When medications or laser procedures fail to adequately control intraocular pressure (IOP), or if side effects prevent the continued use of medications, surgery becomes necessary to achieve the desired level of IOP. Surgery for glaucoma lowers IOP by either increasing the flow of fluid out of the eye or by decreasing the production of intraocular fluid.
It must be remembered that optic nerve damage and subsequent loss of vision caused by glaucoma cannot be reversed. Thus, the point of surgery is not to improve or restore vision, but rather to prevent further vision loss. Currently there is no glaucoma treatment that can restore vision already lost to the disease. For this reason, early diagnosis and treatment are vital.
This article describes Micro-Invasive Glaucoma Surgeries, procedures that may be an option for some patients with certain types of glaucoma. A comprehensive evaluation by a surgeon who performs these procedures is necessary to determine the best option for any given patient.
Traditional glaucoma surgery, known generally as filtering surgery, drains fluid from the eye by creating an artificial pathway through the sclera, the white of the eye. These procedures are very effective at reducing intraocular pressure (IOP), and in some cases are the only good options. However, due to the nature of the artificial connection between the inside and outside of the eye, they are associated with several potential complications, some of which can be serious.
On the other hand, Micro-Invasive Glaucoma Surgery, or MIGS procedures, work by attempting to reestablish good flow through the eye’s normal drainage system or, in some cases, by diverting fluid between the layers of the eye. While typically not quite as effective as filtering surgery at reducing the eye pressure, these procedures are generally faster to perform, offer a much shorter healing period, have less impact on early post-operative vision, and have fewer overall risks, both short and long-term. Additionally, they do not limit the future performance of other procedures, allowing for a stepwise approach to lowering the IOP while not “burning any bridges” for future surgeries that may be needed.
Schlemm's Canal-Based Procedures
Schlemm’s canal is the primary drainage channel by which aqueous humor, the intraocular fluid that is continually being produced, leaves the eye. This canal runs 360 degrees around the front of the eye, where the iris (the colored part of the eye) meets the sclera (the white of the eye). In many cases of open angle glaucoma, it is believed that this canal is not functioning properly. In simplistic terms, the canal is clogged. The actual defect is thought to lie within the trabecular meshwork, a network of tissue that lines the entrance into the canal. Several MIGS procedures aim to bypass the trabecular meshwork and allow fluid to enter Schlemm’s canal directly, thereby increasing drainage of fluid from the eye and lowering the intraocular pressure.
- Micro-bypass stents: Currently there are two types of stents available for implantation in the United States, the Glaukos® iStent®, which now in its third generation is known as the iStent Inject®, and the Ivantis® Hydrus® microstent. Initially approved by the FDA in 2012 and 2018, respectively, these stents are inserted through the trabecular meshwork and into Schlemm’s canal in order to drain aqueous humor more effectively. Currently, implantation of these stents is only reimbursed by Medicare and most insurance companies when combined with cataract surgery. Stents will not be paid for if implanted as a separate procedure. Though the design of each device differs somewhat, studies suggest similar efficacy overall. While one recent study (the COMPARE trial, published April 2019) found the Hydrus to be more effective than the original iStent design, no clear head-to-head data is currently available comparing the Hydrus and the newer iStent Inject. On average, however, these stents lead to pressure reduction of one to two millimeters of mercury beyond that produced by cataract surgery alone and decrease the need for medications by one to two drugs. There is minimal additional risk to the implantation of these stents, and no impact on the normal healing period for cataract surgery. In general, blood thinners do not need to be stopped. The decision regarding which stent to implant is best made based upon the anatomy of the individual eye, conditions found at the time of surgery, and the surgeon’s familiarity with each device.
- Ab-Interno Canaloplasty (AbIC): Another option for increasing the outflow of aqueous via the eye’s natural drainage pathways, this procedure dilates and cleans out Schlemm’s canal. A tiny catheter, or hollow tube, is inserted into the canal through the trabecular meshwork and passed 360 degrees around the entire eye, breaking adhesions and scar tissue that have formed over time. As the catheter is withdrawn, a thick liquid is injected into the canal, further dilating it and forcing out debris. This procedure is effective and reimbursed by insurances either combined with cataract surgery or when performed by itself. Several reasonably sized studies have shown pressure reductions of as much as 30%, or four to six millimeters of mercury, though actual reduction may be more or less than this. Complication rates are relatively low and generally mild, though bleeding is more likely than with stent implantation and therefore discontinuation of blood thinners is recommended for this procedure. Recovery to clear vision may be slightly delayed by a few days due to bleeding.
- Goniotomy: This procedure involves either cutting into or removing a variably sized section of the trabecular meshwork, allowing aqueous humor a more direct pathway into Schlemm’s canal. Overall, efficacy is similar to that of AbIC, though depends upon the extent of meshwork treated. As with AbIC, intraocular bleeding is more likely than with stent implantation. Thus, it is recommended that blood thinners be discontinued for this procedure. Again, recovery to clear vision may be slightly delayed by a few days due to bleeding.
As we have discussed, MIGS procedures attempt to reestablish good fluid drainage through the eye’s natural outflow system, while filtering surgeries abandon that system in favor of a surgically created bypass drain through the sclera, the white of the eye. More recently, however, a third option for aqueous drainage has been developed. Though a bit more difficult to conceptualize, these procedures take advantage of a pressure difference, or gradient, between layers within the eye, allowing fluid to drain away in a novel manner. Currently, none of these implants are FDA approved for use in the US, however their availability is anticipated in the next few years.
Though these procedures do not quite fit the definition of Micro-Invasive Glaucoma Surgery, they do somewhat bridge the gap between MIGS procedures and traditional filtering surgery, and it seems logical to at least mention them here. Like filtering surgeries, aqueous humor is diverted from inside to outside the eye via a bypass through the sclera, forming what is known as a “filtering bleb.” However, much less cutting and manipulation of tissues is required when implanting these devices, with considerably shorter operative and recovery times. Some of the risks of traditional filtering surgery are reduced, though certainly not eliminated. Therefore, unlike other MIGS procedures that can be performed by nearly any eye surgeon trained appropriately, trans-scleral stents are generally implanted by surgeons experienced with glaucoma filtering surgeries and their post-operative management.
There is only one such device approved for use in the US by the FDA: the XEN45 glaucoma implant. Another such implant, known as the PreserFlo shunt, is expected to be approved in 2021.
Choosing a MIGS Procedure
In the United States, the majority of MIGS procedures are performed in combination with cataract surgery in order to gain better control of intraocular pressure and/or to reduce a patient’s dependence on pressure-lowering medications. Given the efficacy and extremely low risk of MIGS procedures, they ideally compliment cataract removal in patients who also have mild to moderate glaucoma. Additionally, these procedures are often an excellent stand-alone option for patients with glaucoma for whom medications and laser trabeculoplasty have failed to adequately control their disease, prior to considering more traditional filtering surgery.
Ultimately, which procedure is chosen depends on several factors, as previously noted. These include overall patient health and use of blood thinners, anatomy of the eye, type and degree of glaucoma, and surgeon comfort and competence with individual procedures. Your surgeon will make a recommendation based upon full consideration of these and other factors.