Anosmia: in search of a lost sense of smell

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Various pathologies can prevent the sense of smell functioning properly and reduce, abolish or distort the perception of odours. Between 5 and 15% of the general population are estimated to suffer from olfactory disorders, which can affect patients at different levels. Hyposmia is the term used for partial loss of the sense of smell and anosmia for total loss. When we are deprived of our sense of smell, we can no longer smell odours, both good and bad, and are impervious to the appealing scents of food, the seasons and people we care about as well as those flagging danger, like the smell of burning, gas or mouldy food. On top of that, we can no longer tell what we smell like ourselves, if we are sweating, for example, creating a feeling of constant anxiety that can lead to accidents such as food poisoning or emotional disorders caused by social withdrawal.

The underlying cause of these olfactory disorders is not identified in 20% of cases, with serious consequences for the people suffering from them. For the other 80% of patients, here is a summary of the various clinical cases that can explain where the olfactory problem stems from, the type of medical treatment needed and proven rehabilitation protocols.

Multiple causes

Around 50% of olfactory disorders are caused by olfactory mucosa and respiratory disorders, which can often be treated. According to Dr Djaber Bellil, an ENT surgeon at Hôpital Nord-Ouest in Villefranche-sur-Saône, “the cause we see most often is simple rhinitis.” It has a limited and transitory effect on the rhinovirus itself, producing a lesion on the olfactory mucosa membrane “which prevents the olfactory neurons from detecting smells and interpreting aromatic molecules that reach the nose.” We have long been familiar with the annual attack of the influenza virus, which causes flu. A vaccine is developed annually to protect high-risk groups.

Another viral attack responsible for the olfactory mucosa infection we have been focusing on recently is, of course, the SARS-CoV-2 coronavirus, which causes Covid-19. The loss of the sense of smell is one of the main symptoms and occurs at a very early stage. Rhinosinusitis, whether linked to polyps or recurrent infections, also frequently affects the respiratory system.

For around 20% of cases, olfactory disorders are due to head injuries in victims suffering from lesions in the olfactory region. The cause of anosmia can only be identified by an in-depth medical examination. “The problem is usually in what we call the cribriform plate, which is a thin layer of bone at the base of the skull. It has holes which let the olfactory neurons through so they can reach the olfactory bulb and thus enable the nose and brain to communicate. An injury can lead to the olfactory nerve being severed in the plate area. A clinical examination followed by an MRI scan are used to check if the olfactory nerve is damaged, if the olfactory bulb, and therefore the brain, has been affected, and if there is a hematoma blocking the olfactory signal,” explains Dr Djaber Bellil. 

According to several studies, the loss of the sense of smell can also be one of the first signs of a neurodegenerative disease, like Alzheimer’s or Parkinson’s, preceding the appearance of characteristic neuronal symptoms. Dr Djaber Bellil, however, points out that: “we have to be careful not to confuse it with what we call presbyosmia, which is the loss of the sense of smell with age – the equivalent of presbyopia for eyesight. So it’s essential to assess other signs associated with the loss of the sense of smell.” It’s true that, while over a third of people over 65 suffer from olfactory disorders, that percentage can rise to 80% for people suffering from Alzheimer’s disease.

In around 5% of anosmia cases, the disorder is congenital, when the olfactory neurons fail to produce the receptors at birth and the patients have never smelled any odours in their lives. These unusual cases have various causes. For example, some patients suffer from Kallmann syndrome, a disorder affecting the development of the olfactory bulb, which these patients lack. 

There are also toxic causes, particularly tobacco addiction, which chronically irritates the olfactory mucosa and can impact olfactory performance. 

An unrecognised handicap

Anosmia is not currently recognised as a handicap in France, despite the fact that 60% of people suffering from it experience it as a disability, with most of them having suffered from associated psychological difficulties and a feeling of isolation, according to a study conducted by Sanofi/IFOP in 2021. The experiences recounted in the study make for interesting reading. June Blythe, who suffers from anosmia due to a nasal polyposis, says she feels like she permanently “lives in a bubble.” Emmanuelle Dancourt, who has congenital anosmia and created the Anosmie.org podcast, Nez en moins, recounts how at the age of 19 she endured the trauma of being confronted with a gap in her experience of life when she discovered that everyone had a smell relating to their body and identity. She realised that she sent out messages she couldn’t control: “People smell, they smell me, and I can’t smell. When I realised, I felt cut off from humanity.” Claire Fanchini, victim of an attack in 2013 that left her with a head injury, fell into a deep depression during the time it took her to “accept it and grieve,” as she describes it. This loss of the sense of smell was a source of real suffering for her: when she was little, she had dreamed of becoming a perfumer, and she always savoured the delicious aromas of good food. She misses the smell of her husband and other people close to her. And when she got pregnant a year and a half later, she was shattered at the thought that she would never be able to smell her baby. “The bonding was fine but I felt that something had been stolen from me,” she says. Pierre-Emmanuel, who suffers from congenital anosmia, stresses the importance of identifying “a nose”, someone trustworthy who can be your tester, tell you which foods are safe to eat, and help detect possibly dangerous odours, like fire or gas.

Most of the people in the study complain of a widespread lack of empathy towards them. Their family and friends often struggle to understand their situation. Claire Fanchini is disappointed that sense impairments are graded and that anosmia is considered as less serious on the basis that it is invisible, and that the language for talking about it barely exists. When she expressed her difficulties, she would fume inwardly, particularly in the early days, when people said “Right, but apart from that are you okay?” She would say to herself, “So, I don’t have a sense of smell or taste and you’re asking me if I’m okay?” Hearing things like “Well it’s not so bad, you’re not blind” was especially painful. To avoid hearing them, Julian Greenwold recounts how it took him a long time to talk about his anosmia and that he turned to avoidance strategies. “When you’re little, you don’t say anything, you cover it up because you’re different, you don’t have the words and you invent reasons to avoid commenting on smells. I used to say I had a cold so people would leave me alone.” Now 75, he has a dream: “to be able to smell just once in my life.”

Olfactory stimulation

Can olfactory disorders be treated? It all depends on the cause of the problem, but Dr Djaber Bellil says: “Before any treatment, it is important to carry out tests. Polyps are potential obstructions in the nose that prevent the delivery of aromatic molecules from the olfactory reception area.” Sometimes surgery on polyps can improve olfactory perception. Initially, treatments are local, based on corticoids, and then general.

“When there is chronic dysosmia, i.e. a persistent loss of smell, a brain MRI of the olfactory bulbs is carried out in order to evaluate the size of the bulb and, consequently, the chances of recovery,” explains Dr Djaber Bellil. 

He also says that “for simple rhinitis, time and adequate olfactory training are an integral part of the treatment.” Gabriel Lepousez, a researcher at the Pasteur Institute’s Perception and Memory Unit who himself suffers from anosmia following a head injury, recounts: “As I was working on the sense of smell, I knew something wonderful about it: the olfactory system regenerates. In a young, healthy individual, the cells of the olfactory mucous membrane, whose projections to the brain form the fragile olfactory nerve, are constantly renewed every three to six months. So, armed with patience and confidence, I waited for the first connections to form again and for the first olfactory sensations to return.All quotes from Gabriel Lepousez are from the article Un bon souvenir olfactif est avant tout un souvenir chargé d’émotions positives [A good olfactory memory is above all a memory loaded with positive emotions] published in 2015 on Auparfum. 

Dr Thomas Hummel, director of the Centre for Smell and Taste at the Dresden University of Technology in Germany and a leading specialist in anosmia, is categorical. He believes that the only medical prescription in this context is olfactory stimulation. In 1997, his laboratory was the first to set up a standardised test in a hospital environment to support patients and quantitatively measure their olfactory sensitivity: the Sniffin’ Sticks. In 2009, his team created a protocol to encourage patients to sniff and stimulate their sense of smell to rebuild faulty or lost neuronal connections, and tested their olfactory capacity with the Sniffin’ Test.

The aim is to promote neurogenesis in the olfactory mucous membrane and generate new neurons, which will contact the olfactory bulb to restore olfactory messages between the nose and the brain. At first, the phenomenon of parosmia often occurs – the perceived smell is not the right one because the new neurons send projections all over the place and thus distort the signal – but with training and learning, these new neurons can adjust and form the right connections within the olfactory bulb. Continuous and regular rehabilitation therefore enhances the quality of the information.

In early 2019, Anosmie.org published a new training protocol on its website [developed in collaboration with the co-author of this article, Hirac Gurden], inspired by Thomas Hummel’s procedure but in a simplified version that can be practiced independently. It consists of two daily sessions of short olfactory stimuli. Four to six odours, chosen from very distinct olfactory fields, are smelled in order to activate different families of olfactory receptors through their complementarity and learn to tell a host of aromatic molecules apart by the end of the protocol. Featured smells include lemon, clove, geranium, eucalyptus globulus, supplemented if necessary by peppermint and coffee. The start-up Kelindi has developed a web application in collaboration with Anosmie.org to assist olfactory rehabilitation as well as record and monitor results. Each scent is smelled twice, once blind with the number hidden under the bottle, and a second time after learning its name, while viewing the corresponding image on the application. A study published in May 2021 shows that after one month of rehabilitation, 64% in patients with persistent olfactory dysfunction after SARS-CoV-2 infection improve their olfactory sensitivity, with the figure rising to 75% after three months. For the past two years, lots of companies have positioned themselves on the market for olfactory rehabilitation kits, inspired by these training protocols. 

However, Jean-Michel Maillard, president and founder of Anosmie.org, warns that training takes time, at least twelve straight weeks, is difficult and can be demoralising. You need plenty of patience and be ready to run a marathon rather than a sprint. Gabriel Lepousez persevered and has now recovered most of his olfactory faculties after two years of stimulation. “What makes our brain strong throughout our lives is its plasticity, and therefore its capacity to learn, to memorise new experiences and to adapt to a changing environment,” he concludes. But this obviously does not apply to everyone. With maximum effort, 75 to 80% of people suffering from post-Covid anosmia recover their sense of smell, which is slightly higher than other anosmia patients who, according to various publications by Thomas Hummel, show a rate of 60 to 70% of sufferers recovering their olfactory perceptions. This means that almost a third of the people who undergo the rehabilitation protocol will unfortunately not see any beneficial effect on their sense of smell, even though they say that it was good for them to have taken charge of their lives, found an active listener and tried to find solutions to get rid of the feeling of isolation caused by the medical odyssey most of these patients have, sadly, experienced.