FND Diagnostic Pitfalls: When Positive Signs Point Elsewhere
FND Nope
Moderator
In a 2023 review published in Neurologic Clinics, Sara Finkelstein and Stoyan Popkirov examine the diagnostic pitfalls of Functional Neurological Disorder (FND). Their paper, Functional Neurological Disorder: Diagnostic Pitfalls and Differential Diagnostic Considerations, documents something that deserves considerably more attention: several clinical findings used to diagnose FND also occur in recognised neurological diseases.
This is not criticism from outside the field. The authors themselves identify these overlaps and caution against excessive diagnostic confidence.
Positive signs are not necessarily specific
Hoover's sign is frequently presented as positive evidence of functional leg weakness. Yet the authors identify supplementary motor area ischaemia as a potential cause of a false-positive result. Parietal lesions can produce motor performance that worsens with attention and improves with distraction, while small thalamic infarctions can produce give-way weakness and unusual arm drift.
These findings matter because they challenge the assumption that inconsistency or improvement with distraction necessarily establishes a functional mechanism. The same observation can have more than one neurological explanation.
The problem extends to sensory examination. Midline splitting, vibration splitting and unusual sensory distributions have traditionally been associated with functional presentations. However, the authors acknowledge that some of these signs have poor discriminatory ability, while small cortical strokes can produce sensory abnormalities that appear non-anatomical.
Finding a positive sign is one thing. Demonstrating that it reliably distinguishes FND from relevant neurological diseases is another.
Movement disorders can look functional
The review identifies numerous examples of overlapping movement characteristics. Variable tremor amplitude occurs in essential and parkinsonian tremor. Irregular tremor occurs in dystonia. Walking backwards can improve dystonic gait, while frontal ataxia can produce inconsistent gait performance. Cortical myoclonus and startle syndromes can produce stimulus-triggered movements, and tic disorders can involve suppressibility and suggestibility.
These are not minor exceptions to an otherwise straightforward distinction. They demonstrate why describing a movement as variable, distractible or inconsistent does not, by itself, establish its cause.
A neurological finding is not functional simply because it fails to behave as expected for a particular disease. The relevant question is whether another disease can explain it.
When suspected functional seizures are something else
The authors cite an earlier investigation involving 64 patients suspected of having functional seizures who underwent conclusive video-EEG assessment. Fourteen received alternative explanations, including temporal lobe epilepsy, mesial-frontal epilepsy, mesial-parietal epilepsy, convulsive syncope and paroxysmal dyskinesia.
That is approximately 22% of this selected group. It is not a general FND misdiagnosis rate, since these were suspected cases undergoing diagnostic assessment. Nevertheless, it illustrates how convincingly other neurological conditions can resemble functional seizures.
The review also acknowledges that certain epileptic seizures arising from deep brain regions may produce little or no detectable activity on scalp EEG. An apparently normal EEG therefore cannot automatically settle the question.
Comorbidity does not settle a diagnostic dispute
The authors discuss the coexistence of functional presentations and recognised neurological diseases, including Parkinson's disease. But coexistence raises a question that deserves more scrutiny.
When someone initially diagnosed with FND subsequently receives another neurological diagnosis, was the original diagnosis correct, was it mistaken, or were both conditions genuinely present?
Simply retaining the FND diagnosis alongside the newly discovered disease does not answer that question. Comorbidity must be established, not presumed. Otherwise, an incorrect diagnosis can survive indefinitely because every subsequent finding is accommodated under another label.
This has obvious implications for research reporting low FND misdiagnosis rates. If subsequent neurological diagnoses are routinely classified as comorbidities without reassessing the original presentation, some diagnostic errors could go unrecognised. The review does not establish that this occurs systematically, but it exposes why the distinction matters.
How confident should the diagnosis be?
Finkelstein and Popkirov recommend considering categories such as possible or probable FND when robust positive findings are absent or the diagnosis rests on a single sign. They also caution against using psychological history, demographic characteristics or other assumptions as substitutes for neurological evidence.
There is another problem worth examining. If individual signs can produce false positives, does combining several signs necessarily establish greater diagnostic certainty? Not without evidence that the combination reliably distinguishes FND from its neurological mimics. Multiple observations do not automatically become independent confirmation of the same explanation.
The question the paper leaves unanswered
The authors document recognised neurological explanations for findings commonly associated with FND. What remains less clear is how clinicians should respond when an established FND diagnosis encounters evidence pointing elsewhere.
What would actually falsify the original diagnosis? When should it be withdrawn rather than preserved as a comorbidity? And how often are supposedly characteristic signs tested against the diseases most likely to reproduce them?
These questions matter because diagnostic confidence is not the same as diagnostic accuracy. A clinician can correctly observe Hoover's sign, distractibility or an unusual sensory pattern while incorrectly identifying its cause.
The review does not demonstrate that FND is invalid. It does, however, document limitations that should prevent positive signs from being treated as infallible. When the same finding occurs in recognised neurological disease, the differential diagnosis remains important, regardless of how confidently the original label was applied.
References
- Finkelstein SA, Popkirov S. Functional Neurological Disorder: Diagnostic Pitfalls and Differential Diagnostic Considerations. Neurologic Clinics. 2023;41(4):665–679.
- Diagnostic accuracy of clinical signs and investigations for functional weakness, sensory and movement disorders: A systematic review. 2023.
- Diagnostic pitfalls in functional neurological disorders. 2022.
