adequate bowel preparation and appropriate luminal distention are essential for inspection , allowing visualization of the entire colonic mucosa , and performance of adequate colonoscopy , defined as an adenoma detection rate 25% . total insufflated air volume during colonoscopy is typically 8 to 18  l . however , air insufflation ( ai ) can cause pain and discomfort , especially when the scope is passed through the sigmoid or transverse colon . several methods to avoid painful colonoscopy have been introduced , and new robotic methods , although not widely used , have also been introduced . this review describes various techniques for painless colonoscopy , including gaseous methods , water - immersion methods , and others . co2 is absorbed across the intestine 160 times more rapidly than nitrogen and 13 times faster than oxygen . the first use of co2 insufflation in endoscopic procedures was in colonoscopy . in a 1984 report , hussein et al .   compared postcolonoscopy discomfort in a co2 insufflation group and an ai group . the co2 insufflation group showed no significant residual abdominal gas on plain radiographs 30  minutes after colonoscopy and much less discomfort than the ai group , which showed large amounts of gas on radiographs . since these data were reported , more than 20 randomized controlled trials ( rcts ) on the effectiveness of co2 versus ai have been published . moreover , three metaanalyses and one systematic review based on previous rcts have recently provided useful information on clinical outcomes of co2 gas in colonoscopy practice [ 5 - 8 ] . according to the latest meta - analysis , based on 24 rcts , and previous meta - analyses and a systematic review , co2 gas insufflation is significantly superior to ai in terms of intraprocedural pain and discomfort . early postprocedural pain at 2 hours after colonoscopy , intermediate pain at 6 hours after colonoscopy , and late pain at 24  hours after colonoscopy were compared , and co2 insufflation was superior to ai at all times for reducing postprocedural pain and flatus . in colonoscopy performance , co2 insufflation did not show an improvement in cecal intubation time or rate compared with ai . regarding safety , end - tidal co2 ( etco2 ) levels during and after the procedure were not significantly different between the co2 and ai groups . sedation during colonoscopy also did not affect the etco2 level in the co2 and ai groups . other than the etco2 level , adverse events related to co2 insufflation during colonoscopy have not been reported , or showed no difference compared with conventional ai colonoscopy . evidence for the safety of co2 insufflation colonoscopy in high - risk conditions , including chronic obstructive pulmonary disease , severe cardiac disease , sleep apnea , and morbid obesity , is still very limited , and caution should be exercised in using co2 insufflation during in these groups . regarding exposure to co2 during colonoscopy , recent evidence showed that there was no difference in intraprocedural or postprocedural pain levels between a scope extubation - only co2 insufflation group and a whole - procedure co2 insufflation group ; thus , both partial - procedure co2 insufflation and extubation - only co2 insufflation can be used to decrease postprocedural discomfort and pain . although co2 exposure can be decreased during colonoscopy by following this study s procedure , the study also had several limitations , such as the small number of enrolled cases and the fact that a colonoscopy expert performed the procedures . in conclusion , co2 insufflation during colonoscopy can decrease the symptoms of abdominal pain and discomfort during and after colonoscopy , but is more effective in reducing postprocedural pain than real - time insertion pain . thus , co2 insufflation can even be helpful for decreasing postprocedural pain in sedated procedures , after the sedative effect wears off . moreover , there was no impact on procedure quality , including cecal intubation and procedure time . although co2 insufflation did not increase procedure - related complications or etco2 levels , there is very limited evidence for safety of co2 use in high - risk patient groups . thus , co2 insufflation , instead of room air , can be used routinely for colonoscopy in patients without specific risk factors . water - assisted colonoscopy was first reported in 1984 as a water immersion ( wi ) method , which facilitated passage through a sigmoid colon with diverticulosis . in this method , the basic concept of wi is that water is used instead of gas to distend the colon to visualize the way forward during intubation . a lumen distended with water appears to be minimally distended and angulation is reduced compared to that with ai . after the scope reaches the cecum , suction removal of water is performed during the withdrawal phase of colonoscopy , allowing inspection of the mucosa . since wi was introduced in the 1980s , several later studies reported advantages and methodologies , such as speeding up intubation and providing warm water instillation to counter colonic spasm . in retrospective studies , wi showed a higher cecal intubation rate than the air method and similar pain scores between sedated and unsedated groups , suggesting the possibility of wi as a pain - reducing technique without sedation . several later rcts comparing pain scores for wi and ai with minimal sedation have been reported ( table 1 ) [ 16 - 26 ] . beyond comparing pain scores , most studies evaluated the impact on colonoscopy procedures , including cecal intubation and the adenoma detection rate . although most published rcts showed reduced pain during colonoscopy , wi could be affected if the bowel preparation was inadequate . the water exchange ( we ) method , modified from wi , was developed by leung et al .   as the least painful , scheduled unsedated colonoscopy procedure . the we method involves continuous water infusion and suction of residual feces and air to clear the view , with no ai . maintaining suction and water infusion cleared the view of residual feces and enhanced the adenoma detection rate , but suctioning dirty water and replacing clean water during insertion is time - consuming . we has major limitations in terms of time , with we during insertion and a longer learning period , but is superior to wi for painless colonoscopy and an improved adenoma detection rate , based on a limited number of rcts . a meta - analysis and systematic review compared ai and water - aided methods , including wi and we , for pain score , adenoma detection rate , and requirements for sedation ; both wi and we were superior to ai regarding procedure pain . recently , several trials have compared water - aided colonoscopy with co2 insufflation , and performed head - to - head comparisons using wi , we , and co2 [ 17,32 - 34 ] . compared with ai and co2 , wi and we significantly reduced colonoscopy insertion pain , and we was the least painful technique . these systems generate internal force and need no or minimal external pushing actions , which helps to limit discomfort and pain during insertion of the scope . although the evidence is very limited , some reports of robotic techniques have shown pain reduction during procedures . the endotics system ( era endoscopy , peccioli , italy ) consists of a sterile , disposable probe ( e - worm ) and a workstation . the probe has a head , a steerable tip , a flexible body , and a thin tail . the workstation allows the endoscopist to control the disposable probe using a hand - held console . the operator can steer the probe head in any direction , elongate the probe body to move it forward , and control rinsing , insufflation , and suction . small - scale studies have been reported with endotics , showing superiority in pain reduction versus conventional colonoscopy , but prolonged insertion times and a relatively low cecal intubation rate need to be overcome . los gatos , ca , usa ) system is similar to a conventional endoscope connected to a pc workstation . the system was designed to traverse the natural shape of the colon , based on a computerized map , so that less pressure is needed , and it can reduce the incidence of looping significantly . only one clinical trial has been reported on the feasibility of the nes system , showing a high success rate for cecal intubation , but still a high looping incidence ( 40% ) . the invendoscope ( invendo medical , garden city , ny , usa ) is a single - use , hand - held controlled colonoscope , with a 10-mm inner sheath . this disposable device is similar to conventional endoscopes , allowing for insufflation , rinsing , and suction with a 3.1-mm working channel . insertion and withdrawal of the colonoscope is controlled by a hand - held control unit . according to the reported evidence , the invendoscope showed a low pain / discomfort score . the endotics system ( era endoscopy , peccioli , italy ) consists of a sterile , disposable probe ( e - worm ) and a workstation . the probe has a head , a steerable tip , a flexible body , and a thin tail . the workstation allows the endoscopist to control the disposable probe using a hand - held console . the operator can steer the probe head in any direction , elongate the probe body to move it forward , and control rinsing , insufflation , and suction . small - scale studies have been reported with endotics , showing superiority in pain reduction versus conventional colonoscopy , but prolonged insertion times and a relatively low cecal intubation rate need to be overcome . the neoguide endoscopy system ( nes ; neoguide systems inc . , los gatos , ca , usa ) system is similar to a conventional endoscope connected to a pc workstation . the system was designed to traverse the natural shape of the colon , based on a computerized map , so that less pressure is needed , and it can reduce the incidence of looping significantly . only one clinical trial has been reported on the feasibility of the nes system , showing a high success rate for cecal intubation , but still a high looping incidence ( 40% ) . the invendoscope ( invendo medical , garden city , ny , usa ) is a single - use , hand - held controlled colonoscope , with a 10-mm inner sheath . this disposable device is similar to conventional endoscopes , allowing for insufflation , rinsing , and suction with a 3.1-mm working channel . insertion and withdrawal of the colonoscope is controlled by a hand - held control unit . according to the reported evidence , the invendoscope showed a low pain / discomfort score . previous rcts have shown that co2 gas is a good option for reducing pain and discomfort during and after a procedure . however , a recent meta - analysis showed that co2 insufflation was more effective in reducing postprocedure pain than pain during the procedure . there is still limited evidence for the safe use of co2 insufflation in high - risk patients . wi , and the recently modified we method , are good options for reducing intra- and postprocedural pain and discomfort compared with ai and co2 insufflation . we is the best modality to reduce patient discomfort and enhance the adenoma detection rate , although the we procedure itself is time - consuming and requires a learning period . several robotic colonoscopy methods have shown favorable results in reducing procedure - related pain and discomfort .
during colonoscopy , air insufflation to distend the lumen and facilitate careful inspection and scope insertion can induce pain and cause discomfort . 
 carbon dioxide ( co2 ) insufflation can decrease abdominal pain and discomfort during and after colonoscopy . 
 the advantage of co2 insufflation is the rapid absorption of the gas across the intestine . 
 another painless option is water - assisted colonoscopy . 
 two methods for water - assisted colonoscopy are available : water immersion and water exchange . in a recent direct comparison , 
 the water exchange method was superior to water immersion , co2 insufflation , and air insufflation with respect to pain during colonoscopy , although it still had the disadvantage of being a time - consuming procedure . 
 cap - assisted colonoscopy is a simple technique involving the use of a small transparent cap attached to the tip of the scope . 
 three studies showed an advantage of this technique in terms of reduced patient discomfort compared with the conventional method . 
 three robotic colonoscopy systems ( endotics system [ era endoscopy ] , neoguide [ neoguide systems inc . ] , and invendoscope [ invendo medical ] ) have been introduced to evaluate pain reduction during colonoscopy , but none has been widely adopted and used in practice . in this review , 
 clinical trials of several techniques and new devices for painless colonoscopy are described and summarized .