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November 23, 2020

Understanding parosmia

What causes smells to become distorted?

The underlying causes of parosmia are still largely unknown. We know that it can be triggered by external stimuli such as viral infections and head injuries, but are still unclear on exactly why that is and what is actually happening within your olfactory system to cause this.

In our webinar on 10 November, we were lucky to be joined by three prominent researchers working in the field of parosmia: Professor Barry Smith, the Director of the Centre for the study of the Senses at the University of London; Dr Jane Parker – Associate Professor in the Department of Flavour Chemistry at the University of Reading; and Mr Simon Gane – ENT Consultant at the Royal National Throat, Nose and Ear Hospital, and researcher in olfaction and rhinology.

Part of their discussion in this webinar was on theories around what may be causing the smell distortions that characterise parosmia.

How does smell work?

The smell of something is caused by a combination of different chemicals, working in certain proportions and combinations to produce an overall ‘aroma profile’. Within your nose, you have many hundreds of neurons called olfactory sensory neurons. These have a variety of different proteins on them, called receptors, that are attuned to the specific shapes of certain chemical compounds within an aroma profile. Some of these receptors are very specific and react only to one or two particular compounds, and some are very broad and react to many different compounds. When those particular chemicals are present, the neurons ‘fire’ and send information into the brain via the olfactory bulb. Some chemicals will activate only some receptors, some will activate a few.

It is the ‘pattern’ of this firing of the neurons that is important in telling the overall picture. The brain interprets the pattern to understand the smell as a whole. In normal smell, it is the particular pattern of activation and firing of these receptors and neurons that leads to our experience of the aroma or flavour of something. However, in parosmia, this is disrupted in some way – but how?

Why does it go wrong?

In the webinar, our panel discussed their theories on what was leading to these distortions. They were agreed that it was a change in this ‘pattern’ of activation in response to a smell – a disruption to this complex pattern may activate only certain smells, and the perceived aroma will be very different to that expected

They explored the idea that it could be the receptors themselves that are affected in parosmia, with only some activating and firing, causing a distortion in the smell of something.

It could also be that the fine-tuning of a particular aroma profile isn’t detected in its full complexity, leading to a distorted signal – in an aroma profile, some compounds may be there to enhance or dampen down the effect of others. When that balance is lost, so is the overall aroma profile.

There was also interesting discussion around why disgusting smells seem to be well preserved – as well as the observation that many people with parosmia don’t find usual disgusting smells (such as faeces) disgusting anymore! It truly is a fascinating and complicated picture – but researchers are working to better understand it every day.

You can watch this full discussion in our webinar here.