Pain physiology and analgesia Flashcards
Define pain
‘Pain is an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage’
Define nociception
Relay of noxious stimulus from the periphery to the central nervous system
What are the 3 types of acute pain?
Somatic
Visceral
Neuropathic
Describe somatic acute pain
- Chemical, thermal or mechanical stimuli to skin, muscles, bones etc
- Usually localised to injury site
- ‘Sharp, stabbing’ sensation – fast pain transmission
Define visceral acute pain
- Inflammation, ischaemia or distension of viscera
- Poorly localised, diffuse
- ‘Burning, dull’ sensation – slower transmitted C fibres
- Possible autonomic components e.g. vomiting, sweating, tachycardia
Define neuropathic acute pain?
- Primary lesion or dysfunction within nervous system
- May be localised or diffuse (depending on degree of nerve injury)
- ‘Burning, tingling’ sensation, may be intermittent
- Can become a component of chronic pain
What is hyperalgesia?
Exaggerated pain sensation in response to noxious stimulus
What is allodynia?
Perception of pain sensation in response to a normally non-noxious stimulus
What is spontaneous/idiopathic/functional pain?
Pain that arises in absence of detectable tissue or nerve injury
Which type of sensitisation may result in chronic pain?
Central
What is peripheral sensitisation?
- Increased responsiveness of nociceptors (result off a decreased activation threshold)
- Occurs with tissue damage and inflammation
- Causes primary hyperalgesia and allodynia at injury site
What is central sensitisation?
- Changes in membrane excitability and upregulation of post-synaptic receptors (especially NMDA receptors)
- Results from intense, prolonged and/or repeated nociceptive input
- Causes secondary hyperalgesia outside/adjacent
Why does acute pain rapidly alter an animals behaviour?
- to minimise damage
- optimise conditions for tissue healing
- stops when healing is complete
What type of pain is relatively easy to treat?
Acute pain
What type of pain is associated with tissue damage (or threat of this) due to surgery, injury or disease?
Acute pain
What is meant by chronic pain being maladaptive?
No useful function
Over what length of time does chronic pain persist?
Beyond the expected time of tissue healing
What are some consequences of poor acute pain management?
- unstable general anaesthesia
- poor animal welfare
- delayed recovery
- development of central sensitisation and chronic pain
What are the 4 stages of nociception?
- Transduction
- Transmission
- Modulation
- Perception
What is pre-emptive analgesia?
Administration of analgesics prior to noxious stimulus to maximise their effect, reduce sensitisation and enhance post operative analgesia
What is multimodal (balanced) analgesia?
Formulation of an analgesic plan that targets as many stages in the nociceptive pathway as possible
Where in the body does transduction, modulation and perception occur?
Transduction = nerve terminal Modulation = Dorsal horn of the spinal cord Perception = brainstem and cerebrum
What are the pharmacological options for management of acute pain?
N = NSAIDs O = opioids P = paracetamol (not in cats) L = local anaesthetics A = alpha-2 agonists N = NMDA antagonists
Which drug group are COX inhibitors?
NSAIDS
Which stages of transmission do NSAIDS effect?
Transduction, modulation and perception
Give some examples of opioids?
Butorphanol Buprenorphine Morphine Methadone Fentanyl
Which opioid has the longest duration of effect?
Buprenorphine
Can opioids be used in food producing animals?
No - but butorphanol can be applied via the cascade
What are the considerations of using pathidine?
Causes histamine release when administered IV, but it is the only licensed full Mu agonist in horses
Why is morphine more often used in horses over methadone even though it is not licenced?
In horses methadone causes a ‘twitchy’ response so morphine is often used instead
Which stage/s of the transmission pathway are affected by paracetamol?
Modulation
In which species can paracetamol not be used?
Cats - toxic at low doses
How do local anaesthetics work as analgesics?
Sodium channel blockers, inhibit transmission of all nerve types
What are some examples of local anaesthetics?
Lidocaine
Bupivacaine
Which local anaesthetic cannot be administered IV and why?
Bupivacaine
- cardiotoxic
What are the effects of alpha-2-agonists?
Analgesia, sedation and muscle relaxation
Which stage of transmission is affected by alpha-2-agonists?
Modulation
Why would you not want to use alpha-2 agonists in patients with some heart conditions?
Cause bradycardia
What are the sedative and analgesic effects of alpha 2 agonists?
Analgesia peaks later than sedation and the duration is shorter
What are some examples of drugs with NMDA antagonistic properties ?
Ketamine
Pethidine
Methadone
Nitrous oxide
Which stages of transmission are affected by NMDA antagonists?
Modulation and perception
What are some possible side effects of ketamine?
Muscle rigidity
Excitation
What are some non-pharmacological methods for acute pain management?
- Wound dressing and external coaptation
- Good nursing care e.g. warmth, stimulation, clean, empty bladder
Why is recognition of acute pain more challenging in veterinary species over humans?
- non-verbal
- behaviour indicators can be difficult to assess
- species/individual differences
What is the multidimensional composite pain scale commonly used in dogs?
Glasgow Composite Measure Canine Pain Scale (Short Form)
What is the Glasgow feline pain scale validated for assessing?
Acute pain in cats
How would the face of a pain free cat differ from a marked pain cat?
- Ear position will be upright and alert in pain free and tucked down in painful
- You can also see that cats tend to squint (orbital tightening) when painful and the muzzle becomes flattened and tense
What are some behavioural indicators in horses that show pain?
- decreased weight bearing
- pawing, flank watching, rolling
- restlessness, reduced appetite, reduced socialisation
How would a painful cow appear on assessment?
- low head position
- low ear position
- tense expression
- arched back
- lame
How would a painful sheep appear on assessment?
- Orbital tightening
- Lowered head and ear position
- Flehming
