The Use of a Thermoexpandable Metal Alloy Stent in the Minimally Invasive Management of Retroperitoneal Fibrosis: A Single Center Experience From the United Kingdom

Introduction

Retroperitoneal fibrosis (RPF) is an uncommon condition characterized by extensive fibrosis surrounding the abdominal aorta and iliac arteries. The fibroinflammatory reaction tends to extend throughout the retroperitoneum and envelop surrounding structures with frequent involvement of the ureters. The incidence is estimated at 1.38 cases per 100,000 persons and most commonly affects those in their fifth and sixth decade of life. ^1,2 It is idiopathic in approximately 70% of cases. Trauma, malignancy, medication (ie, methylsergide, β -blockers), infection, radiation and previous surgery in the area have been associated with the secondary form of the disease.

The classic perivascular distribution of the idiopathic form supports the theory that the disease is an autoimmune-mediated response to antigens in the atherosclerotic plaques of the abdominal aorta. Associated autoimmune conditions were found to be present in 15% of patients in the study by Kermani and associates. ³ Diagnosis is typically established by CT scan that reveals bilateral hydronephrosis with medial deviation at the level of L4–L5 associated with a well-delineated retroperitoneal soft tissue mass enveloping the great vessels and the ureters. ⁴ Magnetic resonance is also useful, producing diffusely low signal intensity on T1-weighted imaging, with high signal on T2 images representing active disease. Surgical biopsy is often needed for histopathologic differentiation of possible underlying malignancy.

Prognosis is generally good for nonmalignant etiology of RPF, and the condition responds well to medical management, which usually involves corticosteroids, alone or in combination with immunomodulating medications (ie, methotrexate, azathioprine, mycophenolate, tamoxifen, cyclophosphamide). ⁵ Surgical interventions are indicated when perivascular and periuretral involvement lead to obstruction, and include vascular stent placement or surgery, placement of percutaneous nephrostomy of ureteral stents, and ureterolysis. ⁵

Although surgical ureterolysis is considered the gold standard for such patients, ⁶ endourologic management of the acute phase and subsequent convalescence period has gained popularity in recent years, largely because of technologic advances in the field. Preservation of renal function is commonly needed for success of medical treatment, and several studies indicate that combination treatment (i.e., medical and surgical) is often necessary.

The Memokath 051^™ (PNN Medical, Denmark) is a semipermanent, thermoexpandable nickel/titanium metal alloy segmental stent that has been shown to be effective in the minimally invasive management of benign and malignant ureteral obstruction from stricture disease. ⁷ These versatile stents can be placed both in a retrograde or anterograde fashion and have been shown to have a better quality of life and stent-related symptoms compared with Double-J stents. ^8,9 Because of their tightly coiled structure, these stents are resistant to extrinsic compression and do not allow tissue in-growth, ¹⁰ while allowing the stent to follow the contours of the ureter, reducing the risk of ischemic injury to the mucosa and allowing proximal ureteral peristalsis. ¹⁰ They can also be removed easily or exchanged should the need arise but do not require routine exchanges, as is the case with Double-J stents. These qualities could make these stents an effective alternative, in the minimally invasive, long-term management of ureteral obstruction.

In this study, we present our experience and the effectiveness of using the ureteric Memokath 051 metal stent in patients with RPF who have ureteral stricture and renal impairment.

Results

The study population included eight female and six male patients (n=14). Mean age was 60.2 years±8.4 standard deviation (SD). Mean length of follow-up was 22.5 months (range 3–56 mos). The majority of patients had idiopathic RPF (n=12, 85.7%), and two patients had a malignant cause (breast carcinoma and lymphoma). A total of 23 renal units had stent placement in 14 patients, 9 (64.3%) of whom had bilateral disease (Table 1). Ten (71.4%) patients had previously received medical treatment, while three (21.4%) had recurrent disease post-ureterolysis. All patients had Double-J stents in situ before referral to us for consideration of Memokath 051 stents and had recurrent stent blockages and stent-related symptoms.

The mean stricture length was 7.6 cm±6.2 SD on the right and 7.7±5.1 SD (P=0.925) on the left. In 13 of the 23 renal units with upper and midureteral strictures that had stents, the vesicouretral junction was not traversed by the stents. Patients tolerated the stents well with minimal discomfort. There was improvement in renal function in all patients, with mean preoperative creatinine 243.4 μmol/L±135.8 SD (2.75 μg/dl±1.54 SD) and mean postoperative creatinine 153 μmol/L±83.7 SD (1.73 μg/dl±0.95 SD) (P<0.001). Overall, 11 of 14 (78.6%) patients were successfully treated with the Memokath stent with 17 of 23 (74%) renal units managed.

Three (21.4%) patients presented with stent obstruction because of migration (two) and blockage (one), necessitating subsequent removal. Mean presentation of complications was at 11.8 months (range 8–14 mos). All these patients had bilateral disease and, of note, none of these three patients had undergone prievious ureterolysis. Both patients with migrated Memokath stents had the stents exchanged and have the replacement stents currently in situ, functioning well, albeit with a mean follow-up of less than 6 months. The patient with a blocked stent from encrustation is being treated with a long-term Double-J stent; we do not recommend placing the metallic stent in active stone formers because of risk of encrustation. None of these three patients went for subsequent ureterolysis because they were high risk for major surgery. Complete resolution of the obstruction was observed in one further patient with migrated stents who has been stent free with normal renal function at 14 months follow-up.

Discussion

Ureteral stents are used for the urgent management of symptomatic patients and to preserve renal function by adequate drainage. According to a series by Katz and colleagues, ⁶ stent placement facilitated later ureterolysis, while Ilie and coworkers ¹¹ reported that ureteral stents relieved obstruction in all patients, at least initially. Heidenreich and associates ¹² used stents or nephrostomy for primary drainage in all patients in their study while treating medically with immunosuppressant regimes. Only four patients (recurrence rate 8%) remained eventually with Double-J stents after a follow-up of up to 120 months. This suggests that the best results come from a combination of medical and surgical treatment.

Ureterolysis is undertaken if medical therapy is contraindicated and in resistant cases. Again, stents are used postoperatively and can be removed 6 to 8 weeks after the procedure. Success rates range 66% to 100% in various studies. ^{13 –15} Chronic stent placement is one of the options that are provided when ureterolysis is technically challenging or fails. ¹⁴

In our study, we used the thermoexpandable ureteric Memokath 051 metal stent in patients who were previously treated and experienced recurrence. Perhaps the most meaningful conclusion from the clinical point of view is the improvement in renal function in all patients as well as the fact that the stent was well tolerated. The latter is also supported in major series with extended follow-up. ^10,16

The whole range of available stent lengths was used in our study, because of the variability and laterality of the strictures (Table 2). Stricture length measurement is performed with the use of a dual-lumen ureteral catheter, which is gradually withdrawn after introduction to mark the proximal and distal boundaries of the stricture on the image intensifier screen. Two measurements are performed as standard.

In total, nine patients had bilateral disease, of whom stent placement failed in three, one of whom had a malignant cause of RPF and the other two had an idiopathic cause of RPF. Unfortunately, because of the small number of cases reviewed in retrospect, no association can be made between etiology of RPF and stent failure. Stent migration was responsible in two patients and stent blockage in one, necessitating subsequent removal. No patient needed percutaneous nephrostomy as an emergency salvage procedure. The average time to presentation of complications was 11.8 months (range 8–14 months).

Stent migration and obstruction are, however, known and documented complications of the Memokath 051 stent. In the largest series with the longest follow-up published to date concerning ureteral Memokaths, Agrawal and colleagues ¹⁶ present their results of 74 stents inserted into 55 patients for a mean follow-up of 16 months (range 4–98 months). They describe stent migration and obstruction from encrustation as late complications, occurring at an average 7.1 months postinsertion.

Obstruction may also be the result of tumor growth proximally or distally, in cases of malignant etiology of ureteral stricture, as described by Zaman and colleagues, ⁷ who found two out of 37 cases treated with Memokath to have blockage of the stent from progressive transitional-cell carcinoma of the ureter. In our series. the obstruction was the result of encrustation in the respective case. It is also possible that in ureters in which compliance is disrupted in an extensive area, such as after stent placement, the increase in the resistance of the liquid flowing in the ureter aggravates hydronephrosis and results in apparent obstruction, as suggested in an experimental model by Kawamura and coworkers. ¹⁷

Stent migration, on the other hand is a complication that is less well understood. Little is known about the conditions and factors that influence stent anchoring and stability. One possibility is overestimation of the initial caliber of the stricture. As a rule, retrograde pyelography is routinely performed to access stricture location and length. Subsequent calibration of the ureteral stricture using graduated ureteral dilators then follows to allow the insertion of the Memokath 051 stent introducer sheath; inability to do so may prompt additional ureteral balloon dilation (in this series using a Uromax™ balloon, Boston Scientific, Natick, MA). It is possible that during this process, the stricture is inadvertently dilated to a greater extent than needed to snugly accommodate the expandable metal alloy. Initial swelling by the hot water used to fully deploy the stent may mask this effect.

Another explanation might be inappropriate placement of the stent in the first place in terms of length and/or location. A short stent inserted halfway into the stricture would migrate more easily than a stent of adequate length in the right position. Accuracy of the initial measurement is important to avoid misinterpretation of the stricture characteristics. This is accomplished by obtaining at least two serial measurements and comparing the approximate values. If there is a discrepancy of >1 cm, then a third measurement is necessary.

One of the patients in our series experienced stent migration from what later proved to be complete resolution of the obstruction. This patient had a benign, left distal stricture measuring 6 cm and is stent free with normal renal function at 14 months follow-up. Whether the effect was from continuation of medical treatment postprocedure or the patency of the ureters definitely merits further investigation and poses interesting questions about stented ureteral stricture dynamics.

There is growing evidence that a combination of medical and surgical management carries the best overall success rate for the disease, with estimated recurrence rates between 5% and 8% in most series. ^3,12,18 Periureteral or perivascular involvement in surgical biopsy or progressive renal failure and uremia are the usual indications for surgical management. While ureterolysis remains unequivocally the gold standard for surgical intervention, we suggest that the ureteral Memokath 051 metal stent may be a promising alternative for initial or salvage postureterolysis minimally invasive management of ureteral obstruction in RPF. To our knowledge, our study is the first reporting the results of the use of metal stents in this group of patients.

Conclusion

RPF remains an elusive disease with vague and nonspecific symptoms and signs. Diagnosis often relies on radiologic findings, and the distinction between malignant and nonmalignant etiology must be made with respect to prognostic implications and treatment outcome. Combined medical and surgical treatment must be recommended for patients who present with severe obstruction and advanced uremia.

Although we acknowledge the relatively small series reviewed in retrospect, in our experience, the thermoexpandable Memokath 051 stent can be considered as a safe, minimally invasive, and effective long-term means of managing ureteral obstruction in RPF. This stent does not need routine exchange unlike Double-J stents and, because of its segmental nature, is well tolerated. A larger, randomized cohort with longer follow-up is warranted to establish the role of these metallic stents in the management of this disease.