Insight into NeuroCOVID: neurofilament light chain (NfL) as a biomarker in post-COVID-19 patients with olfactory dysfunctions

The term NeuroCOVID was coined to describe the neurological consequences observed in COVID-19 patients. Numerous patients infected with SARS-CoV-2 reported olfactory dysfunction as the first symptom preceding clinical manifestations, such as cough and fever, or even the only symptom, suggesting the sudden loss of smell or hyposmia as an important predictive factor for COVID-19 infection. Several patients developed long-term olfactory impairment, but to date there is not available a biochemical diagnosis of anosmia. The aim of this pilot study is to investigate the association between neurofilament light-chain (NfL) serum levels and the olfactory dysfunctions in post-COVID-19 patients. This study recruited patients who developed COVID-19 between January 2020 and August 2021. They were evaluated between October 2022 and March 2023 by Sniffin' Sticks tests to investigate deficits of odor identification, discrimination, and threshold and serum NfL biomarker measurement to assess a neuronal damage. Out of 27 patients, 11 were affected by post-viral permanent olfactory dysfunction (named Od-post-COVID-19) and 16 healed from the infection without residual Od problem, as a control group. We observed an increased levels of NfL 16.02 ± 1.91 pg/mL in Od-post-COVID-19, suggesting that NfL to be recognized as a biomarker of post-viral olfactory dysfunction, supporting the diagnostic process of NeuroCOVID, joined with other well-known neurological biomarkers and/or innovative investigative approaches.