What is chronic pain?
Chronic pain is an unpleasant sense of discomfort that persists or progresses over a
long period. In contrast to acute pain that arises suddenly in response to a specific injury and is usually treatable, chronic pain persists over time and is often resistant to medical treatments.
We have previously discussed chronic pain caused by osteoarthritis. The following are scenarios that cause chronic pain of a non-osteoarthritic nature:
The treatment of the above conditions or scenarios is guided by the understanding of how the disease develops, progresses and behaves, in other words, its pathophysiology. In this article, let us focus on Cancer Pain.
What is cancer?
Cancer refers to abnormal cells that divide uncontrollably within the body of either an animal or human being leading to failure of major organs and have the ability to infiltrate and spread to normal body tissues throughout the body. Cancer is one of the leading causes of death in the world in both humans and animals.
What causes cancer pain?
Cancer pain can be from cancer growing into or destroying nearby tissue. As a tumour grows, it can press on nerves, bones, organs, or release chemicals that can cause pain, or the body can react to the chemicals causing pain. Treatment of cancer helps the pain in these situations. However, cancer treatments, including surgery, radiation and chemotherapy, also can cause pain.
How is cancer pain treated?
There is a range of ways. One is to remove the source of the pain through surgery, chemotherapy, radiation or other treatment. If this isn't possible, pain medications to control the pain are used. These medications include:
1) Therapeutic Approach
a) NSAIDs (Non-Steroidal Anti-Inflammatory Drugs) Some of these drugs have some degree antineoplastic (anti-cancer) effects in both humans and dogs and appear to be mediated through the upregulation and overexpression of COX-2 enzymes by some neoplasms (tumours).
Table 1: Non-Steroidal Anti-Inflammatory Agents (NSAIDs)
Selected NSAIDs Data for Dogs and Cats
Drug Species Route Dose (mg/kg)
Aspirin Dog Intravenous 44mg/kg
Cat Oral 2.5mg/kg
Carprofen Dog Oral 2mg/kg
Subcutaneously 2mg/kg
Cat Intravenous 4mg/kg
Celexocib Dog Intravenous 5mg/kg
Deracoxib Dog Oral 1-5mg/kg
Cat Oral 1mg/kg
Etodolac Dog Oral 10-15mg/kg
Firocoxib Dog Oral 5mg/kg
Ketoprofen Dog Intravenously, 2mg/kg once, then orally 1mg/kg
Intramuscularly, daily
Subcutaneously
Cat Oral 1mg/kg
Meloxicam Dog Oral 0.2mg/kg
Oral 0.1mg/kg
Piroxicam Dog Oral 0.3mg/kg
Cat Oral 0.3mg/kg
Robenacoxib Cat Subcutaneously 2mg/kg
Dog Subcutaneously 2mg/kg
Tepoxalin Dog Oral 10 - 20mg/kg
This site provides a useful calculator to convert laboratory values, dosages, and other measurements to SI units.
b) Opioids While long-term use of oral opioids in animals with cancer pain is limited, canine patients may benefit from codeine or hydrocodone as these drugs have the most favourable pharmacokinetic profile in dogs. Newer extended-release oral and transmucosal opioids may ultimately play a greater role in palliative care and breakthrough cancer pain in pets. Opioids are generally derived from morphine and are potent pain relievers. Examples include morphine, papaveretum, oxycodone, pethidine, hydromorphone, methadone, butorphanol, buprenorphine, meperidine and fentanyl. Most of these drugs are controlled substances because of their addictive potential.
c) Neuromodulatory Agents
Table 2: Neuromedulatory Agents
Neuromodulatory agents Drug Species Route Dose (mg/kg)
Gabapentin Dog/Cat Oral 10-30mg/kg
Pregabalin Dog Oral 3-4mg/kg
Cat Oral 1-2mg/kg
Amitriptylin Dog Oral 1-2mg/kg
Cat Oral 0.5-1mg/kg
Tramadol Dog Oral 5mg/kg
Amantadine Dog Oral 3-5mg/kg
Acetaminophen + hydrocodone Dog Oral 10 mg/kg
Intravenous Constant Release Dog Intravenous 0.25-0.5 mg/kg
Infusion of ketamine
Intravenous Constant Release Dog Intravenous 1mg/kg
Infusion of lidocaine
This site provides a useful calculator to convert laboratory values, dosages, and other measurements to SI units.
d) Bisphosphonates These compounds may palliate osteosarcoma (bone cancer) related pain by decreasing osteoclast activity (bone-forming activity) and inhibiting calcium and phosphorus dissolution, and appear most effective when administered as part of multimodal therapy. Pamidronate is the bisphosphonate most commonly used in dogs. IV infusions are administered every 3 to 4 weeks in patients whose owners elect to forego surgery and chemotherapy. Anecdotally, 60% of dogs respond favourably, and the dosing cycle is repeated until the drug is no longer effective for bone pain. Nephrotoxicity (kidney poisoning) is a dose-limiting adverse effect.
e) Lidocaine Patch Anecdotal experience suggests pain relief with the use of lidocaine patches applied to the skin over the site of the osteosarcoma. Patches are considered safe because they elicit very low plasma levels. However, they must be secured properly in order to prevent ingestion by the patient.
f) Energy-Based Biophysical Modalities These modalities (e.g., therapeutic laser, shock wave therapy, electromagnetic field) are generally considered contraindicated in neoplasia due to possible adenosine triphosphate (ATP) production and activation of cell division.
2) Palliative Care
Where pet owners have opted for palliative care — pain management and disease control versus amputation or chemotherapy—inadequate pain control, rather than the disease itself, will probably be the terminal event leading to euthanasia. Once a collaborative decision is made between the veterinarian and the pet owner that pain can no longer be sufficiently managed, humane euthanasia should quickly follow.
In these difficult cases, it is important to access the entire pain management arsenal because undermanaging these patients' pain is inhumane and results in death (euthanasia). OSA and other bone cancers warrant a multimodal polypharmacy approach to treatment while paying attention to the potential for adverse drug reactions and interactions.
Further reading
References
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