Objective: To highlight the diagnostic complexities of functional movement disorders (FMD) in Parkinson’s disease (PD) and the role of neurophysiological studies in differentiating functional overlay from neurodegenerative movement disorders.

Background: FMD, a subset of functional neurological disorders (FND), present diagnostic challenges, especially when co-existing with neurodegenerative conditions like PD. Functional overlay refers to additional functional symptoms in patients with confirmed neurological disorders, complicating diagnosis and treatment. Understanding the overlap between FMD and PD is critical for improving clinical outcomes.

Method: We present a case of a 62-year-old female presenting with subacute onset of bilateral lower extremity tremors at rest and standing, improving with walking. She has anxiety and multiple somatizations but no prodromal PD symptoms. Her tremor exhibited characteristics suggestive of FMD, including intermittent abduction-adduction leg tremors, with amplitude variability, suppressibility, distractibility, and entrainability. Neurophysiological studies showed tremor bursts without variability with synchronous EMG activity between the antagonist pairs, tremor suppression with contralateral hand tapping, and a high coherence between bilateral leg muscles [figure 1A-C]. A score of 3/10 was consistent with functional tremor (FT) based on laboratory-supported criteria (Schwingenschuh et al., 2011; Schwingenschuh et al., 2016). Although bradykinesia and rigidity were absent, reduced arm swing and suppression of leg tremors only with foot tapping, raised suspicion of underlying PD. An F-DOPA PET revealed decreased dopaminergic uptake in the left posterior putamen [figure 2]. A favorable levodopa response confirmed a coexisting neurodegenerative process.

Results: Diagnosing movement disorders with functional overlay is complex. Lower limb tremors can be seen in PD, dystonic, and functional tremors, but coexisting parkinsonism aids PD diagnosis. Functional neuroimaging and neurophysiological studies help differentiate between functional and neurodegenerative causes. Emerging evidence suggests that FMD may precede or co-occur with PD, highlighting the need for careful evaluation.

Conclusion: Awareness of functional overlay in neurodegenerative disorders through neurophysiological studies is essential to prevent misdiagnosis and optimize patient care.

Figure 1A. Tapping with right foot at 1Hz

Figure 1B. Tapping with right foot at 2Hz

Figure 1C. High coherence between leg muscles

Figure 2.F-DOPA PET:Reduced uptake in left putamen

Table 1. Features of FMD, overlay, and PD

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