Parkinson's disease: diagnosis.

In established PD the Queen Square Brain Bank criteria applied by experts show 90% sensitivity and specificity for the presence of midbrain Lewy bodies. However, in early disease clinical diagnosis is less straightforward. PD diagnosis made in the community by non-experts is associated with a 25% error rate. Nigral abnormalities can now be detected in vivo with 7 tesla MRI and diffusion tensor MRI. Magnetisation transfer can demonstrate melanin loss in the substantia nigra. Transcranial sonography (TCS) detects midbrain hyperechogenicity in both sporadic and genetic PD. PET and SPECT ligands can demonstrate the presence of dopamine terminal dysfunction in early and preclinical disease and an abnormal covariance pattern between levels of resting brain blood flow metabolism in cortical and subcortical regions. In the atypical parkinsonian syndrome multiple system atrophy (MSA) T2-weighted MRI can reveal characteristic changes including reduced putmen signal due to iron deposition and the pontine 'hot cross bun' sign as transverse fibres become visible. Progressive supranuclear palsy (PSP) is associated with midbrain atrophy and 3(rd) ventricular widening. In both these conditions diffusion weighted MRI shows increased striatal water diffusivity but the middle cerebellar peduncle is targeted in MSA and the superior peduncle in PSP. In this review the role of structural and functional imaging for supporting the differential diagnosis of the various degenerative parkinsonian syndromes will be discussed.