Complications of colonoscopy consultation is part of a series discussing the use of gastrointestinal endoscopy in common clinical situations. Provided by the American Society for Gastrointestinal Endoscopy. In writing this guideline, a number of expert-recommended articles were consulted in addition to those retrieved from MEDLINE. The guidelines for the rational use of endoscopy are based on a number of important current reviews and expert consensus. Introduction Complications of colonoscopic consultation are rare, but can be serious and fatal. A group of more than 25,000 diagnostic colonoscopy studies reported an overall complication rate (mainly bleeding and perforation) of 0.35% [1], similar to the 0.3% reported in a recent prospective study of 3,196 cases [2]. The complication rate of colonoscopic polyp removal is elevated to 2.3% [1], but its complication rate is less than that of open colectomy and polyp removal, which has an incidence of 14-20% [3,4] and a 5% mortality rate [3]. The incidence of complications from colonoscopy in community health units is more difficult to determine, as complication rates are reported mainly from experienced centers. In addition, the complication rate of colonoscopy and polyp removal has decreased with the improvement of equipment, electrosurgical techniques and experience in treatment [5]. The complication rate of colonoscopy screening in the asymptomatic population is 0.2-0.3% and includes bleeding, perforation, myocardial infarction and cerebrovascular accidents [6,7]. With the introduction of a large amount of multicenter data such as the Clinical Outcomes Research Initiation (CORI) program, it can be better used for complication assessment in the future. However, despite the ability to obtain more accurate data on complications immediately after the operation, as not reported, late complications are still underestimated. The following methods of colonoscopic polyp removal are available, cold biopsy, thermal biopsy (e.g. biopsy with cautery) and trap biopsy with or without electrical biopsy. Argon ion coagulation is also used to remove large clotless polyps in pieces [8,9]. Complications of polyp removal likewise include those of diagnostic colonoscopy. Additional complications directly related to polyp removal are acute or delayed bleeding, perforation of the polyp removal site, and post-polyp removal coagulation syndrome. Complications related to sedation have been addressed in the guidelines for upper gastrointestinal endoscopy [10]. 1. risk factors for complications Previous colonoscopies and physical examinations have shown that pre-screening medications and abnormal coagulation mechanisms increase the risk of bleeding. Although bleeding occurred similarly in a group of 4735 polyps removed with pure cutting current and coagulation or mixed current, it is generally accepted that the use of pure cutting current may increase the risk of bleeding [11,12]. The incidence of bleeding after polyp removal decreases with increasing experience of the endoscopist [13]. The relationship between the size of the polyp and the occurrence of perforation is unclear [14], however, it is believed that the incidence of perforation is highest in right-sided, non-tipped polyps because these regions have the thinnest colonic walls [15]. 2. prevention of complications Despite best efforts, there is always a risk of complications during colonoscopy or polyp removal; however, some measures can minimize the occurrence of complications. Accurate tightening of the capsule for removal of polyps requires some experience. Inappropriate delay in closing the capsule can cause the polyp stem to dry out, thus preventing complete closure of the capsule. Conversely inadequate cautery before closing the trap can easily cause bleeding. In addition, great care should be taken to avoid inclusion of normal mucosa into the captive apparatus [16]. Injection of saline or norepinephrine into the base of the polyp or under the polyp elevates the polyp and increases the degree of separation of the polyp from the submucosa, which has been used as a technique to reduce the risk of bleeding after polyp removal, especially for removal of large, non-tipped polyps located on the right side of the colon, and also to reduce the depth of thermal injury [17-20]. Mechanical methods such as metal clips or detachable traps have also been used to prevent bleeding associated with polyp removal [21,22]. In patients with coagulation disorders, it is more appropriate to delay checking or correcting coagulation abnormalities. Because these complications are uncommon, there are no controlled studies to prove the merits of these methods. To reduce bleeding, the use of a small captive device without electrocautery may be considered instead of thermal biopsy forceps for small polyps [23]. 3. complications associated with bowel preparation The bowel is cleaned before colonoscopy for better visualization of the colonic mucosa and, in addition, to reduce the concentration of potentially explosive gases in the intestinal lumen. Very few complications of gas explosions in the intestine have been reported [5,24]. One group of studies found flammable gases hydrogen and methane in the intestinal lumen in 10% of patients despite pre-sigmoidoscopy preparation with only standard phosphoric acid soda enemas, while patients with intestinal preparation with polyethylene glycol had no flammable gases [25,26]. Other studies have found a potential risk of intestinal gas explosions with enteral preparation performed with mannitol [27]. Two types of bowel preparations are commonly used: balanced salt solutions containing polyethylene glycol (PEG) and non-polyethylene glycol solutions such as magnesium citrate and phosphate (oral phosphate soda). In the elderly, in patients with renal insufficiency or bruising heart failure, both preparations may cause fatal water-electrolyte disturbances. Other rare complications of oral bowel preparation are vomiting-induced cardia mucosal tear syndrome (Mallory-Weisstears) [31-33], esophageal perforation [34], and aspiration pneumonia [35]. Bowel preparation with phosphates may cause endoscopic and histological changes in the intestinal mucosa of patients with inflammatory bowel disease [36]. 4. perforation Colonic perforation during colonoscopy may arise from mechanical injury to the intestinal wall by colonoscopy, pneumatic injury, or directly due to treatment. Early symptoms of perforation include persistent abdominal pain and distension, and later symptoms are mainly due to peritonitis, including fever and elevated leukocytes, and the presence of free gas under the diaphragm on chest and abdominal plain radiographs.CT is superior to standing plain radiographs [37]; therefore, abdominal CT should be considered in patients with suspected perforation and no free gas on chest and abdominal plain radiographs. There is little difference in the incidence of diagnostic and therapeutic colonoscopic perforations. In a group of 25,000 colonoscopy consultations, the incidence of diagnostic colonoscopy perforation was 0.2% [1], with a 0.32% incidence of perforation in 6,000 colon polyp removals. In contrast, another group of 5000 colonoscopy consultations reported an incidence of 0.12% (4 cases) of diagnostic colonoscopy perforation and 0.11% (2 cases) of perforation in polyp removers [38]. A retrospective analysis of 1555 polyps removed in 1172 patients reported only one silent perforation after removal of a 1-cm sized tipped polyp [16]. No perforation occurred in a group of 591 patients with 1000 polyps removed colonoscopically [39]. In a group of 777 patients with 2019 polyps removed, perforation occurred in 2 cases (0.3%) [40]. While another group of 3196 colonoscopic screening prospectively studied no perforations occurred [2]. 5. bleeding Bleeding after colonoscopy is classified as lower gastrointestinal bleeding, and its occurrence may require blood transfusion, hospitalization, re-colonoscopy or surgery [2]. Bleeding may occur soon after polyp removal or up to 29 d after the operation [41]. The site of bleeding can be determined by endoscopy or erythrocyte nuclear scan [42]. The reported incidence of bleeding after polyp removal ranges from 0.3% to 6.1% [2, 11]. A survey by the American Society for Gastrointestinal Endoscopy (ASGE) found a 0.09% incidence of bleeding in 25,000 diagnostic colonoscopies and a 1.7% incidence of bleeding in 6,000 polyp removals [1]. Bleeding occurred in 48 polyps removed (2.7%) in a group of 1795 polyps removed [38]. One group reported bleeding after polyp removal in 0.64% (0.85% of patients) [12], with 3 of 10 patients requiring blood transfusion. Another series of 591 patients underwent 1,000 polyp removals, with 8 cases of minor bleeding (1.4%) [39]. Other investigators reported bleeding immediately after polyp removal in 1.5% and late bleeding in 1.9% [40]. Although there is a difference in bleeding rates between hot biopsy, cold biopsy and trap electrocautery, no investigator has confirmed this observation. 6. post-polyp removal coagulation syndrome It has been reported that during polyp removal, damage to the intestinal wall due to electrocoagulation can cause burns to the transmural membrane in 0.51-1.2% of patients, causing post-polyp removal coagulation syndrome [16,40]. This syndrome usually occurs 1-5 d after colonoscopy and typically presents with fever, limited abdominal pain, signs of peritonitis and leukocytosis, with no free gas on radiography. five of the six reports reported cases with polyps located in the right wall of the colon and all were non-tipped polyps [16]. It is important to identify this condition as this one does not require surgical treatment. 7. other complications Other rare complications of colonoscopic consultation include splenic rupture [43], acute appendicitis [44], and intra-abdominal bleeding due to mesenteric vascular tears. Chemical colitis can also be caused if the glutaraldehyde used for disinfection is not cleaned [45]. Complications of colonoscopic polyp removal also include bacteremia [46], retroperitoneal abscesses [47], subcutaneous emphysema [48,49], and traps that ensnare the normal intestinal mucosa [16]. Deaths related to colonoscopy have also been reported, with 5 deaths (0.006%) in 83,725 operations reported [50]. 8. treatment of complications All patients with simple perforation need to be considered for surgical management, although perforation usually requires surgical repair and non-surgical management can be considered in some cases [51], and surgery can be avoided in patients with silent perforation or limited peritonitis without signs of abscess formation and where conservative treatment is effective [52,53]. Laparoscopic perforation repair is also feasible [54]. So-called microperforations are those that are detected relatively early (6-24 h after polyp removal) and present with limited abdominal pain and abdominal muscle tension without the irritating symptoms of diffuse peritonitis [55]. The management of such patients is bowel rest, intravenous antibiotics and observation for any worsening of clinical manifestations. Although closure of the perforation with a clip has been reported [56], this method is not currently recommended. Bleeding after polypectomy removal is usually significant and can be treated by colonoscopy. Treatment methods used for GI bleeding, in addition to standard endoscopic treatments (e.g., injection therapy, thermal coagulation, and electrocoagulation), have recently included ligatures, intra-loop ligation, and hemostatic clips for clinical use [57,58]. Non-endoscopic management methods include vascular embolization and surgery [59]. Management of bleeding after polyp removal does not always require admission to the intensive care unit. Post-polyp removal coagulation syndrome is usually treated with intravenous rehydration, broad-spectrum antibiotics and fasting until symptoms disappear [16], and successful treatment by oral antibiotics in an outpatient setting has also been reported [40]. 9. colonoscopic staining If the lesion is not ready for immediate transendoscopic removal after detection or needs to be positioned for endoscopic follow-up, a dye that can be permanently present (e.g., India ink) is used to tattoo the colon near the lesion, thus making subsequent surgical or endoscopic follow-up easy to locate. The injection of permanently present natural ink also requires consideration of the safety of the operation. In a group of 55 patients who had their colon tattooed with India ink, biopsy review after a mean of 36 mo revealed mild chronic inflammation of the colon in six patients and hyperplastic changes in one [60]. A group of 7 patients who underwent surgery 1d to 7w after colon prick marking were found to have histological changes including necrosis, edema, and neutrophil infiltration in the submucosal and plasma muscle layer tissue of the colon [61]. Colonic abscesses with peritonitis have also been reported after colonic stab markings [62]. A review on this topic reported a complication rate of about 0.22% for colonic punctal labeling [63]. Animal experiments in which India ink was diluted (1:100) to the extent that it was visible endoscopically and laparoscopically, and laparoscopic surgery was performed 7 d to 1 mo after injection, revealed no significant histological changes [64]. A new carbon-based permanent marker was recently reported to be injected at 188 in 113 patients with no complications [65]. 10. safety of barium enema angiography after failed colonoscopy If no perforation occurs, the patient has been adequately prepared, and the colonoscopy fails, a same-day barium enema angiography can be considered [66]. In contrast, it is not safe to perform a barium enema within 5 d after colonoscopic polyp removal or deep biopsy of the colon (except rectum) [67,68]. However, there is little information on the importance of this measure. In addition, it is unclear whether CT imaging of the colon (simulated colonoscopy) is safe immediately after colon polyp removal. In conclusion, complications of colonoscopy are rare but unavoidable, with an incidence generally below 0.35%. Because of the possibility of complications, an informed notification system should be established. Operational complications include perforation, bleeding, post-polyp removal coagulation syndrome, infection, preparation-related complications, and death, with more complications occurring with therapeutic colonoscopy than diagnostic colonoscopy. Risk factors for complications associated with colon polyp removal include the site and size of the polyp, operator experience, technique of polyp removal, and the type of coagulation current used. Injection of saline under large, non-tipped polyps reduces the depth of thermal injury, thereby reducing the incidence of complications. Early recognition and timely management of complications can reduce patient mortality. Different treatments are used for different complications, such as supportive therapy for coagulation syndrome after polyp removal, injection or electrocoagulation to stop bleeding after enteroscopy, and surgical repair of simple perforation. Considering risk factors, timely identification of potential complications, and reasonable management to minimize the risk can facilitate patient regression.