Study finds sense of smell may help predict recovery of people with severe brain injuries

Researchers at Cambridge University have found that sense of smell may help when it comes to predicting the recovery of people with severe brain injuries.

Doctors can often find it difficult to determine a patient's state of consciousness following a severe brain injury, but an accurate diagnosis is vital for treatment strategies and can underlie end-of-life decisions. Nevertheless, there is an error rate of up to 40% in determining the state of consciousness in patients with brain injuries.

Scientists from Cambridge trialled a simple ‘sniff test’, which could help doctors diagnose and determine treatment for patients who either have minimal consciousness or are left in a vegetative state following a brain injury.

'Sniff test'

A person’s sense of smell relies on structures deep within the brain and works when we are sleeping, as well as when we are awake. Furthermore, the brain automatically changes the way a person sniffs in response to different smells – for example when faced with an unpleasant smell, breathing becomes shorter and shallower.

Researchers carried out the ‘sniff test’ on 43 patients with severe brain injury, some minimally conscious and some in a vegetative state. All 43 were presented with different jars of smell ten times in a random order, for five seconds, and the volume of air sniffed was measured. One jar contained an unpleasant smell of rotten fish, one a pleasant smell of shampoo, and the third had no smell.

Findings

It was found that minimally conscious patients inhaled significantly less in response to smells but did not discriminate between pleasant and unpleasant. The same patients also modified their nasal airflow in response to the jar with no smell, which suggests they anticipated a smell, or were at least aware of the jar.

The responses from patients in a vegetative state, however, were more varied - some did not change their breathing in response to the smells, whereas others did.

The results of the study, published in the journal Nature, revealed 100% of patients who reacted to the sniff test went on to regain consciousness and over 91% of these patients were still alive three and a half years after injury. 61% of those who showed no response had died.

Dr Anat Arzi, a researcher in the University of Cambridge's Department of Psychology and the Weizmann Institute of Science Israel, who led the study, said: "The accuracy of the sniff test is remarkable - I hope it will help in the treatment of severely brain injured patients around the world."

He added: "We found that if patients in a vegetative state had a sniff response, they later transitioned to at least a minimally conscious state. In some cases, this was the only sign that their brain was going to recover - and we saw it days, weeks and even months before any other signs.”

Dr Tristan Bekinschtein, from the University of Cambridge's Department of Psychology who was also involved in the study, said: "This new and simple method to assess the likelihood of recovery should be immediately incorporated in the diagnostic tools for patients with disorders of consciousness."