Ab interno trabeculectomy demonstrates positive outcomes, even in patients with narrow angles

Take-home message: Narrow angles have been associated with a higher IOP in glaucoma patients, presumably by causing higher outflow resistance. Here, Dr Loewen presents recent research in which both ab interno trabeculectomy (AIT) with the trabectome and phaco-AIT significantly reduced IOP and number of medications required, regardless of the degree of angle opening.

By Dr Hardik Parikh, Dr Igor Bussel, Dr Nils Loewen

Ab interno trabeculectomy with the trabectome (Neomedix, Tustin, CA, USA) is a minimally invasive glaucoma surgery (MIGS) that uses plasma energy to ionise and ablate the trabecular meshwork (TM) to increase conventional outflow.¹ Trabectome surgery and other similar MIGS procedures have been relatively contraindicated in glaucoma patients with narrow angles due to perceived technique challenges and presumed resulting complications. 

Alternative surgical modalities include trabeculectomies and tube shunts; while very effective, they have a high rate of significant complications.² This has resulted in a surge of interest in MIGS, which boast an almost complete absence of vision-threatening complications,³ to safely lower IOP and potentially improve vision via cataract surgery in the same session.

A narrow anterior chamber angle has been considered a relative contraindication in these surgeries due to the following: indirect access to the TM, and a belief that peripheral anterior synechiae, descemetisation of the angle, and fibrosis may form more frequently and increase complications.⁴ This may preclude large numbers of glaucoma patients from trabectome surgery and similar MIGS; women comprise 70% of angle closure glaucoma (ACG) cases, while Asians represent 87% of ACG.⁵

Although numerous studies have described the safety and efficacy of the trabectome in the treatment of glaucoma,^6,7 the relationship between anterior chamber angle grade and outcomes has only recently been formally examined.⁸ In this review, we discuss trabectome patient selection, key steps in surgery, and outcomes that include degree of angle opening.⁸

Patient selection

In our experience, in addition to primary open angle glaucomas, pseudoexfoliation, pigment dispersion and steroid induced glaucomas, a surprisingly a wide range of secondary glaucomas, including following trauma, scleral buckle, uveitis, and failed trabeculectomies⁹ can be successfully operated on.  True remaining contraindications include active neovascular glaucoma, uveitic glaucoma, or glaucoma due to increased episcleral pressure, because the pathology is not primarily in the TM.

Whether or not it is combined with phacoemulsification, trabectome has been shown to frequently lower the IOP to a final average around the mid-teens regardless of baseline IOP.^10–13 Thus, there does not appear to be an ideal baseline. Patients with a higher preoperative IOP have a larger reduction that can be estimated with a formula utilised by the Trabectome Surgery Calculator.¹⁴ The theoretical limit to postoperative IOP is the episcleral venous pressure. The severity of glaucoma does not negatively influence IOP outcome.⁹ On the contrary, glaucomas with a more diseased TM and a higher relative glaucoma disease index show a larger IOP drop.⁹ Some surgeons also use a reduction of medication dependence as a sole indication, and others have used Trabectome to treat ocular hypertension. 

Exclusion criteria

Stable gonioscopic view of the angle is paramount and the patient has to be able to rotate the neck or be able to be stably positioned on the shoulder.  The view has to be with any significant corneal opacity. Surgery should not be attempted in active neovascularisation but is feasible in well-controlled eyes that have been treated with PRP and injectable anti-VEGF agents. As noted above, uncontrolled uveitis and high episcleral venous pressure are other exclusion criteria.¹⁵ Individuals who engage in frequent activities that involve upside-down positioning or playing wind instruments (i.e. activities that increase episcleral venous pressure) should be cautioned about late recurrent hyphemas.

Figure 1: Cross section demonstrating proper engagement of the TM with the trabectome handpiece and TM threaded between the electrode tips and footplate (left). Gonioscopic visualisation of the trabecular meshwork (right). SC = Schlemm’s canal, SS = scleral spur, CBB = ciliary body band.

Review of key surgical steps

It is important to practice visualising the TM to prepare for first trabectome surgeries (Figure 1). It is possible to use fresh pig eyes or human donor eyes for this purpose.^16,17

• First, a paracentesis is created in the left hand position and preservative-free lidocaine is injected. Viscoelastic has to be avoided not to trap ablation bubbles, created optical interfaces or cause carbonisation at the electrodes. It is also not necessary because the continuous irrigation and aspiration provide pristine view of the angle.

• A 1.8 mm incision is then created with the keratome in the kit, that is 2 mm anterior to the limbus to avoid iris prolapse. Flaring the incision on the inside helps to prevent corneal striae from torque during ablation. The patient’s head and microscope are rotated 30 degrees away from the surgeon and the microscope is tilted into the opposite direction by the same amount.

• To properly identify the TM, the posterior lip of the incision is depressed to allow a small amount of fluid to egress to induce hypotony with reflux of blood into Schlemm’s canal. It is easier to engage the TM towards the left and by pointing up at a 45° angle. It is important to avoid pushing outward during ablation not to damage the collector channels.  Instead a slight inward pull can tent the TM and help confirm that the tip is in the proper location.

• Ablation is started at 0.8 mW and titrated up if necessary.¹⁸ Ablation of up to 90 degrees in both directions can be performed but a few clock hours may be all that is needed.

• A viscoadaptive substance can then be injected to fill about 20% of the anterior chamber in order to leave a crescent of viscoelastic tamponading the ablation arc to minimise postoperative hyphema from flowback. If phacoemulsification is to follow, the anterior chamber can be filled completely.

• In narrow angles with synechiae, synechiolysis should be performed before TM ablation using the smooth base plate of the handpiece under continuous irrigation and aspiration.¹⁹

Trabectome success rates

The trabectome has demonstrated considerable efficacy in lowering IOP. Whether or not it is concurrently performed with phacoemulsification, trabectome has been shown to frequently lower the IOP by an average of 36%, finishing at a final mean of approximately 16 mmHg regardless of baseline IOP. The average number of glaucoma medications used by these patients also decreased by one.^10–13

Until recently, it was hypothesised that MIGS would be relatively ineffective after previously failed trabeculectomy because of atrophy after the conventional outflow tract was bypassed. However, a recent prospective study following 73 patients, who had a failed trabeculectomy for 1 year, observed a 28% IOP reduction to 16.2±3.9 mmHg in their trabectome group and a 19% IOP reduction to 15.6±5.1 mmHg in their phacoemulsification with trabectome group. With success rates (defined as <21 mmHg or 20% IOP reduction) at 94% and 92%, respectively, and no reported complications, we concluded that Trabectome surgery could in fact be considered a viable therapeutic option for these patients.⁹

Analysing the impact of angle grade on outcomes

In order to analyse whether the angle grade affects outcomes, we conducted a nonrandomised study to gain insight into whether the degree of angle opening affected final IOP in trabectome surgery only and combined phacoemulsification with the trabectome (phaco-trabectome). After the exclusion criteria were applied, 671 cases were identified within the Trabectome Study Group database and were divided into two groups depending on which procedure they underwent, and further categorised by open (Shaffer grade ≥3) or narrow (Shaffer grade ≤2) angles.⁸

This prospective interventional cohort included all phakic patients with a diagnosis of glaucoma (with or without visually significant cataract), who had 12 months of follow-up. Subjects were excluded if they had missing preoperative IOP data or end-stage visual field damage. The indication for trabectome alone in phakic eyes consisted of IOP above target with progressive glaucoma on maximally tolerated medical or laser therapy. The indication for phaco-trabectome consisted of a visually significant cataract with at least 0.4 logMar (20/50 Snellen) visual brightness acuity testing and the need to lower IOP or the number of glaucoma medications. Visual field status was categorised as early, moderate, or advanced by individual glaucoma specialists based on the most recent Humphrey Visual Field exams (Carl Zeiss Meditec AG, Jena, Germany). All patients had a comprehensive slit lamp and ophthalmoscopy exam prior to surgery.

At 12 months post-operatively, all four groups had statistically significant decreases in IOP. In trabectome eyes with narrow angles, there was a 41% decrease in IOP to 15.7±3.0 mmHg on 0.2 fewer medications. In eyes with open angles, there was a 37% decrease in IOP to 16.4±3.9 on 0.8 (p<0.01) fewer medications (Figure 2A). The outcomes were similar in eyes that had undergone concurrent phacoemulsification (24% IOP reduction to 15.7±3.6 on 0.8 (p<0.01) fewer medications in narrow angles vs. 25% IOP reduction to 17.0±3.4 on 0.7 fewer medications (p<0.01) in open-angles) (Figure 2B).

Figure 3: Kaplan-Meier survival plots for trabectome (left) and phaco-trabectome (right) showing no statistically significant differences in the SG groups (p=0.70 and p=0.56, respectively). Success was defined as a final IOP less than 21 mmHg and greater than 20% reduction from baseline without further surgery.

Success was defined as a final IOP less than 21 mmHg and greater than 20% reduction from baseline without further surgery. At 1 year post-operatively, success rates were comparable between narrow angle and open angle eyes in both trabectome (SG≤2: 72%; SG≥3: 78%) and phaco-trabectome groups (SG≤2: 92%; SG≥3: 94%) (Figure 3). There was no statistically significant difference in success rates in the SG groups in both cohorts. Other criteria for success have been investigated and are reported (Table 1). Scatterplots have also been constructed to demonstrate IOP changes from preoperative baseline to final measurements at 1 year (Figure 4).

Figure 4: Scattergrams for SG≤2 (left) and SG≥3 (right) comparing baseline IOP to final IOP at 12 months.

Table 1: Various criteria for success at 1 year postoperatively for Trabectome and Phaco-Trabectome

SG≤2 (n=43)

SG≥3 (n=271)

32 (74%)

206 (76%)

29 (67%)

157 (58%)

31 (72%)

195 (72%)

22 (51%)

118 (44%)

31 (72%)

175 (65%)

13 (30%)

67 (25%)

30 (70%)

167 (62%)

3 (7%)

16 (6%)

SG≤2 (n=48)

SG≥3 (n=309)

44 (92%)

282 (91%)

27 (56%)

158 (51%)

39 (81%)

258 (83%)

17 (35%)

128 (41%)

35 (73%)

216 (70%)

7 (15%)

80 (26%)

29 (60%)

170 (55%)

2 (4%)

24 (8%)

No differences between narrow angle and open angle eyes

Trabectome has been relatively contraindicated in glaucoma patients with narrow angles for a variety of reasons, including surgical challenges and presumed complications. In this study however, there were no statistically significant differences in IOP and number of medications between narrow angle and open angle eyes in both trabectome and phaco-trabectome groups. Notably, the study was limited by sample size differences and only 1 year of follow-up.

We conclude that indications for the trabectome should be expanded to include a history of failed trabeculectomy and narrow angles.

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Hardik A. Parikh, Dr Igor Bussel, Dr Nils Loewen

E: loewen.nils@gmail.com

Hardik Parikh is a medical student at Rutgers, the State University of New Jersey and a research fellow at the Department of Ophthalmology at the University of Pittsburgh, Pennsylvania, USA.

Dr Igor Bussel is a resident at the Department of Ophthalmology, UPMC Eye Center, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA.

Dr Nils A. Loewen, MD, PhD is an Associate Professor of Ophthalmology at the UPMC Eye Center, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA.