Approximately 10% of patients diagnosed clinically with early Parkinson's disease (PD) have normal dopaminergic functional imaging (SWEDDs—scans without evidence of dopaminergic deficit). A subgroup of SWEDDs are those with asymmetric tremor resembling parkinsonian tremor. Clinical and pathophysiological features which could help distinguish SWEDDs from PD have not been explored. We therefore studied clinical details in 25 tremulous SWEDDs patients in comparison to 25 tremor-dominant.
Electrophysiological tremor parameters and response to a cortical plasticity protocol using paired associative stimulation (PAS) was studied in nine patients with SWEDDs, nine with PD, eight with segmental dystonia and eight with essential tremor (ET). Despite clinical overlap, lack of true bradykinesia, presence of dystonia, and head tremor favoured a diagnosis of SWEDDs, whereas re-emergent tremor, true fatiguing or decrement, good response to dopaminergic drugs and presence of nonmotor symptoms favoured PD. The combination of re-emergent tremor and highest tremor amplitude at rest was characteristic of PD tremor. SWEDDs and segmental dystonia patients exhibited an 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 distinguish between PD and SWEDDs. The underlying pathophysiology of SWEDDs differs from PD but has similarities with primary dystonia.