Pain Mechanisms Flashcards
What is defined as chronic pain?
Pain/discomfort persisting continuously/intermittently for >3/12
Describe how pain fibres travel through the dorsal horn.
The nociceptive fibres enter the horn and relay with secondary neurones in either lamina I, II or V
BUT
Lamina II a.k.a. Substantia Gelatinosa (SG)
Is best thought of as a volume control, or regulator
It synapses with lamina I and V to regulate them
Spinothalamic pathways only come out of Lamina I and V
NOT OUT OF LAMINA II
Describe the physiology of the gate control in pain.
Normal route of pain fibres (C and A-delta)
They come into dorsal horn then they synapse with a second order neurone that travels to the thalamus
BUT
There is also the SG that has inhibitory fibres on that synapse, it tonically inhibits it
And SG is regulated by three ways:
- C and A-delta fibres inhibit it
- Mechanoreceptors (A-beta) excite it (So when you rub
or heat an area up this activates these receptors which
increase the SG effect on the synapse and reduce pain)
- Descending inhibitory control from the brain inhibits SG
(Descending inhibitory control also inhibits the synapse directly)
Define hyperalgesia.
Increased pain at normal threshold stimulation
- Thermal or touch
- Results from peripheral and central sensitisation
Define Allodynia.
It means “other pain”
It refers to:
Pain from stimuli which are not normally painful
Pain which occurs other than in the area stimulated
It is not synonymous with referred pain
Describe what is meant by wind up in terms of the pathophysiology of pain.
Tissue injury (nociceptive pain) and nerve damage (neuropathic pain) may cause persistent activation.
Excess Glutamate release, excess NMDA receptor activation, excess 2nd order firing or wind up
Result: long term changes in nociceptive neurones which become hyperexcitable (respond at lower stimulus intensity) … hyperalgesia
Wind up can lead to receptive field expansion at the peripheral site or allodynia
How can chronic pain be classified?
Nociceptive
Mixed
Neuropathic
(visceral)
There are lots of different ways of classification
What is the main point to take away from pain management?
Treat pain well, hard and soon to stop it from progressing to chronic
What is neuropathic pain?
Pain of neuronal origin - very difficult to treat
Cannot be explained by a single disease process or a single specific location of damage
Neuropathy, cancer, trigeminal neuralgia…
Brain/cord/peripheral nerves
Burning
Electric
Tingling - pins and needles
Shooting
(Hyperalgesia and allodynia common)
What can sensitisation after a peripheral nerve injury lead to?
Decreased threshold of the nociceptor to activation
Increased receptive field of nociceptors
Allodynia
Hyperalgesia
Prolonged post stimulus sensations (hyperpathia)
- Sensation outlives actual damage
Emergence of spontaneous activity
Describe some of the pathophysiology of neuropathic pain. (Ectopic)
After damage in the afferent nerve, the part of the nerve beyond the injury upregulates sodium channels and due to inflammation around it starts firing ectopically
Describe some of the pathophysiology of neuropathic pain. (field expansion)
Activation of one damaged nerve makes bundles near also fire
Ephaptic activity.
Describe some of the pathophysiology of neuropathic pain.
Ectopic firing
Field expansion/Ephaptic firing
Along with sensitisation damaged peripheral nerves develop abnormal sodium channels which fire off dysfunctionally and demonstrate different depolarisation properties
After peripheral nerve injury, large afferent sprout dorsally from lamina 3 to lamina 1+2, which gain access to spinal regions involved in transmitting high intensity, noxious signals instead of dealing with low level stimuli.
How can phantom pain present?
Sensation - Non painful - Common Limb pain - Painful neuropathy - Common - Pain in the missing limb
What is CRPS?
Complex Regional Pain Syndrome
Type 1 - no identifiable lesion - after illness
Type 2 - identifiable nerve lesion (Causalgia)
Sympathetically mediated pain or independent
It is a disorder of the extremities
Usually involves a single limb in the early stages