous humour to flow from the anterior chamber to the collector channels of the outer Schlemm’s canal. Stent positioning is important in these techniques; the stent must be placed near one of the 25–30 collector channels in each eye. In addition some collector channels may be more active than others, and postoperative scarring around the Schlemm’s canal may hinder the procedure.7
Trabectome, which received US Food and Drug Administration (FDA) approval in 2004, is a surgical system that uses electrocautery to remove a strip of trabecular meshwork and the inner wall of Schlemm’s canal, creating a path for the drainage of aqueous humour.8 The technique can be used after failed trabeculectomy.9 Although its indications have been expanding, it is intended for primary and secondary open-angle glaucoma. Several studies have demonstrated the effectiveness of the Trabecotome but to date there have been no randomised controlled trials.7
The iStent inject is a trabecular micro-bypass heparin-coated, non- ferromagnetic, titanium stent that is preloaded into an injector and is inserted in the canal through a corneal incision. In a randomised controlled trial, stent implantation plus cataract surgery achieved pressure reduction with fewer medications that was clinically and statistically significantly superior after 1 year versus cataract surgery alone.10 The Hydrus microstent is a nickel-titanium crescent-shaped scaffold that matches the shape of the Schlemm’s canal and is inserted into the canal via a corneal incision. The Hydrus dilates the Schlemm’s canal for approximately three hours in the nasal quadrant, enhancing aqueous outflow.11 A large international study (NCT01539239) is currently enrolling patients and will compare patients undergoing cataract surgery with and without the Hydrus microstent.
Other options include the gonioscopy-assisted transluminal trabeculotomy (GATT) procedure, a circumferential 360-degree trabeculotomy12 and excimer laser trabeculotomy (ELT), a technique that uses a laser probe to create 8–10 small holes in the anterior trabecular meshwork, and was first validated in 1987.13
Suprachoroidal shunts aim to increase uveoscleral outflow by directing aqueous outflow to the suprachoroudal space. Available shunts include the Cypass supraciliary micro-stent, a polyamide stent containing microholes, which is inserted through the angle into the supraciliary space, designed to take advantage of the negative pressure gradient between the suprachoroidal space and the anterior chamber. In a recently published cohort study, it showed a low rate of surgical complications with decreases in IOP and glaucoma medications.14 The IStent Supra is also currently under investigation.
Reducing the secretion of aqueous humour is another emerging surgical approach to glaucoma. Endocyclophotocoagulation (ECP) involves a diode laser coupled to an endoscope. Preliminary data indicate that this is a safe and effective technique.15
Subconjunctival filtration creates a nonphysiological route for aqueous outflow and works along the same principle as current aqueous shunt glaucoma surgery. The Xen gel stent is a soft collagen-based gelatin implant that is injected through a small corneal incision, and shunts fluid from the anterior chamber to the subconjunctival space. Early studies have shown promising results in patients who are difficult to treat.16 Another subconjunctival filtration stent, the MicroShunt is made of SIBS, a biostable thermoplastic elastomeric material. In an observational study, three years after being treated with the MicroShunt patients had an average IOP reduction of reduced 55%.17
Since no randomised controlled trials have compared different techniques, there has been considerable debate over the value of MIGS. In addition, there are no long-term data on success rates or evidence that MIGS slows visual field progression, and there is insufficient information to guide the selection of patients most likely to benefit from different MIGS procedures. Some experts have argued that MIGS is not as effective as trabeculectomy. However, such statements do not take into consideration the greater safety and patient acceptance of MIGS. Many patients dislike eye drops so much that perhaps MIGS should be considered more as an alternative to topical medication. MIGS devices have the potential transform the treatment of glaucoma, allowing strategies to be tailored to the individual patients, and improving the lives of patients with glaucoma.
Summary of MIGS devices and techniques
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