The glaucomas are a class of ocular disease characterized by progressive optic nerve degeneration with subsequent loss in the retinal nerve fiber layer and retinal ganglion cells. As of today, the only approved treatment for glaucoma is to reduce intraocular pressure (IOP). Glaucoma affects over 2.7 million Americans—that’s a lot!
The most common form of glaucoma is open-angle glaucoma, where the angle between the iris and cornea is wide, but inadequate drainage of aqueous humor causes an increase in IOP.⁹ Many Americans live with this form of glaucoma without realizing it, which justifies the importance of yearly eye exams.
Angle-closure glaucoma results from a complete blockage of the drainage canals, leading to a rapid increase in IOP. The angle between the iris and cornea is reduced as well. This form of glaucoma is a medical emergency.
This quick guide will walk through minimally invasive glaucoma surgery (MIGS) procedures, currently available to treat glaucoma, for ophthalmology residents looking to familiarize themselves with the existing options.
Have no fear: treatments are here!
Minimally invasive glaucoma surgery (MIGS) procedures focus on reducing the IOP. This can be done by enhancing the outflow across the trabecular meshwork and through Schlemm’s canal. The buildup of aqueous humor can be shunted into the subconjunctival space, or production by the ciliary body can be halted using ablation as well.
These procedures have a high safety profile and minimally disrupt the normal anatomy of the eye, making them a desirable treatment choice. Patient outcomes are generally successful and the road to recovery is usually a straight one.
Stent placement allows for the bypass of the trabecular meshwork
The introduction of a stent allows for aqueous humor to flow from the anterior chamber into Schlemms’ canal, reducing the IOP.
A stent can be introduced into a patient with mild-moderate glaucoma. A small incision is made in the drainage tissue of the eye and the stent is placed. Overall, the surgery is a few minutes long.
iStent (Generation 1) and iStent inject (Generation 2)
The iStent is a heparin-coated, non-ferromagnetic titanium stent that measures 0.3mm in height and 1mm in length.² It can be implanted into Schlemm’s canal at the time of cataract surgery. This is a pre-loaded, single-use stent.
The iStent inject is also a heparin-coated, non-ferromagnetic titanium stent, but it consists of a tapered head with a recessed thorax and a terminal flange. The head resides in Schlemm’s canal, the thorax straddles the trabecular meshwork, and the flange resides in the anterior chamber.² This is the smallest device to be implanted into the human body with a height of 360 microns and a diameter of 230 microns.² The iStent inject comes with two pre-loaded devices, which are surgically inserted 2-3 clock hours apart.
Hydrus microstent
The microstent is 8mm in length and 290 microns in diameter with three windows and an inlet that sits in the anterior chamber.² This device is made of nitinol, which is a flexible, biocompatible titanium and nickel alloy. The IOP is lowered by creating a bypass through the trabecular meshwork, and also by maintaining the patency of Schlemm’s canal.
These stents have improved the quality of life and reduced visual morbidity in patients worldwide.
Tissue excision also allows for the bypass of trabecular meshwork
Tissue excision of the juxtacanalicular (JXT) trabecular meshwork, which is composed of a layer of connective tissue lined on either side by endothelium, has benefits as well as risks. The benefit is that this procedure does not require an implant and can be performed as a standalone procedure (or with a simultaneous cataract surgery). A major risk is the closure from trabecular meshwork leaflets.⁶
Trabectome
Trabectome is a surgical system that removes a strip of trabecular meshwork and the inner wall of Schlemm’s canal. By creating a path for drainage of aqueous humor, the IOP is greatly reduced.
The device consists of a single-use, disposable handpiece that performs electrocautery, irrigation, and aspiration. It is connected to a generator power source allowing for bipolar electrodes to ablate the meshwork.2 The irrigation and aspiration prevent the buildup of debris and regulate the temperature. The tip of the Trabectome is bent at a 90° angle for easier insertion into Schlemm’s canal.
Kahook Dual Blade
This device is one of a kind, as the procedure removes the trabecular meshwork completely, reducing the risk of residual leaflets closing off the space again. The sharp tip of the Kahook Dual Blade is designed with a taper to allow for smooth entry through the trabecular meshwork and into Schlemm’s canal. A parallel goniotomy is made with the dual blade into the trabecular meshwork, allowing for the excision of a complete strip which achieves near complete removal.¹
The Kahook Dual Blade is a single-use, disposable instrument that does not require any additional surgical equipment.
Gonioscopy Assisted Transluminal Trabeculotomy (GATT)
GATT is a unique ab interno procedure: most trabeculotomy procedures are ab externo and require a scleral flap to be created, increasing the risk of IOP build-up down the road, as well as scarring and need for postoperative manipulation⁵.
An incision is made into the trabecular meshwork, creating an entry point for the iTrack microcatheter² to advance through Schlemm’s canal. Once the suture has bypassed the canal, the trabecular meshwork is sheared out and the canal is unroofed.⁴
IOP can be reduced by a multitude of other mechanisms.
Although the most common MIGS procedures focus on stent placement and tissue excision of the trabecular meshwork, the treatment options do not stop there.
Ophthalmology residents might wish to focus on mastering goniotomy before moving to other MIGS procedures, but understanding the options available will help when you are ready to add the next set to your armamentarium.
Schlemm’s canal itself can be altered, leading to increased aqueous outflow into the systemic system, thus reducing the IOP. Another mechanism is to reduce aqueous production by the ciliary body itself. If there is no aqueous humor being produced, then the IOP should hypothetically not increase. Although these procedures are not used as frequently, they still get the job done.
Aqueous outflow enhancement through Schlemm’s canal
MIGS procedures also focus on enhancing the aqueous outflow at the level of Schlemm’s canal, rather than the trabecular meshwork. This canal is a unique vascular structure that maintains fluid homeostasis by draining aqueous humor from the eye into systemic circulation.
The processes of cannulation and dilation of Schlemm’s canal reduce outflow resistance and lead to a reduction in IOP, specifically restoring the 360° flow of the canal.
Visco 360/OMNI
The VISCO360 device allows for catheterization and vasodilation of Schlemm’s canal. It is a single-handed, single-use device with a sharp distal tip used to pierce the trabecular meshwork. The catheter is advanced into Schlemm’s canal and a predetermined amount of viscoelastic is injected.⁷ The end result of dilation without tissue damage makes it favorable.
The OMNI System for lowering IOP uses the VISCO360 handpiece and combines with the TRAB360 procedure. The TRAB360 procedure involves trabecular meshwork tissue excision using a single-use handpiece that has a tip that advances 180° into Schlemm’s canal, is retracted, and then re-inserted to tear the remaining 180° of the canal², creating an open flow again. The OMNI System focuses on reducing the IOP through both the trabecular meshwork and Schlemm’s canal.
Ab interno Canaloplasty (ABiC)
The ABiC MIGS restores 360° visco-dilation of Schlemm’s canal through a clear corneal incision. A major benefit of this procedure is that it spares the conjunctiva.
ABiC is performed using the iTrack microcatheter system. This consists of a 250 micron microcatheter with a fiber optic light that is advanced 360° into Schlemms’ canal and slowly withdrawn while performing viscodilation³.
There is also a decreased risk of herniation of the inner wall of Schlemm’s canal.
Ablation of the ciliary body to reduce aqueous humor production
Endoscopic cyclophotocoagulation (ECP) is a cyclodestructive procedure that maximizes the advantage of ablating the ciliary body epithelium to reduce IOP. The procedure is done with a laser endoscope that has the ability to deliver laser energy (Xenon light source) in a titratable manner to the ciliary process.
The probe is inserted through a clear corneal incision and advanced across the anterior chamber to the ciliary body. Once in position, an external foot pedal controlled by the physician will determine the duration of the laser beam. The ciliary processes are then targeted to achieve blanching and contraction⁸.
Overall, this MIGS procedure is an effective tool for treating refractory glaucoma.
MIGS procedures are changing the field of ophthalmology
Glaucoma is common among Americans and is the third leading cause of blindness worldwide, but the severity of the disease can vary greatly. As an ophthalmology resident developing familiarity with the condition, it is especially important to be aware of the signs and symptoms, such as headaches, eye pain, or blurred vision. Yearly eye examinations are recommended as well, especially if there is a family history of glaucoma.
The MIGS procedures have improved not only the medical field but our patients’ quality of life.
Although there are risks to any procedure, patients are able to live a lifestyle without the burden (in many cases) of excessive ocular medications, and with lower IOP.
References
- Capitena Young C. Kahook Dual Blade: Ab Interno Trabeculectomy. EyeWiki. https://eyewiki.aao.org/Kahook_Dual_Blade:_Ab_Interno_Trabeculectomy. Published May 2, 2021. Accessed May 27, 2021.
- Eliassi-Rad, B, Singh, V. Microinvasive Glaucoma Surgery (MIGS). EyeWiki. https://eyewiki.aao.org/Microinvasive_Glaucoma_Surgery_(MIGS). Published June 3, 2020. Accessed May 27, 2021.
- Gallardo MJ, Supnet RA, Ahmed IIK. Viscodilation of Schlemm’s canal for the reduction of IOP via an ab-interno approach. Clin Opthalmol. 2018; 12:2149-2155. Published 2018 Oct 23. Doi: 10.2147/OPTH.S177587
- Glaucoma Associates of Texas. Gonioscopy Assisted Transluminal Trabeculotomy. https://www.glaucomaassociates.com/gonioscopy-assisted-transluminal-trabeculotomy/. Accessed May 27, 2021.
- Lee RMH, Bouremel Y, Eames I, Brocchini S, Khaw PT. The Implications of an Ab Interno Versus Ab Externo Surgical Approach on Outflow Resistance of a Subconjunctival Drainage Device for Intraocular Pressure Control. Transl Vis Sci Technol. 2019;8(3):58. Published 2019 Jun 28. doi:10.1167/tvst.8.3.58
- Maeda M, Watanabe M, Ichikawa K. Evaluation of trabectome in open-angle glaucoma. J Glaucoma. 2013;22(3):205-8.
- Ondrejka S, Körber N. 360° ab-interno Schlemm's canal viscodilation in primary open-angle glaucoma. Clin Ophthalmol. 2019;13:1235-1246. Published 2019 Jul 15. doi:10.2147/OPTH.S203917.
- Siegel M. Endoscopic Cyclophotocoagulation (ECP). EyeWiki. https://eyewiki.aao.org/Endoscopic_Cyclophotocoagulation_(ECP). Published March 8, 2021. Accessed May 27, 2021.
- Types of Glaucoma. Glaucoma Research Foundation. https://www.glaucoma.org/glaucoma/types-of-glaucoma.php. Published June 2, 2020. Accessed May 27, 2021.
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