MSA is a sporadic neurodegenerative disease characterized by a progressive akinetic-rigid syndrome, cerebellar dysfunction, and autonomic insufficiency (2). Typical histopathological changes consist of neuronal loss in the nigrostriatal and olivopontocerebellar pathways with a-synuclein positive, argyrophilic staining, glial cytoplasmic inclusions (3) associated with reactive astrocytes, and activated microglia (4).
PET and SPECT have been used to assess the following aspects of the underlying pathology: metabolic changes, dopaminergic dysfunction, opioid dysfunction, and microglial activation.
In this chapter we use the terminology recommended by the most recent consensus statement (2) designating patients as MSA-P if parkinsonian features predominate and MSA-C if cerebellar features predominate. The consensus criteria have only been stringently applied in more recent papers; previously the less well-defined terms striatonigral degeneration (SND) and sporadic olivopontocerebellar atrophy (sOPCA) have been used to describe these forms of MSA.
18FDG PET measures regional cerebral glucose metabolism (rCMRGlc), reflecting primarily the function of nerve terminal synaptic vesicles. The metabolic rate in a given region, therefore, reflects the activity of afferent projections to and interneurons in a region rather than that of its efferent projections. It is currently not possible to decide whether increases in rCMRGlc detected with PET represent excitatory or inhibitory activity (5).
A number of 18FDG PET studies have investigated the changes in rCMRGlc in MSA. In patients with the parkinsonian type of MSA, decreases of rCMRGlc in the lentiform nuclei and brainstem have been described (6-9). This contrasts with idiopathic Parkinson's disease (PD) where lentiform glucose metabolism is normal or slightly elevated (10,11). On the basis of these reports 18FDG PET seems to be able to discriminate 80-100% of MSA-P and idiopathic PD cases by comparing striatal rCMRGglc (Fig. 1).
MSA patients with cerebellar features show decreases in cerebellar rCMRGglc (8,12), in some cases even without obvious cerebellar atrophy on MRI (13). Glucose hypometabolism in frontal regions (6)—possibly owing to the degeneration of fiber systems connecting the cortex and basal ganglia—is apparent in more advanced patients but usually absent in early cases (12,14).
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