Trabecular bypass procedures have become a viable option for glaucoma treatment. The Kahook Dual Blade (KDB) facilitates ab interno trabeculectomy by employing a single-use, micro-engineered excision blade that makes precise parallel incisions in the trabecular meshwork and inner wall of the Schlemm’s canal to enhance aqueous outflow. A recent study highlights the effectiveness of its application.
Ophthalmic surgeons have long pursued the ideal surgical approach to lowering intraocular pressure (IOP). In the past 5 years, micro-invasive glaucoma surgery (MIGS) has been introduced as an alternative means of improving patient outcomes by facilitating an efficient, effective and safe individualised treatment in the early to moderate stages of glaucoma.
We now have a host of modern ab interno options available to us, including the following:
· Trabecular bypass procedures that open or facilitate flow through the trabecular meshwork (TM);
· Supraciliary procedures that allow fluid drainage from the anterior chamber into the supraciliary space;
· Transscleral procedures that carry fluid from the anterior chamber to the subconjunctival space, creating a low bleb.
Each have advantages and disadvantages, but there is no doubt that the more options we have available for our patients, the better.
This article reviews data that I presented at the 2017 European Society of Cataract and Refractive Surgeons (ESCRS) meeting in Lisbon, Portugal. It compares two minimally invasive ab interno TM/canal-based procedures; specifically, ab interno trabeculectomy using the Kahook Dual Blade (KDB; New World Medical SA) and implantation of the iStent (Glaukos Corp.).
It is commonly believed that the juxtacanalicular system is the site of greatest resistance in the conventional aqueous outflow system in most open-angle glaucoma (OAG) patients.1 The goal of canal-based procedures is to remove or bypass the inner wall of Schlemm’s canal to allow aqueous humour more direct access from the anterior chamber to the collector channels along the anterior wall of Schlemm’s canal.
The KDB facilitates ab interno trabeculectomy by employing a single-use, micro-engineered excision blade that makes precise parallel incisions in the TM and inner wall of the Schlemm’s canal to enhance aqueous outflow (Figure 1).
The blade is introduced into the anterior chamber via a clear corneal incision under gonioscopic view. The tip (a) pierces the TM and the smooth footplate (b) is positioned against the anterior wall of the canal.
The ramp (c) then elevates and stretches the TM, allowing the dual blade (d) to create two parallel incisions as the device is advanced, after which a free strip of TM is fully and cleanly removed through the original incision with minimal residual leaflets.
By removing the desired section of the TM, the device gives aqueous direct access to the collector channels and distal outflow system. The technology is designed to not act upon the scleral wall, which spares the conjunctiva, does not create a bleb, and does not leave a foreign body in the eye.
As with several MIGS devices, use of the KDB requires familiarity with intraoperative gonioscopy. A clear view of the angle is vital, especially as the blade is advanced through the canal. This can be optimised by injecting viscoelastic into the anterior chamber to pressurise the space and deepen the angle surrounding the treatment area.
Proper positioning is also critical: a perpendicular approach to the TM allows for easy accessibility to the angle structures and proper presentation of the dual blades towards the treated TM. Typically, three to six collector channels are exposed with a 3-4 hour treatment using the device.
The iStent is a trabecular micro-bypass product that directly connects the anterior chamber to Schlemm’s canal and creates a permanent opening within it (Figure 2).
The device is a titanium stent-about 1 mm × 0.3 mm in size-that connects at right angles to the canal-implanted portion. It comes with an inserter, which is guided into a corneal wound and into the anterior chamber. The device is then inserted into the canal under gonioscopic view.
The retention arches on the closed side of the body serve to securely fixate the device in Schlemm’s canal. The open half-pipe portion of the body is against the outer wall in order to access a collector channel.
One chalÂlenge with this approach is that the stent may not necessarily be positioned near a collector channel, since there is no assurance of what lies beyond the TM. Commercially available imaging methods do not provide a pre- or peri-operative visualisation of these collector channels. AdditionÂally, some collector channels may be more active than others.
A recent study compared the IOP-lowering effects of cataract extraction (CE) with the titanium stent (iStent) versus CE with ab interno trabeculectomy using a dual blade device (KDB) in patients with mild-to-moderate glaucoma. The study took place at 12 sites (11 in the United States and one in Mexico) and evaluated both postoperative IOP and medication use in 435 eyes treated either by CE+blade (n = 198) or CE+stent (n = 237). Both groups had similar baseline demographics.
Mean postoperative IOP in the excision blade group was lower when compared with the stent throughout the study follow up period (Table 1). At six months, the reduction in mean IOP was 23.7% and 16.4% for the blade and stent groups, respectively (p < 0.05) (Figure 3).
The blade group also exhibited significant reduction in medication use up to 6 months (Figure 4).
While the mean number of medications at 1 day and 1 week postoperatively were similar between the groups, the blade arm began to show a statistically significant reduction in medication use at month 1 compared with the stent arm (64.6% versus 44.6%, respectively; p < 0.05). This difference was sustained through month 6 with a 62.9% reduction in the KDB group (1.7 to 0.6 medications) compared with 46.1% in the iStent group (1.9 to 1.0 medications) (p < 0.05).
Adverse events in both the KDB and iStent groups were comparable, mild and self-limiting (Table 2).
As is expected with canal-based glaucoma procedures, varying degrees of intraoperative blood reflux from Schlemm’s canal was seen in both arms and was self-limited in all cases for both groups.
In this study, the CE+blade produced significant results over 6 months in patients with uncontrolled mild-to-moderate glaucoma. This group exhibited meaningfully greater reductions in IOP and resulted in significantly less medication use when compared with CE+iStent.
This study was retrospective and unrandomised, and therefore subject to bias. In future, prospective randomised clinical trials comparing different MIGS devices are necessary to better evaluate and individualise the MIGS decision pathway.
In my daily practice, I routinely use the excision blade for patients with mild-to-end-stage glaucoma and have had success at treating glaucomatous eyes with follow-ups of over one year. I find ab interno trabeculectomy to be a valid option when trying to decrease medication burden.
More recently, I have started using it in some refractory or advanced cases where I would prefer to avoid more invasive procedures. Preliminary (unpublished) results from this group are promising.
While longer-term data are still needed, the blade procedure offers surgeons an elegant and simple way to lower IOP by excising TM. This intuitive device offers many advantages to surgeons and patients and will continue to propel minimally invasive ab interno glaucoma procedures forward.
We live in a golden age for glaucoma surgery, as intensive innovation of recent years is now translating into better patient outcomes.
1. Johnson M. What controls aqueous humour outflow resistance?. Exp Eye Res. 2006;82:545–557.
Dr Kaweh Mansouri, MD, MPH
Dr Mansouri is a consultant ophthalmologist at the Glaucoma Centre, Montchoisi Clinic, Lausanne, Switzerland, and an adjunct associate professor at the University of Colorado, Denver, United States. Dr Mansouri is a glaucoma specialist, clinician-scientist and active researcher, whose interests include advancing 24-hour IOP monitoring and the development of new technologies in the field of glaucoma imaging and surgery. He consults for New World Medical.