Moorfields Bleb Grading System

Jonathan Clarke, Clinical Research Fellow
Department of Wound Healing, Institute of Ophthalmology
and Moorfields Eye Hospital NHS Foundation Trust.


The trabeculectomy has been modified in a number of ways since first described by Cairns (Cairns 231-33). However the principle remains that a fistula is created between the anterior chamber and the subconjunctival space lifting the conjunctiva and forming a characteristic bleb. The aqueous is reabsorbed by the episcleral vessels and intraocular pressure controlled. Glaucoma affects around 70 million people and is the second most common cause of blindness worldwide. Furthermore it is the leading cause of irreversible blindness with estimates of blindness in around 10% of those affected. Trabeculectomy is the treatment of choice in patients who fail to respond to topical treatment and is the first line of treatment in underdeveloped countries(Quigley 389-93). Over 300 trabeculectomies are performed each year at Moorfields Eye Hospital.


The Moorfields bleb grading system was developed as the importance of bleb appearance to outcome was realised (Picht and Grehn 2-8). Blebs that develop thin avascular zones are at increase risk of leakage and late hypotony as well as sight threatening bleb related infections (DeBry et al. 297-300). Bleb morphology changes with time and it is hoped that the system could be utilised to track these changes and to predict factors that lead to unwanted bleb appearance and associated loss of intraocular pressure. Previous grading systems were unable to describe the varied features of blebs that were considered important in defining surgical success(Cantor et al. 266-71;Picht and Grehn 2-8;Crowston J).

The current system was designed with standard photographs taken from the 5FU trial database. The system of ratification is described below and potential uses for the system.

A Bleb Grading Protocol was developed (See appendix 1). Graders made judgment about the appearance of the bleb from standard mono photograph of superior conjunctiva.

The system relies on the ability to provide standard photographs of the superior conjunctiva. It is hoped that most digital cameras can provide these photographs. The standard photograph is an image with the eye looking inferiorly to display the largest area available of the superior conjunctiva. The canthal margins should be the horizontal limits of the photograph. The images are viewed under magnification.

Standard Photograph Standard Photograph
Figure 1. Example of Standard Photograph


In this grading system the grader is asked to make judgment about the appearance of the bleb. Standard reference photos are provided (see appendix). Where the bleb being graded does not exactly match the reference photographs then the best match should be selected.

When describing a bleb, 3 main aspects have to be considered: area, height and vascularity.

There are six criteria to assess: 2 describing area, 1 describing height and 3 describing vascularity. A grading form and introductory information is provided for the graders.


100 serial, standard photographs taken 4 weeks after trabeculectomy surgery were supplied to the Moorfields Eye Hospital Reading Centre. Grading of the photo was based on the Grading Protocol and performed by two trained independent graders. Each grader regarded 30 of the images at least 2 weeks after initial grading to allow intra-observer reliability to be calculated. The graders were not ophthalmologists and an introductory article concerning trabeculectomy and bleb appearance was provided.

High level of agreement was found between the 2 readers. Inter-observer percentage agreement: Central bleb area (1a): 91%. Maximal bleb area (1b): 91%. Height (2): 94%. Central bleb vascularity (3a): 93%. Maximal bleb vascularity (3b): 85%. Non bleb vascularity (3c): 86%. Subconjunctival haemorrhage: 99%. In the remaining % the disagreement between the readers was no greater than one score. Kappa scores: Kappa scores are measurements of agreement. They range between 0 to 1, with 1 indicating perfect agreement. A value above 0.60 indicates substantial agreement. Inter-observer kappa scores were high: 1a:0.870, 1b:0.857, 2:0.903, 3a:0.899, 3b:0.864 3c:0.779, subconjunctival blood:0.98.

Inter-observer Kappa scores
Figure 2. Inter-observer Kappa scores

The individual components of the bleb are described in detail and the system has been presented at an international meeting (Clarke JCK). The system represents the final stage of development and is a modification of previous work (Marks JR).

Publication of the system in a peer-reviewed journal is in preparation.

Reference List

Cairns, J. E. "Trabeculectomy. Subtitle: "a surgical method of reducing intra-ocular pressure in chronic simple glaucoma without sub-conjunctival drainage of aqueous humour"." Trans.Ophthalmol.Soc.U.K. 88 (1969): 231-33.

Cantor, L. B. et al. "Morphologic classification of filtering blebs after glaucoma filtration surgery: the Indiana Bleb Appearance Grading Scale." J.Glaucoma. 12.3 (2003): 266-71.

Clarke JCK. A System for Grading Filtration Blebs Following Trabeculectomy. A.P.Wells, C. D. Sangermani J. R. Marks J. G. Crowston T. Peto I. Leung P. T. Khaw. Presentation 1201.ARVO 2003. 2003. Ref Type: Abstract

Crowston J. Distant Evaluation of Postoperative Trabeculectomy Cases. . Wells AP, Kirwin J Kennedy C Murdoch I. Clin.Experiment.Ophthalmol. 2000 (Abstr); 134. 2000. Ref Type: Abstract

DeBry, P. W. et al. "Incidence of late-onset bleb-related complications following trabeculectomy with mitomycin." Arch.Ophthalmol. 120.3 (2002): 297-300.

Marks JR. A Comparison of Two Grading Systems for Filtering Blebs. Wells AP, Frangouli A Khaw PT. Invest Ophthalmol Vis Sci (Suppl) 2000. 2003. Ref Type: Abstract

Picht, G. and F. Grehn. "Classification of filtering blebs in trabeculectomy: biomicroscopy and functionality." Curr.Opin.Ophthalmol. 9.2 (1998): 2-8.

Quigley, H. A. "Number of people with glaucoma worldwide." Br.J.Ophthalmol. 80.5 (1996): 389-93.

The Moorfields Bleb Grading System is available on a dedicated website

© Copyright 2008-2017. The Reading Centre, Moorfields Eye Hospital NHS Trust Foundation. All rights reserved.