The filtering bleb is the desired outcome of and most important feature of glaucoma filtration surgery. Nevertheless, for the most part, ophthalmologists have performed trabeculectomy and then regarded the formation or lack of formation of a successful bleb pretty much as an act of God. The oft-repeated admonition among glaucomatologists, that the surgery is only the beginning of the procedure, is finally becoming translated from the realm of abstract thought into reality, with the development of postoperative laser suture lysis and bleb needling procedures. Still, surprisingly little work has been done to characterize different types of blebs, to correlate types of blebs and features of blebs with functionality, and to discover early postoperative signs which will correlate with later bleb appearance and function. The ultrasound biomicroscope offers a solution to many of these problems, and an excellent start has been made by Dr. Tetsuya Yamamoto and his colleagues in Gifu, Japan. Hopefully, their work will stimulate others to investigate further in this important area.

The success of glaucoma filtering surgery depends solely upon whether a well-functioning filtering bleb can be created. Kronfeld classified filtering blebs into three main categories some 50 years ago according to the appearance and function of the bleb. Kronfeld's Type I bleb represents a polycystic, thin-walled bleb. The Type II bleb represents a diffuse, perilimbally-extended bleb. Both types of bleb function well in the majority of cases. Kronfeld's Type III bleb, however, represents a flattened bleb with little or no function. Later, a fourth type, the encapsulated bleb, which looks like a well-demarcated conjunctival cyst and functions poorly, was differentiated as an additional category.

These bleb classifications basically depend on decades-old slit-lamp biomicroscopic findings. The recent development of ultrasound biomicroscopy (UBM) by Pavlin and coworkers enables us to visualize the anterior segment of the eye with microscopic precision, which was previously impossible in the living eye. In addition, the typical appearance of the filtering bleb has greatly changed since the advent of the adjunctive use of antiproliferative agents to the trabeculectomy procedure. Clinically, the filtering blebs in these cases appear diffuse, avascular, thin-walled, and cystic.

We investigated the appearance of filtering blebs following mitomycin-C trabeculectomy using UBM. The images were evaluated for several features, including visibility of a route under the scleral flap, reflectivity inside the bleb and formation of a cavernous fluid-filled space inside the bleb. According to these ultrasound characteristics (Table), we classified the blebs into four main categories: Type L (low-reflective), Type H (high-reflective), Type E (encapsulated), and Type F (flattened).

Our Type L bleb represents a well-functioning, succulent bleb, which is defined by several characteristics of the intrableb structure, including low-reflectivity and an outflow pathway discernible under the scleral flap (Figure). In contrast, Type H, Type E and Type F blebs correspond to less favorable surgical results. The Type H bleb often appears to be a well-functioning bleb, with some vascularity visible using a slit-lamp biomicroscope. Thus, it may be difficult to distinguish the Type H bleb from the Type L bleb without ultrasound biomicroscopy. The Type E bleb corresponds to an encapsulated bleb. Although we can easily recognize a typical encapsulated bleb by slit-lamp biomicroscopy, it is almost impossible to distinguish a small, flat encapsulated bleb from a flattened bleb by slit-lamp biomicroscopy. The UBM image of the Type F bleb is characterized not only by the bleb flatness, but also by an intensified reflectivity of the conjunctiva. In this way, the UBM provides more clinically useful information on intrableb structure than do conventional examination methods, including slit-lamp biomicroscopy.

We recently investigated the relationship between bleb type and associated clinical factors, and presented our data at the 1997 ARVO meeting. Briefly, 67 consecutive eyes of 67 patients were analyzed. All patients had a preoperative IOP ³20 mmHg, a mitomycin-C trabeculectomy, and postoperative UBM imaging. A single researcher classified the bleb typing based on UBM images for each patient in a masked fashion. Type L bleb formation was analyzed by a logistic multiple regression model and was correlated to several clinical factors, such as type of glaucoma or age at surgery. The LOGIST procedure of the PC-SAS statistical package was used for the analysis. The regression model demonstrated that the type of glaucoma and past history of intraocular surgeries correlated significantly with formation of a Type L bleb, eyes with chronic open-angle glaucoma and no previous surgery having the best chance of a well-functioning bleb determined by UBM.

Practically speaking, UBM of filtering blebs in eyes in which IOP breaks out of control provides useful information as to how to deal with the situation. For example, when a scleral route under the scleral flap is invisible in eyes with low intrableb reflectivity, a needling procedure would be indicated to raise the flap. Reoperation would be a better choice when the intrableb reflectivity is very high. UBM imaging should be performed when a trabeculectomized eye suffers from mild IOP elevation with a filtering bleb still discernible by slit-lamp biomicroscopy.

Summarizing, the filtering bleb can be categorized into several distinct types according to UBM images. The information on intrableb structure obtained by UBM gives us a clue as to determining bleb function and methods of managing it when the function begins to decline. Future studies should refine this preliminary knowledge and provide new insights.

Table

Main features of the bleb types

1. Kronfeld PC. The mechanism of filtering operations. Trans pacific Coast Oto-Ophthalmol Soc 1949;30:23-40.

2. Yamamoto T, Sakuma T, Kitazawa Y. An ultrasound biomicroscopic study of filtering blebs after mitomycin C trabeculectomy. Ophthalmology 1995;102:1770-76.

3. Yamamoto T, Ichien K, Sawada A, et al. Clinical factors related to a well-functioning filtering bleb. Invest Ophthalmol Vis Sci 1997;38(Suppl):S165.

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