Pain Flashcards
Definition of pain (from 2019 IASP definition):
An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage.
Or described in terms of such damage
Allodynia:
Pain resulting from a stimulus that would not normally be painful
Hyperaesthesia
Increased sensitivity to stimulation
Hypoaesthesia
Decreased sensitivity to stimulation
Hyperalgesia
Increased or exaggerated response to a normally painful stimulus
Hyperpathia
Abnormal pain response to stimulus applied to an area of decreased sensitivity
Repeated stimulus causes pain (think Chinese water torture)
What are the cellular mechanisms of actions of the opioids?
Receptor: Gi protein coupled
Intracellular pathway: inhibit production of cAMP causing closure of Ca channels. This results in hyperpolarisation of cell and decreased firing
Where in the spinal column are opioid receptors located?
Peri-acqueductal grey matter (PAG)
Rostral ventral medulla (RVM)
How do opioid receptors act in the spinal column?
Increase serotonin and noradrenaline levels in the spinal column (5-HT>NA)
Activate descending inhibitory control pathway
Where is most pain modulation done?
Laminae I & II
List some side effects of IT opioids:
Itch Sedation N&V Urinary retention Constipation Sweating Delirium/confusion Respiratory depression
What factors will increase the risk of post operative respiratory depression following administration of IT opioids?
Choice of IT agent: low dose lipophilic agent (fentanyl) will cause early depressoin, whereas hydrophilic agent may cause early or late resp depression Increasing age Positive pressure ventilation Long acting sedatives concurrently given Opioids via another route Pre-existing respiratory disease
Define chronic pain
Pain that persists longer than expected time of tissue healing
Usually longer than 3 months
Serves no purpose
Requires input from MDT
What systems are involved in pain modulation?
Segmental inhibition
Endogenous opioid system- endorphins, enkephalins
Descending inhibitory system
List symptoms of NEUROPATHIC pain
Hyperalgesia Burning Shooting Stabbing Hyperpathia
How does amitriptyline work in treating neuropathic pain?
Serotonin and noradrenaline reuptake inhibitor
Metabolised to nortriptyline which is more selective NA reuptake inhibitor
Enhances descending inhibition
Enhances opioid effectiveness
(Possibly affects Na channels and NMDA receptors)
Define CRPS
Severe continuous pain
Accompanied by sensory, vasomotor, sudomotor, motor/trophic changes
Pain is restricted to a region - not anatomical or dermatomal
Pain is disproportionate to inciting event
What are the criteria required for diagnosis of CRPS?
Budapest criteria
Continuing pain disproportionate to the inciting event
Other diagnoses excluded
One symptom in 3 of 4 categories
One sign in >2 categories at the time of examination
List risk factors for development of Complex Regional Pain Syndrome
Female sex
Period of prolonged immobilisation
Younger age (although not children so much)
Traumatic insult
Perhaps more common in upper limb compared to lower limb
What features may you find on examination for CRPS?
BUDAPEST CRITERIA:
Pain disproportionate to inciting event
1. Sensory - allodynia; hyperalgesia
2. Vasomotor - temperature asymmetry, skin colour changes, skin colour asymmetry
3. Sudomotor/oedema - oedema, sweating changes or asymmetry
4. Motor/trophic - decreased range of motion, motor dysfunction/weakness/tremor; hair loss/nail changes
No other diagnosis to explain symptoms/signs
What drug treatments are there for CRPS?
Regular analgesia Anti-neuropathic drugs - gabapentin, pregabalin, amitriptyline, duloxetine IV ketamine Corticosteroids Capsaicin (with caution)
What NON-drug treatments are there for CRPS?
TENS Spinal cord stimulator Sympathetic block Somatic nerve block Trigger point injection CBT Neuromodulatory techniques
Define persistent postoperative pain
Pain >2 months or longer than would be expected
Pain despite complete tissue healing
Other causes excluded
Pre-existing conditions excluded
List some common surgical procedures that are associated with persistent postoperative pain
Amputation Thoracotomy Mastectomy LSCS CABG Hip and knee arthroplasty Vasectomy
List some risk factors for development of persistent postoperative pain
Female sex
Increasing age (?maybe younger age as well?!?)
Repeat surgery
Radiation therapy to area/chemotherapy
Pre-operative pain/anxiety
Psychological disposition - anxiety/catastrophising/hypervigilance
What pathological changes occur at spinal cord level during the transition from acute to persistent postoperative pain?
Persistence of nociceptive input to dorsal horn
Central sensitisation via:
-‘Wind up’- temporal summation at the second order neurones due to NMDA receptor activity
-Firing of wide dynamic range neutrons in response to non-nociceptive stimulus
-Increased ion channel expression
-Decreased threshold to fire
-Greater influx of Na with action potentials
Decreased dorsal horn inhibitory neurotransmitters
How is Mg thought to work in the management of pain?
Mg used to keep NMDA receptors blocked and thereby hopefully stops wind up
Ketamine also does similar role as NMDA receptor antagonist
(When NMDA receptors open, large increase in pain transmission)
Define phantom limb pain
Perception of pain or discomfort in a limb that is no longer there
Episodic, in short bouts
Unchanged cortical representation of amputated limb (the brain thinks it is still there)
Incidence of 60-80%
May begin within 24 hours; usually within 1 week
Higher risk if severe pain pre-operatively in the limb
