When we go into our physician complaining of pain, one of the first questions we are asked is to score the amount of pain we are experiencing on a scale of 1-10 so they can determine how bad we feel. Unfortunately, veterinarians do not have that luxury and so we have to observe behaviors in animals that gives us an idea of how much pain our patients are experiencing.
It has been shown that horses show facial expressions as signs of pain and researchers have used this information to develop a horse pain scale. To determine which facial experessions are most related to pain and are most reliable, Dr. Emanuela Dalla Costa from Italy indicated that without an accurate tool to assess pain, we cannot identify when pain occurs, assess its severity or assess the effectiveness of anything we do to treat it. She goes on to say that it is paramount to not only develop a tool to assess pain but to test its scientific validity in order to make sure what we are measuring is pain.
The study involved 39 horses undergoing routine castration and they reviewed 126 photographs of the facial expressions of these horses before and after surgery, and scored them. Then, they tried to determine which aspects of the facial expressions seemed most associated with pain. The most reliable indicators of pain were stiffly backward ears, squinting of the eyes, tension above the eye area and strained chewing muscles. Strained mouths and nostrils were less consistent indicators of pain. The grimace scale was divided into facial action units and each unit received a value corresponding to the amount of pain. Although this study only involved castration pain, it is believed to be helpful in determining pain from other syndromes as well.