Certain physiological parameters, such as heart and respiratory rates, body temperature, pupil size, and blood pressure, have
been used as surrogate measures of pain in veterinary patients, but they do not appear to provide adequate specificity because
they are influenced by other psychologic factors and stressors.8,14 Clinically, behavior appears to provide a better estimate. Observing movement, posture, grooming, appetite and thirst, licking
of the painful area, drooling or dysphagia, vocalization, and growling or hissing when approached may provide semiobjective
information on the degree of pain a given patient is experiencing.5,6,8-10,13,15-17
Few objective methods of pain measurement exist. In veterinary studies, researchers have done force plate analysis, used a
pressure-sensing floor mat (MatScan System—Tekscan Inc.) to detect gait abnormalities, or used pressure algometers, instruments
that measure the pressure required to elicit pain. Investigators currently use these methods of measuring pain in a research
setting. The clinical application and usefulness of such instruments for determining the degree of pain in a single patient
with a given condition is uncertain and awaits further study.
Figure 1. An example of a numerical rating scale, with a gradation from 0 (no pain) to 10 (worst possible pain).
The American Animal Hospital Association (AAHA) has published standards that recommend pain be assessed in all animals, regardless
of the presenting complaint. To comply, each practice team needs to adopt a system for measuring pain in a clinical setting.
Various observer pain scales have been used to estimate pain in animals.5,6,10-12 The most common types are the visual analog scale (VAS) and the numerical rating scale (NRS) (Figure 1).16,17 Both scales are represented by a horizontal line, generally 100 mm long. The NRS has gradations from 1 to 10 (or from 1 to
100), with 1 equaling no pain and 10 (or 100) equaling the worst possible pain. With the VAS, the distance from the end to
the mark is measured and recorded as the pain score. The VAS method has more subjectivity and interobserver variation than
does the NRS, which often incorporates descriptive criteria in many numeric intervals (physiological and behavioral).6,11 Both systems have many weaknesses. A pain assessment system that is readily understood, is easily used, and produces repeatable
results is more important than the type of scale used. The pain score should be recorded upon initial examination of all patients
and regularly reassessed during therapy.
Table1. Examples of Tumors that Can Cause Pain in Veterinary Patients