LARGE-BOWEL OBSTRUCTION
CASE SCENARIO
An 84-year-old woman with a history of Parkinson’s disease presents to the emergency department with mental status changes and progressive abdominal distension. She is currently staying in a group home and is accompanied by her caretaker. She is unable to give a clear history, but her caretaker notes that she has had previous similar episodes of distension that have always spontaneously resolved. She regularly requires docusate, magnesium citrate, and polyethylene glycol to control her constipation.
On examination, the patient is afebrile with normal vital signs. Her abdomen is nontender, but notably distended with a palpable mass in the left lower quadrant. There is no palpable abnormality on rectal examination, although the rectal vault is empty. Her lab work is remarkable for a lactate of 2.1 and creatinine of 2.4, but no leukocytosis.
EPIDEMIOLOGY
Large-bowel obstruction is an uncommon surgical emergency. In Westernized nations, approximately 85% of patients with large-bowel obstruction present with colorectal carcinomas,1 and in fact malignant obstruction is the initial presentation of colon cancer in as many as 20% of colon cancer diagnoses.2 The remaining 10% to 15% of patients obstruct secondary to volvulus, diverticular stricture, or the less common causes listed in Table 10–1.3 Patient age and medical history can provide important clues as to the etiology of the disease, although large-bowel obstruction is most common among the elderly.4
Colonic volvulus is most common among elderly nursing home patients due to a variety of factors, including excessive mobility of the colon, low fiber diet, chronic constipation, and lack of exercise. The sigmoid colon is the most common site of volvulus, representing 65% to 80% of cases of colonic volvulus and with a mean age of presentation of 70 years.3,5 Cecal volvulus represents the majority of remaining cases, with transverse colon or splenic flexure volvulus occurring in rare circumstances.
Acute colonic pseudo-obstruction is a motility disorder characterized by massive colonic dilatation and is most common among patients with concurrent causative medical conditions, although rarely it can develop spontaneously. These patients have most commonly undergone recent operation, with trauma, orthopedic, and abdominal operations representing the majority of cases, but may be admitted for medical conditions as well. Narcotic, anticholinergic, or antidepressant therapies may be causative. Pseudo-obstruction may progress to perforation, with risk substantially increasing at cecal diameter between 9 and 12 cm, and when symptoms last greater than 6 days.6
DIFFERENTIAL DIAGNOSIS
See Table 10–1.
PATHOPHYSIOLOGY
Obstruction of the large bowel is mechanical in nature. Malignancy, being the most common cause of obstruction, can cause progressive distension of the proximal colon that, if untreated, can progress to colonic ischemia and subsequent perforation. Volvulus results in a twisting of the colon that creates a closed-loop obstruction. Volvulus requires a disproportionately long and redundant segment of colon and can torse anywhere from 180 to 540 degrees.5 This, along with significant distension of the affected segment, compromises venous outflow from the colon and results in ischemic damage to the colonic wall. Unlike small-bowel obstruction, in which enteric contents back up into the stomach to alleviate tension across the intestinal wall, in large-bowel obstruction, if the ileocecal valve is competent the obstruction is closed loop and therefore a surgical emergency.
CLINICAL PRESENTATION
The most common presentation of large-bowel obstruction of all types is abdominal distension, typically in association with nausea, vomiting, and obstipation. Feculent emesis is common in complete large-bowel obstruction. Patients may present with mild abdominal tenderness, but significant tenderness or peritoneal signs should raise concern for colonic ischemia. Clinical presentation will vary depending on the etiology of obstruction, with the most common causes detailed in the following.
Malignant Large-Bowel Obstruction
Patients presenting with large-bowel obstruction due to malignant disease are most commonly diagnosed with colorectal malignancy, although obstruction can occur due to ovarian cancer or other malignancies as well. Patients with these etiologies generally present with a history of melanotic stool, chronic weight loss, and progressive, rather than acute, abdominal distension.
Benign Large-Bowel Obstruction
The presentation of patients with benign large-bowel obstruction depends on their diagnosis. Sigmoid volvulus most commonly presents as an acute onset of abdominal distension, although patients will often have a history of vague abdominal discomfort, intermittent cramping, and previous episodes of distension that resolved with the passage of flatus and stool. Feculent emesis and severe distension are late findings, and peritoneal signs are indicative of bowel wall ischemia. Diagnosis of cecal volvulus is less straightforward, and patients are more likely to describe an atypical presentation.
Other mechanical causes of large-bowel obstruction should be evaluated at the time of diagnosis, with a thorough history and physical examination to investigate for a history of diverticulitis, hernia, or other less common causes of obstruction.
Adynamic Disease of the Colon
Acute colonic pseudo-obstruction (Ogilvie’s syndrome) is the most common adynamic cause of large-bowel pseudo-obstruction. The pathogenesis of this syndrome is unclear, but is believed to be an imbalance between vagal parasympathetic tone and thoracic/lumbar sympathetic tone. These patients typically present with progressive dilatation of the colon following a major operation, and if unnoticed can progress to colonic perforation, most commonly cecal due to its larger diameter with increased wall tension secondary to LaPlace’s Law (wall tension = pressure * radius). Adynamic disease should generally only be considered where no cause of obstruction can be identified or expected, and the patient has risk factors for adynamic disease such as prolonged hospitalization, advanced age, recent surgery, or use of associated medications such as opiates or anticholinergics. Adynamic disease will typically involve the entire length of the colon, which is less common in mechanical disease.
WORKUP AND CHOICE OF IMAGING
Although the majority of patients with large-bowel obstruction will undergo helical computed tomography (CT) scanning, the plain radiograph is still a useful diagnostic tool. Initial plain films can demonstrate volvulus, and with the addition of barium, enema, stricture, and malignancy. In a patient with peritonitis and systemic signs of sepsis, an abdominal plain film revealing volvulus can expedite the diagnosis and subsequent transfer to the operating room, without the requirement of a CT scan.
In stable patients, the addition of CT scanning allows for delineation of the site of obstruction, etiology, and extent of the lesion, as well as characterization of bowel ischemia.7,8 Given the high level of detail and the rapid acquisition time, CT is the study of choice in patients with large-bowel obstruction who are not in extremis.
Rectal magnetic resonance imaging (MRI) offers useful information for operative planning in the case of malignant obstruction; however, the time required for image acquisition precludes its use in the emergent setting, i.e., in patients with colonic ischemia.
All patients with large-bowel obstruction should undergo laboratory studies, including a complete blood count (CBC) to evaluate for leukocytosis, a chemistry panel for electrolyte abnormalities, and a lactate level to indicate possible colonic ischemia. Such data assist in informing surgeons of the urgency of operative intervention.
IMAGING FINDINGS
The radiographic hallmark of large-bowel obstruction, regardless of the modality, is marked dilatation of the colon. Depending on the etiology, an abrupt cutoff, tapering, or mass lesion at the site of the obstruction may be visualized.
Plain Film
Normal Colon
The colon on a normal abdominal x-ray is recognizable by its peripheral position, occupying the lateral margins of the abdomen while the small intestine is more centralized. Considered this way, the colon “frames” the abdomen. Haustra are visualized, as well as feces, which has a mottled appearance. Normal caliber of the colon is 5 to 6 cm, although the cecum may be as wide as 9 cm, even in the absence of pathology (Table 10–2 and Figure 10–1).
Figure 10–1 Supine abdominal plain film depicting normal colon. Stool can be visualized as a slightly mottled/bubbly appearance in the ascending colon. Haustra are easily seen in the transverse colon. L, left colon; R, right colon; T, transverse colon.
Malignant Large-Bowel Obstruction
On plain film imaging, obstructive left-sided malignancy will typically demonstrate dilatation of the entirety of the colon, while obstructive right-sided malignancy will demonstrate decompression of the distal colon, and often distension of the small bowel, mimicking a small-bowel obstruction. Distal decompression may be recognized by a paucity of air, particularly in the rectum. Subsequent barium enema can be used to further localize the obstruction and demonstrates an abrupt contrast cutoff at the site of the offending lesion, with nonvisualization of the proximal colon (Table 10–3 and Figure 10–2).
Figure 10–2 Contrast enema revealing obstruction in the ascending colon. Note the relatively decompressed loops of transverse and descending colon, and the abrupt contrast cutoff at the obstruction point. In this image, dilated small-bowel loops can also be appreciated centrally, a common finding in right-sided obstruction. Reproduced with permission from Zinner MJ, Ashley SW. Maingot’s Abdominal Operations, 12 ed. New York, NY: McGraw-Hill; 2013.)
Sigmoid Volvulus
Sigmoid volvulus is often easily identified on both plain film and cross-sectional imaging. Substantial distension of the affected segment is demonstrated, frequently with distortion into the right side of the abdomen. Haustral markings are lost, and as the colon twists it adopts a classic “coffee bean” or “bent inner tube” configuration. The point of torsion sometimes appears on plain film as a tight area of narrowing, called the “bird’s beak” sign. Finally, a “white stripe sign,” or bright line, can be appreciated at the interface between two apposed loops of colon9 (Table 10–4 and Figures 10–3 and 10–4).
