Smell test reveals state of consciousness


Smelling is linked to our consciousness (Image: Globalstock / iStock)

In patients with severe brain damage, it is often difficult to assess their level of consciousness: Do they still get to know something about their environment? And what is the forecast? Now researchers have found that an amazingly simple test could give an answer. If you ask these patients to smell containers with fragrances, people in a minimal state of consciousness react to this by involuntarily taking shorter, shallower breaths. However, this reflex is absent in patients with real and long-lasting coma. However, if one of these patients does respond, then it is highly likely that consciousness will return soon.

When a person suffers severe brain damage from an accident or stroke, they often first fall into a deep coma. After a while, however, this coma can go into a state in which the patient has his eyes open but does not consciously react to his environment. In this phase, it is also very difficult for neurologists to determine whether this person is in a real coma in which only their vegetative functions and reflexes work, or whether the patient has minimal stimuli of consciousness. “The error rate when determining the state of consciousness is up to 40 percent for such patients with brain damage,” explains Anat Arzi from the University of Cambridge and his colleagues. This makes it difficult, for example, to give relatives of such patients a forecast of whether they will wake up or not.

Fragrances provoke respiratory reactions

That is why Arzi and his team have now tried a test method that works on one of the most original human senses: the sense of smell. “The smell is based on brain structures that are closely linked to basic mechanisms of alertness,” the researchers explain, “That’s why we asked ourselves whether the sense of smell could not serve as a biomarker for consciousness.” For their study, they had 43 patients in the Wax coma or minimal state of consciousness subjected to a simple smell test: All participants were told that they were now presented with different smells – without this causing a reaction in the patient. Then the researchers held a bottle of fragrant shampoo, a sample of the smell of rotting fish, and an empty container under their noses. These experiments were repeated several times in alternating order, in parallel the scientists measured the breathing rhythm and breathing volume of the test subjects.

It turned out that the patients who had previously been classified as minimally conscious reacted. Their breaths became shorter and shallower when the pleasant or unpleasant smell was presented to them. As Arzi and his colleagues explain, this corresponds to a reaction that occurs automatically when smells are perceived. It therefore indicates that the patient perceives the smell stimuli. However, some of the subjects also showed breathing reactions when they held the empty container under their noses. This could indicate a more extensive, cognitive response: “If a patient changes their breathing in response to an empty container, then this suggests that they either perceive the container or even respond to the experiment’s advance notice,” the researchers say.

An early indication of a return to consciousness

On the other hand, when they repeated these experiments with patients in a state of unresponsive alertness, these reactions largely failed to materialize – at least when you consider all the results of this group together. However, there were decisive differences within the vegetative coma group: some of these patients reacted to the smells in some rounds, but not in others. After the scientists had followed the fate of these patients over a long period of time, astonishing results emerged: “All ten vegetative coma patients who had shown a respiratory reaction in at least one pass later changed to the state of minimal consciousness,” the scientists report. The olfactory reaction therefore predicted with 100 percent accuracy that a patient would wake from the coma. “We saw this first hint of an awakening brain in some cases days, weeks and months before any other sign of recurring consciousness,” says Arzi. After five years, more than 90 percent of all patients from both groups who had reacted to the smells were still alive. Most of those who failed to respond responded to brain damage.

According to the researchers, these results demonstrate that this simple smell test can provide valuable information on the condition and prognosis of a patient with severe brain damage. “The accuracy of the smell test is remarkable and I hope that this will help treat patients with severe brain damage worldwide,” says Arzi. His colleague Tristan Bekinschtein adds: “This new and simple method to determine the chance of a return of consciousness should be immediately included in the diagnostic tools for patients with consciousness disorders.”

Source: Anat Arzi (University of Cambridge) et al., Nature, doi: 10.1038 / s41586-020-2245-5

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