Pain Mechanisms Flashcards
What is hyoeralgesia?
Increased pain at a normal threshold resulting from peripheral or central sensitisation
What is allodynia?
Pain from stimuli which are not normally painful or pain which occurs in an area not stimulated
Give an example of allodynia
Arthritis in the knee - receptive field for pain is expanded to the thigh so that if the thigh is touched, causes pain
How do hyperalgesia and allodynia occur?
Process of wind-up
- occurs in persistent activation of pain afferents
- get persistent activation and resultant upregulation of NMDA receptors
- get increased glutamate release
- causes excessive second order neurone firing
- results in nociceptive fibres becoming hyper excitable
What are the classifications of chronic pain?
Nociceptive pain
Neuropathic pain
Mixed
What happens in chronic nociceptive pain?
Activation of nociceptors stimulating nociceptive signalling along Aδ and C fibres
How is chronic nociceptive pain commonly described?
A sharp, stabbing pain
What is an example of chronic nociceptive pain?
Arthritis
Long term activation of those fibres will result in wind-up, so many patients will eventually complain of hyperalgesia and allodynia
What happens in chronic neuropathic pain?
Pain of a neuronal origin
No classic stimulation of nociceptors, therefore difficult to treat
No single disease process
What can cause neuropathic pain?
Diabetic neuropathies
Cancer compressing nerves
Trigeminal neuralgia
Where does neuropathic pain commonly occur?
Brain
Spinal cord
Peripheral nerves
How is neuropathic pain commonly described?
Burning
Tingling
Shooting
Hyperalgesia and allodynia common
What are the two main mechanism of neuropathic pain?
Ectopic activity - upregulation of VG Na channels distal to damage, causing increased excitability of the nerve and ectopic action potentials to occur along the pain fibre
Ephatic activity - ectopic beats cause action potentials to be sent to activate adjacent nerve fibre bundles, causing receptor field expansion
What is phantom limb pain and how does it develop?
The painful neuropathy resulting from neuroplastic changes in the CNS
What happens in complex regional pain syndrome?
An injury initiates a pain impulse carried by sensory nerves to the CNS
The pain impulse triggers and impulse in the SNS which returns to the original site of injury
This triggers an inflammatory response causing the vessels to spasm, swelling and increased pain
Pain triggers another response, establishing a cycle of pain and swelling
Results in burning extremity pain and oedema
