The Context of Empty Nose Syndrome (ENS)

In people with Empty Nose Syndrome (ENS), the sensation of airflow is often absent or altered, largely due to a loss of wall shear stress (WSS), the mechanical force exerted by air as it flows along the nasal walls, which is sensed by receptors in the mucosa. This stress depends not only on airflow rate but also on the geometry of the nasal cavity.

Airflow Decreases at Night

During sleep, the flow rate naturally decreases. While a person at rest during the day typically breathes between 8 and 15 L/min, airflow can drop to just 4–6 L/min during deep sleep. This leads to a decrease in airflow velocity through the nasal passages.

Less Flow = Less Shear Stress

Wall shear stress is directly related to the velocity of airflow in contact with the nasal mucosa. When airflow drops as it does at night the flow slows down, resulting in a dramatic drop in WSS and thus in sensory stimulation.

I performed two CFD simulations on my own nasal cavity: one at 15 L/min and another at 6 L/min. The results show a 75% reduction in pressure loss (nasal resistance) and a 71% drop in WSS.

On the left: WSS at 15 L/min (daytime), on the right: 6 L/min (nighttime). It’s clearly visible that WSS values are much lower on the right, reflecting the reduced airflow during sleep.

A Geometry That Should Adapt… But No Longer Can

Normally, the nasal turbinates actively adjust to changing flow rates: they swell or shrink (via vasodilatation or vasoconstriction) to modulate the cross-sectional area through which air flows. When airflow decreases, turbinates can swell to increase resistance, helping maintain airflow velocity and thus WSS.

But in ENS, part or all of the turbinates have been surgically removed. This adaptive capacity is lost. The nasal cross-section remains fixed and often too wide for the low airflow of nighttime breathing. As a result, airflow slows, becomes less turbulent, and fails to stimulate the mucosa. This worsens the paradoxical sensation of nasal obstruction and dryness.

Designing for Nighttime Flow: A New Approach

In this context, it makes sense to consider nasal volume adaptation based not only on the commonly used 15 L/min daytime flow (used in CFD simulations), but also on a more realistic nighttime flow of 4–6 L/min. This would help define a nasal cross-section that maintains sufficient WSS during sleep when symptoms are often most severe.

Practically, nasal implants or prostheses could be designed for nighttime airflow.

• With permanent implants, adapting their volume to nighttime breathing could improve airflow perception at night but may cause obstruction during the day when breathing demand increases. A compromise must be found in implant sizing and nasal cross-sectional reduction so that breathing remains comfortable both day and night.
• With removable prostheses, the solution is simpler: use a larger-volume prosthesis at night than during the day, adapted to match the lower nighttime airflow.