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Sniff Test Can Predict Recovery of Severely Brain Injured Patients

A new study finds that the ability to detect smells predicts recovery and long-term survival in patients who have suffered severe brain injuries.

In fact, a simple, inexpensive sniff test could help doctors accurately diagnose and determine treatment plans for patients with disorders of consciousness, according to researchers at the University of Cambridge in the UK.

The study involved brain-injured patients showing very minimal or no signs of awareness of the external world. It found that 100% of patients who reacted to the sniff test went on to regain consciousness. Furthermore, more than 91% of these patients were still alive three and a half years after their injuries, the researchers reported.

“The accuracy of the sniff test is remarkable — I hope it will help in the treatment of severely brain injured patients around the world,” said Anat Arzi, a researcher in the University of Cambridge’s Department of Psychology and the Weizmann Institute of Science Israel, who led the research, together with Professor Noam Sobel from the Weizmann Institute of Science Israel and Dr. Yaron Sacher from the Loewenstein Rehabilitation Hospital Israel.

It is often difficult for doctors to determine a patient’s state of consciousness after a severe brain injury, the researchers said, noting diagnostic errors are made in up to 40% of cases.

A patient who is minimally conscious differs from one in a vegetative state, and their future outcomes differ, the scientists said. An accurate diagnosis is critical because it informs treatment strategies, such as pain management and can influence end-of-life decisions.

Our sense of smell is a very basic mechanism and relies on structures deep within the brain, the researchers explained. The brain automatically changes the way we sniff in response to different smells. For example, when presented with an unpleasant smell we automatically take shorter, shallower breaths.

Research was conducted on 43 severely brain-injured patients. The experimenter first explained to each patient that different smells would be presented to them in jars, and the breathing through their nose would be monitored using a small tube called a nasal cannula. There was no indication that the patients heard or understood, the researchers said.

Next, a jar containing either a pleasant smell of shampoo, an unpleasant smell of rotten fish, or no smell at all was presented to each patient for five seconds. Each jar was presented 10 times in a random order, and a measurement was made of the volume of air sniffed by the patient.

The researchers found that minimally conscious patients inhaled significantly less in response to smells, but did not discriminate between nice and nasty smells. These patients also modified their nasal airflow in response to the jar with no smell. This implies awareness of the jar or a learned anticipation of a smell.

Vegetative state patients varied — some did not change their breathing in response to either of the smells, but others did.

A follow-up investigation three and a half years later found that more than 91% of the patients who had a sniff response shortly after injury were still alive, but 63% of those who showed no response had died.

By measuring the sniff response in severely brain-injured patients, the researchers said they could measure the functioning of deep-seated brain structures. Across the patient group, they found that sniff responses differed consistently between those in a vegetative state and those in a minimally conscious state, providing further evidence for an accurate diagnostic.

“We found that if patients in a vegetative state had a sniff response, they later transitioned to at least a minimally conscious state,” said Arzi. β€œIn 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.”

In a vegetative state the patient may open their eyes, wake up and fall asleep regularly and have basic reflexes, but they don’t show any meaningful responses or signs of awareness. A minimally conscious state differs because the patient may have periods where they can show signs of awareness or respond to commands.

“When the sniff response is functioning normally, it shows that the patient might still have some level of consciousness even when all other signs are absent,” said Dr. Tristan Bekinschtein in the University of Cambridge’s Department of Psychology, who was involved in the study. “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.”

The study was published in the journal Nature.

Source: University of Cambridge

Sniff Test Can Predict Recovery of Severely Brain Injured Patients

Janice Wood

Janice Wood is a long-time writer and editor who began working at a daily newspaper before graduating from college. She has worked at a variety of newspapers, magazines and websites, covering everything from aviation to finance to healthcare.

APA Reference
Wood, J. (2020). Sniff Test Can Predict Recovery of Severely Brain Injured Patients. Psych Central. Retrieved on December 3, 2020, from
Scientifically Reviewed
Last updated: 8 May 2020 (Originally: 8 May 2020)
Last reviewed: By a member of our scientific advisory board on 8 May 2020
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