Effect of iStent trabecular micro bypass device on outflow system morphology

Abstract

 

Purpose

 

Implantation of the iStent (Glaukos Corp, Laguna Hills, CA) is thought to lower intraocular pressure by creating a direct channel between the anterior chamber and Schlemm’s canal (SC). This retrospective study analyzed the effect of a single iStent on the morphology of SC by comparing the parameters of SC before and after placement, specifically looking at the cross-sectional area (CSA), inner to outer wall distance (IOD) of SC and the thickness of trabecular meshwork (TM).

 

Methods

 

Six patients with primary-open angle glaucoma had the iStent placed. SD-OCT images of nasal and temporal quadrants of each study eye were taken before and after surgery. Ten measurements of SC were manually measured over a consecutive 1mm segment of SC on FIJI ImageJ. The average obtained from the ten measurements represented the mean SC-CSA. Similarly, the SC-IOD and the TM thickness were measured 10 times over a consecutive 1mm segment of SC both in the nasal and temporal quadrants and averaged. The effect of the iStent on these parameters was then quantified by linear mixed effects modeling.

 

Results

 

The total SC-CSA prior to iStent placement was 3321.08 microns, standard deviation (SD) +/-1464.66 microns, standard error-of-mean (SEM) 422.81 microns; post-placement SC-CSA was 4570.923 microns (+/-1212.91 microns and SEM 383.557 microns). The SC-CSA in the nasal quadrant increased from 3398.39 microns (+/-1464.662; SEM 464.14) to 4743.45 microns (+/-1323.24, 540.210). The SC-CSA in the temporal quadrant also increased from 3243.764 microns (+/-1136.909, SEM 754.16) to 4312.135 microns (+/-1160.335, SEM 580.17). The total SC-IOD pre-op was 226.679 microns (+/-51.797) and increased to 264.299 microns (+/- 54.169) post-op (p=0.0139). The nasal pre-op SC-IOD was 248.79 micron (+/- 27.04) and increased to 285.03 microns (+/-52.99) post-op; this was statistically significant (p=0.0433). The temporal pre-op SC-IOD was 204.56 microns (+/-63.23) and increased to 233.20 microns (+/-44.41). The TM thickness was 187.51 microns (+/-69.46) pre-op and increased to 222.29 microns (+/-119.20) post-op. Unless specified, p>0.05 for parameters.

 

Conclusions

 

While changes in SC-CSA and temporal SC-IOD did not reach statistical significance, there was a trend to these parameters increasing post implantation of the iStent. A single iStent implant did significantly increase the width of Schlemm’s canal (SC-IOD measurement) nasally where it is placed.