Pain

Question 1

Definitions

• analgesia
• dysesthesia
• hyperalgesia
• allodynia

Answer 1

Analgesia - absence of pain in response to stimulation which would normally be painful

Dysesthesia - unpleasant abnormal sensation

• hyperalgesia - increased sensitivity to stimuli that would normally provoke pain
• allodynia - pain from stimulus that does not normally cause pain

Question 2

What are the characteristics of

• unmyelinated C fibres
• thin myelinated A delta

Answer 2

Unmyelinated C

• respond to chemical, mechanical, thermal stimuli
• dull, diffuse, ache

Thin myelinated A delta

• mechanical stimuli
• localised sharp pain

Question 3

What are examples of some chemical mediators that act on

• nociceptors
• CNS

Describe how opioid receptors contribute to analgesia

• presynaptically
• postsynaptically

Answer 3

Act on nociceptors

• Histamine
• Prostaglandins
• Bradykinin
• Adenosine
• Seretonin

CNS from DH

• Substance P => pain signal
• Enkephalins, endorphins=> reduce pain signal

Presynaptic
-reduced Ca influx => reduced Substance P release
Postsynaptic
-increased K influx => increased hyperpolarisation

Question 4

Describe the afferent pathway and the efferent analgesic pathway

Answer 4

Afferent - spinothalamic

• Primary afferent => DRG
• Decussation at SC
• Synapse at thalamus
• Synapse at cerebral cortex

Efferent analgesic pathway

• Cortex => thalamus
• Periaqueductal grey, raphe nucleus => seretonergic and noradrenergic fibres onto enkaphalin containing neuron

Question 5

What are the 4 different types of pain

Answer 5

Nociceptive - pain from actual/threatened damage to non neural tissue due to activation of nociceptors

Neuropathic - pain from lesion/disease of somatosensory nervous system

Acute - pain relating to tissue injury or inflammation that resolves when healed
-if inadequately treated => chronic

Chronic - 3months+, cannot be easily explained, physical tissue injury not found

Question 6

Common causes of neuropathic pain

Key features of neuropathic pain

Answer 6

Nerve injury
Trigeminal/postherpetic neuralgia
Post amputation pain
Diabetic neuropathy

Spontaneous pain in absence of noxious stimulus
Pain in areas of sensory loss or allodynia

Question 7

How would you manage acute pain?

How would this differ from chronic pain

Answer 7

Analgesia - oral, parenteral
Immobilisation
Nerve blocks - peripheral, epidural

Analgesic strategies are less effective due to tolerance and dependence