PET and SPECT functional imaging studies in Parkinsonian syndromes: from the lesion to its consequences.

Functional imaging techniques provide major insights into understanding the pathophysiology, progression, complications, and differential diagnosis of Parkinson's disease (PD). The dopaminergic system has been particularly studied allowing now early, presymptomatic diagnoses, which is of interest for future neuroprotective strategies. The existence of a compensatory hyperactivity of dopa-decarboxylase at disease onset has been recently demonstrated in the nigrostriatal and also extrastriatal dopaminergic pathways. Modification of dopamine receptors expression is observed during PD, but the respective contribution of dopaminergic drugs and the disease process towards these changes is still debated. Abnormalities of cerebral activation are seen and are clearly task-dependent, but the coexistence of hypoactivation in some areas and hyperactivation in others is also now well established. Such hyperactivation may be compensatory but could also reflect an inability to select appropriate motor circuits and inhibit inappropriate ones by PD patients. Interestingly, dopaminergic medications or surgical therapy reverse such abnormalities of brain activation.