Our results showed that, in most eyes with functional filtration blebs, we could use 3D AS-OCT to identify the exact filtration openings from the scleral flap margin into the bleb space. The 3D technique allowed us to evaluate the surgical wounds and internal structures of the filtration blebs.
9,10 In cases of bleb revision, we could confirm that aqueous humor could flow out from the patent internal ostium through a filtration opening during surgery. Also, preoperative and postoperative 3D AS-OCT images clearly showed the presence of filtration openings because of the pits and/or troughs in the fluid-filled cavities. C-scan images provided precise identification of the filtration openings along the scleral flap margin.
11 On the basis of our experience, in the present study, we used 3D AS-OCT and custom software to define filtration openings on the basis of pits and/or troughs in fluid-filled cavities in both horizontal and vertical rasters and corresponding C-scan images of scleral flap margins. However, to do this, we had to clearly see the scleral flap margins in the 3D AS-OCT images. It was relatively easy for us to identify the scleral flap margins when the flap was created at the uppermost region, but not in the temporal (or nasal) scleral flaps. This difficulty was caused by problems in identifying a complete scleral flap in the C-scan images. By using our novel custom software, however, we could easily rotate and tilt our 3D AS-OCT images on the screen display. Thus, in this study, our clinical data for glaucomatous eyes after trabeculectomy clearly showed the results of the filtration operations.
First, with the aid of our novel software and 3D AS-OCT, we precisely identified filtration openings in 118 (95%) of 124 eyes after trabeculectomy. In 90 (76%) of these 118 eyes, we found only a single filtration opening. This observation seems to agree with our previous results in which only a single filtration point was present on the scleral flap margin. In eyes after trabeculectomy, aqueous humor can flow out from any point on the scleral flap margins, at least during the early postoperative period. As the postoperative period continues, wound-healing activity causes more sealing of the scleral flap margin. During this sealing phase, the scleral flap margin, with no or scant aqueous flow, is quite likely easily sealed by wound healing. In contrast, sustainable filtration opening forms at the leakiest point on the scleral flap margin. As shown in
Figure 4, most filtration openings were localized on the middle of the scleral flap margins. This predominant localization on the middle may result from the complicated combination of various factors for postoperative wound-healing activities. Among them, the pattern of incision and sutures used for closure of the conjunctival flap affected the localization of filtration openings. The bleb with fornix
-based conjunctival flap had fewer filtration openings around the limbus (TFD ≥ 3.0 mm) compared with blebs with a limbal-based conjunctival flap. This difference may be caused by conjunctival sutures along the limbus that might close the fistula under the scleral flap near the limbus. These findings agree with the characteristic shape of clinically observed blebs with a fornix-based conjunctival flap, which has a more diffuse width and lower height compared with blebs that are limbal-based.
Second, in 28 eyes (24%), multiple filtration openings were observed on the margins of the same scleral flap. The formation of two or more filtration openings may be caused by low wound-healing activity and/or the existence of similar aqueous flow resistance at multiple points. As clinicians, we should pay attention to the fact that the presence of multiple filtration openings cannot always ensure potent IOP-reducing effects, because no significant differences in IOP values were found between F1 and F ≥ 2 groups. Also, some F1 eyes had an elevated IOP (>20 mm Hg). Thus, the presence of filtration openings in the blebs cannot guarantee enough IOP reduction to inhibit progression of glaucomatous optic neuropathy. Further investigation will be needed for determination of the clinical significance for IOP management.
Because our criteria are based on the presence of a fluid-filled cavity and a visible scleral flap margin, we could not identify filtration openings in some eyes with an elevated and/or high-reflectivity bleb (type H) or with a sponge-like structure masking the fluid-filled cavity (type S bleb). Neither type possessed fluid-filled cavities, but the significance of this finding differs for each type. Because AS-OCT has limited penetration,
7,8,10,12 bleb walls with high reflectivity and large fluid-filled cavities cause attenuation of light, which results in shadows. Although, as discussed by Kawana et al.,
10 3D AS-OCT using a 1.3-μm light source has greater penetration than does conventional OCT with an 830-nm light source, our present study suggested that, in some cases of elevated blebs (type H), this limited penetration of 3D AS-OCT still results in a failure to identify filtration openings. Also, because thick bleb walls with low reflectivity are thought to be associated with good IOP control, high reflectivity in thick bleb walls may suggest the occurrence of a scarring response and induce increased IOP because of inhibited filtration.
8,10,13,14 Also, one group of investigators reported that the internal reflectivity of filtration blebs is significantly correlated with IOP values.
15 This result is supported by findings that, in encapsulated blebs, a thick bleb wall with high reflectivity was revealed by AS-OCT
6 and that high reflectivity in AS-OCT images corresponds to the dense scar tissue that we observed in our previous histopathologic studies of excised blebs.
11 The occurrence of encapsulated blebs is thought to be related to inflammatory environment and excessive scarring processes, reported to be caused by the use of preoperative sympathomimetics, a history of argon laser trabeculoplasty, and juvenile glaucoma, which would result in a new increase in the IOP.
16,17
Formation of filtration openings occurs predominantly during early postoperative periods, and one possible application of our knowledge about filtration openings to clinical practice may be in laser suture lysis. Previously, a group in Singapore reported an increased bleb height and decreased reflectivity on an AS-OCT image after laser suture lysis.
18,19 In this present study, with the aid of 3D AS-OCT, we could compare the location of filtration openings. As an interesting finding, our preliminary prospective observation of a small case series of laser suture lysis showed that filtration openings after laser application were the same as those before laser treatment. Also, a scatterplot analysis of filtration openings in type F blebs demonstrated that opening locations were similar despite the pattern of remaining sutures and cut nylon sutures. In view of these data, we hypothesized that laser suture lysis cannot usually change the aqueous outflow route, but it can enlarge the already formed filtration route. If this hypothesis is correct, laser suture lysis may be more effective when a suture near a filtration opening is cut by using a laser. We are planning to conduct a prospective study to elucidate the relationship between laser suture lysis and filtration openings.
With regard to multiloculated blebs, type S blebs evidenced a multiloculated structure with low reflectivity. In previous reports, the description of a multiloculated structure suggested the existence of active filtration.
7,13 However, in our previous experience of bleb revision, during the surgery we confirmed that a multiloculated bleb structure did not always mean active influx into the loculated part of the bleb. However, in general, thick and low-reflectivity bleb walls in 3D AS-OCT images are thought to be associated with good IOP control.
7,8,10,13,14 We therefore believe that both multiloculated structures and thick bleb walls with low reflectivity may indicate inhibited wound-healing activity such as proliferation of fibroblasts, production or accumulation of extracellular matrices, and contraction of scar tissue in the bleb, possibly resulting in good IOP control. Although the sample number was small in the present study, the IOP of four eyes with type S blebs was well controlled, which agrees with the above-mentioned data. This finding also suggests the presence of an active filtration opening, but it is quite likely concealed by the sponge-like structure, through which aqueous humor can easily flow. Also, because the multiloculated structure is often associated with increased bleb height, shadows as mentioned earlier may cause a vague image of the scleral flap margin. More resolving power may be required to establish new criteria for diagnosis of type S blebs.
Since the present study is cross-sectional, the indications for fornix-based or limbal-based were not defined by a unified way. This could affect the identification of the filtration opening, and the issue should be addressed in prospective studies in the future.
In conclusion, our present study showed that, in most eyes with functional filtration blebs, we could identify the precise filtration openings from the scleral flap margin into the bleb space with the use of 3D AS-OCT and our new custom software. The 3D technique allows complete evaluation of surgical wounds and internal structures of the filtration blebs, which suggests the clinical usefulness of 3D AS-OCT in treatment decision making after trabeculectomy.