Enteral Stents

Todd H. Baron, M.D.

Keywords

Enteral stents, malignant obstruction, SEMS (self-expandable metal stents).

Introduction

Enteral stents are defined as stents deployed within the stomach, small bowel and colon. Enteral stents are designed to treat malignant luminal obstruction of the gastrointestinal tract. Although their use is primarily for palliation of malignant obstruction, they can also be used within the colon as a pre-operative modality. This review will provide an overview of the techniques and outcomes of enteral stent placement for malignant disease.

Basic Principles

Gastrointestinal SEMS (self-expandable metal stents) may be placed under endoscopic guidance with the aid of fluoroscopy by gastroenterologists or by interventional radiologists using only fluoroscopic guidance. Endoscopic placement allows more remote locations (distal for the upper tract, proximal for colon) to be accessed (1-3). Although many of the principles for placement of enteral SEMS apply to both endoscopic and radiologic insertion, this article will focus on the endoscopic techniques of placement. Much of the data on the effectiveness and outcome following enteral stent placement comes from series published in the field of interventional radiology. It is assumed that regardless of the method of stent placement, the effectiveness is the same, assuming the insertion rates and complication rates related to insertion are similar.

SEMS are composed of a variety of metal alloys with varying shapes and sizes depending on the individual manufacturer and organ of placement. The stent incorporates deep into the wall of the organ. This reaction allows anchoring of the stent and helps to prevent stent migration. With the use of covered stents this integration does not always occur and a higher rate of stent migration is seen (4). SEMS may produce imaging artifacts on both computer tomography (CT) and magnetic resonance imaging (MRI) localized to the area around the stent that may prevent accurate interpretation. Most SEMS materials appear safe for MRI, but factors such as stent shape, orientation to the magnetic field, and type of alloy composition influence signal intensity in vitro. Therefore, in a patient who has undergone enteral stent placement this information should be obtained before an MRI is performed (5,6).

Upper Gastrointestinal Tract Stents

INDICATIONS

Clear indications for placement of SEMS in the upper gastrointestinal tract are documented malignant obstruction of the stomach, duodenum, or small bowel. Advanced carcinoma of the pancreatic head is the most common malignancy causing malignant gastric outlet obstruction. Other malignancies include unresectable cholangiocarcinoma, primary or recurrent gastric carcinoma, and metastatic disease to the duodenum or proximal jejunum. Recurrent malignancies at sites of gastroenteric anastomoses are also an indication for stent placement (7). Covered stents may be useful for treatment of malignant fistula from the stomach or duodenum to surrounding structures.

CONTRAINDICATIONS

Contraindications to placement of SEMS in the upper gastrointestinal tract are free perforation with signs of peritonitis or tension pneumoperitoneum. An additional contraindication is the documentation of multiple sites of obstruction not within an area that could be covered by one or two stents. Documented peritoneal carcinomatosis is a relative contraindication to stent placement. Known benign disease, including strictures and adhesions, is considered a contraindication to stent placement.

PLACEMENT TECHNIQUES

In the United States, there is only one self-expandable stent approved by the food and drug administration (FDA) for placement in the duodenum for gastric outlet obstruction (Enteral® Wallstent, Microvasive Corporation, Natick, Massachusetts, USA). Many of the published series of SEMS placement in the upper gastrointestinal tract have been those in which standard or modified esophageal stents were used for treatment of gastric and duodenal obstruction (8-10). Outside of the U.S., a variety of covered SEMS designed for gastroduodenal use are available. These include the Choo stent (11) (Solco Intermed Co. Ltd. Seoul, Korea and M.I. Tech Co., Ltd., Pyungtaik-City, Korea), Song stent (12) (Stentech, Seoul, Korea), and Niti-S stent (12) (TaeWoong Medical, Seoul, Korea).

The techniques of insertion are different when using esophageal stents (non-TTS [through-the-scope] delivery systems) as opposed to the Enteral® Wallstent (TTS insertion). The disadvantage of the Enteral® stent is that it is not available in a covered version and is therefore susceptible to obstruction by tumor ingrowth or tissue hyperplasia induced by the stent. The bare metal ends of the Enteral® stent may cause perforation of the bowel wall (14). The common pathway for successfully placing an expandable metal stent, whether TTS or non-TTS, is the passage of a guidewire across the stricture.

Initial Placement of Guidewire
Prior to placing gastroduodenal SEMS, it may be helpful to obtain a radiographic contrast study (upper gastrointestinal barium examination) to assess the anatomy, length of stricture and degree of obstruction. However, such information may not be obtainable in the presence of complete obstruction.

Most lesions producing gastric outlet obstruction will be within the reach of a standard upper endoscope. For lesions distal to the second portion of the duodenum, it is usually necessary to use a colonoscope. Materials that should be readily available include biliary-type catheters and biliary guidewires. Hydrophilic biliary guidewires (Terumo, Tokyo, Japan) are especially useful in order to "cannulate" or access obstructive or nearly obstructive lesions. A stiff 0.035"guidewire (Savary-type wire, or 0.038"Amplatz extra stiff, Cook Medical, Spencer, IN or Amplatz 0.038" stiff guide wire Meditech/ Boston Scientific, Watertown, Mass., USA) is needed for stability in stent placement once the lesion has been accessed. Water-soluble radiographic contrast may also be needed to define stricture length as well as to insure correct passage of catheters within the gastrointestinal lumen. If marking of the tumor margins is desired, injection needles for placement radiopaque contrast are needed.

The procedure should be performed in a room equipped with fluoroscopy. It is imperative to have a gastrointestinal nursing assistant who is facile in complex therapeutic endoscopic procedures such as ERCP with metal stent placement.

The patient should be placed in the left lateral decubitus or prone position. A prone position allows for a better anatomic view under fluoroscopy. The supine position should be avoided because patients with complete gastric outlet obstruction are at high risk for aspirating retained gastric contents. With the use of standard intravenous conscious sedation the endoscope is passed to the site of obstruction. If the endoscope can be passed with minimal difficulty through the obstruction, this should be attempted, but it is important to note that the procedure can be safely completed without passing the endoscope through the stricture. Applying excessive force to the endoscope or aggressively dilating the stricture in order to pass the endoscope through the obstruction is unnecessary and increases the risk of perforation.

If the endoscope passes easily through the lesion, a stiff 0.035" guidewire with a floppy tip is placed through the endoscope channel and passed distally at least 20 cm beyond the point of obstruction. If the endoscope cannot be passed easily through the lesion, a hydrophilic biliary guidewire preloaded through a standard biliary catheter is used to "cannulate" or traverse the stricture as is done during ERCP (Figure 1). Once the wire has passed fluoroscopically through the stricture, recognized by the anatomically correct position of the wire passing into an air filled distal bowel loop (Figure 2), the catheter is advanced over the guidewire through the lesion. Water-soluble radiographic contrast is injected to confirm both proper position and lumenal patency. At this point the guidewire is exchanged for a stiff 0.035" guidewire and the procedure proceeds as described below, depending on the type of stent chosen.

Figure 1	Figure 2

Stent Selection and Placement

Figure 3

Figure 4

Figure 5

The stent chosen should be at least 3 to 4cm longer than the obstruction to allow an adequate margin of stent on either side of the obstruction. Covered stents have the advantage of closing fistula and preventing obstruction from tumor ingrowth or tissue hyperplasia. Dedicated Enteral® Wallstents are uncovered. The advantage of the Enteral® Wallstent is the ability to pass through the working channel of the endoscope and a long enough delivery system to pass through a colonoscope to allow stenting of lesions as far as beyond the ligament of Treitz.

Non-TTS Placement: The endoscope is withdrawn leaving the guidewire in place. The stent is then loaded onto the guidewire and advanced fluoroscopically to the lesion. The endoscope can then be reinserted alongside the stent delivery system to allow endoscopic guidance during deployment. Since the delivery system has no support and poor mechanical advantage it tends to loop in the greater curvature preventing forward advancement. Options to assist passage of the stent include a) preloading a snare into the endoscope, passing the snare over the delivery system, then advancing the endoscope and snare over the endoscope. The snare is closed and the endoscope advanced, advancing the stent (15); b) Using a rat-tooth forceps to grasp the stent and advance it forward (8); c) Modification of the delivery system. This has been applied to the Ultraflex stent. The handle is cut, and a plastic sheath is advanced over the string to extend the delivery system (16); d) placement through a mature gastrostomy tract (17), and e) use of external compression on the greater curvature of the stomach.

TTS Placement: If the Enteral® Wallstent is used, an endoscope with a therapeutic working channel (≥ 4.2mm) is required to allowing passage of the 10Fr delivery system. When stents are placed beyond the proximal and second duodenum a therapeutic channel adult colonoscope is usually required. After the guidewire is in position, the stent is passed over the guidewire through the working channel and is deployed under direct endoscopic guidance while maintaining the proximal position in the desired location while the stent is deployed from the distal end (Figures 3, 4 and 5).

Once the stent is fully deployed the ends of the stent should be carefully inspected fluoroscopically. If either end is not flared or fully expanded to produce a waist, the endoscopist should be suspicious that the stent chosen may have been too short to cover the entire length of the stricture. At this point contrast can be injected into the stent to assess complete patency. If needed, a second (rarely third) overlapping stent may be required to adequately treat the stricture.

The duration of the procedure is highly variable and is dependent on the degree of difficulty one encounters traversing or accessing the stricture. At least one full hour of time should be allotted once sedation is administered.

Concomitant Biliary Obstruction
In patients with malignant duodenal obstruction, coexistent biliary obstruction is commonly present and has usually occurred prior to gastric outlet obstruction (2). Because the biliary tree is usually endoscopically inaccessible through the mesh wall of a self-expandable metal duodenal stent when it has been placed across the papilla, an endoscopic expandable metal biliary stent should be placed if possible in patients with known or impending biliary obstruction prior to duodenal stent placement. Bile then flows effectively through the biliary and duodenal stents as they cross within the duodenum. To treat biliary obstruction following duodenal stent placement across the papilla, a percutaneous transhepatic approach is usually required. Stenting of both the duodenum and bile duct represents the non-surgical equivalent of a double surgical bypass.

LIMITATIONS

Limitations of successful placement include inability to pass a guidewire through the stricture, anatomic difficulties such as severe looping within the dilated stomach, or complicated post-surgical anatomy. Some patients with advanced malignancies and gastroduodenal obstruction may not improve following successful stent placement because of other unidentified sites of malignant gastrointestinal obstruction, diffuse peritoneal carcinomatosis with bowel encasement, or functional gastric outlet obstruction from neural (e.g. celiac axis) involvement by tumor.

Most patients will not be able to tolerate a solid diet. Leafy or raw vegetables should be avoided which could result in stent occlusion.

Severe complications may occur during or late after placement of gastroduodenal and jejunal SEMS. Intra-procedural complications that may occur include complications of conscious sedation, pulmonary aspiration, stent malposition perforation and bleeding. Late complications include distal stent migration, bleeding and perforation as well as fistula formation. Stent migration may be completely asymptomatic or result in bleeding, perforation, or obstruction. Symptomatic stent occlusion from tumor overgrowth, ingrowth, or food impaction requires endoscopic intervention. Obstruction by tumor ingrowth or overgrowth is usually managed with placement of additional stents through the original stent(s) (Video Clip 1).

OUTCOMES

The larger series published in the endoscopic and interventional radiologic literature (4,8-13,18-28) are presented in Table 1. Overall, the technical success rate for placement is high (approximately 90-100%) with clinical success (ability to consume p.o. intake) approximately 80-90%.

Colonic Stents

OVERVIEW

At the present time, all SEMS specifically designed for use within the colon are uncovered. However, covered esophageal stents have been used in the colon to combat problems with tumor ingrowth and to close fistulae (29). Any type of expandable metal stent may be used within the colon including esophageal, tracheobronchial, and biliary stents (30). In the United States, there are two self-expandable stents approved by the food and drug administration (FDA) for placement in the colon for malignant colonic obstruction. These are 1) the colonic Z-stent (Wilson-Cook Medical, Winston-Salem, NC); and 2) the Enteral® Wallstent, (Microvasive Corporation). The advantages of using the Enteral® Wallstent over other stents are the much longer and smaller diameter (10Fr) delivery system that allows passage of stents directly through the working channel of the endoscope. This increases mechanical advantage and lesions as far proximal as the proximal ascending colon may be successfully stented (Video Clip 2) (3). A theoretical advantage of the Wilson-Cook Z-stent (31) is the larger diameter of the lumen compared to the Enteral® Wallstent. One further advantage of the Z-stent is that is does not shorten during deployment.

Most lesions producing colonic obstruction will be within the reach of a standard flexible sigmoidoscope or upper endoscope. For lesions proximal to the descending colon it is necessary to use a colonoscope. If the Enteral® Wallstent is chosen for passage through the working channel of the endoscope, a therapeutic working channel ≥ 4.2mm diameter is required.

INDICATIONS

There are two major indications for placement of colonic stents for relief of colonic obstruction: pre-operative decompression and palliation of advanced malignancy. These will be discussed separately.

Figure 6

Pre-operative Decompression
Primary colorectal cancer that produces left-sided colonic obstruction is the most common indication for placement of SEMS for pre-operative decompression. These patients traditionally are treated with a diverting colostomy and resection followed by a later reanastomosis. Additionally, patients with total colonic obstruction are frequently ill with co-morbid medical conditions and electrolyte disturbances. Therefore, successful placement of an expandable metal stent allows for stabilization of the acute illness, an elective resection with evaluation of extent of disease and comorbid medical conditions and a one-stage operation. The tumor and stent are resected en bloc at the time of resection (Figure 6).

Palliatation of Obstruction
In patients with primary or recurrent colorectal carcinoma where the disease is widely metastatic, or the patient with potentially resectable disease but a non-operative candidate because of co-morbid underlying conditions, colorectal stent placement may serve as a palliative modality of obstruction (Figures 7 and 8). Additionally, patients with local pelvic tumors (ovarian carcinoma) or metastatic disease to the pelvis and colonic obstruction may achieve palliation of obstruction with colonic stenting (32).

Figure 7	Figure 8

Patients with malignancy within the pelvis may suffer from fistulae to surrounding structures such as the vagina or bladder. In this setting, covered esophageal stents have been used to close such fistulae and allow for non-surgical palliation (29).

CONTRAINDICATIONS

Colonic perforation is considered a contraindication to placement colonic SEMS and plain radiographs should be obtained immediately prior to stent placement in order to exclude free perforation. Benign disease is considered a contraindication to SEMS placement, although there are case reports of their use for both pre-operative decompression and for dilation of refractory benign strictures (33).

PLACEMENT TECHNIQUES

Figure 9

Patient Preparation and Positioning
It may be helpful to obtain a retrograde radiographic contrast study (water-soluble or barium enema examination) to assess the anatomy, length of stricture and degree of obstruction (Figure 9) (30). It is important to consider that there may be other sites of obstruction that would negate any effect of stenting a single site of obstruction.

Patients with complete obstruction have usually evacuated any stool below the lesion and therefore a colonic preparation is usually unnecessary. In those patients who have subtotal obstruction in the distal colon, one or two cleansing enemas are usually an adequate preparation. In patients with more proximal lesions and subtotal obstruction, a cautious standard colonoscopy bowel preparation should be given. Prophylactic antibiotics should be considered in patients with complete obstruction and a markedly dilated colon because introduction of air during the procedure may promote microperforation and bacteremia (30).

The patient should initially be placed in the left lateral decubitus position. Rotating the patient into the supine position allows for a better anatomic view under fluoroscopy, if used. Standard intravenous conscious sedation is usually administered, but is not absolutely necessary for treating distal lesions.

Description of Procedure
Placement of SEMS in the rectum and distal sigmoid the using non-TTS stents is analogous to esophageal stent placement. Placement of TTS stents, which are usually necessary for treating more proximal obstruction, is similar to that described previously for treatment of gastroduodenal and proximal jejunal lesions. These two approaches to SEMS placement will be discussed separately below.

Non-fluoroscopic guided stent placement
Non-TTS Stent Placements: For distal left-sided lesions, some authors prefer to assess the entire lesion entirely under endoscopic guidance (31). If the endoscope cannot be passed through the lesion, the stricture is cautiously balloon dilated using a 15mm TTS balloon. A 10 mm endoscope is then passed through the stricture to allow placement of a Savary guidewire as high as possible above the lesion. The endoscope is withdrawn while the stenosis is measured and the position/orientation of the lumen is assessed. After the undeployed stent is passed across the stricture, the endoscope is reinserted to verify and monitor the exact position of the distal end of the stent during deployment. Alternatively, in patients with intrinsic lesions, some authors have used laser therapy to initially recanalize the lumen to allow passage of the endoscope and guidewire for facilitate placement of SEMS (31). Both of these methods allow for stent placement without the use of fluoroscopy.

TTS Stent Placement: If the endoscope passes easily through the lesion, a stiff 0.035" guidewire with a floppy tip is placed through the endoscope channel and passed distally at least 20 cm beyond the point of obstruction. Once the stent passes through the endoscope channel, the endoscope is withdrawn below the distal margin of the stricture and the stent is deployed under direct endoscopic guidance.

Figure 10

Endoscopic/Fluroscopic Stent Placement
If the endoscope cannot be passed easily through the lesion, a hydrophilic biliary guidewire preloaded through a standard biliary catheter is used to traverse the stricture as described previously for upper gastrointestinal stenting. Once the wire has passed through the stricture, recognized fluoroscopically by the anatomically correct position of the wire passing into an air-filled, dilated proximal bowel, the catheter is advanced over the guidewire through the lesion. Water-soluble radiographic contrast is injected to confirm position and lumenal patency. At this point the guidewire is exchanged for a stiff 0.035" guidewire and the stent is deployed (Figure 10).

Outcomes
Table 2 summarizes the major case series published on colonic stents.

Pre-operative Stent Placement
There are several series describing successful pre-operative placement of colonic SEMS allowing for subsequent one-stage resection of the tumor and stent, and avoidance of a colostomy (34-37). Two studies have compared the outcome of patients undergoing endoscopic placement of SEMS for relief of acute large bowel obstruction followed by elective resection to those patients undergoing surgical intervention alone (38,39). Two of 13 patients treated with colonic SEMS required colostomy compared to 10 of 13 patients in the traditional surgical group. When the data was analyzed for the pre-operative patients, a cost savings of 28.8% was seen in the SEMS group because of a decrease in total hospital days, days spent in the intensive care unit, and fewer surgical procedures. A more recent prospective study demonstrated similar findings (39). A primary anastomosis with avoidance of colostomy was achieved significantly more often (85% vs. 41%) in the SEMS group. Total hospital stay, ICU stay and severe complications were significantly lower in the SEMS group.

In a comprehensive systematic review of all colorectal stent literature from 1990 through 2000, colorectal stent placement was successful as a bridge to surgery in 85% of 223 cases and 95% had a one-stage operation (40).

Palliation of Obstruction
Several other series have demonstrated successful palliation of malignant colonic obstruction with successful avoidance of colostomy (30,34,37,41-46). In some series, the stents effectively palliated obstruction for more than one year. The largest series of endoscopic stent placement for palliation of obstructive primary rectal and rectosigmoid obstruction was published by Spinelli et al. (31). Stents were successfully placed in 36 of 37 patients. Three early migrations occurred. Twenty-eight of the remaining 33 patients had good long-term resolution of obstruction without need for further treatment.

Nearly all series have used uncovered stents. One study found an unacceptably high rate of migration using fully covered stents (47). However, in a recent study using partially covered stents in 16 patients with covered stents placed for palliation of malignant left sided obstruction only two stent migrations occurred (29). At a mean follow-up of 21 weeks, no stent occlusion was seen.

In the previously mentioned comprehensive systematic review of all colorectal stent literature from 1990 through 2000, colorectal stent placement was successful as a palliative modality in avoiding a colostomy in 90% of 336 cases (40).

Limitations
Limitations of successful placement include inability to pass a guidewire through the stricture, and anatomic difficulties such as a severely angulated and "fixed" sigmoid which prevents advancing to the site of the lesion. Some patients with widely advanced malignancies and colonic obstruction may not improve following successful stent placement because of other unidentified sites of malignant gastrointestinal obstruction or diffuse peritoneal carcinomatosis with small bowel encasement (30).

Severe complications may occur during or late after placement of colonic SEMS. Intra-procedural complications that may occur include complications of conscious sedation, stent malposition, perforation and bleeding. Two important tips are helpful to avoid intraprocedural perforation. The first is limiting the amount of air insufflation during the exam, especially in patients with a dilated cecum. The second is avoiding aggressive pre- or post-stent dilation (30,40).

Late complications include distal stent migration, bleeding and perforation. Stent migration may be completely asymptomatic or result in rectal bleeding or tenesmus. Removal of distally migrated stents from the rectum is not technically difficult. Proximal migration following successful placement does not occur, but malposition or maldeployment of a stent completely above the stricture is usually of no sequelae, assuming additional stents are placed to relieve the obstruction (personal experience). Stents placed very distally in the rectum may produce tenesmus, rectal pain or fecal incontinence and patients with distal rectal obstruction should be advised of this possibility prior to stent placement. In general, if the stent is placed at least 2 cm proximal to the upper end of the anal canal it does not interfere with anal function. Symptomatic stent occlusion from tumor overgrowth, ingrowth, or stool impaction requires endoscopic intervention. Obstruction by tumor ingrowth or overgrowth is usually managed with placement of additional stents through the original stent(s).

There is little data concerning the safety of SEMS in the colon or rectum in the setting of prior or concomitant radiation therapy. One case report suggests that concomitant chemotherapy and radiation therapy may be safe (48). It is possible that this approach may promote stent migration as the tumor shrinks in response to treatment.

In the previously mentioned comprehensive systematic review of all colorectal stent literature from 1990 through 2000, mortality related to stent placement was 1% of 598 patients. Colorectal stent placement was complicated by perforation in 0 to 7%, stent migration in 3-22%, bleeding in 0-5%, and reobstruction in 0-15% (40).

SUMMARY

For upper gastrointestinal stent placement, palliation of malignant gastric outlet obstruction is a viable alternative to in patients with unresectable cancer and a poor performance status. For colonic stents, pre-operative stenting may allow a one-stage operation and avoidance of colostomy. In patients undergoing palliation of obstruction, SEMS may allow avoidance of surgery altogether. Newer stent designs for both upper and lower gastrointestinal use are needed that are covered while still allowing delivery through the endoscope channel, and are associated with low migration rates.

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Editor:
John C. Deutsch, M.D.
Duluth, MN

Editorial Board:
Manoop S. Bhutani, M.D.
Galveston, TX
William R. Brugge, M.D.
Boston, MA
Peter R. McNally, D.O.
Denver, CO
Iqbal S. Sandhu, M.D.
Salt Lake City, UT
Thomas J. Savides, M.D.
San Diego, CA