Parkinsonian syndromes

Parkinsonian syndromes include idiopathic Parkinson's disease and the atypical parkinsonisms multiple system atrophy and progressive supranuclear palsy. All share loss of dopaminergic nerve terminals in the striatum, which produces the clinical triad of tremor, rigidity and bradykinesia. A common diagnostic problem is distinguishing these conditions, in their early stages, from essential tremor and other tremor syndromes that do not involve dopaminergic degeneration and are managed quite differently.

Dopamine transporter (DAT) SPECT imaging with ioflupane I-123 visualises the density of presynaptic dopamine transporters in the striatum. A normal scan argues strongly against a parkinsonian syndrome and points to essential tremor or a non-degenerative cause; an abnormal scan (reduced or absent striatal uptake) supports a diagnosis of a parkinsonian syndrome but, as the drug label makes explicit, cannot distinguish Parkinson's disease from multiple system atrophy or progressive supranuclear palsy, which all produce similar transporter loss. The same tracer is also used to separate dementia with Lewy bodies, which has striatal dopaminergic loss, from Alzheimer's disease, which does not.

DAT SPECT does not have an equivalent role to amyloid PET in gatekeeping a disease-modifying drug: there is no approved therapy that requires a positive DAT scan before treatment. Its clinical value is diagnostic clarification early in the disease course, when clinical signs are ambiguous and management (dopaminergic therapy vs none) hinges on getting the category right.