Objective: To investigate the MSA subtypes based on the topography of MSA lesions, employing MRI, PET with [¹⁸F]FP-CIT and [¹⁸F]FDG PET.

Background: Based on the dominant clinical features, multiple system atrophy (MSA) is classified into two subgroups: MSA-C and MSA-P. However, the major pathology of MSA appears in three regions: nigrostriatal dopamine neurons, striatal projection neurons and pontocerebellar neurons.

Method: We reviewed 203 cases of probable and possible MSA with MRI and PET studies. For [¹⁸F]FP-CIT PET, the binding ratios were obtained in the caudate and the putamen using the occipital cortex as reference. For [¹⁸F]FDG PET, regional metabolism was analyzed by single-subject SPM analysis using the whole brain as reference.

Results: The number of subjects was 85 for MSA-C(M/F=45/40, age=60.1±7.8 years), and 118 for MSA-P(M/F=44/74, age=62.3±9.4). In the MSA-C group, (i) 48 patients showed OPCA with normal [¹⁸F]FP-CIT binding and glucose metabolism in the striatum; (ii) 24 showed OPCA, subnormal [¹⁸F]FP-CIT binding, and normal glucose metabolism in the striatum; (iii) 13 patients showed OPCA, subnormal [¹⁸F]FP-CIT binding and significant hypometabolism in the striatum. In the MSA-P group, 58 showed parkinsonism without cerebellar ataxia (Group P) while 60 showed both parkinsonism and cerebellar ataxia (Group Pc). In Group P, 4 showed no OPCA, subnormal [¹⁸F]FP-CIT binding and normal metabolism in the striatum, while 54 showed no OPCA, subnormal [¹⁸F]FP-CIT binding and significant hypometabolism in the striatum. In Group Pc, 18 showed OPCA, subnormal [¹⁸F]FP-CIT binding, and normal metabolism in the striatum, whereas 42 showed OPCA, subnormal [¹⁸F]FP-CIT binding and significant hypometabolism in the striatum.
Striatal [¹⁸F]FP-CIT binding did not show significant correlations with cerebellar glucose metabolism. Comparison of striatal [¹⁸F]FP-CIT binding with striatal metabolism revealed two distinct subgroups: one with marked loss of [¹⁸F]FP-CIT binding and normal or slight hypometabolism in the striatum, and the other with subnormal [¹⁸F]FP-CIT binding and proportional hypometabolism in the striatum.

Conclusion: Based on the regional predilection, we propose three topographical subtypes in MSA: the first with dominant OPCA; the second with dominant nigral dopamine neuronal loss, sparing striatal neurons, with or without OPCA; and the third with a proportional loss of nigral and striatal neurons, with or without OPCA.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/topographical-subtypes-of-multiple-system-atrophy-mri-and-pet-studies/