Abdominal Pain – The Basics from a Clinical Perspective:

Before I list the conditions that cause this extremely common symptom, it’s important to go through the mechanism of abdominal pain. Trust me; it’s not as simple as you might think it is. If you have the basics in mind, it will take you seconds to diagnose most conditions of the gastrointestinal system that present with abdominal pain.

Components of Abdominal Pain:

A variety of stimuli can cause pain on the skin, ranging from heat to pinpricks to even deep, intense pressure. With the abdomen, the scenario is completely different. That’s because abdominal pain has two variants which might be present alone or in combination:

  • Visceral Pain: This is the pain associated with abdominal viscera and the visceral peritoneum – the inner layer of the peritoneum that covers the organs within the abdomen.
  • Somatic Pain: This pain is due to stimulation of the parietal layer of the peritoneum and the muscles, skin, fat and fascia of the abdominal wall.

What type of pain is experienced in abdominal conditions and why?

This is what you need to remember: Visceral pain is dull and non-localized whereas parietal/somatic pain is well localized and sharp.

Here’s why visceral pain is dull and non-localized:

  • The abdominal organs or viscera are innervated by slow, unmyelinated pain fibres, the C fibres. Unlike most types of nerve fibres of the nervous system, these fibres are devoid of the myelin sheath due to which their speed of nerve impulse transmission is much less as compared to the myelinated fibres. Moreover, the threshold of the intensity of a stimulus required to stimulate these fibres is much more than that of the other fibres. That’s that but what’s all this got to do with the type of pain experienced in the abdomen? This is a crucial concept so make sure you grasp it! (Guyton was pretty good at explaining the characteristics of nerve fibres, would have been better if he had explained their clinical importance too. Not much of a fan of that dude!)
  • The unmyelinated C fibres are sensitive to tearing, stretching and pressure sensations (such as excessive dilation, ulcers, tears or obstruction of the abdominal organs). There are no proprioceptive organs in the viscera and the receptors for temperature and touch are sparse.
  • The distribution of the C fibres, which carry pain sensations, is not dense due to which pain is non-localized.

Referred pain – why so?

When it comes to the viscera, pain is not only un-localized but it may also be referred. That is because of the plasticity of the CNS and because the pain fibres from the parietal and the visceral components merge on the same second order neuron in the spinal cord. The visceral fibres, when stimulated for a long time, trigger the parietal ones by facilitation. However, the parietal fibres do not have the same effect on the visceral ones (that sounds like good news to me!).

Theuplink (2004), Visceral referred pain areas...
Areas of Referred Pain from the Viscera
(Photo credit: Wikipedia)

Parietal pain is localized and sharp simply because:

  • The parietal peritoneum and the abdominal wall are supplied by myelinated fibres which have a high impulse transmission speed and lower threshold than the C fibres, due to which their stimulation results in sharp pain. The abdominal wall is also rich in proprioceptive organs as well as receptors for sensing temperature and touch.
  • This pain is usually not referred and follows the dermatomal pattern, thus being localized.

What happens if we combine visceral pain somatic pain in the abdomen?

I saw a patient a couple of days ago who had dull, constant pain in the right part of her abdomen but she couldn’t tell me the exact site. After around 2 hours or so, she developed fever and suddenly localized her pain to McBurney’s point (When I discuss the pain of appendicitis, I’ll write about why appendicitis causes pain in this particular region, so stick around!). That’s a pretty classical picture of acute appendicitis. But wait, the appendix is an abdominal organ, why did its pain get localised? That’s because once it got inflamed (the fever proved that the appendix was infected and thus acutely inflamed), it started irritating the PARIETAL peritoneum, which causes somatic or localized pain, thus helping us narrow down our diagnosis. So although this process is painful for the patient, it’s a helpful diagnostic feature for the physician!

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