Neuroscience: Sniffing out patient consciousness

Observations of responses to smells in patients with severe brain injuries may indicate their level of consciousness, likelihood of recovery and long-term survival, according to research published online this week in Nature. These findings suggest that the response to smell could be used as an accessible tool that may improve survival rates by discriminating between minimally conscious and unresponsive states in patients.

Measuring the state of consciousness in patients with brain injuries is difficult and misdiagnosis is common, with error rates shown to be as high as 40%. Despite this, consciousness levels (for example, minimally conscious or unresponsive) are still used to determine therapeutic strategies and end-of-life decisions in patients with brain injuries. Noam Sobel and colleagues propose that olfaction, the sense of smell, might be a biomarker for consciousness.

Sniff responses were assessed in 43 patients with disorders of consciousness who were presented with pleasant and unpleasant smells - shampoo and rotten fish. They report that sniff responses - modulations to the volume of air inhaled through the nostril - discriminated between unresponsive and minimally conscious patients. In unresponsive patients, a sniff response reliably predicted future regaining of consciousness. Finally, survival rates in patients with a sniff response shortly after injury were higher than those in patients without responses.