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