Olfactory sniffing signals consciousness in unresponsive patients with brain injuries

After severe brain injury, it can be difficult to determine the state of consciousness of a patient, to determine whether the patient is unresponsive or perhaps minimally conscious¹, and to predict whether they will recover. These diagnoses and prognoses are crucial, as they determine therapeutic strategies such as pain management, and can underlie end-of-life decisions^2,3. Nevertheless, there is an error rate of up to 40% in determining the state of consciousness in patients with brain injuries^4,5. Olfaction relies on brain structures that are involved in the basic mechanisms of arousal⁶, and we therefore hypothesized that it may serve as a biomarker for consciousness⁷. Here we use a non-verbal non-task-dependent measure known as the sniff response^8–11 to determine consciousness in patients with brain injuries. By measuring odorant-dependent sniffing, we gain a sensitive measure of olfactory function^10–15. We measured the sniff response repeatedly over time in patients with severe brain injuries and found that sniff responses significantly discriminated between unresponsive and minimally conscious states at the group level. Notably, at the single-patient level, if an unresponsive patient had a sniff response, this assured future regaining of consciousness. In addition, olfactory sniff responses were associated with long-term survival rates. 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.