FND, Then Genetic Dystonia: A Pediatric DYT-TOR1A Case

A 2025 case report describes a pediatric patient initially diagnosed with Functional Neurological Disorder (FND) who was later found to have DYT-TOR1A dystonia, a genetic neurological movement disorder. The authors ultimately retained FND alongside the genetic diagnosis, presenting the case as an example of functional and organic dystonia occurring in the same patient.

That conclusion makes the case particularly interesting. Once a neurological disorder capable of producing dystonia was identified, the diagnostic problem changed. It was no longer simply a question of whether the patient's movements looked functional. There was now a demonstrated biological disorder capable of producing abnormal movements, raising the question of how individual symptoms were divided between DYT-TOR1A dystonia and FND.

FND came first

The patient initially presented with abnormal movements that were interpreted as Functional Neurological Disorder. Dystonia can produce involuntary muscle contractions, abnormal postures and repetitive or twisting movements, and functional dystonia can resemble other forms of dystonia closely enough to create considerable diagnostic difficulty.

The authors discuss features that can influence this distinction, including onset, pain, additional neurological symptoms, examination findings, changes over time and response to treatment. They also note that both functional and organic dystonia can worsen with stress. Stress responsiveness therefore offered no simple way of deciding between the two explanations.

A genetic diagnosis emerges

Years after the original FND diagnosis, further investigation identified DYT-TOR1A dystonia. DYT-TOR1A, historically called DYT1 dystonia, is associated with pathogenic variants in the TOR1A gene and commonly begins during childhood or adolescence.

This was not simply another possible differential diagnosis. A genetic abnormality associated with an established neurological movement disorder had now been identified in a patient already experiencing dystonia. The diagnostic landscape had materially changed. The authors nevertheless retained FND as a coexisting diagnosis, leaving an important evidential question: once DYT-TOR1A was known to be present, what findings established that particular remaining movements were functional rather than manifestations of the genetic dystonia?

The attribution problem

Saying that two disorders coexist is one thing. Determining which disorder explains a particular symptom is considerably harder. If FND and DYT-TOR1A dystonia can both produce abnormal movements, assigning some movements to one diagnosis and others to the second requires evidence capable of making that distinction.

This is where comorbidity can create an epistemic problem. The discovery of another neurological disease does not necessarily cause an existing FND diagnosis to disappear. Instead, symptoms can potentially be redistributed between the two diagnoses: the organic disease explains some findings while FND continues to explain others. But that redistribution is itself a diagnostic claim. “Both conditions are present” does not tell us which condition produced a particular movement, nor does it establish that symptoms remaining outside the preferred organic explanation must therefore be functional. The individual attribution still requires justification.

Deep brain stimulation and an important unanswered question

The identification of DYT-TOR1A had practical consequences. The patient was ultimately treated with deep brain stimulation (DBS), an established treatment for severe forms of genetic dystonia, and the authors report a very good response. Treatment response should not be treated as a perfect retrospective diagnostic test, but the sequence is nevertheless important: a neurological disorder capable of producing dystonia was genetically identified, treatment directed at that disorder was undertaken, and substantial overall clinical improvement followed.

That raises another important question: what happened specifically to the movements that had previously been attributed to FND? The reported overall response to DBS does not, by itself, tell us. If movements previously classified as functional persisted while the organic dystonia improved, that would support the distinction the authors were making. If those movements also substantially improved following treatment directed at DYT-TOR1A dystonia, that would be highly relevant to whether their original attribution was correct.

The distinction matters because an overall statement that the patient responded very well to DBS cannot tell us which diagnostic category accounted for the improvement. What would be particularly informative is symptom-level follow-up showing which movements improved, which remained, and how those outcomes compared with the movements previously classified as functional or organic.

What happens to the original diagnosis?

This case illustrates a recurring problem when FND is followed by the discovery of neurological disease. The later diagnosis does not necessarily replace FND because FND can instead be retained as a comorbidity. That may be the correct interpretation in an individual patient, but it also creates an obvious problem of falsifiability.

If the discovery of a disease capable of causing the patient's symptoms does not falsify the functional diagnosis, then the next question should be what evidence could. Without a clear answer, almost any later neurological diagnosis can potentially be accommodated by saying that the patient has both conditions. The possibility of coexistence should therefore not be confused with evidence that coexistence occurred in a particular patient. That remains a diagnostic claim requiring its own support.

Why this case matters

The authors present this patient as having both Functional Neurological Disorder and DYT-TOR1A dystonia. Their conclusion should be reported accurately, but it can still be examined in terms of the evidence used to distinguish one source of abnormal movement from the other.

The important development is that a child whose abnormal movements had already been interpreted through an FND framework was eventually found to have a genetic neurological disorder capable of producing dystonia. That discovery changed the clinical picture sufficiently for the patient ultimately to receive deep brain stimulation, followed by a very good reported response.

The useful question is therefore more specific: once DYT-TOR1A dystonia was demonstrated, what evidence allowed clinicians to determine which movements remained functional? The reported response to DBS adds another layer to that question because knowing precisely which movements improved would provide valuable evidence about their original attribution.

A second diagnosis does not automatically erase the first. But neither should the word “comorbidity” prevent the first diagnosis from being re-examined when new biological evidence and subsequent treatment response change the evidential landscape.

Source

Pediatric DYT-TOR1A dystonia presenting as functional neurological disorder: A case report, Fischer PR, Youssef PE, Brandenburg JE, Ali F, Wilson JL, Medical Reports, Volume 13, 2025, article 100314.