What is pain?

  • It is an unpleasant sensation, with a protective function
  • Definition:
    • unpleasant sensory & emotional experience associated with actual/potential tissue damange

What influences the pain you feel?

  • Individual coping strategies
  • Attitudes
  • Previous pain experience
  • Cognitive understanding
  • Cultural factors
  • Current emotional state

Onset

  • Sudden
    • Mechanical
    • Vascular
    • Traumatic
    • Toxic
  • Gradual
    • Infective
    • Inflammatory
    • Neoplastic
    • Degenerative
    • Endocrine
    • Nutritional

Aggravating/Relieving factors

  • Movement
  • Respiration
  • Ingestion (food)
  • Type
  • Posture

Character/ Quality of pain

  • Dull, aching
  • Burning
  • Gnawing
  • Colicky
  • Crampy
  • Throbbing
  • Stabbing
  • Sharp, tearing
  • Radiating
  • Gripping
  • Shooting
  • Excruciating
  • Sharp lancing

Types of pain

  • Sharp/Bright VS Burning/Dull
  • Fast VS Slow pain
  • A-delta fibres VS ‘C’ fibres
  • Withdrawal VS Immobilization
  • Cutaneous VS Visceral pain
Cutaneous Visceral
Sharp Dull,aching
Well-localised Poorly localised
Not associated with ANS symp Associated with ANS symp
Not referred to other site Freq referred to other site

Referred pain

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Examples:

  • Cardiac pain
    • pain in the chest, axilla, arm (T1, T2)
  • Gall bladder pain
    • pain in the shoulder (C3, C5)
  • Pain of appendicitis
    • pain in umbilicus
  • Ureteric pain
    • pain in testes (loin)
  • Irritation of nerve endings in meninges and cranial vessels
    • headache

Why?

  • Dermatomal rule
    • Pain is referred to a structure, which is developed from the same dermatome from which the pain producing structure is developed.
  • Convergence theory
    • afferents from skin and visceral structure converge on the projection neuron and impulses generated by them ascend in the same tract – but the brain has ‘learnt’ that impulses usually come from the skin and interpret as such
  • Facilitation theory
    • A collateral from the visceral afferent converges on the projection neuron, together with the cutaneous afferent, partially depolarizing the projection neuron
    • A subthreshold stimulus (eg light touch) from the cutaneous neurone can produce an action potential

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Postinjury & Neuropathic pain

  • Persists while the injury heals
    • when the nerves are intact
  • Neuropathic pain
    • Persists even after the injury has healed
    • if the nerves are damaged
    • Difficult to manage, resistant to analgesic
  • Hyperalgesia
    • Minor pain can produce an exaggerated response
  • Allodynia
    • Innocuous pain such as touch can produce pain

Headache

  • Brain itself has very few pain receptors
    • but structures nearby have pain receptors
      • blood vessels at base of brain
      • meninges
      • muscles of scalp
      • neck & face
      • paranasal sinuses
      • eyes & teeth
  • Mediated by
    • mechanical receptors
      • eg stretching
    • chemical receptors
      • eg histamine, serotonin
  • Impulses carried by
    • 5th & 9th cranial nerves
      • by upper cervical cranial roots
  • Types of headache
    • Chronic/recurrent (benign)
      • muscle tension/ tension headache
    • Subacute (over days/weeks)
      • serious intracranial disease
        • encephalitis
        • viral/chronic meningitis
    • Single, severe episode
      • subarachnoid haemorrhage (SAH)
      • migraine
      • acute meningitis
    • Pressure headaches
      • headaches of raised intracranial pressure
      • intracranial mass lesions displace and stretch the meninges & basal blood vessels
        • pain is provoked when these structures are moved physically either by the mass/by raised ICP
        • made worse by coughing, straining, sneezing

Pain mechanism/process

  • Stimulus
    • chemical/mechanical/thermal/biological
  • Pain producing substances (PPS)
    • kinins
    • K+
    • histamine
    • substance P
    • ANG II
    • CCK
    • NE
    • Capsaicin
  • Receptor
    • free nerve endings
      • non specific (no adequate stimulus – stimulus for which the receptors are most specific)
      • adaptation – very little/none
        • image
      • Types
        • Mechanoreceptors
          • in skin
          • definite threshold levels
          • respond to strong stimuli (pin prick, hot temp)
          • fast pain
          • warming signal – withdrawal from noxious stimulus
        • Polynodal nociceptors
          • present in almost all tissues
          • no definite threshold level
          • respond to tissue damange
          • slow pain
          • pain causes immobilization of affected part
            • to prevent further damage
  • Sensory nerves (afferents)
    • A-delta fibres
      • via lateral spinothalamic tract
      • together with temperature sense
    • C fibres
      • via spinoreticulothalamic tract
  • Perception
    • via thalamus
  • Postcentral gyrus (Somatic sensory area 1 – SSA1) & SSA2
    • integration centre
      • postcentral gyrus (SSA1)
      • somatic sensory area 2 (primary & secondary sensory homunculi) / cerebral cortex

Pain pathways
Read from Dr Joachim’s notes on the pathways

  • Neospinothalamic tract
  • Paleospinothalamic tract

1) Neospinothalamic tract (lateral pain system)

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  • Afferent: A-delta fibres
  • To SSA1 & SSA2 of opposite side
    • via lateral spinothalamic tract

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2) Paleospinothalamic tract (Medial pain system)

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  • Afferent: C fibres
  • Goes to limbic cortex (emotions)
    • via spinoreticulothalamic pathway

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Sensory homunculus

In somatic sensory area 1

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  • Postcentral gyrus (Broadmann’s area 1, 2, 3)

In somatic sensory area 2

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  • In Sylvian fissure/lateral sulcus

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Subcortical perception & affect

  • pain can be felt in the absence of the cerebral cortex
    • but it is required for discriminative, meaningful & exact interpretation & some of the emotional components
    • can feel, but can’t understand it
  • the built-in unpleasant affect is mediated by the cingulate gyrus

Role of thalamus

  • Understand type, cause, site, amount of pain
  • Meaningful interpretation of pain is lost in thalamic syndrome
    • damage to posterior thalamic nuclei
    • attacks of prolonged, severe, extremely unpleasant pain
      • spontaneous/in response to trivial stimuli

Chronic pain syndrome – Phantom limb

  • evokes a sensation which is projected to the site of receptor
    • eventhough it may no longer exist
    • eg. amputated limb

Causalgia

  • spontaneous burning pain long after seemingly trivial injuries
  • often accompanied by
    • hyperalgesia
      • abnormally increased pain sense
    • allodynia
      • Pain from stimuli which are not normally painful
  • associated with reflex sympathetic dystrophy
    • shiny skin in the affected area
    • increased hair growth
    • overgrowth of NA sympathetic fibres
  • relieved by alpha adrenergic blockade

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Modulation of pain

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Gatecontrol Theory

  • SG cells in dorsal horn
  • They inhibit impulse transmission in the projection neuron
  • A-delta and A-beta fibres stimulate it
    • stimulation of inhibition!
    • Therefore: inhibition of impulse transmission
  • C fibres inhibit it
    • inhibition of inhibition!
    • Therefore: no inhibition of impulse transmission
    • therefore pain lasts longer

Substances

  • Endogenous opioids
    • Enkephalins, dynorphins
    • a type of analgesic

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  • NMDA receptor blockers
    • esctasy pill

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  • Analgesics
    • narcotics
    • NSAIDs
    • Opiates
    • NMDA receptor inhibitors

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Pain surgery

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Extra:

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