Understanding and recognizing cancer pain in dogs and cats - Veterinary Medicine
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Understanding and recognizing cancer pain in dogs and cats
They may not speak, but our patients with cancer can still tell us they're in pain. Are we getting the message? With careful observation and good client communication, we can identify pain. And with an awareness of the cancers and procedures known to cause pain, we can offer preemptive pain control.


While pain will normally follow a noxious stimulus, certain alterations occasionally result in increased sensitivity to non-noxious stimuli (hyperesthesia), an exaggerated painful response to mildly noxious stimuli (hyperalgesia), or an abnormal painful response to non-noxious stimuli (allodynia).2,6,11 Allodynia and hyperalgesia are most commonly encountered in conditions of untreated or undertreated chronic pain and result from peripheral and central alterations in the transmission, modulation, and integration of nociceptive stimuli. Breakthrough pain occurs when controlled chronic pain suddenly becomes more severe (flare-up).2,6,10,11 Various factors may lead to prolonged and exaggerated pain (hyperalgesia or allodynia). Examples are peripheral sensitization by various substances including prostaglandins, histamine, serotonin, bradykinin, and cytokines (collectively termed the inflammatory soup by certain authors6); central sensitization by other modulators, including glutamate and substance P; and decreased inhibitory modulation.2,6 The term wind-up refers to the temporal summation of painful stimuli in the spinal cord. It arises when chronic pain is untreated or undertreated and is mediated by N-methyl-D-aspartate (NMDA) receptor activation of the C fibers.1,2,6,11,12


An understanding of the principles of analgesia is fundamental to developing a pain control program. Preemptive analgesia is the administration of analgesic therapy before a patient is exposed to noxious stimuli. The goals are better pain control, lower anesthetic and analgesic needs, a smoother recovery, and a lower risk of establishing a chronic pain state.6,11,12 Another principle essential to pain control is multimodal analgesia. This concept takes into account the complexity of pain transmission and uses various methods or classes of drugs with different mechanisms of action to control pain.6,8,11,12 The goal is better pain control than if just a single analgesic agent was used. If we merge both concepts, it becomes obvious that preemptive analgesia is best achieved by combining drugs of various classes, such as a nonsteroidal anti-inflammatory drug (NSAID) with an opioid and an alpha2 agonist, in addition to the use of a local or regional block with a local anesthetic.

The World Health Organization (WHO) has proposed a three-step analgesic ladder for controlling mild, moderate, and severe pain in people.1-3,5-12 With that model, in which adjuvant therapy can be added at any level, mild pain is treated initially with nonopioid drugs, generally NSAIDs. If pain persists after treatment with a nonopioid or if the pain is moderate, a weak opioid (e.g. codeine or tramadol) can be added on the second step. If pain is not controlled with that combination or if the pain is severe, stronger opioids, preferably full mu agonists, are used for the third step of the ladder. It has recently been recommended that two more steps be added to the ladder for patients with advanced, difficult-to-control cancer pain.3 Techniques such as intrathecal administration of opioids, neurostimulation, and peripheral neuroablation can be introduced as a fourth step, and central neuroablative procedures can be added as a fifth step.3 Others have recommended using an analgesic reverse pyramid for certain veterinary patients.8,9 With this approach, when chronic pain is present and central sensitization is suspected, a multimodal approach to reverse such changes is used first, and then the doses of analgesics are gradually reduced or weaker classes of analgesics are given.8,9


One of the biggest problems in controlling pain in veterinary patients is its recognition by clinicians. Since our patients do not communicate what they feel to us, at least not in words, we must use other means to adequately detect pain. One essential component of pain recognition is adequate communication with the owner.5,6,8-12 Owners know their pets better than anyone else and can recognize abnormal behavior, which is a common and reliable indicator that something is wrong.13 Always believing owners who think their pets may be in pain is an important first step, and involving them in periodic reassessment is equally crucial. In general, a simplistic rule applies when clinicians are faced with patients that may be in pain: If an animal seems to be in pain, then it most likely is. A clinically useful way to confirm the presence of pain is to treat the patient for pain and observe clinical improvement.


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