A 'sniff test' could signal consciousness after a brain injury

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A simple "sniff test" could signal if an unconscious person is likely to regain consciousness, a new study finds.

A team of scientists at the Weizmann Institute of Science in Israel revealed that the ability to detect smells predicts recovery and long-term survival in patients who had a severe brain injury. The study, published in the journal Nature, shows that the simple "sniff test," which is inexpensive and convenient, could help doctors to diagnose and develop treatments for patients who have brain injuries and unconsciousness.

Image Credit: pixelaway / Shutterstock
Image Credit: Pixelaway / Shutterstock

Severe traumatic brain injury is a condition when an outside force disrupts the brain's normal function, such as car accidents, a blow or strike to the head, or physical assault. During a TBI event, a patient may experience a period of unconsciousness that may resolve or not. In some cases, patients become unconscious for long periods, while others succumb to the injury.

After a brain injury, it may be hard to determine the state of consciousness of a patient. Doctors and healthcare practitioners may assess the patient using scales, but they do not determine whether the patient is unresponsive or minimally conscious. Knowing if the patient is minimally conscious is crucial to predict whether they will recover. Further, an accurate diagnosis is vital because it can guide treatment strategies and end-of-life decisions.

Sense of smell

The sense of smell is a fundamental mechanism that depends on specific structures in the brain. The brain works by changing the way people sniff in response to various smells. For instance, when a person smells a foul order, he or she takes shallower and shorter breaths. In healthy people, the sniff-response occurs when people are awake or in sleeping states of consciousness,

Olfaction can serve as a biomarker for consciousness. In the study, the team used a non-verbal, non-task-dependent measure known as the sniff response, to determine consciousness in patients with brain injuries. By using the test, doctors have a sensitive measure of olfactory function.

To arrive at their findings, the team measures the sniff response repeatedly over time in patients with severe brain injuries. To do this, they used jars with different smells for 43 severely brain-injured patients. The experimenter left the patient to smell a jar containing a shampoo, an unpleasant smell of rotten fish, or no smell at all. Each jar was given ten times in random order. The volume of air sniffed by the patient was measured.

Study findings

The researchers found that patients who are minimally conscious inhaled less in response to smells but did not discriminate between pleasant and unpleasant odors. The patients also modified the air into their nose in response to the jar with no smell, showing awareness of the jar or learned anticipation of smell.

In vegetative patients, the results varied, with some not changing their breathing in response to both odors, while others did. They also found that if an unresponsive patient had a sniff response, it is tied to being able to regain consciousness in the future.

"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," Anat Arzi, a researcher in the University of Cambridge's Department of Psychology and the Weizmann Institute of Science Israel, said.

Patients who are in a vegetative state may open their eyes, wake up and fall asleep regularly, and have basic reflexes. However, they also do not show meaningful signs of awareness. On the other hand, a minimally conscious patient may experience periods where they can show signs of awareness.

A follow-up research about three and a half years later revealed that over 91 percent of the patients who exhibited a sniff response was still alive, but 63 percent of those who showed no response at all, had died.

"In addition, olfactory sniff responses were associated with long-term survival rates. These results highlight the importance of olfaction in human brain function and provide an accessible tool that signals consciousness and recovery in patients with brain injuries," the team concluded in the study.

Journal reference:
Angela Betsaida B. Laguipo

Written by

Angela Betsaida B. Laguipo

Angela is a nurse by profession and a writer by heart. She graduated with honors (Cum Laude) for her Bachelor of Nursing degree at the University of Baguio, Philippines. She is currently completing her Master's Degree where she specialized in Maternal and Child Nursing and worked as a clinical instructor and educator in the School of Nursing at the University of Baguio.


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