June 23, 2018

Behavior-based Pain Measurement in Animals

A recent review highlights the challenges of using behavior to assess pain and health-related quality of life.
By Natalie Stilwell, DVM, MS, PhD
Pain is traditionally assessed using objective measures, which are less reliable in a clinical setting than in a controlled research environment. Objective measures include heart rate, respiratory rate, and circulating cortisol levels, all of which may be altered by causes other than pain.

In human medicine, it is becoming more common to assess pain through questionnaire-based instruments that ask how a patient feels; however, few comparable options currently exist in veterinary medicine.

Researchers in the United Kingdom recently published a review assessing the current state of behavior-based pain assessment in animals, as well as the methods’ limitations.  

The Challenges of Measuring Pain

In animals, pain often manifests as changes in behavior, including lethargy, aggression, and inappropriate urination. However, these changes are commonly species specific and must be interpreted accordingly.

Similar to human patients who are unable to self-report, animals rely on an observer to be a proxy for reporting pain. In cases of chronic pain, owners are often the most familiar with subtle changes in a pet’s demeanor and behavior. However, an owner’s opinion may be strongly biased; therefore, researchers often attempt to word questionnaires in a manner that discourages subjectivity.

The authors expressed that developing a scientifically robust measure of pain in veterinary medicine is “without doubt the greatest challenge of all.” Measurement of intangible outcomes, such as anxiety and depression, in human psychiatric medicine is similarly challenged and thus tests must pass rigorous validation against a gold standard test, when available. Such tests must also prove to be reliable among multiple time points and users.  

Acute Pain Measurement

Current standards for measuring acute pain tend to use composite scales that assess multiple aspects of pain at once. Several validated instruments are currently available for measuring acute pain in small animal patients, including the Glasgow Composite Measure Pain Scale (CMPS). Some instruments even define an intervention level at which analgesic therapy should be administered.

Facial expression is also currently used to measure pain in several species, including the mouse, rat, rabbit, and horse. Certain composite scale instruments, including the feline CMPS, combine facial expression with physiologic and behavioral parameters to assess pain.

Chronic Pain Measurement

Chronic pain is inherently more complex to assess than acute pain, as it affects social and psychological components of well-being as well as physical health-related quality of life (HRQL). Many generic and disease-specific instruments in human medicine address HRQL, with the aim to best meet the patient’s needs through individualized health care.

In veterinary medicine, HRQL instruments assess a patient’s status either over time or in comparison with other animals of the same species. For assessment of orthopedic pain, guidelines often recommended that observer-reported quality-of-life instruments be used in combination with at least 1 validated tool, such as the Canine Brief Pain Inventory or Feline Musculoskeletal Pain Index. In cases of nonorthopedic pain or comorbidities, a generic rather than disease-specific HRQL instrument may be more appropriate to assess patient pain.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.

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