Pain Flashcards
What is the official definition of pain?
An unpleasant sensory and emotional experience associated with, actual or potential tissue damage
What is the NICE ladder of analgesia?
1 - aspirin, paracetamol or ibuprofen
2 - codeine
3 - Morphine, fentanyl
What are the main side effects of analgesics?
- Constipation
- Nausea + vomiting
- Depression
- Drowsiness
- Changes in cognition
- Tolerance/dependence/misuse
- Sexual dysfunction/loss of libido
- Osteoporosis
- Increased risk of falls
- Renal and hepatic damage
What is analgesic stewardship?
To improve patient outcomes, reduce analgesic related harm and ensure cost-effective use of analgesics to provide optimal pain management
What is codeine?
Naturally occuring analgesic, around one tenth the potency of morphine
but codeine retains most of its activity after oral administration, whereas morphine suffers from considerable first pass metabolism
What is heroin?
All other things being equal, fast access to the brain and rapid offset from the receptor leads to higher abuse potential (better ‘rush’)
Heroin accesses the brain more quickly than morphine due to higher lipophilicity (pharmacokinetics)
Compared to morphine, heroin is about 2 times as potent as an analgesic, but heroin has very low affinity to opioid receptors and so,….
What is morphine?
Active ingredient of opium was isolated by Friedrich Serturner
By 1923 chemists were making analogues and derivatives, in particular to retain the analgesic properties but remove the undesirable side-effects.
Nalorphine
It is an opioid agonist and antagonist. It is used to reverse opioid overdose and in a challenge test to determine opioid dependence.
Dual action is a result of action at two opioid receptors –
a) antagonism at the mu opioid receptor and
b) agonism at the kappa opioid receptor
Benzomorphans
Benzomorphans are used in the development of new drugs and pharmacological tools. They have been studied for their potential to treat drug abuse and moderate to severe pain. Some benzomorphans have a lower risk of drug dependence than other opioids
They have a simplified structure, removal of one/two of the rings from morphine but activity is largely retained
Methadone
No fused rings in the structure. Opioid activity is maintained. Pharmacology typical of a mu opioid receptor agonist.
L-(R)-methadone has the majority of the opioid activity but used clinically as a racemate
Methadone has similar pharmacology to morphine. Both bind and activate the mu opioid receptor to similar levels.
Methadone has much greater lipophilicity than morphine – leads to it being widely distributed around the body.
Has a much longer duration of action in vivo only need to be taken once per day and gives a milder, but more protracted, withdrawal syndrome.
Used for maintenance of opioid addicts.
What are the 2 enkephalins?
The endogenous opioids.
Leu-enkephalin
Met-enkephalin
What is the drug used to knock out elephants?
Etorphine
What is buprenorphine?
Mu opioid partial agonist. Analgesic and treatment for opioid dependence - plateau of effect is reached
Highly lipophilic and almost irreversibly binds to the mu opioid receptor, so has a long duration of action (pharmacokinetic and pharmacodynamic properties working together).
Long duration of action helps minimise any withdrawal syndrome after chronic use/treatment.
What is Diprenorphine?
Mu opioid antagonist. Used as Revivon, to reverse effects of opioid sedation in animals
Cebranopadol
Under development
High affinity and efficacy at mu and nociceptor receptors)
good binding but not high enough efficacy at mu and NOP
What is BU08028?
BU08028 is a drug which acts as an extremely potent partial agonist at both the μ-opioid receptor and nociceptin receptor.
Initial results not promising – longer duration than morphine, but otherwise quite similar
A full agonist in vivo (we wanted a partial agonist)
Less potent than buprenorphine
What was the process for SAR for buprenorphine?
- Had a lead
- Compared the lead to a model for NOP activity
- Made changes based on this and developed a compound with the desired pharmacological profile
- Disappointing results in first set of in vivo assays
- Much more interesting in later in vivo assays
- Now working with a pharmaceutical company to try to move similar compounds to clinical testing
What is nociception?
The physical process of detection and transmission of damaging or potentially noxious stimuli
What are nociceptors?
Structures which detect noxious stimuli
What is the process of nociception?
Noxious stimuli
Primary transduction
Channel opening
Secondary transduction
Change in membrane voltage
Depolarization and action potential generation
Transmitter release
Second order neurone response
What is hyperalgesia?
Increased response to a noxious stimulus
What is allodynia?
Painful responses to a non-noxious stimulus
What is supraspinal control of pain?
Brain stimulation in animals inhibited nociceptive spinal neurones
Similar stimulation sites reduced behavioural response to noxious stimuli
In humans brainstem stimulation caused pain relief
What causes itch?
Afferent input is via Aδ and C fibres from free nerve endings
Inflammation, particularly histamine, can cause it
Analgesics don’t inhibit itch
To cure an itch, you scratch it (mechanical, almost nociceptive stimulation)
Strong central component
What are the 5 desirable effects of analgesia?
Analgesia
Euphoria
Constipation
Sedation
Cough suppression
What are the 7 undesirable effects of analegesia?
Respiratory depression
Constipation (methylnaltrexone)
Sedation
Nausea & vomiting
Tolerance
Itching
Dependence
NSAIDs
Most widely used therapeutic agents
Over 50 types available eg. aspirin, ibuprofen, diclofenac, (paracetamol)
Anti-inflammatory, anti-pyretic, analgesic
All effects related to decreased, prostaglandin synthesis
What are the disadvantages of NSAIDs?
multiple side effects
severe gastric irritation
kidney disorders
paracetamol overdose
What is neuropathic pain?
Pain unrelated to peripheral nociception
Sometimes called pathological pain – serves no purpose
Peripheral nerve damage
(eg. Diabetic neuropathy)
Peripheral nerve terminal damage or infection
(eg. Post-herpetic neuralgia)
Spinal damage
Thalamic stroke
Generally don’t respond to opioids / NSAIDs
What are the analgesic approaches to neuropathic pain?
Tricyclic antidepressants (eg. imipramine), antiepileptic drugs (eg. gabapentin, pregabalin), tramadol
For neuropathic pain – mechanism of action largely unknown
Capsaicin cream
For neuropathic pain and others (eg. Arthritis)
Cannabinoid receptor agonists
Multiple sclerosis – under development for non-neuropathic pain
Glutamate receptor blockers (MK801)
Block afferent transmission and ‘wind-up’– severe side effects
Neurokinin receptor blockers / CGRP receptor blockers
Block afferent transmission and ‘wind-up’ - under development
Vanilloid receptor blockers / ATP receptor blockers
Sodium channel blockers
Block detection of noxious signals
Under development
What is a primary headache?
No detectable underlying cause
Tension-type headache
Migraine
Cluster headache
What is a secondary headache?
Caused by underlying condition
Extracranial: e.g. sinusitis, otitis
Intracranial: e.g. vasculitis, meningitis, abscess, tumour
Whats a tension headache
Pain comes and goes
Feeling of pressure or tightness
Not associated with other symptoms
Bilateral localization
Variable duration, episodic
Patient remains active or prefers to rest
What is a migraine
Typically patient rests in quiet dark room
Episodic severe headaches for 4-72h,
Pulsating, throbbing quality
60-70% unliateral location common
Moderate or severe intensity
Worsened by routine physical activity
> 1 episode per month in 75% of ‘migraineurs’
Plus one of the following:
Nausea and/or vomiting
Photophobia and/or phonophobia (sensitivity to light/sound)
May occur with aura (‘classic migraine’) or without aura (‘common migraine’)
Migraine is the second leading cause of disability
What is a cluster headache
Abrupt onset
Continuous excruciating pain
Associated with tearing, congestion, rhinorrhea, pallor, sweating
Unilateral localization
Duration 0.5-3h, many per day
Patient remains active
prevalence of tension headaches
Lifetime risk of experiencing tension type headache is 70-80%
About 50% of adults aged 40 years experience episodic tension-type headache
Prevalence is higher in women than men
Most patients treated with OTC medicines and do not seek medical attention
Episodic – infrequent episodic, frequent episodic, chronic
Pathophysiology unclear
Theory of increased muscle tension unproven – often posterior neck muscles
Episodes of headache are not associated with nausea or vomiting.
Photophobia or phonophobia may be present (but not both)
how do you treat a tension headache
1st - paracetamol, NSAIDs
2nd - Aspirin + paracetamol + caffeine
Medication overuse headache
Continued use of painkillers causes headache – mechanism unknown
Worldwide prevalence 1-2% of population affected
Headache present > 15 days/month
Drugs causing overuse headaches
Paracetamol, aspirin and non-steroidal anti-inflammatory drugs on 15 or more days per month
Triptans, opioids, ergots or combination analgesic medications on at least 10 days per month
Prevalence of migraine and sex differences
Approximately 10-15% of adult population
* Pre-puberty 6% in both males and females
* Post-puberty, rises to 18% in menstruating women
* Menstrual migraine attacks triggered by falling oestrogen levels
* Post-menopause, migraine frequency falls in many women
What is aura?
Focal neurological disturbance preceding or accompanying headache may include
Motor features
Sensory disturbance, typically visual
Occurs regularly in 15% of migraine patients
Linked to CSD:
Slowly propagating wave of depolarisation – triggered by K+ disturbance?
=> Decrease in regional cerebral blood flow
What are the causes of migraine pain
Genetic Factors: 50% of migraine sufferers have 1st degree relative
Cortical Spreading Depression (CSD)
Wave of depolarization followed by a period of suppression across the cerebral cortex.
Can spread and lead to altered blood flow
Neurotransmitters
Serotonin: Involved in blood vessel tone and pain perception. Low levels during.
Dopamine: Involved in the initiation of migraines
Glutamate: Elevated levels of glutamate found and contribute to neuronal hyperexcitability.
Hormonal changes eg estrogen is thought to influence the activity of neurotransmitters and may trigger migraines.
Stress: Can increase the release of stress hormones, which in turn can trigger changes in brain activity and vascular tone.
Dietary Factors: Certain foods (e.g., chocolate, cheese, caffeine, red wine, and cured meats) contain substances that can either directly influence neurotransmitter activity or trigger changes in blood flow.
Irregular sleep patterns
flashing lights, weather changes
The three theories of migraine pain
Vascular theory
Implicated intracerebral vasoconstriction, followed by extracerebral vasodilatation, as cause of headache
Neuronal theory
Cortical spreading depression (CSD) – an advancing wave of neural inhibition progresses slowly over the cortical surface at 2mm/min - leads to ionic balance disturbance and local blood flow reduction
Inflammation theory
Activation of trigeminal nerve terminals in the meninges and extracranial vessels is primary event in a migraine attack- triggers pain directly
Induces inflammatory changes through the release of neuropeptides and other inflammatory mediators from sensory nerve terminals
How to the triptans work?
The triptans are selective serotonin receptor agonists, with high affinity for the 1Band 1D
1Breceptors are on smooth muscle cells of blood vessels and cause vasoconstriction
5HT1Dreceptors lie on trigeminal nerve terminals and their stimulation blocks activity
Triptans taken in many different ways
The 4 prophylactic migraine treatments
b-blockers
antiepileptic drugs
antidepressants
ca channel block
What’s on the horizon in migraine treatment ?
CGRP antagonists
e.g. Gepants are effective, no vasoconstriction BUT risk of liver toxicity
Three anti-CGRP mAbs have been approved recently for use in NHS:
fremanezumab for prevention of migraine
erenumab
galcanezumab
Glutamate receptor antagonists
Cluster headache: treatment
Acute
subcutaneous sumatriptan - rapid and effective
nasal triptan as optional alternative
Prophylaxis
Verapamil, calcium channel blocker can be considered, off-label use
Verapamil reduces cardiac output, slows heart rate…..
