internal reflectivity of filtering blebs versus ... internal reﬂectivity of filtering blebs versus...

Internal Reflectivity of Filtering Blebs versusIntraocular Pressure in Patients withRecent Trabeculectomy

Lukas Pfenninger, Fabienne Schneider, and Jens Funk

PURPOSE. The authors conducted a morphology analysis offiltering blebs with anterior segment optical coherence tomog-raphy (AS-OCT) in patients shortly after trabeculectomy to findfurther criteria for success or failure of a filtering bleb.

METHODS. Thirty-seven patients (43 eyes) who had recently(�2 years) undergone trabeculectomy were measured 1 to 4times with AS-OCT. In contrast to ultrasound biomicroscopy,AS-OCT eliminates the need for a water bath in contact withthe globe. This is of particular value in a study of morphologicparameters that may be altered by contact and in the postop-erative period when concerns of infection are paramount. Inthis study, the internal reflectivity of the fluid-filled cavity of thefiltering bleb was compared with the intraocular pressure(IOP).

RESULTS. Mean age of the patients was 69.5 years (range, 27–84years). Average preoperative IOP of the 43 eyes was 20.6 mmHg (range, 10–42 mm Hg), average postoperative IOP at thetime of AS-OCT measurement was 10.6 mm Hg (range, 2–40mm Hg), and mean IOP reduction was 10.0 mm Hg (48.5%).Internal reflectivity of the fluid-filled cavity of the filtering blebcorrelated very well to the IOP (R2 � 0.3762, n � 61, P �0.0001).

CONCLUSIONS. AS-OCT is a promising tool with which to analyzethe morphology of the filtering bleb after trabeculectomy. Ahighly significant correlation between the internal reflectivityof the fluid-filled cavity of the filtering bleb and the IOP couldbe shown that may have future impact in evaluating the post-operative success or failure of filtering blebs. (Invest Ophthal-mol Vis Sci. 2011;52:2450–2455) DOI:10.1167/iovs.10-5520

Trabeculectomy is the surgical treatment of choice for pa-tients with uncontrolled glaucoma. The procedure was

introduced by Cairns in 1968.1 The success and outcome oftrabeculectomy depends on the formation of a filtering bleb,which is an elevation of the conjunctiva over the sclerotomysite.2–5 The bleb allows the filtration of aqueous humor, thuslowering the intraocular pressure (IOP).6

Several grading systems have been proposed to classify andcharacterize bleb morphology, which is an indicator of blebfunction and bleb-related complications and, therefore, an im-portant clinical parameter.3,5,7–10

In this study we measured filtering blebs with an anteriorsegment (AS) optical coherence tomograph (OCT; Visante,model 1000; Carl Zeiss Meditec Inc., Dublin, CA). The princi-ples of OCT11 and its application in imaging the AS are welldescribed. In the context of glaucoma,12–20 AS-OCT has alsobeen used in the assessment of anterior chamber angles.17

Unlike ultrasound biomicroscopy, imaging with AS-OCTrequires no contact with the globe.21–27 This is an advantage ina study of morphologic parameters that may be altered bycontact and during the postoperative time when infections aredreaded.

We aimed to estimate the applicability of the AS-OCT toanalyze the internal bleb structure, especially the reflectivity ofthe fluid-filled bleb cavity, and to show a possible correlationbetween AS-OCT results and clinical examination (i.e., mea-surement of the IOP).

METHODS

This study was approved by the local ethics committee (SPUK; Uni-versity Hospital of Zurich, Zurich) and was conducted in accordancewith the ethical standards of the Declaration of Helsinki.

We included 37 glaucoma patients who had undergone trabeculec-tomy at the University Hospital of Zurich within the past 2 years beforeinitiation of the study. Patients with cystic filtering blebs were ex-cluded. Written informed consent was obtained from all patients. Weexamined trabeculectomized eyes with the Zeiss Visante OCT Model1000, which is a high-resolution tomographic and biomicroscopicdevice specifically designed for anterior segment imaging and measure-ment. It has an axial resolution of 18 �m. The principle is based onlow-coherence interferometry using a 1310-nm super-luminescentlight-emitting diode. As does ultrasound B-scan, the Visante OCT ac-quires multiple A-scans and aligns them to construct two-dimensionalimages. Scanning of the AS structures is made according to a measuringprinciple similar to that used for scanning of the retina, which has beendescribed previously.11

Given that intraocular tissues show different optical properties, theOCT is capable of defining the internal structures (e.g., fluid collection,conjunctival/scleral tissue reflectivity) in precise detail. In the study ofIzatt et al.13 the usability of the OCT in the anterior eye was demon-strated in healthy subjects for the first time. Radhakrishnan et al.12

measured healthy subjects with a hand-held AS-OCT. Miura et al.4

examined four eyes with AS-OCT, and the morphologic structure wasdiscussed. Leung et al.14 described the intrableb morphology with theAS-OCT in 14 patients and grouped the intrableb reflectivity into low,medium, and high. Kawana et al.28 analyzed 38 filtering blebs in 31patients of Asian origin with 3D-CAS OCT. A significant correlationbetween the volume of the hyporeflective reticulate area and the IOPcould be shown.28 In our study, the calculated internal reflectivity ofthe fluid-filled cavity of the filtering bleb was compared with the IOPobtained on the same day by applanation tonometry. All measurementswere performed by an examiner masked to the study and to thepatient’s data. OCT scans were conducted at the same background

From the Department of Ophthalmology, University Hospital Zu-rich, Zurich, Switzerland.

Submitted for publication March 14, 2010; revised June 13 andOctober 9, 2010; accepted November 30, 2010.

Disclosure: L. Pfenninger, None; F. Schneider, None; J. Funk,None

Corresponding author: Lukas Pfenninger, Department of Ophthal-mology, Zurich University Hospital, CH-8091 Zurich, Switzerland;[email protected].

Glaucoma

Investigative Ophthalmology & Visual Science, April 2011, Vol. 52, No. 52450 Copyright 2011 The Association for Research in Vision and Ophthalmology, Inc.

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illumination, with the window blinds completely shut and the roomlight turned down to the minimum with a dimmer. Acquisition of databy chart review was undertaken to document relevant clinical data,

including the patient’s initials, age, type of glaucoma, date of trabecu-lectomy, side, morphology of the bleb by slit lamp examination as lastdocumented before OCT measurement, date of AS-OCT-measurement,

TABLE 1. Data for All Included Patients

PatientInitials

Age(y)

Type ofGlaucoma

Date ofTE Side

Morphology ofBleb

Date ofAS-OCT IOP Pre-op

IOP Post-opat AS-OCT

IOP RedPre-Post TE

IOP RedPre-Post TE (%)

BM 27 PDG 5-16-07 OD Prominent, MC� 5-22-07 25 3 22 88BK 55 POAG 5-16-07 OD Flat, MC� 5-22-07 30 9 21 70BR 76 PACG 11-22-06 OS Flat, MC� 5-15-07 18 8 10 55

11-22-06 OS MC� 10-22-07 18 6 12 67BW 73 POAG 9-26-07 OD Prominent, MC� 10-17-07 10 9 1 10DT 57 POAG 2-07-07 OD Prominent, MC� 5-15-07 16 13 3 19EM 72 PEX 10-18-06 OD Flat 5-21-07 16 10 6 38

11-22-06 OS Flat 5-21-07 16 10 14 58GM 84 POAG 6-27-07 OS Prominent 10-18-07 14 8 6 43GP 71 PEX 4-27-07 OD Flat (MC) 5-2-07 36 34 2 6

4-27-07 OD Prominent, (MC�) 5-8-07 36 13 23 644-27-07 OD Prominent, MC� 5-15-07 36 4 32 894-27-07 OD Flat, MC? 10-17-07 36 12 24 67

HU 63 Uveitis 2-13-07 OS Flat, MC� 5-7-07 26 18 8 31IH 79 POAG 3-7-07 OD Prominent, MC� 5-8-07 14 3 11 79

3-7-07 OD Prominent, MC� 5-10-07 14 5 9 643-7-07 OD Flat, MC� 10-22-07 14 10 4 29

KE 66 PEX 11-29-06 OD Prominent, MC� 5-7-07 11 6 5 46KS 63 PACG 12-11-06 OD Flat 5-22-07 12 12 0 0KM 79 PEX 5-11-07 OD Prominent, MC� 5-21-07 18 6 12 67

5-11-07 OD Flat, MC� 10-17-07 18 12 6 33KM 64 PEX 7-2-07 OS Prominent, MC� 10-19-07 30 13 17 57LT 82 POAG 11-29-06 OS Prominent 5-15-07 22 7 15 68

11-29-06 OS Prominent 10-23-07 22 8 14 64MR 78 PEX 2-27-07 OS Prominent, MC� 5-7-07 18 6 12 67MA 46 POAG 10-17-07 OD Prominent, (MC�) 10-23-07 18 10 8 45RO 78 POAG 1-3-07 OD Prominent, (MC�) 5-21-07 16 13 3 19RC 83 PEX 5-11-07 OD Prominent, MC� 5-15-07 27 11 16 60

5-11-07 OD Prominent, MC� 10-22-07 27 12 15 56SA 49 Uveitis 8-23-06 OS Prominent, (MC�) 4-30-07 18 9 9 50

8-23-06 OS Prominent, (MC�) 5-22-07 18 18 0 0SE 58 POAG 9-19-06 OD Prominent, (MC�) 5-2-07 17 13 4 24

9-20-06 OD Prominent 5-16-07 17 10 7 419-20-06 OD Prominent 10-23-07 17 10 7 412-15-06 OS Prominent 5-2-07 16 8 8 502-15-06 OS Prominent, MC� 5-16-07 16 6 10 632-15-06 OS Prominent, MC� 10-23-07 16 8 8 50

SA 59 PEX 5-21-07 OD Prominent, MC� 10-17-07 42 14 28 677-02-07 OS Prominent, MC� 10-17-07 30 14 16 53

SM 70 PEX 3-6-07 OS Prominent, (MC�) 5-9-07 30 9 21 703-6-07 OS Flat 10-25-07 30 40 �10 �33

SG 82 PEX 4-4-07 OD Prominent, MC� 4-30-07 14 8 6 43SW 71 POAG 11-29-06 OD Prominent, (MC�) 10-23-07 37 5 32 87

6-29-07 OS Prominent, (MC�) 10-23-07 20 15 5 25SD 84 PEX 5-9-07 OS Prominent, (MC�) 5-15-07 28 11 17 61SE 69 POAG 8-15-06 OD Prominent 5-7-07 20 6 14 70

3-20-07 OS Prominent, MC� 5-7-07 16 8 8 50SP 74 POAG 10-15-07 OS Prominent, (MC�) 10-23-07 21 8 13 62SH 69 POAG 2-28-07 OD Flat, MC� 5-8-07 20 21 �1 �5

2-28-07 OD Flat, MC� 10-24-07 20 18 2 14VD 81 PEX 3-19-07 OS Prominent, MC� 5-8-07 15 12 3 20WG 82 PEX 5-11-07 OS Prominent, MC� 5-15-07 20 2 18 90WR 80 POAG 3-21-07 OD Prominent 5-2-07 15 8 7 47

3-21-07 OD Prominent, (MC�) 10-19-07 15 11 4 27WE 79 POAG 5-30-07 OS Prominent, MC� 10-19-07 16 11 5 31WI 75 POAG 4-4-07 OD Prominent 5-2-07 19 15 4 21

1-31-07 OS Prominent, MC� 5-2-07 21 11 10 48ZG 52 Uveitis 5-11-07 OS Prominent, MC� 5-15-07 18 2 16 89ZW 84 POAG 12-18-06 OS Prominent, MC� 5-7-07 12 9 3 25

12-18-06 OS Prominent, MC� 10-24-07 12 12 0 0ZR 59 Uveitis 3-14-07 OD Prominent, (MC�) 5-2-07 18 9 9 50

PDG, pigment dispersion glaucoma; POAG, primary open angle glaucoma; PACG, primary angle closure glaucoma; PEX, pseudoexfoliationglaucoma; uveitis, secondary glaucoma after uveitis; TE, trabeculectomy; MC�, numerous microcysts; (MC�), few microcysts; MC�, no to veryfew microcysts.

IOVS, April 2011, Vol. 52, No. 5 Reflectivity, Filtering Bleb, IOP, Trabeculectomy 2451


IOP before surgery, IOP after surgery at AS-OCT measurement, andpostoperative IOP reduction in absolute values and in percentagecompared with preoperative values as last documented before trab-eculectomy (Table 1).

For the measurement, the patient was comfortably seated infront of the Visante OCT and placed his or her chin on the chinrestand the front of the head against the front head support. The patientwas then asked to look down and fixate on an external fixationpoint. The upper lid was gently retracted by the operator to displaythe bleb in the superior bulbar conjunctiva as well as possible, withevery effort made to avoid pressure on the globe or bleb. A real-timevideo integrated into the scanning panel indicated the position ofthe scanning location. Figure 1 shows a photograph of a fullyexposed filtering bleb. The four measurement planes of the AS-OCTare shown with blue arrows.

The optical aperture of the AS-OCT was centered to the maximalelevation of the filtering bleb with the aid of a joystick. Then the

four standard AS-OCT images (vertical, horizontal, oblique to theright, oblique to the left) were obtained simultaneously for eachbleb. (Fig. 2).

Because the Visante OCT has no in-built tool to measure signalintensities, the scanned images were exported as jpg files and thenimported into ImageJ software (developed by Wayne Rasband, Na-tional Institutes of Health, Bethesda, MD; available at http://rsb.info.nih.gov/ij/index.html) for further analysis.

RESULTS

Thirty-seven patients (43 eyes) were analyzed (21 women, 15men). Mean age of the patients was 69.5 years (range, 27–84years). All patients were Caucasian. Mean time to first AS-OCTmeasurement after trabeculectomy was 112 days (range,4–441 days). Mean time to all AS-OCT measurements aftertrabeculectomy was 152 days (range, 4–615 days). AverageIOP reduction at the time of the AS-OCT-measurement com-pared with preoperative values was 10.1 mm Hg (45.7%).Patients were examined 1 to 4 times (average, 1.42 times) withthe AS-OCT in May 2007, October 2007, or both (total AS-OCTmeasurements, 61).

Three parts of the fluid-filled cavity of the filtering bleb andone part of the “background noise” were marked with anellipse, as depicted in Figure 3. Within the filtering-bleb cavity,the ellipses were drawn as large as possible without overlap-ping the filtering bleb wall. Figure 3 shows one patient (BM) 6days after trabeculectomy of the right eye and shows thehorizontal measuring plane of a standard AS-OCT image of theVisante OCT. Several microcysts can be seen on the surface ofthe conjunctiva.

The reflectivity of the content of the ellipses was plotted ashistograms with the ImageJ program. For further analysis, amean of the three pixel density measurements within thefiltering cavity was taken, and the value of the backgroundnoise was subtracted. The background noise had to be sub-tracted from the signal intensity of the filtering bleb because itvaried considerably between patients, though the backgroundillumination in the room was made constant by the completeclosing of the window blinds and the turning down of room

FIGURE 1. Photograph showing the exposed filtering bleb while thepatient is looking down and the upper lid is gently lifted by theexaminer. The four standard measuring planes of the AS-OCT aremarked with blue arrows.

FIGURE 2. The four standard AS-OCT images simultaneously by theAS-OCT. Beside the internal struc-tures of the filtering bleb in the ver-tical measuring planes, the upper lidand the cornea can be seen.

2452 Pfenninger et al. IOVS, April 2011, Vol. 52, No. 5


illumination to the minimum. A lux meter to measure roomlight intensity, as in the study of Wong et al.,29 was not used.In patient BM, the reflectivity of the filtering bleb minus thebackground noise was 0.525 ([17.670 � 20.418 � 19.248]/3 �18.587), showing, in this case, little difference between theinternal reflectivity of the filtering bleb and the backgroundnoise. The corresponding eye pressure at the time of the OCTmeasurement was 3 mm Hg. In Figure 4 (top left), besidepatient BM, three other examples are plotted with differentappearances of the filtering bleb. Patient BW (Fig. 4, topright) also showed low reflectivity of the fluid-filled cavity of

the filtering bleb, with a value of �0.829 compared with thebackground noise and an IOP of 9 mm Hg. Patient WR (Fig. 4,bottom left) showed a rather flat filtering bleb cavity. This filteringbleb had a higher internal reflectivity of 10.8. The correspondingIOP was 11 mm Hg. Patient SW (Fig. 4, bottom right) showed aprominent filtering bleb with a high internal reflectivity of thefiltering bleb cavity of 4.56. At the time of AS-OCT measurement,the IOP in this patient was 15 mm Hg.

Reproducibility of the reflectivity of the backgroundnoise and the fluid-filled cavity of the filtering bleb withinthe same patient and measuring session were tested with

Patient B.M. 6 days after trabeculectomy OD

Background noise

Internal Reflectivityof the filtering bleb

Background noise

FIGURE 3. Same patient and sameOCT scan as shown in Figure 2, 6days after trabeculectomy. The histo-gram of the background noise out-side the filtering bleb and the threehistograms of the internal reflectivityof the filtering bleb are plotted withImageJ.

Patient B.M. 6 days after trabeculectomy OD Patient B.W. 3 weeks after trabeculectomy OD

Patient W.R. 7 months after trabeculectomy OD Patient S.W. 1 year after trabeculectomy OS

Reflectivity minus surrounding noise: 0.439IOP: 3 mmHg

Reflectivity minus surrounding noise: -0.829IOP: 9 mmHg



A B

C D

FIGURE 4. Four examples that showthe variety of filtering bleb morphol-ogy. (A, B) The internal reflectivity ofthe filtering bleb minus surroundingnoise is close to zero. (C, D) Theinternal reflectivity of the filteringbleb minus surrounding noise is 10.8resp. 4.56.



three different OCT scans for all 61 measurements. Forexample, in patient BM, who showed low reflectivity of thefluid-filled cavity of the bleb, scan reflectivity was as follows:first scan—19.982, 18.801, 20.226 (mean, 19.703); secondscan—18.197, 18.672, 19.083 (mean, 18.651); third scan—17.670, 20.418, 19.248 (mean, 19.112). The signals of thebackground noise in the corresponding three scans were18.839, 18.724, and 18.587. The mean internal reflectivity ofthe fluid-filled bleb cavity minus background noise was0.864 in the first scan, �0.073 in the second scan, and 0.525in the third scan (mean, 0.439). In patient GP, who had veryhigh reflectivity in the fluid-filled cavity of the bleb, valueswere as follows: first scan— 43.627, 41.283, 36.264 (mean:40.391); second scan— 41.678, 40.713, 37.731 (mean,40.042); third scan—39.344, 36.262, 38.725 (mean, 38.110).Background noise in the three corresponding scans was 3.782,3.718, and 3.805. Mean internal reflectivity of the fluid-filled blebcavity minus background noise was 36.609 in the first scan,36.324 in the second scan, and 34.305 in the third scan (mean,35.746). As these two examples show, background noise variedconsiderably between patients but was constant within the mea-surement session. Reflectivity in different parts of the fluid-filledcavity of a bleb showed some fluctuation, but overall reproduc-ibility was reasonably good.

In Figure 5, all 61 measurements of the internal reflectiv-ity of the fluid-filled filtering bleb cavities of the 37 patientsare plotted against the IOP measured at the time of theAS-OCT examination by a masked investigator not involvedin the study. It shows a significant correlation, with R2 �0.3762 and P � 0.0001. To prove the stability of the data, weplotted another scatter plot without the two outliers withthe highest IOP of 40 mm Hg and 34 mm Hg and without thetwo filtering blebs with the highest internal reflectivity of35.75 and 28.90. These 57 measurements still showed ahighly significant, though weaker, correlation, with R2 �0.1158 and P � 0.0096.

DISCUSSION

In the clinical setup, several subjective grading systems wereused to evaluate the success or failure of a filtering bleb and to

help the clinician in the follow-up period after trabeculec-tomy.3 In this study, we presented a new and objective tool toguide the clinician in the postoperative period after trabecu-lectomy. We found the AS-OCT to be a promising method withwhich to analyze the morphology of the filtering bleb. Thestudy by Ciancaglini et al.25 reports that functioning blebspresented with a low bleb wall reflectivity measured with theVisante OCT. The study by Yamamoto et al.21 reported asignificant correlation between the reflectivity of the fluid-filledbleb cavity measured by ultrasound biomicroscopy and IOPcontrol. In that study, internal reflectivity was grouped intolow, medium, and high. To our knowledge, our study is thefirst that objectively analyzes the internal reflectivity of thefluid-filled bleb cavity with the aid of drawing histograms andplotting the pixel density against IOP. A significant correlationbetween the internal reflectivity of the fluid-filled cavity of thefiltering bleb and the IOP could be shown that even remainedsignificant when four outlier measurements were excluded.This finding may have future impact in objectively evaluatingthe postoperative success or failure of filtering blebs so thatpostoperative management (e.g., bleb manipulation) can beadapted accordingly.

Limitations of this pilot study are the heterogeneous groupof patients (primary open angle glaucoma, pseudoexfoliation-glaucoma, primary angle closure glaucoma, pigment dispersionglaucoma, secondary glaucoma after uveitis) and the largerange of measuring times and numbers of OCT measurementsafter trabeculectomy, attributed to the fact that the VisanteOCT was available in our clinic only in May 2007 and again inOctober 2007. Therefore, some patients who underwent trab-eculectomy before May 2007 could be measured in May andagain in October, and those who underwent trabeculectomybetween May 2007 and October 2007 could be measured onceor twice only in October 2007.

Further studies with a higher number of patients are neededto confirm this finding. The development of the internal reflec-tivity of the fluid-filled bleb cavity compared with IOP over alonger period should also be analyzed.

In conclusion, we believe that AS-OCT has great potentialfor analyzing the postoperative success or failure of a filteringbleb. Because there is no contact with the eye, even measure-

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Internal reflectivity of the cavity of the filtering bleb minus background noise

IOP vs internal reflectivity of the filtering bleb

y = 0.4412x + 8.2461N = 61R² = 0.3762p = <0.0001

FIGURE 5. Scatter plot of the inter-nal reflectivity of the fluid-filled cav-ity of the filtering bleb versus the IOPat the day of the OCT measurement(n � 61).

2454 Pfenninger et al. IOVS, April 2011, Vol. 52, No. 5


ments as early as 1 day after surgery are possible, in contrast toultrasound biomicroscopy.

Acknowledgments

The authors thank Albert Ehrsam (Carl Zeiss Meditec Inc.) for hisvaluable instruction in the use of the Visante OCT.

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