By David Tuller, DrPH
I have recently written two posts (here and here) about how experts in functional neurological disorder (FND) have a tendency to assert prevalence rates that ignore their own diagnostic criteria. Today I sent a letter to the corresponding author of yet another paper that has similarly engaged in this problematic strategy. I have posted the letter below.
But I want to repeat—pretty much verbatim–a point I made in my last post: I am in no way questioning whether people with the diagnosis have serious disorders and very debilitating symptoms. Of course they do! And of course they deserve the best medical care available, like everyone. If anyone believes FND patients are not struggling with something really awful, an excellent, recently posted essay called “Cadenza for Fractured Consciousness: A Personal History of the World’s Most Misunderstood Illness” will quickly alleviate them of their misconceptions.
As I also wrote in my last post, FND experts come across as sensitive and caring clinicians who clearly want the best for their patients. But whether doctors demonstrate compassion is a separate issue from whether it is acceptable to disseminate prevalence claims that go beyond the established clinical criteria.
Why does it matter if FND experts argue in one paper that the prevalence of FND among outpatients at neurology clinics is “approximately 6%” but elsewhere report that it is 16%, or even state that it is up to one-third? A key reason is that doing so undermines the credibility of the argument that FND is now a rule-in diagnosis and not a diagnosis of exclusion. Lumping together patients with and without rule-in signs in a single category risks turning FND into a wastebasket diagnosis applied to anyone with unexplained symptoms. That happened routinely with the hoary construct of “conversion disorder,” the term FND is meant to replace. Suggesting that patients without rule-in signs have FND is, in effect, misdiagnosing them. And misdiagnoses can lead to an array of unfortunate consequences.
Given that the term “functional” is now frequently used to describe any symptom or condition for which no organic cause has yet been identified, it is understandable that patients with unexplained neurological symptoms might automatically get slapped with an FND diagnosis. But the term now has a more specific and clearly defined meaning. When it comes to prevalence rates, the frequency with which investigators appear to be casually conflating “functional” and FND, as if the two were interchangeable, is likely to generate serious confusion for patients and clinicians alike. In medicine, words matter.
Assistant Professor of Neurology
Yale School of Medicine
New Haven, Connecticut
Dear Dr Tolchin:
In a 2021 paper called “The role of evidence-based guidelines in the diagnosis and treatment of functional neurological disorder” (Tolchin et al), published by Epilepsy & Behavior Reports, you and several colleagues wrote that functional neurological disorder (FND) “is among the most common causes of neurological disability, diagnosed in approximately 16% of outpatient neurology consultations.” The citation for this statement is a 2010 paper called “Who is referred to neurology clinics?—the diagnoses made in 3781 new patients,” which was published by Clinical Neurology and Neurosurgery.
Unfortunately, this paper–one of a number arising from a major research project called the Scottish Neurological Symptoms Study (SNSS)–does not appear to support the 16% prevalence claim.
For years, leading neurologists have noted that FND is not a diagnosis of exclusion but a rule-in diagnosis based on positive signs found during clinical examinations. This approach was enshrined in the fifth iteration of the Diagnostic and Statistical Manual of Mental Disorders, which was published in 2013. In 2016, two experts—including Alan Carson, a co-author of both the 2010 paper and Tolchin et al–wrote the following in the abstract of an authoritative account of the epidemiology of FND, published as a chapter of the. Handbook of Clinical Neurology:
“The recent changes in DSM-5 to a definition based on positive identification of physical symptoms which are incongruent and inconsistent with neurologic disease and the lack of need for any psychopathology represent a significant step forward in clarifying the disorder. On this basis, FND account for approximately 6% of neurology outpatient contacts.”
In the text of this chapter, Carson and his co-author, Alexander Lehn, referenced the findings of the SNSS, which investigated the diagnoses of 3781 outpatients seeking care at four Scottish National Health Service neurology centers. (In fact, the text of the Carson & Lehn chapter indicated that the actual FND prevalence from this Scottish study was 5.4%.)
The prevalence at outpatient clinics cited in Tolchin et al is almost triple that mentioned in the 2016 article on the epidemiology of FND. How could the SNSS have yielded these two disparate prevalence rates for the same illness?
In the 2010 paper, the 16% figure included patients found to have what the authors called “functional” symptoms as well as those found to have “psychological” symptoms—all lumped together into a “functional or psychological diagnoses” category. It is unclear why the authors chose to combine these two groups.
The “functional” group included patients diagnosed with “non-epileptic attacks” as well as “functional sensory” and “functional motor” disorders. In a 2009 paper also based on data from the SNSS, this same group of patients was described as having “conversion symptoms.” These patients were the “approximately 6%” (or 5.4%) identified by Carson & Lehn in 2016 as “typical FND cases” based on “positive identification,” per the DSM-5 definition.
That means the additional 10% were the patients identified as having “psychological” symptoms. In the 2010 paper, this group included those given one of a grab-bag of diagnoses, among them “anxiety and depression,” “pain symptoms,” “fibromyalgia,” “post-head injury symptoms,” “alcohol excess,” and “hyperventilation,” as well as a cohort referred to “non-organic.” Given the prevalence highlighted in the Carson & Lehn chapter, patients in this “psychological” category were apparently not deemed to have “typical FND” based on rule-in signs.
Yet investigators in this domain of inquiry have generally ignored this salient point when they have cited the 16% prevalence figure in papers, including Tolchin et al–even as they have maintained that FND is not a diagnosis of exclusion but one requiring positive identification through rule-in signs.
Jon Stone, the first author of the 2010 paper as well as a co-author of Tolchin et al, noted the following during a 2021 podcast produced by the Encephalitis Society: “Some people think that FND is a condition you diagnose when someone has neurological symptoms but you can’t find a brain disease to go along with it. And that’s absolutely not the case. Some people [i.e. clinicians] do that, but if they’re doing it like that they’re doing it wrong.”
Since FND is now officially a rule-in diagnosis, the appropriate and acceptable prevalence rate to cite from the SNSS is the “approximately 6%” (or 5.4%) noted by Carson & Lehn. Unless unreported data indicate that the 10% with “psychological” symptoms in the SNSS sample could have been diagnosed with FND through rule-in signs, claims that the study found prevalence in outpatient neurology clinics to be 16% are unwarranted.
Perhaps these additional patients with “psychological” symptoms had medical complaints that could be described as “functional.” But “functional” in this context simply means that the symptoms or condition have not been found to conform to standard understandings of known diseases; the term should not be casually conflated with the clinical entity known as FND. It is therefore hard to justify identifying patients as having FND under the DSM-5 diagnostic criteria just because they have symptoms deemed to be “functional.”.
If clinicians who diagnose FND without rule-in signs are “doing it wrong,” per Stone, then those who cite the SNSS data to assert a prevalence rate of 16% are also “doing it wrong.” The field of public health relies on consistency and accuracy in the assessment and determination of disease prevalence. It is confusing, not to mention epidemiologically incoherent, when FND experts seem to overlook their own rule-in rule while tripling the reported rates of this challenging diagnosis.
In a related example, a 2021 paper co-authored by Stone and Carson, among others, noted that “tightly defined FND” represents “at least 5%–10% of new neurological consultations.” A prevalence rate of “approximately 6%” (or 5.4%) obviously falls within that range. However, the authors cite no specific reference for the claim that the prevalence of “tightly defined FND”–presumably FND diagnosed through rule-in signs–ranges as high as 10%, which is almost double the 5.4% cited by Carson & Lehn. And certainly 10% is significantly lower than the 16% mentioned in Tolchin et al and other papers.
Rule in should mean rule in. Absent rule-in signs, patients with neurological symptoms that do not adhere to recognized disease patterns should be given a diagnosis of idiopathic neurological disorder, neurological disorder of unknown etiology, or some related term—not FND. If further information regarding the 10% in the SNSS sample with “psychological diagnoses” is unavailable, the prevalence statement citing that research in Tolchin et al should be corrected to reflect the documented rate of “approximately 6%” (or 5.4%) noted by Carson & Lehn.
Thank you for your attention to this matter.
(I have cc’d William Tatum, the editor of Epilepsy & Behavior Reports, on this letter. Since I plan to post the letter on Virology Blog, a popular science venue, I have also cc’d Vincent Racaniello, the host of the website and a microbiology professor at Columbia University.)
David Tuller, DrPH
Senior Fellow in Public Health and Journalism
Center for Global Public Health
School of Public Health
University of California, Berkeley