Thirsty People Feel More Pain

Going without a drink can make you more sensitive to pain, a study has found. Australian pain expert Dr Michael Farrell of the Howard Florey Institute in Melbourne and team report their findings in today's issue of the Proceedings of the National Academy of Sciences.

“This is another demonstration of the plasticity of pain responses,” he said. “In this particular instance a mild perturbation of electrolyte levels, which is fundamentally what gives rise to thirst … is enough to modify the pain response”. Dr Farrell and the team studied the relationship between thirst and pain in 10 people. The study participants had pressure applied to their thumbs to induce mild pain and were given saline injections to stimulate thirst.

Blood Flow Scan

The researchers used a positron emission tomography (PET) scan to measure blood flow in the brains before and after. The results showed that people who were thirsty felt more pain. Two regions of the brain, the pregenual cingulate and ventral orbitofrontal cortex, which were not turned on by either input alone, lit up suggesting a location where the two sensations were being integrated.

The researchers did not find that pain affected thirst levels, but Dr Farrell says this could be because the participants were not made very thirsty in the first place and any decrease would have been hard to measure. Dr Farrell says the team had speculated there might be circuits in the brain that allow one sensation to modulate another, which is important from the point of view of survival. “Hunger, thirst, tiredness and pain, for example, don't conveniently happen at the same time, so it's important for the body to prioritise,” he said.

Survival instinct

He says pain is accentuated because it is more important to survival than mild thirst. “The sensation with the most immediate implications for survival is pushed to the forefront of attention,” he said. Dr Farrell says the findings suggest it could be wise for people who are about to go through a painful experience should drink more water beforehand. He says evidence from different types of studies also support this relationship between drinking water and pain.

But could people deliberately use dehydration to maximise pain, say via torture?

“We suspect if they got dehydrated enough that the overwhelming sense of thirst would probably make pain less rather than more,” he said. Previous studies in rats have shown that mild thirst makes the animals feel more pain but severe dehydration actually dulls pain, he says. He says this too makes sense from the point of view of survival. “If you were very dehydrated it would pay to suppress pain because it might get in the way of your search for water,” he said.

Ethical limitations

Dr Farrell says it would have been too hard on the study participants, who already spent up to three hours on the table, to test whether drinking decreased pain. “They've got this plastic mask holding their head perfectly still and they've got both arms spread out, one of them with a hypertonic solution going into one vein and the other one getting radioactive isotopes – it would have been intolerable,” he said. He says testing whether dehydration would have dulled pain would be similarly tricky ethically.

By Anna Salleh from ABC Science Online