By David Tuller, DrPH
Functional neurological disorder, or FND, is the new-ish name for the hoary Freudian construct known as conversion disorder. For decades, psychiatrists informed patients that they were “converting” their emotional distress and anxieties into physical symptoms like tremors, seizures, sensory and cognitive deficits, a halting gait, or other physical dysfunctions. The impossibility of proving such claims did not seem to impact psychiatry’s acceptance of the concept or the certainty with which it was applied to patients with unexplained conditions.
Other terms that have been used to describe such symptoms are psychosomatic or psychogenic disorders, hysteria, and the like. These descriptions, which highlight the belief that the conditions are driven by psychological disturbances, have often been regarded as condescending and dismissive. More recently, however, the more neutral-sounding term “functional” has been increasingly adopted to make the diagnosis more palatable to patients, among other reasons. That makes some sense. After all, these syndromes feature deficits in function that resemble standard neurological complaints. And some or many patients who receive a diagnosis of FND, often after years of feeling belittled, overlooked or abandoned by the medical profession, might experience significant relief just from being given an actual name for what they are experiencing. I get that.
I have previously written posts questioning the term FND because it has seemed to be a so-called “wastebasket” diagnosis–just like conversion disorder. Whatever could not clearly be identified as a dysfunction explained by current pathophysiological understanding seemed at risk of being called FND, as it had previously been labeled conversion disorder.
Yet the FND experts have addressed this issue. The change in name has been accompanied as well by changes in the description of the condition in the latest version of the so-called “psychiatric bible,” the Diagnostic and Statistical Manual. Although conversion disorder remains the heading of the category, the DSM-5, published in 2013, now includes “functional neurological symptom disorder” as a synonym. (Note: After posting this earlier, I found out that the DSM-5 last month swapped out the names, with functional neurological symptom disorder becoming the main heading and conversion disorder offered as a synonym.) According to the new criteria, identification of psychological distress, once considered to be a necessary component of conversion disorder, is no longer a requirement for diagnosis—although proponents say it can still be a factor in the genesis of the condition. (In a 2020 presentation on FND, for example, Mark Hallett from the US National Institutes of Health noted in one slide that the condition is “sometimes due in part to a psychological cause.”)
Moreover, FND experts have been advocating the notion that the condition is now a “rule-in” diagnosis based on “positive” signs—not just a dumping ground for all symptoms that don’t fit standard or recognizable neurological or pathophysiological patterns. This need for confirmatory evidence has also been incorporated into the DSM-5 description. The requirement has implications for calculating prevalence rates for FND.
In 2016, Alan Carson and Alexander Lehn, two leading FND experts from the UK and Australia, respectively, made this point explicit in the abstract of a chapter on the epidemiology of the condition:
“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.”
The “positive identification” of FND is conducted through the use of strategies designed, for example, to demonstrate that patients whose attention is distracted can perform physical actions they are otherwise incapable of executing voluntarily. Assuming that these signs accurately identify FND, then the prevalence of the condition in outpatient neurology consultations is, as Carson and Lehn noted, “approximately 6%.” This data point for what they called “typical FND cases” is from a major study of Scottish neurology practices, which Carson and Lehn reference in the text of their chapter. (While they rounded up to 6% in the abstract, they used a more precise figure of 5.4% in the text of the chapter.)
But that doesn’t seem to be the most widely used estimate for this disorder. FND experts have a tendency to cite a higher figure—16%–as the prevalence among outpatient neurology consultations. Here, for example, is a statement from a 2021 paper, whose authors included Alan Carson: “FND is among the most common causes of neurological disability, diagnosed in approximately 16% of outpatient neurology consultations.”
Huh? How can the prevalence of FND diagnoses in outpatient neurology consultations be both 5.4% and 16%?
Where does that 16% prevalence rate come from?
The data for the 16% estimate is also from the Scottish neurological study, whose results were reported in multiple papers. That study investigated the diagnoses rendered to a consecutive series of 3781 outpatients attending neurology clinics. According to a 2009 paper, 1144 of the patients, or 30%, had symptoms that could not be explained at all or only “somewhat” by what the authors called “organic disease.” Of those 1144 patients, only 18% had what the paper called “conversion symptoms” and what Carson and Lehn called “typical FND cases”—presumably, those that could be ruled in through positive signs. (As far as I could tell, the papers did not explicitly mention the use of rule-in signs by the neurologists being surveyed.) That 18% of the sub-group represented less than 6% of the total sample of 3781 patients attending outpatient neurology clinics—the source of the figure cited in the chapter by Carson and Lehn.
But in a subsequent paper in 2010, the investigators presented the Scottish neurological data somewhat differently. In that paper, they highlighted a category of “functional and psychological symptoms” comprising 16% of the 3781 cases. In other words, they apparently lumped together the “approximately 6%” comprising identified FND cases with another 10% of patients assessed as exhibiting “psychological symptoms.” Yet the paper provided no clear explanation for this decision. I assume the investigators assumed that the “psychological symptoms” were linked to the functional deficits, even absent rule-in signs.
Here is a paragraph from the abstract of the 2010 paper:
“The commonest categories of diagnosis made were: headache (19%), functional and psychological symptoms (16%), epilepsy (14%), peripheral nerve disorders (11%), miscellaneous neurological disorders (10%), demyelination (7%), spinal disorders (6%), Parkinson’s disease/movement disorders (6%), and syncope (4%).”
As in the 2021 paper, experts have since repeatedly cited this Scottish research in declaring that the prevalence of FND among outpatient neurology visits is 16%. They have argued, on this basis, that FND is the “second-most” common neurology complaint in outpatient clinics, after headache. Obviously, if they used the Scottish study’s 5.4% figure for “typical” FND cases, as cited.by Carson and Lehn, the disorder would clearly not be the second-most common neurological presentation. According the the relevant data, it would rank much lower on the list, after epilepsy, peripheral nerve disorders, miscellaneous neurological disorders demyelination, spinal disorders and Parkinson’s disease/movement disorders.
The higher prevalence claim is problematic. Either FND is a rule-in diagnosis based on positive signs, in which case it is not the second-most-common neurology presentation in outpatient clinics, according to the data collected by top experts in the field. Or it is not a rule-in diagnosis based on positive signs and can instead be applied to many more people than are identified on that basis. By including in the FND category people with “psychological symptoms” combined with neurological deficits that have presumably not been diagnosed by rule-in strategies, the authors have tripled the reported prevalence.
Some articles and resources on FND refer to yet another statistic, suggesting that up to a third of outpatient neurology consultations involve the disorder. Check out this statement from Alexander Lehn–remember, he co-wrote the chapter that included the 5.4% figure for “typical FND cases”–during a 2020 discussion on an Australian podcast: “One in three presentations to neurology outpatient clinics are due to functional disorders. One in three.”
No equivocation there! One in three is more than six times the 5.4% prevalence rate.
.That statistic presumably comes from the same Scottish study, with its 30% prevalence, or not quite a third, of patients whose conditions were not explained or only “somewhat” explained by what the authors refer to as “organic disease.” (A side note: Given that FND experts themselves are clearly differentiating between “organic disease” and “psychological” causation, why do they insist that others who make the same distinction are engaging in unwarranted and specious “mind-body dualism”? Asking for a friend…)
The 30% figure takes into account all 1144 of the patients whose disorders neurologists could not explain by “organic disease,” including the 5.4% rate of “typical FND cases” and the 10% rate of cases involving “psychological symptoms” but apparently with no rule-in signs. The additional 14% of that 30% cohort presumably have neither rule-in signs nor “psychological symptoms”—just unexplained neurological deficits. That means these 30% and 16% prevalence claims take us right back to the wastebasket diagnosis. With these expansive interpretations of what constitutes FND, anything that appears “functional”—in other words, anything the neurologists can’t explain through standard methodologies—can be counted as FND, whether identified by rule-in signs or not.
From a scientific and medical perspective, this inflation of prevalence data is problematic and confusing. What is the basis for assuming that the 10% with “psychological symptoms”–but apparently without positive, rule-in signs–also have FND? And how about the additional 14% on top of that who have not been identified as experiencing “psychological symptoms” and whose disorders are also unexplained by “organic disease”?
If FND is now a rule-in diagnosis, per the DSM-5 and the leading experts, then people without positive identification should not be given the label or imputed to belong to the category. Yet it seems, from how the data in the Scottish study have been interpreted, that these patients are at risk of being diagnosed with FND even in the absence of any such positive signs–just as they might previously have been tossed into the conversion disorder trash bin.
In short, the documented prevalence of FND among outpatients attending neurology clinics, according to the revised description of the condition and the data collected by top experts, is less than 6%–not 16% and certainly not a third. In the field of oncology, people who have not been identified as having cancer through positive diagnostic tests are not routinely included in cancer statistics. Why are neurologists doing otherwise with FND?
(One final point: Professor Michael Sharpe of PACE fame was the senior author on the Scottish neurological studies cited here. Just saying.)