Pain Flashcards
Define pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
What are the steps of the physiological process of pain perception?
- transduction
- transmission
- modulation
- perception
Describe transduction of pain
- conversion of noxious stimulus (heat, chemical, mechanical) into AP in nociceptor (A-delta or C-fibres)
- responding to stimuli that can potentially damage tissue, sensation may extend actual assault
- primary hyperanalgesia: assault of damaged tissue that recruits ‘sleeping’ c-fibres
Describe transmission of pain
- afferent from periphery into dorsal horn and cross over in spinal cord to ascend in the spinothalamic tract to the thalamus then cortex
- neospinothalamic tract terminates in VPL of thalamus and is mainly A-delta fibres
- paleospinothalamic tract terminates in dorsomedial thalamus and is mainly c-fibres
What are the excitatory substances and receptors involved in pain perception?
excitatory substances: glutamate, substance P, CGRP
pain receptors: AMPA, NMDA, G-protein coupled receptors
Describe modulation of pain
- occurs mostly in dorsal horn: descending inhibition to activate endogenous opioids and higher order brain function
- gate control theory: A-delta fibres synapse on inhibitory neurons which block the c-fibres. Used in TENS for pain management
Describe perception of pain
- conversion of neuronal activity into conscious experience
- past experience, current situation and understanding all modulate the conscious experience
- reticular system elicits autonomic responses (sweating, pallor, tachycardia, nausea etc)
- limbic system links pain perception and mood
Describe A-delta and C-fibres
A-delta: myelinated, respond to sharp localised pain, minority of nociceptors and mainly somatic
C-fibre: unmyelinated, responds to dull, throbbing pain, majority of nociceptors
Describe visceral pain and how it is perceived
- visceral nociceptors respond to distension or ischaemia
- primary afferent activates multiple second order neurons which causes the pain to diffuse rather than localise to one point
- primary afferents converge with second order neurons with somatic input to produce referred pain
Describe some strategies in pain prevention/preparation
- anticipation and simple adjustments
- distraction
- education
- challenging misconceptions
- re-branding
- patient control
- topical anaesthetics
Describe the WHO pain ladder
- used for cancer pain
- used in reverse for acute pain
- medications for neuropathic pain like amitriptyline can be used at each step
Step 1 (mild): simple analgesics Step 2 (moderate): mild opioid eg. codeine/tramadol and continue with simple analgesics Step 3 (severe): strong opioid eg. morphine while continuing simple analgesics
What are the risks for neuropathic pain and its treatment?
- trauma (phantom limb pain)
- diabetic neuropathy
- postherpetic neuralgia
- trigeminal neuralgia
- post-stroke pain
- changes in appearance, sensation and autonomics of limb
treated with TCAs and anti-convulsant
What are the physiological changes caused by chronic pain?
prolonged inflammatory response results in decreased pain threshold in primary afferents:
- increased production of substance P and CGRP
- recruitment of NMDA receptors
- wind-up phenomenon: repeated firing of neurons increases and prolongs excitability even when stimulus is removed resulting in non-responsive pain
- changes in spinal cord gene and receptor expression in DRG and dorsal horn neurons
What are the risk factors for chronic pain?
Non-modifiable:
- female
- older age
- genetic predisposition
- lower economic status
- occupational factors
- previous history of abuse
- compensation
Modifiable:
- past experience of pain
- anxiety and depression
- catastrophizing beliefs
- surgical approach
- attitudes
- communication
What are complex regional pain syndromes and what characterises this?
severe continuous neuropathic pain disproportionate to the trauma:
- abnormal sensation
- vasomotor damage
- sudomotor damage
- motor/trophic change
- regionally restricted eg. hand
