Key points

Introduction

Colonoscopy is considered the preferred method for colorectal cancer screening, especially in the United States. With the recent US Multi-Society Task Force guidelines emphasizing that the goal of screening should be the prevention of colorectal cancer, the role of colonoscopy has assumed even greater significance. The central premise by which colonoscopy prevents colon cancer is the detection and removal of precursor lesions, such as adenomas. Colonoscopy remains an imperfect tool, however, for the prevention of colorectal cancer. One of the major reasons for this is that adenomas can be missed during colonoscopy, even when the procedure is performed by expert endoscopists using meticulous technique. Adenoma detection rate is considered an important quality indicator of colonoscopy performance and its clinical relevance was highlighted in a recent study showing that the adenoma detection rate of endoscopists is independently associated with interval colorectal cancer. Therefore, efforts to improve and optimize adenoma detection rates continue to be an active area of research in the field of colonoscopy.

Introduction

Colonoscopy is considered the preferred method for colorectal cancer screening, especially in the United States. With the recent US Multi-Society Task Force guidelines emphasizing that the goal of screening should be the prevention of colorectal cancer, the role of colonoscopy has assumed even greater significance. The central premise by which colonoscopy prevents colon cancer is the detection and removal of precursor lesions, such as adenomas. Colonoscopy remains an imperfect tool, however, for the prevention of colorectal cancer. One of the major reasons for this is that adenomas can be missed during colonoscopy, even when the procedure is performed by expert endoscopists using meticulous technique. Adenoma detection rate is considered an important quality indicator of colonoscopy performance and its clinical relevance was highlighted in a recent study showing that the adenoma detection rate of endoscopists is independently associated with interval colorectal cancer. Therefore, efforts to improve and optimize adenoma detection rates continue to be an active area of research in the field of colonoscopy.

Reasons for missing adenomas

Several reasons have been attributed for missing adenomas during colonoscopy. These include inadequate bowel preparation, suboptimal technique, shorter withdrawal time, presence of flat/depressed/subtle lesions evading detection, and the inability to visualize the proximal sides of haustral folds, flexures, rectal valves, and ileocecal valve. Incomplete visualization continues to be a vexing problem for endoscopists with a wide range of experience. The haustral folds in the colon are of varying dimensions and can shield significant amount of mucosal surface area from the view of the endoscopist. One of the elements of good inspection technique during the withdrawal phase of colonoscopy is to inspect between folds by torquing the colonoscope and/or deflecting its tip. But this maneuver is limited by the extent of tip deflection and angle of view of the lens at the colonoscope tip as well as the loss of visualization when the colonoscope tip closely approximates the mucosa, leading to a red out. The fact that lesions located on the proximal aspects of colonic folds are prone to be missed during colonoscopy was highlighted in a study that used CT colonography as the reference standard. This study showed that majority of clinically significant adenomas that are missed during colonoscopy are located on the proximal sides of folds. There are 3 methods that aid in inspecting these blind mucosal areas, which include cap-assisted colonoscopy, third eye colonoscopy, and retroflexion of the colonoscope. This review discusses the different aspects of cap-assisted colonoscopy.

Cap-assisted colonoscopy

Mechanism of Action of Cap

Cap-assisted colonoscopy is a simple technique using a small transparent cap attached to the tip of the colonoscope ( Figs. 1–3 ). The cap is shaped like a hollow cylinder and, depending on its length, a portion of it protrudes for a variable distance beyond the tip of the colonoscope. This portion of the cap helps depress the colonic folds while keeping an appropriate distance between the mucosa and the lens at the tip of colonoscope, thus preventing loss of visualization and facilitating the inspection of the mucosal areas on the proximal aspects of folds, flexures, and valves ( Fig. 4 ). Furthermore, the cap can improve mucosal inspection by stretching or splaying colonic mucosa that might have otherwise folded over and obscured small lesions from the view of the endoscopist. The cap can also be of assistance during insertion of the colonoscope by pushing away folds and helping with luminal orientation and better visualization at bends. This helps the endoscopist insert the colonoscope with less air insufflation. Finally, the cap can be used to hook colonic folds at acute bends and help reduce loops in the colonoscope.

Cap with a side hole.

Cap being fitted to the tip of colonoscope.

Cap fitted to the colonoscope and extending for 4 mm beyond the tip.

Colonoscopic view of the cap.

Type of Caps

Caps are of varying lengths, ranging from 2 mm to 10 mm. The most commonly used one protrudes for approximately 4 mm beyond the tip of colonoscope after being tightly secured (see Fig. 4 ). Caps come in different diameters that fit snugly on colonoscopes of various sizes (adult colonoscope, pediatric colonoscope, and magnification colonoscope). The distal end of the cap is horizontal and the edges are rounded to minimize mucosal trauma. The newer version of the cap has a side hole for drainage of fluid and fecal material that might get entrapped within the interior confines of the cap and can, therefore, obscure the view. Another variation of the cap used in one study is the retractable cap ( Fig. 5 ). This cap extends to the desired length beyond the tip of colonoscope by the injection of air with a 10-mL syringe and retracts by aspiration of the air. The maximum extension possible is 7 mm. Various synonyms used for cap in the literature include hood, transparent hood, transparent extension device, and distal attachment.

Transparent retractable cap. Cap retracted ( upper panel ) Cap extended ( lower panel ).

( From Horiuchi A, Nakayama Y. Improved colorectal adenoma detection with a transparent retractable extension device. Am J Gastroenterol 2008;103:341–5; with permission.)

Cap-assisted colonoscopy has been studied for a variety of endpoints that include adenoma or polyp detection rates, cecal intubation rates, cecal intubation time, patient discomfort during colonoscopy, and efficiency in colonoscopy. The literature pertinent to each of these endpoints is reviewed.

Cap-assisted colonoscopy for adenoma/polyp detection

Cap-assisted colonoscopy has been tested for improving adenoma/polyp detection since the late 1990s. The majority of earlier studies, including randomized controlled trials (RCTs) comparing cap-assisted colonoscopy to standard colonoscopy, were conducted in Asia and have shown mixed results. More recently, this technique has been evaluated in Western countries. The first RCT on cap-assisted colonoscopy conducted in United States was by Hewett and Rex. This was a tandem design study with 100 subjects, of whom 52 were randomized to undergo cap-assisted colonoscopy and 48 to regular high-definition colonoscopy as the first procedure, followed by a second colonoscopy by the alternate method. Patients undergoing cap-assisted colonoscopy first had a significantly lower adenoma miss rate compared with those who underwent a regular colonoscopy as the first procedure (21% vs 33%; P = .039). The difference in the miss rate was, however, seen only for diminutive (≤5 mm) adenomas (22% vs 35%; P = .037). In another RCT by Rastogi and colleagues, conducted at a tertiary-care veterans hospital, 420 subjects were randomized to either cap-assisted colonoscopy or regular high-definition colonoscopy. The proportion of subjects with at least 1 adenoma was higher with cap-assisted colonoscopy (69% vs 56%; P = .009). Cap-assisted colonoscopy also detected a significantly higher number of adenomas per subject compared with regular colonoscopy (2.3 vs 1.4; P <.001). Therefore, cap-assisted colonoscopy detected a 13% higher number of subjects with at 1 one adenoma and 59% higher adenomas per subject. A higher number of subjects with right-sided adenomas and total number of right-sided adenomas were also detected by the cap-assisted procedures. A majority of the subjects enrolled in this study were male veterans and more than 90% had an excellent or good bowel preparation, thereby limiting the generalizability of the results. Contrary to the results of these 2 studies, an RCT from the Netherlands did not find an increment in adenoma detection with cap-assisted colonoscopy. This study was conducted at 2 centers by 5 endoscopists, and randomly allocated 1380 participants to cap-assisted colonoscopy or regular colonoscopy. The proportion of subjects with at least 1 adenoma was similar in the 2 groups (28% vs 28%; relative risk [RR] 0.98; 95% CI, 0.82–1.16). The mean number of adenomas per subject was also similar (0.50 vs 0.49; P = .91). A large RCT from Hong Kong involving 1000 subjects showed lower adenoma detection rates with cap-assisted colonoscopy. The adenoma detection rate was 30.5% in the cap-assisted group compared with 37.5% in the regular colonoscopy group ( P = .018). Similarly, the number of adenomas per subject was also lower in the cap-assisted group (0.63 vs 0.96; P = .023). This study, however, had limitations because there was less satisfactory bowel preparation and shorter withdrawal time in the cap-assisted group, factors that could have had a negative impact on the adenoma detection rates. The type of cap used in all these studies had a length of 4 mm protruding beyond the tip of colonoscope. Horiuchi and Nakayama used a specially designed cap, called a retractable transparent hood, and found that although the proportion of patients with adenomas was similar compared with colonoscopy without the device (29% vs 24%; P = .11), the total number of adenomas detected was significantly higher (205 vs 150; P = .04).

Other RCTs have compared cap-assisted colonoscopy with regular colonoscopy but have reported polyp detection rates without information on polyp histopathology. Kondo and colleagues found that the polyp detection rate was significantly higher with the cap (49% versus 39%; P = .04). In another study by Harada and colleagues, the detection rates of polyps was similar with and without the cap (43% vs 42%; P = .86). Similarly, Tee and colleagues reported no difference in the polyp detection rates between colonoscopies performed with and without the cap (33% vs 31%; P = .75).

An exhaustive meta-analysis of the RCTs comparing cap-assisted colonoscopy with regular colonoscopy was recently published. A total of 16 RCTs were included with 8991 subjects, of whom 4501 underwent cap-assisted colonoscopy and 4490 standard colonoscopy. Twelve trials were identified for comparison of polyp detection rates. Cap-assisted colonoscopy detected a significantly higher proportion of subjects with polyps compared with regular colonoscopy (52.5% vs 47.5%, RR 1.08; 95% CI, 1.00–1.17). Number-needed-to-treat analysis showed that 27 patients would need to undergo cap-assisted colonoscopy for 1 additional patient detected with polyps. For adenoma detection rates, 6 trials were identified and no significant difference was seen between cap-assisted colonoscopy and regular colonoscopy (46.8% vs 45.3%; RR 1.04; 95% CI, 0.90–1.19).