Specialists who try to figure out which category these patients fall into get it wrong up 40 percent of the time, making fraught decisions about how much care to provide and how long to continue treatment even harder. Now, scientists have developed a new, simple test to detect patients’ responses to pleasant and unpleasant odors that may help identify which patients are completely unresponsive and which ones are minimally conscious and have a better chance of waking up. The research was published in May 2020 in the journal Nature. “If an unresponsive patient with a brain injury has a sniff response, chances are very high that the person will recover consciousness and will have high survival chances for years,” says lead study author Anat Arzi, a postdoctoral researcher at the University of Cambridge in England. RELATED: Talking TBI, a Glossary of Formal and Informal Terms Used to Describe TBI Treatment, Professionals, And More
How the Smell Test Worked
For the study, scientists repeatedly tested the response to the pleasant (shampoo) and unpleasant (rotten fish) odors in 43 patients at a rehabilitation hospital. They attached a small device known as a nasal cannula to the nose to measure how much air people inhaled when they were exposed to the shampoo and rotten fish scents, compared with their inhalation when they weren’t exposed to any odor at all. People typically inhale deeply when they detect pleasant smells, and sniff abruptly to avoid odors that are unpleasant. Researchers hypothesized that they might detect this type of reaction in brain injury patients who subsequently regain consciousness. After each sniff test, researchers also conducted a battery of tests currently used to assess the state of consciousness in brain injury patients, including exposing them to a variety of other stimuli. Every patient with a sniff response later showed signs of consciousness. And none of the patients in a vegetative state who remained unconscious had any response when they were exposed to the different odors in the study. Patients with a sniff response also had better survival odds. All but 2 of the 24 patients with a sniff response after injury survived after an average follow-up period of slightly more than three years. By comparison, 12 of 19 patients who lacked a sniff response after injury died after an average follow-up of about 1.5 years. While this is encouraging, the study doesn’t help explain why a response to smells might lead to these outcomes. RELATED: Your Everyday Guide to Living Well With TBI
The Importance of a Sniff Response in Unconscious Brain Injury Patients
It’s possible that patients with severe brain injuries may respond to smell before they show a response to other stimuli because unlike other senses, olfactory information has a direct pathway to the cortex, the region of the brain that controls functions like consciousness, attention, and movement. The brain receives other sensory input through the thalamus, which then relays information to the cortex, and this transmission process might be more easily interrupted by a brain injury. “This thalamic circumvention may provide special status for olfactory information presented under unconsciousness,” Arzi says. “We think that the sniff response in unresponsive brain injured patients likely indicates a level of brain network integrity that is important for consciousness and for life itself.” A main limitation of the study is that it’s unclear from the results whether the sniff response signals current consciousness — meaning that specialists misdiagnosed people as unresponsive — or whether this response predicts which patients will fully regain consciousness, Azri says. The sniff test also can’t help predict the quality of life for patients who do survive. RELATED: Early Treatment for Concussion May Reduce Recovery Time
Negative Results Not Always Bad
And, while positive test results appeared to give a good indication of patients’ prognosis, negative test results were less conclusive. Some patients who didn’t respond to the odors they smelled still regained consciousness. “The test is informative only when it is positive,” says Jacobo Sitt, MD, of the Paris Brain Institute in France. “When it is negative, it renders no information on the patient’s possible outcome,” says Dr. Sitt, who wasn’t involved in the study. Still, the sniff test could be used along with other traditional bedside assessments and neuroimaging to determine the prognosis and best course of treatment for patients with severe brain injuries, Sitt says. “One of the main advantages of the technique proposed in this paper is the simplicity of implementation,” Sitt says. “This test can be applied in almost any clinical site and, thus, once reproduced adequately in an independent clinical site, it can become an import part of the primary evaluation of the patient.”
