ISSN 2398-2950      

Pain

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Introduction

  • It is impossible to prove that an animal feels pain but common sense suggests that animals experience pain in a similar 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

Chronic pain

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 (tough, 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 et al (2007) AAHA/AAFP pain management guidelines for dogs and cats. JAAHA 43 (5), 235-248 PubMed.
  • Robertson S A (2005) Assessment and management of acute pain in cats. J Vet Emerg Crit Care 15 (4), 261-272 VetMedResource.
  • Robertson S A (2005) Managing pain in feline patients. Vet Clin North Am Small Anim Pract 35 (1), 129-146 PubMed.
  • Robertson S A & Taylor P M (2004) Pain management in cats-past, present and future. Part 2. Treatment of pain-clinical pharmacology. J Feline Med Surg (5), 321-333 PubMed.
  • Taylor P M & Robertson S A (2004) Pain management in cats-past, present and future. Part 1. The cat is unique. J Feline Med Surg (5), 313-320 PubMed.
  • Lamont L A (2002) Feline perioperative pain management. Vet Clin North Am Small Anim Pract 32 (4), 747-763 PubMed.
  • Silngsby L S, Lane E C, Mears E R et al (1998) Post-operative pain after ovariohysterectomy in the cat: a comparison of two anaesthetic regimens. Vet Rec 143 (21), 589-590 PubMed.

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