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Transcript of Pain Mechanisms
pain depends on the stimulus
pain also depends on where you apply the stimulus
skin, muscle, joints, ligaments, bones
stimuli=heat/cold, stretch, inflammation
internal organs: pelvis, thorax, abdomen
stimuli=stretch, inflammation, ischaemia
PAIN IS ALWAYS
PAIN IS NOT ALWAYS
the result of tissue damage
"Pain is what the patient says it is"
The subjectivity of experience
A nerve is a wire
"Where? What? How bad?"
Sharp, short, localised
Not abolished by morphine
Affective & arousal
Emotional & sympathetic response
Dull, diffuse, prolonged
Spasm & guarding
Abolished by morphine
the dorsal horn
Where peripheral becomes central
The gate control theory
Extracts from ‘Gunshot Wounds and Other Injuries':
“The last of the immediate symptoms of grave lesions of nerves is pain, elsewhere than in the wound..(in some) the pain was of that strange burning nature...it was rare to observe this peculiar and agonizing form of suffering among the immediate consequences of a nerve wound.
The seat of burning pain is very various. Its favourite site is the foot and hand. Its intensity varies from the most trivial burning to a state of torture. The part itself becomes exquisitely hyperaesthetic so that a touch or a tap of the finger increases the pain. Exposure to the air is avoided by the patient with a care which seems absurd and most bad cases keep the hand constantly wet, finding relief in the moisture.
As the pain increases, the general sympathy becomes more marked. The temper changes and grows irritable, the face becomes anxious and has a look of weariness and suffering. The sleep is restless. At last the patient grows hysterical. He walks carefully, carries the limb tenderly with the sound hand, is tremulous, nervous and has all kinds of expedients for lessening his pain.”
features of neuropathic pain
Second order neurone
third order neurone
No single pain center
Lateral thalamic nuclei & somatosensory cortices
Fast & discriminative
"Where? How bad? How long?"
Medial thalamic nuclei, anterior cingulate gyrus,
prefrontal cortices, amygdala (fear)
Slow & affective
Experimental lesion: no pain perceived
pain perceived but no longer bothersome
Effect of opioids
burning/electrical/tingling/pricking/"pins & needles"
may follow nerve distribution
ALLODYNIA: non-painful stimulus is painful
HYPERALGESIA: mildly painful stimulus is very painful
change in skin colour/temperature
mechanisms of neuropathic pain
Easy to recognize
Arbitrarily >3 months
Useless - "a malefic force"
No physiological signs
Difficult to believe in?
A Case of Phantom Limb Pain
plasticity vs elasticity
injury or experience
can change the way pain is felt
"Pain changes the way we feel pain"
Same stimulus produces the same response
Repeated identical stimuli
produce increasing response
rEYNOLD AND HIS RATS
Surgery on unanaesthetized rats
by stimulating the PAG
Pain as punishment
WHO ANALGESIC LADDER
ROLE OF THE ANAESTHETIST
MBBS MRCP(UK) FRCA
Pain Fellow & Specialist Registrar in Anaesthesia
West of Scotland Deanery
HOW IS CHRONIC PAIN DIFFERENT?