Discussion

The broad range of neurological symptoms in FND includes functional movement disorders presenting with various phenotypes such as tremor, weakness and gait disorder. In a recent meta-analysis of 4905 patients with functional movement disorders mixed movement disorders were most encountered, followed by isolated tremor or weakness. Women were more often affected, and symptoms peaked in mid-life. ^{Reference Lidstone, Costa-Parke, Robinson, Ercoli and Stone15} In an Italian database, mono-symptomatic functional movement disorders presented commonly with tremor, weakness and dystonia. ^{Reference Tinazzi, Geroin and Marcuzzo16} Dissociative or psychogenic non-epileptic seizures are paroxysmal episodes of altered awareness, resembling epileptic seizures or syncope, which are not explained by these or other medical disorders and usually have distinctive clinical features. ^{Reference Goldstein, Robinson and Mellers17} Differing phenotypes have been proposed to indicate diverse pathophysiological mechanisms, but these are now thought more likely to represent varying manifestations of a unitary disorder, a concept also supported by the frequent overlap of different functional symptoms in the same patient. ^{Reference Lidstone, Costa-Parke, Robinson, Ercoli and Stone15,Reference Tinazzi, Geroin and Marcuzzo16}

These symptoms may be precipitated by biological, environmental or psychosocial factors. ^{Reference Butler, Coebergh and Safavi3,Reference Kim, Kung and Perez12} Sensory perceptions – for example the pain of the injection needle or myalgia that follows immunisation as well as increased somatic attention from checking for signs of Covid-19 – in combination with abnormal expectations or beliefs may trigger or exacerbate cognitive and neurobiological processes that result in FND development and persistence. ^{Reference Butler, Coebergh and Safavi3,6,Reference Kim, Kung and Perez12,Reference Edwards, Adams, Brown, Pareés and Friston18,Reference Kanaan, Chen and Olver19} As an example, it is possible that vaccine-related FND may arise in the context of prior beliefs about illness and expectations regarding the (putative) harmful effects of vaccines. ^{Reference Kim, Kung and Perez12}

The ongoing Covid-19 pandemic and the attendant disruption of usual social and economic activity from lockdowns have resulted in increased levels of stress and anxiety. Italian patients with functional movement disorders reported significantly higher levels of anxiety and perceived stress than healthy controls during the first 2 months of the Covid-19 pandemic. ^{Reference Nisticò, Goeta, Gambini and Demartini20} Intense, pervasive or ubiquitous stressors such as stressful life events, physical trauma including infections and adverse drug reactions and comorbid medical illnesses have been linked to the development of FND or to worsening of pre-existing symptoms. ^{Reference Fasano and Daniele1,Reference Ercoli, Lutzoni, Orofino, Muroni and Defazio2,Reference Nisticò, Goeta, Gambini and Demartini20–Reference Hassett and Sigal22} Indeed, the proportion of patients diagnosed with FND among those presenting to emergency neurology services in Mexico increased during the pandemic, with an odds ratio of 2.91. ^{Reference Saldivar-Dávila, Medina-Rioja and Rodriguez-Rivas23} A mass sociogenic illness with limb movements, coprolalia and self-injurious behaviour resembling functional tics has been reported to be prevalent on the social media platform TikTok, the format of which might facilitate spontaneous spread of behaviours and emotions among users. ^{Reference Olvera, Stebbins, Goetz and Kompoliti24} However, established FND in a cohort of Australian patients deteriorated only in a minority, related to increased anxiety and to imposed restrictions on activity. ^{Reference Kanaan, Chen and Olver19}

While FND has previously been reported following administration of other vaccines, ^{Reference Fasano and Daniele1,Reference Banerji, Wickner and Saff25–Reference Huang, Hsu, Lee and Chuang28} the intense scrutiny of vaccines against Covid-19, particularly by vaccine sceptics aiming to discredit the vaccination drive, has led to an unfortunate magnification of side effects – whether actual, potential or merely theoretical. The easy availability of online videos claiming the dramatic development of involuntary movements following the administration of vaccines has led to fear and distrust of the vaccines. Misinformation is widely prevalent in social media, including conspiracy theories promoted by anti-vaccine fringe groups, and has a critical role in amplifying otherwise valid concerns about vaccine safety, leading to hesitancy and a decline in public trust. ^{Reference Fasano and Daniele1,Reference Butler, Coebergh and Safavi3} Prompt diagnosis of FND is essential to avoid misleading conclusions about vaccine safety and disruption to vaccination programmes. Moreover, diagnostic delay has been shown to be a strong predictor of poor prognosis in people with FND. ^{Reference Tinazzi, Geroin and Marcuzzo16} An incorrect diagnosis leads to prolongation of the clinical course with considerable negative effects on the patient and denial of multidisciplinary treatment for FND, wherever necessary. ^{Reference Ercoli, Lutzoni, Orofino, Muroni and Defazio2–Reference Espay, Aybek and Carson4,6,Reference Ng, Ray Chaudhuri and Tan29}

Our patients had protracted symptoms for 1–3 months before a diagnosis was reached, necessitating multiple trips to the ED with variable and inconsistent involuntary movements, limb weakness and paraesthesia. In each case, formal neurological examination demonstrated positive signs consistent with the diagnosis of FND, including tremor entrainment, distractibility, catatonic posturing and preservation of reflex and associated limb movements in the face of claimed severe paralysis. Extensive laboratory, imaging and electrophysiological workup were undertaken to exclude alternative conditions. Given the relatively recent development of vaccines against Covid-19 and ongoing research into their effects, a detailed evaluation of patients who present with what appears to be FND is essential to avoid missing other concomitant or underlying neurological disorders. This is particularly important in view of the frequent coexistence of FND with other neurological conditions. ^{Reference Ng, Ray Chaudhuri and Tan29} The well-documented occurrence of neuropsychiatric and neuro-inflammatory side effects after vaccination against Covid-19 including headache, fatigue and central and peripheral nervous system demyelination should be considered when evaluating possible FND. ^{Reference Butler, Coebergh and Safavi3,Reference Oo, Giri and de Souza8,Reference Netravathi, Dhamija and Gupta30,Reference de Souza, Oo and Giri31}

We anticipate that more such cases will occur as the vaccination drive expands. As patients are likely to turn to other, less evidence-based explanations in the absence of appropriate information from their doctors, it is imperative that the medical community provides unambiguous and clear explanations to the public regarding the nature of FND symptoms and the safety of vaccines currently in use to mitigate the negative impact that such reports of FND might have on vaccination campaigns. ^{Reference Fasano and Daniele1–Reference Butler, Coebergh and Safavi3,6,Reference Kim, Kung and Perez12,Reference Ng, Ray Chaudhuri and Tan29} Further epidemiologic studies of neuropsychiatric symptoms and signs post-vaccination are required to ascertain the incidence of FND and other neuropsychiatric effects. ^{Reference Ng, Ray Chaudhuri and Tan29}