Double-contrast barium enema

DCBE is a safe (one perforation per 25 000 examinations) way of showing the configuration of the colon, the presence of diverticular disease, and the absence of strictures or large lesions. However, even high-quality DCBE has significant limitations, including missing large lesions because of overlapping loops (particularly in the sigmoid region), to misinterpreting between solid stool and neoplasm or between spasm and strictures, with particular inaccuracy for flat lesions such as angiodysplasia or minor inflammatory change and small (2–5 mm) polyps. Where colonoscopy services are overstretched, and CT colography is not routinely available, barium enema may be used in “low yield” patients—those with pain, altered bowel habit or constipation; it also shows extramural leaks or fistulae, which are invisible to the endoscopist.

Computed tomography colography

CT colography (“virtual colonoscopy”) has replaced barium enema as the radiological investigation of choice for the colon, with the advantages of being quicker and not filling the colon with dense contrast medium. CT colography does require technical expertise of the radiographer in perfoming it and the radiologist who interprets it. A few patients who are very difficult to colonoscope for reasons of anatomy or postoperative adhesions may be best examined by combining limited left-sided colonoscopy—the most challenging area for imaging but with the highest yield of significant pathology—with virtual colography or barium enema to demonstrate the proximal colon. Virtual colography has the advantage that it can be performed before or after colonoscopy and with the same bowel preparation, although the majority of procedures are now performed with limited or no bowel preparation and “faecal tagging” using water-soluble contrast agents. CT colography requires radiation dosage comparable to that of DCBE, although dedicated CT protocols limit radiation as much as possible.

Colonoscopy and flexible sigmoidoscopy

Colonoscopy and flexible sigmoidoscopy achieve more than contrast radiology or virtual colography because of their greater accuracy and histologic and therapeutic capabilities. Color view and biopsy makes total colonoscopy particularly relevant to patients with bleeding, anemia, bowel frequency, or diarrhea. Flexible sigmoidoscopy alone may be sufficient for some patients, such as those with left iliac fossa pain or bright red per-rectal bleeding. Because of near pinpoint accuracy and therapy, colonoscopy scores for any patient at increased risk for cancer—in whom detection and removal of all adenomas is important for the patient’s future and as a predictor of long-term risk. Colonoscopy is thus the method of choice for many clinical indications and for cancer surveillance examinations and follow-up (Table 6.1). Endoscopy is also particularly useful in the postoperative patient, either to inspect in close-up (and biopsy if necessary) any deformity at the anastomosis or to avoid the difficulties of achieving adequate distension in patients with a stoma.

Combined procedures

The combination of two procedures (colonoscopy and virtual colography or DCBE) has potential advantages. If carbon dioxide (CO₂) insufflation is used for colonoscopy or flexible sigmoidoscopy, the colon will be absolutely deflated within 10–15 minutes and DCBE can follow immediately. As distension is a routine part of virtual colography, it is an ideal procedure to combine with colonoscopy. DCBE can be made difficult if the proximal colon is already air-filled, so problematic to fill and coat with barium. Colonoscopic biopsies with standard-sized forceps are no contraindication to distending the colon for subsequent DCBE or CT colography. Pedunculated polypectomy should also be safe, but the likelihood of deep electrocoagulation during sessile polypectomy, however small, contraindicates use of distension pressure. DCBE perforation is rare, but barium peritonitis can be fatal.

Limitations of colonoscopy

• Incomplete examination can be due to inadequate bowel preparation, uncontrollable looping, inadequate hand-skills, or an obstructing lesion. Unless the ileo-cecal valve is reached and positively identified with clear views of the cecal pole, completion has not been proved.
  
• Gross errors in colonoscopic localization and “blind spots” are possible even for expert endoscopists. Blind areas, with the possibility of missing very large lesions, occur especially in the cecum, around acute bends and in the rectal ampulla. Colonoscopic examination, rigorously performed, can probably approach 90% accuracy for small lesions, but will never be 100%. A “back to back” colonoscopy series, in which the patient was colonoscoped twice by two expert endoscopists, showed only a 15% miss rate for polyps under 1 cm diameter. However, every colonoscopist has experienced the chagrin of seeing a large polyp during insertion, but missing it entirely during withdrawal when the colon is crumpled after straightening the scope.

Hazards, complications, and unplanned events

Colonoscopy, despite its virtues, is more hazardous than diagnostic alternative studies (historically around one perforation per 1500 colonoscopic examinations, although much lower in recently published series, against perforations in 1 : 25 000 barium enemas or CT colography exams). Unskilled endoscopists needing to use heavy sedation or general anesthesia to cover up ineptitude are likely to run greater risks. It should therefore not be regarded as failure to abandon a tough colonoscopy in favor of immediate CT colography, when “pressing on regardless” could result in an avoidable perforation and subsequent complications.

Safety

Safety during colonoscopy comes from gentle technique and avoiding pain (or oversedation, which masks the pain response as well as contributing pharmacological hazards). Before starting a colonoscopy it is impossible to know if there are adhesions, whether the bowel is easily distensible, and whether its mesenteries are free-floating or fixed; pain is the only warning that the bowel or its attachments are being unreasonably strained. The endoscopist must respect any protest from the patient; a mild groan in a sedated patient may be equivalent to a scream of pain without sedation. Moreover, it is hazardous to give repeated doses of sedatives intravenously, effectively anesthetizing the patient without an anesthetist being present. It is safer in such cases to abandon the procedure and reschedule as a formal anesthetic procedure. Total colonoscopy is not always technically possible, even for experts.

If there is a history of abdominal surgery or sepsis, or if the instrument feels fixed and the patient is in pain, the correct course is usually to stop. The experienced endoscopist learns to take time, to be obsessional in steering correctly and managing loops dexterously, but to be prepared to withdraw from any difficult situation and if necessary to try again after position change or other appropriate maneuver. Too often the beginner has a relentless “crash and dash” approach, and may be insensitive to the patient’s pain because it occurs so often.

Despite its potential hazards, skilled colonoscopy is amazingly safe; it is certainly justified by its clinical yield and the high morbidity of colonic surgery (which would often be the alternative). For the less skilled endoscopist, partnership with CT colography in “difficult” cases should reduce the risks—with re-referral to an expert if pathology is found.

Bowel preparation

An informed team member should be available to talk to the patient at the time of booking to explain the procedure, including the importance of successful bowel preparation—although printed instructions and explanations will be sufficient for most patients. The majority of patients find that the worst part of colonoscopy is the bowel preparation and that the anticipation of the procedure (including fear of indignity, a painful experience, or the possible findings) is much worse than the reality of the colonoscopy itself. Anything that will justifiably cheer them up beforehand is ex­­tremely worthwhile, providing that there is understanding and compliance with dietary modification and bowel preparation. Minutes spent in explanation and motivation may prevent a prolonged, unpleasant, and inaccurate examination due to bad preparation. The patient needs to know that a properly prepared colon looks as clean and easy to examine as the mouth—whereas poor preparation can lead to a degradingly unpleasant, less accurate, and slower examination.

Written dietary instructions are well worthwhile, as many patients, anxious to get a good result, find it easier to follow specific instructions “to the letter.” Clear instructions avoid unnecessary anxieties and many telephone calls.

Routine for taking oral prep

Low-residue diet instructions should have been followed, ideally for several days in the case of those with known constipation or slow transit. The patient should be supplied with petroleum jelly or barrier cream to avoid perianal soreness (colorless to avoid endoscope lens contamination as the scope is inserted through the anus). The evening before colonoscopy will be fluid-dominated—input and output—so social events should not be scheduled but there will be plenty of time for watching television or reading between “calls.”

As mentioned above, large-volume solutions are ideally split-administered in two doses, starting on the afternoon or evening before, but it is essential that some oral prep is taken on the morning of the examination so that cecal contents remain fluid and easily aspirated. If an afternoon examination is scheduled and the patient does not have a long distance to travel, both doses can be drunk on the day of examination.

The patient should be encouraged to carry on with normal activities, rather than sitting still during the drinking period; exercise stimulates transit and evacuation. Bowel actions should start within 1–3·hours, but can be much delayed in constipated patients or those who prove to have a long colon.

Bowel preparation in special circumstances

Sedation and analgesia

All aspects of the procedure, including the medication options, should be explained when the colonoscopy booking is arranged. The patient should receive preliminary verbal and written explanation about bowel preparation and what to expect of the procedure (whether from doctor, nurse, or secretary). At this point some patients may judge (in countries where judgment is permitted) that they want full medication, others that they will hope to work normally or to drive afterwards. On arrival for colonoscopy, a few minutes of further explanation will reassure and calm most patients and allow the endoscopist to judge whether the particular individual is likely to require sedation, and if so how much. Most people tolerate some discomfort without resentment if they understand the reason for it. Few people expect to be semi-anesthetized for a visit to the dentist, but on the other hand they understandably expect the intensity and duration of any discomfort to be within “acceptable limits.” Pain thresholds and individual attitudes to pain are not always easy to predict before colonoscopy, because tolerance of the (peculiarly unpleasant) quality of visceral pain varies so much. It is sensible to warn the patient that there will be a few seconds of “wind” or a transient sensation of “urgency.”

During a typical and correctly performed colonoscopy, minor pain is experienced by the patient for only 20–30 seconds. Using moderate or no sedation, and employing the skills, changes of position, and other “tricks of the trade” described below, pain only occurs during looping in the sigmoid colon and while passing the sigmoid–descending colon junction. During the rest of a normal procedure a patient with average pain threshold should experience little more than mild distension or the urge to pass flatus. It is worth pointing out to the patient that pain is useful to the endoscopist because it shows that a loop is forming, but is not dangerous and can usually be stopped in a few seconds (by straightening out the loop that is causing it).

The use of sedation has advantages and disadvantages. The unsedated or very lightly sedated patient can cooperate by changing position, needs no recovery period, and can travel home unaided immediately. The colonoscopist is also encouraged to develop dexterous and gentle insertion technique. On the other hand, some endoscopists who never employ sedation also admit to only 80–90% success in performing total colonoscopy, presumably because some examinations are intolerable. If light “conscious sedation” is used (typically equivalent in effect to 2–3 glasses of wine or beer), the patient is likely to find the examination tolerable or to have amnesia for it. The endoscopist is helped to be thorough by the knowledge that the patient is comfortable, and is also more likely to achieve total colonoscopy in a shorter time. Using heavy sedation, endoscopists can get away with ham-handed and forcibly looping technique—a bad investment in the long term, less likely to achieve complete examinations, more likely to result in complications, and more expensive in instrument repair bills.

It is often said that it is dangerous to sedate, because the safety factor of pain is removed. This is not strictly true, providing that the endoscopist’s threshold of awareness lowers as the patient’s pain threshold is raised—taking restlessness or changes of facial expression as a warning that tissues and attachments are being overstretched.

Most endoscopists use a balanced approach to sedation that will be affected by many factors, including personal experience and the individual patient’s attitude. A relaxed patient with a short colon having a limited examination rarely needs sedation, but an anxious patient with a tortuous colon, severe diverticular disease, or a bad previous experience, may need deep sedation. Patients with irritable bowel syndrome or pain as presenting features are likely to be hypersensitive to stretch and will benefit from opiates.

A very few patients have a morbid fear of colonoscopy, a low pain threshold, or a known “difficult” colon that justifies offering light general anesthesia. General anesthesia is only likely to be hazardous if it allows an inexperienced colonoscopist to use brutal technique while the patient cannot protest. However, even experienced endoscopists are more likely to “push the limits” and to become more mechanistic if patients are routinely anesthetized and “out of it.”

Nitrous oxide inhalation

Nitrous oxide/oxygen inhalation can be a useful “half-way house” between no sedation and conventional IV sedation, for instance in a patient intending to drive after the procedure. The 50 : 50 nitrous oxide/oxygen mixture is self-administered by the patient, inhaling from a small cylinder fitted with a demand valve. Breathing the gas through a small single-use mouthpiece (Fig 6.1) avoids the difficulties that can be experienced in getting a good fit with a face mask, and also the phobia that some patients experience with masks.

Fig 6.1 Nitrous oxide/oxygen mixture is breathed through a mouthpiece.

The patient is shown how to inhale, then “pre-breathes” for a minute or two as the endoscopist prepares to start the procedure, with the intention of achieving gas saturation of the body fatty tissues. Thereafter it takes only 20–30 seconds of gas breathing, when required, to obtain a “high” that makes short-lived pain significantly more tolerable. Nitrous oxide/oxygen inhalation should prove useful for some flexible sigmoidoscopies and, used alone, can be sufficient for motivated patients having total colonoscopy by a skilled endoscopist. Scared patients, prolonged or difficult examinations and examinations by inexpert endoscopists require conventional sedation.

Antispasmodics

Antispasmodics induce colonic relaxation for at least 5–10 minutes and help to optimize the view during examination of a hypercontractile colon. Either hyoscine N-butylbromide (Buscopan®) 20 mg IV (in countries where it can be prescribed) or glucagon 0.5–1 mg IV are effective. Fears about anticholinergics initiating glaucoma are misplaced because patients previously diagnosed are completely protected by their eye drops, and those with undiagnosed chronic glaucoma are best served by precipitating an acute attack, which will cause the diagnosis to be made. Patients should be told to seek medical attention if they experience any eye pain. Glucagon is more expensive, but has no ocular or prostatic side effects.

Intravenous antispasmodics have a relatively short duration of action, leading some endoscopists to give them only when the colonoscope is fully inserted. Experienced endoscopists, sure of a rapid procedure, may give them at the start. There is an unproven suspicion that the bowel is rendered more redundant and atonic by antispasmodics and will be more difficult to examine; on the contrary, we find that the view is improved and have shown that colonoscope insertion is speeded up after using antispasmodics. Benzodiazepines have a weak antispasmodic effect, relaxing most colons except for those that are “irritable” or spastic. In the unsedated patient, therefore, antispasmodics may be particularly helpful and can also be a useful placebo for those who cannot have routine sedation because they need to drive home, but expect an “injection” to cover the procedure.

Insufflation with CO₂ avoids post-procedure problems, especially in patients with irritable bowel disorder or diverticular disease. If air is used such patients can experience problems from air retention, with sudden onset of colic or discomfort after the procedure as the pharmacological effects of the antispasmodics and sedation wear off.

Colonoscopy room

Most units perform colonoscopies in undesignated endoscopy rooms, because the only special requisite for colonoscopy is good ventilation to overcome the evidence of occasional poor bowel preparation. In a few patients with particularly difficult and looping colons it has in the past been helpful to have access to x-ray facilities, especially in teaching institutions. Magnetic imaging (see below) performs the same function without using x-rays; it is increasingly used. We hope that it will spread worldwide to help teaching and the logical performance of colonoscopy.

Colonoscopes

Colonoscopes are engineered similarly to upper gastrointestinal endoscopes, but are longer, have a wider diameter (for better twist or torque control), and have a more flexible shaft. The bending section of the colonoscope tip is longer and more gently curved, avoiding impaction in acute bends such as the splenic flexure. Ideal future colonoscopes ought to have electronic steering to make single-handed insertion easier; present angulation control mechanisms are almost unchanged from those of early gastrocameras and gastroscopes and are poorly suited to the more finicky steering movements during colonoscopy.

The introduction of variable stiffness instruments avoids the need to choose the “right colonoscope for the job” at the stage of purchase or before starting examination of a particular patient—especially one known or predicted to have a long “difficult” colon or severe adhesions. Long colonoscopes (165–180 cm) are able to reach the cecum even in redundant colons and so are our preferred routine choice of instrument (see also “variable-stiffness colonoscopes” below). Intermediate-length instruments (130–140 cm) are considered by some, including most German or Japanese endoscopists, to be a good compromise, almost always reaching the cecum. The only advantage of using 70-cm flexible sigmoidoscopes for limited examinations is that the endoscopist knows from the onset that the procedure will be limited, so avoiding the temptation to go further. However, as flexible sigmoidoscopy can be performed with a longer instrument (a pediatric colonoscope is ideal) there is no reason to purchase flexible sigmoidoscopes for an endoscopy unit, although they may have an essential role in the office of a primary-care physician or an outpatient facility.

Variable-stiffness colonoscopes

Variable-stiffness colonoscopes (Innoflex®, Olympus Corporation) have a twist control on the shaft (Fig 6.2a) that forcibly compresses and rigidifies an internal steel coil similar to that in a bicycle brake cable (Fig 6.2b,c Video 6.2). Compressing the coil stiffens it and the shaft/insertion tube within which it lies. The last 30 cm to the tip of the bending section is left “floppy” at all times. The bonus of using a variable-stiffness colonoscope is that, without having to withdraw and exchange instruments, the endoscopist can select a relatively “floppy” shaft mode to pass looping sections of the colon, then twist to apply “stiff” mode, so discouraging re-looping after the scope has been straightened out, typically at the splenic flexure.

Fig 6.2 (a) Variable-stiffness colonoscopes have a twist control on the shaft. A pull-wire within an internal spring-steel coil (b) compresses the coil and stiffens it (and the scope) (c).

Variable-stiffness scopes thus combine, in one colonoscope, many of the virtues of both standard and pediatric instruments. They prove significantly easier and less traumatic to use in most patients found previously to be “difficult” to examine—especially where the problem was due to uncontrollable looping and discomfort. As any first-time patient may prove to be difficult, a long, variable-stiffness instrument is our “colonoscope of choice.”

Instrument checks and troubleshooting

The functionality of the colonoscope should be checked before examination, because imperfections can be difficult to spot or tedious to remedy during it. Colonoscopy can be difficult enough without adding problems in instrument performance.

Insufflation/lens washing checks are essential before every colonoscopy. Because air flow and water wash share a short common exit channel (see Fig 2.5) the quickest way of simultaneously checking air/water functionality is to depress the water-wash valve and look for a healthy squirt from the scope tip. Once the procedure has started it is difficult to assess inadequacy of air flow and insufflation pressure, the resulting poor view making it seem that the colonoscopy is “difficult” or the colon apparently “hypercontractile.” A great deal of wasted time can be avoided by noticing any such problem before starting, and correcting it or changing instruments.

If there is no insufflation at all, check the light source. Is the air pump switched on? Are the umbilical and water-bottle connections pushed in fully and the water bottle screwed on? Is the rubber O-ring in place on the water-bottle connection? Is the air/water valve in good condition and seated properly (or the CO₂ valve in position where relevant), as it will otherwise allow air leakage? If in doubt, proper air insufflation pressure and flow can be proven by blowing up a rubber glove wound over the scope tip.

Water-wash failure is unusual, except because of an empty water bottle or a faulty air/water valve.

Suction failure can be caused by valve blockage, which should be obvious on careful inspection or changing the valve, or by debris blocking the suction channel. If this is in the shaft it can be dislodged by water-syringing through the biopsy port. Removing the suction valve and covering the opening on the control head with a finger is a quick way of improving suction pressure and can result in rapid clearance of the whole system (as when sucking polyp specimens). Applying the sucker tube directly to the suction-channel opening can also be effective in clearing particulate debris. As a final resort the whole suction system can be cleared by retrograde-syringing using a 50-mL bladder syringe and tubing attached to the suction port on the umbilical. Push the suction valve and also cover the biopsy port during this procedure to avoid unpleasant (refluxed) surprises.

Accessories

All the usual accessories are used down the colonoscope, including biopsy forceps, snares, retrieval forceps or baskets, injection needles, cytology brushes, washing catheters, dilating balloons, etc. Long and intermediate-length accessories work equally well down shorter instruments, so it is sensible to order all accessories to suit the longest instrument in routine use. Other manufacturers’ accessories also work down any particular instrument and, as some are better than others, it is worth taking advice from colleagues when buying replacements.

Carbon dioxide

Few colonoscopists, regrettably, use CO₂ insufflation, although its use has much to commend it. CO₂ was originally used instead of air because of the explosive potential of colonic gases during electrosurgery. However, with the exception of bowel preparation using mannitol, the prepared colon has been shown to have no residual explosive gas. Nonetheless, even for routine examinations, the use of CO₂ offers the striking advantage that it is cleared from the colon 100 times faster than air (through the circulation, to the lungs and then breathed out). This means that 10–15 minutes after finishing an exam using CO₂ insufflation, the colon and small intestine are free of any gas and the patient’s abdomen is deflated, whereas air distension can remain and cause abdominal bloating and discomfort for many hours, which is especially distressing for irritable bowel patients. In the unlikely event of perforation or gas leak (pneumoperitoneum), air under pressure would add to the hazard, whereas rapidly absorbed CO₂ and a well-prepared colon should markedly reduce it.

Any patient with ileus, pseudo-obstruction, stricturing, severe colitis, diverticular disease, or functional bowel disorder should benefit from the added safety and comfort of using CO₂ rather than air insufflation.

Low-pressure, controlled-flow CO₂ delivery systems with fail-safe pressure-reducing features are available commercially. These remove any risk of the patient being exposed to the hazard of high pressure from the cylinder in the event of failure of the conventional flow-meter. A CO₂ insufflation valve can be substituted for the usual air/water valve, but in practice it is easier to connect the CO₂ supply to the water bottle (Fig 6.3) and use the normal air/water valve, as the modest leakage of CO₂ into room atmosphere is of no more consequence than having another person in the room.

Fig 6.3 Connect the CO₂ supply directly to the water bottle.

Magnetic imaging of endoscope loops

There is a need to know what shaft loops have formed during colonoscope insertion and where the tip is. In 1993 two UK groups introduced prototype magnetic imagers to “position-sense” the configuration of the instrument shaft, producing a moving 3D image on a computer monitor. Small coils within the instrument (or in a probe passed down its instrumentation channel) generate pulsed magnetic fields that energize larger sensor coils in a dish alongside the patient, computed to produce a real-time monitor graphic display (Fig 6.4). Two systems are commercially available: ScopeGuide® (Olympus Corporation) (Video 6.3), which uses coils incorporated within the shaft of the scope, and a catheter-based system inserted down the accessory channel of an ordinary colonoscope (Fujinon Magnetic Endoscopic Imaging). These produce fields no stronger than those of a television set and are safe for continuous use, except for patients with cardiac pacemakers.

Fig 6.4 (a) Small coils within the scope generate magnetic fields, (b) energizing larger coils in the receiver dish beside the patient; (c) the signals are then processed as a 3D image on the monitor.

In use, magnetic imaging makes many previously difficult and looping colons much quicker and easier to intubate, and also ensures that the endoscopist knows at all times where the colonoscope tip has reached and what loops have formed. It rationalizes many of the uncertainties of colonoscopy, and can be a boon to both beginners and experts. The magnetic imager is particularly helpful in patients with a long colon, who can be preselected on the basis of a history of constipation or the presence of hemorrhoids, or if they report a delayed response to bowel preparation.

Other techniques

Several other simple and straightforward amendments to standard insertion techniques are beginning to find favour. Water-immersion colonoscopy entails filling the colon with water to “smooth out” the floppy haustra and create a relatively straight colon. Cap-assisted colonoscopy uses a clear plastic cap attached to the tip of the colonoscope to assist with negotiating tight bends and can improve mucosal visualization on withdrawal. There are also many novel technologies in development to improve insertion and inspection (such as computer-controlled bending scopes, called retroscopes), but are not yet in routine use and are therefore currently beyond the remit of this book.

Embryological anatomy (and “difficult colonoscopy”)

The embryology of colon development is complex and somewhat unpredictable, especially in terms of its outcome for mesenteries and fixations, which probably explains the extraordinarily variable configurations into which the colon can be pushed during colonoscopy (Video 6.4). The fetal intestine and colon initially develop as a functionless muscle tube joined at its midpoint to the yolk-stalk. This muscle tube lengthens into a U-shape on a longitudinal mesentery (Fig 6.5a). As the embryo at this 5-week stage is only 1 cm long, the lengthening intestine and colon (Fig 6.5b) are forced out into the umbilical hernia (Fig 6.5c). The gut loop thus differentiates into the small and large intestine outside the abdominal cavity. By the third month of development the embryo is 4 cm long and there is room within the peritoneal cavity for first the small, and then the large, intestine to be returned into the abdomen. This occurs in a fairly predictable manner, with the end result that the colon is rotated around so that the cecum lies in the right hypochondrium and the descending colon to the left of the abdomen (Fig 6.6a).

Fig 6.5 (a) The fetal intestine and colon start on a longitudinal mesentery (b) then rotate as the small intestine elongates (c) and from 5 weeks (1 cm embryo) to 10 weeks (4 cm embryo) are in the umbilical hernia.

Fig 6.6 (a) The embryonic colon extends on its mesentery, (b) then partial fusion of the mesentery and peritoneum occurs at 3 months, (c) although sometimes the colon remains mobile.

With further elongation of the colon, the cecum normally migrates down to the right iliac fossa. At this stage, the mesentery of the transverse colon is free but the mesenteries of the descending and ascending colon, pushed against the peritoneum of the posterior abdominal wall by the fluid-filled and bulky small intestine, fuse with it so that the ascending and descending colon typically becomes retroperitoneal and fixed (although not always—see below) (Fig 6.6b).

Incomplete fusion of the mesocolon to the posterior wall of the abdomen results in a relatively free-floating colon. Such a mobile colon can be a nightmare for the colonoscopist, because there are no fixed points at which to obtain leverage, and few of the usual “tricks of the trade” will work, as most of them depend on withdrawal and leverage against fixations. The explanation for this variation from normal development may be the failure of enteric innervation of the intestinal muscle tube in early embryonic development. An atonic, bulky, and dysfunctional fetal intestine and colon will be retained longer than usual outside the abdomen in the umbilical hernia, until the developing abdominal cavity is large enough to re-accommodate it. Delayed return of a large colon into the abdomen will cause it to miss the “milestone moment” for retroperitoneal fixation and fusion to occur (usually by 10–12 weeks after conception). The long, mobile (and increasingly dysfunctional) colon may present clinically in childhood with straining at stool and bleeding, in teenage years with constipation, or in adulthood with hemorrhoids, variable bowel habit, and flatulence.

Endoscopically such a colon is noted to be unusually capacious, long, and often atypically looping, but it can also be dramatically squashed down and shortened when the colonoscope is withdrawn at the cecum (typically to a length of only 50–60 cm), proving the lack of fixations. Suggestive evidence that this is a genetically determined abnormality of development is the frequency of other first-degree relatives (especially on the female side and sometimes over several generations) known to have disturbance of habit, constipation, or flatulence. If endoscoped or imaged, the colon of such relatives (also their stomach and small intestine) are found to be similarly large, long, and mobile.

How often such failure of fusion, persistent colonic mesentery and mobility occurs is not clear from the literature. A persistent descending mesocolon has been found at postmortem in 36% with an ascending mesocolon in 10%. The persistence of a descending mesocolon explains most of the excessive loops and strange configurations that can be caused by the colonoscope passing the left colon and splenic flexure (Fig 6.7). Occasionally the cecum fails to descend and becomes fixed in the right hypochondrium (Fig 6.8); in others, where a free mesocolon persists, the cecum is mobile and can be pushed into weird configurations by the endoscope (Fig 6.9). Per-operative studies that we have undertaken show that colons in Asian patients are more predictably fixed than those in European patients.

Fig 6.7 Persistent descending mesocolon or mesentery.

Fig 6.8 Inverted cecum.

Fig 6.9 Mobile cecum.

Endoscopic anatomy

The anal canal, 3 cm long, extends up to the squamocolumnar junction or “dentate line.” Sensory innervation, and hence mucosal pain sensation, may in some subjects extend up to 5–7 cm into the distal rectum. Around the canal are the anal sphincters, normally in tonic contraction. The anus may be deformed, scarred, or made sensitive by present or previous local pathology, including hemorrhoids or other conditions. Normal subjects may also be sore from the effects of bowel preparation.

There are two potentially serious consequences from the fact that the hemorrhoidal veins drain into the systemic (not the portal) circulation:

The rectum, reaching 15 cm proximal to the anal verge, may have a capacious “ampulla” in its mid-part as well as three or more prominent partial or “semilunar” folds (valves of Houston) that create potential blind spots, in any of which (as well as the distal rectum) the endoscopist can miss significant pathology. Digital examination, direct inspection and, where appropriate, a rigid rectoscope/proctoscope are needed for complete examination of the area. “Video-proctoscopy” (anoscopy—see below) is a convenient way of visualizing the anal canal, rectal mucosal prolapse, or hemorrhoids, but not the remainder of the rectum (which requires inflation for careful inspection and, where possible, instrument retroversion). Prominent, somewhat tortuous, veins are a normal feature of the rectal mucosa and should not be confused with the rare, markedly serpiginous veins of a hemangioma or the distended, tortuous varices seen in some cases of portal hypertension.

The rectum is extraperitoneal for its distal 10–12 cm, making this part relatively safe for therapeutic maneuvers such as Endoscopic Submucosal Dissection removal (see below) of sessile polyps. Proximal to this it enters the abdominal cavity, invested in peritoneum. Whereas the colon surface is devoid of sensory nerves and pain-free, patients may experience “burning pain” for up to 5–7 cm above the anal verge. This is easily managed for polypectomy by intramucosal local anesthetic injection.

Mucosal “microanatomy” is visible to the discerning endoscopist. This includes the shiny surface coating of mucus, around 30% of the mucosal cells being mucus-secreting and described as “goblet cells” because of their flask-shaped mucus-containing inclusions. The “highlights” reflected off the surface by the protective mucus layer can show up fine underlying detail, such as the arc impressions of circular muscle fibers or the dappled, sieve-like reflections caused by the microscopic crypt or pit openings. Minor abnormalities, such as prominent lymphoid follicles and the smallest polyps or flat adenomas, often first catch the endoscopist’s eye through such reflections or “light reflexes” off the mucus layer. The mucosal columnar epithelium, around 50 cells thick, is transparent (unlike the horny squamous epithelium of the skin surface) and through it can be seen, often in exquisite detail, the paired venules and arterioles that make up the normal submucosal “vessel pattern.”

Colonic musculature develops into three external longitudinal muscle bundles, or teniae coli, and within these, the wrapping of circular muscle fibers. Both muscle layers are sometimes visible to the endoscopist (Fig 6.10). One or more of the teniae may be seen endoscopically as a longitudinal fold, because an unusually thin-walled, capacious colon can bulge out between its teniae. The circular musculature is seen as fine reflective corrugations under the mucosal surface, particularly in “spastic” or hypertonic colons. The distal colon, needing to cope with formed stools, has markedly thicker circular musculature than in the proximal colon, resulting in a tubular appearance (Fig 6.11) broken by the ridged indentations of the haustral folds. The thinner walled transverse colon is kept in triangular shape by the three teniae.

Fig 6.10 The longitudinal muscle bundles (teniae coli) can bulge visibly into the colon.

Fig 6.11 The distal colon is usually circular, with ridged haustrations.

Haustral folds segment the interior of the colon. Those that are prominent in the proximal colon sometimes create “blind spots,” whereas they can be hypertrophied in sigmoid diverticular disease, also creating mechanical difficulties for the endoscopist.

Related posts:

1: The Endoscopy Unit, Staff, and Management 8: Resources and Links 3: Patient Care, Risks, and Safety 5: Therapeutic Upper Endoscopy 4: Upper Endoscopy: Diagnostic Techniques 7: Therapeutic Colonoscopy

Stay updated, free articles. Join our Telegram channel

Join