Trial By Error: Leading FND Site Confirms Criticisms on Prevalence Outlined in Our Letter to Neurology Journal

By David Tuller, DrPH

In two recent posts, here and here, I wrote about our letter on inflated prevalence claims for functional neurological disorder (FND) and about the response from the authors of the study we criticized. The 2021 article in NeuroImage: Clinical, “Neuroimaging in functional neurological disorder: state of the field and research agenda,” asserted that FND was the second-most-common diagnosis at outpatient neurology clinics. The article cited a major 2010 study, Stone et al, in support of this assertion.

In that study, however, only 209 of 3781 participants—that is, 5.5%–were diagnosed with what could now be called FND. That placed FND way down the list of diagnoses from #2–a claim that rested on the parallel and frequently repeated assertion that the prevalence in the 2010 study was 16%.

The authors of the 2021 paper responded to our letter with non-responses, evasions and irrelevancies, in the process documenting the validity of the criticism. I already knew the gist of what their response would be because the popular website neurosymptoms.org had posted a similar accounting of the 2010 study in its FAQ section. (More on that below.)

Neurosymptoms.org is a website created and maintained by Professor Jon Stone, a neurologist at the University of Edinburgh and one of the leading advocates for reframing the Freudian construct of conversion disorder as FND. He was also a co-author of the 2021 paper in NeuroImage: Clinical that was the subject of our letter questioning the prevalence claims. Not incidentally, Professor Stone is a junior colleague of lead PACE investigator Professor Michael Sharpe and the lead author of Stone et al, the 2010 paper whose findings on FND prevalence have been misrepresented since its publication. Professor Sharpe was the senior author of Stone et al.

Adding to the confusion, Professor Stone himself has elsewhere endorsed the lower prevalence for FND. In our letter, we noted a 2023 paper that, citing the same body of research, referred to a prevalence of 5.4%; Professor Stone was a co-author of that 2023 paper as well. (For unexplained reasons, Professor Stone and colleagues have calculated 209/3781 as 5.4% rather than 5.5%.) It is of course the case that the prevalence of FND, as calculated from the data from the 2010 study, cannot be both 5.5% (or 5.4%) and 16%.

So let’s see how neurosymptoms.org now describes this seminal study in its FAQ section on prevalence.(I’ve posted about this before, but it’s relevant now and bears repeating.) The language certainly confirms the concerns about misrepresentation expressed in our letter to NeuroImage: Clinical. Here’s neurosymptoms.org:

“In another 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.”

This statement is evidence of the validity of our criticism. Only 209 out of 3781 participants in the study had “clear FND.” These were patients who were diagnosed with symptoms consistent with conversion disorder, the former name for FND. This statement about “clear FND” is a definitive acknowledgement that the prevalence of FND in this study was not 16%, and that FND was not the second-most-common diagnosis.

Then the passage cites “dizziness and cognitive symptoms,” arguing that these cases might conform to current definitions of FND. But that’s all post-hoc speculation and not grounded in any evidence from the study. Furthermore, having migraine or other symptoms in combination with an “associated functional disorder” is not the same as having FND. And the passage continues: “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.”

This statement is a far cry from a declarative assertion that FND is the second-most-common reason to see a neurologist—the problematic assertion included in the 2021 paper in NeuroImage: Clnical. I’ve been criticized for criticizing FND experts without being a neurologist or neuroscientist or biologist or whatever. But I understand English well enough to know that there’s a really big difference in meaning between “16% of patients have FND” and “6-16% of patients could be said to have a functional disorder depending on how that was defined.”

Professor Stone—and his many distinguished colleagues in the FND field—obviously know there’s a difference as well. They have nonetheless chosen to pretend otherwise in the interests of maximizing reported prevalence rates. Anyone who has knowingly signed on to this blatant distortion of these seminal findings is evidently missing an integrity chip. It is hard to take anything such investigators write at face value—and that’s why I don’t.