Large choroidal vessel thickness drives change in choroidal thickness with IOP following Trabeculectomy

Abstract

Purpose : Choroidal thickness (CT) has been shown to increase with intraocular pressure (IOP)-lowering following trabeculectomy. We studied the relationship between IOP and the change in the large choroidal vessels to determine the physiology behind change in CT.

Methods : 21 eyes of 20 patients undergoing trabeculectomy were examined pre-operatively, then post-operatively at 1 week and 1, 3, and 6 months with IOP checks and Enhanced Depth Imaging SD-OCT (Spectralis, Heidelberg Instruments) centered on the posterior 6mm surrounding the fovea. Two techniques were used to measure the choroidal vessel (CV) thickness, one focusing on the large choroidal vessel layer (LCVL), and the other determining the thickness (diameter) of the visible choroidal vessels themselves. Interstitial thickness was defined (1 – CV thickness). The relationship between the change in CV thickness, choroidal interstitial thickness, LCVL, change in IOP, and change in CT after trabeculectomy was analyzed using linear regression model to calculate the coefficient of determination (R²), at one month and at greatest change.

Results : CV thickness increased with decrease in IOP at one month (p=0.03, 95% CI: -3.1, -0.2) but not at largest change in IOP (P=0.19, 95% CI: -3.3, 0.7). Similarly, interstitial thickness increased with decrease in IOP at one month (P=0.01, 95% CI: -2.8, -0.4), but not at the largest change in IOP (P=0.08, 95% CI: -2.8, 0.2). We found change in LCVL to be linearly correlated with change in CT nasally, temporally, and subfoveally at 1 month (b=1.10-1.49, R²=0.82-0.94, p<0.05) and largest difference (b=1.21-1.40, R²=0.40-0.62,p<0.05). In this sample, CV thickness was not linearly correlated with change in IOP (b=-1.6, R²=0.291).

Conclusions : Our data show that the increase in CT with IOP-lowering following trabeculectomy is due to both changes in intravascular volume and interstitial space. CT increases linearly with the large choroidal vessel layer.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

 

1. SD-OCT on the posterior 6 mm surrounding the fovea.
2. Using ImageJ, the choroid was manually isolated.
3. Contrast enhancement with histogram equalization.
4. A threshold value was set to outline vessels for area measurement, which was averaged over 6 mm choroid to obtain CT.
5. LCVL and CT were measured as the distance between inner sclera and inner point of the large choroidal vessel, and RPE, respectively, using Heidelberg software.