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Minimally invasive glaucoma surgery: Myth or reality?
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Currently, trabeculectomy is the gold standard of glaucoma surgery. Although the original technique has been largely modified in order to increase safety and efficacy, complications and failures are not uncommon, and surgery is reserved for advanced or clearly progressive glaucoma. It is not far from the truth to say that trabeculectomy is like drilling a hole into the eye and hoping that it will heal but not too much.
Complications in trabeculectomy may be devastating and cause a reduction in visual acuity; on the other side, medical therapy also has many problems, and a lot of effort has been made in recent years toward developing forms of minimally invasive glaucoma surgery (MIGS) that may be applied in earlier stages or in milder forms of the disease. The term minimally invasive glaucoma surgery was coined by Iqbal “Ike” K. Ahmed in 2009; it includes a series of surgical procedures characterized by an ab interno approach through a corneal tunnel, minimal damage of normal anatomy, a high safety profile and quick recovery time. In most MIGS procedures the conjunctiva is spared, and in case of failure, there is no problem in performing traditional filtering procedures.
MIGS procedures can be classified according to the site where the aqueous is diverted, so we will have procedures that restore trabecular outflow, suprachoroidal shunts and procedures that aim at creating a subconjunctival filtration track. In evaluating the efficacy of MIGS devices, we have to look at the mean postoperative IOP and its standard deviation more than at the percent reduction of preoperative IOP. Indeed, while the former tells us what IOP we can reasonably expect and gives us a clue about the final resistance to the outflow after surgery, the latter may be misleading because it is strictly dependent on preoperative IOP.
Each group of MIGS procedures has advantages and pitfalls. Generally speaking, approaches that restore the trabecular outflow have a high level of safety, are not indicated if the target pressure is in the lower teens and, depending on episcleral venous pressure, do not prevent nocturnal IOP spikes. Approaches that enhance the suprachoroidal pathway have a high degree of safety but again are not indicated if the target pressure is low, and they possibly have a tendency to fail in the middle to long term due to scarring in the suprachoroidal space. Ab interno subconjunctival outflow procedures (sometimes called “MIGS plus” for the presence of a bleb) are safe and easy to perform because they do not need a gonioscopic approach. However, postoperative IOP is again in the middle teens, and bleb-related problems may possibly occur during the follow-up.
All of these procedures are ideal for combination with cataract surgery because cataract surgery opens the angle and makes MIGS easier. In such cases, however, it is difficult to tell whether and in what proportion the hypotensive effect comes from cataract surgery or MIGS. Moreover, most MIGS procedures require a gonioscopic approach, and this requires modifications in the surgeon’s habits as well as a new learning curve. The development of new devices for angle visualization and a pedal-driven automated inserter could help make surgical maneuvers easier and safer.
We can conclude that there is great potential for MIGS to overcome a variety of problems in current glaucoma management, especially in cataract and glaucoma patients. Indeed, MIGS procedures have a high safety profile but are not indicated when the target pressure is low. If we want very low IOP, we need an ab externo filtering procedure. In this perspective, there is one device that deserves special consideration: the InnFocus microshunt, which cannot be classified as MIGS because of its ab externo approach, but it greatly simplifies traditional filtering procedures. In initial reports, this device has shown a very good level of safety and efficacy. It is quick and easy to implant and does not require changing from the usual ab externo approach, flattening the learning curve. InnFocus reduces the formation of secondary aqueous compared with other surgeries because it does not empty the anterior chamber during insertion; furthermore, it instead diverts aqueous in the sub-Tenon’s space at 6.5 mm from the limbus, further away as compared with classical procedures. This device also prevents postoperative hypotony thanks to the flow resistance determined by the length and inner diameter of the tube.
Currently, in contrast with the high expectations and great interest created by MIGS procedures, there is still a lack in high-quality studies reporting medium- and long-term results, as well as a lack of cost-effectiveness data and little guidance on the criteria for patient selection. A huge amount of work still has to be done in this direction.
- For more information:
- Marco Nardi, MD, OSN Europe Edition Board Member, is vice president of the Italian Ophthalmological Society and full professor of ophthalmology at the University of Pisa. He can be reached at U.O. Oculistica Universitaria, Ospedale S. Chiara, Via Paradisa 2, 56124 Pisa, Italy; email: marco.nardi@med.unipi.it.
Disclosure: Nardi reports contributions to his institution from or fees as a speaker or board member of Novartis, Alcon and Allergan.