Ab Externo Trabeculotomy Performed via Illuminated Microcatheter

A trabeculotomy is a procedure commonly performed on children presenting with congenital glaucoma and open angles. All variations of the procedure incise the TM to increase aqueous humor outflow and decrease intraocular pressure (IOP). The ab externo approach is preferred when corneal scarring is present and gonioscopic view of angle structures is inadequate. Trabeculotomies can also be performed for cases of juvenile open-angle glaucoma, glaucoma associated with Sturge-Weber syndrome, and glaucoma following cataract surgery, with various success rates1,2. In this case, a circumferential, ab externo approach using an illuminated microcatheter and a metal trabeculotome achieves a combined circumferential treatment of approximately 310 degrees. In cases of prior scarring or obstruction in SC, a scleral cutdown can be performed to retrieve the microcatheter.

In this infant, a circumferential, ab externo trabeculotomy with an illuminated microcatheter was primarily used to incise the TM. The procedure should be avoided in situations of poor conjunctival quality, scleral thinning, and angle closure glaucoma. Furthermore, if the child has a history of previous trabeculotomy, trabeculectomy, or cataract surgery with intraocular lens implantation, subsequent scarring from these procedures might prevent complete, circumferential threading of the illuminated microcatheter and result in a partial treatment. If a child is an appropriate candidate for ab externo trabeculotomy, initial steps to consider include instrument checks, location of the scleral flap, and the surgeon’s positioning. In general, operating from a temporal position avoids maneuvering over the patient’s face and allows for preservation of the superior quadrant for potential future superior subconjunctival surgery. It is also important to test the light functionality and set up of the illuminated microcatheter prior to the initial incision.

An 8-0 polyglactin 910 corneal traction suture is used to adduct the eye to allow better access to the temporal subconjunctival space. A temporal conjunctival peritomy is performed using Westcott scissors. The surgeon bluntly dissects Tenon’s capsule away from the sclera underneath. Once the peritomy is completed, wet field cautery is used to control bleeding and prepare space for the scleral flap. A 3mmx2mm rectangular scleral flap is created at 90% depth using a 15 degree blade and crescent blade. A paracentesis is made so that an acetylcholine chloride (10 mg/mL) intraocular solution is introduced for pupil constriction, followed by viscoelastic for anterior chamber depth control. It is important not to introduce too much viscoelastic as increased IOP can compress SC and make it more difficult to thread the illuminated microcatheter. Using a 15 degree blade, a small, superficial incision is made over the back wall of SC at the “gray-white junction” under the scleral flap. Subsequently, a metal trabeculotome is used to probe and correctly identify the SC opening. The metal trabeculotome should not be forced into the SC opening as excessive force can create false passages. After ensuring that the microcatheter is working, it is threaded through either side of SC with non-tooth tier forceps using delicate movements. Toothed forceps may damage the fiberoptic light source or the shape of the microcatheter. By dimming the lights in the operating room, the location of the microcatheter tip is periodically assessed by observing the lighted tip from the fiber optic portion of the microcatheter. As the microcatheter is fed through SC, the illuminated microcatheter tip should be seen advancing the canal under the limbus in a circumferential pattern. A lighted tip that veers posteriorly may mean that the catheter has exited a collector channel or the suprachoroidal space and should be carefully retracted and redirected. In this infant, the illuminated microcatheter travels 270 degrees along SC before approaching an obstruction from a previously scarred trabeculectomy. A scleral cutdown can be performed to retrieve the microcatheter and perform a partial trabeculotomy if the catheter has advanced beyond 180 degrees2. After the microcatheter is exposed, non-tooth tier forceps is used to retrieve the microcatheter. Using another non-tooth tier forceps to hold the other end of the microcatheter, the microcather is gently tugged in a direction orthogonal to the site of insertion. This motion incises the TM and should allow for increased aqueous drainage. The microcatheter is carefully visualized in the anterior chamber during the motion to ensure that the other intraocular structures are not caught by the microcatheter. Historically, a metal trabeculotome has been used to perform a partial trabeculotomy through either direction of SC opening. In this case, a right-sided metal trabeculotome is then inserted into SC opening in the opposite direction of the microcatheter to perform further trabeculotomy of the untreated angle up to the point of prior superior scarring. Once inserted, the trabeculotome is rotated in the direction of the anterior chamber to incise additional TM. This achieves another 40 degrees of TM clearance. The scleral cutdown and scleral flap are closed with several interrupted knots using 8-0 polyglactin 910 suture in a watertight fashion. Viscoelastic is exchanged with balanced salt solution and the eye is left at physiologic pressure. The paracentesis is closed with 10-0 polyglactin 910 suture. The conjunctiva is re-approximated and closed with running 8-0 polyglactin 910 suture.

Following the surgery, the infant demonstrated IOP improvement in the operated eye from pre-operative IOP of 35 mmHg on maximally tolerated glaucoma medications to post-operative IOP of 19 mmHg on no glaucoma medications 1 month afterwards. This reduction was sustained consistently during multiple follow-up examinations and there was a reduction in overall corneal clouding.

This outcome aligns with the reported success rates of trabeculotomy in the literature. In a cohort consisting of 22 eyes affected by primary congenital glaucoma, patients experienced a substantial average decrease of 15.1 mmHg in intraocular pressure following circumferential ab-externo trabeculotomy with an average of 2.7 ± 2.0 years of IOP control². Treatment success in that cohort was seen in 77% (95% CI 53-90%) at 1 year and 58% (95% CI 19-81%) at 5 years.

 

The risks and possible complications for a trabeculotomy are in line with other intraocular surgeries. Of note, there is a common risk of post-operative hyphema that can be mitigated by leaving some viscoelastic in the anterior chamber at the end of the case for a tamponade effect. Additional complications include the risk of Descemet’s detachment, iridodialysis, or cataract related to improper manipulation of the microcatheter or the metal trabeculotome during TM incision. Some patients may need further surgery to reduce IOP as this is a chronic condition.

Alternative angle-based procedures can be used to disrupt the TM and lower IOP in children. Each of these approaches have their own indications as well as advantages and disadvantages. For instance, if gonioscopic view of the angle structures are clear, a traditional goniotomy or ab interno circumferential trabeculotomy can be performed with the assistance of a gonioscopic lens. These alternatives have the added benefit of not requiring scleral incisions.

Furthermore, a trabeculotomy can also be combined with a trabeculectomy for an additional means of promoting aqueous outflow in cases where circumferential trabeculotomy is not possible. Trabeculectomy and glaucoma drainage device implantation can be used in cases that are refractory to angle surgery or in cases of angle closure³.

We do not have any conflicts to disclose.

We do not have any acknowledgments to report.

1. Edmunds B, Beck AD, Hoffmann E, Grehn F. Angle Surgery: Trabeculotomy. In: Grajewski AL, Bitrian E, Papadopoulos M, Freedman SF, eds. Surgical Management of Childhood Glaucoma: Clinical Considerations and Techniques. Springer International Publishing; 2018:57-78. doi:10.1007/978-3-319-54003-0_5 2. Rojas C, Bohnsack BL. Rate of Complete Catheterization of Schlemm’s Canal and Trabeculotomy Success in Primary and Secondary Childhood Glaucomas. Am J Ophthalmol. 2020;212:69-78. doi:10.1016/j.ajo.2019.11.029 3. Weinreb RN, Grajewski A, Papadopoulos M, Grigg J, Freedman S. World Glaucoma Association (WGA) Consensus Series 9 - Childhood Glaucoma. Amsterdam: Kugler Publications; 2013:149.