33 Ab Interno Viscodilation of Schlemm’s Canal: VISCO 360 and ABiC A 75-year-old man presents with moderate primary open-angle glaucoma (POAG) and an intraocular pressure (IOP) of 30 in both eyes. He has been instilling latanoprost and fixed combination timolol and dorzolamide in both eyes. The patient has a target IOP in the mid-teens, and a laser trabeculoplasty was done 6 months ago. The patient had cataract extraction 3 years prior to presentation. The patient has a small but significant nasal step visual field defect in both eyes but does not want to worry about having the risks associated with trabeculectomy. He elects to have ab interno viscodilation of Schlemm’s canal in both eyes. One year after surgery, his IOP is 12 mm Hg OD and 15 mm Hh OS on no medications. Canaloplasty has been performed for over a decade and is well described in Chapter 32. However, with the emergence of minimally invasive glaucoma surgery (MIGS), the utilization of canaloplasty with the placement of a retention suture in the canal has steadily declined, reserved for moderate to severe glaucoma patients. Studies have suggested that the most critical step of canaloplasty is the dilation of Schlemm’s canal with viscoelastic delivered by a catheter.1–4 In the MIGS era, the decline of canaloplasty is likely due to the fact that surgeons are hesitant to perform a surgery that creates an incision in the conjunctiva; however, the IOP lowering of the ab interno stenting procedures reported in other chapters has not been conventionally seen as able to reduce high IOPs (over 30 mm Hg) in patients on multiple (3+) medications to the mid-teens on one medication or none. Moreover, in the United States, insurance reimbursement for the only currently available trabecular stent requires that cataract surgery be performed. Therefore, there has been a need for a MIGS procedure to expand the treatment algorithm. When one evaluates the original data for canaloplasty, it should be noted that there was a subset of patients for whom an obstruction in the canal caused no suture to be placed; however, viscodilation of the canal was performed. This subgroup of patients was not considered as failures, but the resulting IOP was in the mid-teens.3 This led to the thinking that, if a viscodilation is required and a suture is not critical, then perhaps the canal can be dilated from an ab interno approach using either the same catheter, or in some other way, with the canal visualized intraoperatively by a surgical gonioprism. There are two methods of dilating Schlemm’s canal in an ab interno fashion that are currently available. The first utilizes the same iTrack microcatheter described in the Chapter 32, and discussed in Chapter 25, and has been dubbed ABiC (for ab interno canaloplasty). The second uses what looks identical to the TRAB360 device described in Chapter 27, but rather than being used for an ab interno trabeculotomy, the device is filled with viscoelastic and when the probe is pulled back into the device, it injects viscoelastic into the canal. This latter technique has been called VISCO 360. This procedure is typically performed with the patient under topical anesthesia and the surgeon sitting on the temporal side. After a 1.5- to 2-mm incision is made in the clear cornea, with the surgeon ensuring that the incision is not so far posterior as to bleed and obstruct the gonioscopic view, the anterior chamber is filled with a cohesive viscoelastic. If the procedure is combined with cataract surgery, the main phaco wound can be used to do the procedure and it can be done after the lens is placed but before the viscoelastic is removed from the eye. With both the ABiC and VISCO 360 procedures, it may be advantageous to use a miotic medication after the lens is placed but before the procedure is performed. If the procedure is done without cataract surgery, pilocarpine should be placed preoperatively. The TRAB360 device (Fig. 33.1), which is used for the VISCO 360 procedure, has a red stopper on the back of it. This stopper slides to the side, exposing the opening where a cohesive viscoelastic is injected gently until it is noted to be coming out of the tip of the device. The red stopper is then removed. The surgeon is careful not to remove the red stopper before the viscoelastic is injected, otherwise the device will be rendered inoperable. Viscoelastic is then placed on the cornea and the gonioprism is placed on the eye after the microscope is tilted 30 degrees away from the surgeon and the patient’s head is tilted 30 degrees away as well. The tip of the device is then placed in the eye, taking care not to hit the iris or the cornea. The tip is sharp and is used to puncture the trabecular meshwork. Once a small goniotomy is performed with the tip of the device, the blue wheel on the top of the device is turned toward the surgeon to release the nylon catheter into the canal. The wheel is turned until the catheter cannot be advanced further and the surgeon can no longer turn the wheel. The wheel is then turned in the opposite direction, away from the surgeon, and the catheter is brought back through the metal guide, back into the device. While it is retracted, the catheter releases the viscoelastic into the canal. The device is then removed from the eye and the other 180 degrees of dilation is then performed in the same manner. Viscoelastic may be required between sides to improve the view, as transient hyphema is frequent at the site of entry. Saline is then used to flush the viscoelastic from the eye. If combined with cataract surgery, the usual irrigation/aspiration technique can be used to remove the viscoelastic.
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VISCO 360
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