Short bowel syndrome

Overview

Many people in the United States have extensive parts of their small intestine resected each year for different conditions such as inflammatory bowel disease (IBD), cancer, mesenteric ischemia, and others. Necrotizing enterocolitis, midgut volvulus, and other abdominal disasters are becoming increasingly prevalent in children. A functional short-bowel syndrome can result from a variety of nonsurgical treatments. Radiation enteritis is one example of this clinical situation.

Patients with inadequate small intestinal absorptive surface area experience malabsorption, malnutrition, diarrhea, and electrolyte imbalances. Short-bowel syndrome refers to a subset of people who have clinically substantial malabsorption and malnutrition.

According to cadaver studies, the typical length of the adult human small intestine is around 600 cm. The range, according to Lennard-Jones and Weser, is from 260 to 800 cm.  Short-bowel syndrome is caused by any illness, traumatic injury, vascular accident, or other pathology that leaves less than 200 cm of viable small bowel or resulting in a loss of 50% or more of the small intestine.

Short-bowel syndrome is a clinically recognized condition characterized by malabsorption, diarrhea, steatorrhea, fluid and electrolyte imbalances, and malnutrition. The last common etiologic element in all types of short-bowel syndrome is the functional or anatomic loss of substantial parts of the small intestine, resulting in significantly impaired absorptive capacity. Although resection of the colon does not usually result in short-bowel syndrome, the existence of the illness can be an important component in the care of individuals who have lost considerable quantities of small intestine.

Short bowel syndrome definition

The usual length of an adult's small intestine, measured from the duodenojejunal flexure, is between 275 and 850 cm. Every day, around 9 liters of fluid travel through the small bowel as oral fluids, saliva, gastric, biliary, and pancreatic secretions. Approximately 7 liters of those fluids are absorbed in the small colon and 2 liters in the large bowel on average. The big, healthy gut absorbs around 150 kilocalories per day, but in individuals with malabsorption, it can absorb up to 1000 kilocalories per day.

The majority of nutrients are absorbed in the first 100 cm of the jejunum. B12 and bile salts are absorbed in the ileum's final 100 cm; magnesium is absorbed in the terminal ileum and proximal colon; and water and sodium are absorbed throughout the intestine.

Adults with short bowel syndrome (SBS) have fewer than 180 to 200 centimeters of residual small bowel, necessitating nutritional and hydration supplements. Although there is no agreed definition of SBS in children, the necessity for intravenous supplementation has been proposed when less than 25% of the remaining small bowel is predicted for gestational age.

SBS occurs in around 75% of patients after a single, large resection of the intestine, with the remaining 25% occurring after successive resections. Approximately two-thirds of SBS patients survive their initial hospitalization, and a similar amount survive their first year after acquiring SBS. The long-term fate of a patient is mostly determined by his or her age and underlying condition.

Massive small intestine resection jeopardizes digestion and absorption. Without supporting care, adequate digestion and absorption cannot occur, and correct nutritional status cannot be maintained. Crohn's illness, radiation enteritis, mesenteric vascular accidents, trauma, and recurrent intestinal blockage are the most prevalent causes of a short-bowel syndrome in people today. The most prevalent etiologic causes in children are necrotizing enterocolitis, intestinal atresias, and intestinal volvulus.

Congenital short small bowel, gastroschisis, and meconium peritonitis are all disorders related to short-bowel syndrome.

Short-bowel syndrome does not occur in all people who have a considerable small intestine loss. The premorbid length of the small bowel, the segment of intestine lost, the patient's age at the time of bowel loss, the remaining length of the small bowel and colon, and the presence or absence of the ileocecal valve are all important cofactors in determining whether the syndrome will develop or not.

Several invasive or operational techniques and therapies have been developed and used to treat short-bowel syndrome. These include central venous access for total parenteral nutrition (TPN) supply, intestine transplantation, and nontransplantation abdominal surgeries. This section discusses the history of these diverse therapeutic options.

Anatomy

The duodenum is approximately 25 cm long and spans from the pylorus to the duodenojejunal flexure. The duodenojejunal flexure is located on the top left border of the second lumbar vertebra. Just distal to the Treitz ligament, the duodenum rotates anteriorly and caudally to become the jejunum.

The small bowel is split rather randomly into the jejunum (the proximal two-fifths) and ileum (the distal three-fifths), however on the surface, the jejunum seems to be larger, with a somewhat thicker wall. The root of the mesentery, which stretches from the left upper quadrant of the abdomen to the right lower quadrant, attaches it to the retroperitoneum.

The terminal ileum is found in the lower right quadrant. The ileocecal valve separates the small intestine from the large intestine. The cecum is the initial section of the colon. At the hepatic flexure, the ascending colon bends toward the midline and rises along the right paracolic gutter. The transverse colon originates here and travels to the splenic flexure, where it bends slightly medially and inferiorly to become the descending colon.

The descending colon becomes the sinuous sigmoid colon when it reaches the pelvic brim. This section of the colon travels toward the midline and then inferiorly. The sigmoid colon transforms into the rectum below the pelvic peritoneal reflection.

The duodenum receives significant blood flow from branches of both the celiac axis and the superior mesenteric artery. The gastroduodenal artery, superior and inferior pancreaticoduodenal arteries, right gastroepiploic artery, and supraduodenal and retroduodenal arteries are among them. In the case of a sudden closure of the superior mesenteric artery, this dual blood supply aids in the preservation of the duodenum.

Despite their abundant blood supply, the jejunum and ileum rely completely on branches of the superior mesenteric artery, with the exception of the first few centimeters. These are known as the jejunal and ileal arteries or intestine arteries. The terminal ileum is supplied by the ileocolic artery, a branch of the superior mesenteric artery.

The cecum and ascending colon are supplied by the ileocolic artery and the right colic artery, respectively. The middle colic artery provides the primary blood supply for the proximal two-thirds of the transverse colon. The superior mesenteric artery is the source of all of these colic arteries. As a result, the superior mesenteric artery supplies blood to the whole midgut, from the jejunum to the mid transverse colon.

The distal transverse colon and the descending colon are supplied by branches of the left colic artery, which is derived from the inferior mesenteric artery. The inferior mesenteric artery also gives rise to the sigmoid and superior rectal arteries.

Etiology 

SBS can be caused by significant surgical resection or by congenital intestinal disorders. Crohn disease, mesenteric ischemia, radiation enteritis, post-surgical adhesions, and post-operative sequelae are the most prevalent diseases that contribute to SBS in adults. Volvulus, intestinal abnormalities, and necrotizing enterocolitis are the most prevalent diseases in children.

Pathophysiology

The loss of considerable quantities of intestinal absorptive surface area causes physiologic derangements in short-bowel syndrome. Malabsorption of water, electrolytes, macronutrients (proteins, carbs, and fats), and micronutrients is one of the consequences of this loss 

The gastrointestinal tract is an important site for water and electrolyte absorption and transport. The GI tract must cope with its own significant daily secretions in addition to handling exogenously acquired sources of essential nutrients, such as daily water consumption and electrolytes contained in liquid and solid meals.

Sellin demonstrated the monumentality and efficiency of this work by observing that the GI tract processes 8000-9000 mL of fluid every day, with the great bulk of this originating from endogenous secretions. The healthy GI tract's fluid reabsorption is efficient (98 percent), and only 100-200 mL of fecal matter is lost each day. The small intestine is responsible for the vast majority (80%) of this reabsorption.

Disruptions in the key determinants of intestinal fluid absorption reduce the ability to reabsorb this substantial fluid load. The key variables include intestinal mucosal surface area, mucosal health or integrity, small bowel motility status, and the osmolarity of solutes in the intestinal lumen.

These changes can emerge clinically as significant components of short-bowel syndrome, such as diarrhea, dehydration, and electrolyte imbalance. Thus, clinical entities that result in a major reduction of mucosal surface area (eg, substantial small-bowel resection) or damage mucosal integrity might cause short-bowel syndrome (eg, radiation enteritis).

Macronutrients and micronutrients are absorbed along the small intestine's length. The jejunum, on the other hand, contains taller villi, deeper crypts, and more enzyme activity than the ileum. As a result, under normal conditions, approximately 90% of major macronutrients and micronutrients are digested and absorbed in the proximal 100-150 cm of the jejunum. Proteins, carbs, lipids, vitamins B, C, and folic acid, as well as fat-soluble vitamins A, D, E, and K, are all absorbed.

However, if a considerable piece or all of the jejunum is removed, protein, carbohydrate, and most vitamins and minerals can be absorbed normally due to ileum adaptation. Unfortunately, enzymatic digestion declines as a result of the jejunum's irreparable loss of enteric hormones. Biliary and pancreatic secretions become less active. Gastrin levels increase, resulting in hypersecretion of gastric juices. The increased acid production from the stomach may cause damage to the small intestinal mucosa.

Furthermore, the low intraluminal pH produces adverse circumstances for the optimum action of the existing pancreatic enzymes. If a substantial osmotically active solute load of unabsorbed nutrients is supplied to the ileum and colon, diarrhea may occur.

Ileal resection reduces the body's ability to absorb water and electrolytes significantly. Furthermore, the terminal ileum is the location of bile salt and vitamin B12 absorption. Significant ileum length loss nearly often leads in diarrhea. Fat malabsorption, steatorrhea, and loss of fat-soluble vitamins result from continued loss of bile salts following resection of the terminal ileum.

The retention of the ileocecal valve is critical in large small bowel resection. When the ileocecal valve is intact, intestinal transit is delayed, giving more time for absorption. When the ileocecal valve fails, transit time increases and fluid and nutrient loss increases. Additionally, colonic bacteria can colonize the small bowel, causing diarrhea and nutritional loss.

The benefits and drawbacks of colon preservation are discussed below. Following small-bowel resection, intestinal water absorption might be raised to five times its usual capacity.

Furthermore, the colon's resident bacteria have the ability to convert undigested carbohydrates into short-chain fatty acids such as butyrate, propionate, and acetate. These are the colon's preferred fuel source. Surprisingly, the colon may absorb up to 500 kcal of these metabolites each day, which can subsequently be transferred via the portal vein and utilised as a somatic fuel source.

Maintenance of the colon, on the other hand, increases the likelihood of urinary calcium oxalate stone development. Because oxalate is generally bound by calcium in the small intestine, it is insoluble when it enters the colon. Much of this calcium is bound by free intraluminal lipids after extensive enterectomy. Free oxalate is transported to the colon and absorbed. This can eventually cause the urine to become saturated with calcium oxalate crystals, resulting in stone formation. In the absence of a functioning ileocecal valve, colon retention might result in small intestine bacterial overgrowth.

The physiologic changes and adaptation of patients with short-bowel syndrome can be viewed in three phases. 

The acute phase begins immediately following major bowel resection and might extend for 3-4 months. It is linked to malnutrition as well as fluid and electrolyte loss through the GI tract. The GI tract might lose up to 6-8 L of fluid and electrolytes each day. Patients will experience temporary hyperbilirubinemia and abnormal liver function tests.

Enteral feedings can also be started, although they should be done gradually. Patients with a small intestinal length of less than 100 cm will require TPN. The existence of an ileocecal valve or colon may have an impact on the fate of these patients.

The adaptation period usually lasts 2-4 days following intestinal resection and can last up to 12-18 months. Up to 90% of bowel adaptation may occur during this second phase. There is villus hyperplasia, increased crypt depth, and intestinal dilatation. Early continuous feedings of a high viscosity elemental diet may shorten TPN duration.

The GI tract's absorptive capacity is at its peak during the maintenance phase. TPN may still be required for some individuals. Small meals and additional nutritional care for life can help some individuals attain nutritional and metabolic equilibrium. These patients will also need vitamin and mineral supplements, such as vitamins A, B12, and D, as well as magnesium and zinc.

Clinical manifestations 

Malabsorption is the principal detrimental effect of SBS, which is caused by reduced intestinal absorptive surface area and fast intestinal transit. Malnutrition, weight loss, diarrhea, steatorrhea, dehydration, vitamin deficiencies, and electrolyte imbalance are the most prevalent signs, symptoms, and findings. Individuals with SBS have most often had significant small bowel resection as a result of one of the underlying diseases indicated above.

Management

Maintaining a good nutritional status, maximizing the absorptive surface area of the remaining intestine, and avoiding difficulties are the initial steps in treating Short bowel syndrome (SBS). This involves both immediate postoperative parenteral nutrition and enteral nourishment once the patient's ileus has subsided. This promotes gut adaptability, which in turn improves nutrient absorption over time.

Individuals with persistent intestinal failure will require either lifelong home parenteral nutrition or intestinal transplantation if a life-threatening complication of SBS or home parenteral feeding arises. Thus, SBS treatment largely focuses on addressing and preventing complications that arise as a direct result of SBS or home parenteral feeding. Many of the treatments used to control SBS and avoid its repercussions are similar.

• Maintaining some oral feeding, promptly treating infection or inflammation, optimizing home parenteral nutrition, decreasing reliance on soy-based lipid emulsions or switching to fish oil-based emulsions, and non-transplant surgical procedures are used to treat intestinal failure-associated liver disease and liver cholestasis.
• Patients with short bowel syndrome (SBS) who have fewer than 180 centimeters of small intestine or an absent ileocecal valve are more likely to develop gallstones, which are typically constituted of calcium bilirubinate. These gallstones can be avoided or reversed by maintaining enteral feeding, avoiding oral fasting periods, and limiting the use of narcotic and anticholinergic medicines.
• SBS-related dehydration, hyponatremia, chronic renal failure, and nephrolithiasis are treated with intravenous fluid supplementation to maintain adequate hydration and urine flow. More precisely, individuals with SBS should be treated for dehydration and hyponatremia with a urine output of 800 milliliters per day and urine sodium more than 20 millimoles/liter.
• Hypomagnesemia can be induced by magnesium malabsorption owing to distal ileum and right colon loss, as well as fatty acid chelation. Treatment comprises of the above-mentioned hydration and magnesium supplementation at a level greater than 1.5 milligrams/deciliter.
• D-lactic acidosis with an elevated anion gap can arise in SBS patients due to colon microbiota fermentation of unabsorbed carbohydrates. This can result in D-lactate encephalopathy, which manifests as slurred speech, ataxia, altered mental state, psychosis, and, in some cases, coma. D-lactic acidosis is generally diagnosed clinically. Treatment includes discontinuing carbohydrate-containing enteral feeds, administering an antibiotic active against D-lactate producing bacteria, adequate hydration, and thiamine supplements. Metronidazole, vancomycin, clindamycin, neomycin, and tetracyclines are examples of antibiotics.
• Almost all long-term home parenteral feeding patients have metabolic bone disease, which presents as osteopenia, osteomalacia, or osteoporosis. Mineral bone density, serum and urine mineral levels, vitamin D, PTH, and indicators of bone turnover are used to diagnose and evaluate patients. Management may include lifestyle and dietary adjustments, therapy of underlying illness, vitamin D status optimization, parenteral nutrition, and, in certain cases, bisphosphonates.

Surgical management is another major therapy option for SBS patients. Patients receiving parenteral nourishment who have quick transit may benefit from reversing intestinal segments or interposing colon segments into the small intestine.

Those with fewer than 60 cm of small colon and difficulties with parenteral feeding may be candidates for intestinal transplantation. Patients with dilated intestine due to blockage may be treated with intestinal tapering or strictureplasty if the bowel remnant is more than 120 centimeters, or tapering with lengthening if the remnant is smaller than 90 centimeters in adults or less than 30 centimeters in children.

Differential Diagnosis

• Antral G Cells Hyperplasia

• Atrophic Gastritis

• Gastric Outlet Obstruction

• Gastrinoma

• Previous Vagotomy

• Renal Failure

• Retained Antrum Syndrome

• Short Gut Syndrome

Prognosis

Those with a short intestinal length of more than 180 cm usually do not require parenteral nourishment. Those with a height of more than 90 cm will normally need it for less than a year. Those with fewer than 60 cm will almost certainly require parenteral feeding for the rest of their lives.

Complications

Malnutrition, weight loss, steatorrhea, diarrhea, electrolyte imbalance, and vitamin deficiencies are the most common consequences of malabsorption. Other complications include cholelithiasis, transient gastric hypersecretion, bacterial overgrowth, dehydration, hyponatremia, potassium deficiency, magnesium deficiency, renal failure, calcium oxalate kidney stones, cholestasis secondary to intestinal failure-associated liver disease, gallstones, and d-lactic acidosis.

Steatosis is more frequent in adults and older children, but cholestasis with fast development to liver fibrosis is more common in newborns. Intestinal failure-related liver illness is responsible for between 16% and 60% of mortality in newborns on home parenteral feeding.

Conclusion 

Adults with short bowel syndrome (SBS) have fewer than 180 to 200 centimeters of residual small intestine (normal length 275 to 850 cm), necessitating nutritional and hydration supplements. Weight loss, diarrhea, steatorrhea, dehydration, malnutrition, and electrolyte imbalance are common indications and symptoms of malabsorption. Patients are either handled with lifelong parenteral feeding or with intestine transplants.

Finally, SBS care necessitates the formation of an interprofessional team lead by a physician with gastrointestinal experience. A gastrointestinal surgeon can provide surgical suggestions and treat the patient, while a transplant surgeon can broaden the patient's therapeutic choices. Administrative assistance, a nurse coordinator to oversee the patient's day-to-day management, and a dietician are other important team members. Addressing the psychosocial issues of SBS requires the assistance of a psychologist and a social worker.