Pain Flashcards
Nocicpetion
Sensory process that provides the signals that trigger pain
Pain
Feeling/perception of irritating, sore, stinging, aching, throbbing, miserable/unbearable sensations from a part of the body
Congenital analgesia
Don’t sweat, feel no internal/external pain, large scars
Nociceptors found in
Periphery as simple free nerve endings, terminate as naked, unmyelinated endings in dermis
Normal sensory receptors found in
Cell body in dorsal root ganglion
Tissue damage and inflammation leads to
Release of substances (prostaglandins, bradykinin, histamine) - sensitise peripheral nocicpetors and induce hyperalgesia
Types of nociceptor
Transduction of nociceptive stimuli occur at free endings of unmyelinated ‘C’ fibres and thinly myelinated ‘Ad’ fibres
Polymodal/only respond to 1
Physiological basis of nociceptive pain
Activation of peripheral nociceptors, transmission of impulses via A-delta and C sensory afferents to DH, projection to brain via ascending pathways > avoidance, emotion, withdrawal
Transcutaneous nerve recording in human (microneurography)
- Thermal stimuli applied to receptive field of cutaneous thermoreceptor and nociceptor
- Record afferent firing to response to incremental temperatures
- Graph plotting afferent firing frequency vs temp
Dissociated sensory loss/Brown-Sequard syndrome
Unilateral spinal lesion, loss of touch, pressure, vibration and proprioception on same side below lesion, pain opposite side
Phantom pain
Pain and touch sensations with no sensory inputs
Endogenous analgesia/pain modulation
Sensory inputs without pain sensations
Hyperalgesia
Increased pain, tissue that has already been damaged/inflamed is extra sensitive (a reduced threshold for pain, increased intensity of painful stimuli, spontaneous pain)
Allodynia
Touch-evoked pain
Acute pain
Trauma/injury, resolves with healing of underlying injury, protective function, assists wound repair
Chronic pain
Persists beyond normal time of healing, 3-6 months beyond onset, tissue damage, not protective, considerable suffering
Nociceptive pain
Tissue damage and painful stimuli, regulated by opioidergic system
Chronic nociceptive pain
Persists > neuropathic and mixed pain
Benefits of acute pain
Trauma response, protective (avoid further damage)
Adverse acute pain
Humanitarian, CV stress, resp compromise, hypercoagulation
Factors associated with changes in pain perception
Anxiety, depressed, gender, circadian variation, climate
WHO analgesia ladder
1 - morphine, fentanyl, hydromorphone, buprenorphine, methadone
2 - Codeine, dihydrocodeine, tramadol
3 - aspirin, NSAIDs, COX-2-inhibitor, acetaminophen
Opioids
- Presynaptically pain signal transmission
- Postsynaptic membrane hyperpolarised, decreasing probably of AP generation
Non-opioid analegsics - NSAIDS
- Act peripherally
- Act on nociceptive pain
- Inhibit cyclooxygenase, prostaglandin synthesis decreases
Side effects of NSAIDs
GI irritation/bleeding, renal toxicity, CV (cox-2)
Non-opioid analegsics - paracetamol
- Acts centrally
- Analgesic and antipyretic effects
- No anti-inflammatory action
- Inhibition of central prostaglandin synthesis
Paracetamol side effects
Toxic liver damage
Examples of weak opiods
Tramadol and codeine
Examples of strong opiods
Morphine and oxycodone
Efficacy of opiods
Nociceptive pain, less effective in chronic, partially in neuropathic pain
Mode of action of opiods
Activate endogenous analgesic system, stimulate receptors in limbic system eliminated subjective feeling of pain, affect descending pathways that module pain perception, reduce ascending pain signal transmission in spinal cord
Side effects of opioids
Nausea, vomiting, constipation, dizziness/vertigo, somnolence, dry skin, pruritus, resp depression
Best early warning sign of respiratory depression
Progressive sedation
Renal failure and opioids
Don’t use morphine/codeine - sedative metabolites accumulate, Fentanyl or Oxycodone are okay
Epidural analgesia
Directly into epidural space, catheter placed
Uses of epidural analgesia
- Postoperatively (thoracic, abdominal, groin/perineal, lower limb surgery)
- Labour pain
- Chronic pain
Benefits of epidural analgesia
- High quality pain relief
- Improved pulmonary function
- Reduced sepsis and chest infection
- Reduced cardiac morbidity
- Reduced vascular graft failure
- Reduced incidence of DVT
Epidural space
Potential space between dura and vertebral canal wall - connective tissue, fat and blood vessels and nerve roots and tissue in folds
Morphine
Regular immediate release (4 hourly), allow access for breakthrough pain
Celiac plexus block used for
Pancreatic carcinoma and upper abdominal neoplasia
Spinal opiods
Act at dorsal horn, lipophobic morphine reaches brainstem, lipophilic fentanyl remains segmentally localised (subarachnoid space)
Neuropathic pain
Spontaneous pain and hypersensitivity to pain in associated with damage to/lesion of NS
Causes of neuropathic pain
- Post herpetic neuralgia
- Painful diabetic neuropathy
- Trigeminal neuralgia
- Pain after CVA
- Post traumatic/post operative
Dysaesthesias
Unpleasant abnormal sensations ‘ants crawling on the skin’
Neuropathic pain prevalence
2-4% population
McGill Pain Questionnaire (MPQ)
78 ranked descriptors, time consuming but well validated
Drug therapy for neuropathic pain
- NSAIDs (poor)
- Antidepressants
- Anticonvulsants
- Opioids
- Membrane stabilising drugs
- Topical drugs
Antidepressants efficacy (tri-cyclic agents)
Neuropathic pain, complex regional pain syndrome, tension headache (serotonin uptake inhibitors ineffective)
Antidepressants mode of action
Inhibit neuronal reuptake of noradrenaline and serotonin (5-HT)
Side effects of antidepressants
Constipation, dry mouth, somnolence, abnormalities in heart rate/rhythm, insomnia, increased appetite
Anticonvulsants efficacy
Neuropathic pain
Anticonvulsants mode of action
- Gabapentin (binds to presynaptic voltage-dependent Ca channels)
- Pregabalin (interacts with special N-type Ca channels)
- Carbamazepine (blocks Na and Ca channels)
Anticonvulsant side effects
Sedation, dizziness, ataxia, peripheral oedema, nausea, weight gain
Carbamazepine, phenytoin
Na+ channel blockers > dizziness, sedation, rashes, liver damage
Valproate, clonazepam
GABA agonist
Loeser conceptualised pain
Four dimensions - pain behaviours, suffering, pain, nociception
