A novel approach to modeling the effect of sub-conjunctival blebs on flow pressure to enhance clinical bleb grading systems

Abstract

Purpose : Grading systems, such as the Indiana Bleb Appearance Grading Scale (IBAGS), the Moorfields Bleb Grading System (MBGS), and the Wuerzburg bleb classification score (WBCS), are used to classify blebs and monitor their progress. None of these grading systems allow for the effect of the bleb on intraocular pressure (IOP) and are relatively subjective, with the risk of inter and/or intra-operative scale variability. We modelled the effect of the sub-conjunctival bleb on flow pressure using both ex-vivo and in-vitro approaches to enhance the clinical grading systems used in the immediate post-operative period.

Methods : A sub-conjunctival bleb was created by inserting a tube into an ex vivo rabbit eye (n=4) using an ab externo approach through the anterior chamber and into the sub-conjunctival space. Sterile dyed water was injected at a constant rate through the tube into the developing bleb. An in vitro silicone bleb was created by clamping a circular silicone sheet; dyed water was injected at a constant rate and drained through a fixed resistance outlet tube. Photographic readings of the bleb height (H) and radius (R) as well as microfluidic measurements of the pressure (P) of the ex vivo and in vitro models were taken as a function of time. The thickness (T) of the conjunctiva membrane and silicone sheet was measured using a pair of calipers while the Young’s Modulus was obtained from uniaxials tests. Mathematical algorithm software was used to post-process the data and build the bleb model.

Results : The in vitro approach demonstrated that the pressure in the bleb (P) is related to its height (H), radius (R), Thickness (T) and Young’s Modulus (E) as follows: P ∼ E H³ T/R⁴.
Good correlation was observed between the in vitro and ex vivo approaches as shown in Figure 1 (R²=0.9).

Conclusions : The pressure in the bleb has a strong dependence on bleb radius and height, with a weak dependence on conjunctival thickness. These results provide support for an enhancement of bleb categorisation system based on a combination of IOP and bleb dimensions. This enhancement may play a greater role in patient management based on bleb appearance in the early post- operative period.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.

 

Ex vivo bleb data plotted with solid markers and the in vitro model curve in black. Typical error bars are shown on one data point for each case.