Ab Interno Trabeculectomy and Trabeculotomy
Glaucoma is one of the most prevalent causes of blindness in the world and can be treated by controlling elevated intraocular pressure (IOP) in patients. If left untreated, glaucoma can progress from peripheral blindness to complete blindness. There are many forms of glaucoma with primary open-angle glaucoma being the most common. Management usually begins medically with topical pressure-lowering drops first until max medical therapy is reached--usually 4 different classes of IOP-lowering drops as well as an oral carbonic anhydrase inhibitor. Treatment after that depends on the mechanism of the underlying glaucoma, but generally involves laser therapy or various types of surgeries.
An increasingly popular treatment option is a group of surgeries called micro-invasive glaucoma surgery (MIGS), which have shown to have minimal risk with a mild compromise in efficacy. The most attractive features of these surgeries and largely why they are often a primary surgical option are their incredibly high safety profile and very low rates of serious complications. Several different types of MIGS procedures have developed over time including Schlemm’s canal (SC) devices, subconjunctival devices, and the method this article will exploring, ab interno trabeculetomies and ab intero trabeculotomies (collectively referred to AIT for the rest of this article).
The ab interno approach aims to decrease IOP by increasing aqueous outflow through a direct opening in the trabecular meshwork (TM) from within the anterior chamber (AC) to produce direct communication from the AC to the outer wall of SC and the collector channels. This is in contrast to ab externo procedures which create that connection from the conjunctival side, requiring conjunctival dissection and does not involve significant manipulation within the AC. This article will focus on AIT, which includes the Trabectome (NeoMedix Corporation, Tustin, CA), Kahook Dual Blade (KDB, New World Medical, Rancho Cucamonga, CA) goniotomy, gonioscopy assisted transluminal trabeculotomy (GATT), and Omni (Sight Sciences, Menlo Park, CA). For more information about ab interno and ab externo canaloplasty, please visit the Canaloplasty page.
The Trabectome is an ab interno trabeculectomy device that can be used to ablate the TM. This reduces the possibility of filling-in of the incision over time. The Trabectome has a 19.5 mm gauge pointed tip and an insulated footplate, which helps protect the underlying areas from secondary injury during ablation of the TM. An ab interno trabeculectomy with a trabectome device begins with a 1.65 mm incision made temporally on the cornea. Viscoelastic is used to prevent AC collapse. The goniolens is then placed on the cornea to view the nasal angle. The probe is inserted through the incision, targeting the nasal TM across the AC. Approximately 90 degrees of ablation is performed, from 60 to 150 degrees, going through the TM and the inner wall of SC. The probe is removed, the viscoelastic is replaced with BSS, and the wound is hydrated. A 10-0 nylon suture may be used to close the wound.
Kahook Dual Blade (KDB) Goniotomy
KDB is also an ab intero trabeculectomy device using novel dual blade specially designed to perform a smooth incision into SC. The blade then enters and moves along the TM, removing tissue with minimal collateral damage.
Gonioscopy-Assisted Transluminal Trabeculotomy (GATT)
GATT is an ab interno trabeculotomy device that begins with entering the AC through a temporal corneal incision. A goniotomy is created in the nasal angle. A microcatheter or suture is then placed into SC and advanced circumferentially using microsurgical forceps. Traction on the inserted microcatheter or suture creates a full 360 degree trabeculotomy, which does not involve removing any TM.
Omni is also an ab interno trabeculotomy device, so in this sense, it is similar to GATT. During this procedure, the AC is similarly entered through a temporal corneal incision and a cannula is used to cut into the TM, which is visualized using gonioscopy. Once the microcatheter is in SC, it is advanced around the canal 180 degrees. The TM is torn as the microcatheter is taken out and the steps are repeated in the opposite direction to create a complete 360 degrees transluminal trabeculotomy.
Surgical follow up
As with most glaucoma procedures, surgical follow-up after an AIT usually occurs at day 1, week 1, and then at the surgeon’s discretion depending on the patient’s progress. The immediate postoperative period normally entails checking the IOP and bleb appearance (if applicable). Presence of complications such as hyphema, infection, conjunctival/wound leak, shallow/flat AC, hypotony requiring intervention, and choroidal detachment should also be noted.
The ab interno approach has shown to have few complications and relatively less than traditional ab externo trabeculectomies. A blood reflux at the ablation site indicates appropriate depth reach in the TM. Though this is a desirable sign, it results in nearly all patients experiencing a hyphema post-operatively. However, these tend to resolve gradually within one week of the operation. Rarely, a cyclodialysis cleft may be formed accidentally. Traditional ab externo trabeculectomies have more complications including: hypotony (both early and persistent), wound leak, and shallow AC. Rarely, bullous keratopathy or corneal abrasion may be seen as well. A spike in IOP may be seen in either type of procedure. Overall, however, the ab interno approach has generally shown to safe with a complication rate of 4.3%, compared to 35.3% of traditional ab externo trabeculectomies.
The overall success of Trabectome varies between studies and methods used. A meta-analysis showed the success rate--defined as IOP < 21, at least 20% decrease in IOP, and no further surgical intervention required for glaucoma up to two years following the initial surgery--of a Trabectome alone to be approximately 46%, and 85% when combined with phacoemulsification. The overall success rate was found to be approximately 66% at a two-year mark. In the Trabectome alone and Trabectome plus phacoemulsification groups, the reduction in IOP at two years was measured to be approximately 10.5 mmHg (39%) and 6.24 mmHg (27%), respectively. The number of medications needed to manage IOP was decreased by 0.99 and 0.76 medications, respectively.
The ab interno approach has been directly compared to the more traditional ab externo trabeculectomy. In a retrospective cohort study, using the trabectome method for AIT, IOP was reduced by 43.5% in patients receiving AIT and 61.3% in ab externo trabeculectomy patients at two years. Overall, the success rate--defined as IOP < 21 and at least 20% decrease in IOP--of AIT was found to be 22.4% compared to 76.1% success rate yielded by ab externo trabeculectomies. Given the lower success rate, approximately 43.5% of patients required further surgical intervention to control IOP compared to 10.8% of patients receiving traditional trabeculectomies.
Ab externo trabeculectomies are more commonly performed than ab interno trabeculectomies, however ab externo trabeculectomies can often fail to achieve IOP goals. Following the failure of an ab externo trabeculectomy surgery, an additional ab interno trabeculetomy can lower IOP and medications. In a prospective study, Trabectome was performed within one year of a failed ab externo trabeculectomy showed an IOP reduction by 28% and medication count decrease by approximately 0.8 medications. Trabectome combined with phacoemulsification in the same situation yields reductions of IOP by 19% and medications by roughly 0.9 medications. The overall success rates are 81% and 87%, respectively, for an Trabectome and Trabectome plus phacoemulsification one year following a failed trabeculectomy. 
When compared to other methods, KDB goniotomy results in more complete removal of the TM. There is also less damage to the surrounding tissues, which is a drawback of the Trabectome. The results are also favorable with significant reduction in IOP recorded in cadaveric models. In a study by Seibold et al, IOP decreased from approximately 18.3 mmHg to 11.0 mmHg on average.
In regards to GATT, results have shown an IOP decrease of approximately 7.7 mmHg (30.0%) and 11.1 mmHg (39.8%) at 6 and 12 months, respectively. The number of medications required to control IOP reduced by 0.9 and 1.1 on average at 6 and 12 months, respectively. In 9% of patients, further glaucoma surgery was required. Transient hyphemas have been noticed in roughly 30% of patients at 1-week and is the only significant complication. GATT surgeries have shown to be similar in effectiveness as ab externo trabeculectomies with overall success in 68-90% of patients.
In a retrospective study, the efficacy and safety profile of TRAB™360 (Omni predicate device) procedures performed on patients with refractory primary open-angle glaucoma and a pre-operative IOP of at least 18 mmHg was explored. Over the first 12 months following a TRAB™360, approximately 59% of patients showed at least a 20% reduction in intraocular pressure and IOP less than 18 mmHg, with the average number of medications dropping from 1.7 +/- 1.3 to 1.1 +/- 1.0 medications. Results were also promising in patients with even higher baseline IOP (at least 25 mmHg) with 67% experiencing success. There was a 25% failure rate (patient needed further intervention within the first 12 months) and mild adverse effects with patients mostly experience transient hyphema.
Ab interno trabeculectomy and trabeculotomy procedures are a subtype of surgeries that fall under the umbrella of micro-invasive glaucoma surgery (MIGS). The ab interno approach has shown to be effective in a majority of patients when data is analyzed from several different studies looking at individual types of AIT surgeries. When compared to traditional ab externo trabeculectomy, results have generally shown that AIT procedures yield an inferior efficacy. However, these surgeries are still significantly effective and do not carry the same risk profile as traditional glaucoma surgeries, with complications outside of transient hyphema being extremely rare. Most studies are limited to 12-24 months following surgery, and it would be interesting to see patient outcomes over a longer duration.
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