Assistant Professor of Anesthesiology, Veterinary Medicine & Biomedical Sciences, Small Animal Clinical Sciences, Texas A&M University, College Station, TX, USA
The International Association for the Study of Pain (IASP) defines pain as "an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage" (www.iasp-pain.org/Taxonomy#Pain). Failure to recognize pain and provide optimal treatment may result in negative physiological changes including sympathetic nervous system activation, causing increased heart rate, vascular resistance, myocardial workload, oxygen consumption, respiratory rate, ileus, bronchoconstriction, and impair the endocrine system.1 Under-treatment of pain can also result in peripheral central sensitization and the development of chronic or "windup" pain. This sensitization lowers the pain threshold to both noxious (hyperalgesia) and innocuous (allodynia) stimuli.2
Oncologic, osteoarthritic, neuropathic, somatic, and visceral pain are the primary types of pain treated by veterinarians in clinical practice. The objective of this lecture is to discuss acute and chronically painful cases and how they can be effectively managed in practice.
Acute Pain
Acute pan serves as a protective mechanism to a noxious stimulus and is a result of tissue damage caused by trauma, surgery, or disease. Adequately managed, it resolves with the resolution of tissue damage. Inadequate treatment can lead to chronic and "wind-up" pain. Multimodal analgesia is an effective strategy against acute pain as it targets several sites along the pain pathway and allows lower doses of each drug, minimizing adverse effects. Acute pain scales used to assess how a patient responds to therapy include the CSU Acute Pain Scale, University of Glasgow Short Form Composite Measure Pain Score, and UNESP-Botucatu Multidimensional Composite Pain Scale (cats).
Case Example - Intestinal Foreign Body
Signalment: "Jaws" 2-year-old spayed female 30 kg Labrador retriever canine
Presenting complaint: Vomiting, decreased appetite, lethargic
Diagnosis: Intestinal foreign body requiring surgery (resection and anastomosis)
Pain type: Acute, moderate to severe, visceral and somatic pain
Recommended Therapy
Premedication - opioid (methadone 0.2 mg/kg IV or morphine 0.2 mg/kg IV or hydromorphone 0.05 mg/kg IV or fentanyl 5 mcg/kg IV).
Induction - midazolam (0.2 mg/kg IV) and ketamine (2 mg/ kg IV) and lidocaine (2 mg/kg IV) and propofol to effect (<2 mg/kg needed generally).
Maintenance - inhalant and oxygen and constant rate infusion (CRI) of MLK (morphine 0.1 mg/kg/hr + lidocaine 3 mg/kg/hr + ketamine 0.5 mg/kg/hr) or FLK (fentanyl 5–10mcg/kg/hr + LK same doses).
Alternative: Intermittent boluses of morphine, methadone, hydromorphone, or fentanyl as needed, but may create an unstable anesthetic plane.
Post-operative analgesia - Continue FLK or MLK at ½ the dose for 12–24 hours then titrate down every 6-12 hours depending on patients comfort. Alternative: Place Transdermal fentanyl solution or patch directly after surgery when normothermic. Continue Hydromorphone or buprenorphine IV as needed for the first 12–24 hours until patch appears affective.
Comments - Perform local regional techniques whenever possible. With these techniques you can avoid systemic adverse effects by reducing your inhalant and CRI doses dramatically. Recommended techniques: Bupivacaine incisional line block prior to surgery and/or lumbosacral epidural (morphine 0.1 mg/kg + bupivacaine 0.5 mg/kg). Note: total volume of LS epidural should equal 0.2 ml/kg, 0.9% saline may be needed for dilution. Do not exceed 2 mg/kg total dose of bupivacaine within a 6-hour period.
Chronic pain
Chronic pain is defined by the IASP as "pain without apparent biological value which has persisted beyond normal tissue healing time." Pain lasting greater than 3 to 6 months in the absence of ongoing tissue damage and with the development of allodynia or hyperalgesia due to central and peripheral sensitization ("wind-up") can also be considered chronic in nature.3,4 The pain itself can be dull, burning, and its origin can be difficult to localize. The most common clinical conditions associated with chronic pain include osteoarthritis, neoplasia, dental disease, aural disease, and vertebral and spinal cord disease.5 Adequate analgesic therapy in chronically painful conditions can be challenging because of difficulties with pain assessment, expense of analgesic therapy, bioavailability of oral analgesics, and owner compliance.5
Analgesic Management of Chronic Pain
The World Health Organization (WHO) recommends the use of an analgesic step ladder, the "WHO Ladder", when treating chronic pain. The three steps in this ladder differentiate therapy between mild (A), moderate (B), and severe (C) pain. The clinician should begin with step A to avoid unnecessary adverse drug effects and proceed to step B then C when necessary. Options for non-pharmacological therapy should be discussed with the owner and include massage, physical therapy, acupuncture, and transcutaneous electrical nerve stimulation.
WHO Ladder
Step A (mild pain): Well controlled with non-opioid therapy and potentially an adjuvant. Non-opioid therapy includes non-steroidal anti-inflammatories (NSAIDs). It is important that the veterinarian discusses with the owner the potential adverse renal, gastrointestinal, and hepatic effects associated with NSAIDs. The International Society of Feline Medicine and the American Association of Feline Practitioners have created guidelines on the use of NSAIDs in cats which includes but not limited to full examination prior to administration, routine monitoring every 3 to 6 months, dose based on lean body weight, feed moist food and ensure adequate water intake, titrate to the lowest possible dose, educate owners of adverse effects, administer with food, and stop the drug immediately with change in eating, drinking, or urination.6 Patients may tolerate one type of NSAID better than others and it is important to provide at least a 1-week washout period between NSAIDs. Deracoxib and meloxicam are relatively safe long-term NSAIDs in dogs with chronic pain.7,8 Adjuvants used include gabapentin or tramadol. Tramadol is not effective as a sole non-opioid analgesic in dogs. It provides adequate analgesia in cats, however its bitter palatability makes it difficult to administer.
Step B (moderate pain): Can be adequately controlled with a weak opioid, a non-opioid, and an adjuvant. Oral transmucosal (OTM) administration of buprenorphine to dogs and cats would be an example of a weak opioid. The dose required in dogs to produce analgesia (0.12 mg/kg) may make it financially impractical. Additional adjuvants include amantadine or amitriptyline. NSAIDs are still administered.
Step C (severe pain): Requires a strong opioid, a non- opioid, and an adjuvant to provide adequate analgesia. Stronger opioids include pure µ-opioid agonists such as fentanyl, morphine, hydromorphone, methadone, or hydromorphone. Methadone can be administered via oral transmucosal route in both dogs and cats. Other alternatives include lidocaine patches or an intravenous infusion of ketamine, dexmedetomidine, or lidocaine in combination with the opioid. This requires hospitalization and may not be practical in some situations.
Case Example - Chronic Osteoarthritis
Signalment: "Olaf" 14-year-old neutered male 5 kg domestic short-haired cat
Presenting complaint: Less active, no longer jumps to reach food, reluctant to use steps
Diagnosis: Bilateral osteoarthritis of the elbow
Pain type: Chronic, moderate to severe, somatic, and inflammatory pain
Recommended therapy A: Meloxicam 0.1 mg/kg PO on day 1. Titrate to lowest possible dose required. Suggested dose range 0.01–0.03 mg/kg q24h effectively managed OA in 85% of cats studied.9
Recommended therapy B: See therapy A. Buprenorphine OTM 0.01–0.02 mg/kg q6–12h
Or tramadol 1–2 mg/kg PO q12–24h. If needed: Gabapentin 5–10 mg/kg PO q12–24h. Both tramadol and gabapentin have an unpleasant taste.
Recommended therapy C: See therapy A and B. Amantadine 3–5 mg/kg PO q24h and/or amitriptyline 0.5–1 mg/kg PO 24 hours.
Additional therapies: Diet rich in omega-3 fatty acids, glucosamine chondroitin sulfate, manganese, avocado soybean unsaponifiable products, weight reduction, appropriate environment, acupuncture, low level laser, moist heat, physical therapy, and stem and regenerative cell therapy.10
Comments: Titrate NSAIDs to the lowest possible dose required to produce a good quality of life with minimal adverse effects. With evidence of mild renal or hepatic disease in patients start with lower dose (0.02–0.03 mg/ kg). Microlactin, an anti-inflammatory biological response modifier, can be administered at 200 mg/cat PO BID to substitute NSAIDs in patients with chronic kidney disease. Omeprazole (0.75–1 mg/kg PO q24 for <8 weeks) or Misoprostal (5 mcg/kg PO q8h) can be added for persistent GI problems. Deracoxib (1–2 mg/kg PO SID) would also be appropriate in dogs.
References
1. Wright BD. Clinical pain management techniques for cats. Clin Tech Small Anim Pract. 2002;17(4):151–157.
2. Moore SA. Managing neuropathic pain in dogs. Front Vet Sci. 2016;3:12.
3. Grubb T. Introduction: chronic pain. Top Companion Anim Med. 2010;25(1):1–4.
4. Grubb T. Chronic neuropathic pain in veterinary patients. Top Companion Anim Med. 2010;25(1):45–52.
5. Bell A, Helm J, Reid J. Veterinarians' attitudes to chronic pain in dogs. Vet Rec. 2014;175(17):428.
6. Sparkes AH, Heiene R, Lascelles BD, Malik R, Sampietro LR, Robertson S, et al. ISFM and AAFP consensus guidelines: long-term use of NSAIDs in cats. J Feline Med Surg. 2010;12(7):521–538.
7. Roberts ES, Van Lare KA, Marable BR, Salminen WF. Safety and tolerability of 3-week and 6-month dosing of Deramaxx (deracoxib) chewable tablets in dogs. J Vet Pharmacol Ther. 2009;32(4):329–337.
8. Moreau M, Dupuis J, Bonneau NH, Desnoyers M. Clinical evaluation of a nutraceutical, carprofen and meloxicam for the treatment of dogs with osteoarthritis. Vet Rec. 2003;152(11):323–329.
9. Gunew MN, Menrath VH, Marshall RD. Long-term safety, efficacy and palatability of oral meloxicam at 0.01–0.03 mg/kg for treatment of osteoarthritic pain in cats. J Feline Med Surg. 2008;10(3):235–241.
10. Rychel JK. Diagnosis and treatment of osteoarthritis. Top Companion Anim Med. 2010;25(1):20–25.