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Peritonitis and the Pathophysiology of Abdominal Pain

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Overview

Peritonitis describes the inflammation of the peritoneum, the membrane that lines the inner abdominal wall (parietal peritoneum) and covers the abdominal organs (visceral peritoneum). The peritoneal cavity is the potential space between these two continuous layers.

It requires urgent treatment due to the risk of severe infection, septic shock, and death.

The most common causes of infection in peritonitis are gut flora, such as Escherichia coli.

Definitions

Primary peritonitis – where peritonitis happens in the peritoneum itself rather than arising due to pathology elsewhere. An example is spontaneous bacterial peritonitis.

Secondary peritonitis – where peritonitis happens secondary to pathology in nearby structures, such as gastrointestinal tract perforation.

Localised peritonitis – describes inflammation of the peritoneum in a limited area, such is an inflamed appendix irritating the overlying peritoneum.

Peritonism – an outdated word that describes the presence of the signs and symptoms of peritonitis. Some may interchange its use with localised peritonitis.

Intra-abdominal sepsis – any intra-abdominal infection. Often involves both localised and generalised peritonitis.

Visceral peritoneum – the part of the peritoneum that covers the viscera (organs). Pain due to nociceptive stimulation of the visceral peritoneum is known as visceral pain.

Parietal peritoneum – the part of the peritoneum that lines the inner abdominal wall. Pain due to nociceptive stimulation of the parietal peritoneum is known as parietal pain.

The characteristics of abdominal pain and the presence of visceral or parietal pain can help identify if peritonitis is likely.

The Visceral Peritoneum

Overview

The visceral peritoneum lines the viscera (organs) and when stretching, inflammation, chemical irritation, or ischaemia of an organ occurs, this results in visceral pain. It is insensitive to cutting and stabbing, which is why organ biopsies can be taken without causing significant pain.

It is innervated by the autonomic nervous system and when irritated, pain sensation travels via autonomic sensory nerves back to the spinal cord. Since in this route, fewer neurones convey pain signals to the brain, visceral pain tends to be diffuse and difficult to localise and is often described as dull, nauseating, or squeezing. There is also no position that relieves pain and often patients are restless trying to find it.

Autonomic signs

Visceral pain is also associated with autonomic signs due to the pain impulses travelling in the autonomic nervous system. This can include tachycardia, blood pressure changes, sweating, pallor, nausea, vomiting, anxiety, and a sense of impending doom.

Referred pain

The pain sometimes may be referred to elsewhere as the autonomic sensory nerves from the viscera converge with somatic sensory nerves entering the spinal cord at the same point from different areas of the skin (dermatomes). The brain ‘confuses’ the pain signals from the visceral autonomic sensory nerves as pain signals from the more commonly stimulated somatic sensory nerves from the skin. This results in pain being felt as if it is coming from the skin or other somatic regions rather than the viscera themselves. Referred pain is different to radiating pain.

The pain is generally felt according to the embryological origin of the organ. For instance, visceral pain in:

  • Foregut organs may be felt in the epigastric region:
    • These include the oesophagus, stomach, pancreas, liver, gallbladder, and proximal duodenum
  • Midgut organs may be felt in the umbilical region:
    • These include the distal duodenum to the proximal 2/3 of the transverse colon
  • Hindgut organs may be felt in the pubic region:
    • These include the distal 1/3 of the transverse colon to the upper anal canal
  • Retroperitoneal organs may be felt as back pain:
    • These include the kidney and pancreas
  • Irritation of the diaphragm can cause shoulder tip pain

A good example of this is the crushing chest pain felt in myocardial infarction, where the pain is referred (not radiated) and is experienced in the chest rather than where the heart itself is, and its associated autonomic signs (pallor, anxiety, tachycardia etc.).

The Parietal Peritoneum

Overview

The parietal peritoneum lines the inner abdominal wall and receives the same somatic nerve supply as the abdominal wall that it lines. Compared to the visceral pleura, there are increased numbers of nociceptors which are sensitive to pressure, pain, temperature, and laceration. This leads to pain that is a well-localised, sharp pain that often has positions or actions that alleviate or worsen the pain.

Relation to pleural pain

These concepts are similar in the parietal and visceral pleura of the lungs but differ slightly. The visceral pleura of the lungs is different from the visceral peritoneum as it is insensitive to pain and its sensory fibres only detect stretch. The parietal pleura is similar to the parietal peritoneum as it is sensitive to pain, pressure, and temperature and produces well-localised pain.

The Visceral and Parietal Peritoneum: Clinical Significance

Pathology such as inflammation in an organ can initially cause visceral pain that is poorly localised and dull. Over time, as inflammation progresses, the parietal peritoneum becomes involved, resulting in pain that becomes sharp and well-localised. A known example is appendicitis and the nature of its pain.

Initially, inflammation of the appendix results in visceral pain. Since the appendix is a midgut structure, this pain is poorly-localised and felt in the umbilical region. As the appendix becomes more inflamed, the overlying parietal peritoneum is irritated, resulting in pain localising in the right lower quadrant (where the appendix is) and pain that is worsened when jumping, hopping, or driving over speedbumps.

Peritonitis: Causes

The causes of peritonitis include:

  • Infection secondary to gastrointestinal tract perforation or puncture:
    • This can lead to infection as microorganisms escape into the peritoneal cavity, such as from the faeces in the large bowel
    • Chemical irritants may also be released, causing peritonitis (e.g. gastric acid or pancreatic digestive enzymes)
    • Examples causes include:
  • Infection without perforation or rupture:
    • Introduction of microorganisms without the presence of perforation or puncture:
    • Spontaneous bacterial peritonitis
    • Pelvic inflammatory disease
  • Leakage of other fluids such as blood, bile, urine etc. These may be sterile initially but often become infected shortly after, resulting in peritonitis after 24-48 hours
  • Rarely, autoimmune diseases including systemic lupus erythematosus can cause peritonitis

Peritonitis: Presentation

Abdominal pain

Patients are generally unwell (see other features below) and a key feature of peritonitis is abdominal pain. This may be localised and restricted to the area over the affected organ, but can become generalised as the whole peritoneum is involved:

  • Acute abdominal pain and tenderness:
    • The pain is often dull and generalised but then becomes sharp localised as the overlying parietal peritoneum is irritated
    • Movement (e.g. coughing, jumping, or straining) often exacerbates the pain
    • Some patients may lie with their knees flexed to reduce stretching and movement of the parietal peritoneum to reduce pain
  • Abdominal guarding and rigidity:
    • The abdominal wall muscles tense up to cover the underlying inflamed organ
  • Rebound tenderness (Blumberg’s sign):
    • Applying pressure and then quickly releasing it causes pain that is worse when pressure is released. Applying pressure stretches the irritated parietal peritoneum and removing it quickly ‘snaps it’ back into place, resulting in pain.

For example, appendicitis has localised features of peritonitis in the right lower quadrant, however, if it ruptures, its contents escape into the peritoneal cavity, resulting in generalised inflammation and these features being seen throughout the abdomen.

Other features

Alongside abdominal pain, patients with peritonitis are often unwell and distressed. Associated features include:

  • Fever
  • Tachycardia
  • Hypotension – if dehydration or septic shock occurs, can lead to multiorgan failure
  • Ileus:
    • Severe pain leads to excess sympathetic nervous system activity, resulting in reduced bowel movement
    • This may also occur if the bowel is the affected organ (e.g. bowel perforation)
    • Bowel sounds may become reduced/absent

Peritonitis: Investigations

Overview

Specific tests depend on the suspected underlying cause, however, the following tests may be used:

  • Blood cultures:
    • To screen for sepsis
  • Full blood count (FBC):
    • May show leukocytosis suggesting infection
  • Urea and electrolytes (U&Es):
  • Liver function tests (LFTs):
    • May be deranged in hepatobiliary disorders
  • Serum amylase/lipase:
    • If pancreatitis is suspected
  • Erect chest X-ray:
    • May show pneumoperitoneum (free air in the peritoneal cavity) with air accumulating under the diaphragm, suggesting gastrointestinal tract perforation
  • Abdominal X-ray:
    • May show Rigler’s sign (double wall sign), suggesting pneumoperitoneum which can occur secondary to gastrointestinal tract perforation
  • CT scan:
    • Preferred for detecting free air in the abdomen and can help with identifying the underlying cause
  • Abdominal paracentesis microscopy, culture, and sensitivities:
    • Collects a sample of peritoneal fluid for infection

In some cases, urgent surgery may be necessary even if the underlying cause has not yet been found.

Peritonitis: Management

Overview

The key principles of management include:

  • IV broad-spectrum antibiotics and IV fluids:
    • These are often cephalosporins that cover Gram-negative organisms
    • Nearly all patients require supportive IV fluids to correct hypovolaemia
  • Surgery:
    • In the early stages and if the patient is relatively stable, watching and waiting with medical therapy may be sufficient, depending on the likely underlying cause (such as spontaneous bacterial peritonitis)
    • A laparotomy is performed once the patient is stabilised to repair any perforation and wash out the peritoneal cavity
    • Emergency exploratory surgery may be required if the patient is deteriorating quickly, even if the underlying cause has not yet been found

Peritonitis: Complications

The complications of peritonitis can be severe and life-threatening:

  • Fluid and electrolytes moving into the peritoneal cavity:
    • Inflammation leads to increased capillary permeability, resulting in fluid leaking out into the peritoneal cavity
    • This can cause hypovolaemia leading to hypotension, hypoperfusion, organ dysfunction (e.g. acute kidney injury) and multi-organ failure
    • This can also lead to abdominal compartment syndrome
  • Abdominal compartment syndrome:
    • Increased fluid in the peritoneal cavity increases intra-abdominal pressure which when severe enough, results in pressure on abdominal organs and the cardiovascular and respiratory systems. This can lead to shortness of breath and further reductions in organ perfusion and function.
  • Intra-abdominal abscesses:
    • Collections of infected material including pus that most commonly arise at the subhepatic, pelvic, and paracolic spaces.
  • Sepsis and septic shock:
    • This can also cause hypovolaemia due to inflammation leading to increased capillary permeability, resulting in fluid leaking out of blood vessels
    • This can cause hypovolaemia leading to hypotension, hypoperfusion, organ dysfunction (e.g. acute kidney injury) and multi–organ failure

Peritonitis: Prognosis

  • Without treatment, peritonitis is almost always fatal
  • In conditions where peritonitis can be managed with surgery (e.g. appendicitis or gastrointestinal perforation), mortality rates are around 10% for adults and increase up to 40% in the elderly.
  • The prognosis is poorer if hypotension and shock are present and/or if diagnosis and treatment are delayed

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