Target Placement of Multiple iStents
Preoperative office-based gonioscopy is essential in determining a patient’s surgical candidacy for microinvasive glaucoma surgery (MIGS) (University of Iowa Health Care Ophthalmology and Visual Sciences, www.gonioscopy.org). Is the angle open or closed? In the latter, is there appositional or synechial closure? Consideration for placement of multiple micro-bypass stents should be given to those intolerant or unable to take glaucoma medications (i.e., drug-induced systemic or ocular side effects, unable to self-administer medications due to a physical disability), unable to undergo an ab externo surgical procedure (i.e., trabeculectomy, tube shunt surgery) due to immobile bulbar conjunctiva from either prior scarring or systemic disease, or unacceptable risk versus benefit associated with invasive surgery. When two or three micro-bypass stents were combined with phacoemulsification (PE), 77 % achieved a mean IOP <15 mmHg with >80 % taking less medications at 1 year (J Cataract Refract Surg 38:1911–1917, 2012). This reduction may reflect enhanced circumferential flow with increased access to collector channels (CC) draining aqueous into the episcleral venous system (J Cataract Refract Surg 38:1911–1917, 2012). Titratability of multiple stents (n = 1–3) as a stand-alone procedure in lowering IOP further with each additional iStent was demonstrated in a prospective study. Reduction in IOP ≥20 % with unmedicated IOP ≤18 mmHg versus baseline unmedicated IOP was achieved by 89.2 %, 90.2 %, and 92.1 % for one, two, and three stents, respectively, at 1-year post-op (Clin Ophthalmol 9:2313–2320, 2015). Targeted placement of micro-stents should be directed in areas of greater trabecular meshwork (TM) pigmentation associated with close proximity to collector channels (CC) optimizing aqueous outflow via the aqueous veins (AV) to the episcleral vein (EV) (Invest Ophthalmol Vis Sci 50(4):1692–1697, 2009). Identification of initial blood reflux in Schlemm’s Canal (SC) may also serve to target CC (see Chap. 49). For novices, in angles with minimal to no pigment, injection of trypan blue has been shown to selectively stain the TM landmark for visual identification during implantation (Glaukos user meeting, Fort Lauderdale, 2016). However, this latter approach does not provide cues for targeting CC. If the target IOP reduction is not achieved or there is evidence of visual field progression or increased cupping, patients may need to be restarted on medical therapy. Risks, benefits, alternatives, and appropriate educational material should have been provided to the patient regarding angle surgery.
Shareef S, Sharma S. Target placement of multiple iStents. In: Rosenberg ED, Nattis A, Nattis RJ, editors. Operative dictations in ophthalmology. Cham, Switzerland: Springer International Publishing; 2017. pp. 207-14.