| Somatic pain | Visceral pain |
| Well localised | Poorly localised, often midline |
| Sharp, well differentiated | Dull, cramping, poorly differentiated |
| Intensity proportional to tissue damage | Poor correlation between intensity and tissue damage |
| Not usually referred | Frequently referred to somatic structures |
| No autonomic symptoms | Associated autonomic symptoms e.g. nausea, sweating, pallor |
Visceral Pain
Visceral Pain
The syllabi require knowledge of the "anatomy and physiology of pain medicine to include... visceral pain" and the "mechanisms of pain; nociceptive (somatic and visceral)".
Resources
- Visceral pain arises from the internal organs of the thorax, abdomen and pelvis
- It is the most common form of pain produced by disease, yet behaves quite differently to somatic pain
- Primary visceral afferents are mainly unmyelinated C fibres, with few Aδ fibres
- The density of visceral afferent innervation is low, contributing to poor localisation
- They travel alongside sympathetic (and some parasympathetic) nerves, passing through the pre- and paravertebral ganglia without synapsing
- Cell bodies lie in the dorsal root ganglion
- They terminate diffusely over several spinal segments in the dorsal horn (laminae I, V and X)
- Some branches terminate in areas regulating autonomic outflow, explaining the associated autonomic features
- Second order neurones also receive somatic input (see convergence), then ascend in the spinothalamic and dorsal column pathways
- Second order neurones in the dorsal horn receive input from both visceral and somatic afferents (viscerosomatic convergence)
- They respond equally to both, and the brain cannot distinguish the source
- Visceral stimulation is therefore interpreted as arising from the somatic structure supplied by the same spinal segment: referred pain
- The area of referral reflects the embryological origin of the organ
- E.g. the heart develops in the neck and upper thorax; cardiac afferents enter the cord at T1-T5, so pain is referred to the chest, left arm and jaw
| Organ | Typical area of referred pain |
| Heart | Central chest, left arm, neck and jaw |
| Oesophagus | Retrosternal |
| Stomach | Epigastrium |
| Liver and gallbladder | Right upper quadrant, right shoulder tip |
| Appendix and small bowel | Periumbilical |
| Kidney and ureter | Loin to groin |
- Sensitisation is an increase in the magnitude of the pain response to a given stimulus
- Visceral hypersensitivity has three components:
- Peripheral sensitisation of primary visceral afferents
- Central sensitisation of ascending spinal neurones
- Characterised by hyperalgesia and allodynia
- NMDA receptor activation mediates persistent pain and hyperalgesia
- Genetic polymorphisms (e.g. COMT, 5-HTT, β-adrenoceptor genes) and environmental factors contribute
- Dysregulation of descending pathways that modulate nociceptive transmission
- This is thought to underlie chronic visceral pain syndromes such as irritable bowel syndrome and chronic pelvic pain
- Investigate and treat the underlying cause
Non-pharmacological
- MDT approach: physiotherapy and occupational therapy
- Psychological therapies e.g. CBT, hypnotherapy
- Liaison between GP, inpatient and outpatient pain services
Pharmacological
- Conventional analgesics according to the WHO analgesic ladder
- In chronic sensitisation syndromes, TCAs, SSRIs and SNRIs may help by enhancing descending inhibition
Interventional
- Sympathetic blocks, which interrupt the visceral afferents travelling with sympathetic fibres:
- Stellate ganglion block: head, neck, upper limb and thoracic viscera
- Coeliac plexus block: upper abdominal viscera e.g. pancreatic cancer
- Superior hypogastric plexus block: pelvic viscera
- Percutaneous cervical cordotomy (C1-C2) for intractable, unilateral cancer pain