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Department of Ophthalmology, College of Medicine, Chungnam National University, Taejon, South Korea
Department of Ophthalmology, College of Medicine, Sung Kyun Kwan University, Seoul, South Korea
Accepted for publication 16 July 2002
| ABSTRACT |
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Methods: A retrospective chart review was performed on 43 eyes of 40 patients who underwent glaucoma tube shunt implant surgery using double layered e-PTFE membrane and silicone tube to treat refractory glaucoma. The surgeries were performed from May 1991 to September 1995, and the subjects were patients with terminal glaucoma without useful vision on the study eye.
Results: The mean follow up period was 32.9 months. The Kaplan-Meier survival for intraocular pressure (IOP) control (IOP between 6 and 21mm Hg without significant complication) was 80.9% at 1 year, 73.9% at 2 years, and 62.2% at 3 years after surgery. After excluding three eyes of three patients who were dropped within 3 months after surgery and did not have any serious complication or problem in IOP control, the average preoperative IOP was 42.5 (SD 14.6) mm Hg and IOP on the last visit was 17.3 (10.2) mm Hg (p = 0.000, n = 40). The number of antiglaucoma medications before surgery (2.2 (0.6)) was reduced to 0.5 (0.8) on the last visit (p = 0.000). The IOP was controlled within the range of 6–21 mm Hg in 26 eyes (65.0%). In the remaining 14 eyes (35%), we could not control the IOP or additional surgery was needed to control the IOP or to treat severe complications. Two cases of endophthalmitis and three of phthisis were found as serious complications. The other complications were similar to those of other commercially available glaucoma implants.
Conclusion: A comparable clinical result was obtained with this new implant as with the other commercially available implants. This implant with a thin and non-rigid reservoir has a potential to reduce some complications associated with the large volume and rigid consistency of the other implants, although it is not yet proved. This membrane tube implant may be considered as another substitute in the surgery of refractory glaucoma.
Keywords: e-PTFE membrane; glaucoma implant; membrane implant; membrane tube implant; refractory glaucoma
The most common causes for failure after filtration surgery are the blockage of fistula by fibrous tissue ingrowth, adhesion of scleral flap to scleral bed, and fibrous breakdown of conjunctival filtering bleb with resultant failure to maintain the function of the filtering bleb. There have been many trials to halt this process such as providing mechanical barriers and applying antiproliferative agents, etc. As a method of establishing a mechanical barrier to halt the blockage of the fistula by fibrous tissue, various kinds of materials and designs have been developed and applied. Nowadays, use of a semirigid tube as a conduit for aqueous from the anterior chamber has been proved to be safe and effective.1,2 A reservoir portion is needed to facilitate aqueous outflow and prevent blockage of distal end of the tube. There are various types of materials and designs of the reservoir portion, but all of them have been made of rigid material with large volume. We thus need to make a large incision through the conjunctiva and Tenon’s tissue to insert the implant. There are many complications associated with the large volume and rigid consistency of these implants, such as wide area of scar tissue, erosion and exposure of the implant, and limitation of extraocular muscle function, etc.3–6 To reduce these problems, we developed a new implant with the same concept of the previous implants but with a different material, a soft and freely malleable membrane, expanded poly(tetrafluoroethylene) (e-PTFE) as a reservoir portion. Since we had successful results in an experimental animal study with this membrane,7,8 we performed a clinical study with this new glaucoma implant for the refractory glaucoma.
| MATERIALS AND METHODS |
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The patients were admitted 1 day before the operation and underwent a thorough clinical examination; and they were told to discontinue all pressure lowering medications on admission. The operation was performed under the local retrobulbar anaesthesia with 2% lignocaine (lidocaine) the next day. A limbus based conjunctival flap of about 8 mm was made at 6 mm posterior to the limbus. The superior and lateral recti muscles were isolated through this incision. Although we inserted the implant in the upper temporal quadrant if possible, the medial rectus was isolated when we had to insert it in the upper nasal quadrant. In 13 eyes of 13 patients in whom the operator expected poor pressure control (those who had extensive conjunctival scar by trauma or previous surgery), mitomycin C soaking (0.4 mg/ml solution for 3 to 5 minutes) followed by copious irrigation with balanced salt solution was applied. The membranous reservoir was inserted in folded form through the conjunctival opening, and then unfolded underneath the Tenon’s capsule with its both ends to be located under the two recti muscles. The anterior margin of the membrane was fixed onto the sclera at 10 mm posterior to the limbus with 10–0 nylon on both sides. We estimated the appropriate length of the silicone tube, 1.5 mm of tube to be set within the anterior chamber, and then cut the tube with an oblique angle. After paracentesis with a 23 gauge needle at the limbus, the silicone tube was inserted through the needle tract in the bevelled side up position. Then the tube was fixed on the sclera by two anchoring sutures with 10–0 nylon (Fig 2). To prevent the early postoperative overfiltration, we occluded the tube on the outside with a 9–0 nylon releasable suture with the 5–0 nylon intraluminal suture located within the tube. Both releasable and intraluminal sutures were exposed partially through the conjunctiva to be pulled out after postoperative day 3. The criteria for the removal were as follows; if intraocular pressure went over 40 mm Hg within 1 week after surgery, over 30 mm Hg between 1 and 2 weeks after surgery, or over 20 mm Hg after 2 weeks. All temporary occlusion sutures were removed if they remained at the postoperative week 4. The anterior portion of the silicone tube over the limbus was covered by a donor scleral patch graft, size 4 x 5 mm, and it was sutured with 10–0 nylon onto the sclera. The incisions of conjunctiva and Tenon’le were closed with separate 10–0 nylon sutures. The operation was finished with gentamicin (80 mg/ml, 0.5 ml) and betamethasone (4 mg/ml, 0.5 ml) injections in the inferior subconjunctival space. In patients with severe conjunctival scar, subconjunctival injection of triamcinolone (40 mg/ml, 0.3 ml) on the posterior conjunctiva in superior quadrants was applied.
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We classified the case as a success when the patient’ocular pressure was controlled between 6 and 21 mm Hg without or with one or two antiglaucoma medications. Cases in which intraocular pressure was not controlled within the range of 6–21 mm Hg, or cases that needed additional surgery to control intraocular pressure or to treat the complication, if any, were classified as "failures." Intraocular pressure during the first month was not assessed in classifying the result, because there might be a large variation of IOP during this period. Kaplan-Meier survival analysis was done for all 43 study eyes.
Five eyes were not followed for more than 3 months. Two of them had additional procedures done to control severe complications after surgery and they were classified as a failure. The other three eyes did not show any significant complications or serious problems in intraocular pressure control until the last follow up. Because the clinical data such as visual acuity, intraocular pressure, and number of antiglaucoma medications might vary greatly during the first 3 months, we excluded the clinical data of these three eyes in analysing the result. Therefore, the final number was 40 eyes of 37 patients in assessing the result (except for the Kaplan-Meier survival analysis (n = 43)).
We used the SPSS program (Ver 10.0, SPSS Inc, Chicago, IL, USA) for the statistical analysis. In evaluating the numerical values such as change of intraocular pressure and number of antiglaucoma medications, we used the non-parametric test (Wilcoxon paired t test). Pearson’s correlation analysis was performed for estimating the relation between filtering bleb height (measured be A-scan echography) and the intraocular pressure. Kaplan-Meier survival analysis was performed for the successive intraocular pressure control. All subjects were grouped by sex, success and failure, visual outcome, and preoperative diagnosis, then the non-parametric test was applied for the analysis of the difference among the groups. The Mann-Whitney test was used for comparison between the two groups and Kruskal-Wallis test was used for comparison among three or more groups.
| RESULTS |
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The preoperative intraocular pressure was 38.3 (14.9) mm Hg in the successful group, and 50.4 (10.5) mm Hg in the failure group. There were significant differences in preoperative intraocular pressure among the groups (p = 0.013).
In assessing the effect of the application of mitomycin C on the result, there were no differences in age, sex, preoperative intraocular pressure, number of antiglaucoma medications, and the number of previous surgery between the two groups that had and had not undergone mitomycin C soaking. The success was 11 out of 13 in the group that had mitomycin C soaking, and 15 out of 27 in the group that had not had mitomycin C soaking. Although the success rate was better in the mitomycin C group, there was no significant difference between the two groups (p = 0.075). The age, preoperative number of antiglaucoma medications, number of previous surgery, and follow up period did not affect the result (p>0.05, Table 4).
We divided the patients into four groups according to the diagnosis to find if there were any differences in preoperative parameters or result of the surgery in terms of pressure control: neovascular glaucoma group, failed filter group, secondary glaucoma group, and aphakic/pseudophakic glaucoma group. The aphakic/pseudophakic glaucoma group showed the most favourable outcome, a 83.3% success. They were followed by the failed filtration group (75.0% success) and neovascular glaucoma group (63.2% success). The secondary glaucoma group showed the worst success rate, 42.9%. But statistical assessment was not possible because of the small numbers of subjects in each groups (Table 5).
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There were two cases that received full thickness corneal transplantation in this study. In the case of a patient who developed corneal opacity and needed full thickness penetrating keratoplasty after e-PTFE membrane implant surgery as described previously, the graft was well maintained for 5 years until the last follow up visit. On the other hand, in the other case which had corneal transplantation before e-PTFE membrane implant surgery, graft failure developed 2 years after implant surgery.
There were two cases where tube lens touch developed in phakic eyes, and focal lens opacities developed. But they showed no progression of the cataract until the last follow up visit (average of 26 months after implant surgery). Minor retinal haemorrhages in two eyes and intravitreal haemorrhage in one eye were found after surgery, but they resolved spontaneously without any treatment. Exposure of the implant was found in two eyes, and of two cases of endophthalmitis and three of phthisis were found including the cases mentioned earlier; they were classified as failures (Table 6). These three cases of phthisis developed in one patient with neovascular glaucoma and two patients with secondary glaucoma, one from a severe ocular trauma and the other from an epithelial downgrowth after cataract surgery.
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| DISCUSSION |
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Following the surgery of any kind of glaucoma implant, a fibrovascular capsule is known to be formed surrounding the foreign material. This capsule is different from that of filtering bleb formed after conventional trabeculectomy in that the former is a membrane composed of connective tissue positioned between Tenon’s capsule and the implant, located around the equator. The aqueous is absorbed in the orbital capillaries or lymphatics after exiting the intercellular spaces of this capsular membrane.2 The resistance to aqueous outflow may be decided mainly by the permeability of this membrane,13 and the resultant intraocular pressure is controlled by the passive, pressure dependent flow through this membrane.14 So, the main factors that control the intraocular pressure are the surface area, thickness, and permeability through this membrane.15–18 In this study we used the implant with a reservoir portion of 160 mm2 in size, which showed optimal result in both effect and ease of surgical manoeuvre in the previous study.7,8 This is larger than the single Molteno implant, but smaller than the double plate Molteno, Ahmed, Baerveldt, and ACTSEB implants. The membrane formed around this new implant was not different from the tissue formed around the other glaucoma implants in histology.8 The permeability of this membrane might be reduced over time, so topical ß blockers and steroid drops are needed to maintain the intraocular pressure after implant surgery in many cases.19,20 For that reason, we did not consider whether the patient used one to two antiglaucoma medications or not in assessing the result of the surgery.
In comparing the efficacy of controlling the intraocular pressure among many implant types, each study might have different criteria in selecting the patient and classifying the results, so simple horizontal comparison is not appropriate. Our result showed a similar result to those of other commercially available implants in controlling the intraocular pressure by Kaplan-Meier survival analysis. The survival rate in our study were 87.4% at 6 months, 80.9% at 1 year, 73.9% at 2 years, 62.2% at 3 years, and 56.0% at 5 years. These are not much different from the reports that had similar criteria for selecting subjects and classification; survival of 78% at 1 year and 75% at 2 years of Ahmed valve implant,4,6 60.3% and 71% at 2 years of Baerveldt implant,5,21 and 54% at 5 years of Molteno Implant.3
In this study, we could control the intraocular pressure within the range of 6 and 21 mm Hg in 26 (65%) of 40 patients with average follow up of 32.9 months. But if we consider the visual acuity in assessing the result, only 19 eyes (47.5%) showed good intraocular pressure control while maintaining the visual acuity. We looked for all possible clinical variables that might affect the result, and we found sex and the preoperative intraocular pressure had a significant effect on the success rate. Interestingly, females showed 87.5% success rate that was much higher than 50.0% of the males. We tried to find any other differences between female and male groups, but there were no significant differences in clinical variables in both groups. Douglas and associates had reported that males had a significantly poorer prognosis than females in their 94 cases with a Molteno implant,22 but most reports did not showed difference between males and females. Because this study was done with a small number of patients and performed retrospectively, we think a further study with a large number of subjects might yield a different result. Although Siegner and associates reported the preoperative intraocular pressure did not affect the result in their study with the Baerveldt implant,5 we had a contrary result that is consistent with the reports of Mills and associates with a Molteno implant and of Huang and associates with an Ahmed implant, which states that the case with high preoperative intraocular pressure showed high risk of failure after the implant surgeries.3,6 The result of implant surgeries may differ according to the type of glaucoma. Although statistial analysis was not possible in our study, each type of glaucoma showed different success rate. The group with the worst result was the secondary glaucoma group (success rate of 42.9%) which included two cases with primary insult of severe trauma, two corneal disorders (two eyes of Terriens marginal dystrophy of a patient), an epithelial downgrowth after cataract surgery, a case after complicated retinal surgery, and a case with Sturge-Weber syndrome. Except for this secondary glaucoma group, the neovascular glaucoma group showed the worse result (63.2% success) than the failed filter group (75.0% success) and the aphakic/pseudophakic group (83.8% success). This result agreed with the previous reports showing a poor result in neovascular glaucoma patients after implant surgeries.3–6,23 Molteno used anti-inflammatory medications such as steroid, fluphenamic acid, adrenaline, colchicine, and atropine when he performed glaucoma implant surgery to reduce the fibrotic reaction around the implant.24 There have been contradictory reports about the effect of antifibrotic agents on glaucoma implant surgeries.25–30 In this study, when the subjects were divided into two groups, the mitomycin C treated group and the untreated group, there was no difference not only in preoperative variables such as age, sex, preoperative intraocular pressure, and number of preoperative antiglaucoma medications but also in the intraocular pressure at the last visit. The success rate was higher in the mitomycin C group, but it was not significant statistically. As mentioned in the methods section, application of mitomycin C was done in a selected group of patients; in those who had extensive conjunctival scar, simple comparison is not possible, therefore we could not determine the effect of mitomycin C in this study.
Ultrasound echography revealed good posterior filtering bleb in most cases in this study with membrane reservoir. Although we did not perform this examination in routinely, 91.3% of selected cases showed noticeable filtering space with variable height (bleb height of 2.4 (1.3) mm) around the explant. In contrast with the other explant with rigid consistency, we could not find an echo associated with the membrane reservoir. As reported by Lloyd and associates the stated bleb size did not necessarily correlate with the levels of intraocular pressure control,31 and Pearson correlation analysis failed to reveal a statistical correlation between bleb height and intraocular pressure in this study.
In treating refractory glaucoma, the implant surgery offers a better result in long term control of intraocular pressure, but there have been many reports about various complications after implant surgeries from transient hypotony to phthisis. Among many complications, leaking of conjunctival wound, limitation of ocular motility, and exposure of the implant might be associated with the rigid consistency and large volume of the previous implants, and these problems might be reduced if we substitute the reservoir with the soft, thin membrane. Ocular motility dysfunction after implant surgery was not encountered infrequently, and it was improved as the oedema around the eyeball subsided in most cases. Although it can happen in all types of the glaucoma implants, it was more frequent with the Baerveldt implant, which had the largest volume, and up to 77% of ocular motility restriction with this implant was reported by Smith and associates.32,33 We expected less of an ocular motility problem with the new implant, and we detected no ocular motility problem during the study period. Our subjects had poor vision in the operated eyes, so we agree that the chance to find the motility problem might be reduced, and a more objective result could be obtained with the patient group with good visual acuity in both eyes. In the meantime, we expected lower frequency of exposure of the implant or infection with the new implant, but we found no difference in the frequency of these complications compared to the previous reports.
With our soft and freely malleable membrane reservoir, contraction of the fibrovascular capsule surrounding the implant after surgery and the resultant contraction of the reservoir with functional loss might be expected. Although we had no necropsy specimen, we had found no histological difference in the fibrous capsule formed around the e-PTFE membrane compared with the other implants in our previous animal study,7 and we have not found any complication related to such contraction on clinical examination, including ultraound sonography over the fibrous capsule formed around the implant in cases with poor intraocular pressure control.
The complications found in this study were not much different from the other reports with rigid implants in general. The most striking complications found more frequently than in recent reports were phthisis and endophthalmitis. Phthisis was found in three eyes (7.5%) in our patients while the Baerveldt implant resulted in 1.9% with this complication and the Ahmed implant showed 1.3% in other reports.5,6 But phthisis developed in 8% in the initial report of the Molteno implant3; this initial study usually selected the subjects with a poor prognosis, such as terminal glaucoma, and this might be one of the reasons for this high frequency of phthisis as a devastating complication. There were two cases of endophthalmitis encountered in our study. One case had severe intraocular inflammation due to corneal ulcer that had developed before the implant surgery; this inflammation was not controlled and continued after surgery, eventually ending up with endophthalmitis. The other was the case of developed implant exposure after surgery, and even though we recommended replacement of the implant with repair of the surface wound, the patient refused to have a further operation on that blind eye after five disappointing glaucoma procedures. That eye resulted in endophthamitis and phthisis.
Although our subjects were confined to terminal glaucoma cases, we think that our new implant showed comparable intraocular pressure lowering effect and complications with other commercially available implants. This membrane tube implant has some advantages. Because this implant’s reservoir is made of soft and malleable membrane, it can be inserted through a smaller incision in a folded form. Also some complications that are associated with the rigid consistency and large volume might be reduced, although it was not proved in this study. We have a plan to report a comparison study between this membrane tube implant and another implant with random sampling of the subjects. With the encouraging result from this study, we think that this new membrane tube implant can be used as another substitute for the currently used implants without losing efficacy, without increasing the risk of complications, and with possible advantages.
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