Pain & Pain Mechanisms

• Pain

  an unpleasant + emotional experience associated w actual/potential tissue damage ore described in terms of such damage
• Classifications of Pain

  acute

  sub-acute

  recurrent

  chronic

  nociceptive

  neuropathic (peripheral)

  nociplastic
• Sensation & Duration - Transient Pain

  transient pain = short duration, mild, rarely damage to tissues
• Sensation & Duration - Early Sensitisation

  acute - rapid onset w in 1-3 days

  sub-acute - onset w in 3-5 days, recovery phase can take up to 3 months
• Sensation & Duration - Late Sensitisation

  recurrent

  chronic - condition lasting 3+ months
• Nociceptive Pain

  pain that arises from actual/threatened damage to non-neural tissue + is due to the activation of nociceptors

  pain confined to specific anatomical region, intensity may vary

  somatic/visceral pain

  acute/chronic

  predictable + consistent load response
• Mediators of Nociceptive Pain

  nociceptors r specialised receptors that dectects stimuli that may cause harm to the body, which may be mechanical, chem or thermal

  A& fibres mediate sharp localised pain

  C fibres mediate dull + burning pain

  
• Neuropathic (Peripheral) Pain

  pain caused by a lesion/disease of the peripheral somatosensory nervous system

  the nerve itself has been damaged

  pain spread since onset + now in a regional distribution, perhaps in line w a nerve pathway, following myotomal + dermatomal patterns

  stimulus response = unpredicatble

  differentiate w neuropathy
• Nociplastic Pain - Altered Nociception

  pain that arises from altered nociception despite no clear evidence of actual/threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease/lesion of the somatosensory system causing the pain
• Nociplastic Pain - Altered Functions

  can be defined as chronic pain leading towards an altered nociceptive function

  it results in increased sensitivity from the altered function of sensory pathways in the periphery + CNS

  not a daignosis but a descriptor of pain experienced

  encompasses pain from stereotypical terms like dysfunctional pain/medically unexplained syndromes

  e.g. complex regional pain syndrome, chronic low back pain, IBS
• Nociceptive Pain Characteristics

  predictable load to pain relationship

  no neuro involvement

  can inc somatic referred pain

  primary/secondary hyperalgesia (increased pain from stimulus - nervous system overreacts in response to something painful)

  allodynia (pain due to a stimulus that doesn't usually provoke pain - clothing touching skin)
• Neuropathic (Peripheral) Pain Characteristics

  pain in a peripheral nerve pathway

  lancinating (piercing/stabbing)

  often severe

  can inc cutaneous nerves/nerve roots (radiculopathy - radicular pain)

  symptoms of radiculopathy may inc paraesthesia (pins + needles), anaesthesia (numbness) + dynaesthesia (abnormal feeling when touched)

  can inc vascular neuropathy (vascular + blood problems decreasing oxygen supply to peripheral nerves)
• Nociplastic Pain Characteristics

  poor load to pain relationship

  persistent

  central sensitisation

  multituse of psychosocial factors
• Gate Control Theory of Pain

  a spinal cord mechanism in which pain signals can be sent up to the brain to be processed to accentuate (focus) the possible perceived pain or attenuate (reduce) it at the spinal cord itself
• Gate Control Theory of Pain - Gates

  the gate is the mechanism where pain signals can be let thru/restricted

  gate = open, pain signals can pass thru + will be sent to the brain to perceive the pain

  gate closed, pain signals will be restricted from travelling up to the brain + the sensation of pain won't be perceived
• Gate Control Theory of Pain - Pain Stimulus

  if someone experiences a painful (noxious) stimulus, the application of a non-noxious (soothing/light rubbing) stimulus can help activate the gate control mechanism + reduce the pain
• Pain Gate Mechanism

  located in the dorsal horn of the spinal cord

  the interneurons w in the dorsal horn r what synapse to the primary afferent neurons + r where the gate mechanism occurs

  the dorsal horn modulates the sensory info that is coming in from the primary afferent neurons
• Pain Gate Mechanism - Dorsal Horn

  3 primary afferent neurons of the pain gate mechanism

  
• Pain Gate Control Mechanism - A-B Fibres

  A-B fibres, large diameter, have a quick transmission of impulses due to their mylination

  activated by non-noxious stimuli like light touch + pressure

  
• Pain Gate Mechanism - A (Alpha) Fibres

  A (Alpha) fibres, smaller diameter, thinly myelinated

  stimulated by noxious stimuli like pain + temp, specifically sharp, intense + tingling sensations
• Pain Gate Mechanism - C Fibres

  C fibres, similar to A (Alpha) fibres, have the slowest transmission of impulse since they're not myelinated

  activated by pain + temp, namely prolonged burning sensations
• Pain Gate Mechanism - Closing of Gate

  if the interneurons in the substantia gelatinosa r stimulated by the non-noxious large diameter A (Beta) fibres, an inhibitory response is produce + there r no pain signals sent to the brain = pain gate = closed

  
• Pain Gate Mechanism - Opening of Gate

  when the interneurons r stimulated by the smaller diameter A (Alpha) or C fibres, an excitatory response is produced

  pain signals r sent to the brain, these can be modulated, sent back down thru descending modulation + perceived as varying amounts of pain
• Referred Pain

  referred pain = pain felt in a location different from where the actual tissue damage or irritation is occurring

  where the brain misinterprets pain signals, leading to pain being perceived in a different area than its origin

  occurs due to the interconnected network of nerves
• Factors Influencing Referral of Symptoms

  strength of stimulus

  position in the dermatome

  depth (the deeper the structure, the more vague the referral)

  nature of the structure
• General Rules of Pain Referral

  unilateral somatic + neuro structures - unilateral segmental (e.g. dermatomal) referral

  • generally refers distally e.g. down the arm/leg
  • occupies all/part of the dermatome

  central somatic + neuro - central, central unilateral (e.g. buttock/scapular pain or bilateral symptoms

  referred over many segments (multidegmental referral) cos signals can cross over