Mechanisms of MSK referred pain

Question 1

What is referred pain?

Answer 1

Referred pain - pain is topographically distinct from the region in which actual source of pain is located

Occurs in areas w/ cutaneous innervation that differs from that overlying the site of pathology.

E.g. pain from the heart in myocardial ischaemia may be felt in the left axilla or down the inside of the left arm.

Question 2

What are the different types of referred pain?

Answer 2

• Radicular referred pain- dermatomes- spinal nerve root
• Myofascial referred pain- myotomes- muscle & fasica
• Bone/ joint referred pain- sclerotomes
• Visceral referred pain- viscerotomes- internal organs

Question 3

Where is the pain felt in the pain?

Answer 3

Pain is localised in the brain according to a somatotopic map in the S1 sensory cortex.

The Sensory cortex’s representation of the body surface corresponds to the spinal level.

Spinal segmental levels map to dermatomes.

Question 4

What is the pathophysiology of referred pain?

Answer 4

Nociceptors (painful stiumuli receptors) from visera (internal organs) & skin CONVERGE in the same primary afferent spinothalamic neurons.
- Called viscerosomatic convergence

Pain is transmitted through ascending fibres of the spinothalamic tract

The brain interprets the viseral pain as coming from an area of the skin that could be far from the internal organ.

However, the number of fibres w/ these characteristics is limited - this can’t fully explain referred pain.

Has an affect on all fibres leading to different effects:
- Aa fibres - muscle function deficit/muscle weakness
- Ab fibres - sensory loss
- Ay fibres - Reflex loss
- Ad fibres - pain
- C fibres - pain

Question 5

Explain convergence-projection theory & how it could lead to referred neuropathic pain.

Answer 5

1st order neuron:
- also called primary afferent neuron!
- carries impulse from receptors (in the peripheral nervous system) to spinal cord (in CNS)

2nd order neuron:
- i.e. interneuron
- carry impulse from spinal cord to thalamus

3rd order neuron:
- carry impulse from thalamus to primary sensory cortex

NOTE: view diagram on notes!

Afferent fibres coming from the primary site of pain & the area of referred pain e.g. skin converge on a second order neuron in the dorsal horn at the same level of the spinal cord.

There are less second order neurons than the first order neurons, suggesting that first order neurons converge.

The message from the site of primary pain is interpreted by the brain as coming from the area of referred pain.

Question 6

Explain how muscles refer pain

Answer 6

e.g. Injecting saline into tibialis anterior causes referred pain in big toe

• Motor innervation. maps skin innervation.
• We feel muscle pain in the cutaneous area that is at the same cutaneous innervation as the muscles’ innervation.

Question 7

Explain how joints refer pain

Answer 7

A nerve supplying a muscle controlling a joint, also innervates the joint.

Example hip → knee referred pain:

• Different parts of the hip have different innervation.
• Since there are so many different nerves that cross the joint, it could refer from T12 to S1.
• Some of these nerve roots also innervate the knee

Question 8

Explain how the viscera can refer pain.

Answer 8

• Viscera (internal organs) have more than one set of visceral sensory afferents.
• Sympathetic afferents - include the T1-L2 spinal segment.
• Parasympathetic afferents - include vagus & sacral autonomic fibres.