Canis ISSN: 2398-2942

Pain

Contributor(s): Prof Stephen Greene, Jo Murrell, Polly Taylor

Introduction

  • It is impossible to prove that an animal feels pain but common sense suggests that animals experience pain in the same way as man: they have similar physiology and pharmacology; they have some similar reactions to same noxious stimuli; they avoid repeating a painful experience.
  • Pain relief is an essential part of animal welfare.

Pathogenesis

Etiology


Acute pain
  • Trauma, eg head injury, surgery.
Chronic pain
  • Skin disease/ear disease.
  • Degenerative joint disease.
  • Periodontal disease.
  • Neoplasia.

Pathophysiology


Nociception
  • Noxious stimulus → transmission of impulses in response, even if animal unconscious.
Perception of pain
  • Awareness if conscious, not if unconscious/anesthetized.
  • Specific receptors for each stimulus/sensation (touch, sight, hearing) → structurally distinct pathways (but pain pathways less distinct), viz. small myelinated A-delta and unmyelinated C fibers, with other pathways co-opted.
Physiological pain
  • Normal protective transient sensation → localized injury → uses 'pain pathways' (high threshold nociceptors).
Pathological (clinical) pain
  • Pain without stimulus, from normally innocuous stimulation (allodynia).
  • More widespread injury/inflammatory response.
  • No specific receptors - depends on changes in the nervous system induced by the nociceptive activity.
Mechanism of pathological pain
  • Disruption of normal selectivity of nervous system.
  • Nociception/pain sensation occurring through activity of nerve tracts that normally transmit only mechanical pressure, for example.
  • Occurs as a result of physical changes to the nociceptive system and other sensory pathways.
  • Nervous system is plastic - not hard-wired.
  • Sensitization - same stimulus produces more neuronal activity.
Peripheral changes
  • Decreased threshold in A-delta and C nociceptors.
  • Increased sensitivity of sensory receptors.
  • Release of chemicals from injury - excite nerve endings and increase sensitivity.
  • Release of excitatory chemicals from efferent sympathetic nerve endings.
Central changes
  • Input from C fibers changes response of spinal cord dorsal horn cells.
  • Receptor fields expand, dorsal horns respond to other stimuli.
  • Increased excitability of dorsal horn cells.
  • Sympathetic reflexes affected - drive injured afferents - vicious circle.

Diagnosis

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Treatment

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • American Animal Hospital Association; American Association of Feline Practitioners; AAHA/AAFP Pain Management Guidelines Task Force Members, Hellyer P, Rodan I, Brunt J, Downing R, Hagedorn J E & Robertson S A (2007) AAHA/AAFP pain management guidelines for dogs and cats. JAAHA 43 (5), 235-248 PubMed.
  • Capner C A, Lascelles B D X & Waterman-Pearson A E (1999) Current British veterinary attitudes to perioperative analgesia for dogs.​ Vet Rec 145 (4), 95-99 PubMed.
  • Holton L L, Scott E M, Nolan A M, Reid J & Welsh E (1998) Relationship between physiological factors and clinical pain in dogs scored using a numerical rating scale. JSAP 39 (10), 469-474 PubMed.
  • Holton L L, Scott E M, Nolan A M, Reid J, Welsh E & Flagherty D (1998) Comparison of 3 methods of pain scoring used to assess clinical pain in dogs. JAVMA 212 (1), 61-66 PubMed.


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