Control of Cancer Pain in Veterinary Patients

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Control of cancer pain is within the capabilities of most veterinarians and is achievable in most animal patients that have cancer with techniques that are currently available. Great satisfaction can be derived from not only treating the pet's cancer but its pain. Incorporating pain management into oncology practice is good for the well-being of the pet, the owner, the staff, the veterinarians, and the practice.

Section snippets

Importance of alleviating pain

The alleviation of pain is important for physiologic and ethical reasons.6, 7 Briefly, pain can induce a stress response in patients that is associated with elevations in corticotropin, cortisol, antidiuretic hormone (ADH), catecholamines, aldosterone, renin, angiotensin II, and glucose, along with decreases in insulin and testosterone. These changes can result in a general catabolic state with muscle protein catabolism and lipolysis, in addition to retention of water and sodium and excretion

Assessment of pain

Assessment of pain in animals can be difficult and frustrating. Understanding types of pain and their causes can be helpful. Often, veterinarians need to rely on the experience in people to help define the pain in animals. Technicians and other staff members are usually the ones who experience the postoperative period more than the doctors. Pain assessment typically is delegated to these staff members. Recognition and assessment of pain is the first and probably the most difficult step in

Drugs and techniques for alleviation of pain

Throughout this issue, doses are mentioned in the text. It can be assumed that the doses are appropriate to dogs and cats unless otherwise noted.

Drug treatment is the cornerstone of cancer pain management. It is effective and affordable for most patients and owners. The general approach to pain management should follow the World Health Organization ladder, which is a three-step hierarchy (Fig. 1).3 Within the same category of drugs, there can be different side effects for individuals.

Nonsteroidal anti-inflammatory drugs

Nonopioid analgesics include such drugs as carprofen, etodolac, deracoxib, meloxicam, tepoxalin, firocoxib, aspirin, carprofen, ketoprofen, and acetaminophen (Table 1).17 All except acetaminophen are considered NSAIDs. Despite the low anti-inflammatory activity of acetaminophen, it possesses beneficial effects of analgesia; minimal risk for bleeding in thrombocytopenic patients; decreased gastrointestinal effects; and synergism with opioid analgesics, such as codeine. Acetaminophen may also be

Opioids

Opioids are the major class of analgesics used in the management of moderate to severe cancer pain. They are most effective and predictable and have low risk associated with them.24 The most common parenteral opioids used in small animals are morphine, hydromorphone, oxymorphone, fentanyl, codeine, meperidine, buprenorphine, and butorphanol (Table 2). Parenteral opioids should be used in the perioperative period and should be discontinued when a patient can be switched to oral medication.

α2-Agonists

Xylazine, medetomidine, and dexmedetomidine are three α2-agonists approved for use in small animals in the United States. They are noncontrolled parenteral agents and provide excellent visceral analgesia but only for 20 minutes to 2 hours.36, 37, 38, 39 Their effects can be virtually completely reversed with yohimbine or atipamezole, respectively. The α2-agonists should not be the first or sole choice in providing analgesia around the time of surgery to patients that have cancer because they

N-methyl-d-aspartate receptor antagonists

Ketamine has been used for many years as an induction agent to general anesthesia in normal and compromised patients. It has been well established that ketamine provides reasonable somatic but poor visceral analgesia.42 Ketamine has been identified as an N-methyl-d-aspartate (NMDA) receptor antagonist. NMDA receptors are believed to play a role in the processes leading up to central sensitization and wind-up. As an NMDA receptor antagonist, ketamine reduces postoperative pain and cumulative

Tranquilizers

A concern that frequently arises with pain management is concurrent tranquilization and sedation. Most of the drugs used by veterinarians usually produce concurrent sedation. As mentioned previously, opioids have the greatest potential of producing dysphoria instead of sedation. Dysphoria becomes more likely when cats are administered canine doses of opioids and when a patient is already experiencing high anxiety in the hospital. Dysphoric patients can sometimes be treated simply by petting and

Anticonvulsants

Gabapentin is a structural analogue of γ-aminobutyric acid (GABA)49 and was originally introduced as an antiepileptic drug. The mechanism of action of gabapentin is unclear and elusive. Although gabapentin is related to GABA, it does not seem to have any analgesic effect at GABA receptors.

Several rat studies have investigated the effects of gabapentin on signs of neuropathic pain, such as hyperalgesia and allodynia. Other studies indicate a role for gabapentin in decreasing incisional pain and

Tricyclic antidepressants

Tricyclic antidepressants, such as amitriptyline, clomipramine, and imipramine, block the reuptake of serotonin and norepinephrine in the central nervous system (see Table 3). They also have antihistaminic effects. These drugs have been used in human patients for the treatment of chronic and neuropathic pain at doses considerably lower than those used to treat depression.50 Despite the lack of studies verifying the use of tricyclic antidepressants in this manner, clinical experience by many

Local anesthetics

The use of local and regional anesthetic techniques in small animals was common in the early twentieth century. There has recently been increased interest in these techniques, probably because of their ability to provide pre-emptive analgesia and decrease wind-up.51, 52 Local anesthetic techniques can be used instead of general anesthesia in selected cases or, more commonly, in combination.

The most commonly used local anesthetics include lidocaine and bupivacaine. Lidocaine has a short onset

Other pain-relieving modalities

Local or whole-body radiation can enhance analgesic drug effectiveness by reducing metastatic or primary tumor bulk.57 The radiation dose should be balanced between the amount necessary to kill tumor cells and that which would affect normal cells. Mucositis of the oral cavity and pharynx can develop after radiation to the neck, head, or oral cavities, resulting in impaired ability to eat and drink. Mucositis therapies include analgesics, green tea rinses, sucralfate, 2% viscous lidocaine, and a

Specific pain problems

When developing a plan for alleviating pain in a patient that has cancer, it helps to have a paradigm to follow. A simple flow chart (Fig. 2) can help with the sequence of activities related to pain assessment and management.69 This flow chart emphasizes the use of multiple modalities, beginning therapy with the least invasive methods and advancing treatment to meet the patient's needs. Although not all types of pain can be addressed here, pain relief should be considered achievable by

Surgical case examples

The following generic examples, in conjunction with the flow chart, present useful approaches to specific types of pain encountered in oncology practice. The examples recommend specific techniques for the procedure rather than a complete analgesia program. Implicit in the recommendations is an appropriate opioid and an NSAID if not contraindicated.

  • Lateral thoracotomy

    • Intercostal nerve block

    • Interpleural (intrapleural?) local anesthetic

    • Opioid epidural

  • Sternotomy

    • Interpleural (intrapleural?) local

Summary

Control of cancer pain is within the capabilities of most veterinarians and is achievable in most animal patients that have cancer with techniques that are currently available. Once veterinarians and technicians gain a good knowledge base about pain and its therapy, pain control should be achievable by following these simple ABCs:

  • A.

    Assess the pain. Ask for the owner's perceptions.

  • B.

    Believe the owner. The owner sees the pet each day in its own environment.

  • C.

    Choose appropriate therapy following the

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