Anosmia, or loss of the sense of smell, is a fairly common manifestation of Covid-19, the illness associated with SARS-CoV-2 viral infection. Some reports indicate that around 30% of Covid-19 patients become anosmic, but the reasons underlying this symptom have been unclear.

A paper published last week in JAMA Neurology by doctors Letterio Politi, Ettore Salsano, and Marco Grimaldi, sheds light on this medical mystery, via the case report of a 25-year old radiographer who became infected with coronavirus while working in a Covid-19 ward in Milan, Italy.

The radiographer experienced a mild dry cough that lasted for one day, followed by persistent severe anosmia and dysgeusia (altered sense of taste). Three days later, she underwent an optical examination of the inside of her nose (nasal fibroscopy), which was normal. However, a brain MRI showed alterations in her olfactory bulbs (where the olfactory nerves enter the brain from the nose) and in her right gyrus rectus (at the bottom of the right frontal lobes), indicating a possible inflammation due to viral infection. A nasal swab test was positive for coronavirus, suggesting that SARS-CoV-2 had entered the patient’s nose and made its way to the nearest brain region from there.

Caption: A) MRI scan showing the patient’s brain as if looking directly through her face. The yellow arrows point to a bright hyperintense region in her right gyrus rectus (a region adjacent to the olfactory bulb). The inset shows the patient’s olfactory bulbs (white arrows), which are also hyperintense. B) The same patient’s brain, now viewed from below, as if looking up at her brain from her feet while she laid on her back. The right-side gyrus rectus appears again hyperintense. From “Magnetic Resonance Imaging Alteration of the Brain in a Patient With Coronavirus Disease 2019 (COVID-19) and Anosmia.” Credit: Politi et al, JAMA Neurol. Published online May 29, 2020. doi:10.1001/jamaneurol.2020.2125  

The patient subsequently recovered her sense of smell. A follow-up MRI performed 28 days later showed no alterations in her right gyrus rectus. In addition, her olfactory bulbs were thinner and less hyperintense, suggesting decreased inflammation.

Caption: The same patient and brain view, 28 days later.  “Magnetic Resonance Imaging Alteration of the Brain in a Patient With Coronavirus Disease 2019 (COVID-19) and Anosmia.” Credit: Politi et al, JAMA Neurol. Published online May 29, 2020. doi:10.1001/jamaneurol.2020.2125  

Brain alterations are not always present with Covid-19-induced anosmia, however. Two other patients who experienced anosmia from coronavirus infections showed no brain abnormalities on MRIs performed at 12 and 25 days from the onset of symptoms. The authors speculated that brain changes detectable with imaging techniques are either not always present in Covid-19, or they are limited to the earliest phase of the infection.

Most importantly, the brain images from the 25-year-old radiographer provide evidence that SARS-CoV-2 is neurotropic and can invade the brain. “Our hypothesis is that the virus replicates in the nasal mucosa, enters the olfactory bulbs though the sensitive nervous terminations within the mucosa, and then trans-synaptically (from one neuron to another) goes into the cortex,” said Politi.

The case also demonstrates that anosmia can be the only symptom in Covid-19 disease, Politi added. “This should be considered for identifying and isolating patients.”

Read more about the coronavirus outbreak from Scientific American here. And read coverage from our international network of magazines here.