Diagnostic Dogma Patient Advocacy Diagnostic Overshadowing

If FND Is Not ‘We Don’t Know’... What Exactly Do We Know?

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There are a number of FND advocates whom I follow on X, Facebook and Reddit. One of the more interesting posts I saw recently concerned Kelly Shorkey mentioning FND during her testimony at the Allison inquiry. Shawn Buckley, a lawyer, responded by describing FND as essentially meaning: "we don't know."

What interested me was the reaction from an FND proponent. Rather than focusing on the possibility of diagnostic overshadowing being described in the testimony, they immediately challenged Buckley's statement as misinformation.

The response was categorical. The proponent argued that FND does not mean "we don't know", describing it as a recognised neurological disorder diagnosed using positive rule in clinical signs, and invoking NHS guidance in support of that position.

But I think this misses an important distinction. The question is not whether FND is a recognised diagnostic category. The question is what exactly has been established when someone receives that diagnosis?

This is the distinction I have discussed previously in terms of ontology and epistemology. Accepting that a clinical phenomenon exists is not the same as establishing what that phenomenon actually is, why it occurs, or whether apparently similar presentations necessarily share the same underlying process.

Ultimately, the disagreement comes down to the epistemic bar. What evidence are we prepared to accept as sufficient to move from "we recognise this pattern" to "we know what this patient has"?

Consider the reasoning:

  1. A recognisable clinical pattern has been defined as FND.
  2. A patient exhibits that recognisable clinical pattern.
  3. Therefore, the patient meets the definition of FND.
  4. Meeting the definition is treated as knowing what the patient has.
  5. Therefore, FND is not "we don't know."

There is nothing particularly controversial about the first three steps. Medicine classifies syndromes and recognisable clinical patterns all the time. The difficulty appears in step four: classification is being allowed to quietly become explanation.

We have defined a pattern, recognised the pattern in a patient, and given that pattern a name. That can be clinically useful. But naming and recognising something does not automatically tell us what produced it.

This distinction is not unique to FND. Medicine contains many syndromes and diagnostic categories whose mechanisms remain incompletely understood. Uncertainty does not automatically make a diagnostic category illegitimate. The problem arises when the certainty attached to the classification becomes greater than the evidence supporting the explanation.

Once we move beyond recognising and naming the presentation, substantial questions remain. What is the mechanism, and what caused it in this particular patient? Is FND one disorder with a common underlying mechanism, or a category containing heterogeneous processes that produce similar clinical appearances? Why do some patients develop particular symptoms and others do not, and how reliably can the observed phenotype be separated from functional symptoms occurring alongside another neurological or systemic disease? Could apparently similar clinical presentations sometimes arise through different underlying processes?

Understanding the underlying mechanisms may also, in time, fundamentally reshape how FND itself is defined, including whether it ultimately proves to be a distinct disorder at all, rather than a common clinical presentation that can arise from multiple different underlying processes. A definition built around an observed phenotype today may look very different if future evidence reveals distinct mechanisms, causes, or subgroups within what is currently grouped together as FND.

This raises a more fundamental question. Has the relatively low epistemic bar allowed a recognisable clinical presentation to be prematurely elevated into a distinct disorder? Observing and defining a recurring phenotype establishes that the pattern exists, but it does not necessarily establish that the pattern represents a single underlying disorder. Without a settled mechanism, clear causation, or confidence that apparently similar presentations share the same underlying process, the move from "recognisable presentation" to "distinct disorder" may itself be premature.

These are not trivial details surrounding an otherwise completely solved diagnosis. They concern the limits of what the diagnosis itself allows us to claim. This is why simply responding that FND is not "we don't know" risks replacing one oversimplification with another.

Buckley's statement may itself be too broad if interpreted literally. Clearly, something is being claimed: a clinical pattern has been identified, described and classified. But that is very different from saying that the underlying phenomenon is understood.

A more accurate distinction might therefore be: FND is not necessarily "we know nothing". But recognising and naming a phenotype is not equivalent to knowing its cause, mechanism, boundaries, or what is ultimately happening in a particular patient.

That distinction matters because diagnostic labels do not exist only in academic discussions. They change what happens to patients. Once a clinician believes that the explanatory question has been answered, subsequent symptoms can be interpreted through that diagnosis. New abnormalities may receive less attention, further investigation may appear unnecessary, and the diagnostic label can become the explanation against which later evidence is judged. This is where epistemology becomes a practical patient safety issue.

And this is where invoking NHS guidance to support a categorical claim of certainty becomes particularly interesting. The NHS Scotland FND pathway itself explicitly cautions against allowing the diagnosis to close further investigation.

Among its recommendations is:

Keeping a low threshold for investigations.

The reason given is important:

Comorbidity with additional neurological and other medical conditions is common.

That is a significant qualification. Whatever degree of confidence is attached to the FND classification, the guidance itself recognises that clinicians may still need to look further.

More importantly, the same NHS Scotland guidance explicitly warns about what can happen once the label is already present:

People with FND commonly suffer from 'diagnostic overshadowing,' whereby new symptoms may be attributed to FND without adequate evaluation.

That warning matters enormously to the epistemic argument. A diagnostic label can be useful while still having limits. The danger begins when those limits are forgotten and the label moves from describing part of the clinical picture to becoming the assumed explanation for everything that follows. This is precisely why the distinction between classification and knowledge matters.

There is also a broader problem in resource-constrained healthcare systems. A diagnosis that permits relatively rapid clinical classification can provide practical closure in an overloaded system. That does not require bad intentions from clinicians. Time is limited, specialist access is limited, investigations cost money, and healthcare systems inevitably have to decide when further investigation is justified.

Under those conditions, however, a subtle substitution can occur: "Enough evidence to classify" becomes "enough evidence to stop looking." Those are not the same proposition.

This problem is certainly not unique to FND. Premature diagnostic closure and diagnostic overshadowing can occur throughout medicine. But it becomes particularly important when substantial uncertainty remains about mechanism, causation, heterogeneity and the relationship between the diagnostic phenotype and underlying disease.

This is why I think proponents should be particularly careful about declaring that FND is definitively not "we don't know". It may sound like a defence of FND, but overstating what the evidence establishes ultimately weakens that defence. There is nothing scientifically embarrassing about uncertainty. There is something far more problematic about disguising uncertainty as certainty.

Treating unknowns as knowns serves nobody, particularly the patient who ultimately bears the consequences if the classification is wrong, incomplete, or only part of the story.

The useful question is therefore not simply whether FND means "we don't know". It is: What exactly do we know when we diagnose FND, what remains unknown, and where is the evidential boundary between the two?

This matters for advocacy too. Advocacy can very quickly become its own enemy when it loses nuance. In trying to defend the legitimacy of FND, there is a temptation to present uncertainty as settled knowledge and criticism as misinformation. But when the certainty being communicated goes beyond what the evidence can actually establish, the result can be perceived as dishonest, even if that was never the intention.

Good advocacy should not require certainty where certainty does not exist. It should be able to acknowledge what is known, what is hypothesised, what remains unknown, and where legitimate disagreement still exists. Missing that bigger picture does not strengthen the case for FND. It risks undermining trust in the very case advocates are trying to make.