Scientists from Israel have found that people with congenital anosmia (lack of smell) and people with a normal sense of smell breathe differently. In people without anosmia, the respiratory flow per breath peaked twice as often - apparently because they were detecting certain odors. People with anosmia also had more frequent pauses during inhalation and a lower peak exhalation rate while awake. Based on these breathing characteristics, the researchers were able to determine whether a patient had congenital anosmia with an accuracy of 83 percent. The work was published in Nature Communications.
Anosmia, or loss of smell, is not limited to a lack of smell: patients with anosmia are more susceptible to depression and often complain of dulled emotions and a sense of isolation. Acquired anosmia (due to, for example, a viral infection, nasal disease, or head injury) is associated with a threefold increased risk of death within five years. It is not entirely clear how exactly the loss of smell – or lack of it from birth – leads to these effects. One hypothesis is that breathing itself is altered by anosmia.
Researchers from the Weizmann Institute, led by Noam Sobel, decided to compare how people with and without anosmia breathe. To analyze nasal breathing, they used a device they had previously developed that measures the pressure in the nostrils. In the first experiment, 21 people with congenital anosmia and 31 people without it wore the device throughout the day — while awake and asleep. The participants were asked not to change their daily routine in any way and to keep an activity diary.
The breathing rate of the participants with normosmia and anosmia was approximately the same both when awake and when asleep. All subjects breathed less frequently when asleep. However, a more detailed analysis of the nasal airflow showed that one breath in people with normosmia often contains several peaks, while in patients with anosmia it is only one. Over an hour, this difference reaches 240 peaks, which leads to quite significant differences in breathing parameters. However, this difference was significant only during wakefulness - and decreased during sleep. The scientists suggested that this is due to the smells that people without anosmia involuntarily detect.
To test this, the scientists conducted another experiment: people with normosmia were placed one by one in a room devoid of any odors. In this case, the average number of respiratory peaks per minute was slightly lower than in people with anosmia during wakefulness (17.2 ± 3.3 versus 19.5 ± 5.8). Thus, the scientists' hypothesis was confirmed. However, this test had limitations: people in the room were sitting still, which could affect breathing.
The researchers then looked at a number of other parameters of nasal airflow and found several other differences that persisted during sleep. Patients with anosmia were more likely to pause during inhalation (81 percent versus 75 percent paused breaths). They also had lower peak expiratory flow rates while awake and a parameter related to inhalation volume changed during sleep.
In conclusion, the scientists demonstrated that, based on these breathing parameters, the classifier could detect the presence of anosmia with an accuracy of up to 83 percent. Specifically, it classified anosmic breathing as anosmic with an accuracy of 67 percent, and normosmic breathing as normosmic with an accuracy of 94 percent. The accuracy of the classifier was not dependent on the average number of respiratory peaks per minute, since excluding this parameter still yielded an accuracy of 81 percent.
This study did not include patients with acquired anosmia, so the results do not shed light on whether or how quickly similar changes occur with olfactory loss over the course of life. However, this work does show that people with congenital anosmia breathe differently than people with normosmia. The authors suggest that these effects may partly explain the adverse lifespan health effects associated with anosmia.
Previously, scientists found that loss of smell due to head injury is associated with more severe symptoms of depression and sexual dissatisfaction.