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A simple sniff of a pleasant or rotten odor may help guide decision making by physicians and families of patients who have suffered traumatic brain injury.

Brain injury can occur as a result of a heart attack, trauma, stroke, or surgical accident.  When a patient slips into a nonresponsive state, difficult decisions are sometimes made only the basis of a guess, or a non-conclusive imaging study.

A study recently published in the journal Nature suggests one of our most basic senses may offer sophisticated guidance on whether a human will eventually awaken from a comatose, or semi-comatose state. Currently, the error rate for determining whether a patient will regain consciousness can be as high as 40 percent.

Because our sense of smell is deeply entangled in our respiratory function, researchers wondered if a sniff test might have value for predicting whether a patient would eventually regain consciousness. As it turns out, offering apparently unconscious patients the opportunity to whiff pleasant and pungent smells could be an inexpensive, straightforward method to obtain important clinical insight.

In this relatively small study, 43 patients who had suffered brain injury were offered whiffs of both pleasant and noxious odors.  Researchers explained the experiment to each patient.  None of the patients demonstrated awareness of the environment or the study.  Each was then offered a succession of opportunities to whiff a jar with a pleasant smell of shampoo, another with a noxious odor of rotten fish, and a third jar with no smell at all.  The breathing of each patient was monitored through a nasal cannula.

The results of the study were encouraging:

  • Of the patients who reacted to the sniff test, all progressed to regain consciousness and 91 percent remained alive three years later.
  • For those patients who did not sniff, only 35 percent went on to eventually regain consciousness.
  • In the case of one patient, the sniff test indicated the patient was returning to consciousness two months before the patient otherwise showed any other signs of improved cognitive function

Noted study author Noam Sobel of the Weizmann Institute “It can be explained in a few sentences: If you don’t know if two people are conscious, give them an odor. If they sniff it, they’re conscious; if they don’t, they’re not. What’s more, if they do sniff it, they’ll probably live for at least three years. And if they don’t sniff it, then their chances of surviving are much lower.”

While more research is needed to replicate these results, this simple bedside test, along with neuroimaging, could help doctors reduce medical mistakes involving patients with brain injury who cannot speak but might be able to sniff—for themselves.

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