Cancer has become a leading health concern in companion animal medicine and contributes significantly to mortality in this population. Significant strides have been made across the fields of chemotherapy, radiation and surgery that allow rapid detection and elimination/reduction of cancer. Along with these advances has come a realization of the burden that cancer and its treatment places on the body. This burden, assessed as quality of life for our patients, is often a significant concern among clients seeking help in treating their companion’s cancer. In many instances, inability to control these side effects may contribute to a decision to euthanize even when the cancer itself is being successfully managed. One of the largest manifestations of this concern relates to pain caused by cancer and its treatment.
Causes of pain in cancer patients is multifactorial. A tumor itself can cause pain by directly invading tissues, compressing nerves, and activating nerve endings in any part of the body. In addition, cancer can worsen preexisting pain and contribute to chronic debilitation through a variety of signaling mechanisms. Surgery and other invasive diagnostic and therapeutic modalities lead to acute pain in cancer patients that may be more likely to become chronic pain because of tumor activity. Pain negatively affects quality of life and has deleterious effects on metabolism and immunity. Painful patients are generally less mobile and less likely to eat, contributing to a cycle of continued debilitation and worsening quality of life.
Luckily, recognition of and treatment for pain has taken great strides. More modalities than ever exist for keeping patients comfortable through the variety of treatments and experiences they are faced with when diagnosed with cancer. As with all types of pain, the approach to cancer pain relies on a step wise approach that includes recognition, categorization, multimodal treatment, and reassessment. Recognition of cancer related pain has improved dramatically through the improved skills of practitioners who are actively looking and the institution of validated pain scoring systems. These scoring systems are now available for acute and chronic pain situations in companion animals and can help greatly in modifying pain treatment.
Categorization of pain guides treatment selection. Pain is most usefully categorized based on intensity (mild, moderate, severe), time (acute, chronic, intermittent), and origin (somatic, visceral, neuropathic). It can also be classified as adaptive (appropriate response to condition) or maladaptive (inappropriate nervous system response duration or intensity). Pain resulting from cancer and its treatment often spans all of these classification systems as time progresses. Presence of multiple conditions may also contribute to pain in cancer patients and should be evaluated. A very common example is the patient with a tumor that also has significant osteoarthritis pain.
Treatment of cancer pain can be complex and should always include multiple modalities. Directed therapies that may remove the source of tumor related pain such as surgery, radiation therapy, and osteoclast inhibitors should be implemented early in the disease course if possible. Early and preventative analgesic strategies should be used for painful procedures such as surgery or biopsy. Loco-regional anesthetic techniques can be key to this effort and are still underutilized. Opioid type drugs and anti-inflammatory medications such as NSAID’s and corticosteroids remain the first line of treatment, especially for pain that is acute or intermittent and of somatic origin. These drugs are helpful but lose much off their utility as cancer pain becomes chronic, neuropathic, and maladaptive. Adjuvant drugs that address pain related excitatory changes in the nervous system have become an exceptionally valuable tool and are often included in cancer care. These include pharmacologic modifiers of both inhibitory and excitatory systems such as GABA, NMDA, serotonin/norepinephrine, and calcium/sodium channels. Non-pharmacologic modalities such as acupuncture and physical rehabilitation have an increasingly realized and important role in treating and preventing pain in cancer patients by maintaining whole body health, muscle tone and metabolism. Importantly, pain should be reassessed often as the cancer patient progresses through treatment. Changes to pain generation and in intensity of preexisting pain are common. Therapy should be adjusted so that it continues to meet the needs of the cancer patient through a variety of experiences and may need adjustment both up or down depending on changing conditions.
In summary, pain is widespread and multifactorial in veterinary cancer patients. Its prevention and treatment can contribute significantly toward maintaining quality of life and improving quality time spent with their human companions. It should be a high priority to seek out and characterize sources of pain in cancer patients. This allows early intervention and aggressive treatment with multimodal analgesic protocols that include directed therapies, pharmacologic management, and multiple supportive modalities.