Pathophysiology of Pain
Pain associated with the abdomen falls into one of three types and it is important to identify which type at the earliest possible stage.
Visceral pain
The autonomic nervous system innervates the abdominal organs and produces vague, poorly localised pain in response to organ stretch (Fig 1).
Fig 1 Visceral pain
If these organs are affected by peristalsis then this pain can appear intermittent or ‘colicky‘ in nature. Organs are innervated bilaterally so pain is often felt in the midline even if the organ is not actually positioned in the midline. Visceral pain is normally localised by the patient to the embryonic site of the organ, which may be different from the actual site, for example, the periumbilical pain of early appendicitis (Fig 2).
Fig 2 Embryonic development of the gut
Parietal (somatic) pain is due to irritation of the parietal peritoneum and is well localised to the site of the organ, for example the right iliac fossa (RIF) pain of later appendicitis [4].
Referred pain may be felt at a distant or ‘referred’ site to the organ due to misinterpretation of stimuli by the brain, again based upon embryonic development of the nervous system. An example of this would be diaphragmatic irritation due to blood, which can be felt at the shoulder tip [1].
Pointers toward referred pain include an apparent lack of expected abnormalities on examination. For example: a normal shoulder examination in a patient with shoulder pain.