The clinical benefit of imaging striatal dopamine transporters with [I-123]FP-CIT SPET in differentiating patients with presynaptic parkinsonism fromthose with other forms of parkinsonism

[I-123]FP-CIT (N-omega -fluoropropyl-2 beta -carbomethoxy-3 beta-{4-iodophe nyl}nortropane) has been developed successfully as a radioligand for single -photon emission tomography (SPET) imaging of dopamine transporters, which are situated in the membrane of dopaminergic neurons. Imaging of these tran sporters has shown promise as a clinical tool to detect degeneration of the dopaminergic nigrostriatal pathway. Several "presynaptic parkinsonian" syn dromes, such as Parkinson's disease or multiple system atrophy, are charact erised by degeneration of the nigrostriatal pathway. [I-123]FP-CIT SPET ima ging studies have shown the ability to detect loss of striatal dopamine tra nsporters in such syndromes. However, in clinical practice it is sometimes difficult, but important, to discriminate patients with "presynaptic parkin sonism" from those with other forms of parkinsonism not characterised by lo ss of presynaptic dopaminergic cells (e.g. psychogenic parkinsonism or drug -induced postsynaptic parkinsonism). In these inconclusive cases, it may be of value to confirm or exclude the existence of degeneration of nigrostria tal dopaminergic cells by using imaging techniques such as [I-123]FP-CIT SP ET. Using [I-123]FP-CIT SPET, we have imaged the striatal dopamine transpor ters in a group of patients with inconclusive forms of parkinsonism, and, m oreover, have been able to perform clinical follow-up of these patients 2-4 years after imaging. In 33 inconclusive cases, ratios of specific to non-s pecific binding were calculated for the caudate nucleus and putamen followi ng [I-123]FP-CIT SPET imaging and compared with ratios obtained in healthy controls. In nine of the patients, degeneration of the nigrostriatal pathwa y was found scintigraphically and in all these cases, presynaptic parkinson ism was confirmed by clinical follow-up, In the other 24 subjects no degene ration was found scintigraphically, Forms of parkinsonism other than the pr esynaptic were confirmed at follow-up in 19 cases, and in three cases no co nclusive diagnosis was established, but presynaptic parkinsonism was exclud ed clinically. A clinical diagnosis of presynaptic parkinsonism was establi shed in two cases: one case of multiple system atrophy tin this patient los s of dopamine D-2 receptors was found with [I-123]iodobenzamide SPET perfor med 2 weeks after [I-123]FP-CIT imaging) and one case of Parkinson's diseas e. Our data suggest that the positive predictive value of [I-123]FP-CIT ima ging is very high, and although the negative predictive value is lower, dop amine transporter imaging offers the prospect of a quick, objective method to confirm or exclude presynaptic parkinsonism in inconclusive cases.