Trial By Error: Update on Our Letter Concerning the Prevalence of Functional Neurological Disorder

By David Tuller, DrPH

In August, I submitted a letter to NeuroImage: Clinical concerning inflated rates of prevalence of functional neurological disorder, on behalf of myself and nine colleagues. After the letter went through peer review and a round of revisions, I was informed in early November that it had been accepted, as I noted in a blog post.

Our letter was a response to a 2021 article that cited a seminal 2010 study in asserting that FND was the second-most-common presentation at out-patient neurology clinics. This assertion was untrue—the 2010 article found that the number of cases qualifying as conversion disorder, as FND was then called, was way down the list of diagnoses. I had initially been informed that our letter might be published within a couple of weeks. As it turns out, the journal is planning to release it along with an expected response from the authors of the 2021 paper, which certainly makes sense. I’ve been told that the exchange is likely to be published this or next month.

(Notwithstanding my concerns about exaggerated claims made by FND investigators, it is important to make clear that patients with FND diagnoses experience tremendous suffering. A patient who goes by the moniker @FnDPortal has written a compelling and sometimes harrowing essay, Cadenza for Fractured Consciousness: A Personal History of the World’s Most Misunderstood Illness, that is well worth a read in order to understand some of the struggles that can accompany the condition.)

If the NeuroImage: Clinical article—“Neuroimaging in functional neurological disorder: state of the field and research agenda”—had been the only one to include this error, I likely wouldn’t have bothered to counter it. However, it is just one of the more than fifty papers in peer-reviewed journals that have misrepresented the FND prevalence found in the 2010 article and related studies from the same research project, the Scottish Neurological Symptoms Study. The #2 statement is often combined with the parallel claim that 16% of outpatient neurology patients received FND diagnoses. In the SNSS, headache, at 19%, was the most common presentation. However, only 209 out of 3781 study subjects, or 5.5%, received diagnoses that could be categorized as conversion disorder. Any claim that the SNSS found FND to be the second-most-common diagnosis, with a prevalence of 16%, is indisputably false.

I’ve been trying to correct the record on this matter for more than a year, with letters to investigators and journals requesting corrections. (A sharp-eyed source first alerted me to these discrepancies.) This request has been either rebuffed or ignored–so I am pleased that NeuroImage: Clinical has accepted our letter and that accurate information on this matter will soon be in the published record. In the meantime, I’ve included below the text of the preprint of the letter, which can also be accessed here. (The final version is largely the same, but with a few tweaks.)

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Letter to NeuroImage: Clinical

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) as evidence for the assertion that functional neurological disorder (FND) is the “2^nd most common outpatient neurologic diagnosis.” Although studies have yielded varying FND prevalence rates, the claim that it is the second-most common diagnosis at outpatient neurology clinics represents an erroneous interpretation of the findings of the referenced 2010 paper.

FND is the current name for what was formerly called conversion disorder, the diagnosis previously given to patients believed to have psychogenic motor and gait dysfunctions, sensory deficits, and non-epileptic seizures. According to the 2013 edition of the Diagnostic and Statistical Manual of Mental Disorders and as noted in Perez et al, FND is not a diagnosis of exclusion but requires the presence of specific “rule-in” clinical signs believed to be incompatible with known neurological disease. Some of these clinical signs have long been used by neurologists and other clinicians to help them identify cases of conversion disorder.

Stone et al’s 2010 paper was one of several arising from the Scottish Neurological Symptoms Study (SNSS). The study reviewed records from multiple outpatient neurology clinics and reported that 209 of 3781 attendees, or less than 6%, received diagnoses compatible with conversion disorder–in other words, what would now be called FND. In terms of ranking, this group of patients—labeled in the SNSS as having “functional” symptoms or diagnoses–was far down the list. The study found higher rates of many other conditions, 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 a different SNSS paper (Stone et al, 2009) to support the assertion that FND prevalence at outpatient neurology clinics was 5.4%—far lower than the percentage needed to be the “2nd most common” diagnosis. Moreover, the authors of another paper (Foley et al, 2022) have recently issued a correction for the same misstatement of FND prevalence from the SNSS findings as the one identified in Perez et al.

The assertion that the SNSS found FND to be the “2nd most common” diagnosis at outpatient neurology clinics is based on a parallel and commonly repeated claim that the study found the prevalence in these settings to be 16% (e.g. Ludwig et al, 2018). That rate is almost three times the 5.4% prevalence recently highlighted in Mason et al. The extra patients included in this greatly expanded FND category were another 10% collectively identified inthe SNSS as having “psychological” symptoms or diagnoses. These “psychological” patients fell into a range of clinical 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 paper cited in Perez et al reported that a combined grouping of the patients with “functional and psychological” symptoms or diagnoses had a prevalence of 16% but did not provide any evidence that the 10% included under the “psychological” label met, or could have met, the explicit FND requirement for rule-in clinical signs.

FND is not synonymous with the broader “functional and psychological” category in the SNSS and should not be presented as if it were. The post-hoc reinterpretation of previously reported data in a way that conflates FND with other complex conditions—almost tripling its apparent prevalence in the process–is an example of the phenomenon known as diagnostic creep. In any event, the SNSS results are a matter of record. Whatever future studies might determine about FND rates, the published findings cited by Perez et al and addressed in this letter do not support either the claim that it is the “2^nd most common” diagnosis in outpatient neurology clinics or the related claim that its prevalence at these venues is 16%.

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
Department 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, 2018. Stressful life events and maltreatment inconversion (functional neurological) disorder: systematic review and meta-analysis of case-control studies. 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.