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Architectural analysis of modified trabeculectomy blebs using spectral-domain optical coherence tomography in a tertiary care centre

1 Department of Ophthalmology, Deben Mahata Medical College and Hospital, Purulia, West Bengal, India
2 Department of Ophthalmology, Anand Hospital and Eye Centre, Jaipur, Rajasthan, India
3 Scientist E, ICMR-RMRCNE, Dibrugarh, Deben Mahata Government Medical College, Purulia, West Bengal, India
4 RIO, Kolkata, West Bengal, India

Date of Submission11-May-2021
Date of Decision15-Sep-2021
Date of Acceptance27-Sep-2021

Correspondence Address:
Nabanita Barua,
21, Rabindranath Tagore Road, P.O.: Bediapara. P.S.: Dumdum, Kolkata - 700 077, West Bengal
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mjdrdypu.mjdrdypu_343_21


Purpose: Slit-lamp examination and intraocular pressure (IOP) monitoring have been used as substitute marker for assessment of bleb functionality for ages. With the advent of optical coherence tomography (OCT), a noninvasive, fast imaging techniques, it can now being used to look for the internal morphology to predict the long-term functional viability of the bleb. Materials and Methods: It is a single centric, observational study analyzing 30 eyes with trabeculectomy and combined procedure (postoperative 2 months to 1 year). Any patient with resurgery, previous conjunctival disease, or systemic antimetabolite therapy was excluded. After all baseline evaluation, imaging of all blebs was done by spectral-domain OCT (RTVue Inc., Fremont, CA). The architecture was described and functional correlation was evaluated. The bleb wall thickness, vertical subconjunctival fluid height, and qualitative assessment of wall reflectivity were recorded for analysis. All data were analyzed Epi-info 7 software. Pearson's correlation and scatter diagram were used to look for nature of correlation between two variables, P < 0.05 was considered significant. Results: Sample was age and sex matched. The mean IOP was 12.5 ± 2.502 mmHg in functional bleb. We found a significant inverse correlation between vertical subconjunctival depths and bleb width by scatter diagram (P < 0.0001). In subgroup analysis, we found functional blebs had lower bleb wall thickness (P = 0.007) and larger vertical subconjunctival fluid pockets (P = 0.003). Conclusions: In vivo architecture of filtering blebs can be objectively described using spectral OCT. Various quantitative and qualitative parameters can be analyzed to document functionality of bleb. Functional blebs had thinner bleb wall and larger subconjunctival fluid, whereas nonfunctional blebs had thick wall with high reflectivity.

Keywords: Bleb analysis, failing bleb, optical coherence tomography evaluation of bleb

How to cite this URL:
Barua N, Goel S, Ghosh P, Mukherjee S, Chakraborti C. Architectural analysis of modified trabeculectomy blebs using spectral-domain optical coherence tomography in a tertiary care centre. Med J DY Patil Vidyapeeth [Epub ahead of print] [cited 2022 Dec 7]. Available from: https://www.mjdrdypv.org/preprintarticle.asp?id=336818

  Introduction Top

Trabeculectomy is one of the universally accepted surgical methods of treating glaucoma since 1968.[1] Various modalities have been suggested to assess the functionality of bleb. Intraocular pressure (IOP) measurement and slit lamp examinations are taken as indirect markers for viability of bleb. Achieving target pressure without any supplementary topical medication is the ultimate goal.

There exist various classification systems based on the morphological analysis: the Indiana Bleb Appearance Grading Scale and Moorfield and Kronfeld classification system are all helpful but these are all qualitative and observer dependent.[2],[3]

Imaging bleb is important to provide finer clue about internal milieu of bleb. This offered an edge over morphological classification as quantitative measurement was possible. With the advent of anterior segment optical coherence tomography (OCT), noncontact, high resolution topographic cross-sectional imaging of anterior segment structures has become possible. This documents internal morphology which could be a better predictor of the functionality of a bleb.[4],[5],[6]

  Materials and Methods Top

The study was conducted in accordance with the ethical standards of the 1964 Declaration of Helsinki and approved by the Local Clinical Research Ethics Committee. Thirty eyes of twenty-two patients who had undergone only trabeculectomy and combined procedure (phaco with trabeculectomy) were included in the study. They were enrolled from glaucoma clinic of the institution over a period of 2 months where they visited as a part of routine postoperative follow-up. Complete ophthalmological examinations were done like vision, refraction, slit-lamp examination for morphological classification, IOP measurement by Goldmann applanation tonometry, and fundus evaluation. Day of examination varied from immediate 2nd month to 1-year postsurgery. This is single centric cross-sectional study, done by single observer to avoid observer bias.

After baseline investigations, all patients were evaluated by OCT after due consent. With RT Optovue OCT with anterior segment module, bleb photographs were taken. Inclusion criteria were operated trabeculectomy/combined surgery from 2 to 12 months. Patients with resurgery, previous failed bleb, evidence of previous conjunctival disease, on antimetabolite therapy were excluded.

Follow-up schedule was at 2-month, 6-month, and 1-year postoperative. All baseline were done at each visit, only 1-day assessment of OCT paramenters was done. OCT was repeated only in case of impending bleb failure. No intergroup comparison between parameters in between groups (combined surgery versus only trabeculectomy) was done as number of patients was less.

All surgeries were performed by a single surgeon at a tertiary care center following local anesthesia (either a retrobulbar or peribulbar injection of 1% lidocaine). All were fornix-based trabeculectomies. A limbal peritomy was created centered at the 12 o'clock position. In high-risk cases, an MMC-soaked cellulose sponge (0.4 mg/ml) was applied subconjunctivally for 2 min and then removed and washed with 30 ml of irrigating fluid. A partial-thickness scleral flap was created followed by a sclerotomy and iridectomy. The scleral flap was closed using 10-0 nylon sutures. The remaining conjunctiva and tenon's capsule were closed using two 8-0 polyglactin sutures at the limbus. In high-risk cases, one releasable suture was also applied. High-risk cases were aphakia, neovascular glaucoma, pigmentary glaucoma, congenital glaucoma, posttraumatic, poststeroid induced, young patients.

In a patient with associated significant cataract, combined surgery was planned. In this procedure, superior peritomy and MMC application were done before phacoemulsification with a temporal clear corneal incision. After completion of phacoemulsification with foldable intraocular lens implantation, single 10-0 nylon suture was applied to clear cornea then superior trabeculectomy was completed.

All patients received a topical antibiotic four times daily for 1 week and a topical corticosteroid four times daily, which was then tapered over a period of 6–8 weeks. Releasable suture was removed manually after 1 week in all cases where filtration was insufficient or IOP too high. Bleb was considered functional if IOP was <18 mmHg without medication at 6 months.


OCT procedure

All subjects were scanned using the RTVue system (Optovue, INC., Fremont, CA). It takes 26,000 A scans/s with speed of 256–4096 scans/frame. It has a depth resolution of 5 μm and a transverse resolution of 15 μm, scan beam wavelength is 840 ± 10 nm, exposure power at the pupil is 750 μW.

Measurements of bleb wall and vertical subconjunctival fluid space were taken for assessment [Figure 1]. Bleb wall thickness or width is defined as the distance between the first reflective signal from the conjunctiva to the top of the subconjunctival fluid space. As bleb wall thickness may vary along the scan, only the maximum distance was measured (a). The subconjunctival fluid space was measured as the maximal thickness of the hyporeflective area directly beneath the bleb wall in the vertical cross-sectional image (b).
Figure 1: Various scans in optical coherence tomography to visualize bleb with inset showing measurement taken in this study, (a) bleb wall thickness, (b) vertical subconjunctival fluid space

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Bleb wall reflectivity was also graded according to the colorization of the images as follows by the device: Grade 1, black-blue low reflectivity; Grade 2, blue-yellow moderate reflectivity; Grade 3, red high reflectivity. It was used for qualitative assessment only. Low bleb wall reflectivity meant reflectivity of the bleb wall was lower than that of healthy conjunctiva. High bleb wall reflectivity means wall had homogenously higher reflectivity than the surrounding healthy conjunctiva. Areas of low reflectivity suggested the presence of fluid in the wall.[7]

All the data were recorded and calculated using Epi-info 7 [Centers for Disease Control and Prevention (CDC), Atlanta, Georgia, USA]. During statistical analysis, we have excluded 2 flattened and 2 encysted blebs to analyze all functional blebs. P values of 0.05 were considered as statistically significant and P value of was 0.001 considered highly significant. Pearson's correlation coefficients were used to assess the correlations between continuous variables. Scatter diagrams were created to look for nature of correlation between 2 parameters. The scatter diagram graphs pairs of numerical data, with one variable on each axis, to look for a relationship between them. If the variables are correlated, the points will fall along a line or curve. The better the correlation, the points will be concentrated along the line.

  Results Top

Sample was age and sex matched. [Table 1]a and [Table 1]b showed gender distribution and sample characteristic. We had 15 cystic and 11 diffuse blebs in this study [Table 2]. The study demonstrated 4 types of blebs cystic, diffuse, encysted and flattened [Figure 2]. [Table 3]a showed mean IOP control in cystic bleb type was 13.33 ± 5.4 mmHg, in diffuse variety was 11.45 ± 3 mmHg. This difference was statistically significant (P = 0.031 [<0.05]), which meant control was better with diffuse blebs. The mean bleb wall thickness or width was higher in diffuse (0.17 ± 0.12 mm) was higher than cystic bleb (0.12 ± 0.8 mm), P value is 0.031 (<0.05).
Table 1:

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Table 2: Distribution of subjects according to bleb type and sex (n=26)

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Figure 2: Four types of bleb found in study. 1: Cystic, 2: Diffuse, 3: Encapsulated, 4: Flattened

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Table 3:

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Mean subconjunctival fluid (vertical height in mm) in cystic variety was 0.13 ± 0.08, in diffuse bleb measurement was 0.08 ± 0.04. This difference was statistically significant (P = 0.014 [<0.05]), which meant that cystic blebs had larger fluid cavity.

[Table 3]b shows IOP has no significant correlation with bleb wall thickness or width and subconjunctival vertical fluid height, however, the direction was inverse. To further study the nature of direction, we analyzed data in scatter diagram. The result showed the correlation of direction was not statistically significant. Pearson's correlation coefficient was (−) 0.105, P value = 0.611 (>0.05) [Figure 3]. Similar analysis was done with IOP and vertical wall thickness. Results were similar, statistically not significant; Pearson's correlation coefficient was (−) 0.084, P value = 0.684 (>0.05) [Figure 4]. In this study, there was no relation between IOP and bleb wall thickness and subconjunctival vertical fluid height [Figure 3] and [Figure 4].
Figure 3: Scatter diagram showing correlation of intraocular pressure with bleb wall thickness

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Figure 4: Scatter diagram showing correlation of intraocular pressure with subconjunctival vertical fluid height

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We analyzed vertical subconjunctival depths against bleb width by scatter diagram. It was statistically highly significant. Pearson's correlation coefficient was (−) 0.656, P < 0.0001 [Figure 5]. It showed strong inverse correlation between the parameters. Thinner was the bleb wall, larger was the fluid pocket. This is usually a feature of functional bleb: Larger fluid pocket and thinner bleb wall.
Figure 5: Scatter diagram showing correlation of bleb wall thickness with subconjunctival vertical fluid height

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Correlation between external bleb appearance and internal architecture:

As it is evident from [Table 3]a that IOP is well controlled in both cystic and diffuse bleb (no statistically significant difference found in between the two, P=0.05). The bleb wall thickness is higher in diffuse than cystic bleb, P = 0.031 (<0.05). Height of subconjunctival fluid pocket was higher in cystic bleb, P = 0.014 (<0.05). [Table 4] showed that there was no statistical significant correlation of IOP with bleb wall width or subconjunctival fluid, but direction is inverse. However, height of subconjunctival fluid pockets is inverse correlated with bleb wall, so in case bleb wall becomes thinner, larger subconjunctival fluid pockets will be seen. It is also seen from [Table 5] that wall thickness or width is more in nonfunctional bleb with smaller fluid pockets and higher bleb wall reflectivity as compared to functional bleb.
Table 4: Difference between Visante and RTVue FD-optical coherence tomography[14]

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Table 5: Comparison between functional and nonfunctional bleb

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Clinical correlation between cystic bleb and IOP is seen but subconjunctival fluid height as measured by OCT does not correlate with IOP. From [Table 3]a and [Table 5], we could see IOP is very well maintained in cystic and diffuse bleb as compared to encapsulated and flat blebs. The difference of bleb wall thickness or width and subconjunctival fluid pockets was statistically significant in two groups. Now, in this study bleb, wall thickness or width and subconjunctival fluid pockets did not correlate well with IOP though they have shown inverse direction. Hence, in case of high IOP, there will be smaller fluid pockets, which is a feature of nonfunctional bleb.

We had 26 functional and 4 nonfunctional blebs in the study tenure. Although there were less number of nonfunctional blebs, to look for difference between two groups, we compared the data between these groups [Table 5]. IOP was significantly higher in nonfunctional group (26 ± 5.888 mmHg as opposed to 12.5 ± 2.502 mmHg in functional group; P < 0.0001). Bleb wall thickness was significantly thinner (0.1442 ± 0.054 mm) in functional group as compared to nonfunctional (0.2975 ± 0.2598 mm) bleb (P = 0.007, P < 0.05). Vertical fluid height was significantly higher (0.1115 ± 0.0461 mm) as compared to nonfunctional bleb (0.0375 ± 0.0096 mm) (P = 0.003, P < 0.05). Hence, in our study, functional blebs had thinner bleb wall thickness and larger subconjunctival fluid pockets.

  Discussion Top

In the evaluation of functionality trabeculectomy, assessment of slit lamp-based grading systems, measurement of IOP has been well documented. A diffuse, mild elevated bleb, the presence of microcysts, and low conjunctival vascularization are suggestive of a successful bleb. In contrast, excessive elevation and dense vascularization are poor prognostic indicators. Various imaging-based studies have measured (OCT, ultrasound biomicroscopy, or confocal microscopy) of bleb and surrounding conjunctival tissue to quantitatively measure internal parameters.[5],[6],[7],[8]

Yamamoto et al. have analyzed the trabeculectomy blebs into four types looking at the internal architecture and the reflectivity by ultrasound biomicroscopy. They were low reflective, high reflective, encapsulated, flattened; good IOP control was found in the first group.[9] In our study, rest 3 kinds of bleb were associated to poor IOP control. All nonfunctional blebs had high reflectance. These findings also corresponded with previous studies.[10],[11],[12],[13]

Savini et al. reported the use of the Stratus OCT (Carl Zeiss Meditec, Dublin, California, USA) to image functional filtering blebs. Trabeculectomy without antimetabolites was associated with type A blebs (thick wall and a single large fluid-filled space), mitomycin-C trabeculectomy with type B blebs (thin wall and multiple large fluid-filled spaces), mitomycin-C phacotrabeculectomy with type C blebs (multiple flattened fluid-filled spaces).[10] We also have found all these types of functioning blebs.

At present, we have 4 different types of commercially available OCT for anterior segment imaging, Stratus OCT (Zeiss), Visante (Zeiss), SL-OCT (Heidelberg), RTVue FD-OCT (Optovue).[14] [Table 4] highlights the difference between Visante and RTVue FD-OCT (used in study).

It is evident from [Table 4] that Visante (Carl Zeiss Meditec, Inc., Dublin, CA) is slower but its longer wavelength penetrates more deeply. It is fastest TD OCT and provided 16-mm scan width and almost 6-mm scan depth in tissue are sufficient for AC biometry. Depth resolution is approximately 17 μm full-width-half-maximum in tissue. RTVue (Optovue, Inc., Fremont, CA), on the contrast, FD-OCT, is 13 times faster and can provide a depth resolution that is over three times higher (5 μm) than the Visante. It is predominantly used for posterior segment imaging. When used with Cornea Anterior module (CAM-S, CAM-L), RTVue- can also be used for anterior segment imaging. Due to faster image acquisition, it reduces motion error and generates higher definition (more A-scans per image). Two types of scans can be taken for 3D construction of bleb, parallel and perpendicular to limbus. Because of the short wavelength (840 nm), parallel scans cause lot of scatter, more of perpendicular scans are required.[15],[16]

In this study, we described and quantified the internal features of different bleb morphologies with reference to the observed clinical appearance of the modified or augmented bleb. The higher scanning resolution in RT-OPTOVUE OCT images allows us to differentiate two levels of intra-bleb fluid-filled spaces, the subconjunctival fluid collections, and the supra-scleral fluid space. The study parameters can be compared to only modified blebs as we have not included only trabeculectomies in the study.

Bleb wall thickness was significantly thinner in functional group as compared to nonfunctional bleb (P = 0.007, P < 0.05). Vertical fluid height was significantly higher as compared to nonfunctional bleb (P = 0.003, P < 0.05). Hence, in our study, functional blebs had thinner bleb wall thickness and larger subconjunctival fluid pockets. We also established inverse relationship between bleb wall and vertical fluid pocket. Functional blebs have larger fluid pocket and thinner bleb wall (P < 0.0001). Güven Yilmaz et al. found functional blebs exhibited significantly greater base width, height, and wall thickness compared with nonfunctional blebs (P < 0.05).[17]

Now in this study bleb wall thickness or width and subconjunctival fluid pockets did not correlate well with IOP though they have shown inverse direction. Ciancaglini et al. found no differences between functional and nonfunctional blebs in any of the biometric parameters of the bleb wall except reflectivity.[18] Singh et al. found significantly thicker walls in functional blebs versus nonfunctional blebs.[19] According to their study hypothesis, the bleb thickness is the important sign of successful bleb and height was more attributable to tissue expansion. Bleb wall consists of two components, conjunctival and subconjunctival tissue. In successful blebs, subconjunctival tissues absorb more fluid and expand and bleb wall thickness. In their study, the height and the extent of the internal cavity also were not significantly correlated with the IOP. These findings correlated well with our study results.

There are various recent studies stating that width of filtration opening at an early stage is an important prognostic factor for long-term surgical success of trabeculectomy.[20],[21],[22] Future cohort study with more bleb parameters may be needed to establish this. In our study, we have taken measurement of fluid pockets and bleb wall to look for the ultimate viability of bleb.

  Conclusions Top

OCT with anterior segment module is more widely available as compared to only anterior segment OCT, can be used for multiple purpose and more cost effective, offers valuable information comparable quantitatively. According to this study, bleb with thin wall, good height, fluid pockets controls IOP well. Hence, the absence of the subconjunctival fluid collections, appearance of high reflective spike in immediate postoperative might give clue toward impending failing bleb. Combining both clinical and imaging information could provide a new perspective toward understanding ultimate outcome after trabeculectomy and need for early intervention like augmentation of frequency of drugs, bleb needling.


There are various limitations of the current study like small sample size. More patients needed to included for similar analysis to look for intergroup comparison. We can also include only trabeculectomy patients in the study to look for difference of parameters between only trabeculectomy and modified trabeculectomy. We have used OCT RT-Optovue. It uses short wavelengths, anterior chamber angle structures not visualized in all cases. It also does not correct for refractive error. There were lot of limbal scatter in parallel scans, so we needed more perpendicular scans to acquire internal scans.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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Vesti E. Filtering blebs: Follow up of trabeculectomy. Ophthalmic Surg 1993;24:249-55.  Back to cited text no. 2
Cantor LB, Mantravadi A, WuDunn D, Swamynathan K, Cortes A. Morphologic classification of filtering blebs after glaucoma filtration surgery: The Indiana bleb appearance grading Scale. J Glaucoma 2003;12:266-71.  Back to cited text no. 3
Wells AP, Crowston JG, Marks J, Kirwan JF, Smith G, Clarke JC, et al. A pilot study of a system for grading of drainage blebs after glaucoma surgery. J Glaucoma 2004;13:454-60.  Back to cited text no. 4
Pavlin CJ, Harasiewicz K, Foster FS. Ultrasound biomicroscopy of anterior segment structures in normal and glaucomatous eyes. Am J Ophthalmol 1992;113:381-9.  Back to cited text no. 5
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Tominaga A, Miki A, Yamazaki Y, Matsushita K, Otori Y. The assessment of the filtering bleb function with anterior segment optical coherence tomography. J Glaucoma 2010;19:551-5.  Back to cited text no. 7
Van Buskirk EM. Cysts of Tenon's capsule following filtration surgery. Am J Ophthalmol 1982;94:522-7.  Back to cited text no. 8
Yamamoto T, Sakuma T, Kitazawa Y. An ultrasound biomicroscopic study of filtering blebs after mitomycin C trabeculectomy. Ophthalmology 1995;102:1770-6.  Back to cited text no. 9
Savini G, Zanini M, Barboni P. Filtering blebs imaging by optical coherence tomography. Clin Exp Ophthalmol 2005;33:483-9.  Back to cited text no. 10
Powers TP, Stewart WC, Stroman GA. Ultrastructural features of filtration blebs with different clinical appearances. Ophthalmic Surg Lasers 1996;27:790-4.  Back to cited text no. 11
Addicks EM, Quigley HA, Green WR, Robin AL. Histologic characteristics of filtering blebs in glaucomatous eyes. Arch Ophthalmol 1983;101:795-8.  Back to cited text no. 12
DeBry PW, Perkins TW, Heatley G, Kaufman P, Brumback LC. Incidence of late-onset bleb-related complications following trabeculectomy with mitomycin. Arch Ophthalmol 2002;120:297-300.  Back to cited text no. 13
Leung CK, Weinreb RN. Anterior chamber angle imaging with optical coherence tomography. Eye (Lond) 2011;25:261-7.  Back to cited text no. 14
Ramos JL, Li Y, Huang D. Clinical and research applications of anterior segment optical coherence tomography – A review. Clin Exp Ophthalmol 2009;37:81-9.  Back to cited text no. 15
Tominaga A, Miki A, Yamazaki Y, Matsushita K, Otori Y. The assessment of the filtering bleb function with anterior segment optical coherence tomography. J Glaucoma 2010;19:551-5.  Back to cited text no. 16
Güven Yılmaz S, Değirmenci C, Palamar M, Yağcı A. Evaluation of filtering bleb function after trabeculectomy with mitomycin C using biomicroscopy, anterior segment optical coherence tomography and in vivo confocal microscopy. Turk J Ophthalmol 2015;45:132-7.  Back to cited text no. 17
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Watanabe-Kitamura F, Inoue T, Kojima S, Nakashima KI, Fukushima A, Tanihara H. Prospective 3D investigation of bleb wall after trabeculectomy using anterior-segment OCT. J Ophthalmol 2017;2017:8261364.  Back to cited text no. 20
Tsutsumi-Kuroda U, Kojima S, Fukushima A, Nakashima KI, Iwao K, Tanihara H, et al. Early bleb parameters as long-term prognostic factors for surgical success: A retrospective observational study using three-dimensional anterior-segment optical coherence tomography. BMC Ophthalmol 2019;19:155.  Back to cited text no. 21
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