Micro-invasive Glaucoma Surgery (MIGS) was first introduced in 2004 with the approval of Trabectome, but then exploded onto the scene in 2012 with the approval of the iStent (Glaukos, Laguna Hills, CA) and turned the glaucoma world on its head. Since then many new procedures and devices have emerged, each of which provides different risk, benefit, and safety profiles. One of those devices is the Kahook Dual Blade (KDB, New World Medical, Rancho Cucamonga, CA) which was introduced in the United States in 2015 (Figure 1).
Figure 1: The Kahook Dual Blade
Goniotomy with the KDB works by removing the trabecular meshwork (TM) in a minimally invasive manner. Remember that in glaucoma, the TM, specifically the juxtacanalicular TM, is the site of greatest resistance to aqueous outflow. Therefore, removing it should in turn lower intraocular pressure (IOP) by improving aqueous outflow.
KDB clinical outcomes
KDB goniotomy has gained acceptance since its introduction due to its favorable safety profile and excellent clinical results. Unfortunately, given its relative novelty, many studies have limited follow-up to only about one year. In a one-year study by Sieck et al of 197 eyes, success rate (defined as IOP reduction >20% or reduction of at least one medication) was approximately 72% with combination phacoemulsification and KDB goniotomy (phaco+KDB) and 69% with KDB goniotomy as a standalone procedure. In that same study, average IOP in the phaco+KDB group was significantly reduced from16.7mmHg on 1.9 meds pre-operatively (pre-op) down to 13.8mmHg on 1.5 medications post-operatively (post-op). With standalone KDB, the IOP was reduced from 20.4mmHg on 3.1 medications pre-op to 14.1mmHg on 2.3 medications post-op (1). Sieck et al then presented their two-year results on 382 eyes (74 KDB only eyes, 308 phaco+KDB eyes). Average IOP at 2 years was 13.5 mmHg, down from 16.3mmHg for phaco+KDB and 14.5mmHg, down from 21.8mmHg pre-op for KDB only. They also evaluated success rates in different glaucoma subtypes. Pseudoexfoliation glaucoma patients had the highest success rate at 86.1% followed by primary open angle glaucoma at 65.8% (2).
Earn up to 7 hours of CME from the industry's leading names. Eyes On 2022—three days of live, virtual education for ophthalmologists. Register now
Comparison to other MIGS and goniotomy techniques
Again, because of their relative novelty, there have not been many studies which directly compare different MIGS procedures to one another. This can sometimes make deciding between procedures more difficult for both physicians and patients. Dorairaj et al did complete a 6 month multicenter retrospective trial which compared phaco+KDB to phaco+iStent. Phaco+KDB had significantly more patients with IOP reduction >20% at 6 month follow-up (56.1% with KDB and 43.9% with iStent; p<0.05). Phaco+KDB also lowered IOP and medication usage more than Phaco+iStent in this study (3).
There are several techniques by which surgeons perform trabeculotomy and goniotomy. These have long been used in pediatric glaucoma to lower IOP but the usage of all techniques in adult open angle glaucoma has expanded as well. Other goniotomy or trabeculotomy techniques that you may have heard of include Gonioscopic-Assisted Transluminal Trabeculotomy (GATT), Trabectome (Neomedix Corporation, Tustin, CA), OMNI/Trab 360 (Sight Sciences, Manlo Park, CA), and traditional goniotomy with an MVR blade. GATT and traditional goniotomy are different from a KDB goniotomy in that in the first two procedures, the TM is cut, torn, or incised either with a suture (as in GATT) or an MVR blade or needle (as in traditional goniotomy). These first two procedures, therefore, leave residual TM leaflets that my scar closed later on. Trabectome removes TM with an electrocautery device but has also been shown to leave TM leaflets and can cause thermal injury to the surrounding tissue. In contrast, KDB goniotomy uses a novel dual blade design to more completely remove the TM from the nasal angle without causing significant damage to the surrounding tissues. This has been demonstrated to be true histologically in comparison to Trabectome and MVR blade (4). A study published in 2019 compared 6-month surgical outcomes of 360 trabeculotomy (GATT or Trab 360) to KDB. In their study, at 6 months, surgical success was similar between GATT or Trab360 eyes and KDB eyes. However, significantly more eyes in the KDB group achieved IOP ≤ 18mmHg (80% vs 59.3%; p=0.04) and ≤ 15mmHg (61.4% vs 25.9%; p=0.003). Mean IOP and medication reduction were also similar between the two groups (5). Another study of 73 eyes compared phaco+KDB to phaco+Trabectome at 12 months. At one year, success rates, defined as IOP of ≤ 21 mmHg and IOP reduction of at least 20% from baseline, or reduction of at least 1 glaucoma medication, was similar between the groups at 88.9% and 77.3% for phaco-KDB and phaco-Trabectome groups, respectively (6).
Good candidates for KDB
So now that we’ve briefly reviewed the data, who should you consider performing KDB goniotomy on? There are multiple things to consider when deciding on a surgical approach for any patient. As a versatile MIGS option, KDB can be performed in combination with cataract surgery, but also in pseudophakic patients or in phakic patients as a stand-alone procedure.
One obvious factor is their type and severity of glaucoma. Additional factors you should consider are their IOP goal or target, their starting IOP and the patient’s personal surgical goal (IOP control, medication independence, halt progression of disease, etc.). KDB is approved for the treatment of open angle glaucoma and ocular hypertension. It can be used in patients with any disease severity as well. Remember though that while angle-based procedures which work on the TM can successfully lower IOP, aqueous outflow still depends on the downstream outflow system and therefore cannot lower IOP below episcleral venous pressure (i.e. 8-10mmHg). Thus, KDB and other goniotomy procedures may not be ideal for patients with a very low IOP goal. Nor are they indicated for use in patients with elevated episcleral venous pressure.
KDB has shown particular success in patients with secondary open angle glaucoma such as pseudoexfoliation, pigment dispersion, and steroid-induced glaucoma (2). In all of these instances, the disease process occurs at the TM, therefore removal of the TM makes sense (2,7). Lastly, while primarily used in patients with open angles, KDB can be used in some, but not all patients with angle closure. In patients with relatively small areas of peripheral anterior synechiae (PAS) or in those with relatively recent angle closure, goniosynechiolysis followed by KDB goniotomy can often be performed safely. Goniosynechiolysis can be performed via your preferred method but can also be performed with the footplate of the KDB.
Details regarding their glaucoma often drive the decision-making process, but with that said, always remember to consider the whole patient. To perform most angle surgery, you need a cooperative patient with a clear cornea. During the KDB procedure specifically, which may take at least a few minutes even for an experienced surgeon, the patient has to rotate their head approximately 30-45° away from you, the surgeon, then look in a specific direction and not move. Therefore, avoid patients with a history of neck injury or those who can’t turn their neck or are unable to stay still. Lastly, remember to choose a patient with a clear cornea. Small scars or opacities outside of the visual axis you can usually work around but you ideally want as clear a view as possible. Can you see their angle easily during in-clinic gonioscopy? If so, then the cornea is probably clear enough for your case.
Remember to review their systemic medications pre-op as well. If a patient is on blood thinners such as aspirin, this does not need to be stopped. For patients on warfarin or other newer anticoagulant medications, consider a temporary cessation a few days prior to surgery with the approval of their cardiologist or primary care physician as they are likely at higher risk for hyphema.
Poor candidates for KDB
KDB is not indicated in patients with elevated episcleral venous pressure, active angle neovascularization, or angle dysgenesis. Any patient with non-identifiable angle structures is also not a good candidate. It may be performed in certain cases of chronic or acute angle closure, although we would not recommend starting out with these cases. Eyes with minimal angle pigmentation can make identification of angle structures difficult. So, avoiding these eyes for your first few cases and choosing those with more dense pigmentation of the TM is certainly helpful. As described above, any angle procedure, including KDB should be avoided in patients with corneal opacities which obscure your view of the nasal angle and in those who cannot turn their head and neck or follow instructions appropriately. Patients on blood thinners are not contraindicated but special consideration should be given in these situations, especially if the patient is monocular.
Pre-op counseling - What to tell patients before surgery?
As with all surgeries, accurate and thorough pre-operative counseling is key. A good rule to live by when possible is under-promise and over-deliver. We describe the procedure to patients as a way to open the natural drainage system of the eye, like unclogging a sink drain, as we have found that most patients will understand that metaphor. For patients undergoing KDB along with phaco, inform them that it will lengthen their procedure slightly, review the additional risks, and go over what to expect the day of surgery. Always make sure they understand why you are doing this procedure as well, such as “we are doing this to try to get your pressure lower” or “to try to get you off of some of your glaucoma drops.”
Lastly, always review what they can expect after surgery. Hyphemas can be asymptomatic, significantly limiting visually or range anywhere in between. We explain to patients before surgery that they should expect some bleeding during surgery, but that it is an expected event after this type of surgery. The blood may blur their vision post-op for a few days up to a week or more. It is important to remind them this isn’t their friend’s cataract surgery where they were 20/20 day 1, and they should expect blurred and fluctuating vision for at least a few days. If the hyphema is absent or minimal and they’re seeing great, they will think you are wonderful and if they have a significant hyphema, they were prepared for it. Also, review activity restrictions post-op, especially in patients on blood thinners or those at high risk of bleeding. Warn patients of the risk of re-bleeds, which happens around 3-5 days post-op, if at all. Further, many surgeons use carbachol or Miostat at the end of a KDB case. This will usually lead to a miotic pupil, dim vision, and a possible headache post-op day 1. Warn your patients of the expected headache or brow-ache so that you aren’t fielding emergency headache calls the night of surgery and having to explain it to an anxious patient. Lastly, we like to warn patients that IOP may fluctuate post-op due to bleeding, inflammation, possible steroid response, etc. This way if they have an IOP spike post-op week 1 or their IOP is not as low initially as you had hoped, again they were warned and are not surprised by this.
Learning how to perform KDB Goniotomy and tricks of the trade
Before your first case, as with any surgery, it’s important to practice. This includes watching instructional videos, reading up on surgical techniques, and doing wet labs. Your New World Medical surgical representative should have a great wet lab to work with you on before your first few cases and they can usually be present with you in the operating room (OR) when necessary (check with your surgical center regarding COVID regulations and representatives). Contact your local rep for assistance with this.
After teaching this procedure to multiple residents, fellows, and new attendings, we have noted that each struggle or succeed in different areas. One area where surgeons struggle is in properly identifying the TM, especially in lightly pigmented individuals. So, let’s review a few tricks you can use to help you identify the proper landmarks in this instance. The first is simple, practice gonioscopy in clinic and in the OR. Practice identifying all angle structures and have someone confirm your findings. On routine phacos, turn the patient’s head and your microscope just like you would for any angle surgery and practice identifying the structures. You can even take a blunt instrument like a Sinsky hook and touch the TM or practice the gliding movement. Intraoperatively you can find the TM if you’re struggling with two easy tricks, use Trypan blue or look for blood reflux. Trypan blue stains the trabecular meshwork, making it, you guessed it, blue, which in turn makes for easy identification of the TM. If you are performing your KDB prior to phaco, just use Trypan blue intracameral before placing viscoelastic to stain the TM (Figure 2).
Figure 2: Trypan Blue staining of the trabecular meshwork. Photo courtesy of Bac Nguyen and New World Medical.
If you are performing your KDB after phaco though, you may notice the TM is no longer blue, because the irrigation during phaco rinsed it away. In this instance, you can remove viscoelastic, place more Trypan, then refill with viscoelastic. Another way to identify the TM is to look for blood reflux into the Canal of Schlemm (SC) (Figure 3). In some patients this is prominent, but in others, you may need to lower the IOP. Burp or remove some viscoelastic and then look at the TM again and you should notice it has a reddish hue.
Figure 3: Blood Reflux in the Canal of Schlemm
Another area where new surgeons struggle is balancing the gonioprism on the eye with their non-dominant hand. This is just as it is described, a very delicate balancing act and not an intuitive skill. It takes practice. So as mentioned above: practice, practice, practice, BEFORE your first cases. Too much pressure on the gonioprism and you will get corneal striae which will block your view. Too little pressure and you will get air bubbles in the cornea-gonioprism interface and have no view (Figure 4). You definitely don’t want to lose your view while the blade is in position either. Never advance a blade that you cannot see. Also, if you have blood on your surgical field, either from a wound near conjunctival vessels or from a sub-tenon’s block, for instance, ensure the blood is wiped away from the cornea prior to placing your viscoelastic and gonioprism. Blood in the interface will obscure your view and is difficult to clean off.