Trial By Error: Letter to Journal about Inflated Claims of FND Prevalence

By David Tuller, DrPH

I have spent some time trying to correct the record on the reported prevalence of functional neurological disorder (FND). As I have documented, leaders of the FND field have spent the last decade misrepresenting the findings of a seminal 2010 study, Stone et al, to claim that this diagnosis is the second-most-common presentation at outpatient neurology clinics, at 16%.

These twin claims about the prevalence rates found in Stone et al are categorically false—as I have pointed out over and over again. In fact, Stone et al reported that less than 6% of patients at the clinics surveyed were diagnosed with what would now be called FND, the new name for what used to be called conversion disorder; this clinical entity was therefore way down the list of diagnoses. To assert otherwise is to engage in what is clearly a form of the phenomenon known as “diagnostic creep.” There are no decent excuses for such flagrantly anti-scientific behavior as mis-citing key data from a seminal study in your field of expertise to almost triple the apparent prevalence of the condition of interest.

At least 50 papers in multiple journals have included one or both of these misstatements about the rates of FND found in Stone et al. I have sent letters to a number of journals asking for corrections. Several colleagues have co-signed these appeals. As I reported in June, one journal—NeuroImage: Clinical–agreed to correct a 2021 paper from Perez et al that had indisputably mis-cited the 2010 paper. I then began referencing that upcoming correction in letters I sent to other journals. I also sent a letter directly to the lead and senior authors of the 2021 paper, thanking them for agreeing to the correction in NeuroImage: Clinical and asking them to initiate similar corrections in other papers they co-authored. (They did not respond.)

I subsequently heard from NeuroImage: Clinical that the authors had withdrawn their agreement to the correction. Wow! I’d never heard of such a thing, and neither had my co-signatories, but there it was. The authors had acknowledged that a correction was warranted—and then, apparently, unacknowledged that self-evident reality. Perhaps they had not fully grasped that correcting this obvious error in one paper meant correcting it across the board. Who knows? No explanation for this unusual reversal was provided.

In any event, the journal invited us to submit a letter for publication instead. We have done that, and it is now in the process of being reviewed. If the letter is accepted, I assume the authors will be given a chance to respond. In the meantime, the letter has been posted on a pre-print server and is now in the public domain, so I have included it below.

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Inaccurate Reference Regarding FND Prevalence Requires Correction: Comment on “Neuroimaging in functional neurological disorder: state of the field and research agenda” (Perez et al, 2021), published in NeuroImage: Clinical

To the editor:

An article in NeuroImage: Clinical, “Neuroimaging in functional neurological disorder: state of the field and research agenda” (Perez et al, 2021), cited a prominent paper (Stone et al, 2010) to assert that functional neurological disorder (FND) is the “2^nd most common outpatient neurologic diagnosis.”

This assertion about the findings of the 2010 paper is unambiguously false. It requires a correction. Indeed, when we first alerted the journal about this matter, the authors agreed to publish a corrigendum—an offer they subsequently withdrew.

The 2010 paper was one of several arising from the Scottish Neurological Symptoms Study (SNSS), which reviewed records from 36 outpatient neurology clinics and reported that 209 of 3781 attendees–less than 6%–received a diagnosis of “functional” symptoms or what could also be called conversion disorder, the former name for FND. The study found higher rates of many other diagnoses, including 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%).

Earlier this year, a paper in the European Journal of Neurology (Mason et al, 2023) cited another SNSS paper (Stone et al, 2009) to note an FND prevalence at outpatient neurology clinics of 5.4%—far below the level needed to be “2^nd most common.” Of note, the lead and second authors of the SNSS papers were co-authors of this 2023 article. Moreover, the authors of another study (Foley et al, 2022) have recently corrected the same misrepresentation of the SNSS findings as the one identified in Perez et al, 2021.

The “2^nd most common” assertion is based on a parallel claim that the SNSS found an FND prevalence of 16% (e.g. Ludwig et al, 2018)–almost triple the 5.4% rate cited recently in Mason et al, 2023. The extra patients included in this expanded FND category were the 10% collectively identified in the SNSS as having “psychological” symptoms; they fell into a hodge-podge of diagnostic sub-categories, among them hyperventilation, anxiety and depression, atypical facial/temporomandibular joint pain, post-head injury symptoms, fibromyalgia, repetitive strain injury, and alcohol excess. The SNSS reports included a combined “functional/psychological” grouping but did not provide evidence that the 10% with “psychological” symptoms met or could have met criteria for FND, which is not a diagnosis of exclusion but requires specific confirmatory clinical signs.

The post-hoc reinterpretation of previously reported data in a way that conflates FND with other complex conditions and almost triples its apparent prevalence is an example of the phenomenon known as diagnostic creep. But the SNSS findings are a matter of record. It should not be considered contentious to point out that these findings do not support the claim that FND is the “2^nd most common” diagnosis at outpatient neurology clinics, or that the prevalence is 16%. The accurate assessment and transparent reporting of prevalence rates are core public health functions. The failure to correct self-evident errors is harmful to the scientific process and to the public’s trust in the integrity of the medical literature.

Sincerely–

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

References:

Foley C, Kirkby A, Eccles F, 2022. A meta-ethnographic synthesis of the experiences of stigma amongst people with functional neurological disorder. Disability and Rehabilitation. DOI: 10.1080/09638288.2022.2155714.

Ludwig L, Pasman J, Nicholson T, et al. Stressful life events and maltreatment in conversion (functional neurological) disorder: systematic review and meta-analysis of case-control studies, 2018. Lancet Psychiatry; 5:307–320.

Mason I, Renée J, Marples I, et al, 2023. Functional neurological disorder is common in patients attending chronic pain clinics. Eur J Neurol. DOI: 10.1111/ene.15892.

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

Stone J, Carson A, Duncan R, et al, 2009. Symptoms ‘unexplained by organic disease’ in 1144 new neurology out-patients: how often does the diagnosis change at follow-up? Brain; 132(10): 2878–2888.

Stone J, Carson A, Duncan R, et al, 2010. Who is referred to neurology clinics?—the diagnoses made in 3781 new patients. Clin Neurol Neurosurg; 112(9):747-751.