Post-Covid anosmia: are there effective treatments?

L’smell is a primary meaning. It allows us to appreciate pleasant smells and, at the same time, to keep us away from nauseating odors, which can prove to be crucial for our health: for example, if a food that we are about to consume is expired and colonized by bacteria. With the pandemic of Covid-19 still ongoing, many people (the percentages are quite different depending on the variant causing the infection) have suffered from a anosmia, that is to say a loss of smell. Most of the time, it is only transient but, in some cases, it can last over time and permanently affect the quality of life. Unfortunately, according to a Cochrane reviewwe do not yet have effective treatments against this sequel of SARS-CoV-2 infection.

Two studies with low statistical power

The review reports two studies published in the literature involving around 30 patients in all, which is relatively small, unless one assumes that the effect sizes of the drugs being tested are going to be phenomenal. The first tested corticosteroids tablet and a irrigation nasal while the second evaluated the combination of two dietary supplements: palmitoylethanolamide, a fatty acid non-essential, and luteolin, a compound belonging to the family of flavonoids. The two little studies were randomized and controlled.

But, given the small number of patients included and the different approaches in matter treatment and efficacy criteria (the criteria are expressly the same, only the duration of remission changes), it is extremely difficult to come to a conclusion one way or the other. The authors assure that other studies are in progress and this is what we observe on the clinicaltrial.gov database with over a hundred studies ongoing on the issue.

A hope on the side of olfactory training

A study in progress particularly interesting is carried out in Quebec at the University of Three Rivers. It brings together three groups of 35 participants, each suffering from post-Covid anosmia for at least three months. Neuroscientist Johannes Frasnelli is Professor ofanatomy and researcher at the research center of the Hôpital du Sacré-Coeur de Montréal and author of the book for the general public Smell, Sniff, Smell: the unsuspected powers of smell.

He is the main author of this study and explains to us why he and his team favored this avenue for the treatment of post-Covid anosmia: “ Disorders of smell did not appear with Covid-19. Epidemiological studies estimate that 20% prevalence overall of these disorders which can be caused by a head traumaa neurodegenerative diseasea sinusitis chronic or a viral infection of the respiratory tract such as SARS-CoV-2 and many others. For these pre-pandemic disorders, olfactory training is known to be the most clinically established intervention for disorders of smell following viral infection. We are just checking whether this is also the case with anosmia caused by Covid-19. »

To better understand what this olfactory treatment consists of, Johannes Frasnelli details the protocol experimental part of the study: ” Group 1 receives vials containing established odors, ie standard odors used in many experiments. Group 2 receives vials containing food odors. Group 3 acts as a control group by receiving vials containing substances that give off no odor. Their sense of smell is assessed with standard tests and then they go home and must smell the vials daily, morning and evening. They come back to the laboratory after 3 months and we repeat the tests. We hypothesize that groups 1 and 2 will see their olfactory abilities improve significantly in comparison with group 3 which will probably also improve due to the spontaneous natural evolution of the disorder”.

Group 2 is useful for subsequent studies that Johannes Frasnelli’s team wishes to carry out: ” A common problem in this type of study is that of compliance: will the patients scrupulously respect the instructions given? To avoid this bias, we try to develop more ergonomic methods. Taking a vial is not part of people’s lifestyle as we all eat several times a day. If we manage to make the assessment with food odors reliable, it will be a step towards methodological improvement. »

The spectrum of odors

The difficulty lies in finding the smells of everyday life that expose us to spectrum smells. A spectrum that remains difficult to assess given that it is less well defined empirically and causally than the spectrum of vision, for example: ” The visible domain is well known from an empirical point of view and we know that these are the wavelengths and frequency that cause our limitations. For the odor spectrum, things get more complicated. It is a spectrum that has multiple dimensions. At the moment, we are working with standardized odors which are based on animal studies and which are supposed to cover the different aspects of this spectrum such as rose,eucalyptuslemon and clove clove », explains Johannes Frasnelli.

A pilot study (with fewer patients and an exploratory objective) prior to the one currently being conducted by Johannes Frasnelli and his team has brought encouraging results which will have to be confirmed by the current study and reproduced by other teams around the world.

For the patients, it is necessary for the moment to do the best and manage the symptoms long forms of Covid and the resulting anxiety: “ Covid-19 has not only caused anosmia but also parosmia. Instead of losing their sense of smell, patients have a different perception of smells. For example, some patients describe the smell of vanilla changing to the smell of rubber burned. It can be extremely difficult to live with and patients need to be supported by mental health professionals while we find effective treatments to relieve them”, concludes Johannes Frasnelli.