When my cat Blixa was a kitten he would let me know when he wasn't feeling well.

He'd let out a gut-wrenching yowl then hurl through the house like a little black and white vomit comet.

But other times he would seemingly bounce back from procedures, such as tooth extraction, that would leave a human laid up on a couch for days.

As he got older, he became less vocal and it became increasingly difficult to tell when he was in pain.

Diagnosing pain in cats is tricky, even for vets.

"People think cats are classically predators, but they are little enough to be potentially prey as well, so it is in their interest to hide pain," said Carolyn O'Brien, a feline specialist at Melbourne Cat Vets.

It's not that cats are more "stoic" than other animals like dogs, said Fernando Martinez Toboada, a veterinarian at the University of Sydney who specialises in anaesthesia and pain management.

"Perhaps they show [signs of pain] in a more subtle way and we're not capable of reading them," Dr Toboada said

Acute pain following an injury, accident or surgery is particularly hard to spot.

"Pain is individual. Circumstances where it can be painful for one cat might not be painful for another cat," he said.

So what signs do vets look for?

If you've ever tried wrangling your cat into a carrier you've probably already worked out your furry friend is not keen on going to the vet.

"Part of the reason why people have traditionally been a bit reluctant to take their cats in to seek medical care is because they know the cat's going to feel pretty anxious," Dr O'Brien said.

Out of their comfort zone they go into full flight or fight mode. In his youth, Blixa could scale walls and run around the ceiling of the surgery.

"Cats have been notoriously difficult patients to handle and we now realise it's because of a lot of the anxiety they suffer." Dr O'Brien said.

Their anxiety can make it difficult to see behaviours that the cat does in the comfort of its own home.

"Frozen to the spot can sometimes be their way of coping," she said.

I'll just stare into space and the human won't see me. ( Wikimedia: Canadianknowledgelover )

This means vets can't rely on physiological signs such as respiratory or heart rates or blood pressure to assess their pernickety patients' pain levels.

"We know that it is very difficult to interpret the rate of breaths in a clinic environment, as compared to the home. It is really, really tricky," Dr O'Brien said.

Physiological signs could also be influenced by underlying health conditions or drugs.

Instead, vets need to rely on reports of behaviours at home such as laying low, being grumpy or not as smoochy and refusing food.

Vets also look for signs such as guarding or particular postures.

"One classic sign we often tend to see in animals if they tend to have a painful abdomen is their back arches," said Dr Toboada.

"If you see animals really flat, not wanting to move, that very often can indicate movement makes them uncomfortable."

Subtle signs of acute pain in the face

To help vets assess pain levels, Canadian researchers have developed a new tool known as the Feline Grimace Scale (FGS)

This scale interprets subtle signs of acute pain in the cat's face.

Dropped ears, squinty eyes and drooping whiskers were common factors observed in a group of cats ranging from bog standard mogs to Maine Coons admitted with non-emergency acute pain, according to their research.

The ears flatten when a cat is in pain. ( Supplied: University of Calgary )

Originally developed for assessing pain on babies and non-verbal humans, grimace scales have been validated across a range of animals from rabbits to rats. But this is the first time it's been used for cats, said Paulo Steagall of Montreal University.

"The FGS has been developed and validated for use by vets but owners could use changes related to pain in cats to know that something might not be right and take their pets to a veterinarian," Dr Steagall.

For a vet, being able to accurately assess pain levels is critical for ensuring a cat is given the right amount of pain medication when it needs it.

A cat in pain will squint its eyes. ( Supplied: University of Calgary )

Dr O'Brien said the new scale was much more user-friendly than other scales and she uses it in her clinic.

But it's not without its limitations.

"We can't always say that just because a cat has got the ears down or the whiskers back that it's definitely painful, but certainly if we can see a cat that is happy and confident and is looking towards the front of the cage…we can interpret that that cat is likely to be comfortable," she said.

A cat in pain will have upturned whiskers. ( Supplied: University of Calgary )

The test hasn't been used to assess breeds such as Persians that have squashed faces like Grumpy Cat, nor do we know if changes to a cat's face as they age influence the results.

While the scale is "really well designed" the effect of drugs on the cat's facial expression is also a major limitation, said Dr Toboada.

"Not all the sedatives have analgesic [pain-relieving] properties, so we might be masking the result."

Once kitty goes home after surgery it's a good idea keep it inside for a couple of days.

"They might feel a bit wobbly post-op from the anaesthetic, but we would expect most cats would be back to themselves within 24 hours," Dr O'Brien said.

What about chronic pain?

According to Dr Tobaoda, around 90 per cent of animals in pain at home are going to be in some degree of chronic pain rather than acute pain.

While vets use scales to identify acute pain, they rely heavily on pet parents to spot when their cat is suffering from chronic pain as a result of conditions such as osteoarthritis and kidney disease.

Blixa at 19. ( ABC: Genelle Weule )

Signs to look out for are:

Lack of interest in food

Lack of interest in Not moving around or jumping up onto things as much

onto things as much Changes to how they use litter trays or scratching poles

"But everything is a lot more subtle than acute pain," said Dr Toboada, who is particularly interested in helping animals with this type of pain.

The grimace scale could be a "game-changer" in the future with more work.

"We know the grimace scale for rats that was validated for acute pain can be used effectively for chronic pain," Dr Toboada.

In his senior years, my Blixa slowed down (no more scaling walls) thanks to the effects of osteoarthritis and a myriad of other old cat diseases.

He remained pretty perky on a low dose of anti-inflammatories until one day exactly a year ago.

At the ripe old age of 22 he let out his trademark yowl and walked slowly towards me. It was time.