The most common cause of small bowel obstruction (more than half) is postoperative adhesions. Other causes include: tumors, hernias, torsion, foreign bodies, inflammatory bowel disease, and radiation bowel disease. Abdominal surgeries that predispose to small bowel adhesive obstruction include appendectomy, colorectal surgery, and obstetrical and gynecological surgery. Statistics show that lower abdominal and pelvic surgeries are more likely to cause small bowel obstruction than upper gastrointestinal surgeries.
In recent years, the number of women performing cesarean sections has increased greatly, and according to the authors’ intraoperative exploration of other abdominal procedures with a history of cesarean section, these women rarely develop significant adhesions, probably related to the distal separation of the incision of the enlarged uterus from the bowel and the dynamic contraction of the uterus after delivery, which is less likely to adhere to the peritoneum. Therefore, patients with a history of cesarean section presenting with intestinal obstruction should not limit their thinking to postoperative adhesions and misdiagnose other conditions.
Small bowel obstruction leads to dilatation of the proximal bowel and accumulation of fluid and gas. In the early stages, peristalsis increases in front of and behind the site of obstruction, and frequent loose stools and vomiting (flatus) may occur. The higher the site of obstruction, the more pronounced the vomiting and the more frequent the abdominal distention (crampy); this is not the case in terminal ileal obstruction, where the abdominal distention is more pronounced and sometimes difficult to distinguish from colonic obstruction based on symptoms alone.
SBO may be partial or complete, simple or strangulated. If left untreated, strangulated small bowel obstruction has a 100% mortality rate; if operated within 36 hours, the mortality rate decreases to 8%; if operated beyond 36 hours, the mortality rate is 25%. Although the incidence of strangulated small bowel obstruction is not high, it can have serious consequences if it occurs, especially if the diagnosis or management is delayed; in view of this, vigilance and timely diagnosis of strangulated small bowel obstruction are essential.
Simple small bowel obstruction presents as intermittent abdominal cramps, but if the abdominal pain increases and becomes persistent, or if it is persistent from the beginning, the possibility of strangulated bowel or ischemia should be considered. Typical signs of intestinal ischemia also include fever, tachycardia, signs of peritonitis, and even shock; once these signs are present, the intestine is mostly necrotic. However, in the early stage, despite the obvious abdominal pain, there are no obvious abdominal signs, and the X-ray examination is mostly unremarkable, even the dilatation of the intestine and air fluid level (air fluid level) are not obvious, so that some physicians relax their vigilance and misdiagnose it as medical abdominal pain.
Diagnostic imaging and evaluation
The normal adult small intestine is 3-6 meters in length. Although the small intestine is freely peristaltic, the beginning of the jejunum and the end of the ileum are fixed due to local structural constraints, usually the jejunum is located in the left quadrant and the ileum in the pelvic midline, which helps to determine the site of obstruction by imaging.
In normal adults, there is a small amount of gas in the small intestine, and one to two non-dilated, inflated small intestinal collaterals with a luminal width of no more than 3 cm can be seen on X-ray plain film; in CT scan, the normal value is no more than 2.5 cm (due to the magnification effect of X-ray film).
When SBO is suspected, radiographs are usually the first choice to determine the location and extent of the obstructed intestinal collaterals. the sensitivity of radiographs for diagnosing SBO depends on the amount of gas in the proximal intestine of the obstruction; if there is no gas or very little gas, SBO may be missed. CT scan is recommended. In addition to the advantages mentioned above, CT scan is particularly suitable for critical, non-standing cases.
CT scans not only provide a clearer visualization and measurement of the dilated bowel, but also allow observation of the thickness of the bowel wall, the condition of the bowel wall, the mesentery and blood vessels, the presence of occupying lesions, the presence of ischemic signs, and the presumed location and cause of the obstruction. Of course, CT scans are also less sensitive in diagnosing mild, partial or incomplete SBO. The decision to operate or be conservative is easily made by the surgeon with the visualization of the CT scan.
Ultrasound can be used to screen for acute abdominal pain. The ultrasound presentation of SBO is characterized by dilated bowel, pneumatization and fluid accumulation in the intestinal lumen, and visualization of intestinal mucosal folds. Because of the real-time display, ultrasound can identify paralytic intestinal obstruction from mechanical intestinal obstruction by observing intestinal peristalsis. Ultrasound, like CT, can also clearly show ascites. MRI is not suitable for the diagnostic evaluation of acute abdominal conditions such as intestinal obstruction because it is too time-consuming. Arteriography is indicated for strangulated intestinal obstruction, in other words, intestinal obstruction due to vascular disease or ischemic bowel disease.
X-ray plain film
First of all, critically ill patients in shock or pre-shock are not suitable for radiographs. In exceptional cases, patients may be placed in the supine position and lateral films may be taken. Otherwise, the patient should be taken in the standing position.
It takes about 3 to 5 hours of complete obstruction before dilated intestinal collaterals appear. Therefore, X-ray examination performed within 3 hours of abdominal pain may yield negative results and should be taken or reexamined after 3 to 5 hours.
The diagnosis of SBO is made when the small bowel collaterals are more than 3 cm in diameter, provided that the lumen is filled with fluid, which may show a narrow soft tissue mass shadow. Usually, the more distal the obstruction, the greater the number of gas-fluid levels (gas-fluid level) and the stepped (stepladder) manifestation may appear. The mucosal folds surrounding the jejunal lumen are clustered at a spacing of 1-4 mm, which increases after obstruction, while the lumen in the terminal ileum is flat. In case of peristalsis, string-of-beads sign may appear due to interruption of gas in the intestinal lumen.
Most closed-loop small bowel obstructions are caused by adhesions. X-rays may show a coffee bean sign (inflated collaterals) or a pseudotumor sign (fluid-filled collaterals).
Gallstone obstruction mostly occurs in the terminal ileum and presents as a calcified intraluminal stone shadow. In particular, not all gallstones are radiopaque, and the proximal segment of gallstone obstruction is also predominantly fluid-filled and less gas-filled, and therefore may be missed on plain radiographs alone.
CT scan
CT scan can be used for the diagnosis of any intestinal obstruction or suspected intestinal obstruction, especially in cases of suspected narrowing. Enhanced CT scan is preferable if available.
Due to the accumulation of gas and fluid in the intestinal lumen and the filling of the intestinal canal, an oral contrast agent is usually not required for the scan; however, an oral contrast agent can more accurately show the site and extent of the obstruction.
A small bowel diameter of more than 2.5 cm is considered abnormal; dilated proximal intestinal collaterals and deflated distal intestinal collaterals can be diagnosed as SBO. feces sign is seen at the proximal end of the obstruction and is an uncommon but reliable sign of mechanical intestinal obstruction. The site of obstruction is the migration of dilated and deflated intestinal collaterals. The degree of collaterals deflation and the amount of residual intestinal material distal to the obstruction reflect the degree of obstruction, and if oral contrast enters the deflated segment, the obstruction is partial or incomplete.
Unless the adhesive band is very wide, CT scans usually do not show this. For the diagnosis of adhesive small bowel obstruction, first, the patient must have factors causing the adhesions, such as laparotomy or inflammation; then the CT scan excludes other causes of obstruction, showing only abrupt changes in the lumen diameter. Adhesive bands are commonly found in the terminal ileum, deep in the abdominal wall incision scar, in the area of the original surgery, or in the area of inflammation.
Closed-collar small bowel obstruction presents with U- or C-shaped small bowel collaterals that converge at the site of torsion, a radiolucent mesentery and stretched vessels within it, and tightly twisted vessels that some call the whirl sign. At the site of obstruction, the deflated intestinal collaterals are round, oval, or triangular in shape; in the longitudinal view of the torsion, they show the beak sign, which is a fusiform thinning of the intestinal canal at the site of obstruction.
CT features of strangulation include circumferential thickening of the intestinal wall with high attenuation, a target sign, and congestion or hemorrhage in the closed collaterals. A serrated beak may be seen at the site of obstruction. The mesentery is edematous and shows diffuse swelling and blurring. Enhancement scans may reveal: delayed, weak or no enhancement of the intestinal wall. CT angiography (CTA) and interventional DSA can directly demonstrate vascular occlusion, stenosis, and intestinal ischemia.
CT manifestations of intestinal torsion include transposition of the bowel and displacement of the superior mesenteric artery. Intussusception (intussusception) presents as a target sign in cross-section with alternating low-attenuation and high-attenuation layers of the intestinal wall. Sausage-shaped or reniform masses may be seen at some levels.
CT is also useful in the diagnosis of ventral hernias in rare areas, especially in obese patients, and can clearly demonstrate the contents of the hernia.
In cases of Crohn disease: narrowing of the intestinal lumen, thickening of the intestinal wall, and often target-like or double halo manifestations in the acute phase due to lamination of the intestinal wall. On enhancement scans, there can be significant enhancement due to mucosal and plasma inflammation, and the intensity of the enhancement correlates with the activity of the disease. In the chronic phase, the stratification of the intestinal wall disappears, showing the typical homogeneous attenuation and visible fat deposits in the intestinal wall.
In cases of radiation enteropathy, thickening of the intestinal wall, narrowing of the intestinal lumen and fibrosis of the mesentery can be seen, with the majority of the obstruction located in the pelvic floor.
In cases of intestinal tuberculosis, CT scans may show mild irregular thickening of the intestinal wall if the inflammation is not severe; if the inflammation is severe, the intestinal wall may be markedly thickened to form a heterogeneous inflammatory mass, and sometimes large regional lymph nodes with low central attenuation may be seen.
Small bowel tumors are uncommon, some of which present as SBO. adenocarcinoma or adenoma present with intestinal wall thickening and intraluminal obstruction, and primary non-Hodgkin’s small bowel lymphoma rarely causes obstruction, but most often lymphoma of the mesenteric lymph nodes infiltrates the bowel wall and causes luminal obstruction.
SBO secondary to other abdominal diseases may also be seen with corresponding images: e.g. acute appendicitis and its peripheral inflammation, cholelithiasis, metastatic carcinoma, and fecal stone (bezoar).
Treatment
Non-surgical or surgical treatment?
Surgical treatment is preferred when a preoperative diagnosis can be made or when there is a high suspicion of certain unavoidable causes, such as small bowel tumors.
Of course, the patient’s symptoms, signs and imaging are crucial in diagnosing the cause of SBO and deciding whether to operate. Even in the case of small bowel tumors, appropriate preoperative preparation can be performed. However, acute surgical treatment is considered first in all of the following cases
Severe persistent abdominal pain: regardless of the presence of clear abdominal signs, small bowel ischemia (strangulated intestinal obstruction) should be considered first and an immediate enhanced CT scan, CTA or DSA is recommended.
Acute peritonitis: it may be caused by ischemic necrosis and perforation of the small intestine.
Abdominal distension, ascites, and loss of bowel sounds: provided that paralytic intestinal obstruction has been excluded, the small intestine may be necrotic.
High degree of abdominal distension: cystometry >25cmH2O, if intra-abdominal pressure decreases through gastrointestinal decompression, perforation and release of ascites, and there is no blood flow obstruction in the small intestine, non-surgical treatment can be continued; otherwise, dissection and decompression should be performed.
Shock during treatment: If there is no shock at the time of admission and shock occurs during non-surgical treatment (shock of non-medical factors), the indication for surgery is absolute. People with simple intestinal obstruction rarely present with shock unless there is severe dehydration for a long time.
In SBO combined with shock, severe conditions such as intestinal ischemia should be considered first. From a simple logical analysis, the condition of untreated SBO is still stable, and the condition at least remains stable, or even tends to remit or improve during non-surgical treatment. If shock occurs, after excluding medical factors such as drug allergy and cardiogenic shock, it is certain that a fatal lesion of the intestinal canal has occurred. It is added to emphasize that elderly people are less responsive, and in the presence of intestinal tube blood flow disorders, abdominal pain symptoms and abdominal signs are not obvious, but show a deterioration of the general condition.
Imaging helps the surgeon to make a correct judgment and thus a rational decision. Again, the importance of CT scan is emphasized. Imaging should be performed as much as possible when the condition is still stable and suitable for moving, and the skills to determine bowel crisis should be acquired.
Most SBOs can be treated non-operatively first, except in emergency situations. Non-operative treatment is both conservative and a pre-surgical preparation if necessary. Fluid resuscitation (fluid resuscitation) is necessary due to loss of body fluids and decreased intake in the intestinal lumen.
Continuous nasogastric tube (NG tube) suction and application of growth inhibitors may reduce the symptoms of abdominal pain, bloating and vomiting and improve the success rate of conservative treatment.
Increased bacterial colonization and bacterial translocation in the intestinal cavity after obstruction, however, rarely show signs of significant infection and prophylactic application of antibiotics does not seem to have a definite effect in most cases.
The administration of analgesics is generally not advocated during nonsurgical treatment to prevent masking the condition, unless it has been determined that surgery is imminent or that conservative treatment will be successful (e.g., partial small bowel obstruction after surgery); antispasmodics may relieve colic but do not help the bowel to overcome resistance and release the obstruction. Antiemetics may be given to patients who continue to have nausea and vomiting after an indwelling NG tube.
The most common case in which non-surgical treatment can be successful is obstruction due to post-surgical intestinal adhesions. Generally after no more than 7 days, most small bowel obstruction can be relieved; those who are not relieved should be considered for surgical treatment; patients who are relieved may be completely relieved (especially those with partial obstruction), or they may be the result of fasting and gastrointestinal decompression, whose own mechanical factors have not been lifted; the latter can be confirmed by trying to eat and imaging such as gastrointestinal imaging, in which case surgical treatment is still needed , a long period of conservative treatment is unhelpful and dangerous.
It is important to know that for cases where conservative success is not possible, early surgery not only reduces the patient’s pain, but also decreases the incidence of complications and morbidity and mortality; on the contrary, unrelieved primary disease and prolonged consumption can cause deteriorating nutritional status, endostasis instability, and organ dysfunction in patients, so delaying surgery is bound to increase the incidence of complications and morbidity and mortality after surgery.
Some physicians are too cautious about the surgical decision for elderly patients and patients with medical comorbidities, and some patients with SBO are treated conservatively for too long, while surgery is eventually unavoidable or even serious complications occur after surgery. After a prolonged delay in surgery for mechanical small bowel obstruction, the patient may have reduced or disappeared abdominal pain as the bowel gives up efforts to overcome resistance and stops paroxysmal peristalsis, or what appears to be paralytic intestinal obstruction may appear to be fairly stable on the basis of fasting and fluid replacement, further inducing the surgeon to delay surgery.
In cases where the surgeon loses patience or the condition suddenly worsens, the outcome of surgery may be catastrophic with poor bowel healing and prolonged depletion resulting in the insufficiency of vital organs. Avoiding this situation, a dynamic CT scan can reveal that the obstruction is not relieved and will increase the surgeon’s confidence to operate.
The most common cases in which non-surgical treatment can be successful are obstructions due to post-surgical intestinal adhesions. However, some adhesive obstructions are impossible to release and some obstructions are caused by tumors; surgical treatment is preferred when certain causes of unavoidable surgery can be diagnosed preoperatively or are highly suspected. If the cause has been unclear, surgery should mostly be performed for small bowel obstruction that has not improved for a conservative period of time, or for recurrent small bowel obstruction (even if it is partial).