Gonioscopy-Assisted Transluminal Trabeculotomy (GATT): Technique and Early Results

Gonioscopy-Assisted Transluminal Trabeculotomy (GATT): Technique and Early Results

Authors: Davinder S. Grover, Mahmoud A. Godfrey, Ronald L. Smith, James C. Fellman
Affiliation: Glaucoma Associates of Texas, Dallas, Texas, USA
Journal: Ophthalmology, Volume 121, Issue 5, 2014, Pages 855–861
DOI: 10.1016/j.ophtha.2013.11.001

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Abstract

Purpose:
To describe a new, ab interno, circumferential trabeculotomy technique — gonioscopy-assisted transluminal trabeculotomy (GATT) — that spares the conjunctiva and sclera, and to report early results in patients with open-angle glaucoma.

Design:
Retrospective, consecutive, interventional case series.

Participants:
Eighty-five eyes of 85 patients with open-angle glaucoma, including primary open-angle glaucoma, pseudoexfoliation glaucoma, and pigmentary glaucoma.

Methods:
GATT was performed using either an illuminated microcatheter (iTrack 250A; iScience Interventional, Menlo Park, CA) or a 5-0 polypropylene suture. After making a small goniotomy under gonioscopic view, the microcatheter or suture was passed circumferentially through Schlemm’s canal and then withdrawn, completing a 360° trabeculotomy. Preoperative and postoperative intraocular pressure (IOP), glaucoma medications, and complications were analyzed.

Main Outcome Measures:
IOP and number of glaucoma medications at 6 months, and perioperative complications.

Results:
The mean preoperative IOP was 27.3 ± 7.2 mmHg on 3.2 ± 1.1 medications.
At 6 months, mean IOP decreased to 15.7 ± 3.5 mmHg (P < 0.001), and mean number of medications decreased to 1.1 ± 1.0 (P < 0.001).
Hyphema occurred in 30% of eyes and resolved spontaneously in all cases. There were no cases of hypotony, infection, choroidal effusion, or vision-threatening complications.

Conclusions:
GATT is a minimally invasive, ab interno circumferential trabeculotomy that lowers IOP effectively while sparing the conjunctiva and sclera. It can be performed in eyes with previous surgery and shows promising early outcomes in open-angle glaucoma.

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Introduction

Glaucoma remains one of the leading causes of irreversible blindness worldwide. Elevated intraocular pressure (IOP) is the primary modifiable risk factor associated with disease progression. Traditional surgical approaches, such as trabeculectomy and tube shunt implantation, are effective at lowering IOP but involve significant risks, including hypotony, infection, and bleb-related complications. These procedures also require manipulation of the conjunctiva and sclera, limiting future surgical options.

In recent years, there has been growing interest in minimally invasive glaucoma surgery (MIGS), which aims to reduce IOP through ab interno approaches that minimize tissue disruption and improve safety profiles.
Several MIGS techniques target Schlemm’s canal and the trabecular meshwork, the principal sites of aqueous outflow resistance in open-angle glaucoma.

Gonioscopy-Assisted Transluminal Trabeculotomy (GATT) represents a modification of traditional trabeculotomy, performed entirely through an ab interno approach under gonioscopic visualization.
Unlike conventional trabeculotomy, which requires a conjunctival peritomy and scleral flap, GATT preserves external tissues and can achieve a 360° opening of Schlemm’s canal.

The purpose of this study was to describe the GATT technique and report early surgical and clinical outcomes in a series of patients with open-angle glaucoma treated using either an illuminated microcatheter or a 5-0 polypropylene suture.

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Methods

Study Design

This was a retrospective, consecutive, interventional case series of patients with open-angle glaucoma who underwent Gonioscopy-Assisted Transluminal Trabeculotomy (GATT) between January 2011 and June 2012 at the Glaucoma Associates of Texas (Dallas, USA).
All procedures were performed by the same surgeon (D.S.G.). The study adhered to the tenets of the Declaration of Helsinki and received institutional review board approval.

Patient Selection

Patients were included if they had:

• Open-angle glaucoma confirmed by gonioscopy

• Intraocular pressure (IOP) inadequately controlled on maximally tolerated medical therapy

• A clear cornea permitting gonioscopic visualization

Patients were excluded if they had:

• Angle closure glaucoma

• Neovascular or uveitic glaucoma

• Prior incisional glaucoma surgery involving the angle

Demographic data, glaucoma type, preoperative IOP, number of medications, best-corrected visual acuity (BCVA), and surgical complications were recorded.

Surgical Technique

The GATT procedure was performed as follows:

1. Anesthesia and Setup:
   Surgery was performed under peribulbar or topical anesthesia.
   A temporal clear corneal incision was created to allow access to the nasal angle. The microscope and patient’s head were tilted to optimize visualization.

2. Goniotomy:
   Under direct gonioscopic view, a small goniotomy (approximately 1–2 mm) was made in the nasal trabecular meshwork using a microsurgical blade (MVR blade or similar).

3. Cannulation of Schlemm’s Canal:
   A flexible, illuminated microcatheter (iTrack 250A, iScience Interventional) or a 5-0 polypropylene (Prolene) suture was inserted into Schlemm’s canal through the goniotomy.
   The device was advanced circumferentially 360° through the canal. In cases using the illuminated catheter, the glowing tip was visualized through the sclera to confirm canalization.

4. Completion of Trabeculotomy:
   Once the catheter tip re-emerged at the original goniotomy site, both ends of the microcatheter (or suture) were grasped with forceps and pulled through the anterior chamber, creating a full 360° trabeculotomy.

5. Closure:
   The viscoelastic material was removed, and the corneal incision was sealed with stromal hydration. No conjunctival incision was required.

A simultaneous phacoemulsification was performed when indicated for visually significant cataract (combined cases were analyzed separately).

Postoperative Care

Topical antibiotic drops were prescribed for one week, and topical steroids were tapered over four to six weeks. Glaucoma medications were discontinued postoperatively and reintroduced if IOP exceeded target range during follow-up.

Outcome Measures

Primary outcome measures were:

• Mean intraocular pressure (IOP) at each postoperative visit

• Mean number of glaucoma medications

Secondary outcomes included:

• Postoperative complications (e.g., hyphema, hypotony, IOP spikes)

• Visual acuity changes

Follow-up visits were scheduled at day 1, week 1, month 1, month 3, and month 6 postoperatively.

Statistical Analysis

Continuous variables were analyzed using paired t-tests. A P value of <0.05 was considered statistically significant.
Data are presented as mean ± standard deviation (SD).

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Results

Patient Characteristics

A total of 85 eyes from 85 patients were included in the study.
The mean age was 64 ± 14 years (range, 22–87 years).
Diagnoses included:

• Primary open-angle glaucoma (POAG): 61 eyes (72%)

• Pseudoexfoliation glaucoma: 16 eyes (19%)

• Pigmentary glaucoma: 8 eyes (9%)

Thirty-three eyes (39%) had previously undergone cataract extraction.
Thirteen eyes (15%) underwent combined GATT with phacoemulsification, while the remaining 72 eyes (85%) had GATT alone.

The mean preoperative intraocular pressure (IOP) was 27.3 ± 7.2 mmHg (range, 18–50 mmHg) on an average of 3.2 ± 1.1 glaucoma medications (range, 1–5).

Intraocular Pressure Outcomes

Postoperative IOP decreased significantly at all follow-up intervals.

At 6 months, 82% of eyes achieved an IOP ≤18 mmHg, and 62% achieved IOP ≤15 mmHg.
There was no significant difference in mean IOP reduction between the illuminated microcatheter group and the polypropylene suture group.

Glaucoma Medication Use

The mean number of medications decreased from 3.2 ± 1.1 preoperatively to 1.1 ± 1.0 at 6 months (P < 0.001).

• 45% of eyes were medication-free at last follow-up.

• 78% required two or fewer medications.

Visual Acuity

Best-corrected visual acuity (BCVA) remained stable in 91% of eyes.
In combined phacoemulsification–GATT cases, mean BCVA improved due to cataract removal.

Complications

Transient hyphema (≥1 mm layered blood in the anterior chamber) was the most common postoperative finding, occurring in 30% of eyes.
All cases resolved spontaneously within 1–2 weeks without surgical intervention.

Other complications included:

• Transient IOP spike (>30 mmHg): 9% (all managed medically)

• Mild corneal edema: 6% (resolved within 1 week)

• No cases of hypotony, infection, choroidal effusion, or vision-threatening complications.

Reoperations

Two eyes (2.3%) required additional glaucoma surgery (trabeculectomy) within 6 months due to inadequate IOP control.

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Discussion

This study describes a novel, ab interno, circumferential trabeculotomy technique — gonioscopy-assisted transluminal trabeculotomy (GATT) — and demonstrates that it effectively reduces intraocular pressure (IOP) in patients with open-angle glaucoma while sparing the conjunctiva and sclera.

The mean IOP reduction of approximately 42% at 6 months, combined with a significant decrease in the number of glaucoma medications, compares favorably with traditional trabeculotomy and other minimally invasive glaucoma surgeries (MIGS). Importantly, the procedure can be performed through a small clear corneal incision without disturbing the conjunctiva, preserving future surgical options.

Comparison with Other Techniques

Traditional trabeculotomy, performed via an external approach, involves creating a conjunctival peritomy and scleral flap to access Schlemm’s canal, often achieving only a limited (90°–120°) trabeculotomy.
In contrast, GATT allows 360° treatment using an internal approach, which may facilitate a more uniform reduction in outflow resistance across the entire canal circumference.

The observed IOP reduction to the mid-teens in most patients parallels results achieved by more invasive filtering procedures but with fewer serious complications.
The hyphema rate (30%) was similar to that reported in other trabeculotomy studies and resolved spontaneously in all cases.
No hypotony or infection occurred, emphasizing the procedure’s safety profile.

Mechanism of Action

GATT directly addresses resistance in the trabecular meshwork and the inner wall of Schlemm’s canal. By opening the canal circumferentially, aqueous humor can access multiple collector channels, potentially improving physiologic outflow.
This approach aligns with other Schlemm’s canal–based procedures such as Trabectome, canaloplasty, and viscocanalostomy, but differs in that GATT fully disrupts the trabecular meshwork over 360°, producing a complete trabeculotomy rather than enhancing canal patency.

Advantages of GATT

• Ab interno approach: preserves the conjunctiva and sclera.

• Applicability: can be performed in eyes with previous incisional surgery.

• 360° treatment: allows for complete circumferential access to Schlemm’s canal.

• Safety: minimal risk of hypotony, infection, or bleb-related complications.

Limitations

This study’s limitations include its retrospective design, limited follow-up duration (6 months), and lack of a control group. Longer-term studies with larger populations are needed to confirm the durability of IOP reduction and to assess long-term safety.
Additionally, GATT is suitable primarily for eyes with open angles and may not be appropriate for secondary glaucomas associated with angle closure or extensive peripheral anterior synechiae.

Future Directions

The promising results of GATT suggest it may play a significant role within the evolving spectrum of MIGS procedures. The ability to lower IOP without bleb formation and with minimal tissue trauma supports its use as an intermediate option between medical therapy and traditional filtration surgery.

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Conclusion

Gonioscopy-Assisted Transluminal Trabeculotomy (GATT) is a minimally invasive, ab interno 360° trabeculotomy that effectively lowers intraocular pressure and reduces medication dependence in patients with open-angle glaucoma.
It can be performed with either an illuminated microcatheter or a 5-0 polypropylene suture and carries a low risk of serious complications.

By sparing the conjunctiva and sclera, GATT preserves future surgical options and represents a valuable addition to the glaucoma surgeon’s armamentarium.

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References

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2. Harms H, Dannheim R. Epicritical consideration of 300 trabeculotomies. Albrecht Von Graefes Arch Klin Exp Ophthalmol. 1970;179:105–116.

3. Beck AD, Lynch MG. 360° trabeculotomy for primary congenital glaucoma. Arch Ophthalmol. 1995;113:1200–1202.

4. Mendicino ME et al. Long-term surgical outcomes of 360° trabeculotomy versus goniotomy in congenital glaucoma. J AAPOS. 2000;4:205–210.

5. Lewis RA et al. Canaloplasty: circumferential viscodilation and tensioning of Schlemm’s canal using a flexible microcatheter. J Cataract Refract Surg. 2007;33:1217–1226.

6. Grover DS et al. Gonioscopy-assisted transluminal trabeculotomy (GATT): technique and early results. Ophthalmology. 2014;121:855–861.

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