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Pain: Underlying Mechanisms, Rationale for Assessment

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A-delta and C-fibers innervate joint nocioceptors; ... Basis for Joint and Bone Pain: ... (joint bomechanics) 'mechanical nocioceptors activated ... – PowerPoint PPT presentation

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Title: Pain: Underlying Mechanisms, Rationale for Assessment


1
Pain Underlying Mechanisms, Rationale for
Assessment
  • Jessie VanSwearingen, Ph.D, PT
  • Associate Professor
  • Department of Physical Therapy
  • University of Pittsburgh School of Health and
    Rehabilitation Sciences

2
PAIN
  • an unpleasant sensory and emotional
    experience.primarily associate with tissue
    damage or describe in terms of such damage or
    both.
  • Intl. Assoc. for Study of Pain

3
The Report of Pain
  • 3 components of the patients experience
  • sensory discriminative (localize, quality)
  • motivational / affective (emotional)
  • cognitive / evaluative (meaning)

4
The Report of Pain
  • Relation of pain and tissue damage
  • not consistent or constant
  • All pain is truly experienced
  • (-- a helpful clinician belief)

5
Terminology of Pain
  • Nocioception neural response related to
    potentially tissue damaging stimuli
  • Pain conscious experience of nocioception

6
Terminology of Pain
  • Experience of pain
  • dysthesia - experience abnormal noxious sensation
  • paraesthesia - abnormal nonpainful sensation
  • hyperpathia- exaggerated pain response to noxious
    or nonoxious stimuli)
  • allodynia - perception of nonoxious stimuli as
    painful

7
Terminology of Pain
  • hyperalgesia increased pain response to painful
    stimuli
  • hypoalgesia - decreased sensitivity to noxious
    stimuli
  • hyperesthesia and hypoesthesia - increase or
    decrease, respectively, in sensitivity to
    nonnoxious stimuli

8
Characteristics of Pain
  • Nocioceptive pain - directly related to the
    activation of peripheral nocioceptors
  • somatic - aching , squeezing, stabbing
  • visceral - cramping, knawing, rise and fall

9
Characteristics of Pain
  • Neuropathic - pain assumed to be related to
    aberrant sensory processing (PNS or CNS)
  • Deafferentation- sympathetic maintained
    alterations in peripheral transmission or central
    representation
  • burning, lancinating, electrical

10
Characteristics of Pain
  • Idiopathic - pain persisting without identifiable
    organic basis or excessive pain for organic
    processes
  • (presumes some clinical correlation)
  • (psychogenic - no clinical observations
    correlating with the pain)

11
Nocioceptive Pain
  • Activation of nocioceptors by tissue-damaging
    stimuli. Mechanisms
  • neurogenic inflammation - vasodilation
    inflammatory cells antidromic (polymodal)
    nocioceptor release of Substance P and others
    from nerve terminals
  • endogenous substances - directly activate
    nocioceptors - histamine, Subs. P, bradykinin,
    ACH, serotonin, K
  • prostoglandins - sensitize nocioceptors produce
    lower thresholds for noxious stimuli (also
    serotonin, ADP, NE, interleukin, NGF) role in
    development of chronic pain

12
Neuropathic Pain
  • Peripheral tissue injury leading to aberrant
    somatosensory processing?pathophysiologic
    changes, which sustain a pain experience.
  • Mechanisms
  • peripheral generators
  • sympathetic maintained
  • central (mechanism) generators

13
Neuropathic Pain Example Axonal Injury
  • 1) multiple axon sprouts ? neuroma
  • 2) axon sprouts ?spontaneous activity (peripheral
    generator)
  • pain
  • 3) neuromas sensitive - tenderness mechanical
    and chemical sensitivity
  • chronic pain
  • 4) ephases - spread of impulses in juxtaposed
    nerve fibers incl sympathetic nerves
    (sympathetic maintained)
  • 5) ectopic generation of impulses in DRG,
    ?transmitter release?dorsal horn neurons expand
    receptive fields (hyperalgesa)

14
Neuropathic PainCNS Activity
  • Increased activity in spinal cord, thalamus,
    cortex following peripheral nerve injury
  • central sensitization of neurons
  • abnormal feedback (sympathetic outflow? stimulate
    peripheral nocioceptors)

15
Clinical Events of Pain
  • Hyperalgesia
  • 1º -site of injury peripheral nocioceptor
    sensitization
  • 2 º -surrounding region peripheral and central
    mechanisms
  • central - hyperexcitable neruon activated by
    nocioceptor

Pain episodes with the same phenomena may not
have the same mechanism
16
Referred Pain
  • Phenomena
  • stimulation of peripheral nerve fascicles, report
    of pain throughout the extremity
  • pain from muscle or visceral injury accompanied
    by cutaneous hyperalgesia
  • convergence-projection theory
  • convergent input of nocioceptors from different
    sources on to the same projection neurons or
    central neurons

17
Basis for Joint and Bone Pain Joint Nocioceptors
  • 1. nocioceptors - polymodal cutaneous receptor
    c-fiber, unmyelinated (capsule, type IV)
  • 2. free- nerve endings - A -delta nocioceptors
    in intl and extl joint ligaments
  • 3. Synovium - small diameter, neuropeptide
    containing fibers
  • A-delta and C-fibers innervate joint
    nocioceptors
  • ? Also sensitive mechanical and chemical stimuli

18
Basis for Joint and Bone Pain Joint
Mechanoreceptors
  • 1. Large diameter, fast-conducting afferents,
    serving..
  • 2. Corpuscular receptors - low-threshold,
    dynamic receptors capsule outer layer -type I,
    subsynovial layer - type II, dynamic receptors on
    surface of joint ligaments
  • 3. Mechanically Insensitive Afferents (MIAs) -
    C-fiber afferents, become sensitive to mechanical
    stimuli only with inflamed joint

19
Basis for Joint and Bone Pain
  • Sleeping Nocioceptors (MIAs)
  • insensitive to pain or mechanical stimuli
  • become spontaneously active
  • active during non-noxious movement
  • enlarged receptive fields
  • (central targets unknown)

20
Basis for Joint and Bone Pain Spinal Cord
Mechanisms
  • Dorsal horn neurons
  • nocioceptive specific (NS)
  • wide dynamic range (WDR)

articular inputs
  • Basis for arthritic pain being
  • poorly localized
  • poorly discriminated
  • Basis for referred pain and hyperalgesia

21
Basis for Joint and Bone Pain Spinal Cord
Mechanisms
  • Noxious joint inputs reach cortical targets.
  • inputs from inflamed joints appear to take paths
    to widespread supraspinal targets
  • With persistent nocioceptive input, dorsal horn
    neurons ? in sensitivity
  • enhanced responsiveness
  • enlarged receptive fields

22
Basis for Joint and Bone Pain Spinal Cord
Mechanisms
  • With acute joint inflammation,
  • (sensitivity)
  • 1) dorsal horn neurons with little response to
    movement show large response
  • (enhanced receptive fields)
  • 2) respond to stimuli remote from the site of
    inflammation
  • 3) become spontaneously active

23
Basis for Joint and Bone Pain Somatosensory
Cortex
  • Chronic inflammation
  • receptive field changes
  • increased background activity
  • prolonged response to non-noxious stimuli
  • (reduced inhibition of pain afferents in the
    dorsal horn - decreased descending inhibitory
    pain projections with inflammation)

24
Neurogenic Inflammation
  • Example axon reflex
  • - localized vasodilation and exudation in
    response to an irritant
  • - intact sensory innervation
  • - mediated by release of neuropeptides from
    C-fiber terminals ?
  • change in vascular tone and permeability
    production of inflammatory cells immune response

25
Neurogenic Inflammation
  • Partially attenuated by Substance P depleter (eg
    capsaicin)
  • In Rheumatoid Arthritis, reduced neuropeptide Y
    (vasoconstrictor) in the sympathetic nerve
    terminals ?? no stop to the inflammatory
    response

26
Summary of Pathophysiology of Joint and Bone Pain
  • Chemical Nocioception
  • pain activation of nocioceptors
  • primary hyperalgesia - sensitization of
    nocioceptors
  • swelling, vascular response to neuropeptide
    release

27
Summary of Pathophysiology of Joint and Bone Pain
  • Mechanical Nocioception
  • (joint bomechanics) mechanical nocioceptors
    activated
  • primary hyperalgesia, sensitized mechano -
    nocioceptors
  • mechanoreceptors, incl remote musculotendinous
    site receptors, induce dorsal horn plasticity

28
Summary of Pathophysiology of Joint and Bone Pain
  • Secondary Hyperalgesia / Neuropathic Model
  • altered central pathways with chronic arthritis
    (both altered dorsal horn neurons responding and
    the pattern of the response)
  • changes in threshold for response
  • changes in projection targets
  • changes in the responsiveness
  • referred pain to other joints cutaneous areas
    and deeper tissues

29
Clinical Assessment of Joint and Bone Pain
  • Looking for
  • anatomic origin - define the tissue damage
  • mechanisms of pain production
  • associated disease

30
Clinical Assessment of Joint and Bone Pain
  • Finding a hyperalgesic joint in the region of
    pain
  • - likely to be the origin
  • - other signs crepitus, swelling (implies
    nocioceptors activated)
  • but in osteoarthritis mechanoreceptors could
    elicit mechanical nocioception and sensitize
    primary afferents..

Joints which dont move and joints that move
properly can be painful.
31
Clinical Assessment of Joint and Bone Pain
  • Recognizing Bone pain
  • (causes vascular, infection, neoplastic,
    metabolic)
  • not influenced by posture or movement
  • worse at night
  • well localized, over the painful site (eg
    vertebra)
  • (eg. Osteonecrosis osteoporotic fracture)

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