By David Tuller, DrPH
*NOTE: Kim, an FND patient and advocate, has left an extensive set of criticisms in the comments. I urge those interested in this debate to read them.
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 16 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 what was formerly called conversion disorder, a Freudian diagnosis in which psychological distress was said to have been “converted” into physical symptoms. Experts in the FND field assert categorically that the condition 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 very 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 new study. That this deeply flawed paper passed through peer review at a BMJ journal was “very disappointing,” he said.
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.
David, you’ve seriously misrepresented this paper, and once again, undermined an entire patient community in the process.
First, let’s be clear: this study does not diagnose FND. It explores signs of functional limb weakness (FLW) in Long Covid patients, not a formal FND diagnosis. Calling this “diagnosing patients with a disorder they don’t report symptoms for” is a strawman. I repeat, not once is FND even mentioned in the study.
Your claim that FND is “just a theory” is both factually wrong and deeply irresponsible. FND is recognised by the National Institutes of Health as a legitimate neurological condition, and the diagnosis is embedded in NHS-endorsed neurological clinical care pathways across England and Wales. This is not niche or speculative medicine. It is a diagnosis made in neurology clinics, based on positive signs, by trained clinicians. What you’re really doing here is accusing those clinicians, and patients themselves, of subscribing to a “theory”, when in reality they’re following evidence-based medical standards and lived experience. It’s not just disrespectful, it’s damaging. It’s 2025. We’ve moved on. This kind of misrepresentation has to stop.
You dismiss the mismatch between signs and self-reported symptoms as “bonkers.” Yet it’s well known that patients with complex conditions often struggle to accurately report or describe symptoms. Some may report heaviness or fatigue rather than weakness; others under-report due to stigma or misunderstanding. The real insight here is that signs may appear before patients perceive or describe symptoms. That’s a valuable area for research, not mockery.
You also ignore a key finding: these FLW signs improved with recovery, indicating they are linked to patients’ clinical state, not incidental or fabricated.
Your mocking tone and phrases like “house of cards” don’t just critique this study. They contribute to a culture that delegitimises FND. This has real-world consequences. Patients already face stigma and barriers to care. As a public commentator, you have a responsibility to engage fairly with both the research and the lived experience behind it.
Finally, if you want to challenge poor science, fine, but this isn’t it. Your commentary muddies the water more than it clarifies it. And let’s be honest, you don’t speak for us. You rarely engage with people who live with this condition, and show little insight into what it’s like to manage it day to day. If your crowdfunders are paying you to provide accurate information, they might want to ask why you’re choosing instead to misrepresent an entire condition and the community around it.
A particularly critical and disturbing failure of both the basic claim and the peer-review process.
> “Experts in the FND field assert categorically that the condition is a “brain network” disorder, but that is a theory, not a fact.”
It isn’t even that much. It is a poorly evidenced and largely incoherent hypothesis. The sort of unoriginal low-grade idea that should have been shredded and discarded at the preliminary barnstorming session at which it was first proposed.
That it was not, and has instead survived and prospered to the influential extent it has, says nothing good about standards at the (allegedly) highest level of modern medicine.
> “They contribute to a culture that delegitimises FND.”
It justifiably delegitimises the underlying diagnosis and concept of FND, which has never been robustly established.
It does not – and Mr Tuller has explicitly said this several times – in any way delegitimise or downplay the experience of patients who are being dragged unwittingly into this diagnostic delusion.
You are attacking the wrong target. Which is exactly what the creators of the FND concept want you to do.
And that has very serious real world consequences indeed.
Hi Sean,
You talk about the “brain network disorder” concept as an incoherent and low-grade hypothesis, but it’s worth pointing out: no one is claiming it’s the full answer. People with lived experience of FND are well aware there’s uncertainty, and we actively advocate for better research to deepen that understanding. But to imply it should have been “shredded and discarded” says more about your disdain for the field than the state of the evidence.
You also suggest that “delegitimising FND” doesn’t hurt patients, only the concept. But that’s a false separation. FND provides a recognised and structured framework within neurology, which enables access to care, treatment, and multidisciplinary support. It’s not a vague or interim label, it’s a clinical diagnosis with established pathways. Dismissing the diagnosis entirely leaves us with nothing. It’s not just a debate about scientific models, it’s about real lives. So yes, delegitimising FND does harm patients, especially when it’s done in such an absolute, dismissive way.
You don’t have to love the current state of FND research. We don’t either. But denying it entirely, or ridiculing the clinicians and patients involved in shaping it, isn’t progress. It’s erasure.
It looks to me like the first commenter here is claiming that positive signs of FND may be demonstrable before a patient is aware of symptoms of FND themselves but that seems to be at odds with what is relayed in a 2023 paper -https://www.nature.com/articles/s41582-022-00765-z – which suggests to me that the techniques used for positive signs reveal that normal function is possible in patients who are displaying symptoms. It seems then that the symptoms of malfunction, and I assume at least the report of these symptoms by the patient, must come first because the so-called ‘positive’ signs are simply establishing that normal function appears to be present in someone who clearly cannot function normally. The plea of FND proponents appears to be – these patients aren’t malingering or feigning because they report and have clearly observable malfunction and genuine distress which can’t be explained by known conditions for which tests are available, whilst still retaining normal neurological capacity – but if patients don’t report and don’t have observable malfunction and show no such genuine distress, then the ‘positive’ signs surely just indicate that they have no problem at all with regard to the condition under consideration. I am NOT suggesting that FND sufferers’ symptoms aren’t real and distressing, or that they don’t have serious bio-pathological illness, but it does seem that patients need to have reported the symptom in question and for it to be evident before the so-called ‘positive’ signs of FND can be interpreted in any way. I therefore agree with David’s assessment of the paper about limb weakness, but I’d be genuinely interested to hear what the authors of that 2023 paper think about it.
Hi CT,
You’re right to bring up the 2023 Neurology paper, it’s an important one, but think there may be a misunderstanding about what “positive signs” in FND actually indicate. They’re not just showing that normal function is possible. They’re revealing inconsistencies that can’t be explained by structural disease, and they do so even before someone might frame their experience as a symptom. That’s part of why they’re called “positive” signs. They’re not about ruling things out, but about identifying specific clinical patterns (like Hoover’s sign, for example) that point toward an FND diagnosis.
The idea that symptoms must be consciously reported before positive signs have value doesn’t align with how FND may present for many people. A patient may experience heaviness, imbalance, or fatigue but not recognise it as “limb weakness”, yet the sign is there clinically. That appears to be what this paper was getting at.
As for the “genuine distress” point, again, distress isn’t a diagnostic requirement for FND anymore. It used to be, under older models like conversion disorder, but modern criteria no longer require it. The positive signs are enough when interpreted by a trained neurologist.
And I have to gently push back on the idea that agreeing with David’s take is the logical outcome of all this. His reading of the paper misrepresents both what the authors did and how FND is diagnosed. There is no indication that any of the patients were clinically diagnosed with FND. The study refers only to signs consistent with functional limb weakness, and even then, only in the context of Long Covid. No diagnosis was made or confirmed. He also frames it as diagnosing people with a disorder they “don’t report symptoms for,” but that assumes a very narrow and outdated view of both symptom awareness and diagnostic reasoning.
Finally, my name is clearly visible on the comment. Choosing to call me “the first commenter” feels like a subtle way to diminish or depersonalise what I said. You’re welcome to disagree with my points, but please extend the basic courtesy of addressing me properly.
Dear David,
I think you have picked up on a red herring here. Positive signs for FND are a matter of debate but there is nothing anomalous about finding them in the absence of symptoms. It would not alter their validity.
The fact that so much is left open-ended / unexplained by the paper, leads me to conclude that it is not a well-constructed piece of work.
I therefore question how it was able to get through peer review, but that in itself is another can of worms: https://www.nature.com/articles/d41586-025-02172-y
In response to the comment directed to me above, from memory the 2023 paper I mentioned in my comment above gives the Hoover sign as an example of a sign that shows that normal neurological function (by that I mean no sign of known pathological illness/condition) is present in someone who exhibits the symptom of apparent limb weakness. I understand that that is how it shows that the supposed genuine symptoms are inconsistent with known structural disease. As I indicated above, I’d love to hear the opinion of one of the authors of that paper on this. What comes first, the symptoms or the positive sign? That would seem to be the question.
I take the point about genuine distress – the criteria have changed – but if people raising points about FND don’t mention the genuine distress that FND sufferers suffer then, from what I’ve seen, they tend to get accused of either not acknowledging that distress or negating it. That’s why I mentioned it.
As for not calling the first commenter by their name….as I understand it, people have the right to remove their online comments (or ask for them to be removed) if they wish so that they disappear. Since it appeared to be a real name of an identifiable person that was being used rather than a pseudonym or unidentifiable abbreviation, I was simply trying to protect their right to do that (if they wished to) out of respect. I didn’t expect to be criticized for doing that.
To make it clearer and avoid confusion, perhaps in the first sentence of my (7.05 AM) comment above I should have written “normal neurological capacity” rather than “normal neurological function”.
As someone who was incorrectly and lazily diagnosed with FND in spite of physical evidence of an autoimmune process (it turns out I have genetic immune regulatory defect) the misuse of FND to explain things not yet understood is concerning. Whilst I don’t doubt some patients have FND (and they deserve treatment and care and research) I think it is right that we challenge the expansion of the FND label without appropriate evidence. It would stop what happened to me happening to others in my situation. And don’t underestimate the severity of that, it put my life in danger and I had to fight for a long time to get the proper diagnosis. People with long covid are already facing massive hurdles because of the as yet unknown mechanisms surrounding their illness without the waters being muddied. As a result of my own misdiagnosis I have been reluctant to speak about my symptoms or to show any emotional affect at appointments for fear of it happening again. Some of the reasons I was told I had FND were absurd and used circular logic like this paper. Whilst I see defensiveness (and rightly so) on the part of FND patients about anything percieved to refute FND as legitimate, don’t they want people who don’t have FND to avoid misdiagnosis? I also don’t think questioning the murky science in this area is necessarily questioning the condition itself, just criticising our lack of scientific understanding and the certainty with which medical professionals often talk about it.
In addition to my previous post, we once believed patients stomach ulcers were caused by stress and now we know many are treated with antibiotics because they are caused by bacteria – with either explanation the disease was a “real” entity its just with the correct explanation categorisation and effective treatment can happen. Even if the cause of FND is functional, the mechanism will exist in signalling pathways in some fashion and should obey the laws of science and eventually be measurable – rubbish vague studies wont help with that.
To the good professor who appears to think that positive signs for FND CAN point to FND in the absence of symptoms:
Can I please direct you to a 2023 paper (-https://www.nature.com/articles/s41582-022-00765-z) to read, if you haven’t already done so. If your opinion is unchanged having read that paper, could you perhaps explain to me why you’re of that opinion (since authors of that paper haven’t yet responded to my expressed desire for them to explain it to me). I’d be most grateful for your input on this.
Hi Kim,
Thanks for your continued advocacy for people with a current FND diagnosis. I posted the link to this blog in the FND Hub I’m in and there are no responses in that hub, so far. I wonder if it would be helpful to people who have been diagnosed with limb weakness (via the Hoover’s sign) when they are not reporting weakness if you can comment on the post about this there. Likewise I would be interested to know what the many FND charities make of one of David’s earlier blogs about the editorial approach re FND/MUS etc in a current journal – sorry, I can’t remember the name of it at the moment (post concussive symptoms are a damn nuisance, eh?) since although I’ve seen the errata request from people with M.E. via their petition I haven’t yet seen any responses from the FND charities or the FND Society. I have more comments to make about that blog but am back at work now after a short break to cover for my sister while she and her family took a much needed holiday and am short on spoons, as usual.
Bean – thank you for your advocacy too. And ditto to everyone else who has commented.
Without accurate DNA sequences in public genetic databases many such problems arise (Genetic Constants In DNA and RNA – 6).
Hi Kim,
I’m confused (cognitive vestibular interactions and post concussive symptoms can do that to a person) so I hope you can help to clarify things for me. I can see FND clearly mentioned in the second sentence in the abstract yet you say that FND isn’t mentioned in the study. So who is right, you or the authors? As an advocate my main concern about this study hasn’t yet been mentioned in these comments and that is that there was no patient or public involvement (PPI) in it. PPI is considered best practice in the UK and many other countries (as long as it isn’t done as a tick box exercise, of course) so my feeling is that all patients, whatever our conditions, deserve best practice and these patients didn’t get it so I’d welcome your thoughts about that regarding this paper and any others you think are of note because of the absence of robust PPI. I also note that there is no mention in the comments that FN(C)D is in DSM5 or that the agnostic stance has, apparently, been detrimental to FND research and it would be interesting to know how many people with a current FND diagnosis were told it’s in DSM at the point of diagnosis or during the (somewhat mythical) ‘management’ consultation.
Another cause for concern/confusion is the mention of inconsistencies that can’t be explained by structural disease. We’ve known for ages that people don’t need structural damage to the brain to get less common conditions like Mal De Debarquement Syndrome (MdDS, my main diagnosis, which is thought to be a FND by some doctors although I have yet to get a clear answer from any of them regarding why they think that or need us all to be in DSM whether we’re in remission or not, or feeling better when we’re exposed to passive motion, or pregnant or for many other reasons), Migraine, Visual Snow Syndrome, Anxiety, Tinnitus, Sopite Syndrome, post concussive symptoms, Motion Sickness, binocular vision dysfunction, etc etc. So I am not sure how or why this is relevant to FN(C)D and would really value your input about this since I’ve never understood this.
The paper that mentioned the DSM agnostic stance stuff indicated that the rule in signs are no longer necessary to make a FND diagnosis so I’m not sure where that leaves people who are not sure if their diagnosis has been made accurately and report that they feel they’ve been ‘sweeped under the carpet, out of sight’ once they’ve been given this diagnosis. I do know some people have had very positive experiences following an accurate diagnosis of FND (especially if they’ve been able to access appropriate therapies (and avoid inappropriate ones) in a timely fashion). However there are many others who have been diagnosed and discharged without any support and others who have been on the receiving end of badly given diagnoses (eg during a 3 minute consultation in a corridor and then told to ‘google it’ or who have not been told the diagnosis but find it later, buried in their notes, often under more relevant diagnoses such as migraine or small fibre neuropathy or chronic pain etc etc) so it would be useful to know if anyone is measuring the impact of poorly delivered diagnoses and whether or not this adds to the economic burden of FND and/or impacts the chances of recovery. I realise some of this is ‘off topic’ from this paper but I value your patient expertise and would really welcome your thoughts.
From the rare conditions (10k and counting, with just under half being neurological conditions) perspective, I have met many people (usually female but not always) who have initially been misdiagnosed with FND/conversion disorder prior to getting their accurate diagnoses but very few of them have been empowered to get their misdiagnoses recorded officially. I have also encountered people who have been told that if they don’t accept a conversion disorder diagnosis/FND diagnosis their primary care will be withdrawn. I have no idea what any of us can do about this but it does not enhance our diagnostic odysseys and when ’eminent’ neurologists cross their arms and say things like ‘questioning the FND dx proves that it’s the right diagnosis’ they bring the whole field into disrepute which is hardly going to empower anyone to better health. If you have any suggestions as to what we can do about this, I’m all ears.