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Newly trained surgeon shares perspective on trabecular micro-bypass system
The learning curve is short and succinct, and initial experiences are positive.
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Various new glaucoma procedures have recently entered the market to fill the void of microinvasive surgical options to lower IOP. These ab interno procedures do not scar the conjunctiva, are usually quicker to perform, and have fewer complications compared with traditional glaucoma surgery such as trabeculectomy or tube shunts. They provide a moderate lowering of IOP that makes them well suited for cases of mild to moderate glaucoma. The newest of these procedures is the iStent Trabecular Micro-Bypass from Glaukos.
The iStent device is a 1-mm-long “L-shaped” implant that is placed in the trabecular meshwork via a preloaded injector. The long arm consists of an open half pipe with retention arches and a self-trephinating tip that sits in Schlemm’s canal, and the short arm (snorkel) extends slightly into the anterior chamber. The iStent creates a patent opening in the trabecular meshwork that increases aqueous outflow and decreases IOP.
The iStent is indicated for use in conjunction with cataract surgery. Following lens placement, the patient’s head and the microscope are tilted to allow visibility of the angle, a gonioprism is placed on the cornea, and the iStent is guided into place in Schlemm’s canal. Of course, things are never this simple the first time you perform a procedure, especially not when the OR is filled with company representatives and extra staff curious to see the new procedure.
I recommend, in advance of the first case, practicing with a gonioprism on at least a dozen standard cataract procedures. This will go a long way toward ensuring smoother cases. By doing so, we discovered that the nurse needed to release the tape on the patient’s head, and we noticed that the microscope oculars lowered noticeably when we rotated the scope. I now start with the patient’s bed higher, an additional handle cover on hand to re-orient the microscope, and a spare towel to place under the patient’s shoulder in case he has difficulty rotating his head. I also have the patient practice head tilting before surgery.
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In addition, the better the surgeon knows the angle landmarks, the easier the procedure becomes. A great refresher is provided at gonioscopy.org.
The technique
My technique begins after I have placed the IOL. I use a small amount of Miostat (carbachol, Alcon) directly over the nasal iris, and then I add a small amount of additional viscoelastic in the nasal angle to provide better access and visibility. Avoid over-inflating the anterior chamber because this can collapse Schlemm’s canal and make insertion more difficult. The nurse then releases the tape on the patient’s head, and I rotate it 45° away from me while I rotate the microscope 45° toward me. I grasp the inserter like a pencil, with my index finger on the release button, and inspect the inserter to ensure the implant is on the tip. Make sure your hand is properly positioned on the inserter initially so you do not have to readjust your grasp inside the eye. Once the inserter is in the anterior chamber, the gonioprism is placed on the cornea. The inserter is then advanced across the pupil to the nasal angle, aiming for the trabecular meshwork and the hyperpigmented collector channels when possible. It is helpful to increase the microscope magnification and illumination to more easily identify these structures.
Approach the top half of the trabecular meshwork at a 15° angle, with the stent parallel to the tissue and the tip directed toward the patient’s feet. Use the tip of the implant to engage the trabecular meshwork and slide it into Schlemm’s canal with a gentle but decisive motion. Hesitation will release blood and obstruct the view, making the procedure more difficult. The stent should glide in easily with minimal resistance. Resistance will be encountered if the scleral wall is hit. If this occurs, back up slightly and, with a slight lifting motion, slide the implant into Schlemm’s canal. If unable to easily slide the implant into position, the surgeon can back out completely, select a different site 1 to 2 clock hours superior or inferior to the original site, and try again.
Once the implant is in place, I allow the eye to rotate back into position and carefully and slowly press the release button, avoiding any lateral movements. Blood reflux into the anterior chamber indicates that the implant is in the proper position. Gently tap the tip of the snorkel to ensure it is properly seated. The snorkel tip should be clear and shiny, and the retention ridges should not be visible. On occasion, you may think that the implant is in place only to realize after releasing it that it is not. Regrasping the implant with the inserter is surprisingly easy, and you should practice this before performing your first case. To facilitate this, correctly identify right vs. left and right side up vs. upside down implants.
Finishing up
Remember that “the enemy of good is better.” Although we all strive for perfection, if the implant is in a good position, congratulate yourself and finish the case. Because I am compulsive and not smart enough to listen to my own advice, I have mucked around after a good procedure, only to undermine the results and have to redo the implantation. Postoperatively, I prescribe steroid and NSAID drops slightly longer than for patients undergoing cataract surgery alone.
My initial experiences with the iStent are positive, and I look forward to the long-term results from my patients. It will be interesting to see how combining this procedure with other treatments designed to decrease aqueous inflow will further affect IOP. The learning curve is short and relatively shallow for anyone who does cataract surgery, and the benefit-to-risk ratio is excellent for my patients. The iStent is an exciting new addition to our armamentarium for the treatment of mild to moderate glaucoma.