Rotten fish smell could detect awareness in people with brain injuries

Rotten fish

More useful than you might think

Tim Platt/Digital Vision/Getty Images

A waft of rotten fish might help doctors looking for signs of awareness in people who are unresponsive after serious brain injury.

It can be hard to know if people in this condition are in a vegetative state, when they cannot see or feel anything, or if they have some awareness but are almost paralysed.

Doctors investigate this with tests such as asking people to follow a moving finger with their eyes, but for those in this “minimally conscious state”, awareness typically fluctuates over time, so they may be wrongly assumed to be vegetative.


Anat Arzi at the Weizmann Institute of Science in Rehovot, Israel wondered if people with some awareness would show any response to smells, as non-injured people reduce the air they inhale in the presence of bad smells, both consciously and unconsciously. We even show a small “sniff response” to odours in our sleep.

“The brain has the capacity to process information when we are not consciously aware, probably for survival reasons,” says Arzi.

Her team investigated 43 people in a rehabilitation facility in the first few weeks or months after a brain injury. About half were thought to be in a vegetative state, and the rest were thought to be minimally conscious. A small tube entering the nose could measure the volume of air they inhaled.

Some people in both groups showed about a ten per cent reduction in inhaled air when presented with a strong odour – whether it was of liquid that smelled like rotten fish or simply a fruity shampoo.

The test was most revealing for the 24 people classed as vegetative. Of those, 16 were later diagnosed as minimally conscious because they had started showing some responses, such as eye movements. Ten of these had showed a sniff response, but none of those who stayed in a vegetative state had had one.

“If you don’t have a sniff response we can say nothing. But if you have a sniff response it’s highly informative,” says Arzi. “Maybe we can provide with the sniff test a simple tell to be used at the bedside that will reduce misdiagnosis rates.”

Journal reference: Nature, DOI: 10.1038/s41586-020-2245-5

More on these topics: