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When Entrainment Misleads: Organic Tremor Mistaken for Functional Tremor

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A tremor that changes with distraction, varies in intensity and appears to follow the rhythm of another movement is often interpreted as evidence of functional tremor. These observations are used as positive diagnostic signs, supposedly allowing clinicians to identify a functional disorder rather than merely exclude other neurological conditions. But what happens when those signs are misleading?

In a 2018 case report published in Clinical Neurophysiology Practice, researchers documented a 62-year-old woman whose tremor appeared to satisfy published clinical and physiological criteria for functional tremor. Further testing revealed a persistent organic tremor, while involuntary mirror movements offered an alternative explanation for what had appeared to be entrainment. The paper, authored by S.H. Merchant, D. Haubenberger and M. Hallett, exposes a serious diagnostic problem: a clinical sign can appear functional without the underlying movement sharing that mechanism.


A Patient Who Appeared to Meet the Criteria

The woman had experienced worsening tremors affecting both arms and her voice over several years. Her medical history included childhood poliomyelitis, post-traumatic stress disorder, depression and a dissociative psychiatric disorder. She reported that her tremor fluctuated with her emotional state, particularly during episodes when she felt disconnected from herself.

Previous investigations had already identified motor abnormalities consistent with her history of polio, while brain MRI showed periventricular white matter changes. Despite this neurological history, she was referred for evaluation because a functional explanation was suspected.

During her first examination, clinicians observed features suggesting an organic movement disorder, including tremor at rest and during posture, jerky movements during coordination testing and small involuntary finger movements. During a second examination on the same day, however, the tremor appeared more variable, partially distractible and capable of following the rhythm of movements performed with her opposite hand.

Those findings supported a functional interpretation under the published diagnostic criteria. The patient also reported experiencing dissociation during the second examination, when her tremor became noticeably different and more severe. Rather than resolving the diagnostic uncertainty, the contrasting examinations prompted the researchers to investigate the movements using objective physiological recordings.


What the Recordings Revealed

The researchers used surface electromyography (EMG) and accelerometers to measure muscle activity and tremor frequencies under different conditions. They examined the tremor at rest, during sustained posture, with added weights and while the patient performed rhythmic tapping movements with her opposite hand.

The recordings identified a persistent tremor component in both hands, approximately 4–5 Hz at rest. Crucially, when the patient tapped with her opposite hand at 2 Hz or 3.5 Hz, the original tremor component continued at its own frequency. Additional activity appeared at the tapping frequency, but the underlying tremor had not actually adopted that rhythm.

This distinction changed the interpretation. The apparent entrainment could be explained by mirror movements, involuntary movements occurring in one limb alongside voluntary movements of the other. The researchers also considered motor overflow, in which activity associated with an intended movement spreads to muscles not directly involved in that movement.

In other words, the examination appeared to show the tremor synchronising with voluntary tapping, but the recordings demonstrated that two movement components were occurring simultaneously. The original tremor persisted independently.

The researchers could not establish the precise cause of the organic tremor. Possible contributors included essential tremor, neurological consequences of polio and medication effects. Nevertheless, the objective recordings demonstrated a persistent organic tremor component that the apparent functional signs had failed to explain.


The Problem With Entrainment

Entrainment is widely used in the assessment of functional tremor. Clinicians ask a patient to perform rhythmic movements with an unaffected limb while observing whether the tremor changes frequency to match. When this happens, it is treated as evidence supporting a functional diagnosis.

This case demonstrates why that interpretation requires caution. The researchers found that mirror movements could imitate entrainment, creating the appearance of a positive functional sign without the underlying organic tremor actually changing its frequency.

Even more importantly, the authors acknowledged that the patient appeared to satisfy the published clinical criteria and physiological criteria for functional tremor. Yet detailed neurophysiological analysis identified an organic tremor that persisted throughout the testing.

A positive diagnostic sign is only as reliable as its ability to distinguish the proposed disorder from alternative explanations. If another neurological mechanism can produce the same observation, the observation alone cannot establish which mechanism is responsible.

The paper also raises concerns about the influence of psychiatric history on neurological diagnosis. The authors explicitly warned that psychiatric comorbidities can introduce diagnostic bias. In this case, the patient's psychiatric diagnoses and symptom fluctuations could easily have reinforced a functional interpretation, despite the presence of independently measurable neurological abnormalities.

The study does not establish that all components of the patient's presentation were organic, and the authors did not rule out coexisting functional movements. It is also a single case report, meaning it cannot establish how frequently this particular diagnostic error occurs. What it does establish is that apparent entrainment can be misleading and that objective testing can reveal an underlying organic tremor that clinical examination alone may not distinguish.

That is a significant limitation for any diagnostic approach that treats such observations as sufficient evidence of a functional mechanism. The issue is not whether a clinician can observe entrainment, but whether the observed movement genuinely represents entrainment of the underlying tremor rather than a separate physiological process.

The researchers recommended more careful physiological assessment, including lower-amplitude tapping to reduce motor overflow and EMG coherence analysis to help distinguish movement components. Their findings also suggest that potentially treatable organic tremors may be overlooked in patients already considered to have functional tremor.

There is an additional irony: one of the authors was Mark Hallett, a prominent researcher in functional movement disorders. The warning about misinterpreting entrainment therefore comes from researchers working within the very field that promotes its diagnostic use.


Conclusion

This case illustrates how easily a diagnostic observation can be mistaken for an explanation. The patient's tremor was variable, partially distractible and apparently entrainable. Her psychiatric history made a functional interpretation seem plausible. Yet objective recordings identified an organic tremor that continued independently of the movements supposedly demonstrating entrainment.

The lesson is straightforward: observing a positive sign does not automatically establish the mechanism producing it. When alternative neurological processes can imitate a sign used to diagnose functional tremor, those alternatives deserve investigation rather than dismissal. Otherwise, the diagnostic label risks becoming a substitute for understanding what is actually happening.


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