Referred Pain and Dermatomes
Referred Pain: Pathophysiology & Mechanisms
Referred pain is a phenomenon where pain is perceived in an area remote from the site of the actual nociceptive stimulus. This is a highly tested concept across critical care and anaesthesia boards, heavily relying on neuroanatomical principles.
The definitive, evidence-based explanation for referred pain is the Convergence-Projection Theory.
- Mechanism: Visceral nociceptive afferents and somatic nociceptive afferents (from the skin or muscles) converge onto the same second-order wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord.
- Cortical Misinterpretation: Because somatic pain is evolutionarily much more common than visceral pain, the higher cortical centres (somatosensory cortex) systematically misinterpret the visceral input as originating from the corresponding somatic dermatome.
- Sensitization Link: Prolonged visceral pain can trigger central sensitization (as discussed in previous sections), leading to secondary hyperalgesia in the referred somatic area, making the skin genuinely tender to touch.
Visceral Referred Pain Patterns
The specific spinal segments associated with visceral organ referred pain.
Visceral Organ | Innervation (Spinal Level) | Classic Referred Pain Location | Clinical Eponym / Sign |
Diaphragm (Central) | Phrenic Nerve (C3-C5) | Ipsilateral shoulder/neck | Kehr’s Sign (often splenic rupture) |
Heart | Sympathetic afferents (T1-T4) | Left chest, axilla, medial left arm | Angina Pectoris |
Gallbladder | Sympathetic afferents (T5-T9) | Right upper quadrant, right scapula | Boas’ Sign |
Ureter | Sympathetic afferents (T11-L2) | Flank radiating to groin/testicle | Renal Colic |
Dermatomes and Clinical Pitfalls
A dermatome is defined as the area of skin supplied by the afferent nerve fibers of a single posterior spinal root. Understanding dermatomal distribution is crucial for mapping regional anaesthesia blocks and diagnosing radiculopathies.
Essential Landmarks:
- C5: Clavicles
- T4: Nipple line
- T10: Umbilicus (Target level for obstetric labor analgesia)
- L4: Medial aspect of the leg and medial malleolus
- S1: Lateral aspect of the foot and heel
The Dermatomal Overlap
While peripheral nerve lesions cause discrete, sharply defined areas of anaesthesia, dermatomes have significant physical overlap. Destruction of a single spinal dorsal root rarely produces complete anaesthesia in its corresponding dermatome due to overlap from adjacent roots above and below (ANZCA, 2020).
The two most prominent maps are the Foerster map and the Keegan and Garrett map.
Understanding the methodological difference between these two maps is essential for correctly diagnosing radiculopathies and performing sensory assessments for regional anaesthesia.
Methodological Comparison
Feature | Foerster Map (1933) | Keegan & Garrett Map (1948) |
Methodology | Surgical sectioning of adjacent dorsal nerve roots (rhizotomy) to isolate a single root, mapping the residual sensation. | Clinical observation of hypoalgesia secondary to single nerve root compression (herniated discs). |
Visual Layout | Demonstrates massive, irregular overlap of adjacent dermatomes. | Depicts neat, linear, non-overlapping bands spiraling down the limbs. |
Basis of Map | True anatomical/physiological cutaneous innervation. | Embryological limb bud development. |
Clinical Accuracy | High / “Good” evidence (Basis for the ASIA impairment scale). | Intermediate evidence (Oversimplifies actual innervation). |
The Foerster Map: The Clinical Gold Standard
Otfrid Foerster developed this map by evaluating patients who had undergone surgical dorsal rhizotomies for spasticity or chronic pain.
By cutting the nerve roots above and below a specific level, he was able to map the exact area of skin uniquely supplied by the remaining intact root.
- The Principle of Overlap: Foerster definitively proved that tactile dermatomes overlap extensively. Because of this overlap, the transection or severe injury of a single dorsal nerve root rarely results in total anaesthesia of the corresponding skin area (except occasionally at C2).
- Forester map has been adopted as the foundational framework for the American Spinal Injury Association (ASIA) impairment scale, which is the gold standard for classifying spinal cord injuries in ICU and trauma settings.
The Keegan and Garrett Map: The “Textbook” Map
Jay Keegan and Frederick Garrett published their map in 1948, based primarily on mapping areas of diminished sensation (hypoalgesia) in patients suffering from surgically confirmed intervertebral disc herniations.
- The Embryological Aesthetic: They visualized dermatomes as continuous, unbroken strips extending from the spine down the length of the limbs, theorizing this was how nerves stretched during embryonic limb bud development.
4. References
- Giamberardino, M. A. (1999). Recent and forgotten aspects of visceral pain. European Journal of Pain, 3(2), 77-92.
- Lee, M. W., McPhee, R. W., & Stringer, M. D. (2008). An evidence-based approach to human dermatomes. Clinical Anatomy, 21(5), 363-373. (Critically contrasts the clinically relevant Foerster dermatome maps against the Keegan & Garrett maps).
- McMahon, S. B., Koltzenburg, M., Tracey, I., & Turk, D. C. (2013). Wall and Melzack’s Textbook of Pain (6th ed.). Elsevier.
- Standring, S. (2020). Gray’s Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.