INTRODUCTION
M icroinvasive glaucoma surgery has emerged in recent years as an alternative to traditional glaucoma surgeries. The trabecular micro-bypass stent, commonly known as the iStent (Glaukos Corporation, Laguna Hills, CA, USA),is a titanium stent inserted into the trabecular meshwork (TM)to create a direct channel between the anterior chamber and Schlemm’s canal to increase aqueous outflow. It is typically implanted during phacoemulsif i cation for patients with openangle glaucoma and visually significant cataracts. The iStent has shown to be more effective in reducing intraocular pressure(IOP) and lowering the number of post-operative hypotensive medications than phacoemulsif i cation alone[1-2].
The standard surgical technique for iStent insertion can be performed before or after phacoemulsification based on the preference of the surgeon. For optimal viewing, the head of the patient is rotated 35° away from the surgeon while the surgical microscope oculars are rotated 35° toward the surgeon. An ophthalmic viscoelastic device is inserted into the anterior chamber to widen the nasal angle. The iStent applicator is brought across the anterior chamber toward the nasal angle under direct gonioscopic view. The stent is released from the applicator, punctures the TM, and is inserted into Schlemm’s canal[3].
With dimensions of 1 mm in length and 0.3 mm in height, it can be challenging to successfully place an iStent under direct gonioscopic view. Intraoperative challenges include poor angle visualization and stent malpositioning[1,4]. The purpose of the present study is to describe a novel technique of creating a landing strip within the TM to assist with iStent implantation.
SUBJECTS AND METHODS
Our retrospective study included all patients who received an iStent with phacoemulsification from May 2014 to February 2015. All surgeries were performed at Wills Eye Hospital by a single experienced surgeon (Moster MR). The Wills Eye Hospital Institutional Review Board approved the study following the principles of the Declaration of Helsinki.
Figure 1 Landing strip technique for iStent implantation. A: A 25-gauge microvitreoretinal blade is used to bisect the nasal TM to create a landing strip; B: The landing strip improves visualization of the TM by providing a visual clue for the precise location of iStent implantation; C:The iStent applicator is pressed along the landing strip; D: The iStent is released from the applicator and directly implanted into the TM.
Landing Strip Surgical Technique Although the iStent can be placed prior to phacoemulsif i cation, the authors’ preference is to place the stent after phacoemulsif i cation. When the iStent is implanted after pseudoemulsification, the same temporal corneal incision used for phacoemulsif i cation is accessed. An ophthalmic viscoelastic device is injected if needed based on the discretion of the surgeon to deepen the anterior chamber.The patient’s head is rotated approximately 35° away from the surgeon while the microscope is tilted 35° toward the surgeon.A 25-gauge microvitreoretinal blade is used to bisect the nasal TM to less than 1 clock-hour, effectively creating a landing strip (Figure 1). Additional viscoelastic material is injected to widen the landing strip. The landing strip allows for improved visualization of the TM and serves as a visual landmark for precise implantation. The iStent applicator enters via the same temporal incision used for phacoemulsification and crosses the anterior chamber toward the nasal angle under direct gonioscopic view. The leading edge of the iStent applicator is pressed along the landing strip through the TM. The landing strip is visualized adjacent to the stent, confirming that it is positioned correctly, and the iStent is released from its applicator. The viscoelastic device is removed and the wound is hydrated with balanced salt solution.
Statistical Analysis Medical records were reviewed and data was analyzed using SAS Analytics Pro software, version 9.4 (SAS Institute Inc, Cary, NC, USA). The t-test was used to compare the data before and after operation. P＜0.05 was considered statistical signif i cance.
RESULTS
Stent implantation via this technique was performed in 34 eyes of 30 patients. A summary of patient demographics is shown in Table 1. Pre-operative and post-operative characteristics are shown in Table 2. Of the 34 eyes, 33 (97.1%) were taking at least 1 glaucoma medication prior to surgery. The mean follow-up time was 6.1±4.8mo (median, 5.5mo; range, 1d to 16mo). Three eyes (8.8%) from 3 patients were seen at 1-day follow-up, then lost to further follow-up. At 1-month follow-up(n=31) and 6-month follow-up (n=17), the number of ocular hypotensive medications signif i cantly decreased (P＜0.001 and P=0.05, respectively). At 1-month (n=31), 6-month (n=17),and 1-year (n=12) follow-up, 20 patients (64.5%), 7 patients(41.2%), and 4 patients (33.3%) were able to discontinue all glaucoma medications, respectively.
Table 1 Demographic and clinical characteristics of the study population n (%)
1One patient had trabeculectomy 18y prior to iStent placement; 2All 13 patients had previous laser with selective laser trabeculectomy.
The mean IOP decreased compared to baseline at 1-month,6-month, and 1-year follow-up (P=0.99, 0.58 and 0.76,respectively), but the difference was not statistically signif i cant(Table 2). There were 3 eyes (8.8%) with increased IOP to 28,42 and 48 mm Hg on post-operative day 1, which decreased to 12, 11 and 8 mm Hg, respectively, after removal of viscoelastic material through the corneal incision at the slit-lamp. Of these 3 patients, 2 ultimately required trabeculectomy due to elevated IOP despite maximal medical therapy. The first patient continued to have elevated IOP ranging from 38 to44 mm Hg at subsequent follow-up visits, so trabeculectomy was performed 3wk after iStent surgery. After trabeculectomy,the post-operative IOP was 12 mm Hg at 1-month followup. The second patient had trabeculectomy 5mo after iStent surgery when IOP increased to 52 mm Hg despite maximal medical therapy. The post-trabeculectomy IOP was 14 mm Hg at the most recent follow-up visit for the second patient.
Table 2 Mean pre-operative and post-operative number of glaucoma medications, IOP and visual acuity Mean (SD)
BCVA: Best corrected visual acuity; logMAR: Logarithm of minimum angle of resolution; SD: Standard deviation.
The only intra-operative complication was hemorrhage in 1 eye (2.9%). The most common post-operative complications were transient hyphema (n=7; 20.6%), transient corneal edema(n=3; 8.8%), and subconjunctival hemorrhage (n=2; 5.9%),which all resolved by 1-month follow-up.
DISCUSSION
The purpose of the present study was to describe a novel technique of creating a landing strip to assist with iStent implantation. Our results showed that implantation using this technique significantly decreased the mean number of postoperative medications at 1-month and 6-month follow-up,consistent with findings from other studies[1,4-5]. At 1-year follow-up, there was a reduction in the number of hypotensive medications, although this result was no longer statistically significant, possibly due to the small sample size (n=12).Over 60% of patients were able to discontinue all medications post-operatively at 1-month follow up. Since patients often experience challenges adhering to hypotensive medications[6],reducing medications with stent implantation may help overcome these challenges. However, several studies have shown that the mean number of medications increases as follow-up time lengthens[4,7], which was seen in our study.Although the mean number of medications increased as follow-up time increased, 33% of patients had appropriate IOP without any medications at 1-year follow-up.
Our study showed that there was a decrease in IOP at 1-month follow-up and this IOP-lowering effect continued at 6-month follow-up. Other studies have also found that there is a delay in IOP reduction, as IOP continues to decrease at 6-month followup[4,8]. The mechanism of the iStent is to increase outf l ow of aqueous humor. The mean IOP at baseline in our study was 19.7±4.1 mm Hg, so reduction in IOP would theoretically be less in normotensive eyes compared to hypertensive eyes.In addition, it is possible to place more than 1 iStent during cataract surgery. Several studies have shown that implanting multiple iStents resulted in sustained IOP reduction with signif i cantly fewer medications after 12mo of follow-up[9-10].
There were 3 patients with elevated IOP on post-operative day 1, and 2 patients ultimately required trabeculectomy.Stent failure in these patients may have occurred due to aqueous outflow obstruction past Schlemm’s canal or postoperative TM dysfunction. Despite the possibility of failure,it is reasonable to consider iStent implantation to lower IOP since there are few adverse events or late-onset complications associated with the iStent[1-2].
In our study, the only intra-operative complication was hemorrhage seen in 1 eye, which supports the safety of iStent implantation seen in other studies[1-2]. The most common postoperative complication was a hyphema. Previous studies found that hyphema was common after iStent implantation[7-9],including a case series where hyphema was seen in 7 of 10 eyes[7].
Our study is limited by the small sample size and short followup period. Further follow-up is needed to determine the longterm outcomes of this technique. In addition, previous studies have shown that there is a reduction in IOP and number of glaucoma medications from phacoemulsification alone[11].A major limitation of our study is the inability to discern the effect of phacoemulsif i cation alone versus phacoemulsif i cation with iStent implantation on reduction of IOP and number of glaucoma medications. Although several studies have shown that iStent may be more effective in reducing IOP and number of hypotensive medications than phacoemulsif i cation alone[1-2],further research needs to be conducted. Another limitation is our study population mainly consisted of Caucasians and patients with primary open angle glaucoma. As a result, it is difficult to extrapolate our results to other races and types of glaucoma. Additional research is needed to determine the outcomes of this technique in other populations. Despite these limitations, the landing strip technique appears to be an effective way to assist iStent implantation.
ACKNOWLEDGEMENTS
Conflicts of Interest: Zheng CX, None; Moster MR,Consultant for Alcon, Allergan, Solx, Merck; Received Grants to Institution from Aerie, Aeon, New World Medical, Alcon,Glaukos, Bausch and Lomb, Iridex; Received Payment for Lectures from Alcon, Allergan, Ista, Merck, Bausch and Lomb,New World Medical, TBI, Solx; Gogte P, None; Dai Y, None;Manzi, RS, None; Waisbourd M, None.
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