Correspondence: Inaccurate reference leads to tripling of reported FND prevalence

Highlights

• •Perez et al asserted that FND is the "2nd most common" diagnosis in outpatient neurology.
• •Stone et al (2010), cited by Perez et al, does not support the “2nd most common” claim.
• •In Stone et al, a broad “functional/psychological” category was the second most common
• •FND is not synonymous with the “functional/psychological” category in Stone et al.

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) as evidence for the assertion that functional neurological disorder (FND) is the “2nd 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 in the 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 “2nd 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

Declaration of Competing Interest

David Tuller's academic position at University of California, Berkeley, is largely funded through crowdfunded donations to the university from patients with ME/CFS, Long Covid, and other complex chronic conditions.

Data availability

The data used were data previously published by others.