Referred pain

Question 1

Back pain types

Answer 1

• Radicular pain
• Facet joint pain
• Sacroiliac joint pain
• Discogenic pain
• Myofascial pain
• etc. etc

Question 2

Referred Pain Learning Outcomes

Answer 2

• Dorsal horn convergence
• Somatotopic representation
• Insight into the necessity to understand neuroanatomy / innervation when considering peripheral “pain generators”
• Radicular pain
• Somatic referred pain • (Visceral pain)
• Central mechanisms of (pain) perception

Question 3

Referred pain

Answer 3

In clinical terms, referred pain may be defined as pain perceived as occurring in a region of the body topographically distinct from the region in which the actual source of pain is located

pain perceived as arising or occurring in a region of the body innervated by nerves or branches of nerves other than those that innervate the actual source of pain.

Question 4

Where can the Pain come from

Answer 4

Nerves for: myotomes, dermatomes, “sclerotomes” (Bone and Joint structures eg ligament)

Question 5

Referred pain – key point

Answer 5

Pain is perceived in the dermatome of the segmental level of the innervation of the structure

If you know the innervation of the organ or structure, you can predict the pattern of referral of pain

Question 6

Learning Points referred pain

Answer 6

1. Pain is localised by the brain according to a somatotopic map in the S1 sensory cortex
2. S1 cortex representation of body surface corresponds to spinal segmental level
3. Spinal segmental level maps to body surface dermatomes
4. Direct activation of segmental nerve root will refer to dermatome as radicular pain
5. Dorsal Horn Convergence: any afferent input to that spinal segment may refer to that segmental dermatome:
   
   1. Muscle via nociceptive afferents that run with motor nerve
   2. Joint via nociceptive afferents from joint structures
      3. Visceral input via visceral “SNS” autonomic fibres

Question 7

Fundamental referred pain learning outcomes

Answer 7

1. Pain is localised by the brain according to a somatotopic map in the S1 sensory cortex
2. S1 cortex representation of body surface corresponds to spinal segmental level
3. Spinal segmental level essentially maps to body surface dermatomes
4. Direct activation of segmental nerve root will refer to dermatome as radicular pain
5. However, the deeper structures beneath the layer of cutaneous innervation do not necessarily have a nerve supply that arises from the same spinal segmental level as the skin (dermatome)
6. Recall that each muscle has a specific motor innervation from a spinal segmental level – e.g biceps = C5; C6. Although we tend to consider the nerve supplying that muscle as a motor nerve, the same nerve – e.g. musculocutaneous nerve supplying biceps contains afferent C-fibre sensory afferents from the same spinal segmental level. This can be used to create a myotomal map of muscle sensory innervation
7. Similarly, alongside the proprioceptive sensory fibres supplying each synovial joint, there are nociceptive C-fibres.
8. Similar C-fibres innervate ligaments and periosteum of bones such that it is possible to create a sclerotome map of bone and joint innervation
9. Visceral sensory afferents co-locate with the sympathetic (and to a lesser extent the parasympathetic) innervation of the organ. The spinal segmental levels of visceral innervation can be used to construct a viscerotome map.
10. The fundamental concept is therefore Dorsal Horn Convergence: any afferent noxious input to that spinal segment may refer to the dermatome:
    1. Muscle via nociceptive afferents that run with motor nerve
    2. Joint via nociceptive afferents from joint structures
    3. Bone via periosteal nociceptive afferents
    4. Visceral input via visceral “SNS” autonomic fibres

Question 8

Referred Radicular Pain:

Answer 8

Pain arising from pathological activation of the dorsal root ganglion (DRG) at a spinal segmental level. Classically from a disc prolapse causing “sciatica”, but also from degenerative compression or malignant infiltration of nerve roots. Neuropathic activation of the DRG and associated activity in the spinal dorsal horn at the same segmental level produce pain referred the distal extremity supplied by the nerve root involved

Question 9

Referred Somatic Pain:

Answer 9

Distal to the dorsal root ganglion, fibres from the ‘ nerve root innervate spinal ligaments, facet joints, disc annulus etc. Pathology in these structures produce nociceptive input that converges in the dorsal horn with cutaneous sensory afferents. Pain can therefore be referred distally along the distribution of the nerve root.

Question 10

Referred Muscle Pain – myofascial pain:

Answer 10

Nociceptive afferent activation within muscle converges in the dorsal horn of the spinal cord at the same segmental level as the motor supply originates (in the ventral horn).

Question 11

Referred Visceral Pain:

Answer 11

Nociceptive afferents from the gut coalesce in the visceral plexi: coeliac, mesenteric, hypogastric, renal etc. Afferent fibres run with the visceral autonomic nerves: recall that the autonomic nervous system comprises:

Sympathetic nerves:
From lateral horn of spinal cord at segmental levels T1 – L2; then forming chain of paraspinous sympathetic ganglia

Parasympathetic nerves:
Glossopharyngeal (IX), vagus (X) and S2,3,4 provide afferent innervation of viscera.

Visceral pain is therefore perceived at the segmental level where their sympathetic innervation arises in the spinal cord.

Diaphragmatic pain is perceived via its somatic innervation – recall that the motor supply is phrenic nerve C3,4,5 and pain is therefore referred to the equivalent dermatome – i.e shoulder pain.

Question 12

Dorsal horn signaling

Answer 12

Not 1:1 relay in dorsal horn

Question 13

Sensory nociceptive afferents

Answer 13

Majority of fibres are of cutaneous origin
• ~ 80% cutaneous
~10% muscle afferent
~10% articular affer