A new study by Canadian researchers has found that men and women remember pain differently and that the memory of earlier pain makes men hypersensitive to later pain, but not women.

Scientists believe our memory of pain could be key to understanding and treating chronic pain.

Of mice and men

Dr. Loren Martin and his colleagues were actually investigating another question when they discovered this surprising result. They were measuring how multiple sources of pain changed pain perception.

In experiments, in mice they used a heat probe that created a mild level of heat on the mouse's feet. Then they gave the mice a dose of vinegar to upset their stomachs. The mice, unsurprisingly, didn't like it.

The surprise came when they they repeated the experiment. The male mice showed more stress when brought back to the location of the experiment, and had stronger responses to the heat stimuli — they were more sensitive to the pain. The female mice showed no extra stress or sensitivity.

Researchers shone a mild heat probe at the mice's feet to induce a slight pain in the experiment. (Sana Khan)

They then ran a similar experiment on humans.

They used the same combination of stimuli — heat on the forearm, and an uncomfortably tight blood pressure cuff over the bicep. They left the cuff on for about 20 minutes and had the participants do arm exercises to increase the pain.

The participants came back the next day and were sent to the same environment where they wore the blood pressure cuff to repeat the heat probe experiment. This time, the men reported feeling more pain from the heat probe, while nothing changed for the women. It was the same sex difference that they'd seen in the mice.

Men also reported feeling more stressed, which paralleled what they found in male mice, when they measured the amount of stress hormones in their blood.

Men and women experience pain differently

"In males, the pain seems to be driven by a stress response," Martin told Quirks & Quarks host Bob McDonald. Martin is an associate professor in the department of psychology at the University of Toronto, Mississauga.

"When a man or a mouse returns to that pain environment, it stresses that male and that was what drove the pain sensitivity."

The experimental set up for humans (from research paper)

Martin thinks the explanation for this might be due the fact that women are more used to the sensation of pain from their experience with childbirth to their monthly menstrual cramps, and as a result, have a higher tolerance of repeated pain than men.

Women are still more sensitive to pain in general than men, Martin pointed out, but the study showed that men are hypersensitive to subsequent experiences of pain.

Memory and chronic pain

Martin thinks this could have important implications for the treatment of chronic pain. "One of the prevailing theories in chronic pain is that it doesn't go away because we can't forget the pain," he said. "There's a learning aspect in that our brain has learned the pain."

Chronic pain affects 20 to 25 per cent of the general population and it increases with age. "It's a pretty big problem," said Martin.

When Jeanine McDonald heard a pop in her low back as she bent down to pick up a lid from a box, she had no idea she'd ruptured a disc and would wait three months for surgery. Then a second disc ruptured and left her in more debilitating chronic pain, the kind that millions of Canadians live with daily. (Jonathan Hayward/Canadian Press)

There's a huge social and psychological component to chronic pain as well, in addition to the physiological, he added.

He thinks that the anticipation of pain based on a person's previous experience may make it more likely for the injury to go unresolved and develop into chronic pain.

"We still need more research to show that memory is a driving force," said Martin. But for now, he thinks physicians should try to find out a patient's past history and experiences when treating chronic pain, because our prior experience with pain might affect how we respond in future.