With the start of the new school year, my young son is about to enter pre-K and has been practicing his ABCs and 123s. Of course, we use song to help him remember his letters and numbers in harmony. Because of this, I find it a good time to discuss the ABCs of ABiC. Of course I will not do it in song, primarily because I would sound like Alfalfa from the Little Rascals singing, “I’m in the mood for love” to Darla. But, what I can do is describe the steps and surgical pearls of ABiC, so that you can find rhythm in the operating room.
ABiC can be as easy as “ABC”. All you have to do is follow the steps – and understand what to do when a step of “off key”.
STEP 1: Performing corneal incisions (Figures 1 and 2)
Although simple and overlooked, creation of the corneal wounds can allow for your case to glide as smoothly as Fred Astaire. It can can also cause major difficulties. You want to create your clear corneal incision with the same architecture that you would create your tri-planar temporal clear corneal incision. The wound needs to be long enough to ensure that your chosen anterior chamber stabilizing ophthalmic viscosurgical device (OVD) does not burp out of the wound easily, but not too long and anterior that it interferes with the introduction of micro forceps into the anterior chamber.
The second corneal wound is the paracentesis. I prefer to use a 27-gauge needle and direct the needle precisely towards the nasal angle. The resultant tunnel that the needle creates firmly secures the iTrack microcatheter in the anterior chamber and prevents lateral movement and migration away from the angle. (Figure 3).
Some colleagues use a 15-degree blade or side-port blade, and still angle towards the nasal region of the eye. Once the iTrack microcatheter is in proper position in the anterior chamber, secure the hub to the patient’s forehead or cheek. (Figure 4) One important tip is to avoid nicking the limbal vessels, as blood from these vessels tends to flow onto the surface of the cornea, which stains the gonio lens-coupling agent. This, in turn, can lead to an obstructed view of the angle and make manipulation of the angle or the microcatheter difficult (Figure 5).
Figure 1. Typical tri-planar temporal clear corneal incision.
Figure 2. Side-port incision directed towards the nasal angle. Incision can be created with a number of different instruments including a 27-gauge needle, super-sharp blade, or 15-degree blade. Regardless of instrument used, direct the incision, as described and avoid the limbal vessels.
Figure 3. Prior to inserting, ensure the iTrack microcatheter is fully primed with Healon or Healon GV. Direct the catheter towards the nasal angle and secure the hub to the patient’s forehead or cheek.
Figure 4. Once the iTrack microcatheter is secured in the anterior chamber, wrap the iTrack and secure the hub to the forehead. I personally always make my incision at the twelve o’clock position and intubate the inferior canal first. This means I need to manipulate the catheter in a forward direction when operating on a right eye and backward when operating on a left eye.
Figure 5. Note the blood underneath the goniolens and the poor view of the angle.
STEP 1.5: iTRACK™ MICROCATHETER PREPARATION
After removing the iTrack microcatheter from its packaging, your technician should soak the iTrack in balanced salt solution or water. The iTrack microcatheter is coated with proprietary substance, which becomes “slippery” after being hydrated and allows for the iTrack to glide through the canal. After soaking, prime the iTrack microcatheter with either Healon or Healon GV (my preference). The microcatheter is then introduced into the nasal angle through the side-port incision and directed towards the nasal angle. Once the microcatheter is properly placed in the anterior chamber, secure the hub of the microcatheter to the patient’s forehead or cheek. I typically create my side-port incision at the twelve o’clock position and intubate the inferior canal. This means that I have to manipulate the microcatheter in a backhand fashion when I am operating on a right eye, since I am right-handed. It is important to be become accustomed to manipulating devises both in a forward and backward motion, as the canal can have abnormalities that can prevent complete canal intubation in one direction, and intubation in the opposite direction may be necessary.
(As a side note, I recently encountered a patient with a diseased angle and was unable to implant a Hydrus micro-stent (Ivantis, CA) in the inferior angle of a left eye and had to re-direct and implant the device in the superior angle using my left hand.)
STEP 2: OTOMY CREATION
After tilting the patient’s head and microscope to provide optimal visualization of the nasal angle, a small goniotomy is needed to allow for introduction of the iTrack microcatheter into Schlemm’s canal. The same 27-gauge needle can be used to create a small horizontal incision in the pigmented portion of the trabecular meshwork. After creating the small goniotomy (~10 mm) depress the posterior lip of the incision so the outer wall of the canal is observed. If the eye is hypotonous, blood will likely be in the canal and can regurgitate into the anterior chamber. A small amount of blood will not interfere with the next step of the procedure, but a significant amount of blood can obstruct the angle. Should the latter be encountered, the blood can be pushed aside with more viscoelastic and provide a good view of the angle structures. If injection of viscoelastic fails to provide an adequate view, evacuate the blood from the anterior chamber using an irrigation/aspiration hand-piece and inject a fresh bolus OVD. I also prefer to use a visco-dispersive OVD (Viscoat, Alcon Laboratories, Ft. Worth) as I feel it helps maintain the chamber depth and form, but some colleagues prefer using a high molecular weight visco-cohesive OVD like Healon GV (Abbott Medical Optics, Santa Ana, CA), as it may provide a clearer view.
STEP 3: CANAL INTUBATION
Using the temporal clear corneal incision, introduce surgical micro-forceps (like MST forceps) into the anterior chamber and place the goniolens on the cornea to view the nasal angle. Next, grasp the iTrack microcatheter at an oblique angle approximately one centimeter from the tip (Figure 6). Slide the microcatheter along the pigmented portion of the trabecular meshwork proximal to and toward the otomy site. The iTrack microcatheter should slide into the canal. Intubate approximately one centimeter of the canal before re-grasping the microcatheter. With continued intubation, ensure the iTrack microcatheter is in the canal by watching the red fiber optic light at its tip. There may be times that the canal has a weak spot and the iTrack can misdirect and enter the suprachoroidal space. The beauty of the iTrack microcatheter is the fiber optic tip. It allows surgeons to safely intubate Schlemm’s canal and to know where they are in the canal throughout the procedure.
Not all canals are created equal. Some canals are fully patent and canal intubation proceeds without difficulty. Some canals, however, are diseased and feature various abnormalities the can be encountered upon circumnavigation of the iTrack microcatheter. One can encounter weak areas, which can cause the iTrack microcatheter to dive into the suprachoroidal space (Figure 7). Additionally, abnormalities like adhesions, strictures, septae (though controversial), can also preclude complete canal intubation. If “obstructions” are encountered I recommend infusing viscoelastic in the canal to dilate the area and allow for passage of the iTrack microcatheter. Other times, grasping the iTrack microcatheter closer to the otomy and pushing it with slightly more force can transcend these abnormal areas within the canal. This is possible because the iTrack microcatheter has a central guide wire that provides strength. If unsuccessful at bypassing adhesion or strictures, one can create a new side-port incision 180 degrees away from the primary paracentesis, reposition the catheter through the new paracentesis and intubate the canal in the opposite direction. Most times, the canal can be completely catheterized either in one direction or by using the iTrack microcatheter to intubate the canal in a clockwise and counterclockwise direction.
Figure 6. The iTrack microcatheter is grasped about 10 mm away from the fiber optic tip and at a slightly oblique angle.
Figure 7. Note the fiberoptic light shinning through the sclera.
STEP 4. WITHDRAWL OF iTRACK MICROCATHETER AND VISCODILATION
Once the iTrack microcatheter has been circumnavigated 360 degrees, it can be withdrawn by pulling on the proximal portion of the iTrack outside the eye. I prefer using a second instrument like a Lester hook to serve as a fulcrum to avoid linearization or the otomy and potential damage to the trabecular meshwork. I typically remove the iTrack microcatheter at a rate of one clock hour per second and have my technician infuse Healon GV using the ViscoInjector connected to the fiberoptic probe. I have them infuse the viscoelastic by clicking the injector twice per clock hour.
Compared to traditional (ab externo) Canaloplasty, we have become a little more aggressive with the amount of viscoelastic we inject during ABiC. We are able to be do so because, unlike traditional canaloplasty where we have a relatively hypotonous eye when performing viscodilation, with ABiC we have an anterior chamber that has been pressurized with viscoelastic. With traditional canaloplasty, we have to hypo-pressurize the eye after creating our deep dissection to prevent blowout of the Descematic window. Because eyes had low pressures, they were at higher risk for developing Descematic detachments. In the case of ABiC, because the eye is pressurized, there is significantly less risk of detachment.
Once the iTrack microcatheter has been removed and the canal viscodilated, the OVD in the anterior chamber must be removed as ABiC has disrupted or removed any tissue in the trabecular meshwork, save the small otomy. At the termination of each case, my own routine is to instill a drop of pilocarpine 1% to facilitate outflow, a drop of brimonidine 0.15% to prevent an immediate post-operative pressure spike and an steroid/antibiotic combination drop.
ABiC is a very elegant procedure and can be easily mastered with the right preparation and understanding of each step of the procedure. Many surgeons have coupled ABiC with other procedures like trabecular micro-bypass or trabecular ablation. It leaves such a small footprint on the eye, that any glaucoma procedures can be performed after ABiC if necessary. In fact, ABiC can be repeated with success as well as traditional canaloplasty. It can be performed as a stand-alone procedure in phakic or pseudophakic patients. Additionally, it can be coupled with cataract surgery to reduce intraocular pressure and medication burden. It is a very diverse tool that allows us to manipulate the outflow system in a myriad of patients. Based on my experience, it is an important tool in improving a patient’s quality of life whilst also helping to preserve their vision.