Research suggests a patient’s ability to smell can predict their recovery outcome

New research suggests that a severely brain injured patient’s ability to smell can identify whether they are unresponsive or minimally conscious, predict whether they will recover consciousness, and their chances of long-term recovery and survival.

Determining whether a patient is minimally conscious or unresponsive is crucial in realistically assessing their options, creating a treatment and pain management plan, and providing their loved ones with a prognosis as to their chances of recovery.  After a severe brain injury, it can be extremely difficult to assess whether a patient is unresponsive or minimally conscious.  In fact, research suggests an error rate of up to 40%.

The reason it is so important to get an accurate diagnosis is that the two states, while appearing similar, tend to have dramatically different outcomes.  An unresponsive patient is one with no consciousness of themselves or their surroundings, while a patient with minimal consciousness has either some awareness of their environment or an awareness of ‘self’, despite that consciousness being severely altered.  Minimal consciousness is often entered into after being in a coma or vegetative state.  It can be transitional or permanent, but in most cases, minimal consciousness is seen a stage on the road to recovery.  Being able to accurately identify whether a patient has entered a minimally conscious state allows a medical practitioner to assess their chances of recovery and the likelihood they will regain consciousness.

The research was carried out by University of Cambridge’s Department of Psychology and the Weizmann Institute of Science Israel.  Researchers hypothesised that a severely brain-injured patient’s ability to detect smells could not only determine their degree of consciousness, but also predict their chance of recovery and long-term survival.

The research involved 43 patients with severe brain injuries, each patient showed either very minimal signs of awareness or no awareness of the external world.  Researchers presented different smells to the patients in jars, while monitoring breathing through their nose using a nasal cannula.  Jars containing either a smell of shampoo, a smell of rotten fish, or no smell at all, was presented to the patient for five seconds.  Each jar was presented multiple times, in a random order, while the researchers measured the volume of air the patient sniffed, changes to inhalation and nasal airflow.

100% of the 43 patients involved in the research who reacted to the smells went on to regain consciousness.  Of those, 91% were still alive three years post-injury.  Of the patients that showed no response to the smells, 63% had died after three and a half years.

Anat Arzi, a researcher in the University of Cambridge’s Department of Psychology and the Weizmann Institute of Science Israel, who led the research, said: “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.”

By measuring the sniff-response in severely brain injured patients, the researchers could measure the functioning of deep-seated brain structures in a simple way.  Olfaction signals that the brain structures involved arousal are responsive.  The ‘sniff test’ provides a measure of olfactory function and therefore, of consciousness.

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.  This simple test could dramatically reduce the error rate when assessing consciousness of brain injured patients and help doctors assess the condition and prognosis of patients with severe brain injuries.

Dr Tristan Bekinschtein, from the University of Cambridge’s Department of Psychology, was involved also in the research.  He believes “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.”