When Suppressible Movements Were Called Functional: A Genetic PKD Case

A 2024 paper describes a 14-year-old boy with involuntary movements who was initially diagnosed with Functional Neurological Symptom Disorder (FNSD), specifically a functional movement disorder. Further investigation eventually identified a rare monogenic movement disorder: paroxysmal kinesigenic dyskinesia (PKD) associated with a pathogenic variant in the PRRT2 gene.

What makes this case particularly useful is that the criticism of the original functional diagnosis does not have to be imported from outside the paper. The authors themselves describe the case in the context of FNSD misdiagnosis and identify a specific problem with the earlier reasoning: psychiatric comorbidity and the apparent ability to suppress abnormal movements had contributed to the functional interpretation, while the specific clinical features required to positively support FNSD were absent.

A functional movement disorder diagnosis

The patient presented with involuntary movements and was initially diagnosed with a functional movement disorder. According to the authors, aspects of the presentation that contributed to this interpretation included psychiatric comorbidity and suppressibility of the movements. These features may appear suggestive of a functional presentation, but the later assessment challenged whether they were sufficient to establish one.

This distinction is important. A feature can be compatible with FND without being specific enough to demonstrate FND. The authors explicitly warn against diagnosing FNSD solely because psychiatric symptoms are present or because abnormal movements can be suppressed. In this patient, they state that specific features indicative of FNSD were absent.

The diagnosis changes

Further evaluation led to a very different explanation. The patient was diagnosed with paroxysmal kinesigenic dyskinesia, a neurological movement disorder characterised by brief attacks of abnormal involuntary movement, commonly precipitated by sudden voluntary movement. Genetic investigation identified a pathogenic variant involving PRRT2, providing a biological explanation for the movement disorder.

The paper therefore does not present this simply as a case in which another possible diagnosis was added to the differential. The authors explicitly describe the boy as initially diagnosed with FNSD and subsequently found to have a rare primary monogenic movement disorder. The article itself is framed around the problem of misdiagnosis.

Suppressibility was not enough

One of the most interesting aspects of the case is suppressibility. Voluntary suppression of an abnormal movement can intuitively look like evidence that the movement is under voluntary control, and therefore potentially functional. But neurological movement disorders can also show varying degrees of suppressibility. The presence of suppressibility therefore does not automatically establish the mechanism responsible for the movement.

In this case, that distinction became clinically important. A feature that had helped support the functional interpretation existed alongside a genetic movement disorder capable of producing the patient's symptoms. The lesson drawn by the authors is not that suppressibility has no diagnostic value, but that it cannot substitute for the specific positive clinical features required to establish FNSD.

Psychiatric comorbidity was not enough either

The same problem applies to psychiatric history. Psychological or psychiatric difficulties may coexist with neurological disease and cannot independently establish that neurological symptoms are functional. The authors specifically identify reliance on psychiatric comorbidity as one of the problems illustrated by this case.

This matters because psychiatric information can easily become diagnostically asymmetric. Once a patient has anxiety, depression or another psychiatric diagnosis, unusual neurological symptoms may appear more compatible with a functional explanation. But the presence of psychiatric comorbidity does not provide evidence against a genetic, structural, metabolic or other neurological disorder. It is background information, not a neurological rule-in sign.

Where were the positive signs?

This is where the case becomes particularly relevant to modern claims that FND is a positive diagnosis rather than a diagnosis of exclusion. The authors explicitly endorse that principle. They argue that FNSD should be diagnosed by recognising specific clinical features that positively support the diagnosis rather than merely failing to identify another disease.

But they then apply that standard to their own case and conclude that the relevant specific FNSD features were absent. That is important. The problem was not simply that clinicians happened to miss a rare genetic disease. According to the authors' own analysis, the evidence used to support the functional diagnosis was insufficient in the first place.

The distinction is subtle but fundamental. A functional diagnosis that is genuinely based on positive evidence should stand or fall according to that evidence. Psychiatric comorbidity, unusual movements and suppressibility may influence clinical suspicion, but suspicion is not the same thing as demonstration.

The value of follow-up

The authors also emphasise extended clinical follow-up and clinically directed genetic testing. A neurological presentation can evolve, and features that are ambiguous during an initial consultation may become considerably more informative over time. In this case, continued investigation eventually produced evidence for a monogenic movement disorder that changed the diagnosis.

That has an important implication for diagnostic closure. An FND diagnosis should not transform subsequent abnormalities into irrelevant findings simply because the patient already has an explanatory label. If new features emerge, the original diagnosis should remain open to reassessment just as any other medical diagnosis would.

Why this case matters

This case is useful precisely because it does not require an argument that every functional diagnosis conceals an undiagnosed neurological disease. The authors make a much narrower and more defensible point: this particular child was diagnosed with FNSD without the specific positive features needed to establish it, while psychiatric comorbidity and suppressibility were given diagnostic weight. Further investigation subsequently identified a genetic movement disorder.

It is therefore a practical demonstration of the difference between something that looks compatible with FND and evidence that actually rules FND in. Those are not the same evidential standard.

The authors ultimately make the point remarkably clearly: complex cases cannot simply be classified as functional because alternative explanations have not yet been found. If FND is genuinely a diagnosis of inclusion, then the positive evidence supporting it matters most precisely in cases where the presentation is unusual and the differential diagnosis is difficult.

Source

Misdiagnosis of functional neurological symptom disorders in paediatrics: Narrative review and relevant case report, Baglioni V, Esposito D, Bernardi K, Novelli M, Zaccaria V, Galosi S, Pisani F. Clinical Child Psychology and Psychiatry. 2024;29(3):1026–1042. DOI: 10.1177/13591045241240805.