Understanding and recognizing cancer pain in dogs and cats
Pain is common in people with cancer, with the reported incidence varying between 30% and 90%; a higher frequency of pain is reported with more advanced disease.1-4 Pain negatively affects quality of life as well as many important physiological functions, so controlling it in all patients should be a top priority. It is said that more than 70% of human patients suffering from cancer pain can be relieved with an opioid-based regimen.1-4
Similar data on cancer pain and its treatment in companion animals do not exist, but a conservative estimate is that at least 50% of veterinary cancer patients experience some degree of pain. While certain veterinary patients with cancer pain may be relieved adequately by treating the underlying malignancy, other patients will require symptomatic therapy to improve their quality of life, the primary objective of veterinary oncology.
To adequately manage cancer pain in dogs and cats, it must be recognized early, and patients should be frequently assessed for pain by both the attending clinician and the pet owner. Many barriers remain for the appropriate treatment of cancer pain in animals, including its poor recognition with many neoplastic conditions, the difficulty of objectively assessing the degree of pain, inappropriate reassessment once therapy is implemented, fear of using or lack of knowledge about the analgesic arsenal, and poor communication with the client.5-12PAIN TERMINOLOGY AND A BRIEF PHYSIOLOGY OVERVIEW
Pain can be described in many different ways, based on various criteria.1,2,5-7,10-12 When classified according to temporal aspects, pain is described as acute, chronic, or intermittent. When classified by intensity, pain can be mild, moderate, severe, or excruciating. Another classification is by origin or type of pain, with somatic, visceral, and neuropathic pain having different causes, associated symptoms (as described by human patients), and response to therapeutic intervention. These three types of pain may occur alone or in combination in the same patient.1 Finally, cancer pain can be a direct result of the primary tumor (e.g. invasion, tissue destruction, pressure), a result of distant metastases (e.g. bone, meninges, pleura), a result of painful paraneoplastic syndromes (e.g. hypertrophic osteopathy, peripheral neuropathies), or the result of either diagnostic (e.g. biopsies) or therapeutic procedures (e.g. aggressive surgery, chemotherapy, radiation therapy). Pain in patients with cancer may also be secondary to unrelated conditions common in older animals (e.g. osteoarthritis).
Nociception is the transduction, conduction, and processing of afferent nerve signals generated by stimulated nociceptors, resulting in the perception of pain.2,6,11 Signals from noxious stimuli (mechanical, thermal, or chemical) are transmitted mainly through two types of nerve fibers: the small myelinated A delta fibers and the small unmyelinated C fibers.2,6,11 Their nerve endings are located in the skin, subcutaneous tissues, periosteum, joints, muscles, and viscera.6 The larger myelinated A beta fibers normally transmit non-noxious stimuli, such as touch, vibration, pressure, movement, and proprioception.6 However, the non-noxious input from these fibers may be incorrectly processed in an altered central nervous system, resulting in the perception of pain (allodynia).2,6,11 The A delta and C fibers enter the dorsal horn of the spinal cord through the dorsal root ganglia, where they synapse with second-order neurons of the gray matter.2,6,11,12 Some integration and modulation may take place in the dorsal horn, through interactions with excitatory and inhibitory interneurons. The resulting nociceptive information is carried to the brain through the spinothalamic tracts, where it can be integrated, processed, and recognized in multiple areas of the brain.2,6