Aetiology

There are many possible causes for lower gastrointestinal (GI) bleeding.

Anatomical

Anatomical abnormalities of the colon, either congenital or acquired, may result in lower GI bleeding. Diverticular disease occurs in the older population (although there is increasing prevalence in younger patients), whereas bleeding from a Meckel's diverticulum is more common in children.[7][8]

Diverticular disease:

  • Diverticulae are acquired lesions of the colon, typically located in the colonic wall at the sites of penetrating nutrient vessels

  • Diverticulae may be present anywhere in the colon, though they are more commonly located in the left colon. They have a greater tendency to bleed if located in the right colon

  • Diverticular disease may account for up to 42% of lower GI bleeding; it is the most common cause of lower GI bleeding in older patients[1][9]

  • Risk factors for developing diverticular disease include lack of dietary fibre, chronic constipation, and non-steroidal anti-inflammatory drugs (NSAIDs).[10]

Meckel's diverticulum:

  • A congenital anomaly of the GI tract, due to incomplete obliteration of the vitelline duct, resulting in an ileal diverticulum

  • Occurs in approximately 2% of the population[8]

  • Located within 100 cm of the ileocaecal valve, on the anti-mesenteric border of the ileum.

Vascular

Colonic angiodysplasia:

  • A common cause of lower GI bleeding among older people

  • Occurs anywhere in the GI tract, the most common location being the right colon

  • The pathogenesis is unknown, although it may be related to ageing. Most patients are aged over 60 years[11][12] 

  • Risk factors for bleeding from angiodysplasia include multiple angiodysplasia lesions, chronic renal disease, and recent anticoagulant therapy.[12][13][14]

Ischaemic colitis:

  • Results from mucosal hypoxia, caused by hypoperfusion of the intramural vessels of the intestinal wall. The hypoperfusion is caused by vascular disorders (e.g., atherosclerosis or vasculitis) or disorders with increased blood viscosity

  • Therapy is directed at treating the underlying cause.

Radiotherapy for abdominal or pelvic cancers:

  • Can result in vascular damage with subsequent mucosal ischaemia, thickening, and ulceration

  • Acute radiation injury occurs within 6 weeks of radiotherapy, while chronic injury occurs 9 weeks to 4 months after radiation injury.

Bleeding from a Dieulafoy's lesion in the colon:

  • A rare cause of lower GI bleeding[15][16]

  • Small mucosal lesions, which subsequently erode an underlying vessel, leading to bleeding.

Aorto-enteric fistula, vasculitis, hereditary haemorrhagic telangiectasia, and blue rubber bleb nevus syndrome are other vascular causes of lower GI bleeding.

Inflammatory

Patients with inflammatory bowel disease (including Crohn's disease and ulcerative colitis) may present with bloody diarrhoea. Life-threatening bleeding from inflammatory bowel disease is rare.

Infectious

Any type of infectious colitis may cause haematochezia (bright red rectal bleeding) The most common types are enterohaemorrhagic Escherichia, Salmonella, Histoplasma, and Cytomegalovirus colitis. Stool studies, including stool cultures, are useful in making the diagnosis. 

Bacterial:[17][18]

  • Campylobacter

  • Clostridium difficile

  • Enterohemorrhagic Escherichia coli

  • Salmonella

  • Shigella

  • Vibrio parahemolyticus

  • Yersinia.

Parasitic:[19][20]

  • Cryptosporidium

  • Entamoeba histolytica.

Viral:

  • Cytomegalovirus

  • Herpesvirus.

Neoplastic

Colorectal cancer and colon polyps are causes of rectal bleeding. Bleeding from these causes is usually occult and results from mucosal defects on the surface of the tumour. The bleeding is rarely severe and is generally painless and intermittent. Positive predictive value of rectal bleeding for colorectal cancer is 8.1% in those aged 50 years and over. For rectal bleeding accompanied by weight loss or change in bowel habit, pooled positive likelihood ratios for colorectal cancer are 1.9 and 1.8, respectively.[21] Anal cancer is a rare cause of rectal bleeding.

Anorectal

Internal haemorrhoids:

  • A source of bleeding in 2% to 9% of patients with acute lower GI bleeding.[22]

Solitary rectal ulcer:

  • An uncommon cause of heavy rectal bleeding

  • Local ischaemia plays a role in the pathogenesis of this entity

  • Internal rectal prolapse or lack of inhibition of puborectalis muscle during straining, are also thought to play a role.

Rectal varices:

  • An uncommon cause of lower GI bleeding

  • Found in patients with portal hypertension

  • They develop in the rectal mucosa, between the superior haemorrhoidal veins and the middle and inferior haemorrhoidal veins

  • They are treated similarly to oesophageal varices, with either sclerotherapy or portosystemic shunts.[23]

Anal fissures:

  • Can present with rectal bleeding

  • Patients report pain during defecation

  • Rectal bleeding may also be present with bowel movements.

Miscellaneous

Miscellaneous causes of lower GI bleeding include the following:

  • Post-polypectomy bleeding

  • NSAID colopathy

  • Elastic tissue disorders

  • Upper GI bleeding (rapid transport)

  • Prostate biopsy site bleeding

  • Endometriosis.

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