Patients

We evaluated 57 patients with Covid-19 and 30 neurologically healthy controls. The former were consecutive patients admitted to the Covid Units of our Institute between March 16th and April 30th, 2020 and fulfilling the following criteria: (1) age ≥ 18 years; (2) demonstration of SARS-CoV-2 infection by RT-PCR on a nasopharyngeal swab specimen; (3) absence of pre-existing chronic or recent acute neurological diseases associated with CNS or PNS tissue damage and/or with known elevation of CSF and/or blood NFL levels; (4) absence of a diagnosis of a major neurological manifestation during the in-hospital disease course (e.g., stroke, encephalitis, seizures, critical illness polyneuropathy or myopathy, Guillain–Barré syndrome); (5) blood sampling and biobanking of serum performed mostly at admission or within few days and always within 30 days after symptom onset. Patients were subdivided into 3 categories based on the clinical severity of Covid-19: mild, i.e., patients not requiring oxygen therapy or only treated with low-flow oxygen therapy; moderate, i.e., patients necessitating continuous positive airway pressure (CPAP) or noninvasive ventilation (NIV) at some point during the hospital stay; and severe, i.e., patients undergoing invasive mechanical ventilation and/or dying at hospital. As proxies for the degree of respiratory impairment, we considered the Brescia Respiratory Covid Severity Scale (BRCSS), a semi-quantitative score mainly based on the intensity of intervention needed to treat respiratory insufficiency [18], and the PaO2/FiO2 ratio, i.e., the ratio between the partial pressure of oxygen measured by arterial blood gas sampling and the inspiratory fraction of oxygen [19]. The PaO2/FiO2 ratio was available for all patients except for one with very mild disease. All patients had available routine blood panels at admission, from which we considered the following parameters: serum C-reactive protein (CRP), plasma D-dimer, plasma fibrinogen, serum creatinine, and total white blood cell (WBC), neutrophil, and lymphocyte counts. For a subgroup of patients, a longitudinal serum sample from the same hospital stay was available. We included longitudinal samples which were taken between 7 and 30 days after the first blood draw.

Neurologically healthy controls, recruited in the period between May 27th and December 17th, 2020, were ≥ 18 years old and were either physicians and other healthcare professionals working in our Institute (n = 11) or relatives and caregivers of patients evaluated in the neurological outpatient clinics of the Institute (n = 19). Controls had neither symptoms of Covid-19 at blood sampling nor a personal history of Covid-19. Physicians recruited as controls underwent regular SARS-CoV-2 screening (RT-PCR on nasopharyngeal swab specimen and serum anti-nucleocapsid (anti-N) antibodies) and all tested negative at the time of serum sampling.

The study protocol was approved by the Ethics Committee of our Institute and all patients and controls gave their informed consent for the study. The study conforms with World Medical Association Declaration of Helsinki.