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Neuropsychiatry
Systematic review
Incidence and prevalence of functional neurological disorder: a systematic review
  1. Sara A Finkelstein1,2,
  2. Clare Diamond3,
  3. Alan Carson4,
  4. Jon Stone4
  1. 1 Functional Neurological Disorder Unit, Division of Behavioral Neurology and Integrated Brain Medicine, Department of Neurology, Massachusetts General Hospital, Boston, Massachusetts, USA
  2. 2 Harvard Medical School, Boston, Massachusetts, USA
  3. 3 Bristol Trials Centre, University of Bristol, Bristol, UK
  4. 4 Centre for Clinical Brain Sciences, University of Edinburgh, Edinburgh, UK
  1. Correspondence to Professor Jon Stone; jon.stone{at}ed.ac.uk

Abstract

Background Robust epidemiological data regarding population incidence and prevalence of functional neurological disorder (FND) would be helpful with regards to resource allocation and planning for this disorder, particularly given high symptom burden and high healthcare utilisation. We therefore aimed to systematically review and synthesise available data on FND incidence and prevalence.

Methods PubMed was searched to identify original research articles that reported on the incidence or prevalence of FND. Risk of bias assessment for each study was conducted. Incidence and prevalence rates of FND were additionally estimated by extrapolating data from low risk of bias studies on functional seizures alone.

Results Thirty-nine articles were included. Nineteen reported on FND incidence, 21 reported on prevalence. Comparison between studies was difficult due to methodological differences and significant heterogeneity of incidence and prevalence estimates was found. The incidence of FND was estimated at 10–22/100 000, while minimum prevalence of FND was estimated at 80–140/100 000, with a possible range of 50–1600/100 000. Incidence of paediatric FND was estimated to be between 1 and 18/100 000.

Conclusions The range of incidence and prevalence varies widely across studies, with significant heterogeneity among studies and most studies likely provide underestimates due to methodological challenges. However, using our best method as a conservative estimate, there are likely a minimum of 50–100 000 people with FND in the UK, as an example country. Given that FND appears to be more prevalent than many other well-known and well-funded neurological disorders, incidence and prevalence data suggested here indicate the need for greater research and clinical funding allocation to FND programmes.

Data availability statement

All data relevant to the study are included in the article or uploaded as supplementary information. Not applicable.

https://creativecommons.org/licenses/by/4.0/

This is an open access article distributed in accordance with the Creative Commons Attribution 4.0 Unported (CC BY 4.0) license, which permits others to copy, redistribute, remix, transform and build upon this work for any purpose, provided the original work is properly cited, a link to the licence is given, and indication of whether changes were made. See: https://creativecommons.org/licenses/by/4.0/.

https://doi.org/10.1136/jnnp-2024-334767

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    WHAT IS ALREADY KNOWN ON THIS TOPIC

    • A number of studies have tried to delineate the incidence and prevalence of functional neurological disorder (FND).

    WHAT THIS STUDY ADDS

    • This study provides a contemporary synthesis of incidence and prevalence data on FND in a systematic review, with a best estimate of 50-100,000 people living with FND in the UK, as an example country.

    HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

    • Having a more accurate estimate of FND incidence and prevalence would help direct appropriate healthcare policy, planning and resource allocation, as well as prioritisation of research funding across government agencies and foundations.

    Background

    Functional neurological disorder (FND) describes symptoms and signs of genuinely experienced alterations in the voluntary motor, somatosensory and perceptual systems, which are distressing or impairing, and manifest one or more patterns or deficits typical of the disorder. FND can manifest as a number of different symptoms, including (functional/non-epileptic) seizures, limb weakness, movement disorders (eg, tics, jerks, tremors, gait), speech, swallowing and communication disorders, dizziness (also known as persistent postural perceptual dizziness), somatosensory or special sensory changes and cognitive symptoms. Data show that individuals with FND comprise 5%–15% of patients presenting to neurology services.1 2 FND is often associated with a high degree of physical and psychological symptoms including comorbidity with chronic pain disorders, fatigue and anxiety/affective disorders.3 4

    Epidemiological data, including both incidence and prevalence, can help determine how resources should be allocated in managing specific diseases and disorders. Incidence is defined as the number of new cases of a disease/disorder within a defined population, over a period of time. Prevalence is defined as the total number of cases of a disease/disorder within a population, and can be divided into period prevalence and lifetime prevalence. Period prevalence is the total number of active cases captured over a period of time, while lifetime prevalence is defined as the proportion of people within the population who experience the disease/disorder at any point in their lifetime.

    A systematic review found mean annual costs related to FND to lie somewhere between US$5000 and US$87 000 per patient5 or >US$1.2 billion in annual emergency department and inpatient costs in the USA,6 suggesting that FND is associated with high healthcare utilisation. However, cost estimates and appropriate resource allocation for FND have been hampered by the fact that robust epidemiological data regarding population incidence and prevalence of this disorder are sparse, with no contemporary systematic reviews attempting to synthesise what data are available. Lack of clarity on the epidemiology of FND is likely due to a number of factors, including changes to the definition and method of diagnosis of FND over the past several decades, limited clinician knowledge and documentation bias, research that has generally focused on discrete symptom subtypes of FND (eg, functional seizures or functional motor disorders) and because it is a diagnosis that must usually be made in secondary care.

    To address this gap in knowledge, we aimed to systematically review the available incidence and prevalence data on FND at a population level in order to make a determination regarding the minimum incidence and prevalence of this disorder, exploring sources of heterogeneity between estimates.

    Methods

    Eligibility criteria

    To be included in this systematic review, studies had to: (i) be original research; (ii) report on patients with FND (described as functional or psychogenic neurological symptoms/disorder, hysteria or conversion disorder, or, in the case of functional seizures, dissociative or non-epileptic) in adults and/or children; (iii) report either on incidence or prevalence of FND, or provide data from which incidence and/or prevalence could be calculated, within a defined geographic area for which the population was known. Studies in which only one FND subtype (eg, functional seizures, functional movement disorder, functional limb weakness) were considered were included, but studies focused exclusively on functional cognitive disorder, persistent postural perceptual dizziness (PPPD), functional speech or functional sensory disturbances (loss of vision, hearing or tactile sensation) were not. PPPD and functional cognitive disorder were not included as these have only recently been defined as FND subtypes and in the case of the latter diagnostic guidelines have only recently been operationalised. Studies more broadly examining ‘functional’ or ‘somatoform’ disorders were included only if the data included specific frequency of FND symptoms such as functional motor symptoms or functional seizures. Search was not limited by date of publication or study quality. Studies looking at special subpopulations only (eg, refugees) and non-English language studies were excluded.

    Search strategy

    A search of the PubMed database was updated to 15 August 2023. The reference lists of all relevant studies, as well as reference lists of relevant review articles identified in the PubMed search, were additionally screened (snowball search).

    For the PubMed search, we defined two main search concepts of “functional neurological disorder” and “epidemiology”, and combined them. To search for articles related to FND, we searched for any of the terms “psychogenic”, “conversion disorder”, “non-epileptic”, “functional movement disorder” or “functional neurologic*” in the title/abstract. To search for articles related to epidemiology, we searched for any of the terms “incidence”, “prevalence” or “epidemiology” in the title/abstract.

    Study selection

    A two-step selection process was completed by one reviewer (SAF) and used to identify studies for inclusion. First, all titles and abstracts were screened for relevance. Second, the full texts of the relevant studies were reviewed, and inclusion and exclusion criteria were applied. Reasons for exclusion were documented. In cases where it was unclear whether a study should be included, study was discussed among the author group with consensus reached. All studies meeting eligibility criteria, regardless of degree of risk of bias, were included in the synthesis of results.

    Data extraction

    Data extraction was completed in duplicate by two independent reviewers (SAF and CD) using a standardised data collection form. Incidence and prevalence data were taken directly as reported from the studies and synthesised in data tables by study. In several cases, incidence and/or prevalence was not reported, but data on frequency of FND (or FND symptom subtype) and population of the catchment area were recorded, allowing incidence and/or prevalence to be calculated and reported in data tables by study. Bibliographic data (author, publication year), country, setting, patient age and sex, methods and source(s) of data ascertainment, inclusion and exclusion criteria and definitions/diagnostic criteria for FND (including for functional seizures whether electroencephalogram (EEG) confirmation was required) were additionally extracted. For the purposes of this review, any study including patients <16 years of age was considered to include a paediatric population. Information that was missing or unclear was marked as ‘not reported’.

    Risk of bias assessment

    In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidance for systematic reviews7 and because significant methodological heterogeneity was expected among studies, we used a component approach to assess study quality and risk of bias. Components that were deemed relevant to study quality were based on authors’ clinical and research knowledge of the field, as well as drawing from several checklists for assessing observational studies (Joanna Briggs Institute Critical Appraisal Checklist, Agency for Healthcare Research and Quality Methodology Checklist, Crombie’s items).8–10 Studies were evaluated in four domains: research methodology, cases captured, case definition and ascertainment and quality assurance (see table 1 for further description of domains). Each domain was rated using a qualitative traffic light system: green indicating low risk of bias, yellow indicating medium risk of bias and red indicating high risk of bias. Articles were systematically assessed in a standardised manner, and rechecked to ensure consistency of application of traffic light system across studies. Domains of ‘research methodology’, ‘cases captured’ and ‘case definition and ascertainment’ were given a ‘green light’ if all areas of interest within the domain were evaluated as high quality/low risk of bias OR if all areas but one were evaluated as high quality except for one rated as moderate quality/medium risk of bias. Domain was marked as a ‘yellow light’ if two or more areas of interest within the domain were evaluated as moderate quality/medium risk of bias. Domain was marked as a ‘red light’ if any of the areas of interest within the domain were evaluated as low quality/high risk of bias. For the fourth domain of ‘quality assurance’, a green light was given if there was any inclusion of a process designed to check quality of data once it had been collected, a yellow light was given if there was some part of the case selection process built in that would help assure good case selection and a red light was given if there was no mention of any method of quality check of the data or quality assurance built in to case selection process. If data were unknown or missing, a medium risk of bias was noted.

    View this table:
    Table 1

    Domains used to assess quality of studies

    Data analysis

    Assuming a normal distribution in the population sample, 95% CIs were calculated for prevalence studies where possible using the formula Embedded Image where p’ was the proportion of people with FND (number of people with FND divided by the total sample size, n).

    Data extrapolation

    As we anticipated data heterogeneity and studies with high risk of bias, we additionally sought to estimate incidence and prevalence through other means. Studies with the highest methodological quality, based on risk of bias assessment, reporting on incidence or prevalence of functional seizures alone, were used to extrapolate overall FND incidence and prevalence (ie, encompassing seizure, motor, sensory subtypes, although not necessarily accounting for more newly defined FND subtypes including functional cognitive disorder and persistent postural perceptual dizziness). This was accomplished by assuming that functional seizures represent 30% of all patients with FND, determined by a weighted average (based on number of patients in each study taken) derived from 13 studies with relatively larger samples sizes where different FND subtypes including functional seizures were documented.11–24 Of note, there may be differences in percentage of functional seizures between paediatric and adult populations, geographic/ethnocultural groups or depending on when the study was conducted; however, this seemed a reasonable estimate based on looking at a number of studies with heterogeneous populations, and correlated well with the authors’ own clinical observations.

    Results

    The PubMed search returned 969 unique abstracts published between 1956 and 15 August 2023. Based on a review of the abstracts, 884 articles were excluded because they did not relate to the research question, were review articles or were non-English language articles. The full texts of the remaining 85 articles were reviewed, of which 24 were determined to meet inclusion criteria. Reasons for exclusion were documented (figure 1). A snowball search was conducted by reviewing the reference lists of included articles as well as reference lists of relevant review articles found during the initial search. This yielded a further 32 articles that underwent full-text review; 17 were excluded with reasons for exclusion documented, while the remaining 15 met inclusion criteria. This yielded a combined total of 39 included articles (see figure 1 for summary of study screening and identification).

    Figure 1
    Figure 1

    Study selection flow chart. FND, functional neurological disorder.

    Incidence

    A total of 19 studies reported on incidence of FND; seven studies did not limit results by FND symptom type,20–26 nine studies reported only on functional seizures,27–35 two reported only on functional limb weakness36 37 and one study reported only on stroke-like symptoms.38 Of the studies not limiting results by symptom subtype, two documented in detail FND symptoms that were present (including seizures, dizziness, movement disorders, weakness, somatosensory, speech, visual and auditory symptoms),22 24 three documented presenting/main FND symptom as motor (including movement disorders and weakness), seizure, sensory or mixed20 21 23 and one did not characterise patient symptoms.26 Studies from Europe, North America and Australia/New Zealand were represented and included both adult and paediatric populations. Incidence rates and demographic data from these studies can be found in table 2. For studies that reported incidence rates for each year of the study separately in addition to the overall average incidence rate by year, only the average incidence rate by year was reported in the table.

    View this table:
    Table 2

    Summary of studies of incidence of FND

    Of the three studies that did not limit results by FND symptom type (including both motor symptoms and seizures, among other symptoms in some cases) in either adult or combined adult and paediatric populations, similar incidence was reported across studies (10.7–22/100 000).20 25 26 All three used only one data source, although data sources varied (psychiatric case registry26 vs single neurologist25 vs patients presenting to hospital).20 Interestingly, within a single study by Stefansson et al, using similar data sources (psychiatric registries)—one from Iceland and one from the USA—the incidence rate of one population was double the other.26 In the case of the study by Stevens, while only data from a single neurological practice was used, the author notes that they were in the unique position of being the only neurologist in the district, and as such presumably all referrals would have been directed to them. Case definitions varied significantly across studies ranging from ‘hysterical neurosis’26 to ‘neurological symptoms without recognised disease’,25 to use of an in-house coding system for functional neurological symptoms also inclusive of hypochondriasis.20 Of interest, incidence of multiple FND subtypes combined, across these three studies, overlaps with incidence from several studies looking at incidence of functional seizures alone.

    Of the four studies reporting on incidence of that did not limit results by FND symptom type in paediatric populations alone, incidence ranged from 1.3 to 18.3/100 000.21–24 While all four studies included multiple data sources for cases, the study with the highest incidence rate (Yong et al) was the only one to include paediatric neurology as a data source; notably, 77/97 cases (79%) were derived from paediatric neurology sources in this study (other cases from general paediatricians and psychologists).24 The study by Yong et al was also the only one conducted in the wake of the COVID-19 pandemic, while the other studies were conducted in the 15 years prior.

    Of the nine studies reporting on incidence of functional seizures alone, in either adult only or a combined adult and paediatric population, incidence ranged from 0.91 to 19.8/100 000.27–35 Studies reporting the highest incidence of functional seizures used either multiple data sources30 or data from the emergency department,35 while studies reporting the lowest incidence used only data from video-EEG/telemetry units (including long-term monitoring epilepsy unit).29 31 Notably, in the study by Sigurdardottir et al, which reported the second-lowest incidence, only cases in which patients were having events that ‘resembled epileptic seizures’, where there was diagnostic uncertainty, were considered. Studies with the highest reported incidence also considered both clinical and EEG data in order to determine diagnosis of functional seizures,28 30 35 while studies with lowest reported incidence only considered cases of EEG-documented functional seizures.29 31

    Prevalence

    Twenty-one studies reported on prevalence rates, 16 on period prevalence27 39–53 and six on lifetime prevalence.53–58 Of the period prevalence studies, nine studies did not limit results by FND symptom type,39–45 52 53 four reported on functional seizures only,27 47 48 51 one reported on functional seizures and functional weakness,46 one reported on functional parkinsonism only49 and one reported on functional dystonia only.50 Of the studies not limiting results by symptom subtype, one documented in detail FND symptoms that were present (including seizures, weakness, movement disorders, somatosensory and speech symptoms),40 two documented primary FND symptom (seizure or motor)45 52 and six did not characterise patient symptoms.39 41–44 53 Of the lifetime prevalence studies, five did not limit results by FND symptom type53–57 and one reported on functional seizures only.58 Of the studies not limiting results by symptom subtype, two documented in detail FND symptoms that were present (including syncope/seizure, movement disorders, weakness, dizziness, swallowing, speech, visual, somatosensory and auditory symptoms)55 57 and three did not characterise patient symptoms.53 54 56 Studies reporting on data from Europe, the Middle East, Africa, South America and Asia were represented, as were both adult and paediatric populations. Prevalence rates and demographic data from these studies can be found in tables 3 and 4.

    View this table:
    Table 3

    Summary of studies of period prevalence of FND

    View this table:
    Table 4

    Summary of studies of lifetime prevalence of FND

    Of the eight studies reporting on prevalence that did not limit results by FND symptom type, in either adult only or in a combined adult and paediatric population, prevalence ranged from 48 to 1571/100 000. Notably, the study reporting the lowest prevalence (48/100 000) by Singh and Lee,40 reported that their method of case finding, which entailed asking general practitioners to recall patients they had seen with FND (without chart review) was likely to be unreliable. Two other studies41 52 with lower prevalence (144.5 and 297/100 000, respectively) only had two FND cases each. The study with the highest prevalence (1571/100 000) by Fink et al, used prospective data collection from general practices and encompassed both a patient survey and interview by a psychiatrist.43 Studies with the next three highest prevalence rates (607–1214/100 000)42 44 53 gathered data through door-to-door community surveys asking about symptoms, with two44 53 involving an interview/examination by a psychiatrist to confirm cases but no involvement from neurology.

    Of the four studies reporting on prevalence of functional seizures separately, in combined adult and paediatric populations, prevalence ranged from 23.8 to 2000/100 000. In one study with a lower prevalence (25.8/100 000) by Inaida et al, only cases in which an epilepsy diagnosis was revised to a functional seizure diagnosis were considered.47 The study with the lowest prevalence (23.8/100 000), by Villagrán et al, used a patient registry and included cases with the International Classification of Disease, 10th edition diagnostic code for either functional seizures (F44.5) or ‘convulsions not elsewhere classified’ (R56.8). The studies with higher prevalence used either patient self-report obtained through community survey alone (Rief et al—2000/100 000)46 or a community survey that also incorporated clinical and EEG data (Ferruzzi et al—1002/100 000).51 Notably, the study by Rief et al also reported the prevalence of functional limb weakness to be 2000/100 000, which, if taken together with the functional seizure prevalence, would indicate a minimum prevalence of 4000/100 000 for FND more broadly. None of the studies required EEG-documented functional seizures for case inclusion.

    Risk of bias assessment

    Risk of bias for both incidence and prevalence studies across four domains is summarised in tables 5 and 6.

    View this table:
    Table 5

    Risk of bias assessment of functional neurological disorder incidence studies

    View this table:
    Table 6

    Risk of bias assessment of functional neurological disorder prevalence studies

    Multiple case definitions for FND were used across studies, including older Diagnostic and Statistical Manual of Mental Disorders (DSM) definitions and the entity of ‘hysterical neurosis’. For incidence studies, of the 11 studies not assessing functional seizures alone, three used a case definition consistent with the current DSM-5 diagnostic criteria.23 24 36 For prevalence studies (excluding those assessing functional seizures alone), one used a definition consistent with current DSM-5 criteria for FND.50 In general, the more up-to-date the definition used for the study, the lower the risk of bias with regard to case ascertainment was deemed to be, as it was likely both that case definitions overlapped with other conditions as they are currently defined and that cases were missed due to the strong focus on psychiatric morbidity needing to be present. Of the studies considering only functional seizures, three of nine incidence studies27 28 35 and two of five prevalence studies27 47 considered cases that were either EEG-documented or based on clinical diagnosis (of note, whether EEG-documented diagnosis only was considered was unclear in two of the other prevalence studies). Functional seizure studies that only considered EEG-documented cases were paradoxically considered to have higher risk of bias in that they likely underestimated incidence because accessing EEG confirmation is often not possible—either due to infrequent events or lack of access to services—and those accessing EEG services are often patients in which diagnosis of epilepsy versus functional seizures is more uncertain (potentially excluding more clear-cut cases of functional seizures). As noted in the results, studies with lower incidence of functional seizures all required EEG-documented events, while studies that considered both EEG and clinical data for case inclusion reported higher incidence. Validity of inclusion of patients without EEG-documented functional seizures is demonstrated by other work in the field, most notably, a large randomised controlled trial of treatment for functional seizures (Cognitive Behavioural Therapy for Adults with Dissociative Seizures or CODES trial) that included this group (for further discussion of issues surrounding this, please see referenced study).59

    Case ascertainment methods, which speak to accuracy of diagnosis, included review of database diagnostic codes, chart review, patient self-report, structured interviews and clinical examination. Studies in which patient self-report was the primary method of case ascertainment were considered to have a high risk of bias, as they likely overestimated prevalence of FND. Six of the 10 prospective incidence studies24 28 30 36 37 60 and three of the 16 prospective prevalence studies48 55 58 employed clinical examination by a neurologist in order to confirm the diagnosis. Studies in which risk of bias was considered to be lowest for case ascertainment employed a multistep approach in which potential cases were identified through surveys or other methods, followed by a clinical examination to confirm the diagnosis.45 48 55 58

    Sources of cases, which speak to opportunity to capture all potentially relevant cases within the sample but not accuracy of diagnosis, varied; for incidence studies, the majority used either neurology or psychiatry secondary care populations. Three incidence studies derived cases from emergency department populations,20 35 38 while five incidence studies included primary care sources (including paediatrics).22–24 30 34 Cases derived from multiple sources were considered to be the most likely to capture all relevant patients. Cases derived from neurology populations were also considered likely to capture the majority of relevant patients given patients with FND primarily present with neurological symptoms, while acknowledging that these studies were still likely to miss a proportion of cases being seen only by psychiatry, primary care, etc. Nearly all of the prevalence studies used prospective data collection, with the most common method of data collection being a door-to-door community-based survey. While this had the advantage of screening a large number of people (and in some cases essentially screening everyone) in the population of interest for FND, many of these studies did not use rule-in physical examination signs to confirm the diagnosis nor involve consultation with a neurologist or neuropsychiatrist (with some exceptions).48 49 51 55 58 Other sources of data for prevalence studies included primary care, healthcare databases and neurology.

    Precision of point estimates with regard to prevalence studies was difficult to ascertain (tables 3 and 4). The majority of studies did not report CIs for prevalence estimates and instead estimates were calculated by the authors of this review. Of note, calculations were based on an assumed normal distribution, which could not be confirmed as data sets and SD were also not provided for included studies.

    Estimated FND incidence and prevalence based on rate of functional seizures

    Of incidence studies looking at rates of functional seizures alone, studies by Duncan et al 28 and Villagrán et al 27 were determined to have the lowest risk of bias in populations including both children and adults, while the study by Hansen et al 34 had the lowest risk of bias in the paediatric population. Assuming that patients with functional seizures represent approximately 30% of all patients with FND (see rationale and references in ‘Methods’ section), extrapolating from the functional seizure incidence rates from the aforementioned studies yielded an overall estimated incidence of FND between 10 and 16/100 000 for all ages, and 8/100 000 for the paediatric population.

    Of studies looking at prevalence rates of functional seizures alone in both adults and children, the study by Villagrán et al was determined to have the lowest risk of bias. Extrapolating from the functional seizure prevalence rate from this study, assuming that patients with functional seizures represent approximately 30% of all patients with FND, yielded an estimated prevalence of FND of 79/100 000.

    Evidence synthesis

    Due to high heterogeneity, a meta-analysis was not possible. The outcomes of the review were a review of the range of findings of incidence and prevalence, as well as data extrapolation.

    Discussion

    Many barriers exist to assessing incidence and prevalence and then accurately aggregating data from FND studies (figure 2). The diagnostic criteria for FND have changed over time, and significant changes have occurred between the DSM-IV-text revision (and older versions of the DSM) and DSM-5 that likely led to a change in how many cases were being captured: shift from a ‘rule out’ to ‘rule in’ diagnosis with positive signs on examination and dropping the need for a stressor preceding symptom onset. As such, diagnostic precision as to what would be considered FND based on current diagnostic criteria likely was increasingly poor the older the study, leading to underestimation of cases in some instances (eg, due to only including cases with a preceding stressor, which, as an example, in one study led to exclusion of about one-third of cases)55 and overestimation in others (eg, including non-core FND symptoms such as pain under the umbrella of conversion disorder). In many of the included studies, neurologists or neuropsychiatrists were not involved in diagnosis, likely lowering precision. Diagnostic coding, which in many studies was used to identify cases, is likely inconsistent in clinical practice, with some patients possibly being lumped under the broader category of ‘somatoform disorders’, and some patients perhaps not having a diagnostic code for FND applied (eg, coding functional seizures less specifically as ‘convulsions’ or functional leg weakness as ‘leg weakness no cause’). Other reasons for diagnostic coding inconsistency may include lack of clinician knowledge regarding the diagnosis or confidence in making the diagnosis, or reluctance to apply what some view as a stigmatising label to a chart record. FND has also been shown to be underdiagnosed—for example, 8% of patients diagnosed and treated for status epilepticus in two large epilepsy trials were ultimately discharged with a diagnosis of prolonged functional seizures.61 There is also the more general barrier to assessing incidence and prevalence accurately due to logistical difficulties in obtaining large sample sizes. While several studies in this review had drawn from populations in the 100 000s or even in some cases >1 million range, many others were much smaller. By comparison, if we examine epidemiological studies of other neurological and psychiatric disease, overall population may be much larger—such as in a study examining prevalence of multiple sclerosis that included data from over 100 million people62—or on par with some of the studies included in this review—such as in a study examining prevalence of depression, in which around 400 000 participants were surveyed.63

    Figure 2
    Figure 2

    Factors leading to underestimation or overestimation of prevalence of functional neurological disorder (FND). ALS, amyotrophic lateral sclerosis; EEG, electroencephalogram; MS, multiple sclerosis.

    More recently, as functional neurological symptoms have become increasingly recognised, there has been a trend towards compartmentalising them by neurological subspecialty, and roughly half of the studies included in this review reported data on patients with only one type of FND symptom (eg, functional seizures). This is out of keeping with how many patients with FND present, that is, with a mixed symptom picture, and makes estimates of FND symptom subtypes, such as functional seizures, more difficult to ascertain. Case ascertainment was also often from only one point of healthcare access that patients with FND are likely to encounter (eg, only considering cases from psychiatry), which will likely lead to epidemiological underestimation as patients with FND present to a wide array of providers including primary care and many specialists such as emergency physicians, internists, stroke specialists, orthopaedic surgeons, physiatrists, neurologists or psychiatrists. Furthermore, even if only examining cases from, for example, neurology, capturing all cases presenting to a given specialty within a defined geographic area presents its own challenges (eg, in the case of parallel public and private healthcare systems), with a number of studies likely missing a large number of cases. There was significant variability with respect to age range included, and some studies used a population denominator of the entire population for incidence and prevalence calculations, while the numerator only considered adult cases. There were studies included from a wide geographic, socioeconomic and cultural range, which may have introduced additional heterogeneity. Socioeconomic and cultural factors such as stigmatisation of FND on both the part of patients and healthcare providers (that may have affected willingness to present to healthcare and/or code for the disorder), the lens through which FND symptoms are understood, access to care and threshold for seeking out care, may have all influenced epidemiological estimates. These sources of heterogeneity make comparison of data between studies challenging. Looking at all the factors above we consider that the existing data are likely to lead to an underestimate of true incidence and prevalence rates.

    Incidence of FND

    There was only one study looking at incidence in all age groups that did not limit results by FND symptom type,26 which found disparate rates between Icelandic and American populations, of 11 and 22 per 100 000, respectively. This study was conducted in 1976, using an out-of-date definition for FND and taking data only from psychiatric registries. Two additional studies looking at incidence of FND without limiting by symptom type in adults both determined similar incidence rates of 11/100 000.20 25 However, the risk of bias in case ascertainment was high in the study by Stevens25, and the study by Beharry et al 20 likely included a high number of cases that were not true incident cases (ie, not the first presentation of FND). When data were extrapolated from functional seizure studies with a lower risk of bias, incidence for FND appeared to be somewhat similar, between 10 and 16/100 000. Synthesising these data, a reasonable estimate for minimum incidence of FND is 10–22/100 000. A reasonable estimate for the minimum incidence of functional seizures alone, based on lowest risk of bias studies, is 3–5/100 000.

    Looking at paediatric data, low risk of bias studies that did not limit results by FND symptom type by Ani et al 22 and Kozlowska et al 23 reported incidence between 1 and 4/100 000. Incidence was notably higher in a more recent study by Yong et al, also noted to have low risk of bias, in which the rate of paediatric FND was found to be 18/100 000 with a sample taken from a secondary care paediatric neurology service only. The higher rate may represent decreased barriers to diagnosis over time, such as through increased awareness of FND among healthcare providers. This study was also conducted during the COVID-19 pandemic, and it has previously been reported that incidence of FND may have increased during this period.64 When data were extrapolated from Hansen et al,34 a study on functional seizures alone in the paediatric population with the lowest risk of bias, incidence was estimated at 8/100 000. Taking these data together, incidence of FND in the paediatric population alone was between 1 and 18/100 000, and likely at the higher end of that estimate.

    Prevalence of FND

    Prevalence between studies was difficult to compare, as many studies had high risk of bias with respect to case definition, ascertainment and completeness or appropriateness of cases captured (either due to likely overestimation or underestimation of cases). Period prevalence rates varied by two orders of magnitude, ranging from 10s to 1000s per hundred thousand. Calculating the point prevalence by extrapolating from a ‘low risk of bias’ study looking at prevalence of functional seizures alone yielded an estimate of 79/100 000, which may provide a more accurate estimate. Looking at the studies themselves, if we exclude the study with the lowest reported prevalence (48/100 000) by Singh and Lee40 (which is likely reasonable given the presumed gross underestimate of cases as reported by the authors), the next lowest estimate is 144.5/100 000 (Faravelli et al). Synthesising these data, the minimum point prevalence of FND may lie between 80 and 140/100 000, while the overall estimate based on all data sources varies more widely between 50 and 1600/100 000. To put this in context, based on the Global Burden of Disease Study, the prevalence worldwide of multiple sclerosis is 30/100 000, of motor neuron disease is 5/100 000 and of brain and other CNS cancers is 11/100 000,65 while the prevalence of depression is 3440/100 000 and of anxiety is 3780/100 000.66

    Based on the lower, more conservative estimate of prevalence determined above, we can use whole population data to estimate that within the UK (population 67.6 million in 2022)67, there are likely a minimum of 50 000–100 000 people living with FND, and possibly up to 1 million if higher prevalence estimates are used. Within the USA (population 334.9 million in 2023)68, at a minimum there are likely 265 000–480 000 people living with FND, and possibly up to 5 million people.

    Additional groups

    Incidence and prevalence of FND may vary when considering special populations. A study by Garrett et al estimated the incidence and prevalence of FND in active-duty US military personnel. They found 7644 incident cases over the years 2000–2018, with an estimated yearly incidence between 29.5/100 000 and 37.2/100 000,69 roughly two to three times that estimated in our review. This may reflect the lower age group or military setting but was a particularly interesting study that did not meet criteria for this review. Another scoping review examining the frequency of functional seizures in forcibly displaced persons noted one study showing an incredibly high FND symptom burden, with 28% of women experiencing functional seizures.70 While direct comparison of these data with other incidence and prevalence studies included in this review is difficult due to methodological differences, it demonstrates that the rate of FND in vulnerable populations may potentially be significantly higher than rates reported in the general population.

    Strengths and limitations

    One major strength of this study was its inclusiveness of studies considered, particularly in reporting on data from studies looking at different FND symptom subtypes, which are often siloed in the literature (eg, movement disorder vs functional seizures). We additionally used studies of FND frequency, from which we extrapolated incidence and prevalence estimates, thus incorporating valuable data that otherwise get ignored. Studies from a wide range of geographical regions and both high-income and low-income countries were included, which also reflects a wide variety of cultures. Limitations of this study include the reliance on indirect methods of calculating incidence and prevalence rates for FND, due to the challenges posed by the source data. Data extrapolation, using functional seizures to calculate overall prevalence of FND, also assumed that 30% of patients with FND present with functional seizures. While this was based on a number of studies and seemed a reasonable estimate, a systematic search was not conducted to thoroughly verify the accuracy of this percentage, nor were studies evaluated for quality or bias. Most studies did not include data on patients with functional dizziness (or persistent postural-perceptual dizziness, the most common type of chronic dizziness71) or functional cognitive disorder, two newly recognised FND subtypes,72 which likely led to an underestimate of FND cases. Furthermore, functional speech and communication disorders and functional movement disorder subtypes were not included specifically in the search terms. While a number of studies did include patients with these symptoms, and we are unaware of any recent reviews focusing solely on these specific symptom subtypes, it is possible that some data were missed. However, we believe it is unlikely that this would have altered the overall results. With respect to generalisability of results, it is difficult to determine due to the heterogeneity of the data. Incidence studies were only available from high-income countries, which may not be generalisable to middle- and low-income countries. For the purposes of this review, only one database (PubMed) was searched. While in the authors’ experience additional database searches are unlikely to yield further articles on the topic of FND, it is possible that some articles were not included in this study due to the failure to search other databases.

    Future directions

    Future studies aimed at estimating FND incidence or prevalence should ideally use the current DSM-5 diagnostic criteria and include all motor FND (including both weakness and movement disorders) cases as well as cases of functional seizures. Consideration should also be given to including newer FND subtypes, such as PPPD and functional cognitive disorder. Delineating these cases from patients with primary chronic pain disorders, fatigue and functional somatosensory symptoms alone (for which there are no robust rule-in signs) would capture a large proportion of cases while minimising the likelihood of including patients who may fit better into another diagnostic category (eg, fibromyalgia). Including multiple sources of data (eg, emergency care, neurology, psychiatry) would help capture the majority of cases. Consideration could also be given to including data from neuro-ophthalmology clinics and neurotology clinics in order to capture cases of PPPD and functional visual disorders. Cases are best ascertained by a neurologist or neuropsychiatrist with training in identifying this disorder.

    Data availability statement

    All data relevant to the study are included in the article or uploaded as supplementary information. Not applicable.

    Ethics statements

    Patient consent for publication

    Ethics approval

    Not applicable.

    Acknowledgments

    The authors would like to thank Dawn Golder, UK Executive Director of FND Hope, for suggesting this research topic.

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    Footnotes

    • X @jonstoneneuro

    • Correction notice Since this paper first published, the results section has been updated so that the sentence 'incidence of FND was estimated at 10–22/100 00' now reads 'incidence of FND was estimated at 10–22/100 000'.

    • Contributors All authors contributed to the planning, conduct and reporting of the work described in the article. SAF is the guarantor.

    • Funding JS received funding as a National Research Scotland Career Research Fellow and through the Wellcome Trust Institutional Translational Partnership Award. AC received grant funding from the Chief Scientist Office Scotland and the Medical Research Council UK.

    • Competing interests JS provided expert witness testimony in personal injury and negligence cases, received royalties from UpToDate and received support from Sun Pharma to attend an educational course. JS is the Secretary of the FND Society, serves on the medical advisory boards of FND Hope, FND Hope UK and FND Action, and is a member of the European Academy of Neurology Coordinating Panel on FND and the UK FND Network Board. JS is also the creator of the freely available website www.neurosymptoms.org. AC is a paid editor of the Journal of Neurology, Neurosurgery, and Psychiatry, provides expert witness testimony and is the past President of the FND Society and the British Neuropsychiatric Association. SF has received speaker’s honoraria for lectures on FND.

    • Provenance and peer review Not commissioned; externally peer reviewed.