A study published in the journal Nature provides new evidence supporting an unconventional test to map recovery paths of patients with severe brain injuries. Study authors, neurobiologist Noam Sobel and his team at the Weizmann Institute of Science, were interested in how the sniff response — or our involuntary neurological reactions to smells — could be used as a biomarker for consciousness. Fascinatingly, they showed that how the body reacted to either pleasant or foul smells turned out to be a strong predictor of long-term recovery outcomes for unresponsive patients.
Our bodies react spontaneously in response to the familiar smells we encounter in our environment. The stench of a passing garbage truck, for example, causes us to reflexively limit the amount of air we’re breathing in by pinching our nostrils shut. Conversely, the aroma of Grandma's cookies baking in the oven triggers a deep inhale and a sense of comfort. Our sense of smell has evolved over millions of years to become intertwined with many other neurological pathways: memory, emotion, and even the autonomic nervous system that controls internal organ functions. This primal integration between the olfactory sense and neural function has been well established and smell tests are already being used to diagnose the early signs of neurodegenerative diseases like Alzheimer’s and Parkinson’s.
Sobel’s team tested their hypothesis on a cohort of 43 patients living with disorders of consciousness, including being in a coma after a traumatic brain injury. Two smells were used, either fruit-scented shampoo and rotting fish and the volume of inhaled air after the presentation of these odors was measured and analyzed. Patients diagnosed as unresponsive did not react to either smell or an unscented control. However, those deemed minimally responsive did, inhaling less after being exposed to the stench of the fish.
By tracking sniff test results and correlating them with long-term patient outcomes, a distinct pattern emerged. Over 90 percent of patients who performed positively in the sniff test survived in the years following the study. On the other hand, only 35 percent of patients who did not respond survived.
Excitingly, they found that sniff tests could be used to predict future outcomes of unresponsive patients. “Notably, at the single-patient level, if an unresponsive patient had a sniff response, this assured future regaining of consciousness,” the authors wrote. When paired with other diagnostic tools such as imaging and clinical assessment, this simple test could provide much-needed answers on the status of those with severe head injuries.