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Postoperative IOP is related to intrascleral bleb height in eyes with clinically flat blebs following deep sclerectomy with collagen implant and mitomycin
  1. Nikolaos Mavrakanas,
  2. Efstratios Mendrinos,
  3. Tarek Shaarawy
  1. Glaucoma Unit, Department of Ophthalmology, Geneva University Hospitals, Geneva, Switzerland
  1. Correspondence to Dr Tarek Shaarawy, Glaucoma Unit, Ophthalmology Department, Geneva University Hospitals, Rue Alcide-Jentzer 22, 1211, Geneva 14, Switzerland; tarek.shaarawy{at}hcuge.ch

Abstract

Aim To investigate the relationship between intrascleral bleb height and intraocular pressure (IOP) following deep sclerectomy with collagen implant (DSCI) and mitomycin C (MMC) in eyes with clinically flat blebs.

Methods The records of 25 eyes of 22 consecutive patients presenting with clinically flat blebs following DSCI with MMC for primary or secondary open angle glaucoma were reviewed. Anterior segment optical coherence tomography (AS-OCT) scans were used to evaluate postoperative intrascleral bleb height and its relation to IOP control. Eyes requiring postoperative bleb manipulations, needling or goniopunctures were excluded.

Results The mean age of the patients was 71.9±12.6 years, and the mean preoperative IOP was 25.3±5.6 mm Hg. The mean time of the AS-OCT examination from the operation was 8±4.9 months, and the mean IOP at that time was 13.8±4.2 mm Hg (p<0.001). All operated eyes manifested an intrascleral bleb with AS-OCT. The mean intrascleral bleb height was 0.58±0.16 mm. IOP and intrascleral bleb height were found to be inversely correlated (p<0.001, r=−0.626). None of the eyes had subconjuctival blebs, and 17/25 eyes showed microscopic conjuctival fluid collections.

Conclusion The authors report a positive inverse correlation between intrascleral bleb height and postoperative IOP in eyes presenting clinically flat blebs following DSCI with MMC, suggesting an important role for intrascleral filtration in lowering IOP. Further studies are warranted to evaluate this relationship at different postoperative time points and possibly with different types of implants.

  • Anterior segment OCT
  • intrascleral bleb
  • deep sclerectomy
  • collagen implant
  • mitomycin
  • bleb height

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Introduction

Anterior segment imaging is a rapidly advancing field in ophthalmology. New imaging modalities, such as anterior segment optical coherence tomography (AS-OCT), have recently become commercially available. The Visante OCT (Carl Zeiss Meditec, Dublin, California) is a non-contact, high-resolution, imaging technique that uses low-coherence interferometry to provide in vivo cross-sectional images of structures of the anterior segment with a spatial resolution of 10–20 μm.1 2 Recently AS-OCT has been used to evaluate filtering blebs after glaucoma surgery.3–9

Deep sclerectomy with collagen implant (DSCI) is a form of non-penetrating filtering surgery for medically uncontrolled glaucoma.10 It provides stable control of intraocular pressure (IOP) at long-term follow-up with few postoperative complications.11 The basis of non-penetrating filtering surgery is to create filtration through a naturally occurring membrane, the trabeculo-Descemet membrane, which acts as an outflow resistance site, allowing IOP reduction without postoperative ocular hypotony. A deep sclero-keratectomy is performed to expose this membrane and also creates an intrascleral space which is intended to form an intrascleral bleb postoperatively. The intrascleral bleb may function as an aqueous reservoir or filtration site in cases where a subconjunctival filtration bleb is not formed.

Slit-lamp visualisation of filtering bleb morphology is useful for recognising signs of unfavourable subconjunctival bleb evolution. However, it is a qualitative assessment. The intrascleral blebs cannot be seen or evaluated by slit-lamp examination. We were therefore interested in examining postoperative intrascleral blebs to see if their presence and size were related to IOP reduction and control postoperatively.

To the best of our knowledge, there are only two studies in the literature that evaluated bleb characteristics following non-penetrating deep sclerectomy by AS-OCT.6 8 The purpose of this study was to investigate the correlation between intrascleral bleb height and IOP following DSCI and Mitomycin C (MMC) in eyes that showed clinically a flat bleb (no subconjunctival bleb), during the postoperative follow-up period.

Patients and methods

We included 25 eyes of 22 consecutive patients (male/female: 9/13) presenting clinically flat blebs (bleb height: O, Indiana bleb appearance grading scale) following DSCI with MMC for primary or secondary open angle glaucoma. Uncontrolled glaucoma was defined as well-documented progression of glaucomatous visual-field defects and optic nerve morphology under maximal tolerable medical treatment (two or more antiglaucoma medications). Exclusion criteria were previous eye surgery, laser trabeculoplasty, postoperative bleb manipulations, needling and goniopuncture.

Surgical procedure

All surgeries were performed by a single experienced surgeon (TS) using an operating microscope. The conjunctiva was opened at the limbus. A 5×5 mm superficial scleral flap was performed at a depth of one-third scleral thickness, hinged posteriorly. This superficial scleral flap was extended 1–1.5 mm anteriorly into the clear cornea. The initial incision was made with a No11 blade, and the horizontal dissection with a crescent microsurgery knife. Sponges soaked in Mitomycine 0.02% were placed for 45 s in the scleral bed and between the sclera and the Tenon capsule, and these sites were then irrigated with balanced salt solution for 60 s.

A deep sclerokeratectomy was performed by dissecting a second deep 4×4 mm scleral flap based at the anterior extremity of the superficial flap with two lateral incisions and a posterior deep scleral incision made with a No 11 blade. The deep scleral flap was then dissected anteriorly with the blade to unroof Schlemm canal and another 1–1.5 mm further anteriorly to remove the sclerocorneal tissue over the anterior trabeculum and Schwalbe line. Then, the deep scleral flap was removed with scissors. At this stage, percolation of aqueous through the trabeculum was evident. The juxtacanalicular trabeculum and Schlemm canal endothelium were then removed using a fine smooth forceps. A collagen implant (Aquaflow, STAAR Surgical AG Nidau, Switzerland) was placed in the sclerokeratectomy site to preserve the intrascleral space. The superficial scleral flap was then sutured down with 10/0 sutures.

Imaging technique

Visante-OCT (Visante-OCT Carl Zeiss Meditec, Dublin, California) was used to evaluate the height of the postoperative intrascleral blebs. The patient was asked to look downwards at an angle from the horizontal of about 30°. The upper eyelid was gently retracted by the examiner to expose the superior bulbar conjunctiva as fully as possible without pressing on the globe. Single-scan (figure 1) and high-resolution scan protocols (figure 2) were performed to search for the location of greatest bleb height on each study eye. Using a two-dimensional image recorded by the Visante OCT, the maximum height of the intrascleral bleb was manually measured with the OCT caliper tool and automatically calculated with the device's software.

Figure 1

(A) Single scan of the anterior segment with the Visante optical coherence tomograph (OCT) showing the intrascleral bleb (arrow) and the trabeculodescemetic membrane (arrowhead) following deep sclerectomy with collagen implant. (B) High-resolution scan of the anterior segment with the Visante OCT. The maximum height of the intrascleral bleb is manually measured with the OCT calliper tool and automatically calculated with the device's software. Note the presence of small intraconjunctival fluid collections (arrowheads).

Figure 2

Correlation between intrascleral bleb height and intraocular pressure (IOP) in 25 eyes of 22 consecutive patients presenting clinically flat blebs following deep sclerectomy with collagen implant with mitomycin C.

The study was performed in conformance with the principles of the Declaration of Helsinki and with the Swiss federal laws. All patients provided signed informed consent before surgery. Because AS-OCT is performed for routine clinical care in our department, and no additional procedure was performed for research purposes alone, no specific consent was required by the Ethics Committee of the Geneva University Hospitals. However, all patients were informed of the aims of recording these data, and their oral consent was obtained before undergoing AS-OCT.

Statistical analysis

Normality was verified using the Shapiro–Wilk test. Differences in terms of IOP and number of medications before and after surgery were compared using a paired Student t test. Data are presented as mean±SD. We searched for a relationship between postoperative IOP and maximum intrascleral bleb height using the Pearson correlation coefficient. In addition, time postsurgery was plotted against intrascleral bleb height and against postoperative IOP using the Pearson correlation coefficient. Differences or correlations were considered statistically significant when p was less than 0.05.

Results

The patients' mean age was 71.9±12.6 years (range: 45–88), and the mean preoperative IOP was 25.3±5.6 mm Hg (range: 21–38). The mean number of medications per patient was reduced from 2.48±0.65 to 0.6±0.64 (p<0.001) after DSCI with MMC. The mean time of the OCT from the operation was 8±4.9 months (range: 2–18), and the mean postoperative IOP at that time was 13.8±4.2 mm Hg (range: 4–22) (p<0.001). The mean bleb height was 0.58±0.16 mm (range: 0.31–0.88). A positive inverse correlation was found between the IOP and the height of the intrascleral filtration bleb (p<0.001, r=−0.626) (figure 2). No significant results were found when the time postsurgery was plotted against intrascleral bleb height (r=0.08, p=0.71) and against postoperative IOP (r=0.33, p=0.11).

None of the eyes showed subconjunctival blebs at the AS-OCT scans, which was in accordance with the biomicroscopic appearance of flat blebs that we observed in those eyes. Some cases (17/25) showed small intraconjuctival fluid collections. There were no cases of cystic or encapsulated blebs in this series of eyes.

There were no significant complications in this series of patients, specifically no shallow or flat anterior chamber, no choroidal detachment and no endophthalmitis. One patient showed slight cataract progression, one patient a conjunctival incision leak in the first postoperative days and one patient a corneal abrasion. The conjunctival incision leak was treated with a soft contact lens and, at follow-up in 5 days, revealed no Seidel. The corneal abrasion re-epithelialised in 48 h with topical tobramycin 3 mg/ml and lubricants.

Discussion

Bleb morphology after glaucoma filtering surgery appears to be clinically important. Current clinical bleb grading systems evaluate parameters such as bleb vascularity, height, area, wall thickness, microcystic changes, encystment, leakage and degree of demarcation by means of slit-lamp observation and photography.12–14 Modern anterior segment imaging modalities have not been clinically used to formally grade filtering blebs and, in particular, might be useful in evaluating filtering bleb morphology when the bleb appearance does not correlate with IOP.

Muller et al8 and Labbé et al6 used slit-lamp-adapted AS-OCT and Visante OCT respectively to investigate the internal structure of filtering blebs following deep sclerectomy. However, this group of eyes was not homogenous, as some of them had undergone trabeculectomy, some non-penetrating deep sclerectomy with or without intraoperative MMC or collagen implant, and some eyes having also goniopuncture during follow-up.6

In the current study, we investigated a homogenous group of patients, all having DSCI with MMC and postoperatively clinically flat blebs and report for the first time visualisation of the intrascleral bleb with measurement of bleb height with the Visante OCT. A positive inverse correlation between intrascleral bleb height and postoperative IOP was found. No significant results were found when time postsurgery was plotted against intrascleral bleb height and against postoperative IOP. A temporal factor can therefore be excluded.

Visualisation of bleb morphology following deep sclerectomy has been reported with ultrasound biomicroscopy (UBM).15 16 Khairy et al found a poor correlation between the intrascleral filtering bleb height and the level of IOP at the time of UBM examination. However, these results may reflect a suboptimal dissection of the trabecular-Descemetic membrane (TDM), as UBM showed a mean TDM thickness of 0.26 mm (the thickness of the human TDM is less than 0.13 mm). No scleral implant or antimetabolite was used in these cases.

Bleb height may be an important factor relating to the success of filtering operations. In a study of subconjunctival bleb height after trabeculectomy, the majority of failed trabeculectomy blebs (17/21, 81%) were low, and more than half of the successful blebs were moderate to high.3 A recent study of trabeculectomy blebs with AS-OCT before and after laser suture lysis showed an increase in total bleb height and increased bleb wall thickness after the laser suture lysis.4 However, Ciancaglini et al found that, among AS-OCT morphological parameters, only bleb wall reflectivity was significantly related to the filtering capability in eyes following trabeculectomy, with successful blebs having a lower bleb wall reflectivity, and that there was no significant difference in mean bleb height between successful and failed blebs.

Deep sclerectomy may lead to IOP reduction via a number of possible mechanisms: a subconjunctival filtering bleb, an intrascleral filtering bleb, suprachoroidal filtration and enhanced episcleral vein outflow of aqueous from Schlemm canal.17 18 According to our study, the formation of an intrascleral filtering bleb after deep sclerectomy seems to be important for surgical success. During deep sclerectomy, a 5–8 mm2 layer of sclera is removed, and the space of this excision becomes the site of an intrascleral filtering bleb when the operation is successful. Different materials have been implanted in the scleral resection bed to help maintain a patent intrascleral space, such as collagen implants,11 reticulated hyaluronic acid19 and non-resorbable acrylic implants.20 21 However, very good surgical results of deep sclerectomy have been reported without implants, albeit somewhat not as good as those with implants.22 In intrascleral filtering blebs, the mechanism of aqueous resorption may be different from that in subconjunctival filtering blebs. The aqueous which flows through the trabeculo-descemetic membrane into the scleral bleb created in deep sclerectomy in the rabbit has been reported to be resorbed by new aqueous drainage vessels, whether or not a collagen implant was used.23 Similar results were obtained using the same animal model with anterior segment fluorescein and indocyanine green angiography.24

Anterior segment-OCT imaging is a safe and easy method of following bleb morphology in the postoperative period, offering a unique opportunity to study the healing and remodelling of blebs over time. UBM is also useful in assessing the anatomical results of non-penetrating glaucoma surgery and determining prognostic elements.25 Anterior segment OCT has a higher resolution than UBM, which has an axial resolution of about 25 μm and a transverse resolution of about 50 μm.1 AS-OCT has additional advantages for bleb imaging because it is non-contact, does not require any coupling medium or supine positioning of the patient and requires minimal experience. also, unlike UBM, the position and orientation of the scan can be localised and visualised from the real-time video panel.

Posterior segment OCT has also been used to image filtering blebs.26 27 The depth of tissue penetration (depth of field) of the posterior segment OCT is limited to 2 mm so that structures such as scleral flap and fluid space above the sclera are not visualised. The major advantage of the Visante AS-OCT over posterior segment OCT is its deeper tissue penetration (6 mm vs 2 mm respectively), as it uses a longer wavelength of superluminescent diode for imaging (1310 nm versus 820 nm respectively) and thus allows for visualisation of deeper bleb structures.1

Our study has a number of limitations. We investigated the relation between bleb height and IOP at a single time-point within a variable period after surgery (2–18 months). Second, Visante OCT does not provide signal intensity measurements, and so we did not measure bleb wall reflectivity, a parameter described by others as related to bleb functionality. Before a validated approach for OCT pixel intensity measurement becomes available, it is difficult to comment objectively on intrableb reflectively. It is important to remember that while the intrableb structures can be visualised clearly with OCT, some important parameters such as bleb vascularity and leakage still require slit-lamp examination.

In conclusion, we found a significant positive inverse correlation in DSCI with MMC between intrascleral bleb height and postoperative IOP in eyes with clinically flat blebs. These results suggest that intrascleral filtration is a potential mechanism of aqueous humour resorption following DSCI. Anterior-segment OCT is a promising tool for imaging bleb structures that are invisible biomicroscopically and enhances our understanding of bleb functioning following glaucoma surgery. Further studies are warranted to evaluate the relationship of intrascleral bleb dimensions to IOP control over the postoperative time course following non-penetrating deep sclerectomy and possibly with different types of intrascleral bed implants.

References

Footnotes

  • This study was presented in part at the 6th International Glaucoma Symposium, 28–31 March 2007, Athens, Greece and at the World Glaucoma Congress, 18–21 July 2007, Singapore.

  • Competing interests None.

  • Patient consent Obtained.

  • Provenance and peer review Not commissioned; externally peer reviewed.