Approximately 10% of patients diagnosed clinically with early
Parkinson's disease (PD) have normal dopaminergic functional imaging (Scans Without Evidence of Dopaminergic Deficit [SWEDDs]). An important subgroup of SWEDDs are those with asymmetric
rest tremor resembling parkinsonian
tremor. Clinical and pathophysiological features which could help to distinguish SWEDDs from PD have not been explored. We therefore studied clinical details including non-motor symptoms in 25 tremulous SWEDDs patients in comparison to 25
tremor-dominant PD patients. Blinded video rating was used to compare examination findings. Electrophysiological
tremor parameters and also response to a cortical plasticity protocol using paired associative stimulation (PAS) was studied in 9 patients with SWEDDs, 9 with
tremor-dominant PD (with abnormal
dopamine transporter single photon emission computed tomography findings), 8 with segmental
dystonia, and 8 with
essential tremor (ET). Despite clinical overlap, lack of true
bradykinesia, presence of
dystonia, and head
tremor favored a diagnosis of SWEDDs, whereas re-emergent
tremor, true fatiguing or decrement, good response to
dopaminergic drugs, and presence of non-motor symptoms favored PD. A single
tremor parameter could not differentiate between groups, but the combination of re-emergent
tremor and highest
tremor amplitude at rest was characteristic of PD
tremor. SWEDDs and segmental
dystonia patients exhibited an abnormal exaggerated response to the PAS protocol, in contrast to a subnormal response in PD and a normal response in ET. We conclude that despite clinical overlap, there are features that can help to distinguish between PD and SWEDDs which may be useful in clinical practice. The underlying pathophysiology of SWEDDs differs from PD but has similarities with
primary dystonia.