Introduction to haematochezia and melaena

Blood may be present in diarrhoea of either large or small intestinal origin; however, sometimes the passing of blood is the predominant presenting clinical feature. Melaena refers to a dark tarry appearance to the stool which is caused by the presence of digested blood.

In humans, blood must be present in the GI tract for at least 8 hours to turn a black colour, so a slow colonic transit time can cause melaenic stools from bleeding from a colonic area. Generally though, melaena is due to bleeding from the proximal GI tract or swallowed blood from a nasal, oral, oesophageal, pharyngeal or respiratory tract lesion. The use of salicylates, bismuth or charcoal can colour the faeces black as well.

Haematochezia is the presence of bright red blood in the faeces. If fresh blood is present on the outside of the faeces it is more likely to be from the distal colon or rectum.

Confirming the presence of melaena should be performed using a faecal occult blood test after the patient has been fed a meat-free diet for 3 days. In patients with undiagnosed anaemia, a faecal occult blood test should also be performed, as smaller amounts of GI bleeding can result in an ap­parently normal coloured stool.

Differential diagnoses for melaena include:

Swallowed blood, e.g. from oro-nasal lesions

Oesophageal disorders, e.g. ulcers, erosions, neoplasia

Gastric causes, e.g. erosions or ulcers, severe gastritis, sharp foreign bodies, neoplasia, drugs (NSAIDs, corticosteroids, iron overdose)

Small intestinal disorders, e.g. ulcers, severe inflammation, neoplasia, oreign bodies, hookworms

GI ischaemia, e.g.

Liver disease

Pancreatitis (severe acute)

Kidney disease causing uraemia

Differential diagnoses for haematochezia include:

Colitis or proctitis

Colonic or rectal tumour/polyp, stricture, infarct or foreign body

Caecal inversion

Colonic vascular ectasia

Ileocaecal intussusception

Parasites, e.g. hookworms • Small intestinal bleeding with rapid transit time (i.e. insufficient time for oxidation of haemoglobin)

Diagnosing the cause of GI bleeding

The patient should be examined for evidence of non-GI tract bleeding such as a cough resulting in haemoptysis or epistaxis. The mouth and pharynx should be examined for oral causes. Regurgitation may occur with oesophageal disease and haematemesis with gastric bleeding.

Haematology may reveal anaemia. When chronic, an anaemia due to GI bleeding may become microcytic, hypochromic and non-regener- ative. Even occult bleeding can cause a rapid drop in the packed cell volume and severe bleeding can cause a dramatic decrease within hours. The blood urea nitrogen may increase due to digestion of blood. A plate­let count, buccal mucosal bleeding time and coagulation panel should be performed to check for primary or secondary bleeding disorders. Un­like most other cases of GI bleeding, cases of idiopathic haemorrhagic gastroenteritis often have an increased packed cell volume. This is likely because the fluid losses are more pronounced than the loss of red blood cells, although the diarrhoea often appears severely haemorrhagic and there is often not obvious clinical dehydration.

Procedures to localize the bleeding may include ultrasound, endo­scopy and exploratory surgery.