Editing Short bowel syndrome


''The malabsorption and malnutrition that occur with <180cm of small bowel in adults''

== '''Aetiology''' ==

* Resectional
** Single massive bowel resection (75%)
*** Intestinal ischaemia
*** Midgut volvulus
*** Traumatic SMA injury
*** Necrotizing enterocolitis
** Multiple resections (25%)
*** Usually Crohn disease
* Non-resectional
** Enterocyte failure
*** Autoimmune
*** Microvillous inclusion
*** Tufting enteropathy
** Dysmotility
** Bariatric procedures
** Intestinal-intestinal fistula

== '''Pathophysiology''' ==

* Intestinal failure - inadequate bowel function to maintain nutrition and hydration, including macro- and micro-nutrients, electrolytes and fluid volumes
* Phases:
** Acute hypersecretory phase - 3-4 weeks
*** Malabsorptive and osmotic (and sometimes bile salt) diarrhoea due to lack of absorption of fluid and electrolytes and macronutrients
** Adaptation - 1-2 years
*** Requires enteral nutrition
*** This is where the structural and functional changes below occur
** Late
*** Plateaued adaptations
* Adaptive hyperplasia - the bowel's adaptive capacity after resection
** Stimulated by nutrients within the intestinal lumen
** Structural adaptation
*** Dilation and elongation of the remnant bowel via muscle hyperplasia
*** Villus lengthening and increase in microvilli
*** Increase in enterocyte number
*** Crypt cell proliferation
*** Angiogenesis for improved perfusion
*** Slowed transit time
** Functional 
*** Increased expression of transporter proteins and exchangers
*** Increased production digestive enzymes
*** Improved digestion from enterocytes
* Requirement for parenteral nutrition is influenced by length, location and health of the remaining intestine, but commonly:
** Measure from DJ flexure. 
** >180cm: no PN
** 60-180cm: PN for less than 1 year
** <50-60cm: permanent PN
** Interestingly, UTD doesn't list any such guidelines, merely that it is highly dependent on function of the remaining bowel
* Bowel anatomy factors:
** Jejuno-colic anastomosis - most common anatomy - prognosis depends on length of jejunum remaining
** Jejuno-ileocolic anastomosis (retention of some ileum and ICV - best prognosis
** End-jejunostomy - worst prognosis
** Proximal bowel resection is tolerated better than distal - the ileum can adapt and restore absorptive capacity better than the jejunum
** Presence of at least half the colon is functionally equivalent to having an extra 50cm of small bowel



== '''Presentation''' ==

* Typical problems
** Diarrhoea
** Fluid and electrolyte deficiency
** Malnutrition
*** Especially iron, magnesium, zinc, copper and vitamins
*** Resection of the distal two-thirds of ileum and ICV commonly leads to B12 and bile salt deficiencies, along with diarrhoea and anaemia
* Other complications
** Gallstones - increased incidence caused by disruption of enterohepatic circulation
** Nephrolithiasis - caused by hyperoxaliuria
** Rapid transit
** Bacterial overgrowth
** TPN complications (liver disease)
** Peptic ulcer disease
** Metabolic bone disease
** Catheter complications

== '''Early phase management''' ==

* Replace fluid and electrolytes
** Volume losses may be >5L/day - need fluid balance
* TPN promptly as required
* Commence enteral diet as soon able to tolerate it, via feeding tube
** Debate over best initial diet
** Initially, high-carbohydrate high-protein diet maximises absorption
** Avoid milk products
** Begin at iso-osmolar concentration, which can be increased later
** Provide nutrients in simplest forms - elemental or polymeric diet
** Provide fat-soluble vitamins and zinc, magnesium and calcium
* Restore intestinal continuity as soon as possible

== '''Adaptation phase management''' ==

* Transition to oral diet as soon as able to tolerate it
** High in complex carbohydrates and modest fat/oxalate
** Isotonic rehydration solutions like St Mark's, to maintain a urine output of at least 1L/day
* Control diarrhoea
** Can be caused by hypergastrinaemia and gastric hypersecretion, which is triggered by massive small bowel resection - manage with PPI
** Ileal resection - bile salt diarrhoea - cholestyramine
** Anti-motility agents such as loperamide - judiciously
** Octreotide also seems to reduce diarrhoea in the early phase of short bowel syndrome, although there is an unresolved question of whether it inhibits gut adaptation
* Probability of weaning TPN is <6% if not done within two years of last intestinal resection
* There are also experimental drugs for promoting mucosal growth - GLP-1 analogues like liraglutide - UTD recommends use only if unable to be weaned from TPN, as current case numbers very low for evidence base

== '''Late phase management''' ==

* Medical
** TPN/fluids/micronutrients as required at steady state
*** B12
*** Fat-soluble vitamins
*** Calcium, magnesium, zinc, selenium
** Teduglutide (GLP-2 analogue) - if unable to wean from TPN despite aggressive therapy - initiates and maintains small bowel adaptive response to resection and improves nutrient absorption
** Prevent and treat bone disease and other complications
* Surgical strategies - limited success
** Preserve existing bowel length and restore continuity
** Delay intestinal transit time
*** Construction of various valves and sphincters
*** Anti-peristaltic segments - moderate success
*** Abandoned techniques:
**** Colonic interposition
**** Recirculating loops of bowel
**** Retrograde electrical pacing
** Increase absorptive area
*** Bianchi procedure - intestinal tapering and lengthening procedure - serial transverse enteroplasty - possibly gets good results in well-selected patients- 90% show improved nutrition, and can lengthen by up to 55% in some patients - but high-risk of complications. Need a distended gut for this to work, which is not commonly seen in adults. Increase length but more importantly slow transit. Starkey has never seen it. 
** Intestinal transplantation
*** Standard of care for patients in whom intestinal rehabilitation has failed and are at risk of life-threatening TPN complications (impending liver failure, thrombosis of more than two major access veins, frequent severe line infections, dehydration)
*** One year survival of 77%; 5 year survival 58%, even in modern times. Lots of perioperative mortality but then ongoing attrition.
*** Can be done in combination with liver - 5 year survival 54%
*** No role for transplantation in the acute setting - wait for extent of disease to become clear
*** Recognised by USA medicare system in 2000 - progressive uptake since then. Most of the initial indications were related to complications of home TPN, 'failure of TPN', see above.
*** There are about 5000 people worldwide, as of June 2023, who have had a transplant at a recognised centre, based on registrary data, and only 100-200 per year nowadays. 13 Australians transplanted as of June 2023.



== '''Prognosis''' ==

* Long-term outcome determined by age and underlying intestinal disease

[[Category:Small bowel]]