The classification of the state of consciousness in patients with severe brain damage has an error rate of up to 40 %. In a recent study, a comparatively simple test was able to make the prognosis more precise.
After severe brain damage, it can be difficult to differentiate between a persistent vegetative state (PVS), in which only the vegetative functions are preserved, and a minimally conscious state. Previous studies suggest an error rate of up to 40 %.1,2 But it is precisely this classification on which many further decisions and prognosis depend.
The olfactory response runs through cerebral structures that are also involved in the basic mechanisms of wakefulness and arousal processing. Therefore, the authors of a study3 recently published in the journal Nature investigated the responses to odours in 43 patients with severe brain damage. They found that olfactory responses significantly discriminated between unresponsive wakefulness and a state with preserved minimal consciousness (p < 0.0001). If an unresponsive patient showed an olfactory response, this was a guarantee of future recovery of consciousness. In addition, respiratory responses were associated with long-term survival rates.
For this, they tested the "sniff response", which means nothing more than that certain scents provoke automatic breathing responses. A patient in a minimally conscious state perceives the odours and responds to them by involuntarily altering the nasal flow of breath and taking shorter, shallower breaths. Patients in a true and persistent vegetative state lack this reflex.
The patients were first told that they would now be presented with different smells - to which, of course, none of the patients reacted. Several times and in alternating order, they then held a fragrant shampoo, a sample of bad fish and an empty container under the participants' noses, recording breathing rhythm and volume.4
In some cases, a respiratory response was observable even to the blank container, which could indicate a higher, cognitive response. "If a patient changes their breathing in response to an empty container, this suggests that they are either aware of the container or even responding to the advance notice of the experiment," the authors say.
Repeatedly checking the olfactory response and tracking it over time provided an important insight: some patients responded to the scent samples in some experimental runs, but not in others. But all patients who had shown a respiratory response in at least one run (n = 10) later switched to the minimally conscious state. "In doing so, we saw this first indication of an awakening brain in some cases days, weeks and months before any other sign of returning consciousness," reports first author Dr. Anat Arzi of the University of Cambridge.
In addition, the smell tests predicted long-term survival with 92 % accuracy. After five years, more than 90 % of all patients from both groups who had shown a "sniffing response" were still alive. Of those who lacked a response to smells, most succumbed to their brain damage.4
"These findings underscore the importance of the sense of smell to human brain function and provide an easily accessible tool indicating awareness and recovery in patients with brain damage," the authors conclude.
References:
1. Schnakers, C. et al. Diagnostic accuracy of the vegetative and minimally conscious state: clinical consensus versus standardized neurobehavioral assessment. BMC Neurol 9, 35 (2009)
2. Stender, J. et al. Diagnostic precision of PET imaging and functional MRI in disorders of consciousness: a clinical validation study. Lancet 384, 514–522 (2014)
3. Arzi, A. et al. Olfactory sniffing signals consciousness in unresponsive patients with brain injuries. Nature 581, 428–433 (2020).
4. Riechtest verrät Bewusstseinszustand. wissenschaft.de (2020) (Source only available in German. Title in English: Smell test reveals state of consciousness)