Trial By Error: Letter on Inflated FND Prevalence Rates Accepted for Publication

By David Tuller, DrPH

As I have regularly noted, patients with diagnoses of functional neurological disorder (FND) 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.

But having empathy for FND patients does not mean giving carte blanche to experts to disseminate misinformation. For more than a decade, leaders in this domain have been citing a seminal 2010 study to argue in dozens of articles in the medical literature that FND is the second-most-common diagnosis at outpatient neurology clinics and/or that its prevalence in these settings is 16%. Within the field itself, these repeated claims seem to have attained the status of “fact”—even though they misrepresent the data. The only appropriate prevalence for the category from the 2010 study was less than 6%, which placed it way down the list of diagnoses.

I have written several posts regarding this prevalence-inflation, to which I was first alerted by a sharp-eyed colleague. Investigators and journals have rebuffed or ignored requests for corrections. But the journal NeuroImage: Clinical has now accepted a letter that highlights the issue. Besides me, the signatories are David Davies-Payne, Jonathan Edwards, Keith Geraghty, Calliope Hollingue, Mady Hornig, Brian Hughes, Asad Khan, David Putrino and John Swartzberg. (Their various affiliations are on the letter itself.)

The journal sent out our initial submission for peer review.  We revised it per the requested changes. Now the revised version has been deemed ready for publication. It should be up within weeks. Although I have disagreed with some editorial decisions, I appreciate the overall care with which the journal has handled the matter. This letter might be the first in a peer-reviewed journal to challenge these inflated prevalence claims and explain how they were constructed.

To me, this inflation of numbers from a well-known body of research just seems weird. I mean, it’s very easy to check. It doesn’t take much to compare these statements to the actual numbers reported in the 2010 study, which was part of a larger project called the Scottish Neurology Symptoms Study. The hyped-up claims are based on combining a whole bunch of participants labeled in the study as having “psychological” conditions (around 10%) with the much smaller number identified as having “functional” symptoms (less than 6%)—what we would now call FND. Conflating these categories and asserting that the entire group of 16% represented FND is unwarranted. (The largest diagnostic group was headache, at 19%.)

As we note in our letter, this sort of exaggeration contributes to the phenomenon known as diagnosis creep. FND experts might believe the higher rates (#2 prevalence, at 16%) are a better reflection of their current clinical experience. But that belief does not justify re-interpreting published data and lumping together unlike groups in a way that bolsters their questionable statistical assertions. Epidemiology graduate students at Berkeley would not get away with this sort of data manipulation. It is certainly not what would be expected from experienced investigators.

In the meantime, the final submitted version of our letter is available on a pre-print server. Here’s the top part:

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%)...

You can read the rest here…