Question explored with the scientific record
Causes of FND
The short version: FND is not caused by a single thing. The evidence points to a mix of physical triggers, nervous system vulnerability, and life stress, but the studies that exist are mostly descriptive, not designed to prove causation.
The evidence here is a collection of observational studies, not controlled trials that could isolate a cause. The largest study, from 25 Italian centers, found that 84% of 410 patients with functional motor disorder had non-motor symptoms like anxiety, fatigue, and pain [4]. About 79% reported a major life event or trauma, and 59% reported some form of abuse [1]. But these are associations, not causes. A person who develops FND is more likely to have these things in their history, but many people with trauma never develop FND, and some people with FND have no trauma history at all.
The physical side matters too. One study found that among people with suspected cauda equina syndrome whose scans were normal, 11-12% were later diagnosed with FND, compared to 0% of those with actual nerve compression [3]. That suggests the body can produce real symptoms from pain and stress without structural damage. Another study found that FND and multiple sclerosis are sometimes mistaken for each other, and about 13% of MS patients had symptoms their neurologist said were not fully explained by MS [5]. The mechanism here is likely a nervous system that has learned a faulty pattern, not a structural injury.
What is missing from this evidence is any study that tracks people before they develop FND to see what predicts it. No prospective trials. No comparison of vaccinated versus unvaccinated groups, which matters because some vaccines have been linked to transient neurological symptoms that could mimic or trigger FND [2]. The evidence cannot rule out that immune triggers, including vaccines, play a role in some cases. It simply has not been studied.
My call: FND is real and its causes are a mix of physical triggers, nervous system vulnerability, and life stress, but the evidence is too weak to assign blame to any single factor. Confidence: moderate for the association with trauma and stress, low for any specific physical trigger.
Sources used 5
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Neuropsychiatric phenotypes in functional movement disorder
Episodic functional movement disorder shows hyperkinetic movements, hyperarousal, and trauma associations, whereas constant FMD shows gait weakness and reduced motor agency, suggesting FMD is part of a broader neuropsychiatric syndrome requiring transdisciplinary management.
DOI: 10.1017/s1092852923002353 -
Inactivated COVID-19 vaccine induced acute stroke-like focal neurologic symptoms: a case series
This retrospective, single-center case series from Naresuan University Hospital in Thailand describes 19 healthcare workers who developed transient stroke-like focal neurologic symptoms within minutes to hours after receiving the inactivated COVID-19 vaccine (CoronaVac), with no…
DOI: 10.1186/s12883-022-02739-6 -
The clinical features and outcome of scan-negative and scan-positive cases in suspected cauda equina syndrome: a retrospective study of 276 patients
Scan-negative cauda equina syndrome arises as an end pathway of acute pain, sometimes with partly structural findings and vulnerability to functional disorder.
DOI: 10.1007/s00415-018-9078-2 -
Clinical Correlates of Functional Motor Disorders: An Italian Multicenter Study
A large Italian multicentre cross-sectional study describing the clinical and non-motor features of functional motor disorders (FMDs), comparing isolated versus combined FMDs, detailing phenotypes, associated functional neurological symptoms, and factors linked to the combined p…
DOI: 10.1002/mdc3.13077 -
Functional neurological disorder and multiple sclerosis: a systematic review of misdiagnosis and clinical overlap
MS and FND are not infrequently mistaken for one another, and comorbidity may be more common than acknowledged.
DOI: 10.1007/s00415-021-10436-6