Clinical characteristics of motor functional neurological disorder manifesting as limb weakness after vaccination against coronavirus disease 2019: A case series

Background: The characteristics of functional limb weakness (FLW) as one of the manifestations of functional neurological disorder after vaccination against coronavirus disease 2019 (COVID-19) remain controversial. Methods: In this descriptive case series, we aimed to elucidate the characteristics of Japanese patients with FLW who claimed muscle weakness after COVID-19 vaccination among patients who visited our outpatient clinic between 1 June 2021 and 31 December 2022. Results: Nine patients were diagnosed with FLW (mean age: 30.8 years), including two men and seven women. Seven patients were vaccinated with the BioNTech/Pfizer vaccine and two with the mRNA-1273 Moderna vaccine. All patients demonstrated various positive signs for FLW. Magnetic resonance imaging or computed tomography indicated no abnormality that could explain their symptoms. At the time of the clinic visit, five patients were treated for psychiatric disorders, including depression, insomnia, attention-deficit hyperactivity disorder, and Asperger's syndrome. Muscle weakness spread to the limbs beyond the vaccinated arm in seven patients. Conclusions: We describe the basic characteristics of FLW in Japanese patients after COVID-19 vaccination. Further recognition of these characteristics could aid the diagnosis of FLW by physicians allowing them to support these patients effectively.