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Moorfields
Bleb Grading System
Back to Glaucoma
Jonathan Clarke, Clinical Research Fellow
Department of Wound Healing, Institute of Ophthalmology and Moorfields
Eye Hospital NHS Foundation Trust.
Introduction:
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.
Background:
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.
Figure 1: Example of Standard Photograph
Principles:
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.
Validation:
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
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
www.blebs.net
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