Gonioscopy-Assisted Transluminal Trabeculotomy (GATT) - a glaucoma surgery

Article initiated by:

Ana IM Miguel

All authors and contributors:

Michael Balas, Tal Sharon, Mohammed Sfaxi, Patrick Gooi, Matthew Schlenker, Ana IM Miguel

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Summary

Glaucoma is a major cause of irreversible blindness in the world. Many patients with glaucoma will need surgery during their lifetime, and classical surgery has risks and a significant failure rate. Micro-invasive glaucoma surgeries (MIGS) aim to decrease the complication rate while decreasing intraocular pressure (IOP). Gonioscopy-assisted transluminal trabeculotomy (GATT) is a conjunctiva-sparing approach to decrease IOP that strives to enhance the natural physiological outflow from the eye. We aim to identify GATT’s surgical technique, indications, contraindications, and possible role among other glaucoma surgeries.

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Disease Entity: glaucoma

Figure 1. Different mechanisms of MIGS devices, along with some examples of techniques. **withdrawn from the market. GATT: gonioscopy-assisted transluminal trabeculotomy. Abic: ab interno canaloplasty. ECP: endocyclophotocoagulation. ECPL: endocyclophotocoaguloplasty. KDB: Kahook Dual Blade. *MIMS: « Minimal invasive & Stent-less Treatment Glaucoma Innovation » (mimics a less invasive technique of deep sclerectomy). iTrack and OMNI can be considered in the “excise/cleave” group (when there is a trabeculotomy). Adapted from Fellman et al.

Glaucoma is a group of optic neuropathies with typical optic nerve head cupping and apoptosis of retinal ganglion cells and their axons with respective vision loss. It is the second leading cause of blindness. Among other classifications, glaucoma can be divided into open-angle and closed-angle. A study identified unilateral blindness in 15.5% of patients with primary open-angle glaucoma (POAG) after 7.5±5.5 years. Therefore, timely diagnosis and treatment are essential to prevent irreversible blindness. The main glaucoma treatment is decreasing IOP; through medication, laser, or surgery. Up to twenty to 50% of glaucomatous patients will need surgery in their lifetime. Unfortunately, classical filtering surgery risks potentially blinding complications, including endophthalmitis and hypotony.

Consequently, new surgical approaches have arisen, aiming to be less invasive while effectively decreasing IOP. GATT has unique features that we believe are useful for fighting glaucoma. Figure 1 summarizes different types of less invasive approaches (adapted from Fellman et al.).

Historical considerations

The idea of using a Nylon suture to perform a trabeculotomy was first described by Smith et al. in 1962. In 2012, Chin et al. published a study in Japan using the Nylon suture to perform a 360° trabeculotomy. Later in 2014, Fellman and Grover described  GATT using a Prolene suture, using large cohorts afterward to support its value. Since then, GATT surgery has been increasingly performed. Techniques such as AbiC, OMNI, and iTrack were later developed.

Anatomical review

We present a brief anatomical review of the iridocorneal angle to understand better and perform the GATT surgery. Figure 2 illustrates the iridocorneal angle structures. The iridocorneal angle is limited by the cornea, anteriorly, and the iris, posteriorly.

Figure 2. Iridocorneal angle scheme and respective gonioscopy findings.

In an open-angle, the visible structures are, from anterior to posterior:

1. Schwalbe’s line, a pigmented 50-150 µm-wide line that shows the transition between the Descemet membrane and the trabeculum.

2. Non-pigmented trabeculum, adjacent to the Schwalbe’s line.

3. Pigmented trabeculum, the functional part of the trabeculum. It is more pigmented inferiorly (possibly due to gravity) and in dark irises. The trabecular meshwork is responsible for 70 to 90% of the aqueous humor outflow. Its inner face contacts the Schlemm’s canal, and its external face contacts the aqueous humor. A trabecula is “a meshwork” or “a piece of a spongy substance”. The trabecular meshwork contains three portions:

3.1 The Uveal meshwork, adjacent to the aqueous humor, is formed by trabeculae with large holes, from 25 to 75 µm.

3.2 Corneoscleral meshwork, from the scleral spur to the scleral sulcus, with smaller oval holes as they approach the Schlemm’s canal, offering slightly more resistance to the aqueous humor.

Figure 3. Illustration of a GATT procedure and its relationship with the iridocorneal anatomy. In this figure, Schlemm’s canal is filled with blood (red), aiding in its identification. A goniotomy was performed (whiter image) to allow the entry of the Prolene suture into the Schlemm’s canal, and there is some blood escaping from the ends of the goniotomy into the anterior chamber.

3.3 Justacanalicular tissue. This endothelial tissue has the highest resistance to the aqueous humor outflow, due to its minute pores and giant vacuoles (from 0.5 to 2 µm).

4. Schlemm’s canal. Visible when the pressure of the episcleral veins is higher than the intraocular pressure (IOP), namely when there is eye compression to cause reflux of blood from the episcleral veins to the Schlemm’s canal. This 360° endothelial channel receives the aqueous humor, that travels to the collector channels (or episcleral veins) and aqueous veins. During the GATT procedure, the suture will first pass inside this canal, and then it will rip it open in a portion or all of the canal (180° Vs 360°, as explained below), causing direct communication between the Schlemm’s canal and the anterior chamber.

5. Scleral spur; a thick white band, easy to identify. The scleral spur is the most anterior portion of the sclera and the insertion site of the longitudinal ciliary muscle.

6. Ciliary band, a thick band, rose to dark brown, that shows the ciliary body.

7. Sometimes iris vessels are seen in the angle. They can be physiological if they follow the angle radially and do not cross the trabecular meshwork.

8. Iris processes may be seen; they differ from the wider peripheral anterior synechia (PAS, which would need a gonios-synechio-lysis before performing a GATT). Iris processes do not cross the trabecular meshwork, unlike PAS. Figure 3 illustrates the GATT procedure at the iridocorneal angle.

Indications and pre-operative considerations

GATT is indicated for any patients with trabecular meshwork dysfunction. It is particularly well-suited for trabecular meshwork dysgenesis such as in juvenile open angle glaucoma, or secondary trabecular meshwork  dysfunction, such as seen in uveitic glaucoma, pseudoexfoliative glaucoma, and pigmentary glaucoma, among others. It can be combined with cataract surgery. It can be helpful irrespective of the glaucoma staging, as long as there is a reasonably controlled disease, when there is clearly trabecular meshwork dysgenesis or dysfunction, or when the traditional approaches are deemed too risky.

Compared to more traditional techniques such as trabeculectomy, GATT offers several advantages, including a lower complication rate, faster recovery, and reduced costs and operating time. Furthermore, GATT has the advantage of sparing the conjunctiva, preserving the option for more invasive glaucoma surgeries if later needed. GATT can also be performed as a standalone procedure or in combination with cataract surgery, providing flexibility in treatment planning.

The main indications for GATT surgery are :

-         Uveitic glaucoma

-         Pseudoexfoliative glaucoma

-         Pigmentary glaucoma

-         Congenital, pediatric and juvenile glaucoma

-         Steroid-induced glaucoma

-         Primary open-angle glaucoma

-         Glaucoma secondary to vitreo-retinal surgery (namely with silicone oil)

-         Can be combined to cataract surgery

-         Closed-angle glaucoma, if associated with cataract surgery

Pre-operative considerations for GATT encompass several key aspects, including a detailed medical and ocular history, use of medication, slit-lamp examination, gonioscopy, measurement of IOP, and evaluation of the optic nerve head and retinal nerve fiber layer. Patients should continue their glaucoma medications until the day of surgery and may be prescribed topical antibiotics and anti-inflammatory medications preoperatively. Anticoagulants or blood thinners should be stopped one week before surgery, though 2 days may be sufficient for apixaban and rivaroxaban. It is crucial to discuss the GATT procedure's potential benefits, risks, and alternatives with the patient and obtain informed consent. The patient should be prepared for important postoperative rest (an elevated head position while sleeping , no bending past the waste, and no lifting more than 10 pounds to decrease the bleeding risk). Finally, thorough surgical planning should be undertaken, including selecting the appropriate instrumentation, preparing the operating room, placing the patient in the reverse Trendelenburg position, and ensuring that the surgical team is familiar with the steps of the GATT procedure while being prepared to manage any intraoperative complications.

Contraindications

Identifying contraindications for GATT is essential for optimizing outcomes and minimizing complications. GATT is not recommended for patients with narrow or closed angles, except if associated with cataract surgery. Further contraindications include the presence of extensive peripheral anterior synechiae (PAS) and neovascularization, as these factors may result in excessive bleeding, inflammation, and surgical failure. Patients with significant corneal opacities or compromised endothelium are also unsuitable candidates, as opacification impedes proper intraoperative visualization of angle anatomy (which has been solved thanks to the endoscopic GATT technique, as described below). This procedure may not be ideal for patients with unstable intraocular lenses (IOLs). GATT should be avoided when there is fixation loss on the visual field since there may be IOP spikes and bleeding during the early postoperative period, but classical surgery is has also a preoccupying risk of fixation loss in patients close to losing fixation. Finally, GATT should be avoided in patients on anticoagulation medications that cannot be reversed preoperatively. In cases where GATT is contraindicated, alternative surgical interventions should be considered to ensure effective glaucoma management.

In summary, contraindications for GATT procedure include the following:

-         Medical inability to hold blood thinners

-         Inability to respect the postoperative rest

-         IOP > 40mmHg despite maximal medical treatment (except in secondary trabecular meshwork changes, in which GATT can be effective: relative contraindication)

-         Advanced glaucoma with central fixation loss (relative contraindication)

-         Closed-angle (except if combined with cataract surgery)

-         Corneal opacities (except if one uses the EATT technique; see below)

-         Extensive PAS (except amenable to goniosynechiolysis; we should plan for backup glaucoma surgery in case the goniosynechiolysis is not possible) or angle neovascularization.

Surgical technique

Before starting, one should ensure that the equipment, material, staff, security checklist, and positioning of the patient are done (a reverse Trendelenberg position is recommended to decrease blood reflux and improve the view). A temporal approach for the surgeon is preferred by the authors, but a superior position can also be performed. Then, we will perform a 180° GATT procedure. The video below shows the steps needed for the GATT surgery:

Figure 4. Entry of the 27-G needle into the angle

We start with:

1.      Paracentesis, if combined phacoemulsification is planned (all incisions are corneal and not limbal, to avoid bleeding from the limbal vessels that would hinder visualization), then intracamerular lidocaïne, followed by viscoelastic in the anterior chamber (AC) and in the angle.

Figure 5. Insertion of the Prolene suture into the AC (notice the mushroom head of the suture)

2.      With a 2.2mm knife, we perform the main wound. If a combined cataract procedure is planned, we do it at now until the IOL insertion (some surgeons prefer performing GATT before phacoemulsification). Then, a 27-G needle is inserted, directed to the nasal angle (figure 4).

3.      Cut the needle from the 5-0 Prolene and use 6 cm of it, and then use a low-temperature cautery applied 90 degrees from the Prolene 5-0 to induce a mushroom head of the suture. Then, insert it into the AC using the needle entry (figure 5).

4.      Use the angular position: tilt the head of the patient away from us at least 30 degrees, tilt the microscope towards us ≈40 degrees, and ask the patient to look away as a last resort if unable to achieve proper positioning from the head and microscope positioning alone. Then use a gonio prism (such as a Swan-Jacob lens) to see if the angle visualization is good.

Figure 6. Goniotomy.

5.      An MVR blade or 27 gauge needle enters through the main wound and into the trabecular meshwork (TM) to do a goniotomy (the blade is kept parallel to the TM or slightly anterior, starting to the right of the surgeon’s visual field and performing a goniotomy with one straight cut to the left of the surgical view and for of 1-clock-hour), as seen in figure 6. A left handed surgeon will start a goniotomy to the left of the view; the goniotomy should be on the side of the view that allows for maximum real estate to watch the suture go through Schlemm's canal, or to reserve space for a 2^nd goniotomy if needed.

6.      We can confirm the opening and extension of the goniotomy with gentle pressure posteriorly with the MVR blade.

Figure 7. Injection of OVD into the goniotomy.

7.      We can inject viscoelastic (OVD) into the goniotomy to dilate the Schlemm’s canal, which can decrease intraoperative bleeding, and improve the view. We should inject it to displace any bleeding away from the direction we will be inserting the prolene suture (to the right for a right handed surgeon) (figure 7).

8.      Insertion of Prolene into Shlemm’s canal. Using micro-surgical iris forceps to hold the suture, we insert it in the goniotomy and into Schlemm's canal (figure 8). If the suture is in a good position (we can appreciate the suture through the transparency of the Schlemm’s canal), we do 10 passes (or 20 small passes) to perform a trabecutolomy of 180 degrees (usually, we will feel a resistance). If any pain, we stop to ensure the suture has not gone suprachoroidal, or that it has come out of Schlemm’s canal and on iris.

Figure 8. Insertion of the 5-0 Prolene into the Schlemm's canal.

9.      We pull the suture centripetally to perform a trabeculotomy under a direct angular view. Then, after realigning the head of the patient and microscope, using the iris forceps again, we pull the suture entirely into the center of AC and outside the eye (figure 9). Finally, we remove the OVD (we can leave up to 10-40% of OVD in the anterior chamber to avoid postoperative bleeding, but care must be taken to prevent postoperative IOP pikes). The wounds should be thoroughly hydrated to ensure they are water-tight, and the eye should be left with an IOP>30mmHg to decrease the risk of bleeding.

Figure 9. Removing the suture to open the Schlemm’s canal and thus perform a trabeculotomy.

The videos below show examples of GATT:

Surgical variations

a) 180° versus 360° GATT

Accumulating evidence suggests that 360° treatment of the Trabecular Meshwork (TM) or Schlemm’s canal (SC) may not be necessary to maximize the effect and achieve surgical success. The most common hypotheses for this are a potential dose-response threshold effect beyond 120-180° of TM and non-uniform drainage along the 360° circumference of the TM, which results in increased efficacy with treatment to specific segments of TM and less contribution to the effect with treating other areas (in healthy eyes a segmental active outflow occurs mainly in nasal and inferior quadrants). While success rates are similar with 360° and 180° trabeculotomy, the rates and severity of complications, particularly hyphema, may be higher with 360° cleavage of SC’s roof. Therefore, according to the above-stated, the authors tend to perform hemi-GATT (unroofing of 180° of SC) procedures as a routine. Performing a 180°-GATT may also leave the remaining 180° for future intervention if needed.

b) Inferior versus superior GATT

Not much evidence exists comparing superior to inferior hemi-GATT procedures. Waldner et. Al. showed recurring trends of the lower hazard ratio for failure and better IOP reduction with performing inferior hemi-GATT rather than superior, though no statistical significant difference was found. This is consistent with previous in-vitro and in-vivo anatomical studies, showing segmental outflow. Further studies are required to assess this subject better. Likely there is variability in the distribution of collector channels between individuals. Ideally, one could determine whether an individual has superior or inferior dominance in their collector channel distribution, and subsequently, the hemi-GATT would be performed in the corresponding region (this point is still under discussion by glaucoma experts). We have described this variability by performing intraoperative venography, as seen in the videos below.

Figure 10. iTrack device.

c) Using devices for GATT

GATT can be performed using devices such as iTrack and OMNI. OMNI allows the injection of viscoelastic as the catheter is retracted. iTrack (figure 10) also allows this, and it has a light that will enable visualization of the end of the cannula, allowing, for instance, detection and correction of a suprachoroidal or collector channel path. However, there is a cost for these devices.

Figure 11. EATT procedure. This endoscopic camera allows a visualization from the inside out; in the figure, there is a depiction of the final stage of the GATT: a trabeculotomy, with the suture being removed from the eye by the main incision (center of the image) and causing a trabeculotomy (right of the image).

d) Endoscopic video assisted transluminal trabeculotomy (EATT)

One can consider an endoscopic approach to GATT when the cornea precludes a gonioscopic view. Preoperatively a UBM and anterior segment OCT should be performed to assess the angle anatomy, and when there is a question of significant PAS that could not be released easily, one should discuss with the patient and be ready to consider a subconjunctival filtering surgery option if the EATT is not possible.

For this technique, a separate incision is performed inferiorly for a right eye and right-handed surgeon, and superiorly for a left eye and right-handed surgeon (opposite for a left-handed surgeon). The microscope does not need to be tilted for this procedure, nor does the patient’s head need to be tilted. An endoscopic probe is inserted into the eye, and all the steps can be performed utilizing this endoscopic view.

e) Using intra-operative OCT

With the possibility of performing intra-operative OCT, we can have another means of verifying if the Prolene 5-0 is in the proper position and see the trabeculotomy site (figure 12).

Figure 12. Intra-operative OCT during GATT surgery. On the left, we see the final part of a GATT surgery; on the right, the respective OCT (we can appreciate the trabeculotomy and the suture already outside the Schlemm’s canal).

Figure 13. Good angular view. On the left side of the image, we identify a top-down view and out of focus (non-ideal), as opposed to the right side with en face view (ideal).

Pearls for success

The biggest pearls for success with this surgery are maintaining a good view and avoiding the suprachoroidal trajectory.

Figure 14. Avoiding the suprachoroidal path. On the left side of the image, a supra-choroidal path is identified; this was facilitated by a posterior goniotomy (on the image bottom right, the green strokes show the goniotomy performed, while the blue strokes show the ideal goniotomy position). We withdrew the suture, injected abundant OVD, conducted a more anterior goniotomy, and inserted the suture with a more anterior bias. The suture went successfully into the Schlemm’s canal.

To ensure a good view, first, we must stop blood thinners and position the patient in reverse Trendlenburg, as placing the head above the heart diminishes episcleral venous pressure and blood reflux. We should be thorough in obtaining an excellent angular view (increasing the inclination of the head of the patient 30-45° away from the surgeon, the oculars of the microscope tilted closer to the surgeon at least 30°), and if needed asking the patient to look away from the surgeon; the gonioscopic lens should be held without excessive pressure to avoid corneal folds. Figure 13 distinguishes a top-down view from a good angular perspective. Don’t hesitate to use abundant OVD to expel bleeding and improve the surgical sight (ideally, from left to right of the field to direct the bleeding away from our surgical viewpoint for a right handed surgeon).

Figure 14 depicts pearls to avoid and manage a suprachoroidal path.

Complications

Intra-operative complications

Intra-operative complications include:

-         Intra-operative bleeding is common, in particular in patients who are on antiplatelet agents, anti-coagulated, have clotting disturbances, or have an inflammatory condition. The mechanism for bleeding is blood reflux from episcleral vessels through the collecting channels into the unroofed Schlemm’s canal (SC) according to the pressure gradient. The bleeding is usually more prominent when the intra-ocular pressure is lower than the venous pressure – while aspirating or evacuating the ophthalmic viscosurgical device (OVD), etc. Adding OVD may be beneficial. When a significant hyphema is present at the end of the procedure, IOP elevation might be needed. Use high-pressure infusion or surgical wound hydration with irrigation to increase the AC pressure. For good tamponade and prevention of continuous bleeding after the surgery, it is acceptable to leave some OVD in the anterior chamber (AC), instruct the patient to avoid heavy lifting and maintain a high head position. Consider a segmental GATT of 90-180 degrees to reduce the bleeding risk.

-         Misplacement of the suture – the suture may be diverted - posteriorly to the supra-choroidal space, which may cause pain to the patient and lead to supra-choroidal cleft, or anteriorly to the AC. In most cases, if the initiation of the nasal SC cannulation was successful, and advancement of the suture within the canal was observed, one may suspect resistance to suture advancement down the line (inner narrowing of the canal, external pressure on the canal, previously implanted device, or scarring). When the suture is misdirected, and a false passage is created, it may damage the adjacent tissues, creating cyclodialysis, iridiodialysis, etc.. The GATT procedure in this case will have a partial effect. If there is a need for further cannulating or unroofing of the SC, recannulation of the SC could be performed by entering the suture in the opposite direction.

-         High resistance to suture advancement or a stop in suture advancement –a gentle redirection of the advancement trajectory can be tried, and advancing the trabeculotomy while centrally pulling the suture can be applied (“walk the dog” technique). If these techniques are not helpful, a ripcord of the suture (unroofing) should be initiated. In extreme circumstances, one should consider advancing the suture in the opposite direction.

-         Flattening of the anterior chamber – can occur when OVD is evacuated from the AC during the procedure (with manipulation of surgical wounds, compression of the cornea with the gonioprism, or, rarely with a displacement of it to the vitreous cavity. If the AC remains flat despite refilling of cohesive OVD and removing any external pressure on the eye, a posterior pressure should be suspected and managed according to the cause (choroidal effusion, suprachoroidal hemorrhage, retrobulbar pressure, speculum pressing the globe, etc.).

-         Descemet membrane injuries and detachment - may be caused by surgical instruments, usually near the goniotomy site in the nasal cornea (caused by the intraocular forceps advancing the suture), in particular in narrow angles and when the eye is not adequately tilted or near the surgical entry site. This may lead to corneal edema or scarring. A rare incidence of Descemet detachment was reported with suture advancement, or rarely during viscodilation.

-         Lens-related complications – in phakic eyes, surgical instruments could accidentally touch, compress, or push the lens, and the anterior capsule may also be injured. In pseudophakic eyes, the AC is usually deeper, yet an IOL displacement may be caused, in the setting of unstable IOLs .

Postoperative complications

The significant concerns after a GATT procedure are:

Figure 15. Differential diagnosis of IOP spike causes post-GATT (clinical probability x days of follow-up). During the 1^st day post-GATT, residual OVD is undoubtedly associated with the IOP spike, then hemorrhage and steroid response are the leading causes; later in the follow-up, we may consider insufficient IOP lowering from the surgery.

-         Bleeding (e.g. – microhyphema, hyphema, and vitreous hemorrhage) is caused by blood reflux from the episcleral veins into the anterior chamber (AC). This could be explained by the pressure gradient caused by a relatively hypotonic AC (leaky surgical wounds, OVD evacuation or aspiration, and compression by the gonioprism or other surgical tools). Hyphema is a transient, self-resolving sequel rather than a complication since the correlation with long-term results is debatable – while in some studies hyphema was found to be a risk factor for developing an IOP spike, in others, it was not . Re-bleedings may occur and are not uncommon, usually within the first postoperative month. In rare cases, a AC washout or vitrectomy may be needed.

-         IOP spike – a transient IOP elevation, generally defined as IOP > 30 mmHg or >10% elevation from baseline pre-operative IOP. A spike may be seen during the first postoperative month but is more common between postoperative day 4 and the second postoperative weeks. A spike is usually treated with an increasing dosage of hypotensive drugs in a stepwise manner, according to the severity of glaucoma and the spike. An anterior chamber tap may be considered a bridging therapy and may be repeated. Adding topical hypotensive agents, oral carbonic anhydrase, and systemic hyperosmotic drugs are generally the main path to control the spike. Rarely, a secondary glaucoma surgery is required (a cyclophotocoagulation, a tube-shunt, trabeculectomy, etc.).

-         Inflammation (mostly iritis or anterior uveitis) is usually well controlled with anti-inflammatory drugs – topical steroids, non-steroidal anti-inflammatory drugs, or both, with rapid tapering during the first month. Sometimes dosage adjustments or rate of tapering are needed, especially for patients with uveitic glaucoma, significant steroid response, or significant bleeding.

-         Goniosynechiae or Peripheral anterior synechiae (PAS) – may form, in particular in cases of shallow AC (in primary angle closure, in phakic eye, or due to hypotony or choroidal effusion), in eyes with significant inflammation or predisposed eyes. Pilocarpine 2% is sometimes added to the postoperative regimen to avoid it. Laser iridoplasty may also be done in such cases.  

-         Cystoid macular edema (CME) – may be present after surgery, and as in pseudophakic CME (Irvine-Gass syndrome), is usually controlled with topical anti-inflammatory agents but may require intravitreal injections of corticosteroids or anti-VEGF agents.

-         Wound leaks – are uncommon. Usually, watchful monitoring is acceptable (with or without a bandage contact lens and or aqueous depressant drops addition), though sometimes suturing the surgical wounds is necessary.

-         Infection or endophthalmitis is a severe and rare complication of any intraocular procedure.

-         GATT (unlike other MIGs) was not found to cause endothelial cell density depletion. An iatrogenic Descemet detachment may rarely occur (at the surgical wound or adjacent to the goniotomy), leading to scarring or corneal edema, temporary or permanent.

-         Shallow anterior chamber – unfrequent; etiologies are similar to other glaucoma surgeries: cyclodialysis, choroidal effusion or hemorrhage (extremely rare), wound leak, and anterior synechiae.

-         Hypotony is a rare complication of GATT and is usually secondary to an accidental creation of a cyclodialysis cleft during the procedure or possibly ciliary body shutdown in predisposed eyes. Hypotony is IOP < 6 mmHg or low IOP with shallow AC, maculopathy, or choroidopathy. Post-GATT hypotony is usually self-resolving within days but may need intervention such as topical steroids and atropine or injection of viscoelastic (OVDs).

-         Procedure failure – Lack of achieving surgical success (success definition varies among different studies, but in most cases defined as IOP < 21 mmHg and >20% reduction from baseline IOP or reduction in IOP-lowering drugs) at POM6, loss of light perception or need for a secondary IOP-lowering surgical procedure. Failure can occur in any glaucoma surgery, and its rate for GATT will be discussed in Outcomes.

Postoperative follow-up

There is high variability in the postoperative follow-up protocol. Most protocols report 1-2 follow-up visits in the first postoperative week (usually postoperative day (POD) 1 and POD 3-7, every 1-2 weeks until postoperative month (POM) 1, and every 1-3 months until POM 6. Complications or particular concerns during the surgery or the postoperative period may yield more frequent visits. Later follow-ups usually vary according to glaucoma severity, IOP control, need for treatment adjustment, and other considerations at the surgeon's discretion .

Outcomes

GATT surgery has a promising success rate while sparing conjunctiva for future incisional surgery. Its success varies from 46% at 18 months to 89% at 2 years and 75% at 3 years, and a meta-analysis estimated a combined surgical success of 85%. It is vital to note slight variations in the success definition across these studies; the authors recommend using the World Glaucoma Association Guidelines for glaucoma surgical trials for comparability.

Faria et al. . described a success rate of 87% at 12 months of GATT and 91% of combined cataract surgery (phaco-GATT) at 1 year in a cohort of POAG patients, with patients older than 60 years having a higher risk of failure (hazard ratio of 10.96; P=0.026). The mean IOP decreased from 24.9±8.5 to 12.1±2.1 mm Hg (P<0.001). Another study showed a smaller (63%) success rate at 12 months, smaller in African patients (42%, P<0.05), but with a 44% IOP decrease. Differently, a retrospective study showed inferior success in phaco-GATT compared with GATT at one year of follow-up, respectively 83.8% and 87.5%. Other studies showed similar qualified success of 86.21% in phaco-GATT and 83.48% in GATT at 24 months .

Although GATT is classically performed in mild to moderate glaucoma, studies showed its effectiveness also in advanced glaucoma (83.7% of success in moderate to advanced open-angle glaucoma at 19.4 months).

In primary congenital glaucoma, performing GATT surgery avoided filtering surgery with a cumulative probability of qualified success of 95.5% and complete success of 66.7% at 1 year of follow-up[48]. Patients with juvenile-open angle glaucoma (JOAG) also had good results with GATT, even with severe JOAG, considering high complete and qualified success rates of 74.3% and 91.4% at 12 months and 58.6% and 81.2% at 18 months.

Patients with pseudoexfoliative glaucoma (PXG) are good candidates for GATT, with a higher success than POAG patients according to some studies, but similar according to a recent study, in which the cumulative success probability was higher during the first year in PXG (97.6%) than POAG (86.8%, P=0.01); but without no significant difference at 2-year nor 3-year visits (P=0.24). The success rate is high in PXG, 89.2% at 2 years follow-up.

Uveitic glaucoma is one of the subgroups in which GATT is known for its relative success (81% of cumulative success at 12 months and a mean IOP decrease of 68% ).

GATT is also useful for secondary glaucoma after silicone oil and steroid-induced glaucoma.

In contrast, a combined phaco-GATT approach is preferred in primary closed-angle glaucoma (ACG). Still, a prospective study showed an acceptable success of 73% at 12 months and 78% two years after isolated GATT in PACG.

Trabeculectomy’s success has been described between 56-65% at five years (by the TVT study[56]) and 79% at five years (by the CIGTS study), and trabeculectomy has many risks and complications that may be frequent (up to 50%) and potentially blinding.

Studies comparing GATT with trabeculectomy show a trend toward higher success in trabeculectomy (59% Vs 46% complete success at 18 months in POAG, non-statistically significant, but statistically significant difference in % of IOP lowering), despite the higher risk of complications with the latter. Comparing GATT with XEN, a retrospective study found superior results with GATT at 24 months (complete surgical success of 34.2% in XEN and 50.5% in GATT groups, p = 0.039; and qualified success of 97.4% in XEN and  89.7% in GATT, p=0.025).

Comparing GATT with conventional trabeculotomy (CT), a retrospective study[60] showed a far superior success in phaco-GATT versus phaco-CT at 2 years (40.4% Vs 96.6%, p < 0.001) but a more negligible difference in isolated GATT versus CT (40.8% and 54.2%, p=0.55), which could indicate a synergic effect of phaco with GATT (as described with other MIGS).

Unsurprisingly, studies comparing phaco-GATT with phaco-iStent showed higher success in the first group ( success of 86.4% Vs. 35.1% at 1 year).

GATT was effective even in eyes with previously failed trabeculectomies, with a success rate of 60-70% at 24 months (and a cumulative proportion of reoperation of 0.29), and of 61.5% in a case series with 17.8±4.1 months (patients with PXG had more mean IOP lowering after GATT than patients with POAG, 45.6% Vs. 34.8%), among other studies with similar results.

Conclusion

In conclusion, GATT is a safe, effective, and valuable glaucoma surgery.

References

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