Kahook Dual Blade: Ab Interno Trabeculectomy
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Glaucoma is one of the leading causes of blindness worldwide. All current therapies for glaucoma target intraocular pressure (IOP) lowering either through the use of oral or topical medications, laser or surgical therapy. The trabecular meshwork (TM), specifically the juxtacanalicular meshwork, has long been thought to be the site of greatest resistance to aqueous outflow. Incision and/or removal of the trabecular meshwork, i.e. Trabeculectomy or Goniotomy, has been successfully used to lower IOP in congenital glaucoma for years and more recently has been shown to be advantageous in adult patients as well.  
The Kahook Dual Blade (KDB, New World Medical, Rancho Cucamonga, CA) is a novel goniotomy blade created to produce a more complete removal of TM through a minimally invasive approach. It was launched in the United States in 2015 and was designed to achieve near complete removal of TM with minimal, if any, surrounding tissue damage. This is in contrast to other ab interno trabeculectomy techniques (Gonioscopy-Assisted Transluminal Trabeculotomy (GATT) and traditional trabeculotomy), where the TM is incised and therefore residual TM leaflets remain post-procedure. In addition, the Trabectome (Neomedix Corporation, Tustin, CA) has been shown to both have residual leaflets and cause thermal injury to nearby tissues. A pre-clinical study on human donor corneo-scleral rims demonstrated histologically the more complete TM removal with less surrounding tissue damage with the KDB compared to both Trabectome and an MVR blade trabeculotomy. Minimizing residual trabecular meshwork leaflets theoretically may lead to less fibrosis overtime and therefore potentially enhance long term outcomes compared to other similar procedures.
The dual blade has several key design features to achieve a more complete goniotomy. First is the sharp tip designed with a taper to allow for smooth entry of the blade through TM and into Schelmm’s Canal (SC). The heel of the device fits easily within SC and allows smooth advancement of the blade within the canal while preventing collateral damage during treatment. The ramp of the blade generates a gentle stretch of the TM as the blade is advanced. Finally, the dual blades create parallel incisions of the TM allowing excision of a strip of TM which achieves a near complete removal of TM, instead of incision only.
An additional benefit of the KDB, is that it is a single use, disposable instrument which does not require any additional special surgical equipment. Further, there is no implant related risks as no implant is left behind with this procedure.
Indications and Pre-operative Considerations
Goniotomy with the Kahook Dual Blade is indicated for the treatment of open angle glaucoma and ocular hypertension. Patients with uncontrolled IOP or those who have controlled IOP but other factors such as multiple drug allergies, drug intolerance, or non-compliance may be considered for this procedure. It is approved as a standalone procedure or in combination with phacoemulsification (phaco) cataract surgery. When used as a standalone procedure it can be performed in pseudophakic or phakic patients. In phakic patients, special care should be taken to avoid damage to the lens capsule. While it is indicated with any disease severity it may be inappropriate for patients with a very low target IOP given that successful goniotomy still depends on patent aqueous outflow pathways downstream. Therefore, it should not be expected to lower IOP beyond episcleral venous pressure (i.e. 8-10mmHg). Patients with more distal scarring and outflow malfunction may also have limited success with these procedures.
Specific success has been seen in patients with secondary open angle glaucoma such as pseudoexfoliation glaucoma and pigmentary glaucoma. Since the mechanism of elevated IOP in these patients is related to accumulation of extracellular material and pigment within the TM respectively, it is intuitive that removal of the TM should improve IOP. The KDB goniotomy has also shown success in congenital glaucoma and has been used in patients with uveitic and/or steroid induced glaucoma with reported success. Prior success or failure of selective laser trabeculoplasty has not been shown to affect success rate with the KDB.
Other patient factors that should be considered are patient cooperation and quality of the gonioscopic view. Patients should ideally have a clear cornea, the ability to avoid eye movements for several minutes at a time and clearly identifiable angle structures. In order to produce adequate angle visualization, patients must be able to rotate their neck by approximately 30-45 degrees. While not a definite contraindication, special consideration should be given to anti-coagulated patients given the risk of hyphema.
KDB goniotomy is contraindicated in patients with active neovascularization, angle dysgenesis, and some forms of angle closure glaucoma. However, if peripheral anterior synechiae can be lysed to visualize the TM, KDB goniotomy may still be performed successfully. These cases usually require concomitant gentle goniosynechiolysis which can be performed either with the side of the dual blade or via the surgeon’s preferred technique. In cases of small PAS, these areas can be skipped and goniotomy can be performed only in the surrounding open areas. As noted above, patients with angle dysgenesis or non-identifiable angle structures are not ideal candidates for this procedure. Poor visualization of the angle, which may be due to corneal quality, lack of angle pigmentation or patient cooperation, is also a relative contraindication.
The Kahook Dual Blade goniotomy is most commonly performed under topical anesthesia. In children or less cooperative adults, it can be performed under general anesthesia. For patients that require general anesthesia, it is important to remind the anesthesia team that rotation of the patient’s head will be required before performing such maneuvers. Preoperative pilocarpine and/or intracameral miotic agents can be used in patients undergoing the procedure without phacoemulsification to facilitate visualization of the angle, but is not required.
A corneal paracentesis is first created and the anterior chamber and angle are deepened with viscoelastic. Non-preserved lidocaine may be instilled intracamerally before viscoelastic for further anesthesia. A temporal clear corneal incision of at least 1.5mm is then created. More viscoelastic is added if necessary to deepen the nasal angle. Care should be taken not to over-inflate the eye as this can cause collapse of and difficulty entering the canal. Likewise, under inflation of the anterior chamber may result in corneal striae with gonioscopy. Next, the patient’s head is tilted approximately 30-45 degrees away from the surgeon and the microscope is tilted 45 degrees towards the surgeon. Viscoelastic is placed on the underside of a direct gonioprism and the device is inserted through the corneal incision sideways with the surgeon’s dominant hand while the gonioprism is placed on the ocular surface with the non-dominant hand so that the nasal TM is in direct view. If anatomic structures are difficult to identify, Trypan blue can be used to stain the TM, or the IOP can be lowered temporarily to produce blood reflux into SC. The sharp tip of the blade is inserted through the TM and into SC. After the TM is pierced, the heel of the device is seated against the wall of SC and advanced in a clockwise or counterclockwise manner for approximately 3-5 clock hours depending on surgeon preference. As the device is advanced, the ramp gently stretches the TM while the dual blades create parallel incisions to generate a strip of TM. If a large trabecular strip is present this
can be removed with intraocular forceps. Small and/or peripheral strips however can be left in place. Viscoelastic is then irrigated from the anterior chamber and Carbachol intraocular solution may be injected intracamerally to produce pupillary miosis. All wounds are hydrated and checked to ensure water-tight closure and a 10-0 nylon is placed in the main wound if necessary. It is recommended that IOP be kept slightly high, around 20-25mmHg, at the end of the case to prevent further blood reflux, although this is patient and surgeon dependent.
When performed along with cataract surgery the goniotomy can be performed either before or after phacoemulsification at the surgeons’ preference as one study showed no difference in visualization as rated by surgeons whether the procedure was performed before or after phacoemulsification. When surveyed, 98% of surgeons agreed that the use of the blade was straightforward and 98-99% agreed the advancement of the blade within SC was efficient. 
In cases where phacoemulsification is performed along with KDB goniotomy, antibiotic, NSAID and steroid drops are administered and tapered over one month post-operatively per surgeon preference. Pilocarpine 1% ophthalmic solution may also be used to maintain tension on the scleral spur and maintain patency of the TM cleft in the early post-operative period. This is then tapered weekly over one month. A topical NSAID is not used in cases where phaco is not performed concomitantly. Pilocarpine should be avoided in high myopes or patients with a history of retinal detachment. Patients should be warned of the risk of blurred vision post-operatively either due to inflammation, blood reflux/hyphema, or refractive shifts with pilocarpine use. Patients are traditionally seen at post-operative day 1, week 1 and month 1, or sooner if complications occur. IOP spikes, if they occur, are treated if significant by re-starting ocular anti-hypertensives. In these cases, the topical steroid can be tapered faster or switched to a different formulation if a steroid response is suspected.
The use of ocular anti-hypertensives in the post-operative period varies by surgeon and by patient. Some surgeons prefer to stop all anti-hypertensives and only re-start if the IOP increases above goal. Others prefer to continue patients on their pre-operative medications or a modified pre-operative regimen and slowly taper drops off only after the patient is healed and finished with topical steroids.
Most complications associated with KDB gonitomy are mild and self-limited. The most commonly reported events are IOP spike and hyphema. The rate of IOP spikes with phaco combined with KDB was approximately 6% versus an approximate 12% rate in phaco combined with iStent (Glaukos Corporation, San Clemente, CA) according to one study. All IOP spikes resolved either spontaneously or with topical medications.
Intraoperative hyphema is a common occurrence (approximately 40% incidence in most studies) due to blood reflux from SC. These are typically transient and have no effect on final IOP or best corrected visual acuity. In a survey of surgeons, intraoperative blood reflux was seen in 39% of 120 eyes, with 9% of patients showing retained blood at post-operative day one and only 0.8% having residual hyphema after post-operative week one. Other studies have noted a smaller incidence of hyphema post-operatively which may be due in part to inter-physician reporting variabilities. Regardless, this incidence is less frequent than quoted rates of hyphema with trabectome (100% intraoperative and 78% at post-operative day one) and GATT (30% at post-operative week one). One study found a slight trend towards higher rates of hyphema in anti-coagulated patients but this was not statistically significant. Hyphema requiring anterior chamber washout is rare.
Other rare complications include corneal edema, rebound iritis, cyclodialysis cleft, posterior vitreous detachment and posterior capsular opacification, the incidences of which are 2%. Descemet’s tears have been reported at a rate of 4%. Careful entry and exit through the corneal wound with the blade held sideways can help to prevent these. There have been no cases of hypotony or endophthalmitis after KDB goniotomy in published studies.
Goniotomy with KDB continues to gain acceptance and widespread use amongst clinicians because of its favorable risk profile compared to both filtering surgeries and other MIGS procedures in addition to the documented success rates. Studies published since it came onto the market in 2015 have demonstrated these benefits. Given the relative novelty of this blade however, most published studies are limited to 1 year of follow up.
A prospective multicenter study by Dorairaj et al. of 52 eyes that underwent phaco in combination with KDB showed an IOP reduction of 26.2% at 1-year follow-up (P<0.001). Pre-operative IOP average in this study was 16.8 ± 0.6 mmHg at baseline and 12.4 ± 0.3 mmHg at month 12. The same cohort had 63.5% of patients with at least 1 medication reduction at month 12. In another multi-center study by Greenwood et al, 58.3% of patients had an IOP reduction of at least 20% and 61.7% were using at least 1 fewer IOP lowering medications after 6 months of follow up. A retrospective review of 100 patients who underwent goniotomy either with or without phaco in adults showed a significant decrease in IOP in all eyes (from 16.5 ± 5.0 mm Hg to 14.4 ± 3.7 in the combined group and 24.3 ± 9.1 mm Hg to 16.7 ± 7.6 in the standalone group) without a significant difference between the two groups at 18 months (P =0.5) . Another recent retrospective review of 197 eyes which underwent either KDB goniotomy alone or combined with phaco showed a significant decline in IOP and glaucoma medication usage at 1 year follow up. Success, defined as 20% decrease in IOP, was achieved in 71.8% of patients who underwent phaco-KDB and 68.8% of patients who underwent KDB goniotomy alone.
Several studies have compared results of phaco plus KDB goniotomy to phaco plus iStent procedures. The KDB Goniotomy Study Group demonstrated that their phaco-KDB cohort experienced a significantly greater decrease in IOP (23.7% vs 16.4%, p<0.001) and reduction in medication usage (1.1 vs 0.9 medications, p=0.001) when compared to the phaco-iStent group at 6 months follow up. These results were further substantiated by a prospective randomized controlled trial by Falkenberry et al. comparing phaco-KDB and phaco-istent. At 12 months follow up, the primary outcome (20% or greater IOP reduction or medication reduction of 1 or more) was achieved by 74/79 patients (93.7%) in the phaco-KDB group and 65/78 patients (83.3%) in the phaco-iStent group (P =0.04). Finally, a large retrospective analysis of 315 eyes by ElMallah et al. demonstrated that mean IOP reductions were significantly greater in their phaco-KDB group than in their phaco-iStent group at all time points including month 12 [- 5.0 (0.3) mmHg vs. - 2.3 (0.4) mmHg, respectively, p < 0.001].
While the comparison between phaco plus KDB goniotomy and phaco plus iStent procedures has the most clinical data to date, there have been other studies comparing phaco plus KDB to other phaco-plus procedures. For example, Hirabayashi et al. compared phaco plus KDB to phaco plus 360° trabeculotomy (via either Trab360 or gonioscopy-assisted transluminal trabeculotomy [GATT]). They found that mean IOP reductions and medication reductions were similar between groups at 6 months. However, more eyes undergoing KDB than Trab360/GATT achieved target IOP ≤18 mmHg (80.0% [56/70] vs 59.3% [16/27], P=0.040) and ≤15 mmHg (61.4% [43/70] vs 25.9% [7/27], P=0.003). 
An important factor when considering any phaco plus procedure is its potential to impact refractive outcomes. Sieck et al. published a retrospective review of patients who received either phaco plus KDB or phaco alone and showed no significant difference in the rate of refractive surprise between the two groups. They found that refractive surprise greater than ±0.5 D occurred in 26.3% of eyes in the phaco-KDB group and 36.2% in the phacoemulsification group (p = 0.11). Refractive surprise greater than ±1.0 D occurred in 6.6% for the phaco-KDB group and 9.7% for the phacoemulsification group (p = 0.08). This data helps alleviate potential concerns regarding KDB’s impact on refractive outcomes when paired with cataract surgery.
Of note, all of the studies mentioned have included patients with mild, moderate, and severe glaucoma, however few, if any studies have evaluated whether success rates differed amongst these groups. Similarly, the majority of patients in these studies had primary open angle glaucoma, but cohorts also included patients with pigmentary, pseudoexfoliative, and angle closure glaucoma. Subgroup analyses were not routinely performed or were not of adequate power.
However, there have been recent studies investigating the use of KDB in unique clinical scenarios including CACG, uveitis, and severe refractory glaucoma. A retrospective analysis of 42 eyes with CACG undergoing phaco plus KDB showed IOP reduction of 12.3 mmHg (p<0.0001) as well as a reduction of IOP-lowering medications by 2.2 (p<0.0001) at a 12 month follow up. Furthermore, 92.9% of eyes achieved IOP ≤18 mmHg, 100% achieved IOP reduction of ≥20%, 95.2% required ≥1 fewer medications for IOP control, and 85.7% (36/42) were medication-free. In a small retrospective study of 16 uveitic eyes undergoing KDB either with or without phaco, Miller et al. showed 10 eyes (62.5%) maintained an IOP at or below their goal through their most recent follow-up visit (mean follow-up time of 9.6±5.6 months) and the mean number of glaucoma medications was significantly reduced by 1.5±1.4 medications (P=0.004). Finally, a retrospective analysis of 53 eyes with severe or refractory glaucoma demonstrated an IOP reduction of >20% in 57.7% of eyes at 6 months. Furthermore, 63.5% and 92.3% of eyes achieved an IOP≤14 and ≤18 mm Hg, respectively, and the mean number of glaucoma medications was reduced by 1.2±1.3 (36.6%) compared with baseline (P<0.001).
KDB goniotomy is a safe and effective option for managing glaucoma in appropriate patients. Compared to other goniotomy/trabeculectomy procedures, it results in less surrounding tissue damage and a more complete removal of TM. Clinical studies have demonstrated the efficacy of KDB goniotomy both with and without phacoemulsification for reduction of both IOP and medication dependence. However, given the relative novelty of this blade, most studies are limited to 12 month follow up. When compared against other MIGS devices such as the iStent, KDB produced equal, if not improved IOP lowering. Longer term follow-up and future prospective studies are needed to better elucidate the clinical outcomes of this device as well as determining the best candidates for the procedure.
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