Why Defer Trabeculotomy or Goniotomy
Recently there has been a large increase in the number of cataract and glaucoma surgeons who are performing a type of minimally invasive glaucoma surgery that goes by various names, all of which have the term “goniotomy” or “trabeculotomy” in them. Two of the more common ones are Gonioscopy-Assisted Transluminal-Trabeculotomy (GATT), Kahook Dual Blade (KDB) Goniotomy. What all of these procedures share in common is that they can be done through a small incision (such as that created during cataract surgery), they are quick, and they work by tearing through or removing a portion of the natural drainage system in the eye.
What is particularly interesting, however, is that they are also based on surgical techniques first described over half a century ago. Essentially, an incision is made in the cornea or on the sclera (white surface of the eye), the eye’s drainage canal identified, and either a suture or metal instrument is threaded into the canal after which the inner wall of the canal and Trabecular Meshwork is torn open. The trabecular meshwork is the eye’s “drainage grate” and is believed to be the primary reason IOP elevates in the majority of those with open angle glaucoma.[1] It makes sense, then, that destroying this grate could result in better flow of fluid out of the eye.
Short-Lived IOP Lowering Effect
Initial reports demonstrated a significant decrease in IOP after trabeculotomy or goniotomy. Despite impressive early results, however, these procedures largely fell out of favor in adults as the IOP lowering effect tended to be short-lived.[2] Indeed, in a paper published in 1977, the authors state that “In contrast to the results of congenital glaucoma, trabeculotomy is a poor choice of operation for the control of adult-onset glaucoma”[3] It is thought that the torn trabecular leaflets likely scarred down over time potentially worsening the outflow issue beyond pre-trabeculotomy.[4]
Underwhelming Result
In 2005, Dr. Baerveldt patented the Trabectome[5], an instrument designed not to tear through the trabecular meshwork, but to actually remove it, eliminating any torn edges that could scar back down. The Trabectome, however, never gained wide acceptance as it was an expensive piece of equipment that few surgery centers purchased. Additionally, the results were underwhelming: final IOP after surgery tended to stay in the high teens.
More recently the Kahook Dual Blade (KDB) has given the Trabectome some serious competition. Like the Trabectome, the KDB removes trabecular meshwork reducing the likelihood that residual leaflets will obstruct outflow over time. Unlike the Trabectome, however, the Kahook Dual Blade is a single use instrument that does not require a large upfront investment. Given the ease of performing this procedure it has rapidly grown in acceptance among eye surgeons.
Poor Choice of Operation
In what I find to be a fascinating example of how institutional memory is truly short-term, the latest MIGS craze is Gonioscopy-Assisted Transluminal-Trabeculotomy (GATT). In this procedure a small incision is made through the cornea and a suture or catheter is threaded through the trabecular meshwork into the Canal of Schlemm. Once the suture has been passed around the eye it is pulled, effectively ripping through the trabecular meshwork. Similar to the procedure that has been around for over 40 years, the trabecular leaflets are still there and at risk of scarring down. Why is it not still “a poor choice of operation for the control of adult-onset glaucoma”?[3] It’s as if glaucoma surgeons have developed a collective amnesia as to why it made sense to develop the Trabectome and Kahook Dual Blade in the first place.
Why might this be?
KDB goniotomy is inexpensive and fast to perform and reimburses really well. It also does generally produce a notable drop in IOP immediately after surgery which seems to last for at least a year in most patients. So, the short-term benefit to both the patient and glaucoma surgeon is clearly there. Long term, however, it’s hard to imagine that the same issues with long-term failure in adults that was noted decades ago won’t still be an issue a few years after GATT. Eye surgeons who perform GATT, however, consider a year or so of IOP reduction an acceptable result given the ease and relative safety of performing this minimally invasive glaucoma surgery (MIGS). What’s wrong with that?
Destruction of the Trabecular Meshwork
Whether one cares about residual trabecular leaflets or not, GATT, Trabectome, and KDB all suffer from destruction of the trabecular meshwork. This might not be a problem if the trabecular meshwork was simply a clogged drainage grate. However, there is growing evidence that the trabecular meshwork is much more than just a drainage grate and should be preserved if possible. Indeed, there is an impressive amount of work from Murray Johnstone demonstrating dynamic pulsatile flow[6] In other words, the trabecular meshwork acts as a pumping mechanism.[7] Remove the trabecular meshwork and the pump is broken. Additionally, it is now known that a complex system of mechanosensory regulation exists in which outflow is determined by stress placed on the trabecular meshwork.[8] Finally, there are multiple new pharmaceutical agents such as RHOPRESSA® (netarsudil ophthalmic solution 0.02%) that would be unlikely to have any beneficial effect if the trabecular meshwork has been destroyed as their mechanism of action is on the trabecular meshwork.[9] No trabecular meshwork, no benefit from Rhopressa. Why eliminate an entire class (or future classes) of TM-dependent treatment options unless there are no other reasonable options?
The Better Option
For this reason, I prefer to offer my patients surgical options such as canaloplasty that spare the trabecular meshwork. Only if I cannot achieve adequate IOP reduction with meshwork-sparing surgery will I consider KDB goniotomy or GATT.
References:
1 Lutjen-Drecoll E. Functional morphology of the trabecular B meshwork in primate eyes. Prog Retin Eye Res 1999;18:91–119.
2 Minckler DS, Baerveldt G, et. al. Clinical Results with the Trabectome for Treatment of Open-Angle Glaucoma. Ophthalmology. 2005;112:962–967
3 Luntz MH, Livingston DG. Trabeculotomy ab externo and trabeculectomy in congenital and adult-onset glaucoma. Am J Ophthalmol. 1977;83:174-179.
4 Amari Y, Hamanaka T, Futa R. Pathologic investigation failure of trabeculotomy. J Glaucoma. 2015 Apr-May;24(4):316-22. doi: 10.1097/IJG.0b013e31829e1d6e.
5 Baerveldt, G. and Chuck, R. “Minimally Invasive Glaucoma Surgical Instrument and Method” US Patent (2005): Available at https://www.google.com/patents/US6979328
6 Johnstone M, Martin E, Jamil A. Pulsatile flow into the aqueous veins: manifestations in normal and glaucomatous eyes. Exp Eye Res. 2011;92:318-327.
7 https://www.youtube.com/watch?v=51MheK88170
8 WuDunn D. Mechanobiology of trabecular meshwork cells. Exp Eye Res. 2009;88:718–723.
9 Lin CW, Sherman B, Moore LA, et. al. Discovery and Preclinical Development of Netarsudil, a Novel Ocular Hypotensive Agent for the Treatment of Glaucoma. Journal of Ocular Pharmacology and Therapeutics. Volume: 34 Issue 1-2: March 2018.
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David Richardson, MD
Medical Director, San Marino Eye
David Richardson, M.D. is recognized as one of the top cataract and glaucoma surgeons in the US and is among an elite group of glaucoma surgeons in the country performing the highly specialized canaloplasty procedure. Morever, Dr. Richardson is one of only a few surgeons in the greater Los Angeles area that performs MicroPulse P3™ "Cyclophotocoagulation" (MP3) glaucoma laser surgery. Dr. Richardson graduated Magna Cum Laude from the University of Southern California and earned his Medical Degree from Harvard Medical School. He completed his ophthalmology residency at the LAC+USC Medical Center/ Doheny Eye Institute. Dr. Richardson is also an Ambassador of Glaucoma Research Foundation.
