43 – Acute and Chronic Pain Management in Horses

Question 1

What are the consequences of pain?

Answer 1

• Anxiety or fear
• Behavioural changes
• Contralateral limb laminitis
• Ileus: cecal impactions
• QOL concerns

Question 2

**Pain pathway

Answer 2

1. Transduction: touching something hot or cold
2. Transmission: brain gets pain message and transmits it to CNS for appropriate response
3. Modulation: brain and dorsal horn of SC decides how much to react to transmission
4. Perception: life experience and cultural environment influence this (ex. males are less pain tolerant)

Question 3

**How do you recognize and assess pain in horses?

Answer 3

• Stiffly backwards ears
• Orbital tightening
• Tension above eye area
• **Prominent strained chewing muscles
• Mouth strained and pronounced chin
• Strained nostrils and flattening of profile

Question 4

**Obel laminitis grading scale

Answer 4

• Grade 1: shifts weight at rest, sound at walk
• Grade 2: stilted at walk, turns with great difficulty, but forelimb can be lifted
• Grade 3: reluctant to walk, 1 forelimb can only be lifted with great difficulty
• Grade 4: only moves if forced

Question 5

Drugs used for mild pain

Answer 5

• NSAIDs

Question 6

Drugs used for moderate pain

Answer 6

• NSAIDs
• Gabapentinoids
• Opioids
• Alpha-2 agonists
• Local anesthetics
• CRI

Question 7

Drugs used for severe (treatable) pain

Answer 7

• Opioids
• Opioids + alpha-2 agonists
• Local anesthetics
• CRI
• Surgery

Question 8

‘Drugs’ used for severe (uncontrollable) pain

Answer 8

• Euthanasia

Question 9

NSAIDs effects

Answer 9

• Anti-inflammatory
• Anti-pyretic
• Analgesic

Question 10

Examples of NSAIDs in horse med

Answer 10

• Colic
• Flunixin meglumine (VISCERAL PAIN)
• Phenylbutazone (MUSCULOSKELTAL PAIN
• Ketoprofen
• Firocoxib

Question 11

Flunixin meglumine (effects and side effects)

Answer 11

• Side effects
  o GI, renal, intestinal healing delayed
• Anti-endotoxic
• Anti-inflammatory
• Prevents intra-abdominal adhesion formation

Question 12

Phenylbutazone (effects and side effects)

Answer 12

• Musculoskeletal pain: acute and chronic
• Can cause right dorsal colitis: consider weight of patient, RESTRICT owner access
  o Typically, clients overdose (issue if given for a long time!)
• Side effects
  o GI, renal

Question 13

Fibrocoxib (type and effects)

Answer 13

• COX 2 selective
  o Spares the GI and renal systems
  o Less potential complications/side effects
• Osteoarthritis pain
• *Long term pain management
• Need to use the horse formulation (used to use dog formulation)

Question 14

Acetaminophen

Answer 14

• NOT an NSAID
• Adjunct therapy
• Analgesic
• Antipyretic
• Dose 30mg/kg PO BID
• Best if used in conjunction with NSAID
• *laminitis, lameness

Question 15

What are gabapentinoids mechanism of action for pain?

Answer 15

• Changes to VG calcium channels
• Changes to NDMA receptor expression and binding
• *gabapentin: poor oral bioavailability in horses
• Ex. pregabalin

Question 16

Pregabalin

Answer 16

• Better oral bioavailability in horses
• Chronic pain: laminitis, trauma
• Dose 2-4mg/kg PO BID – TID
• Can take a couple of days to see affect
• Usually start at high dose then taper
• Can alter mentation (ex. small animal=not as excited to see owner)

Question 17

*Alpha-2 agonists examples and uses

Answer 17

• Xylazine, detomidine, romifidine
• *Sedatives
• Visceral analgesics
• Reduces GI motility
• CV effects (not a big issue in most cases)
• *synergism with opioids (ex. alpha-2 agonists + butorphanol)=when they work the best!

Question 18

Alpha-2 agonist sedation effect via

Answer 18

• Activation of CNS alpha-2 receptors

Question 19

Xylazine uses

Answer 19

• Short half life (~30mins)
• Moderate to severe pain
• Subjectively assess pain severity (‘diagnostic aid’)
  o If only lasts 15mins=horse is probably more painful then you think

Question 20

Detomidine duration of action

Answer 20

• 1-1.5 hrs
• If still not working=add butorphanol

Question 21

What is the most commonly used opioid in horse?

Answer 21

• Butorphanol
  o Kappa agonist, mu antagonist

Question 22

Butorphanol uses

Answer 22

• Analgesic
• Mild to moderate visceral pain (NOT severe pain)
• Sedative
• Reduces GI motility
• Analgesic dose: 0.1-0.2 mg/kg q2-4hrs
• *cost prohibitive
• *good for standing procedure (reduces ‘excitability’ due to xylazine)

Question 23

What are the other opioids used in horses?

Answer 23

• Buprenorphine
• Morphine (full mu agonist)
• Fentanyl patch (full mu agonist)
• Hydromorphone (full mu agonist)

Question 24

Buprenorphine in horses

Answer 24

• Partial mu agonist
• Longer duration of action
• Local anesthesia: include in nerve blocks
• Not used often, but can be if needed

Question 25

Morphine in horses

Answer 25

• Intramuscular, CRI, epidural
• Side effects can be severe if IV

Question 26

What are the side effects with morphine IV in horses?

Answer 26

• Excitement
• Disorientation
• Ataxia
• Ileus
• If epidural: ‘irritated’: rub hair away
• *can be administered with alpha-2 agonist or acepromazine

Question 27

NMDA antagonists: ketamine

Answer 27

• Decreases glutamate binding the NMDA receptor in dorsal horn
  o ‘knocks’ out memory of pain
• *acute and chronic pain (block central sensitization)
• Intramuscular (low dose), *CRI or epidural
• Side effects:
  o Excitement (not great for fracture repair), stiff walking

Question 28

Lidocaine uses (MOST COMMON)

Answer 28

• **Potentiates analgesic actions of opioids and alpha-2 agonists
• Used locally and systemically
• Prokinetic GI motility (usually works within first 24hrs)
  o Treatment for post-op ileus
  o Anti-inflammatory
• Decreases requirements for inhalant and infectable anesthetics
• Concurrent administration with flunixin allows for decrease in flunixin dose (improves intestinal healing)
• *used for colic surgeries CRI
• 5-20 mins onset

Question 29

Mepivacaine duration of action

Answer 29

• 2-3 hours

Question 30

Bupivacaine duration of action

Answer 30

• 6hrs
• Ex. used for lameness exams

Question 31

Routes of administration in horses

Answer 31

• Parenteral
• IV or IM
• CRI
• Trans-dermal: fentanyl
• Epidural

Question 32

What drugs are given IV or IM?

Answer 32

• NSAIDs
• Alpha-2 agonists
• Butorphanol
• Morphine
• Buscopan

Question 33

What drugs are given CRI?

Answer 33

• Butorphanol/morphine
• Lidocaine
• Ketamine
• Dexmedetomidine

Question 34

What drugs are used for epidural and what is the location?

Answer 34

• Ketamine
• Morphine
• Alpha-2 agonists
• *C1-C2
• *epidural catheter (coccygeal vs. cervical)

Question 35

Laminitis

Answer 35

• Common (rotation of coffin bone with capsule=separation of lamina)
  o Sinking or rotation of P3
  o Inflammation has no place to go=challenging to control the pain
• Don’t need to know specific drugs used

Question 36

How do you decide what to use?

Answer 36

• Acute vs. chronic pain
• If NSAID not controlling pain=add another medication
• *multimodal
• Treat before onset of pain always
  o Pre-op analgesia or local blocks
• Colic pain: surgical option?
• Laminitis: how long has it been treated for?
  o Adjunct therapies
  o Treat underlying cause

Question 37

What are some adjunct therapies for laminitis?

Answer 37

• Icing feet
• Pads
• Deep bedding (sand)

Question 38

What might be some underlying causes of laminitis?

Answer 38

• Endocrine disease
• Sepsis
• Contralimb

Question 39

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