Trial By Error: A Letter to Psychological Medicine about Inflated FND Rate Claims

By David Tuller, DrPH

The journal NeuroImage: Clinical, an Elsevier title, recently agreed to correct the false statement that a 2010 study found functional neurological disorder to be the second-most-common diagnosis at outpatient neurology clinics. To the journal’s credit, it responded positively within days of receiving a letter about the matter from a group of us, although the correction has not yet been published.

Last week, I sent a letter to the lead and senior authors of that paper, Dr David Perez and Professor Selma Aybek, alerting them that we had identified at least nine other papers that included the same mistake and for which one or the other of them was the lead or senior author. I suggested that it would be helpful if they themselves informed the journals of the need for such corrections. Not surprisingly, I did not hear back from either of them.

Earlier today, I sent a letter from our group to the journal Psychological Medicine about a 2021 paper for which Dr Perez, a neuropsychiatrist at Massachusetts General Hospital in Boston, was the senior and corresponding author.  This journal, of course published the arguably fraudulent PACE trial recovery paper in 2013. It has essentially served as a house organ for members of the CBT/GET ideological brigades. It will be interesting to see how it handles this request.


Dear Professor Murray and Professor Kendler:

For years, leading neurologists have noted that functional neurological disorder (FND), the new term for what was formerly called conversion disorder, is not a diagnosis of exclusion but a rule-in diagnosis requiring positive signs found during clinical examination. This approach was enshrined in the fifth iteration of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), which was published in 2013. Nonetheless, FND experts appear to be overlooking this definition when making claims about rates of the diagnosis at neurology clinics.

For example, a 2021 article in Psychological Medicine from Diez et al, “Reduced limbic microstructural integrity in functional neurological disorder,” included the following sentence: “For much of the 20th century, functional neurological (conversion) disorder (FND) was marginalized across neurology and psychiatry despite being the second most common reason for neurological referral and incurring significant healthcare costs” (1).

This sentence cited three references; the specific reference for the assertion that FND is the “second most common reason for neurological referral” was a seminal and widely cited paper from Stone et al called “Who is referred to neurology clinics?—the diagnoses made in 3781 new patients,” which was published in 2010 in Clinical Neurology and Neurosurgery” (2). 

Yet Stone et al, one of several papers arising from a research project called the Scottish Neurological Symptoms Study (SNSS), does not support the claim. The journal NeuroImage: Clinical has recently agreed to issue a corrigendum regarding the exact same assertion and reference in a paper called “Neuroimaging in functional neurological disorder: state of the field and research agenda” (3). Dr Perez, the corresponding and senior author of the Psychological Medicine article, was the lead author of that NeuroImage: Clinical paper, so he is aware that the statement is erroneous and is not supported by the data from Stone et al. (I have cc’d Dr Perez and the co-signatories of this letter.)

According to Stone et al, the second-most-common category of presentations at neurology clinics, after headache, was a grouping called “functional and psychological symptoms,” at 16%. An examination of this heterogeneous grab-bag of conditions indicates that it does not easily equate to what is called FND, per the DSM-5 criteria.

In the SNSS, 209 of the 3781 patients, or 5.5%, were diagnosed with “functional” symptoms such as sensory or motor disorders or non-epileptic seizures. These were identified in a related 2009 paper as cases of “conversion” symptoms (3); they would now be indisputably classified as FND. At the 5.5% rate, FND would be way down on the list of diagnoses mentioned in Stone et al, after headache (19%), epilepsy (14%), peripheral nerve disorders (11%), miscellaneous neurological disorders (10%), multiple sclerosis/demyelination (7%), spinal disorders (6%) and Parkinson’s disease/movement disorders (6%).

The other 10% of the 16% in Stone et al’s second-most-common grouping, who were defined collectively as having “psychological” symptoms, fell into a hodge-podge of sub-groups, including hyperventilation, anxiety and depression, atypical facial/temporomandibular joint pain, post-head injury symptoms, fibromyalgia, repetitive strain injury, and alcohol excess, among others. Also lumped in with this “psychological” cohort were cases identified as “non-organic” and “no diagnosis.” Stone et al presented no evidence that any of these patients met or could have met the rule-in criteria for a more strictly defined FND diagnosis. Arguments that members of this 10% also had FND are grounded in speculation and assumption, not fact.

In subsequent articles, two of the co-authors of Stone et al have repeatedly endorsed the much lower rate for the specific clinical entity known as FND. In 2016, Professor Alan Carson, the second author of Stone et al, wrote the following with a co-author in the abstract of an account of the epidemiology of FND, published as a chapter of the Handbook of Clinical Neurology (4):

“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.” The text of the chapter mentioned the SNSS data and gave the exact rate for “typical FND cases” as 5.4%. (Since this “typical FND” group included 209 out of the 3781 study participants, it is unclear why the figure given is 5.4% and not 5.5%.)

In 2018, several FND experts, including Professor Carson and Professor Jon Stone, the lead author of Stone et al, published a paper in JAMA Neurology called Current concepts in diagnosis and treatment of functional neurological disorders” (5). Referencing the SNSS, the paper included the following statement: In a well-designed consecutive series of 3781 outpatients of neurology clinics, 5.4% had a primary diagnosis of FND.” (Again, it is not clear why this figure was not 5.5%.)

More recently, Professor Carson and Professor Stone were co-authors of a paper called Functional neurological disorder is common in patients attending chronic pain clinics,” published on May 23rd in the European Journal of Neurology (6). Citing the SNSS findings, this article reported that “the prevalence of typical FND in patients attending neurology outpatient clinics is 5.4%.” (Again, we believe the proper figure is 5.5%.)

Finally, Professor Stone is one of two co-authors, along with Professor Michael Sharpe, of the section about conversion disorder on the medical education site UpToDate. (It is unclear why UpToDate is still using this outdated term for the diagnosis.) The section indicates that it was updated in June of 2022. 

In the epidemiology sub-section, the article states that “the point prevalence of conversion symptoms in clinical settings ranges from 2 to 6 percent.” The section cites three studies, including Stone et al. Here’s what it notes about the latter: “A prospective study of 3781 neurology outpatients found that conversion disorder was present in 6 percent.” This statement is obviously inconsistent with any claim that the same study found FND to be the second-most-common presentation, which itself is based on the declaration that the prevalence was 16%.

While unexplained symptoms or ailments found among SNSS participants who did not have a conversion disorder diagnosis might be called “functional” disorders in the current lexicon, they cannot reasonably be said to be equivalent to FND. Functional disorders are diagnoses of exclusion; FND is definitively not, given the DSM-5 requirement for rule-in clinical signs. The data from Stone et al have not changed since 2010. So why has this research since been framed, in Diez et al and other publications, as evidence that FND is the second-most-common presentation at neurology clinics?

The popular site, maintained by Professor Stone, sheds some light on how a claim of “second commonest reason to see a neurologist,” at a rate of 16%, could be derived from the SNSS data. According to the “frequently asked questions” section of the site:

“In [an] older study of 3781 new appointments across Scotland, there were 209 patients who had clear FND and another 200 who had additional functional disorder diagnoses including dizziness and cognitive symptoms which could also be included now within FND. Other patients presented with diagnoses like migraine, but the neurologists thought the main issue was an associated functional disorder. So, anything from 6-16% of patients could be said to have a functional disorder depending on how that was defined. The upper limit of that estimate would make it the second commonest reason to see a neurologist.”

First, this passage confirms the relevant point. Only 209 patients out of 3781, or 5.5%, had “clear FND”–way below the level that would be required for this diagnosis to be the second-most common. Second, the 16% figure represents merely the “upper limit” of a broad possible range of estimated rates—and not rates for “clear FND” but for the fuzzier and more expansive construct of “functional disorder depending on how that was defined.”

It is not appropriate to retroactively re-interpret the data from the SNSS and effectively triple the reported rate of “clear FND” from 5.5% to 16%–thus vaulting this diagnosis into second place on the list. FND experts presumably believe the higher number is a better reflection of current diagnostic rates. notes, for example, that recruitment of patients for the SNSS occurred two decades ago and that “recognition of FND has improved” since then. But this argument, even if valid, does not justify the decision to inflate the study’s reported FND rate beyond what the data indicated.  

Professor Stone made a salient observation 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 then they’re doing it wrong.”

If clinicians who regard FND as a diagnosis of exclusion and ignore the need for rule-in signs are “doing it wrong,” per Professor Stone, then surely those who cite Stone et al or other SNSS papers to assert that FND is the second-most-common reason to see a neurologist, with a 16% rate, are also “doing it wrong.” It is confusing, not to mention epidemiologically incoherent, when FND experts report divergent rates in different papers while citing the exact same set of data.

This is especially so when the lead and second authors of Stone et al have on multiple occasions endorsed statements about the rate of “typical FND” in the SNSS that do not support the greater claims disseminated in dozens of publications, such as Diez et al. (It is particularly perplexing that these same investigators have also been co-authors of studies endorsing the inaccurate but higher prevalence claims.) Unless the changes in the DSM-5 and the requirement for positive rule-in clinical signs are meaningless, the statement that Stone et al or the SNSS found FND to be the second-most-common diagnosis is categorically untrue. The citation in Diez et al—or rather, the self-evident mis-citation–should be corrected.

Thank you for your attention to this matter. (I have cc’d the paper’s lead and senior authors as well as the co-signatories of the letter.)


Todd Davenport
Department of Physical Therapy
University of the Pacific
Stockton, CA, USA

David Davies-Payne
Department of Radiology
Starship Children’s Hospital
Auckland, New Zealand

Jonathan Edwards
Department of Medicine
University College London
London, England, UK

Keith Geraghty
Centre for Primary Care and Health Services Research
Faculty of Biology, Medicine and Health
University of Manchester
Manchester, England, UK

Calliope Hollingue
Center for Autism and Related Disorders/Kennedy Krieger Institute
Dept of Mental Health/Johns Hopkins Bloomberg School of Public Health 
Johns Hopkins University
Baltimore, MD, USA

Mady Hornig
Department of Epidemiology
Columbia University Mailman School of Public Health
New York, NY, USA

Brian Hughes
School of Psychology
University of Galway
Galway, Ireland

Asad Khan
North West Lung Centre
Manchester University Hospitals
Manchester, England, UK

David Putrino
Department of Rehabilitation Medicine
Icahn School of Medicine at Mt Sinai
New York, NY, USA.

John Swartzberg
Division of Infectious Diseases and Vaccinology
School of Public Health
University of California, Berkeley
Berkeley, CA, USA.

David Tuller (corresponding author)
Center for Global Public Health
School of Public Health
University of California, Berkeley
Berkeley, CA, USA


1. Diez I, Williams B, Kubicki MR, et al. Reduced limbic microstructural integrity in functional neurological disorder. Psychological Medicine 2021; 51(3): 485-493.

2. Stone J, Carson A, Duncan R, et al. Who is referred to neurology clinics?—The diagnoses made in 3781 new patients. Clinical Neurology and Neurosurgery 2010; 112: 747–751.

3. Perez D, Nicholson T, Asadi-Pooya A, et al. Neuroimaging in functional neurological disorder: state of the field and research agenda. NeuroImage: Clinical 2021; 30: 102623.

4. Stone J, Carson A, Duncan R, et al. Symptoms ‘unexplained by organic disease’ in 1144 new neurology out-patients: how often does the diagnosis change at follow-up? Brain 2009; 132: 2878-88.

5. Carson A, Lehn A. Epidemiology. Handbook of Clinical Neurology 2016; 139: 47–60.

6. Espay A, Aybek S, Carson A, et al. Current concepts in diagnosis and treatment of functional neurological disorder. JAMA Neurology 2018; 75(9): 1132–1141.

16 thoughts on “Trial By Error: A Letter to Psychological Medicine about Inflated FND Rate Claims”

  1. Wow. I love the way you methodically and surgically cut through these ‘mistakes’ in research.

    You do wonder how supposed professionals keep getting this wrong. However, these ‘mistakes’ often bolster poor work.

    Thank you.

  2. L B commented:

    “You do wonder how supposed professionals keep getting this wrong. ”

    I’m guessing that many of these authors have simply been following the herd and haven’t bothered to read the literature properly for themselves. I could perhaps understand that for overworked juniors or newcomers; it’s not good but I could understand it. But what excuse can there be for professors and the like, people who call themselves experts in the field?

    It suggests to me that there might be a wider problem in medical science with many doctors and other healthcare professionals not really understanding how science is supposed to work and not appreciating that the literature always needs to be properly read, scrutinized and challenged, that that is a major part of what science is about. (It might also explain their indignation and apparent feelings of being harassed or abused when their work is challenged.) If medicine is taught more as an art than a science, and medical students are expected to look up to their superiors and not to question what they say, then any inclination they may have had towards questioning and challenging the accepted truth may well be effectively drummed out of them at medical school and beyond. To my mind, medical students and doctors should be actively taught to distinguish the skill-acquiring art of medicine from the quite different rigor that’s needed for medical science. While the differences between these disciplines aren’t spelled out to them, we’re probably likely to see a lot more of what amounts to eminence- rather than evidence-based based medical ‘science’, if we can even call it that. (I’d argue that diagnostics is where these different skill sets should ideally come together, and especially so for medically unexplained symptoms, where there’s a real need to question the status quo. But medics are instead encouraged to abandon any remnants of scientific curiosity they might still have and to shut the cases down as mental health problems. That may well please health economists but it does doctors and patients a huge disservice. )

  3. Lady Shambles

    CT: you said “L B commented:

    “You do wonder how supposed professionals keep getting this wrong. ” ”

    If I’m right, and from David’s exemplary work I think I must be, then many of the authors are misquoting their *own* work. I’d be interested to know what you think about that?

  4. Lady Shambles – I refer you to my first comment above. It’s all very perplexing. I’m not sure if we’d need a detective or a shrink to get to the bottom of it.

  5. We could perhaps call this the ‘Great FND Fiasco’ (or GFF for short). If they can have FCD, surely we can have GFF?

  6. Lady Shambles

    CT: that is very nearly ‘GuFF’ isn’t it? Which is apt. “Guff (n) Foolish talk or ideas.”

  7. I saw this online - What the dickens? (What a joke!)

  8. Mike Fraumeni

    All of the work David and colleagues are doing here is very important of course ie: “Prevalence data is important for public health because it gives an estimate about the burden of a disease in a population, which in turn guides decisions about resources for public health interventions,” Schneider said.

  9. Indeed Mike, here’s an example of the second most common claim for FND prevalence in neurology clinics being made - . It may well have been made in good faith, given that the claim has been so widely disseminated in the medical literature, but made it was in the context of trying to obtain greater resources for FND. Of course conditions such as FND should get their fair share of resources but securing more via inflated prevalence claims could strip funding away from other deserving illnesses. I imagine that’s especially likely with a public healthcare system.

  10. Here are links to some (archived) UK Government and Parliament petitions:


    Make of them what you will.

  11. Lady Shambles

    CT said: “What the dickens? (What a joke!)” I’m guessing you were referring to the list of vested interests held by one of two of the main UK proponents of FND? Jon Stone.

  12. Mike Fraumeni

    CT, yes, resource allocation distribution in healthcare should be made as much as possible based on the most reliable evidence available and any medical society or their members trying to inflate data or provide false impressions are bordering on unethical behaviour I might think.

  13. Lady Shambles – my comment reflected the thoughts going through my mind on coming across that article (- The title reminded me somewhat of Charles Dickens’ ‘A Christmas Carol’. Was that what the author intended, I wonder? If so, who might be imagined in the corresponding role of Scrooge and who in the role/s of the ghosts of FND past, FND present and FND future/’yet to come’?

  14. Lady Shambles

    CT said ” If so, who might be imagined in the corresponding role of Scrooge and who in the role/s of the ghosts of FND past, FND present and FND future/’yet to come’?” Our thoughts seem to be alighting on different aspects. The ‘yet to come’ aspect from your contemplations is taking me to dystopian lands. Let’s hope fiction doesn’t materialise into fact. The sooner FND, and its synonyms, is removed from the medical lexicon the better it will be for patients imo.

  15. Lady Shambles – au contraire, I’m rather hoping that fiction does materialize into fact and that we witness a Dickensian-style conversion that sees the end to a disorder that truly belongs to the past.

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