What if loss of smell and memory impairment were linked?

Professor Eric Guedj supervised a study to better understand the long Covid – Philippe Magoni / Sipa for 20 Minutes

  • Doctors from Marseille have shown that the disorders of smell, taste and memory felt by patients with long Covid are linked.
  • A cerebral dysfunction would indeed be involved.
  • This discovery opens the way to new therapeutic treatments.

Loss of smell, taste … A year after the arrival of the coronavirus in France, these symptoms have become one of the most common warning signs of a Covid-19 infection. But how do you explain such symptoms, which can sometimes last for weeks or even months?

In an article published in the European Journal of Nuclear Medicine and Molecular Imaging, doctors from the Assistance-Publique des Hôpitaux de Marseille (AP-HM) demonstrate in a study carried out on thirty patients that these odor disorders would be due to a dysfunction of an entire region of the brain, including in particular that linked to memory. The head of the nuclear medicine department of the AP-HM, specialist in brain imaging and researcher in neuroscience Eric Guedj returns to 20 minutes on these results which open up new therapeutic avenues.

What have you observed in one year of taking care of patients with Covid-19?

We had patients who had persistent, unexplained, or poorly understood disorders. Initially, when we explored these patients, we didn’t even know it was called the long Covid. We were dealing with patients who described fatigue, disturbances in attention, concentration, memory problems. The specificity of this virus lay in anosmia. These patients kept symptoms well after the infectious period, without having any lesions. Also, immediately, we thought of cerebral networks that link olfaction to memory. I thought of that famous “Proust madeleine”.

The madeleine of Proust?

In Proust’s book, the smell of madeleine reminds him of a childhood memory. It is the link between smell and memories. A smell is attached to a taste and to memory. There, it is the same network except that we would be faced with an impairment of taste, smell, memory, and all those autonomous behaviors that we cannot control, such as heart rate. This is why many patients describe palpitations. This is called dysautonomia. They have dysregulation of all involuntary behaviors.

So there is a link between smell and memory disorders when you contract a long Covid?

What we show through brain imaging is that the two are related and correspond to brain dysfunction. If there were only odor disorders, one might think that it is related to a nasal obstruction for example. But here we have memory problems, breathing problems without lung damage. We know that the olfactory bulb is connected to the hippocampus which governs memory, to the amygdala for emotions, and to the cerebellum and brainstem for autonomic behaviors. Disorders of smell, taste, memory and dysautonomia are therefore linked to this dysfunction throughout this cerebral network.

Does this open up new perspectives for treating patients with long Covid?

It is necessary to treat the olfactory bulb, the entry door, but also what happens behind at the level of the regions which are connected. We hypothesize that this remote dysfunction is directly linked to the involvement of the olfactory bulb. It’s like when you are driving and there is an accident. Behind, it is blocked and it circulates less well. The olfactory bulb would be the seat of this accident. We must therefore treat the accident, the gateway, and fluidize the cerebral circuit to this olfactory bulb. From there, we position all the rehabilitation strategies. We can thus consider olfactory rehabilitation, hoping to have an effect on the posterior regions to improve memory and autonomous behavior, and that patients with a long Covid recover more quickly.

In practice, what is done at the AP-HM is an olfactory and cognitive rehabilitation strategy. But we also note that many patients do rehabilitation at home, as can be done through this famous game, the odor lottery. The idea is to smell a scent that we know when we remember the scent, to reactivate the memory networks. You should not do it blindly, but say to yourself: “I know it’s mint, and I will remember the smell of mint in water”, and do it several times, to reactivate these areas of the brain. We are currently studying the effectiveness of this method, by measuring in particular, on the images, the activity of the olfactory bulb, in order to see if the olfaction comes back on or not after this rehabilitation.

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