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Diarrhoea is defined as the passage of a lose liquid stool. Urgency, which often accompanies diarrhoea, is the sensation of the need to defecate without delay! Urgency can be caused by a large volume liquid stool (where the rectum becomes overwhelmed) or it can also be cuased by rectal irritation.
Frequency purely refers to the number of stools passed, and is not necessarily related to diarrhoea or urgency.

Important factors

  • Bloody diarrhoea is always pathological, and will nearly always be caused by some form of colitis.
  • Diarrhoea that occurs in the morning, but is followed by normal stools later in the day is rarely pathological.
  • Diarrhoea that develops in a hospitalized patient may be due to C. difficile infection – you should always check the stools for C. difficile.
  • Consider parasitic infection as a cause if there is a history of foreign travel.
  • Remember many acute episodes are infective in nature.
  • If you are unsure of a diagnosis remember it can have an endocrine cause – namely hyperthyroidism.

Acute Causes

Infective Diarrhoea

  1. Bacterial – there are four main bacterial causes:
    1. E. coli (most common)
    2. Salmonella – associated abdominal pain and occasionally vomiting
    3. Shigella – associated abdominal pain and occasionally vomiting
    4. Campylobacter
  2. Viral
    1. Rotovirusthis is THE most common cause of diarrhoea
    2. Norovirus – an umbrella term for a range of similar viruses
  3. Parasitic
    1. Amoebic dysentery – caused by Entamoeba histolytica
    2. Giardiasis – caused by Giardia
    3. Cryptosporidium

Diarrhoea acquired when travelling

  1. Cholerathis is highly dangerous. The cholera bacteria will secrete a toxin that draws fluid into the lumen at the rate of up to 1L an hour. Patients can lose up to 24L of fluid in a day! Basic treatment involves drinking salt and water. A sign of this condition is ‘rice water stool’ where the patients stools are so runny they look like an opaque white liquid.
  2. E. coli – tends to be more dangerous than the strains acquired at home
  3. Giardiasis – this is a small parasite that infects the duodenum and jejunum. Not only does it cause diarrhoea, but it also damages the mucosa in these regions, and so can also result in malabsorption.


  1. Antibiotics – there are 5 ways in which antibiotics can lead to diarrhoea:
    1. Clindamycin this is a broad spectrum antibiotic (and the same affect may be seen in other broad spectrum AB’s) and it will kill almost all bacteria in the gut. The problem is that this then allows resistant C. difficile (if present) to proliferate and cause diarrhoea.
    2. Erythromycin – this increases gut motility. It is sometimes even used to treat constipation
    3. Penicillins – breakdown products of this act as an osmotic laxative
    4. Tetracyclin – this has an effect on fat absorption (i.e. it inhibits it) and thus leads to diarrhoea
    5. Neomycin – this affects bile salt absorption and thus the bile salts act as an osmotic laxative and draw fluid into the lumen.
  2. Laxatvies – remember that up to 5% of patients with diarrhoea that you cannot identify a cause for may be taking laxatives – a sort of psychological condition. Also remember that pretty much any drug can cause diarrhoea, some common examples are:
    1. Digoxin
    2. Magnesium salts
    3. PPi’s – particularly omeprazole
    4. Cimetidine

Chronic Causes

Metabolic disorders

  • Hyperthyroidism
  • Thyrotoxicosis
  • Anxiety
  • Peptides secreted by unusual tumours (e.g. VIP, serotonin, substance P, calcitonin)

Small bowel disease

  • Crohn’s disease – pain and diarrhoea are prominent. Blood and mucous are less common. Often occurs in young adults with a long history. There may also be weight loss and malnourishment.
  • Coeliac disease history of wheat and cereal intolerance. Often presents in adulthood with chronic diarrhoea and weight loss, and abdominal pains
  • Blind loop syndrome – this produces a frothy, foul smelling liquid stool due to bacterial overgrowth and fermentation. It is often associated with previous surgery, and may be a complication of Crohn’s disease.

Large bowel disease

  • UC – causes intermittent bloody / mucousy stools, and may also present with colicky pain. Common in young adults. There may be a short history in the initial presentation.
  • Colon cancerolder patients, may be streaky stools (streaks due to blood and mucous). Often, a change in frequency is the only feature. A mass may be palpable, and faecal occult blood will be positive
  • IBS – this can present with both diarrhoea and constipation, or a mix of both. There may also be bloating and colicky pain, but there is never blood.
  • Spurious – compacted faeces in the rectum may cause an obstruction, that then only allows watery faeces to pass the blockage. It is common in the elderly, often those with mental illness, and also with constipating drugs.
  • Very rarely, polyps and diverticular disease may cause constipation.


  • FBC – to check for leukocytosis (for infective causes and colitis) and anaemia
  • Anti α-gliadin Abs – test for coeliac’s disease
  • Thyroid function tests – check for hyperthyroidism
  • Stool culture – check for infections; don’t forget microscopy for parasites
  • Proctoscopy / sigmoidoscopy – cancer / colitis and polyps
  • Flexible sigmoidoscopy / colonoscopy – if protoscopy does not deliver enough detail.
  • Small bowel enema – can see Crohn’s coeliac’s and Whipple’s disease
  • ERCP – can see pancreatic insufficiency.

For more information, see the article Altered Bowel Habit


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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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