Acute lower GI bleeding includes a wide clinical spectrum, ranging from minute bleeding to massive haemorrhage with haemodynamic instability. Lower GI bleeding is approximately one fifth as common as upper GI bleeding and accounts for approximately 20 to 30 hospitalisations per 100,000 adults per year.    The incidence of lower GI bleeding increases with age.
In the West, the 2 most common causes of the acute lower GI bleeding, resulting in significant blood loss, are colonic diverticular disease and angiodysplasia. Patients with severe bleeding or significant comorbid states require rapid identification and aggressive resuscitation. Haemodynamically insignificant bleeding may frequently result from haemorrhoids and colonic neoplasms. Rare causes of bleeding include solitary rectal ulcer, vasculitis, and endometriosis.
Colonoscopy is the mainstay of the evaluation in patients in whom anorectal or upper GI causes have been ruled out. This is performed to localise the bleeding source and to enable haemostasis. Endoscopic haemostasis is successful in most cases. Mesenteric angiography or nuclear imaging is only performed in patients in whom colonoscopy is not feasible or where there is persistent bleeding and a negative colonoscopy. In up to 25% of patients, the source of bleeding cannot be definitively identified.  
- Meckel's diverticulum
- Radiation-induced telangiectasia
- Dieulafoy's lesion
- Aorto-enteric fistula
- Hereditary haemorrhagic telangiectasia
- Blue rubber bleb nevus syndrome
- Anal cancer
- Rectal ulcer
- Rectal varices
- Post-polypectomy bleeding
- Non-steroidal anti-inflammatory drug (NSAID) colopathy
- Upper GI bleeding (rapid transport)
- Prostate biopsy site bleeding