Pain Threshold and Tolerance
Threshold: when you feel pain
Tolerance: when you can’t take it anymore
Measuring this is still subjective because actually measuring motivation (doesn’t tell us about their actual pain)
Rating scales
VAS better than NRS because in NRS you remember the previous number you used
FACES scales are problematic because they start happy instead of neutral
Problems with these scales: hard to say when we are measuring, context matters (could be in pissy mood), “pegs” people don’t understand the worse and use 10 too much (to send a message) (people have different ideas of what worse pain looks like), is is useful for WITHIN but not BETWEEN patients.
We always imagine pain as worse than it really is
Clinical pain score
Combines:
- ongoing pain
- allodynia
- hyperalgesia
- functionning
The score you give actually is a lot of things that have nothing to do with what we assign the score to (not just nociceptors)
FACS
By Paul Ekman
Facial Action Coding System
Encodes for facial muscle movements associated with pain expression
Melzack
Came up with:
- Pain descriptors by intensity
- Peak intensity diagram (sprained ankle to causalgia)
- MPQ McGill pain questionnaire (20 categories, pick 1 of each. Splitting attempt between inflammatory/neuropathic). Made a shorter version with descriptors + PPI + visual analog
DN4
Short pain questionnaire
10 questions, 1 point per question, need 6+ to be neuropathic pain
Owestry Disability Index
Test if your pills are working
Come up with a disability score
10 sections
Pain Catastrophizing
Catastrophizing = rumination + magnification + helplessness
High catastrophizing = more likely to get chronic pain, and to not get better
WOMAC questionnaire (3 sections)
Western and MacMaster
3 sections: pain, stiffness, disability
QST Quantitive Sensory Testing
Detect threshold and tolerance
Use mechanical/thermal stimuli and have method to measure it
Psychophysics: go up and down around threshold until 50% chance to find exact
German way: do series of QST, express patient values in z-scores, how many SD away you are from average. Try and split patients into categories based on symptoms ( need an average and everyone trained the exact same)
Biomarkers of pain
Don’t actually have any but:
tissue damage, cardiovascular (heart rate, BP), stress (cortisol), neural (EEG, imaging (best biomarker)), chemical (don’t work), molecular (DNA variant, mRNA levels) (don’t work because DNA doesn’t change with pain, and mRNA would need spinal tissue but can’t)
fMRI as biomarker?
YES:
- only one accepted
- differences in brain activity (brain agrees with the ratings so don’t need ratings)
NO:
- Found big overlap with imagined/pretend pain suggesting brain can be tricked, and so people could fake it
Why we use animals
Pain uses rats
Science in general uses mice
Challenges to using animals
Different animal ethics
Deontological: rule is the only to decide if something is right or wrong (and animal research is against the consent rules)
Consequentialist (utilitarianism): end justify the means
Thermal Assays
Mechanical Assays
Chemical assays
Inflammatory Assays
Inject inflammogens and then do mechanical or thermal stimulus. Can cause allodynia and hyperalgesia
Neuropathic assays
Surgery, cut a nerve in the foot and then measure thermal or mechanical
Axotomy (complete denervation) vs Partial denervation. Measure Autotomy (bite off own foot) when feels like phantom limb and count how many phallenges left in the morning
3 big criticisms for Status Quo (animal modelling)
Individual differences in pain perception
Real question isn’t what causes pain but why pain in some people but not others?
Only 7% of surgical patients get chronic post-surgical pain (which is a lot of people still)
Morphine does for sufficient analegesia is 2-80mg/kg which is a 40 fold (huge difference)
Reasons:
- Organismic: nature, about you, genetic background, sex, age, psychological traits,…
- Environmental: nurture, what happens to you, past experiences, gender, psychological states, diet,…
Biopsychosocial model
Disease better explained by interplay of 3:
biologival, sociocultural, psychological
Heritability of pain