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When Ignorance Becomes a Diagnosis: FND Experts Warn of Diagnostic Failure

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What happens when a neurologist encounters a neurological presentation they do not recognise? According to a 2020 paper published in Neurology: Clinical Practice, there is a genuine risk that the patient will be labelled with Functional Neurological Disorder (FND). Not because the clinician has established what is wrong, but because they have failed to recognise what might actually be wrong. The warning comes from researchers working within the FND field itself, including David L. Perez, in a paper titled Cautionary Notes on Diagnosing Functional Neurologic Disorder as a Neurologist-in-Training.

The authors acknowledge several diagnostic problems that deserve far more attention than they generally receive. Weak functional signs can occur in recognised neurological and pain conditions. Neurologists can disagree about whether a presentation is functional. Incongruence requires considerable clinical expertise to judge. An FND diagnosis can prematurely terminate appropriate investigations. Most importantly, an unfamiliar neurological presentation must not be labelled FND merely because the examining clinician has never encountered it before.

“Bizarre” neurologic presentations should not be labeled as FND simply because the clinician has not previously encountered such a symptom complex.

That statement should be obvious. Apparently, it needed to be written down in a medical journal. The distinction matters because the clinician's unfamiliarity with a disease tells us something about the clinician, not necessarily the patient. An unusual presentation is not evidence of a functional disorder simply because it falls outside somebody's experience. A diagnosis that depends on what an individual doctor happens to recognise is vulnerable to the limits of that doctor's knowledge.

The authors also warn that marginally positive functional examination signs have low specificity. Collapsing or give-way weakness, for example, can occur in patients with acute or chronic pain. Tremor variability can occur in Parkinson's disease, particularly with changes in arousal or emotional state. Some signs considered supportive of FND also lack robust agreement between examiners. These are not obscure objections invented by critics. They are limitations explicitly acknowledged in the clinical literature.

This raises a question that deserves a proper answer: when a sign appears in both a recognised neurological condition and a presentation labelled functional, what exactly has the sign established? It may describe an observable feature of the patient's examination, but that does not automatically establish its cause. If the same feature can arise through different pathological processes, its presence alone cannot distinguish those processes. Treating a weak or nonspecific sign as decisive diagnostic evidence is not sophisticated clinical reasoning. It is overinterpretation.

The paper distinguishes weakly positive signs from signs considered more diagnostically specific, such as Hoover's sign. However, even this distinction does not eliminate the broader problem of interpretation. We have previously examined the limitations of positive signs in our discussion of Hoover's sign and apraxia. A positive sign must be interpreted within the complete neurological presentation, including possible alternative explanations. Calling something a rule-in sign does not make the rest of the differential diagnosis disappear.

Perhaps the most revealing admission concerns incongruence. The authors acknowledge that diagnosing a functional movement disorder partly depends on judging whether the presentation is incongruent with other movement disorders, and that making this judgement requires substantial expertise. They also acknowledge that experienced neurologists can disagree in their diagnostic impressions. Consider the implications. If two appropriately trained neurologists can examine the same presentation and reach different conclusions about whether it is functional, the diagnostic judgement is not as straightforward or reproducible as the confident language surrounding FND sometimes suggests.

There is an uncomfortable circularity waiting to happen. A clinician encounters something unfamiliar, judges it inconsistent with recognised disease, and then uses that perceived inconsistency to support an FND diagnosis. But the original problem was that the clinician did not recognise the presentation. Unless alternative explanations have been adequately considered, the reasoning risks converting ignorance into evidence. The paper explicitly warns against this mistake, yet it provides no measurement of how frequently it occurs in everyday clinical practice.

The authors further caution that an FND diagnosis should not lead to premature discontinuation of an appropriate diagnostic workup when important clinical features remain unexplained. This matters enormously. A patient may present with several neurological abnormalities, some of which resemble features described in FND literature. Those similarities do not establish that every symptom has the same explanation, nor do they justify ignoring abnormalities that remain outside the proposed diagnosis. The paper gives examples involving epileptic seizures and neurodegenerative disorders, including a longitudinal study in which three of 89 patients initially identified with functional limb weakness subsequently developed neurodegenerative disease.

The authors discuss these findings primarily through the possibility of neurological comorbidity or functional symptoms preceding a neurodegenerative diagnosis. But that interpretation is not the only question worth asking. When another neurological disease is subsequently identified, investigators must still establish whether the original functional diagnosis was correct, whether the disease was missed, or whether the observed signs reflected a phenotype shared across conditions. Simply invoking comorbidity does not settle the matter. This distinction is central to our examination of FND, autism and diagnostic overlap.

It also connects directly to the common phenotype hypothesis discussed on this site. The hypothesis asks whether some findings described as functional may instead represent shared clinical expressions of different underlying neurological, physiological or systemic processes. The 2020 paper does not test or establish that hypothesis. What it does establish is that the authors themselves recognise overlap between certain functional signs and recognised medical conditions. That is precisely why the distinction between describing a phenotype and identifying its cause deserves serious investigation.

The discussion of psychological explanations is equally revealing. The authors warn neurologists against assuming that patients want to be ill, are seeking secondary gain, or have symptoms caused by stress or trauma. They acknowledge that initial neurological assessments generally cannot establish the complex relationships between psychological experiences and neurological symptoms, and that such factors may be irrelevant for some patients. In other words, the clinician cannot simply discover an unusual neurological presentation, notice that the patient has experienced stress, and declare the mystery solved.

There is also a striking tension within the paper. The introduction repeats the familiar assertion that FND misdiagnosis rates are low and comparable with those of other neurological conditions. Yet the authors proceed to describe multiple ways in which diagnostic errors can occur, including nonspecific signs, limited clinical knowledge, disagreement between specialists, premature diagnostic closure and inappropriate interpretation of unfamiliar presentations. Those observations do not, by themselves, disprove published misdiagnosis estimates. They do, however, expose an important limitation: this commentary provides no evidence that the recognised safeguards are consistently applied across routine clinical practice, or that failures are reliably detected and counted.

That distinction matters. A low reported misdiagnosis rate is not proof that every clinician applies the diagnostic criteria correctly, that every patient receives adequate investigation, or that an incorrect diagnosis will eventually be discovered. A misdiagnosis that is never recognised cannot appear as a confirmed diagnostic reversal in a follow-up study. This is a methodological concern, not a claim that any particular published rate is necessarily wrong. It is also why the problems explored in Inside the Reasoning of an FND Professional cannot be dismissed by simply repeating population-level reassurance.

There is another practical problem. The authors recommend that trainees seek experienced guidance when uncertain, while also acknowledging that even well-trained neurologists can disagree. What happens to patients who cannot access a subspecialist, who are discharged without meaningful follow-up, or whose later symptoms are interpreted through an existing FND label? The paper recommends continued neurological involvement, but does not establish how consistently this happens. A diagnostic framework that depends heavily on clinical expertise needs to be judged not only by how it performs in specialist centres, but by what happens when that expertise is absent.

None of this means that every patient labelled with FND has an undiscovered neurological disease. It means that the diagnostic label cannot be treated as proof that the necessary clinical reasoning occurred. The paper's warnings exist precisely because clinicians can get this wrong. The question is not whether the literature contains instructions to avoid these mistakes. The question is whether those instructions are followed, how failures are identified, and what happens to the patients when they are not.

The uncomfortable conclusion is that the diagnostic risks are not merely allegations made by dissatisfied patients. They are acknowledged in the professional literature itself. Weak signs can mislead. Incongruence can be misjudged. Neurologists can disagree. Unfamiliar disease can be mistaken for a functional presentation. Investigations can stop too soon. These are substantial weaknesses in the diagnostic process, and patients bear the consequences when they are ignored.

A neurologist not recognising a disease does not make that disease functional. It makes the disease unrecognised. That should never have needed explaining.

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