Contributions of PET and SPECT to the understanding of the pathophysiologyof Parkinson's disease

Positron emission tomography (PET) and single photon emission computed tomo graphy (SPECT) provide the means to studying in vivo the neurochemical, hem odynamic or metabolic consequences of the degeneration of the nigrostriatal dopaminergic system in Parkinson's disease (PD). The extent of striatal do paminergic denervation. can be quantified with radiotracers as [F-18]FDopa for PET and [I-123]tropanes for SPECT. There are other radiotracers such as [C-11]Dopa and meta-tyrosines as well as PET tracers for uptake sites, Str iatal uptake of [18F]FDopa and [I-123]tropanes is markedly decreased in PD, more in the putamen than in the caudate nucleus, and inversely correlates with the severity of motor signs and with duration of disease. PET and SPEC T make possible the assessment by noninvasive means of the changes in dopam ine receptor density, the effect of neuronal transplants or neuroprotective treatments in PD patients, or the nigrostriatal dopaminergic function in a t-risk subjects. Activation studies using cerebral blood flow and metabolis m measurements during a motor task reveal an impaired ability to activate t he supplementary motor area and dorsolateral prefrontal cortex in PD. This functional disability is reversed by the use of dopaminergic medication or by surgical treatment by pallidotomy or deep brain stimulation. The differe ntial diagnosis between PD and multiple system atrophy, progressive supranu clear palsy or corticobasal degeneration is not yet clearly established by PET and SPECT, even though these syndromes have some particular neurochemic al and metabolic profiles. On the other hand, PET and SPECT are useful for distinguishing PD from Dopa-responsive dystonia, or for assessing the integ rity of the nigrostriatal dopaminergic pathway in atypical cases of postura l tremor or iatrogenic parkinsonian syndromes. (C) 2001 Editions scientifiq ues et medicales Elsevier SAS.