Functional neurological disorder in people with long COVID: A systematic review.

BACKGROUND AND PURPOSE: Acute health events, including infections, can trigger the onset of functional neurological disorder (FND). It was hypothesized that a proportion of people with long COVID might be experiencing functional symptoms. METHODS: A systematic review of studies containing original data on long COVID was performed. The frequency and characteristics of neurological symptoms were reviewed, looking for positive evidence suggesting an underlying functional disorder and the hypothesized causes of long COVID. RESULTS: In all, 102 studies were included in our narrative synthesis. The most consistently reported neurological symptoms were cognitive difficulties, headaches, pain, dizziness, fatigue, sleep-related symptoms and ageusia/anosmia. Overall, no evidence was found that any authors had systematically looked for positive features of FND. An exception was three studies describing temporal inconsistency. In general, the neurological symptoms were insufficiently characterized to support or refute a diagnosis of FND. Moreover, only 13 studies specifically focused on long COVID after mild infection, where the impact of confounders from the general effects of severe illness would be mitigated. Only one study hypothesized that some people with long COVID might have a functional disorder, and another eight studies a chronic-fatigue-syndrome-like response. DISCUSSION: Neurological symptoms are prevalent in long COVID, but poorly characterized. The similarities between some manifestations of long COVID and functional disorders triggered by acute illnesses are striking. Unfortunately, the current literature is plagued by confounders, including the mixing of patients with initial mild infection with those with severe acute medical complications. The hypothesis that long COVID might in part correspond to a functional disorder remains untested.