what is the definition of pain
an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage
what are the two types of pain and what are their receptors
- immediate pain which is transmitted through A delta fibres
- Persisting pain which is transmitted through c fibres
what is a common consequence for diabetic patients in relation to pain
diabetic patients can get peripheral neuropathy which means they cannot feel their feet so commonly get problematic ulcers which can cause the foot to become gangrenous
A-alpha nerve fibre
motor fibres to muscles and proprioception
A-beta fibres
touch and pressure
A-gamma fibres
motor fibres to muscle spindles
A-delta fibres
temperature and pain
- myelinated
- sharp and localised pain
- fast conduction
- polymodal
- not usually visceral
B fibres
autonomic nervous system
C fibres
temperature and pain
- unmyelinated
- dull throbbing pain
- slow conduction
- polymodal
what are the 4 factors which summarise the physiology of pain
- transduction
- transmission
- modulation
- perception
what is transduction
conversion of a noxious stimulus (heat, mechanical, chemical) into an action potential in a nociceptor
- heat more than 45 degrees or less than 15
- chemical = K+, ATP, Bradykinin, histamine, substance P
- mechanical
transduction
nociceptors are the free nerve endings of A delta and C fibres
they respond to stimuli that will potentially cause damage to the tissue
- primary hyperalgesia (pressing on a bruise)
give examples of substances which activate nociceptors
- potassium
- protons
- serotonin
- bradykinin
- histamine
give examples of substances which lower the threshold for nociceptors
- prostanoids
- leukotrienes
- substance p
- CGRP
- Glutamate (MAIN)
No single “pain receptor” but glutamate binds to
- AMPA
- NMDA (sleeping but important!)
- G-protein couple receptors
The Neospinothalamic tract
which terminates in the ventral posterior lateral nucleus (VPL) is mainly composed of Aδ fibres.
The Paliospinothalamic
tract which terminates in the dorsomedial (DM) and intra laminar areas is composed of C fibres.
Descending Inhibition (3 mechanisms)
- GABA & Glycinergic interneurones
- Descending inhibition PAG-RVM-DH
- Endogenous opioids
+ DISTRACTION
What is the gate control theory
based on the theory of presynaptic inhibition of pain information produced by mechanical stimulation.
basically your nociceptor fibres transmit the pain signals to the dorsal horn but if someone rubs your leg then the mechanoreceptors that deal with this sensation synapse on an inhibitory neurone and therefore cut off this signal before it gets there (presynaptic inhibition which closes the gate to noxious information)
Pain perception
- Reticular system elicits an autonomic response
* Limbic system links perception of pain with mood
Visceral Pain
- Visceral nociceptors respond to distension or ischaemia
- Visceral primary afferent will activate multiple second order neurones
- Pain more diffuse (less well localised)
- Converge on second order neurones with somatic input
- Referred pain (convergence)
Associated features of pain
- Sweating
- Pallor
- Nausea
- Tachycardia
- Hypertension
step 1 on pain ladder - mild pain
simple analgesics
step 2 on pain ladder - moderate pain
use mild opioid like codeine or tramadol
continue the simple analgesics
step 3 on the pain ladder - severe pain
use strong opioid such as morphine
continue simple analgesics
medications for neuropathic pain
amitriptyline and gabapentin
Neuropathic Pain
A pain arising as a direct consequence of a lesion or a disease affecting the somatosensory system
• Allodynia
pain response to things that should not elicit pain such as touch
causes of neuropathic pain and what can be found on examination
Traumatic (phantom limb pain)
Diabetic neuropathy Postherpetic neuralgia Trigeminal neuralgia Post-stroke pain
changes in colour of limb to changes in sensation
dysaesthesia
unpleasant/abnormal sensation
hyperalgesia
over reactive pain to something that would normally be painful but is over dramatic
neuroglia
pain in the anatomical distribution of a nerve or nerves
Neuroplasticity behind chronic pain
- Prolonged inflammatory response results in decreased pain threshold in primary afferents
- Increased production of substance P & CGRP
- Recruitment of NMDA receptors • Wakes up WDR
- “wind-up” phenomenon
spinal cord changes to neuroplasticity in chronic pain
Spinal Cord - changes in gene & receptor expression in DRG & dorsal horn neurons
Risk Factors for Chronic Pain
female age genetics low socio-economic status occupational history of abuse compensation
modifable:
- anxiety and depression
- surgical approach
- attitudes
- communication
what is the Budapest criteria
- Patients must report continuing pain disproportionate to the trauma
- Patients must report at least one symptom in three of the four following categories:
(a) Sensory: hyperalgesia (that is, exaggerated pain to a painful stimulus, such as pinprick) and/or allodynia (that is, pain elicited by a normally non-painful stimulus, such as light touch)
(b) Vasomotor: skin colour and/or temperature changes/asymmetry
(c) Sudomotor/oedema: swelling and/or sweating changes or asymmetry
(d) Motor/trophic: weakness, tremor, dystonia, decreased range of motion and/or trophic changes/asymmetry involving nails, skin and/or hair - Patients must display one sign in two of the categories above
- Signs and symptoms must not be better explained by another diagnosis.
