Trial By Error: A Letter About the Inflated Prevalence Rate of Functional Neurological Disorder

By David Tuller, DrPH

I have recently written two posts (here and here) about how experts in functional neurological disorder (FND) have a tendency to assert prevalence rates that ignore their own diagnostic criteria. Today I sent a letter to the corresponding author of yet another paper that has similarly engaged in this problematic strategy. I have posted the letter below.

But I want to repeat—pretty much verbatim–a point I made in my last post: I am in no way questioning whether people with the diagnosis have serious disorders and very debilitating symptoms. Of course they do! And of course they deserve the best medical care available, like everyone. If anyone believes FND patients are not struggling with something really awful, an excellent, recently posted essay called “Cadenza for Fractured Consciousness: A Personal History of the World’s Most Misunderstood Illness” will quickly alleviate them of their misconceptions.

As I also wrote in my last post, FND experts come across as sensitive and caring clinicians who clearly want the best for their patients. But whether doctors demonstrate compassion is a separate issue from whether it is acceptable to disseminate prevalence claims that go beyond the established clinical criteria.

Why does it matter if FND experts argue in one paper that the prevalence of FND among outpatients at neurology clinics is “approximately 6%” but elsewhere report that it is 16%, or even state that it is up to one-third? A key reason is that doing so undermines the credibility of the argument that FND is now a rule-in diagnosis and not a diagnosis of exclusion. Lumping together patients with and without rule-in signs in a single category risks turning FND into a wastebasket diagnosis applied to anyone with unexplained symptoms. That happened routinely with the hoary construct of “conversion disorder,” the term FND is meant to replace. Suggesting that patients without rule-in signs have FND is, in effect, misdiagnosing them. And misdiagnoses can lead to an array of unfortunate consequences.

Given that the term “functional” is now frequently used to describe any symptom or condition for which no organic cause has yet been identified, it is understandable that patients with unexplained neurological symptoms might automatically get slapped with an FND diagnosis. But the term now has a more specific and clearly defined meaning. When it comes to prevalence rates, the frequency with which investigators appear to be casually conflating “functional” and FND, as if the two were interchangeable, is likely to generate serious confusion for patients and clinicians alike. In medicine, words matter.


Benjamin Tolchin
Assistant Professor of Neurology
Yale School of Medicine
New Haven, Connecticut

Dear Dr Tolchin:

In a 2021 paper called “The role of evidence-based guidelines in the diagnosis and treatment of functional neurological disorder” (Tolchin et al), published by Epilepsy & Behavior Reports, you and several colleagues wrote that functional neurological disorder (FND) “is among the most common causes of neurological disability, diagnosed in approximately 16% of outpatient neurology consultations.” The citation for this statement is a 2010 paper called Who is referred to neurology clinics?—the diagnoses made in 3781 new patients,” which was published by Clinical Neurology and Neurosurgery. 

Unfortunately, this paper–one of a number arising from a major research project called the Scottish Neurological Symptoms Study (SNSS)–does not appear to support the 16% prevalence claim.

For years, leading neurologists have noted that FND is not a diagnosis of exclusion but a rule-in diagnosis based on positive signs found during clinical examinations. This approach was enshrined in the fifth iteration of the Diagnostic and Statistical Manual of Mental Disorders, which was published in 2013. In 2016, two experts—including Alan Carson, a co-author of both the 2010 paper and Tolchin et al–wrote the following in the abstract of an authoritative account of the epidemiology of FND, published as a chapter of the. Handbook of Clinical Neurology:

“The recent changes in DSM-5 to a definition based on positive identification of physical symptoms which are incongruent and inconsistent with neurologic disease and the lack of need for any psychopathology represent a significant step forward in clarifying the disorder. On this basis, FND account for approximately 6% of neurology outpatient contacts.”

In the text of this chapter, Carson and his co-author, Alexander Lehn, referenced the findings of the SNSS, which investigated the diagnoses of 3781 outpatients seeking care at four Scottish National Health Service neurology centers. (In fact, the text of the Carson & Lehn chapter indicated that the actual FND prevalence from this Scottish study was 5.4%.) 

The prevalence at outpatient clinics cited in Tolchin et al is almost triple that mentioned in the 2016 article on the epidemiology of FND. How could the SNSS have yielded these two disparate prevalence rates for the same illness?

In the 2010 paper, the 16% figure included patients found to have what the authors called “functional” symptoms as well as those found to have “psychological” symptoms—all lumped together into a “functional or psychological diagnoses” category. It is unclear why the authors chose to combine these two groups.

The “functional” group included patients diagnosed with “non-epileptic attacks” as well as “functional sensory” and “functional motor” disorders. In a 2009 paper also based on data from the SNSS, this same group of patients was described as having “conversion symptoms.” These patients were the “approximately 6%” (or 5.4%) identified by Carson & Lehn in 2016 as “typical FND cases” based on “positive identification,” per the DSM-5 definition. 

That means the additional 10% were the patients identified as having “psychological” symptoms. In the 2010 paper, this group included those given one of a grab-bag of diagnoses, among them “anxiety and depression,” “pain symptoms,” “fibromyalgia,” “post-head injury symptoms,” “alcohol excess,” and “hyperventilation,” as well as a cohort referred to “non-organic.” Given the prevalence highlighted in the Carson & Lehn chapter, patients in this “psychological” category were apparently not deemed to have “typical FND” based on rule-in signs.

Yet investigators in this domain of inquiry have generally ignored this salient point when they have cited the 16% prevalence figure in papers, including Tolchin et al–even as they have maintained that FND is not a diagnosis of exclusion but one requiring positive identification through rule-in signs.

Jon Stone, the first author of the 2010 paper as well as a co-author of Tolchin et al, noted the following during a 2021 podcast produced by the Encephalitis Society“Some people think that FND is a condition you diagnose when someone has neurological symptoms but you can’t find a brain disease to go along with it. And that’s absolutely not the case. Some people [i.e. clinicians] do that, but if they’re doing it like that they’re doing it wrong.”

Since FND is now officially a rule-in diagnosis, the appropriate and acceptable prevalence rate to cite from the SNSS is the “approximately 6%” (or 5.4%) noted by Carson & Lehn. Unless unreported data indicate that the 10% with “psychological” symptoms in the SNSS sample could have been diagnosed with FND through rule-in signs, claims that the study found prevalence in outpatient neurology clinics to be 16% are unwarranted. 

Perhaps these additional patients with “psychological” symptoms had medical complaints that could be described as “functional.” But “functional” in this context simply means that the symptoms or condition have not been found to conform to standard understandings of known diseases; the term should not be casually conflated with the clinical entity known as FND. It is therefore hard to justify identifying patients as having FND under the DSM-5 diagnostic criteria just because they have symptoms deemed to be “functional.”.

If clinicians who diagnose FND without rule-in signs are “doing it wrong,” per Stone, then those who cite the SNSS data to assert a prevalence rate of 16% are also “doing it wrong.” The field of public health relies on consistency and accuracy in the assessment and determination of disease prevalence. It is confusing, not to mention epidemiologically incoherent, when FND experts seem to overlook their own rule-in rule while tripling the reported rates of this challenging diagnosis.

In a related example, a 2021 paper co-authored by Stone and Carson, among others, noted that “tightly defined FND” represents “at least 5%–10% of new neurological consultations.” A prevalence rate of “approximately 6%” (or 5.4%) obviously falls within that range. However, the authors cite no specific reference for the claim that the prevalence of “tightly defined FND”–presumably FND diagnosed through rule-in signs–ranges as high as 10%, which is almost double the 5.4% cited by Carson & Lehn. And certainly 10% is significantly lower than the 16% mentioned in Tolchin et al and other papers.

Rule in should mean rule in. Absent rule-in signs, patients with neurological symptoms that do not adhere to recognized disease patterns should be given a diagnosis of idiopathic neurological disorder, neurological disorder of unknown etiology, or some related term—not FND. If further information regarding the 10% in the SNSS sample with “psychological diagnoses” is unavailable, the prevalence statement citing that research in Tolchin et al should be corrected to reflect the documented rate of “approximately 6%” (or 5.4%) noted by Carson & Lehn.

Thank you for your attention to this matter.

(I have cc’d William Tatum, the editor of Epilepsy & Behavior Reports, on this letter. Since I plan to post the letter on Virology Blog, a popular science venue, I have also cc’d Vincent Racaniello, the host of the website and a microbiology professor at Columbia University.) 

Best–David Tuller

David Tuller, DrPH
Senior Fellow in Public Health and Journalism
Center for Global Public Health
School of Public Health
University of California, Berkeley

11 thoughts on “Trial By Error: A Letter About the Inflated Prevalence Rate of Functional Neurological Disorder”

  1. Thanks David, that’s a great letter. I do hope you get a response.

    And next time I drink too much I’ll remember to chalk it up to FND.

  2. Or rather, I’ll try to remember, but I might struggle a bit. Some of us FND patients do. I think they call it FCD these days.

  3. To me, David is presenting a vey strong and important argument here that really needs to be understood. Perhaps it would help if people appreciated how the 16% figure, and other figures or phrases to indicate prevalence, may have influenced how doctors understand/regard FND and been used to try to extract extra funding and/or services* (*see below) . If patients have a genuine Hoover’s sign (supposedly a sign of ‘functional weakness’), that can’t be denied. The cause of the positive sign can be challenged -there may be biomedical reasons for why it happens – but that the patient has ‘tested’ positive for this sign can’t be denied. If a patient has seizures that don’t show up on EEG, that is a fact. It may be that the EEG isn’t a good enough test and perhaps doesn’t reach the parts of the brain involved, but it is a fact. Without the resources to investigate these things ourselves, we cannot prove FND researchers and proponents wrong, all we can say is that they might be wrong, as medical opinion has been wrong in the past with respect to other now proven biomedical problems. But if researchers who insist that FND is no longer a ‘diagnosis of exclusion’ (and that as such it must be diagnosed via rule-in signs) also tell us that 16% of neurology outpatients have FND, or that it is the ‘second most common’ presentation in neurology based on the Stone et al 2010 paper, then the argument is that they will be promoting the misdiagnosis of 10% of neurology outpatients according to their own rule-in definition of FND (for which their recognized FND experts have stated the prevalence is around 6%). If they say that “a third” of neurology outpatients have FND, then the argument is that they will be promoting the misdiagnosis of over a quarter of neurology outpatients, according to their own definition. That then surely becomes a major patient safety issue, doesn’t it?

    * eg take a look at this - . (Perhaps the ‘second most common’ claim here is based on some other paper [not the Stone et al 2010 paper] – without a reference we can’t know for sure – but we can argue that, with a <6% rate, rule-in FND can't be the 'second most common' so, regardless of where the claim comes from, this petition appears to be at odds with what leading FND experts insist.) This petition is current, but there have been previous petitions regarding FND, including the following UK Government and Parliament petitions, submitted during previous governments and now closed, which appear to indicate or at least suggest FND prevalence greater than the 6% rate:
    -, - , -

    Who put these petitions out there? Who knows? (It's possible that different people or organizations were behind them.) But they suggest to me the damage that could be done if governments and health providers took them on trust and didn't check out the figures for themselves.

  4. “If clinicians who diagnose FND without rule-in signs are “doing it wrong,” per Stone, then those who cite the SNSS data to assert a prevalence rate of 16% are also “doing it wrong.” ”

    .. Nice!

  5. Lady Shambles

    CT said ” …. the argument is that they will be promoting the misdiagnosis of 10% of neurology outpatients according to their own rule-in definition of FND (for which their recognized FND experts have stated the prevalence is around 6%). If they say that “a third” of neurology outpatients have FND, then the argument is that they will be promoting the misdiagnosis of over a quarter of neurology outpatients, according to their own definition. That then surely becomes a major patient safety issue, doesn’t it?”

    Surely it does! As a patient, not a FND patient however, I’d be very sceptical of any diagnosis from the neurology stable which isn’t properly understood, described, measured and accepted as a neurological diagnosis. By which I mean I’d be inclined to refuse any diagnosis that led me to be corralled in any of these brackets, be it the 1/3 of, the 16% of or the supposedly ‘positively signed’ 5.4%. Frankly it sounds as though the whole thing is a made up mush from neuros who don’t want to admit their lack of understanding of many diseases which might present with ‘neurological symptoms’. As someone who has had the misfortune to meet more neurologists than they’d ever have cared to do, and found them somewhat unprepossessing at best (there might be exceptions, I haven’t met them), I’m reluctant to take their word for it that any of these supposed categories are real, except in their own aberrant belief systems. Of course, as David has already warned, that does not mean that the people lumped under these various headings aren’t suffering a great deal. It would behove neurologists to actually try and unpick the biology of these presentations rather than waste time, money and funding on this vanity project of ‘wastebasket diagnoses to save our reputations’ .

  6. Lady Shambles commented:

    “I’m reluctant to take their word for it that any of these supposed categories are real, except in their own aberrant belief systems.”

    I am too, but I think people really need to know that David isn’t saying that these categories are real, (or at least I don’t think he is). NB I’m not saying that Lady Shambles thinks that David thinks that they’re real, but it looks to me as though others may have been leaping to that conclusion. To me, it’s abundantly clear that this is all about flagging up that some FND experts/researchers appear to have been inconsistent (between definition and prevalence) in quite an astonishing way and on a level that I suspect could put many patients at considerable risk of misdiagnosis, going by their own definitions/mantra and figures. I believe that these experts/researchers have serious questions to answer about these apparent inconsistencies and the potential harms that could arise from them, so thanks again David for highlighting and pursuing this, and please don’t let any seeming lack of interest/engagement and/or criticism deflect you from this purpose.

  7. Thanks, CT–right, I’m not making any claims about the 6%. I’m just pointing out that they are making inconsistent claims about prevalence by conflating what they call “functional” symptoms and the clinical entity now called FND, whatever that clinical. entity is.

  8. David Tuller said:

    “I’m not making any claims about the 6%.”

    Thanks David, that couldn’t be clearer.

  9. Sometimes we forget that the practice of science, at a fundamental level, is to challenge oneself and others in order to move in the direction of increasing accuracy.

    IMO, this letter Dr. Tuller sent is an example of science at work to increase accuracy.

    Virology is a fascinating realm and so is neurology because in both realms small things are considered.

    “To become a neurosurgeon, at OHSU a physician must accomplish the following … ” (Small Brains Considered).

    IMO, when scientists are at the top of their game they relish constructive criticism which moves their discipline along toward increasing accuracy.

  10. Steve Hawkins

    Anyone who’s been for a professional neurological examination knows that there is hardly anything in their diagnostic kit with which to rule anything in but the most obvious, catastrophic, nerve damage. The field is wide open to be exploited by the likes of ‘Jon’ Stone and Wesseley et al, who are happy to make up diagnoses and quack ‘treatments’ to keep their geese laying golden eggs until they retire.
    This is a great shame, because there is a great deal of really clever neurological research going on, some of which is now regularly discussed on’s own ‘TWiN’ podcast. It’s just that hospital consultants simply have no access to anything that can examine humans in the way researchers examine fruit flies and mice. 🙁

  11. As I understand it, this blog isn’t an argument about whether the rule-in 6% genuinely have FND or not, rather it’s about whether researchers who insist that rule-in is essential for a diagnosis of FND are abiding by their own definition when supplying prevalence figures. It’s a matter of trust, isn’t it? If researchers don’t abide by their own definitions in such matters, then can they be trusted with regard to other things they write or say?

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