Introduction

Although optometrists in various parts of the world, including South Africa are treating glaucoma with topical medications, there are instances when some patients warrant referral to an ophthalmologist with special interest in minimally invasive glaucoma surgeries or micro-invasive surgeries (MIGS).

Where do MIGS fit in?

Traditionally, the first-line therapy for primary open glaucoma (POAG) relies on topical glaucoma medications or selective laser trabeculoplasty (SLT) to lower the intraocular pressure (IOP). If these treatment options do not work, the patient is referred to a glaucoma surgeon to consider aggressive filtration surgical procedures such as trabeculectomy or tube shunts.  However, these traditional filtration surgeries come with risks such as bleb-related complications, diplopia and hypotony.

The majority of glaucoma patients in the average optometric practice have neither completely mild nor advanced glaucomatous disease; rather, they lie somewhere in the middle.  Some of these patients are often taking multiple glaucoma medications, are not ideally controlled and could benefit from a treatment modality more aggressive than topical medication, but less so than current filtration surgeries that are available. This is where MIGS fit in.  Recently, (MIGS) has become a new addition to the glaucoma treatment armamentarium.

MIGS procedures work by using microscopic-sized equipment and tiny incisions. Microscopic-sized tubes are inserted into the eye to drain fluid from inside the eye to underneath the conjunctiva.  While they reduce the incidence of complication, some degree of effectiveness is also traded for the increased safety.

Indications for MIGS

Patients that are candidates for micro-invasive glaucoma surgery are: 

• Patients with mild to moderate glaucoma
• Primary open-angle glaucoma, pseudoexfoliation glaucoma, pigmentary dispersion glaucoma
• Glaucoma that is uncontrolled with maximum pharmacologic treatment or there are barriers preventing adequate medication dosing.
• Ages greater than 18
• Patients with clinically significant cataract, as MIGS may be performed simultaneously.

Contraindications for MIGS

Relative contraindications for this procedure may include:

• Angle-closure glaucoma
• Secondary glaucoma that is moderate to advanced
• Previous glaucoma surgery
• Severely uncontrolled IOP
• Previous refractive procedures
• Monocular patients

Classification of MIGS

MIGS can be classified by the anatomical structure it targets in an attempt to allow or decrease resistance to aqueous outflow.

TRABECULAR MESHWORK AND SCHLEMM’S CANAL DEVICES

• Trabectome (Neomedix): The procedure involves ablating 120 to 180 degrees of the trabecular meshwork and the inner wall of Schlemm’s canal to lower resistance to aqueous outflow. The most common postoperative complication with Trabectome is transient hyphaema.
• iStent (Glaukos): Approved in 2012 for use in conjunction with cataract surgery, this device is designed to serve as a bypass through the trabecular meshwork to facilitate outflow of aqueous.

Two other devices, the iStent Inject and iStent Supra are currently in FDA clinical trials.  The iStent Inject resembles a punctal plug, with the end inserted into Schlemm’s canal and the head in the anterior chamber.  This allows aqueous to flow through the lumen of the device and into Schlemm’s canal.

The iStent Supra is a supraciliary device designed to release aqueous through the uveoscleral outflow pathway. The 4 mm tube is made of polyethersulfone and titanium and targets the large absorptive capacity of the suprachoroidal space.

• Ab interno canaloplasty (ABiC) (Ellex): This procedure uses an illuminated microcatheter through a corneal incision of 1.8mm to address all aspects of outflow resistance, including the trabecular meshwork, Schlemm’s canal and the collector channels. In contrast to traditional canaloplasty, ABiC does not require a tensioning suture to lower IOP.
• Kahook Dual Blade (New World Medical): This is a device that excises or removes the trabecular meshwork and the inner wall of Schlemm’s canal to lower resistance to aqueous outflow and enable better IOP control. The dual blade has demonstrated a more complete removal of trabecular meshwork compared with a micro vitreoretinal blade or Trabectome, leaving behind no residual trabecular meshwork leaflets, which are prone to close. This allows for sustained IOP reduction and control. The most common postoperative complication is transient hyphaema.

• Hydrus Microstent (Ivantis): The Hydrus is an 8 mm stent with the 1mm inlet segment resting in the anterior chamber, and the 7 mm scaffold segment residing in Schlemm’s canal.

SUPRACILIARY DEVICES

Cypass Micro-Stent (Alcon): This device, approved in 2016 for use in conjunction with cataract surgery, is designed to access the supraciliary space, bypass the conventional outflow pathway and target the uveoscleral outflow path. The increase of aqueous through the uveoscleral outflow path has the potential to lower IOP more than devices that target compromised outflow through the trabecular meshwork, according to research. Once the device is implanted, a small cyclodialysis cleft is typically visible with gonioscopy around the edges of the devices.  Postoperative complications include transient hypotony and hyphaema.

SUBCONJUNCTIVAL DEVICES

Xen gel stent (Allergan): Approved in 2016 as a stand-alone procedure or in conjunction with cataract surgery, this device lowers IOP by creating a drainage pathway that takes an intrascleral course from the anterior chamber to the subconjunctival space. It is designed to expand and conform to the shape of the surrounding tissue after implantation. The device typically will be visualised in the anterior chamber with gonioscopy, with 1 mm visible in the anterior chamber and 2 mm visible in the subconjunctival space.  A bleb will be noted on the first day postoperatively.

The Optometrist’s role in MIGS

Collaborative care is common between optometry and ophthalmology in the postoperative management of cataract surgery.  Similarly, this will be true with pre- and postoperative management of MIGS.  With the exception of a few additional aspects, post-op care following MIGS is essentially the same as following cataract surgery.

GONIOSCOPY

It is imperative to have a thorough understanding of the angle anatomy and be proficient with gonioscopy, not only to visualise the angle preoperatively to check for abnormalities and assess candidacy, but also to evaluate proper placement of MIGS devices postoperatively and identify potential problems.

MANAGING COMPLICATIONS

IOP spikes, similar to those after cataract surgery, are a particular concern because glaucoma patients already suffer from a condition that has compromised the health of the optic nerve head.  The severity of both the disease and the IOP spike will dictate the level of treatment. Most of the time, these patients can be managed by adding one or two topical glaucoma medications.  Aggressive tapering of postoperative steroid should be considered as well.  In some cases, if the IOP spike is aggressive and early in the postoperative period, such as day one, clinicians can consider anterior decompression.

Anterior decompression is an effective way to decrease IOP rapidly, but does come with risks, namely endophthalmitis and decompression retinopathy.  It should be reserved for patients presenting with emergent IOP spikes and at risk for optic nerve head damage. The patient should be placed on a topical glaucoma medication after anterior decompression and the IOP monitored for two to seven days.

The angle of the eye has an abundant vasculature, and it is not uncommon for a patient to experience a mild, transient hyphaema in the early postoperative period. Patients with hyphaema will typically present within the first few weeks with a complaint of cloudy vision.  On examination of the anterior segment, the practitioner will see what appears to be an aggressive anterior chamber reaction, which in actuality is the presence of red blood cells in the anterior chamber.  A small amount of blood may also be noted in the angle anatomy.

The key factor in managing a hyphaema after MIGS is patient education.  No intervention is needed in most cases, and practitioners should educate the patient that the cloudy vision will decrease and resolve over the next week.  In the case of a large hyphaema, the patient may need to be referred back to the surgeon to have an anterior chamber washout.

Placing stents in the angle of the eye puts the stent in close proximity to the iris. Although uncommon, a tuft of iris can occasionally obstruct the lumen of a stent. If the stent is obstructed on gonioscopy and IOP is elevated, the optometrist should refer the patient back to the surgeon to remove the obstruction.

The rate of hypotony is low with most MIGS, as many of them do not bypass episcleral venous pressure, which means that IOP will not decrease below 10 mm Hg. The exceptions are devices acting via the supraciliary and subconjunctival spaces.

Optometrists assessing these MIGS devices need to observe the anterior chamber to make sure it is deep and there is no iridocorneal touch. A fundus exam will rule out choroidal effusion. If the anterior chamber is formed and no choroidal folds exist, the optometrist can monitor the patient without a referral back to the surgeon. If the anterior chamber is flat or shallow, or if iridocorneal touch or folds exist, the optometrist must refer the patient to the surgeon for an anterior chamber reformation. Finally, in the setting of hypotony, it is important for patients to discontinue use of their topical glaucoma medications.

Clinicians must establish a new baseline IOP once IOPs have stabilised following MIGS. This postsurgical IOP will become the new baseline used to monitor the patient and make decisions about the need for future treatment. Optometrists should also obtain new visual fields and retinal nerve fibre layer analysis after MIGS to better monitor the patient for progression.

Conclusion

MIGS procedures are designed to augment one of three portals of aqueous outflow: to the subconjunctival space, to Schlemm’s canal and collector channels, or to the suprachoroidal space for enhanced uveoscleral outflow. In patients with mild to moderate glaucoma with an IOP that has been unable to be controlled by medications or who have poor medication compliance, MIGS appears to be a viable option. Optometrists must counsel patients on the availability of these surgeries earlier in the glaucoma continuum.

References

1.Schweitzer J. Mastering MIGS, Today and Tomorrow. Review of Optometry. 15 July 2017.

2.Samples JR, Singh K, Lin Sc, et al. Laser trabeculoplasty for open-angle glaucoma: a report by the American Academy of Ophthalmology. Ophthalmology 2011; 118(7):2296-2301.

3.Grover DS, Godfrey DG, Smith O, et al.Gonioscopy-assisted transluminal trabeculotomy, ab interno trabeculotomy. Ophthalmology 2014; 121:855-861.