By David Tuller, DrPH
A recently published study about functional neurological disorder (FND) has reported some perplexing data. Of almost 300 patients diagnosed with Long Covid, 100 were identified as demonstrating one or more “positive signs” for “functional limb weakness,” a form of FND. Yet only 14 of those 100 patients reported experiencing limb weakness in the first place; the other 84 did not.
Hm. What does it mean to identify positive signs of functional limb weakness in the absence of reported limb weakness? Who knows? Certainly the investigators themselves make no credible attempt to explain this conundrum.
The study—“Contemporary positive signs of functional limb weakness in post-acute sequelae of SARS-CoV-2: an exploratory analysis of their utility in diagnosis and follow-up”–was published in June by BMJ Neurology Open, a major joural. It is retrospective, with data drawn from the medical records of Long Covid patients who attended a neurology clinic in Tokyo, Japan, from 2021 to 2014. At the clinic, they received comprehensive neurological exams, including testing for positive signs for functional limb weakness. (According to current practice, FND diagnoses require affirmative clinical indications, often referred to as “positive signs” or “rule-in signs,” such as intact reflexes in a limb said to be weak or paralyzed, that are purportedly incompatible with known pathophysiological processes.)
During the exams, the neurologists tested for functional limb weakness using six different signs, described in detail in a supplementary file. Apparently, the discrepancy between the number of patients found to have these positive signs and the much smaller number who actually reported limb weakness during these exams did not raise any particular concerns among the investigators.
Instead, they seem to have assumed that positive signs for functional limb weakness in people with Long COVID indicate cases of FND–even in the absence of evidence that patients are experiencing the relevant symptom. The investigators then suggest that these alleged cases of FND are likely implicated in generating and/or perpetuating Long Covid symptoms like fatigue and headache. “Some of the most common neurological symptoms of long COVID may be caused by FND,” they conclude.
Given that five in six of those with positive signs of functional limb weakness did not report limb weakness, this line of argument is kind of bonkers. The most urgent question arising from this study is: Do these signs mean anything at all? (Several of the signs have long been used in neurology; a couple of them were much more recently identified. As I have previously discussed, the evidence for the overall accuracy of these various signs is shaky.)
I suppose it is possible that some patients in the study might not have been that specific and might have referred to limb weakness as “fatigue.” But it seems highly unlikely this would have occurred in 84 out of 100 cases. After all, these patients underwent comprehensive neurological exams that included tests for functional limb weakness. Presumably, the neurologists conducting these exams asked questions that would have, or should have, elicited an accurate accounting of a distinctive symptom like limb weakness.
FND is the current name for the psychiatric condition formerly called conversion disorder, in which psychological distress was said to have been “converted” into physical symptoms. Experts in the FND field assert categorically that is a “brain network” disorder, but that is a theory, not a fact. The reality is that the etiology and pathophysiological processes causing the symptoms remain unknown. What is clear is that people with FND suffer from extremely distressing and disabling symptoms that resist easy explanation. Those with the condition are ill-served by research that fails to abide by basic rules of scientific reasoning.
My UC Berkeley colleague, infectious disease physician and professor emeritus John Swartzberg, shared my low opinion of this piece of work. That this deeply flawed paper passed through a BMJ journal’s peer review process, he said, was “very disappointing.”
The paper is marred by sentences like this: “Assuming that patients with positive signs had FND, the prevalence of FND coexisting with long COVID is likely to not depend on which variant of COVID patients were infected with but solely on the number of patients infected with COVID-19, as observed in this study.” Since those with positive signs of functional limb weakness were much more likely not to have reported any limb weakness than to have reported it, the assumption that “patients with positive signs had FND” is hard to justify. And any further claims built on that unjustifiable assumption cannot be taken seriously.
And there are passages in which, given the uninterpretable results on the positive signs, the argument reads like a parody:
“In summary, our study showed that long COVID, accompanied by positive signs, is not rare and that this phenomenon indicates the possibility of the coexistence of long COVID and FND. Therefore, some patients with long COVID may present with symptoms of FND. If positive signs are observed in long COVID patients, they are a useful indicator of the coexistence of FND in those with long COVID.”
The paper is a house of cards built on unwarranted assertions and pirouettes of logic. (I’ve addressed a core concern in this post but not the only one.) In any event, BMJ journals have not distinguished themselves when it comes to ME/CFS and Long COVID. This latest problematic publication is not remotely surprising.