Functional Bowel Disease (IBS)
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Functional bowel disease (also known as Irritable bowel syndrome – IBS) refers to a group of symptoms – including abdominal pain and discomfort, bloating and change in bowel habit – for which no underlying cause or pathology can be identified.

As such, it is typically a diagnosis of exclusion. 

These symptoms are relatively common, and this term encompasses a large amount of diseases that have no identifiable underlying pathology present.
Functional bowel disorders are probably a result of an abnormal brain-gut relationship.
The terms Irritable Bowel Syndrome and Functional Bowel Disease are often used interchangeably, however they are slightly different. Irritable Bowel Syndrome is the main type of functional bowel disorder. Other disorders (e.g. functional abdominal bloating, functional diarrhoea) will often be variations of IBS, generally with fewer or slightly different symptoms.

Red flags

Signs and symptoms suggestive of a more serious underlying cause:

  • Rectal bleeding
  • Age >50 at first presentation
  • Family history of bowel or ovarian cancer
  • Iron deficiency anaemia
  • Unexplained weight gain or weight loss

Do NOT give a diagnosis of IBS in patients with these symptoms without thorough prior investigation.

Differential diagnosis


These can be divided in to general and specific. General symptoms are present ‘across the board’ i.e. some of these symptoms are present in all types of functional bowel disorders. Specific symptoms are confined to one specific type of disorder.


  • Nausea alone
  • Vomiting alone
  • Belching
  • Chest pain unrelated to exercise
  • Postprandial fullness (fullness after eating)
  • Abdominal bloating
  • Abdominal discomfort and pain (particularly in the right iliac fossa)
  • Passage of mucous from the rectum
  • Frequent bowel actions, with urgency first thing in the morning
  • Often patients are young
  • There is no blood loss
  • No weight gain

Non-GI manifestations

  • Gynaecological
    • Painful periods (dysmenorrhoea)
    • Pain after sex( dyspareunia)
    • Premenstrual tension
  • Urinary symptoms
    • Frequency
    • Urgency
    • Nocturia
    • Incomplete emptying of bladder
  • Other
    • Back pain
    • Headaches
    • Bad breath / unpleasant taste in mouth
    • Fatigue
    • Poor sleeping


  • About 20% of the general population fulfil these criteria, but only 10% of the population will consult their doctor as a result of GI symptoms. An important question for researches to ask is: ‘Do patients who seek medical help have different underlying pathological disease from those who do not?’ – basically, does seeking medical help just mean you are a worrier, or does it mean you genuinely have worse symptoms.
  • IBS is the most common cause of GI referral in the UK – it is also a major cause of absenteeism at work.
  • Women are 2-3x as likely to be affected as men.
  • There is a wide overlap with non-ulcer dydpepsia, chronic fatigue syndrome (CFS), dysmenorrhea and urinary frequency.
  • A significant number of patients have a history of sexual or physical abuse.

The Rome Criteria

The Rome criteria are a set of criteria that attempt to define the symptoms of functional bowel disorders. Using the criteria, you can put sufferers into different categories in an attempt to tailor treatments to them.
The criteria state that in the preceding 12 months, there should be at least 12 consecutive weeks of abdominal pain and discomfort with at least two of the following:

  • Pain relieved on defecation
  • Onset associated with a change in frequency of stool
  • Onset associated with change in appearance of stool.
  • It is important to remember that the symptoms of functional bowel disease vary widely, and as a result, you may not be able to categorise all patients. This does not mean that a patient does not suffer from functional bowel disease.

Types of functional bowel disorders

  • IBS
  • Functional abdominal bloating (pain/gas/bloat syndrome)
  • Functional constipation
  • Functional diarrhoea
  • Functional abdominal pain


About 50% of patients referred to hospital for their symptoms will have some sort of diagnosable psychiatric condition. Anxiety, depression, somatisation and neurosis are common problems. However, most patients who present in general practice do not have a co-existing psychiatric condition.

  • Neurosis is a condition that causes psychological distress, but unlike psychosis, or some personality disorders, it does not prevent or affect rational thought. In neurotisism, symptoms are interpreted more negatively than in the general population.
  • Somatisation – is where a person who has psychological distress may display this in the form of some sort of physical ailment.

Many functional bowel disorder patients will have abnormal health behaviours; i.e. they have a tendency to consult for many minor ailments that otherwise people would not bother about – they are worriers. These patients will also have reduced coping ability.

Infectious diarrhoea precedes the onset of symptoms in 7-30% of patients.

Most patients will have a colicky LLQ pain that is relieved by defecation. They will also tend to have diarrhoea and constipation regularly. Although most patients have both, it is useful to categorise patients as having mostly diarrhoea or mostly constipation.
Those with diarrhoea tend to pass low volume stools regularly, sometimes with mucus but never with blood. They also tend to have few symptoms during the night.
It is also quite common for patients to experience increased bloating throughout the day, despite the fact that it has not been proven that they produce any more gas than a normal patient.

The non-GI symptoms of these conditions can often be just as bad, if not worse than the GI ones. Often, this condition is also associated with chronic fatigue syndrome, fibromyalgia, and temporomandibular joint dysfunction.

Irritable Bowel Syndrome

  • This is characterised by abdominal pain, often relieved by defecation, with altered bowel habit, and the sensation of abdominal distension. There will often be a change in stool consistency and frequency and there may be the passage of mucus in the stools.
  • These symptoms are generally common in those with IBS, but uncommon in those with actual structural bowel disease.
  • Tests and investigations do not confirm diagnosis (but they can rule out other conditions). Often a diagnosis can be made based on history and clinical diagnosis alone without the need for further investigation.
  • Other factors that may help aid diagnosis include:
    • Upper-gut symptoms – such as bloating after eating
    • Women may have menstrual and bladder problems
    • Increased prevalence of psychosexual problems
  • Clinical exam
    • This will often produce tenderness in the left iliac fossa
    • Rectal exam and sigmoidoscopy may be useful and are usually performed. Colonoscopy should be reserved for the over 40s to rule out the possibility of colorectal cancer. The mucus will be normal. Any signs of mucosal inflammation mean that it is NOT IBS.
    • There may be pigmented mucosa in the rectum (melanosis coli) and this is a sign of long-term laxative use – and thus shows how chronic the problem is.
    • The sigmoid colon may be palpable because it is full of faeces.
    • There may also be tenderness under the ribs.
    • Any symptoms that suggest an underlying pathology, (e.g. weight loss, rectal bleeding) should be treated exactly as they would be in any other patient – i.e. investigate these symptoms in their own right, even if there is a long history of IBS.


  • Functional bowel disease is a diagnosis of exclusion – meaning that more serious disorders should be excluded first
  • In the absence of red flags (above), then the following investigations are recommended:
    • FBC
    • Coeliac serology (usually tTG)
    • CRP
    • ESR
  • Other tests to consider:
    • TSH
    • FOBT
    • Iron studies (ferritin)
    • Stool MC+S with PCR (if diarrhoea)
    • Faecal calprotection
      • Tests for Inflammatory bowel disease
      • May be appropriate for patents with IBS with diarrhoea


Currently, there is no fully understood pathology of IBS. There are several theories currently discussed:

  • Neuromuscular dysfunction – this is the most popular theory. Basically this theory states that patients with IBS have some sort of neuromuscular abnormalities that affect normal gut motility. The problem is that there isn’t much evidence for this. There is some evidence that shows there is increased colonic activity in those with IBD, but how this relates to symptoms is uncertain.
  • Visceral hypersensitivity – this is another popular theory and states that nervous sensitivity in the gut is somehow enhanced in people with IBD. This would make sense, because many IBD patients report increased pain response to rectal distension (e.g. during the colonoscopy). The sensitivity appears to be visceral specific – i.e. there is no increased cutaneous hypersensitivity. It is also uncertain as to whether this hypersensitivity exists as a result of abnormal mechanoceptor functioning, or as a result of abnormal sensory processing by the brain and spinal chord.
  • Psychiatric disease – there is a lot of evidence to show that psychiatric disorders and abnormal illness behaviours are more prevalent in those with IBD than in the general population. How exactly psychiatric disease could cause IBD is uncertain, but it is known that visceral sensitivity is increased in depression.
  • Diet – it is usual for us to try and link a GI problem with diet. And particularly in the case of constipation, we have a case for the link – functional constipation is uncommon in areas with a high fibre diet. However, following a high fibre diet is unlikely to reduce the symptoms of functional constipation.
    • Food allergy is an area that can be difficult to explore. In many instances, allergy to a certain food can produce symptoms very similar to that of IBD. In some cases (e.g. eggs shellfish) the causal factor is obvious as the symptoms arise after eating that particular food. However, in other cases (such as lactose or fructose intolerance) where the allergen is present throughout the diet, it is much harder to identify food allergy as the cause. Also, these cases, the ‘allergen’ does not actually produce an immune response. Therefore it is possible that a certain proportion of patients who are labelled ‘IBD’ do in fact just have a food intolerance.

Often the disease follows a relapsing and remitting course – the relapses are often triggered by stressful life events.


  • Reassurance and explanation of the condition. Often stress, anxiety and depression can exacerbate the condition.
  • In 50% of cases, a placebo will relieve the symptoms
  • Advice regular exercise
  • Avoid carbonated drinks
  • Persistent diarrhoea
    • Loperamide – 2mg
      • 1/4 of a tab may be effective
  • Persistent constipattion
    • Osmotic laxative – e.g. movicol, lactulose
    • Avoid cycling between use of laxative and anti-diarrhoea agents
    • Some fibres – e.g. bran, may make IBS worse
    • Avoid stimulant laxatives
  • In patients who have obvious pelvic floor dysfunction there are various techniques that can be employed to alter the defecation process and improve defecation.
  • Anti-depressants may be prescribed in low doses, and these can improve mood, and thus reduce anxiety.
    • Amytriptiline is the most commonly used drug in this case – e.g. 10mg nocte
    • SSRIs may also be used, but there is less evidence for their efficacy
  • Analgesics – are generally ineffective and should be avoided (e.g. paracetamol, NSAIDs)
    • Peppermint oil may be an effective analgesia
    • Buscopan 20mg QID PRN as an antispasmodic may be effective
    • Mebeverine 135mg TDS PRN may also be effective as an antispasmodic
  • Low FODMAP diet
    • Fermantable oligosaccharides, disaccharides, monosaccharides and polyols
    • A low FODMAP diet reduced fermentable substances in the diet
    • Difficult to implement and maintain
    • Usually introduced with the assistance of a dietician
    • There are also lots of useful apps!
    • Should be trialled for 6-8 weeks
    • If symptoms significantly improve, then re-introduce individual foods one at a time to try to find a particular culprit, and to reduce the restrictions placed on the diet
  • Lactose free diet
    • Consider a 4 week trial of exclusion of lactose to see if symptoms improve
      • Hard cheese does not contain lactose and can still be eaten. It is an important source of calcium and protein!
    • Lacteeze is a medication that can be taken with lactose to relieve symptoms of an individual meal
    • Lactose hydrogen breath testing is available, but sensitivity and specificity is variable. Not required for a diagnosis – which can be made clinical if symptoms improve on lactose free diet
  • Consider psychology referral
  • Consider gastroenterology referral if there is concern about the diagnosis, or if symptoms do not respond to treatment
  • Surgery should generally be avoided, however in a very small group of patients with very severe constipation, an iliorectal anastomosis may provide relief of symptoms.

It is very important not to dismiss these patients – because they do have real symptoms!

Functional abdominal bloating, aka pain/gas/bloat syndrome

  • This is characterised by symptoms of abdominal fullness, audible bowel sounds, and excessive flatus with no evidence of maldigestion or malabsorption. There is also no evidence of poorly digested fermentable carbohydrate.
  • There may also be nausea and premature feelings of fullness.
  • This is not generally associated with diarrhoea or constipation.
  • There will be greater than normal discomfort upon distension of the rectum for colonoscopy that suggest increased gut sensitivity.
  • There is no evidence that the amount of gas produced is above normal.
  • Treatment options for this are better than for other variants of functional bowel disorders. Many patients find that a very mild dose of an SSRI (10-25mg at night) and perhaps a smooth muscle relaxant can improve symptoms.
  • There is some evidence to suggest that there are reduced levels of alpha-actin fibres in circular layers of smooth muscle in some patients.
  • This condition is also associated with chronic idiopathic intestinal pseudo-obstruction (CIIP)


Functional diarrhoea

This is diarrhoea without the other symptoms of functional bowel disease. The symptoms include:

  • The passage of several stools in rapid succession, usually first thing in the morning. There may be no further defecation that day, or it may occur after meals.
  • The first stool of the day is often formed, the later ones tend to be mushy
  • Urgency
  • Anxiety about bowel movements often leading to restriction of movement / travelling
  • Exhaustion after the ‘morning rush’

The condition is only usually diagnosed after excluding other more serious diagnoses such as IBD or secretory diarrhoea. To help differentiate functional diarrhoea from other types, some things that are not normally found in functional diarrhoea are:

  • Large stool volumes
  • Rectal bleeding
  • Nutritional deficiency
  • Weight loss

In cases where it is especially difficult to make a diagnosis, patients may be admitted to hospital for 3-day stool analysis, to test for pathogens, fats, stool weight, osmolality and creatinine content.
It is also worth looking in stools for laxatives, as some patients may take laxatives, and then go to the doctor complaining of diarrhoea!
It is more important to look for pathological causes in this condition than in other functional conditions – this is because the symptoms resemble more those of a pathological condition.

In some patients there is an obvious relationship with mental state – and anxiety visibly worsens symptoms.
This is often treated with loperamide and a tricyclic antidepressant taken at night (such as clomipramine 10-30mg)


  • Murtagh’s General Practice. 6th Ed. (2015) John Murtagh, Jill Rosenblatt
  • Oxford Handbook of General Practice. 3rd Ed. (2010) Simon, C., Everitt, H., van Drop, F.
  • Beers, MH., Porter RS., Jones, TV., Kaplan JL., Berkwits, M. The Merck Manual of Diagnosis and Therapy

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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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