By David Tuller, DrPH
For more than a year, I have been criticizing experts in the field of functional neurological disorder for misrepresenting the findings of a seminal study, in effect tripling the reported prevalence rate of the condition. These untrue claims about the Scottish Neurological Symptoms Study (SNSS)—specifically, that the prevalence of FND among outpatient neurology clinics was 16% and/or that it the second-most-common diagnosis–have been repeated in more than 50 papers in the last dozen year. (I did not discover this discrepancy by myself; a shrewd observer brought it to my attention.)
FND is the updated name for the Freudian entity called conversion disorder, although the two differ in diagnostic criteria. Conversion disorder required the presence of some identified trauma or psychological distress that purportedly triggered the symptoms; FND requires the presence of rule-in clinical signs said to be inconsistent with known neurological disease. In fact, long before these changes were included in 2013 in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, neurologists used such rule-in signs to identify cases of psychogenic or “functional” gait and motor disorders, non-epileptic seizures, and sensory deficits—the three main categories of conversion disorder.
In the SNSS, the rate of patients in the study identified as having conversion disorder symptoms was 5.5%, not 16%–209 out of 3781 participants. And at 5.5%, it was way down the list of presentations at outpatient neurology clinics.
However, the FND field since then has expanded its purview beyond these three main categories of conversion disorder to encompass a range of cognitive and related symptoms–part of an overall argument that FND is a common presentation rather than relatively rare. Of the 16% in the SNSS identified as having FND, two-thirds were identified as having “psychological” diagnoses and symptoms that could not fully be explained by neurological disease. However, as we noted in our letter, there is no evidence that any of these additional 10% met or could have met the rule-in criteria for FND. The fact that their symptoms could not fully be accounted does not justify a post-hoc, categorical reinterpretation of their diagnoses as FND.
