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Pain
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Introduction
  • 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.
Practical approach
  • Assume that pain occurs; therefore it must be treated.
  • Essential to know:
    • Signs of pain.
    • When analgesics are usually needed.
    • What drug/dose/route is most effective Pain: management.

Presenting signs
  • Increase in abnormal behavior patterns.
  • Decrease in normal behavior patterns.
  • Decreased weight.
  • Decreased food intake.
  • Decreased water intake.
  • Decreased activity.
Pathogenesis Top

Etiology

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

Pathophysiology

Nociception
  • Noxious stimulus right_arrow 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) right_arrow 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 right_arrow localized injury right_arrow 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 Top

Clinical signs
  • Increased abnormal behavior patterns.
  • Decreased normal behavior patterns.
    Tip Species specific - need to learn specific behavior signs for each species, eg dog, cat, horse.
  • Decreased weight.
  • Decreased food intake.
  • Decreased water intake.
  • Decreased activity.

Treatment Top

Standard treatment

Analgesia
  • Combinations of:
    Either Opioids Analgesia: overview.
    Or Non-steroidal anti-inflammatory drugs (NSAIDs) Analgesia: overview.
    Or Local analgesics Analgesia: overview.
Supportive treatment
    • Support injured area, eg Robert Jones bandage.
    • Treat underlying disease.
    • Reduce inflammation and edema (corticosteroids).
    • If acute abdominal pain, treat the disease - spasmolytics, alpha-2-agents. Avoid morphine and other powerful analgesics before diagnosis made for fear of masking signs.
    • Warmth, but do not overheat.
    • Comfortable bedding.
    • Drain bladder.
    • Oxygen.
    • Food/water.
    • Tender loving care and attention to alleviate boredom and fear of a strange environment.

Monitoring
  • Return of normal behavior - indicating adequate pain relief.
    Tip Listen to the owner; they know what the animal is like normally.
    Tip Listen to nurses; they spend time with the animal.
  • Use a scoring system to give objective 'measurement' of pain.

Subsequent management

Sequelae Top

Sources Top
Publications
Refereed papers
  • Capner C A, Lascelles B D X & Waterman-Pearson A E (1999) Peri-operative analgesia in dogs. Vet Rec 145 (4), 95-99.
  • 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 , 469-474.
  • 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 , 61-66.

Vetstream contributor(s)
  • Dr Stephen Greene DVM MS DipACVA , University of Illinois, Veterinary Medical Teaching Hospital, 1008 West Hazelwood Drive, Urbana, IL 61802, USA.
  • Dr P M Taylor MA VetMB PhD DVA MRCVS University of Cambridge, Department of Clinical Veterinary Medicine, Cambridge CB3 0ES, UK.
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