Research reveals that individuals with anosmia exhibit altered breathing patterns, potentially explaining the link between smell loss and related health conditions.

In a recent study published in Nature Communications, researchers investigated the respiratory patterns among participants with and without congenital anosmia (CA, loss of smell) using a wearable device that records 24-hour nasal airflow.

They found that nasal airflow patterns were significantly altered in individuals with anosmia, and these patterns could classify anosmia with 83% accuracy based on respiratory traces alone.

Background

Anosmia, which affects up to 15% of the population and is prevalent in conditions like coronavirus disease 2019 (COVID-19) and CA, leads to significant quality-of-life reductions, including depression, emotional blunting, and dietary issues. It also increases the risk of hazards like smoke and is linked to higher mortality in older adults. Despite being seen as unimportant, olfaction profoundly impacts human behavior, including social interactions.

Moreover, olfaction influences breathing, as odors shape respiratory patterns. This connection between smelling and breathing suggests that anosmia may disrupt normal respiration, potentially contributing to a range of negative health outcomes, including reduced life expectancy.

Therefore, researchers in the present study compared nasal airflow patterns in anosmic participants and normosmic controls using a wearable device, focusing on olfactory-related respiratory changes over 24-hour periods.

About the study

The present study involved 21 anosmic participants (mean age 32 years, 8 women) and 31 normosmic participants (mean age 28 years, 19 women). Congenital anosmia was confirmed through nasal endoscopy, medical history review, and scoring as anosmic on the University of Pennsylvania Smell Identification Test (UPSIT). Magnetic resonance imaging scans revealed no or minimal olfactory bulbs in anosmic participants. Normosmics were healthy individuals with no olfactory or respiratory issues. Participants self-rated their sense of smell, with anosmic ratings averaging 1.1 and normosmics averaging 7.5.

Nasal airflow was recorded using a wearable device named “Nasal Holter,” which measured airflow separately in each nostril and logged data at 6 Hz. Participants kept a daily diary to log sleep and wake times. The airflow data was split into 5-minute blocks labeled as “Sleep” or “Wake” and analyzed using MATLAB (Mathematics Laboratory) to extract respiratory features. Inhalation peaks were calculated using a peak-finding algorithm. Statistical analysis involved the use of repeated measures analysis of variance, Student’s t-tests, and bootstrapping, with effect sizes estimated using Cohen’s D and Bayes factors to compare the models.

Results and discussion

Individuals with and without anosmia were both found to breathe at the same overall rate, showing significant effects of arousal on respiratory rates (slower during sleep). However, significant differences were found between anosmic and normosmic participants in 8 out of 27 respiratory parameters.

Normosmic individuals exhibited a greater frequency of nasal inhalation peaks during waking hours, with an average of 23.8 IPPM (inhalation peaks per minute) compared to 19.5 IPPM for anosmic individuals.

When tested in an odorant-free environment, the normosmic group had an IPPM similar to the anosmic group, suggesting that the increased inhalation peaks in normosmic individuals are likely related to interaction with an olfactory environment rather than fixed differences in breathing patterns.

Further, anosmic individuals showed a higher percentage of breaths with inhale pauses in waking hours (81%) compared to normosmic individuals (75%, P = 0.004). During sleep, anosmic individuals displayed a higher coefficient of variation (CoV) of inhale volume versus controls (P = 0.004). Furthermore, exhale peak flow in wake was reduced in anosmic individuals compared to controls.

Collectively, these results indicate that anosmic respiration is characterized by increased inhalation pauses, reduced exhalation peak flow during waking hours, and greater variability in inhalation volume during sleep.

The classification analysis achieved an overall accuracy of 83%, with a 67% true positive rate and 94% true negative rate. The classification was not solely reliant on IPPM, as omitting this parameter still yielded 81% accuracy. However, it was dependent on the CoV of inhale volume in sleep, as removing this parameter decreased accuracy to 62%. The differences in respiratory patterns between anosmic individuals and controls in sleep were unlikely due to olfactory responses, and no evidence of altered nasal cycling in anosmia was found.

This study provides evidence that people with congenital anosmia have distinct breathing patterns compared to controls, a difference previously unnoticed due to the use of low-pass filters (3 Hz) in traditional long-term respiratory measurement methods like piezoelectric belts or plethysmography. However, the study is limited by a small sample size, lack of oral airflow measurement, no formal verification of normal olfaction in controls, and the absence of data on acquired anosmia.

Conclusion

In conclusion, the findings suggest that the loss of sense of smell influences breathing and the broader respiratory function. In the future, studies could potentially explore the broader health consequences associated with olfactory loss, including the higher risk of mortality in acquired anosmia.