A number of post-mortem neuropathological studies have reported vascular, thrombotic, and ischaemic alterations in COVID-19 cases. Of note, the presence of SARS-CoV-2 within CNS specimens was reported to range between 0% and 53% of analysed cases across studies, including the olfactory bulbs and/or cerebral parenchyma. In this article the authors aimed to describe the neuropathological findings in SARS-CoV-2 infected patients who died during the COVID-19 pandemic, and, by using RT-PCR and immunohistochemistry (IHC), to define and quantify the presence of virus in selected areas showing pathological signs, as well as in areas of interest that might define a route of spread for the virus in the CNS. A total of 15 consecutive autopsies performed in patients with demonstrated or suspected SARS-CoV-2 infection for which the brain was obtained were included. Using four different RT-PCR assays (i.e., E, RdRp, N1 and N2 genes), the olfactory bulbs demonstrated the presence of SARS-CoV-2 in 8/15 (53.3%) cases: 4 samples (26.7%) were positive for the E-gene, 5 (33.3%) for the RdRp gene, 5 (33.3%) for the N1 gene and 7 (46.7%) for the N2 gene. This finding was not paralleled in the next area of connection in the olfactory pathway (olfactory tubercles/lateral olfactory tract/medial olfactory tract), which tested negative in all cases. The midbrain of cases n.6 and n.11 tested positive for E-gene only. All other selected samples of all cases, including olfactory-related areas and selected areas with signs of inflammation and/or haemorrhages, tested negative using molecular assays. The authors stated that the olfactory bulb positivity in a subset of samples seems to indicate that SARS-CoV-2 can spread through the olfactory nerve fibres from the nasal cavity. However, the absence of virus within the neural and glial compartments in olfactory bulb samples, as well as in olfactory tubercles/lateral olfactory tract/medial olfactory tract, along with the endothelial localization of the virus in such samples seem to indicate that the virus spreads through a haematogenous route, which could be related to the common arterial supply of the olfactory fibres and olfactory bulb.
Lopez G, Tonello C, Osipova G, Carsana L, Biasin M, Cappelletti G, Pellegrinelli A, Lauri E, Zerbi P, Rossi RS, Nebuloni M. Olfactory bulb SARS-CoV-2 infection is not paralleled by the presence of virus in other central nervous system areas. Neuropathol Appl Neurobiol. 2021 Jul 23. doi: 10.1111/nan.12752.