MIGS - Small Incisions with Big impact!

What is MIGS?

One must have heard about MIVS which stands for Minimally Invasive Vitrectomy Surgery. MIGS could be called its sister concern and stands for Minimally Invasive Glaucoma Surgery. MIGS have been present for an appreciable amount of time in the past, they have garnered a lot of interest in recent years. Touted to be the next paradigm shift in the management of glaucoma and are being likened to what anti-VEGFs were for vitreo-retina.

Surgery

How are they different from a traditional trabeculectomy? Well, majority if not all. are independent of a surgically induced filtering bleb. They are group of surgeries which are performed using a smaller incision than the traditional trabeculectomy. Means the healing is faster with carries lesser risk for infection and bleeding.

Why do we need MIGS?

Before we start talking about MIGS let us know more about why it is needed. We know the success of a traditional trabeculectomy depends on the health of the filtering bleb.

An ideal bleb is diffuese, minimally levated, microcystic and relatively avascular. But do we always get these features? Even in the best of hands, a surgical bleb may fail. And failure can be due scarring, blebitis, overfilteration, underfilteration, encapsulation, overhanging among others. These factors can be manipulated to a great extent but may still be dependent on the preoperative dry eye, ocular surface disease, use of anti-glaucoma medications with preservatives and steroids etc.

This probably lead to the thought of why not circumnavigate the filtering bleb and think of a novel approach altogether?

What is MIGS again!

MIGS are surgeries or devices which have the following basic characteristics: minimally invasive, ore effective, higher safety profile, quicker to perform and flatter curve of learning. Also these surgeries can be very well combined with a clear cornea phacoemulsification.

MIGS is indicated for patients with glaucoma less severe than that requiring traditional trabeculectomy

Anatomy of the angle of Anterior Chamber

Let us revise the anatomy of the angle of the anterior chamber (AC). The angle of the AC is formed by the trabecular meshwork, scleral spur, ciliary body and the iris. The trabecular meshwork in turn can be divided into 3 parts from internal to external - uveal, corneoscleral and juxtacanalicular (providing maximum resistance to outflow) meshworks.

The aqueous draining through the Schlemms canal then passes through the internal and external collector channels and subsequently into the aqueous veins. It is important to note that drainage channels are most common in the inferonasal quadrant.

With MIGS we want to potentiate the physiological drainage pathway without the need of a sub-conjunctival filtering bleb.

Types of MIGS

MIGS may target either one or multiple anatomical landmarks. The iStent, Hydrus, Kahook Dual Blade, BANG (Bent abinterno needle goniectomy), Trabectome, GATT (gonioscopy assisted transluminal trabeculotomy), ABiC (abinterno canaloplasty) target the angle of AC. The CyPass targeted the suprachoroidal space. Xen and Express shunt are placed in the sub-conjunctival space and are partially bleb dependant.

MIGS has become increasingly prominent in discussions because of its ability to address a range of anatomical landmarks

Indications for MIGS

These MIGS procedures can act in the entire glaucoma continuum. From mild to moderate glaucomas, open angle and pigmentary glaucomas, angle closure glaucoma which opens after Yag PI and without goniosynechae in certain cases. Only those eyes which require modest reduction of IOP should be chosen. Also eyes with ocular surface disease and poor adherence to anti-glaucoma therapy can be considered as indications in certain cases.

Contraindications for MIGS

Eyes with angle closure and peripheral anterior synechae, unstable IOL and cataract, neovascular glaucoma and uveitis are contraindications for MIGS. This is because for MIGS, the angle of AC must be free from any pathology which may be found in above cases.

Classification

MIGS can be grossly classified on the basis of the target anatomical space into sub-conjunctival, Schlemms canal, uprachoroidal and ciliary body based. Theses can further be classified into abinterno and abexterno procedures.

Complications

Complications include and are not limited to hyphema, iridodialysis, cyclodialysis, malposition of implant, lost implant, transient IOP spike, iritis, corneal edema and luminal obstruction.

Advantages

The use of MIGS is advantageous in a lot of ways. Firstly it potentiates the physiological outflow of aqueous ie the drainage from AC into the Schlemms canal. Most are independent of a filtering bleb therefore conjunctiva can be spared for a subsequent trabeculectomy surgery, if the need be. MIGS procedures are inherently minimally invasive and hence can be easily combined with a clear corneal phacoemulsification.

Disadvantages

MIGS have a learning curve. Majority if not all require a surgical gonioscope. Though short term data is aailable, long term data on the MIGS is awaited. The cleft created in cases like BANG and KDB may close with peripheral anterior synechae. Importantly, the access to Schlemms canal is limited circumferentially.

MIGS look promising in the future management of glaucoma.

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