How to Know When a Cat is Hurting

Recognizing when a cat is in pain is simple in only the most extreme of cases. Thousands of years of natural selection have made cats VERY good at masking pain. After all, it’s generally not a good idea to advertise the fact that you’re not at your best when a predator or potential mate might be found around the next bend in the trail.
Pain encompasses more than just the “I hurt” sensation, but also the overall distress that it can cause. As the World Small Animal Association’s Global Pain Council puts it:
Pain is a complex multi-dimensional experience involving sensory and affective (emotional) components. In other words, ‘pain is not just about how it feels, but how it makes you feel’, and it is those unpleasant feelings that cause the suffering we associate with pain.
I wish I had tools to help my patients like the one physicians use to relate just how bad their patients' suffering is… modified for cats, of course. “Okay Frisky, just put your paw on the face that best expresses how you feel today.”
Wong-Baker FACES® Pain Rating

(Click image for larger view)
But veterinarians and owners have to rely on a cat’s behavior to evaluate pain. Fortunately, we’ve just received a little help in this regard with the publication of a new paper entitled “Behavioural signs of pain in cats: an expert consensus.”
A panel of 19 “international veterinary experts in feline medicine” concluded that the following 25 signs were “sufficient to indicate pain, but no single sign was considered necessary for it.”
In other words, if your cat is doing any of the following, he is probably hurting. Even if he’s not, pain could still be a problem.

Difficulty jumping
Abnormal gait
Reluctance to move
Reaction to palpation [applying light pressure with the hands]
Withdrawn or hiding
Absence of grooming
Playing less
Appetite decrease
Overall activity decrease
Less rubbing toward people
General mood
Hunched-up posture
Shifting of weight
Licking a particular body region
Lower head posture
Blepharospasm [squinting]
Change in form of feeding behavior
Avoiding bright areas
Eyes closed
Straining to urinate
Tail flicking 

While this list is helpful, it only goes so far. For instance, a cat who has an abnormal gait might certainly be in pain, but other non-painful conditions (e.g., neurologic disorders) could also be involved.
In cases where I have failed to find another reason for a cat’s change in behavior and I’m left with undiagnosed pain as the most likely cause, I often rely on a tried and true veterinary test: response to treatment. I’ll put my patient on a few days of buprenorphine—my favorite kitty pain reliever—and if his behavior returns to normal, we now know that pain is to blame.
Guidelines for recognition, assessment and treatment of pain: WSAVA Global Pain Council members and co-authors of this document: Mathews K, Kronen PW, Lascelles D, Nolan A, Robertson S, Steagall PV, Wright B, Yamashita K. J Small Anim Pract. 2014 Jun;55(6):E10-68.
Behavioural Signs of Pain in Cats: An Expert Consensus. Merola I, Mills DS. PLoS One. 2016 Feb 24;11(2):e0150040.