In this longitudinal study, we observed progressive increases in PDRP and PDCP expression in iRBD, as well as concurrent declines in caudate and putamen DAT binding. While in the iRBD cohort, progression rates were similar for the two PD networks, expression values for the PDRP were greater than PDCP at each timepoint, a finding consistent with earlier cross-sectional studies6. Compared to healthy control subjects, PDRP expression was already elevated at baseline, whereas PDCP values did not reach abnormal levels until the final imaging timepoint. Likewise, the rate of decline in DAT binding was similar for the caudate and putamen, with relative preservation of nigral dopaminergic input to both structures initially, followed by modest reductions to 80–85% of the normal mean at the final timepoint. Importantly, four iRBD participants converted to PD/DLB during the subsequent 7 years of clinical follow-up. Thus, our imaging data were mainly shaped by the evolution of distinct PD network topographies during the prodromal phase of the illness. Indeed, network progression in iRBD was accompanied by the formation of abnormal functional connections linking PDRP nodes to one another and to PDCP nodes. These connectivity changes are likely maladaptive as suggested by the appearance of abnormal network configurations soon after the diagnosis of iRBD.
In PD, increases in PDRP expression have been attributed mainly to changes in cortico-striato-pallido-thalamo-cortical motor circuits taking place as a downstream consequence of nigrostriatal dopaminergic degeneration7,13,14. That said, the abnormal network elevations in iRBD may also be influenced by changes involving monoaminergic and/or cholinergic pathways in the brainstem and basal forebrain. Indeed, measurements of local metabolic activity at key network nodes (Fig. S1) suggest that while the earliest regional abnormalities were localized to basal ganglia and cerebellum, PDRP core regions in the ventral thalamus and dorsal pons reach significance only the final timepoint. While longitudinal metabolic changes in cortical PDRP regions were not identified, significant declines were observed in the inferior parietal PDCP node. It is unclear, however, whether this change was the result of loss of cholinergic afferents to the region or α-synuclein deposition, or both. In the absence of rigorous neuropsychological testing, it is also unclear how metabolic changes in this region relate to concurrent cognitive performance in individuals with iRBD.
That said, the results of the connectivity analysis generally accord with the α-synuclein propagation hypothesis15. In early iRBD, gain and loss of connections were observed in the PDRP space, involving mainly those linking the basal ganglia to the brainstem and cerebellum. We note that the initial gain of functional connections was between metabolically active core nodes, with subsequent development of abnormal connections linking core nodes to their less active counterparts in the network periphery. Indeed, a similar pattern of connectional gain was previously reported in a cross-sectional analysis of connectivity changes in PD patients10. On the other hand, loss of the normal core-to-core and core-to-periphery connections was observed both early and later on in the course of iRBD. For example, we noted early loss of normal connections linking the cerebellum and contralateral putamen in iRBD, suggesting that this pathway is likely disrupted years before the onset of parkinsonian symptoms. Other potentially relevant connections, such as those linking the thalamus to the paracentral lobule (which includes the SMA) and the amygdala to the parahippocampal gyrus, are lost later in the course of iRBD. How these changes related to the post-conversion phenotype of individuals with iRBD remains to be elucidated. When considered in aggregate, however, the findings point to dynamic network remodeling in iRBD, beginning in the PDRP core zone and spreading outward over time into the periphery.
Interestingly, during iRBD progression, the abnormal connections that were gained were not limited to those between PDRP nodes. Indeed, a significant stepwise increase in abnormal connections linking PDRP to PDCP nodes was noted over time (Fig. 2; Fig. 3A, middle). These connections were predominantly cortico-cortical, initially linking frontal lobe regions, with the subsequent appearance of abnormal connections linking the amygdala and parahippocampal gyrus to the frontal operculum. Over time, abnormal connections become evident linking frontal, temporal, and parietal regions with occipital association cortex. That said, gain and loss of functional connections linking PDCP nodes was of small magnitude compared to those between PDRP nodes or between PDRP and PDCP nodes.
We additionally found that the gain and loss of individual connections likely had a detrimental influence on the structure and function of these networks. In this regard, we found evidence of reduced small-worldness for both PDRP and PDCP, beginning at the first timepoint. This change is attributed to the abnormal reductions in connections between nearest neighbors (low clustering coefficient) that were evident in both networks, as well as increases in the average number of edges separating individual nodes from the others (high characteristic path length). These topological changes suggest an imbalance between regional specialization (segregation) and parallel processing (integration), which may reduce the efficiency of information flow through both networks. It is also noteworthy that assortativity, the link-averaged correlation coefficient of degree centrality for pairs of connected nodes in a network16–18, is abnormally increased in iRBD beginning at the second timepoint for PDRP and the third timepoint for PDCP. This metric captures changes in connectional diversity that render networks more vulnerable to random attacks, fragmentation, and critical transitions19–21. We have previously described progressive increases in PDRP assortativity in multiple groups of PD patients22. The current data point to analogous network changes in iRBD, starting years before the onset of motor symptoms. Taken together, the decline in small-worldness and subsequent increase in assortativity seen in longitudinal iRBD are compatible with progressive compromise of information flow through PD-specific networks in prodromal stages of the disease.
In addition to the changes in PD network expression, we found that DAT binding declined over time in both the caudate and putamen, at similar rates for the two regions. In that regard, FPCIT PET provides complementary information concerning the evolution of dopaminergic deficits in iRBD and the relationship of these changes to the onset of parkinsonism23. While presynaptic dopaminergic deficits, particularly those localized to the putamen, are a recognized risk factor for phenoconversion12,24, these measurements, as well as other potential predictors such as severity of motor symptoms, autonomic dysfunction, and cerebrospinal fluid analysis, do not accurately foretell subsequent transition to clinically manifest PD25,26. That said, the longitudinal PET data presented in this study suggest that changes in PDRP expression and putamen DAT binding, when evaluated together, may predict phenoconversion more accurately than either measure alone. Indeed, in our earlier longitudinal PET studies of newly diagnosed PD patients11,12, we found that these individuals typically had PDRP expression values above + 1.5 and reductions in putamen DAT binding below 40% of the normal mean value11,12,24. Given estimates of the rate of change in these measures, we used the final pre-conversion scans of each iRBD participant to calculate the time needed to reach the prespecified cut-offs for early PD. While in close agreement with observed phenoconversion times determined independently by movement disorders experts, predictions made based on the model need to be replicated in prospective studies of larger iRBD cohorts.
In this regard, we note that only four of the 13 (31%) longitudinal iRBD subjects converted to PD/DLB over a total of 9.6 years (longitudinal imaging for an average of 4.3 years and follow-up clinical observation for an additional 5.3 years). The relatively small number of phenoconversions in this longitudinal cohort can be explained by the milder initial clinical features in these subjects compared to the 12 of 17 (71%) who phenoconverted in the cross-sectional study. Indeed, on enrollment, the phenoconverters in the former group had shorter mean iRBD duration than their cross-sectional counterparts (3.6 vs. 9.6 years), lower initial motor ratings (2.0 vs. 5.4 points), and longer time from initial imaging to phenoconversion (8.3 vs. 4.8 years). It is therefore likely that the iRBD participants in the longitudinal imaging study were further from phenoconversion at the time of enrollment than their counterparts in other studies.
It is noteworthy that other investigators have identified iRBD-related spatial covariance patterns (termed iRBDRPs) in FDG PET scans from iRBD subjects27,28, or from de novo PD patients with iRBD (termed PD-RBDRP)29,30. PD-RBDRP expression measured in iRBD subjects predicted future phenoconversion to PD in iRBD, although this pattern exhibited significant topographical overlap with PDRP. More recently, a somewhat different multivariate mapping technique was applied to [99mTc]-HMPAO SPECT cerebral perfusion scans acquired in iRBD and healthy volunteer subjects31. The resulting pattern was reported to be a good predictor of future phenoconversion in individuals with iRBD, although prospective validation of the marker is needed in independent pre-conversion samples. This approach contrasted with that employed in the current study in which we used the FDG PET-based PDRP, a highly reproducible network biomarker of PD, which was additionally validated in [99mTc]-ECD SPECT perfusion scans32,33. This approach led to a significant correlation between baseline PDRP expression and the time to phenoconversion, although further validation is desirable. It is also worth noting that new methods have recently been developed to measure PD network expression in individual subject scans. For example, dynamic [18F]-FPCIT PET can be used to quantify PDRP and PDCP expression in early phase perfusion scans, while caudate and putamen DAT binding can be measured in the late phase images34. Non-invasive measurements of network expression are also possible given that elevations of the resting-state fMRI-based PDRP have been documented in both PD and iRBD 5,35,36. This approach may prove valuable as a non-invasive, widely accessible means of assessing the risk of phenoconversion in individuals with iRBD.
Given that metabolic pattern expression was found to be a sensitive measure of iRBD progression in our study, we performed a preliminary power/sample-size analysis using the PDRP expression as a progression biomarker in a hypothesized 2-year (2-timepoint), blinded disease-modification trial of individuals with iRBD randomized to active treatment versus placebo. The results show that randomizing 114–308 subjects may be sufficient to detect reductions in progression rate of 30–50% respectively with 80% power (two-sample Student’s t-test; G*Power 3.1). This suggests that PDRP measurements can enhance statistical power in such trials.