A systematic search was performed, with no restriction regarding the idiom or the year of publication, since the inception of database till October 01, 2018 using PubMed, MEDLINE, Web of Science, EMBASE, and Cochran Central Register of Controlled Trials databases. Both MeSH and non-MeSH terms were included in the search.

All studies comparing colonic stent vs surgery for acute malignant large bowel obstruction were included. Relevant studies about colonoscopy, acute obstructive abdomen due to neoplasia were also included.

Studies were excluded from this review according to the following criteria: use of the stent for benign treatment, stents placed by an interventional radiologist; unclear or missing data for the outcomes variables.

Randomized trials (Evidence 1A) were prioritized as well as previous reviews on the topic.

Currently, indications for stent placement in patients with MCO are: Stent as a “bridge to surgery” to avoid ES[57]; Palliative CRC patients[58]; Extra colonic tumors causing acute abdominal obstruction (e.g., advanced gastric cancer, ovarian cancer)[59-61].

Signs of systemic toxicity or septic shock as these are signs of colonic ischemia or perforation[62]. Intra-abdominal abscess. Excessively dilated cecum (> 9 cm) as endoscopic insufflation may precipitate colonic perforation. Distal rectal lesions that would require the stent to cross the dentate line as this can induce severe pain, tenesmus, and rectal bleeding[34,63]. Persistent coagulopathy (relative)[64].

Rarely, extrinsic lesions can compress the colon causing MCO. The most frequent causes of extrinsic obstruction are primary pelvic malignancies (ovarian, uterine, and bladder cancer), advanced gastric cancer or metastatic lesions to the pelvis[65]. Extrinsic obstruction occurs more frequently in the left colon, especially in the distal region, and in these cases endoscopic tissue biopsy is not technically possible and the exact etiology and extent of obstruction is often not clear[60,61].

Patients with extrinsic colonic obsltruction, in the vast majority of cases, have advanced disease with reduced life expectancy and no potential curative surgical ressection. .Nevertheless, the technical and clinical success of stenting in these cases, is less effective then when apllyied to primary colorrectal tumors, and there no ideal option[59,65].

Whether it is in palliative patients or in those who will use the stent as a bridge for elective surgery, there is a high chance of colonic perforation if chemotherapy is associated, especially with angiogenesis inhibitors such as bevacizumab[66,67]. The European Society of Gastrointestinal Endoscopy does not recommend the combination therapy of stent with antiangiogenic drugs[62].

Chemotherapy for metastatic CRC (CRCM) has evolved in the last decade from cytotoxic agents to molecular targeting agents[68]. Currently, four cytotoxic agents [5-fluorouracil (5-FU), capecitabine, irinotecan, oxaliplatin][69,70] and five molecular targeting agents [bevacizumab (BV), cetuximab, panitumumab, aflibercept, and regorafenib] are used as chemotherapeutics for CRCM, given in combination or alone[71-74].

New chemotherapy schedules can take approximately 24 mo[75-77], twelve months longer than that used in the classic 5-FU + leucovorin scheme[78]. Chemotherapy decreases the risk of tumor ingrowth compared to the use of SEMS alone; however, chemotherapy is considered a significant risk factor for long-term complications, including perforation and stent migration[79]. Regarding the use of bevacizumab, studies have reported that it is an independent risk factor for late complications and even without a stent increased the risk of perforation by 19.6 times[80,81].

Considering the benefits of increased survival and QOL achieved by new chemotherapies, there still is a role for stents in palliation, however, with parsimony[66,82].

Covered stents are mainly used in the establishment of colonvesical fistulas, coloenteric and cervicovaginal malignancies[86]. Although the theoretical advantage of covered stents is that they have a lower risk of tumor ingrowth, they are also more likely to migrate compared to uncovered stents.

In a randomized trial[84] including 151 patients with AAO by CRC, there was no difference in the clinical success rate for the placement of covered stents compared with uncovered stents (96% vs 92%). There was a higher rate of migration (21% vs 2%) and a trend towards less tumor ingrowth in covered stents (4% vs 15%). There were no differences in relation to adverse events or obstruction by debris.

Although patients may have AAO, bowel preparation should be attempted and preparation depends on location and degree of obstruction: For partial obstruction in the distal colon, two water-soluble enemas (250-500 mL) are sufficient; For partial obstruction of a proximal lesion, oral colon preparation may be attempted and discontinued if symptoms such as abdominal pain or emesis occur[89]; For complete colonic obstruction, oral preparations are contraindicated due to high risk of perforation. Rectal water-soluble enemas should be considered[90].

Prophylaxis is not mandatory for patients undergoing stenting. However, in patients with complete obstruction, we suggest prophylaxis considering the risk of micro perforation and bacteremia during insufflation[62,91].

Lower endoscopic procedures can typically be performed anywhere on the sedation spectrum, from sedation to general anesthesia. However, this is not applied to patients who need to use a stent because they are in an obstructive emergency. General anesthesia with active airway management should be mainly performed to prevent bronchoaspiration with feculent emesis for example and also does not move at the time of the procedure increasing the chance of perforation[92,93].

Stents are always placed under endoscopic guidance with the aid of fluoroscopy[62,94] (Figure 4). During colonoscopy, limited insufflation should be used to minimize the risk of perforation due to the risk of a closed loop between the obstructive lesion and the ileocecal valve. The use of carbon dioxide has largely supplanted air for this procedure, and most complex therapeutic cases[95]. A water-immersed colonoscopy is another technique that can be used to minimize bowel distention[96].

Upon reaching the lesion, an attempt can be made to cross the stenosis with the endoscope. However, if the endoscope does not traverse through the obstruction easily, a 0.035-inch guidewire may be passed through stenosis under fluoroscopic guidance.

The first RCT[48] comparing stenting versus ES in palliative patients included balloon dilation of the stenosis prior to stent placement, which is no longer considered and acceptable practice. If endoscope passage through the stenosis is not possible, fluoroscopic guidance is preferred to balloon dilation as the latter is associated with increased risk of perforation[84].

After confirming the length of stenosis, either through the passage of the endoscope or with contrast injection under fluoroscopic guidance, technique of stent placement depends on the type of stent being used.

For this method, a therapeutic endoscope is needed to introduce the stent through the working channel (Figure 5). If the stenosis cannot be traversed, contrast injection helps to delineate the stenosis and to confirm guidewire placement trough the stenosis under fluoroscopy.

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Figure 5 Stent placement by Through the scope technique.

The stent is then passed over the guidewire to the proximal margin of the tumor and then implanted under fluoroscopic guidance and endoscopic visualization of the distal portion of the stent. Each end of the stent must be at least 2 cm longer than the stenosis (4 cm of safety margin), as these stents typically shorten after deployment and expansion[62,85,97] (Figure 6).

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Figure 6 The stent is passed over the guidewire to the proximal margin of the tumor and then implanted under fluoroscopic guidance and endoscopic visualization of the distal portion of the stent. A: Malignant lesion causing colonic stenosis; B: Stent deployment; C: Stent immediately after deployment; D: Fecal contents coming through the stent after decompression.

To prevent migration, it is not recommended that the endoscope be passed through the stent once the stent is placed, although endoscopic/fluoroscopic visualization should be used to rule out early complications.

After the guidewire placement, endoscopic visualization is still preferred, however, not absolutely essential[64]. This technique may be helpful when there is an acute angulation or others factors limiting endoscopic visualization. The stent is inserted over the guidewire and implanted under fluoroscopic guidance (Figure 7).

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Figure 7 Stent placement by Over-the-wire technique.

The correct position of the stent reveals a waist in the center of the stent that crosses the tumor with a widening of the proximal and distal ends. If either end of the stent is not adequately expanded to produce a waist, it may be too short to cross the stenosis. In such cases, a second or third overlapping stent can be used without removing the first to completely cross the stenosis[98].

This may occur late or immediately after the procedure and is associated with poor outcomes[9]. Several factors can increase the risk of perforation including radiotherapy and chemotherapy as well as colonic anatomy[3,66,67]. A meta-analysis in 2018, including palliative patients revealed a perforation rate of 9.5%[53].

Occurs in approximately 10% when used as a bridge to surgery and in 1% of palliative patients, usually one week after insertion. The main causes of migration include incorrect stent selection, stent dimensions being too narrow, small, or short, mild stenosis that is not obstructive, and improvement of stenosis due to radiotherapy or chemotherapy[7,32,53]. Other less common factors that may precipitate stent migration include extrinsic lesion, dilation of stenosis, or use of covered stents.

Occurs in approximately 11.1% of palliative patients. This occurs due to tumor overgrowth at the proximal or distal margins of the stent or through tumor in growth through the cells of the stent[53]. Possible endoscopic treatments for stent obstruction include laser ablation of the tumor, argon plasma coagulation or placement of new stent[48,102].

Immediate post-procedure bleeding may occur due to irritation of the colon mucosa by stent flanges, tumor friability, trauma due to either stent passage or guidewire placement, or endoscope trauma. Late bleeding may be due to stent related ulcerations or erosions in the colonic mucosa[64].

Mild abdominal pain is common and may be prolonged up to five days after stenting. For this, the use of simple analgesics can be helpful. Opioid analgesics may be required within 48 to 72 h of stenting due to expansion of the stent with consequent worsening of pain[34,85]. For low rectal lesions, stent-induced irritation of the nerve endings near the squamous-columnar junction should be avoided.

Despite the high technical success of stent implantation, failure of colonic decompression can occur. This often results in urgent surgery and is considered a serious adverse event[9,103]. The most common reasons are described below[91]: Other additional sites of intestinal obstruction; Stent shorter than stenosis length; Incomplete stent expansion; Stent migration; Underlying motility disorder; Fecal impaction.

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Figure 8 Specimen and stent. A: Specimen after surgery showing the stent crossing the lesion; B: Image of the stent and the malignant lesion after resection.

In a recent systematic review and meta-analysis performed by Arezzo et al[15], used only RCTs with a total of eight articles: Mortality rate of 9.9% in ES group and 9,6% was demonstrated in the SEMS group; Adverse events rate of 51.2% was demonstrated in the ES group versus 33.9% in the patients with stent; Temporary colostomy rate of 33.9% was demonstrated in the stent group vs 51.4% of patients undergoing ES; The definitive colostomy rate of 22.2% was demonstrated in the stent group versus 35.2% demonstrated in patients of ES group; Regarding the success of the primary anastomosis, there was a 70% rate for the stent group versus 54.1% for the ES group; The need for surgical intervention due to adverse events was 10.9% in the patient with SEMS versus 8.7% in the ES group; The operating time: 172 min in patients submitted ES and 146 minutes in the SEMS group; The hospitalization time was, on average, 14.5 d for patients submitted to ES and 15.5 d for those submitted to STENT; Tumor recurrence was 40.5% in patients in the stent group and 26.6% in the ES group (Table 2).

A recent systematic review and meta-analysis performed by Ribeiro et al[53] in 2018, compared the use of stents to surgical intervention, only in palliative patients, as a definitive treatment. Only RCTs were included, with a total of four articles including 125 patients: 30-d mortality rate of 6.4% in patients submitted ES vs 6.3% in the stent group; Analyzed survival was 244 d in the ES group and 279 d in the SEMS group; Clinical success was 84% in the SEMS group and 96% in the ES group; 30-d adverse event rates were 36.5% in the stent group and 24% in the ES group; Technical success was favorable to the surgery group (84% for stent group and 97% for ES group); Rate of permanent colostomy was higher in the surgery group (86.1% versus 14.3%); Length of intensive care stay was not statistically significant between groups; The mean time of hospitalization was 35.5 d for patients undergoing ES and 17.5 for the stent group; Perforation was the most common complication found in the stent group, representing 42.8% of total adverse events, with six of sixty-three patients (9.5%) having perforation, one (1.5%) migration and seven (11.1%) obstruction.