Pain physiology Flashcards
What is pain?
An unpleasant sensory and emotional experience associated with actual or potential tissue damage
What is the protective role of pain?
Limits further damage to the individual
What is the adverse affect of pain?
Stimulates an acute catabolic stress response allowing would recovery and repair
Describe the affect of pain on the central nervous system
via acute catabolic stress response causes:
- anxiety
- depression
- sleep impairment
What is the response of the CVS system to pain?
via acute catabolic stress response:
- increased blood pressure
- increased heart rate
- increased ischaemic heart disease
What is the response of the respiratory system to pain?
via acute catabolic stress response:
- inhibits cough
- hyperventilation
What is the response of the gastrointestinal tract to pain?
via acute catabolic stress response:
- ileus
- nausea
- vomiting
What is the response of the genitourinary system in response to pain?
via acute catabolic stress response:
- urinary retention
- uterine inhibition
What is the response of muscles to pain?
via acute catabolic stress response:
- restless (o2)
- immobility (DVT)
What is the metabolic response to pain?
- increased catabolic: cortisone, glucagon, growth hormone, catecholamines
- decreased anabolic: insulin, testosterone
- decreased plasminogen: increases coagulation, DVT
What is nociception?
The neural mechanism by which an individual detects the presence of a potentially tissue harming stimulus
What are the 4 processes of nociception?
- Transduction of sensation into APs
- Transmission
- Modulation
- Perception
Describe pain fibers
- Free nerve endings found in the epidermis of the skin
- Don’t have specific apparatus and are multi-modal
- have myriad channels for various ligands
Which nerve fibers respond to temperature?
- C
- A-delta
Which nerve fiber responds to cold temperatures?
A delta
Which nerve fiber responds to warmth, heat
C
What is the difference between nociceptors and other receptors?
Nociceptors are classed as high threshold
- only need them firing at important times
Where do primary afferent nociceptors synapse?
In the dorsal horn of the spinal cord
Where do secondary afferent nociceptors synapse?
In the brain
What are the features of C fibers?
- slow
- unmyelinated
- conveys the dull, diffused aches
- abolished by morphine
What are the features of A-delta fibers?
- fast
- myelinated
- shorp, short and localised pain
- protective (reflex withdrawl)
- not abolished by morphine
How is the dorsal hown organised?
Into laminae numbered I -> X
Different nerve fibers are found in different sections of laminae
Where do A-delta fibers enter the dorsal horn?
lamina I
Where do C fibers enter the dorsal horn?
lamina I & II
Which area of the dorsal horn does pain usually enter?
Superficial dorsal horn - lamina I and II
Where do A-beta fibers enter the dorsal horn?
Deep lamina - III and IV but also feed into lamina II
Which interneurons are present in lamina II
- excitatory: glutamine
- inhibitory: GABA. Glycine
Where is pain modulated?
- spinal (gate control theory)
- supraspinal (conditioned pain modulation)
Describe the gate control theory of pain
- in absence of stimulus from c fibers, inhibitory interneuron inhibits the ascending pain pathway
- with strong pain, C fiber inhibits the inhibitory interneuron and stimulates the ascending pain pathway
- Pain can be modulated by simultaneous somatosensity input via A-beta fibers (i.e. nonpainful stimulus can activate inhibitory interneuron to inhibit the pain pathway despite c fibers action)
What is the clinical application of the gate control theory?
Use of the tens machine in chronic pain and contractions during labour.
Describe conditioned pain modulation
Activation of serotinergic and noradrenergic pathways releases serotnonin, noradrenalin and enkaphalins at the spinal level
noradrenaline is more consistent analgesia
How can conditioned pain modulation be activated
- cold water emersion
- psychologically
How do opiods work?
Stumulate the nuclei in the fourth ventricle, which stimulates the release of noradrenaline and serotonin at the spinal cord level
How is pain perceived by the brain?
Perception of painful stimuli does not result from the brain’s passive registration of tissue trauma, but from its active generation of subjective experiences through a network of neurons known as the neuromatrix
Requires:
- sensory discriminative (determines site, severity and duration)
- affective motivational
- cognitive activation
What is hyperalgesia?
Abnormally high levels of pain from noxious stimuli
What is primary hyperalgesia?
pain sensitivity that occurs directly in the damaged tissues
What is secondary hyperalgesia?
pain sensitivity that occurs in surrounding undamaged tissues
What is allodynia?
Pain from a stimulus that is not normally painful (e.g. cotton wool)
How does pain hypersensitivity occur?
Through:
- peripheral sensitisation
- central sensitisation
What common substances can activate a nociceptor?
- potassium
- serotonin
- bradykinin
- hydrogen
- histamine
- atp, adenosine
What common substanses can cause sensitisation of a nociceptor?
- prostaglandins
- leukotrienes
- substance p
- noradrenaline
- neurokinin A
- CGRP
- nitric oxide
- reactive oxygen species
How do peripheral nociceptors get sensitised?
During an inflammatory response, causing the release of chemicals that can alter the nocicpetor from being a high threshold nociceptor to a low thershold.
What causes central sensitisation of nociceptors?
Through the loss of interneurons resulting in the CPM becoming hypoactive and therefore a reduction in noradrenaline and serotonin
What are the different types of pain?
- acute (trauma)
- chronic (> 3 months or longer than would normally expect for the injury)
- cancer (different resources)
- non-cancer
- nociceptive (tissue damage)
- neuropathic (nerve damage)
How can we differentiate nociceptive pain?
- somatic (skin, muscle bone)
- visceral (internal organs)
Describe common features of somatic pain
- site: well localised
- radiation: dermatomal
- character: sharp, aching, gnawing
- periodicity: constant +/- incident
- associations: rarely
Describe common features of visceral pain
- site: vague distribution
- radiation: diffuse to body surface
- character: dull, cramp, dragging
- periodicity: often periodix
- associations: nausea, sweaty, HR and BP changes
List examples of neuropathic pain
- post-stroke pain
- lumbar radicular pain
- diabetic peripheral neuropathy
- post-herpetic neuralgia
- chronic post-surgical pain
How is neuropathic pain often described?
- shooting
- electric shock like
- burning
- tingling
- numbeness
How is neuropathic pain differentiated?
- central or peripheral
Central harder to treat but treatment is the same for both types
How is back pain classified?
Usually has both a nociceptive component and a neuropathic component
What is the difference between radicular pain and referred pain?
in back pain both are experienced in the legs
- radicular pain travels the full length of the leg in a narrow band
- referred pain can be felt in various areas of the back of the legs due to noxious stimulation in the lumbar segments
How can radicular pain and referred pain in relation to the back be differentiated clinically?
- does the pain travel below the knee?
- character of pain (burning, stabbing, etc.)
- association with sensory of motor deficits
- provocation by straight leg raise
How can severity of pain be assessed?
Using rating scales:
- numerical rating scale
- verbal rating scale
- visual analogue scale (particularly useful in children or elderly)
Functional impairment:
- how many of the following are affected by pain; walking, general activity, normal wor, sleep, mood, enjoyment of life, social interaction
What are the treatment goals of pain management?
- How the patient feels (‘says’)
- how the patient responds (‘does’)
What are the 6 P’s of pain treatment?
1st line:
- preventative
- patholgy (treat it)
2nd line
- physical therapy
- pharmacotherapy
- procedural
- psychologically based
Describe the who analgesic ladder
1 - paracetamol, NSAIDs
2 - codeine, dihydrocodeine, dextropropoxyphene
2/3 - tramadol
4 - morphine, coycodone, fentanyl, buprenorphine, methadone, hydromorphone, diamorphine, pethidine
What are the steps for prescribing opiods?
- assess whether pain is responsive to opiods
- screen patient for risk of dependency and monitor for misuse/addiction
- define & agree on successful outcome
- appropriate pain prescription (drug. dose, duration)
What is multi-modal drug therapy?
Combined therapies to manage pain
Give examples of neuropathic analgesics
- anti-depressants (amitriptyline)
- anti-convulsants (gabapentin, pregabalin)
- anti-arrythmics (lidocaine)
Others:
- ketamine
- capsaicin
- clonidine
- cannabinoids
What are the yellow flags in patients who need pain management?
Harmful beliefs - 4P’s
- progressive pathology
- passive:
> belief that pain and activity are harmful
> sickness behaviour: extended rest
> overprotective family or lack of support - dePression
- problems:
> social (withdrawal)
> work ( poor satisfaction, heavy work, unsociable hours, time off)
> financial
> legal claim/compensation
Why is psychological input helpful in pain management?
psychology deals with the normal brain under duress
Use STarT back screening tool
What are red flags in lower back pain?
- weight loss
- pmhx of malignancy
- fever
- signs of systemic inflammatory disease
- anatomical change
- possible #/history of trauma
- cauda equina syndrome
- neurological signs (radiculopathy)
What are procedural treatments of pain?
- epidural injections
What is ischaemic pain?
pain associated with lack of adequate blood supplyto active tissues (e.g. angina pectoris)
What causes the triple response?
The release of histamine following mild trauma to the skin
What causes the red reaction of the triple response?
Capillary dilatation mediated by histamine
What causes the flare in a triple response?
Redness in the surrounding area due to arteriolar dilatation mediated by axon reflex
What causes the wheal of the triple response
Oedema due to exudation of fluid from capillaries and venules mediated by histamine
What is the cardiovascular response to pain?
The cardiovascular system responds to the stress of unrelieved pain by increasing sympathetic nervous system activity which, in turn, increases heart rate, blood pressure and peripheral vascular resistance. The body will respond to the increase in activity by activating the parasympathetic division, which works to return to homeostasis
