Small Bowel Resection

Anatomy and Physiology

The small bowel consists of 3 histological sections: duodenum, jejunum, and ileum. The duodenum is the most proximal segment, approximately 25 to 30 cm, extending from the pylorus. Its dual blood supply comes from the celiac axis and superior mesenteric artery (SMA). The gastroduodenal artery off the celiac axis branches into the anterosuperior and posterosuperior pancreaticoduodenal arteries,, which travel anterior and posterior to the duodenum and pancreatic head. These arteries form arcades with the anteroinferior and posteroinferior pancreaticoduodenal arteries,, which arise from the SMA.

The duodenum is further separated into 4 sequentially named portions. The first portion is the duodenal bulb. It is intraperitoneal and travels in a transverse rightward direction off of the pylorus. The second portion is the descending duodenum. It locates retroperitoneally. This portion contains the ampulla of Vater (duct of Wirsung) and duct of Santorini. Its proximity to the biliary tree and pancreas makes any surgical interventions in this portion uniquely challenging. The third portion is the transverse duodenum, and it is also retroperitoneal. Once the duodenum passes through the space between the aorta and SMA, it becomes the fourth portion, which turns upward after crossing the midline. The ligament of Treitz is a thin muscular band that connects the duodenum and jejunum to the surrounding connective tissue. This serves as the marker for the transition from the duodenum to the jejunum.

The jejunum is the second segment of the small bowel. It is approximately 100 cm long and is characterized anatomically by its circular muscular folds and long vasa recta providing blood supply. The jejunum is the maximum site for nutrient absorption except for B12, bile acids, and folate, absorbed in the ileum. Iron is also excluded as it is absorbed primarily in the duodenum. Water and salt absorption almost primarily takes place in the jejunum.

The ileum is the final segment of the small bowel, encompassing the final 150 cm before meeting the cecum. It is characterized by its short vasa recta.

The blood supply for the jejunum and ileum comes from numerous branches coming off the SMA. There are roughly 5 to 6 jejunal branches and 6 to 8 ileal branches that form a highly redundant arcade of vessels, contained all within the small bowel mesentery. The terminal ileum and cecum received blood supply coming off the ileocolic artery. The ileocolic artery branches from the SMA separate from the ileal and jejunal branches, although they shares branches with the prior. No watershed areas exist within a small bowel with healthy vasculature. 

The surgical anatomy of the small bowel can be structurally separated into 3 layers: mucosa, muscularis, and serosal layers. In reality, there are several histologically distinct layers within the bowel wall with unique and important functions. These sublayers are indistinguishable to the naked eye, though, and therefore not pertinent knowledge in the context of small bowel resection.

The innermost layer lining the lumen of the bowel is the mucosa. This consists of numerous folds of glandular epithelial tissue. It is richly vascularized with a high rate of tissue turnover and minimal strength for holding sutures. The submucosal nerve plexus makes up the outermost sublayer of the mucosa. Deep to this is the muscularis, with separate circular (inner) and longitudinal (outer) muscle separated by the myenteric nerve plexus. This layer is primarily responsible for mixing and propelling enteric contents through the bowel and has its intrinsic basal electrical rhythm that controls peristalsis. The mucosa contains a thin band of the muscular mucosa, which has a minor effect on bowel motility. The outermost layers are the adventitia (for retroperitoneal structures) and serosa (additional layer for intraperitoneal structures). The adventitial tissue is largely fixed to surrounding retroperitoneal tissue, whereas serosa creates a clear boundary between the gut and surrounding tissue. The serosa connects to the mesentery along the inner wall of the bowel. These layers are composed of several layers of connective tissue and will provide an immobile strength layer to a small bowel anastomosis or secure bowel to surrounding connective tissue. [5][6][7]

Indications

There are many reasons for performing a small bowel resection (SBR), and common indications are listed below. While the general principles of an SBR are nearly universal, modifications of the surgical technique may be required based on the pathology. Some common indications are listed below, with an abridged explanation of how each pathology needs to be specifically addressed.

• Obstruction not amendable to adhesiolysis
• Malignancy: Suspected malignancies will typically require 8-10 cm margins.
• Non-traumatic perforation: The differential for adult patients includes ulcerative disease and malignancy. SBR may be preferred over the repair to allow for tissue diagnosis by a pathologist
• Traumatic perforation: Defects can be repaired if they encompass less than 50% of the small bowel loop; otherwise, they require resection.
• Ischemic necrosis: This occurs secondary to emboli, thrombus, or low-flow malperfusion. Bowel ischemia is often progressive if the underlying diseases state is not corrected, requiring multiple resections over several days and carrying high morbidity
• Inflammatory bowel disease (IBD): Resection is typically the last treatment choice reserved for strictures not amendable to stricturoplasty or conservative management. The inflammatory nature of IBD increases the risk of complications
• Enterocutaneous fistula not amendable to closure with conservative measures
• Necrotizing enterocolitis with perforation
• Symptomatic Meckel diverticulum or diverticular disease [8][9][10]

Contraindications

There are no absolute contraindications that can be applied to the population in general. Active IBD is a relative contraindication, as local inflammation increases the risk of anastomotic leak. Few additional relative contraindications exist since SBR is usually only considered to remove a diseased segment without additional therapeutic options. [11]

Equipment

A standard laparotomy tray will typically contain all of the necessary instrumentation for an open SBR. Self-retaining table-mounted retractors such as the Omni-Flex or Bookwalter retractor systems can be helpful if the abdomen is to be explored. For a laparoscopic SBR, standard atraumatic graspers are preferred for handling the bowel to prevent iatrogenic serosal injury. Laparoscopic electrocautery or cutting devices such as the Ligasure or Harmonic Scalpel are required for mesenteric dissection. [12]

Preparation

Small bowel resection can be done during an open or laparoscopic procedure depending on the timing and indication for the surgery. For elective resections, bowel preparation with a low residue diet for several days before the surgery is generally undertaken. Mechanical bowel prep may be added but is not standard. Conditions requiring SBR are often emergent, and therefore pre-op planning is limited to supportive care for the critically ill patient. Collectively, the following preparation strategies are recommended to minimize the post-procedural complications with small bowel resection: 1) optimizing the nutritional status, 2) weaning the immunosuppressive agents, 3)abdominopelvic abscess drainage, specifically in chronic conditions, including Crohn disease. [13]

Technique or Treatment

Small bowel resection involving the ileum or jejunum is straightforward as long as the bowel is adequately mobile. Duodenal resections are exponentially more complex due to the duodenum’s retroperitoneal position and proximity of the ampulla of Vater, biliary tree, and pancreas. For this reason, duodenal resections will almost always require an extra-anatomic reconstruction, often specific to the pathology being addressed. Given the variety and complexity of procedures to treat duodenal pathology and the marked difference between resecting duodenum versus the more distal bowel, it is reasonable to separate discussion of those procedures for their specific pathologies. 

Management of various pathologies within duodenum demand utilizing different surgical approaches. Accordingly, duodenal tumors, including asymptomatic lesions that are found incidentally during esophagogastroduodenoscopy, are exponentially challenging. These lesions should be biopsied, and those with high malignant potential, including symptomatic tumors and adenomas, should be removed. While smaller lesions with less than 1 cm diameter are candidates for endoscopic polypectomy, larger tumors with greater than 2 cm size should be removed surgically. Considering the tumor's location within the duodenum, transduodenal polypectomy or segmental duodenal resection might be selected. The highest challenging duodenal tumors are those located in proximity to the ampulla of Vater and should preferably be managed with pancreaticoduodenectomy or Whipple procedure. For the 1-2 cm lesions, specifically, the lesions which are not exceeded beyond mucosa, other less-invasive procedures, including endoscopic polypectomy, are preferred. Adenomas that are removed endoscopically are subjected to long-term routine surveillance to diagnose the recurrent lesions timely. In patients with recurrent adenomas within the second portion of the duodenum, pancreaticoduodenectomy is indicated.[14] 

 The entire small bowel should be examined before resection. If the surgery is being performed open, the portion being resected should be eviscerated and placed on a towel to limit contamination from spillage. Resections are planned based on the intended anastomosis: stapled or hand sewn. For a stapled anastomosis, defects are made through the mesentery close to the bowel wall, away from large vessels. Gastrointestinal anastomosis staplers (GIA), usually 60 mm linear cutting with a soft tissue load, are placed across the bowel through the defects. Staplers are then fired at a slightly oblique angle above and below the diseased segment, cutting and sealing the bowel in unison. The diseased, isolated segment of the bowel is then freed from the mesentery using electrocautery, with identified vessels suture ligated with silk suture. Once removed, the sealed ends are most commonly reconnected using a side-to-side, functional end-to-end anastomosis. In this technique, the proximal and distal segments are aligned in parallel with the mesenteric borders adjacent to one another. It is important to inspect the base of the mesentery to ensure there is no abnormal rotation of the tissue. Sero-muscular sutures should be placed on the mesenteric side for support and to prevent migration. Small enterotomies are then made on the anti-mesenteric corner of each segment. A common channel is then created by placing one limb of a GIA stapler into each lumen through these enterotomies. Firing the stapler then creates a common channel between the 2 segments. The common channel should be inspected to ensure there is no intraluminal bleeding at the staple line. Once hemostasis is confirmed, the enterotomies are then closed, usually by exclusion by firing another TA or GIA stapler across the top of the anastomosis. Edges should be checked to ensure the serosa is incorporated into the staple line on both sides. As the corners of the staple line have a higher risk of ischemia, Lembert stitches using absorbable sutures should be placed in the corners, allowing the tissue to invert beneath the suture. Lastly, the mesenteric defect is closed with permanent or absorbable sutures to prevent internal herniation. Care must be taken to avoid strangulating the edges of the mesentery, as the main blood flow to the anastomosis will come from this tissue.

A 2-layer, hand-sewn anastomosis will increase operative time but remains the preferred method for many surgeons. First, appropriate mesenteric defects are created. The mesenteric tissue between the 2 defects is then clamped and suture ligated. Two non-crushing clamps are placed across the bowel at each of the desired locations of transection, and the bowel is divided sharply. A 1-cm stump should be left beyond the end of the clamps for suturing. Anastomoses will typically be done in an end-to-end fashion. However, side-to-side and end-to-side methods can also be employed. The latter can be beneficial when a size discrepancy exists between the 2 segments, such as when 1 segment is more distended and edematous or making an ileocecal anastomosis.

With the diseased bowel segment now cut and removed, the cut ends of the small bowel are held aligned, and stay sutures are placed in the corners. The clamps holding the bowel ends are rotated in opposition to one another to bring the posterior outer serosal layers together. A series of silk interrupted Lembert sutures are then placed through the seromuscular layers to conjoin the 2 segments. This is continued until the stay sutures are reached at each end. Next, absorbable sutures are placed in full-thickness bites to form the posterior inner layer along with the same distance. The sutures should be placed so that the mucosa inverts and contacts the opposing mucosa throughout. With the posterior layer complete, absorbable Connel (U-shaped) stitches are placed in the corner, and the pattern is continued to close the anterior inner layer. Suturing of the inner layers can be safely done with either interrupted or continuous patterns. Continuous sutures have been shown in animal models to cause decreased perianastomotic tissue oxygen tension compared to interrupted sutures; however, limited human trials have not shown any significant difference in anastomotic leak rates when comparing the 2. Once that anterior inner layer is closed, silk Lembert stitches are placed in interrupted fashion across the anterior outer surface, completing the 2-layer anastomosis.

If a single layer is preferred, the steps above are performed, with the Lembert stitches being excluded. A small bowel anastomosis can be safely created with single or double-layer closure, largely depending on surgeon preference. Single-layer closures are faster when compared to a double layer. A Cochrane review concluded the 2 styles were equivocal in safety; however, a meta-analysis of available randomized trials comparing the 2 was only moderate quality. Therefore, the clinical discretion of the surgeon is always warranted. A single-layered closure is often preferred in patients with inherently narrow bowel lumens, such as neonates. A double-layer closure may cause a too-high risk of obstruction if the lumen is small. Overall, it isn't easy to compare the 2 techniques on a broad scale due to heterogeneity in all the factors that may affect the overall outcome. These include suture type, surgeon experience, patient factors such as treating disease, and other comorbidities.

SBR for mass or neoplasm will generally require oncologic margins of 8 to 10 cm and associated mesentery and lymphatic tissue; whereas, benign processes only require limited resection with sparing of the mesentery to preserve blood flow. If an oncologic resection is required, the primary vessel supplying the segment must be traced back to the mesenteric root and divided at its base. The adjacent mesentery is then fully resected in a wedge shape extending from the small bowel to the vessel's origin. This should allow for adequate lymph node harvest.

Under most circumstances surrounding an SBR, the surgeon will plan to return the small bowel to normal anatomic continuity. Numerous situations exist where this may not be preferred. The surgeon may opt for an extra-anatomic reconstruction to bypass or exclude a segment of diseased or non-functional bowel that cannot otherwise be removed. With a distal ileum resection, creating an ileocolonic anastomosis may be risky if there is a large size discrepancy between the two ends and/or a hostile environment from inflammation or infection. In this case, an end-ileostomy will be the safest option for the patient.

Creating an ileostomy starts with choosing a site for the ostomy, away from other incision sites and above the beltline. When possible, this site should be chosen and marked pre-operatively with the patient awake and sitting upright. A circular segment of skin is excised at the chosen spot, and sharp dissection is used to create a passage through the subcutaneous tissue to the rectus sheath. The rectus sheath is incised, and the muscle is bluntly dissected down to the level of the peritoneum, which is entered sharply. With the distal ileum adequately mobilized, it is grasped with a Babcock and gently delivered through the stoma, along with its supporting mesentery. Care must be taken to keep the terminal ileum in its correct orientation. Once the ileum is through the stoma, the main abdominal incision should be closed and covered before opening the bowel. The stapled/sutured edge of the ileum should then be excised to produce a fresh bleeding edge. Absorbable sutures should be placed between the seromuscular layer of the bowel and the subcutaneous tissue in each quadrant to orient the ileostomy. Once secured, interrupted sutures can then be placed circumferentially, taking full-thickness bites of the bowel and securing it to the dermis. When tightened and tied, these should help to avert the end of the ostomy. Once complete, the ostomy should be checked for patency before application of a stoma appliance.[15],[16]

Complications

Superficial wound infections are common in contaminated cases. Allowing the wound to close by secondary intent can reduce the risk of soft tissue infection, abscess, and potential wound dehiscence.[17][18][19]

Anastomotic breakdown or leak is the most feared common complication after SBR, with incidence varying from 1% to 24% based on numerous factors. Risk can be reduced by focusing on operative techniques to ensure adequate blood supply with minimal tension. This must be combined with appropriate preoperative and postoperative supportive care to mitigate the effects of systemic patient disease and prevent protein-calorie malnutrition.

Unlike high-risk colonic anastomoses that can be protected with the proximal diversion of the fecal stream, diversion is not an option in SBR. Proximal diversion of anything proximal to the terminal ileum would impair the guts' absorptive function. Proximal decompression with a nasogastric or gastric tube can help alleviate early healing anastomosis. The anastomotic breakdown will inherently allow enteric contents to permeate into the peritoneal cavity, causing abscesses, peritonitis, and sepsis, and often progressing to abdominal wound dehiscence.

Fistulization is a common subacute complication of anastomotic breakdown after SBR. Enterocutaneous fistulas are both common sequelae of anastomotic leaks, as well as a common reason for additional SBR if they are persistent. Enteroenteric and enterocolonic fistulae are not uncommon findings in a hostile abdomen of a malnourished or hemodynamically impaired patient. These are likely under-recognized due to the lack of external clinical signs and can contribute to malabsorption by allowing enteric contents to bypass a variable portion of the small bowel’s absorptive surface.

SBR, as with any intraabdominal surgery, can contribute to adhesions which increase the future risk for obstruction.[20]

The absorption of fat-soluble vitamins is dependent on the diffusion process. First, the vitamins need to be emulsified and incorporated into micelles with other lipids to be absorbed. The emulsification depends on bile secretions and pancreatic enzymes. In SBS, there is a rapid intestinal transit reducing the time for emulsification resulting in fat-soluble vitamin deficiency.[21]

Clinical Significance

Massive SBR can lead to short bowel syndrome (SBS), a condition characterized by malnutrition and malabsorption secondary to loss of functional small bowel and more rapid intestinal transit. In addition to weight loss and protein-calorie malnutrition, patients suffer from diarrhea, steatorrhea, electrolyte abnormalities, and deficiencies in fat-soluble vitamins. This primarily occurs in patients that have suffered a very-long segment SBR or multiple sequential resections. SBS is highly morbid, with 1 in 3 patients perishing within the same hospitalization as a diagnosis. An additional 1 in 3 will die within the first year after diagnosis from malnutrition coupled with the inherent medical comorbidities attributable to treatment of the disease. [2]The treatment goals for SBS are not just for a patient to be alive and well, but to maintain adequate nutrition with enteral feeds only. Patients who are forced to rely on prolonged parenteral nutrition will be subjected to high monetary costs, lifestyle limitations, as well as medical issues such as liver disease, and increased risk for complications from chronic central venous catheters.

While the bowel’s absorptive capacity improves with healing, small bowel remnant length is the most important prognostic factor for patients with SBS. Most adults have between 600 and 800 cm of the small intestine, and small bowel absorptive function is generally not impaired until greater than half of the bowel is gone. In adults, patients with greater than 180 cm of remaining small bowel will likely have enough absorptive surface to avoid long-term parenteral nutrition. Likewise, an adult with less than 60 cm of the small bowel will likely be dependent on parenteral nutrition indefinitely. The overall clinical severity is also affected by other factors affecting the absorptive capacity of the small bowel. The presence of the ileocecal valve and terminal ileum is a strong, positive prognostic factor for weaning off parenteral nutrition. The underlying disease state necessitating resection may also impair the function of the remaining bowel. For example, improving the function of chronically inflamed or irradiated bowel may plateau quickly, leaving a patient with SBS despite a relatively long segment.

Along with necrotizing enterocolitis in premature infants, congenital abnormalities such as volvulus or malrotation pose the highest risk of acquiring SBS in neonatal populations. Children with greater than 60 cm of the bowel can usually be sustained with enteral nutrition alone. However, the prognosis for survival and adequate enteral nutrition is not accurately predicted based on overall bowel length due to variations in total bowel length based on gestation age. Overall survival for a child with SBS is around 70%. There tends to be marked increases in the risk of death and parenteral dependence as the percentage of predictive bowel length for gestational age approaches 10%, showing the amazing ability for survival even in the face of a near-total loss of small bowel.

The goals of SBS management start with preventing weight loss and micronutrient deficiency while treating the underlying illness care to prevent the loss of any additional small bowel. Patients are typically in a critically ill, catabolic state at the time of diagnosis. Parenteral nutrition, therefore, is a mainstay of early SBS management to limit malnutrition. Early return to enteral feeds once ileus has resolved is advised, even if a diagnosis of SBS is expected. Enteral feeding is thought to stimulate intestinal adaptation by both directly stimulating enterocytes and by inducing endocrine and paracrine effects signaling for hypertrophy of the remaining small bowel mucosa. Drugs such as loperamide, diphenoxylate and atropine, and opiates will slow gut function, increasing the potential absorptive time of enteral feeds. Reducing gastrointestinal secretion and controlling diarrhea are also important goals for maximizing absorption.

Surgical options to treat small bowel syndrome are used sparingly in the adult population and reserved for patients unable to sustain with enteral feeds. Children are more likely to be surgical candidates, but they are also more likely to thrive with enteral feeds. Regardless, numerous surgical options exist to either lengthen the small bowel or improve function.

Approximately 50% of patients with SBS will require additional intraabdominal surgery at some point after diagnosis for various intestinal problems. With any surgery on a patient with SBS, preserving the remaining small bowel should be the main concern of a surgeon.

More common in children, strictures can cause pseudo-obstruction, leading to bowel dilatation and bacterial overgrowth. These should be treated with stricturoplasty or resection of the redundant bowel. If a patient has dilated bowel (at least 3 to 4 cm), a STEP (serial transverse enteroplasty procedure). [22] can be effective in lengthening the small bowel. Increases of up to 55 cm of intestinal length have been reported. The Bianchi procedure is another option for lengthening that has largely been replaced by the STEP. Patients with rapid transit can benefit from reversing 10 to 15 cm segments of the bowel. Short segment colonic interposition grafts have also been attempted with anecdotal reports of improvement.

Enhancing Healthcare Team Outcomes

Small bowel resection is done for many reasons. The surgery is usually performed by a general surgeon. However, the workup of patients with small bowel pathology usually involves an interprofessional team that includes a radiologist, pathologist, surgeon, gastroenterologist, primary care provider, and nurse practitioner. Patients should be educated on the potential complications of surgery such as bowel leaks, strictures, deep vein thrombosis, abscess, and wound infections. Prior to surgery, the patient should be informed about the need for bowel preparation.