Introduction

REM-sleep-behavior-disorder (RBD) is a sleep disorder characterized by the absence of muscle-atonia during REM sleep [1]. RBD represents an early (prodromal) clinical manifestation of evolving motor neurodegenerative synucleinopathies. RBD patients have up to an 80% risk of developing Parkinson’s disease (PD) after 10 years from RBD-diagnosis [2]. It is know from lesion and connectivity studies in animals [1] that RBD displays a network dysfunction of brainstem nuclei (Bn) involved in arousal and motor function (e.g. the locus-coeruleus area, reticular-formation, raphe-nuclei, pedunculotegmental nuclei, and substantia nigra SN). The hypothesis that Bn are involved in RBD is also supported by an ex-vivo human clinical-pathological staging model of PD progression (the so called Braak hypothesis [3]), which predicts that neuro-degeneration of Bn of the arousal and motor systems occurs much earlier (mainly pre-symptomatic stages 1-3) than in the striatum and in the neocortex (symptomatic stages 4-6). A few human MRI studies of non-idiopathic RBD [4-8] and one recent study of idiopathic RBD [9] report the presence of structural changes in pontine/midbrain areas compatible with animal RBD studies and with the Braak hypothesis. Yet, a precise identification of the Bn involved and of their connections in-vivo is still missing, thus limiting our understanding of idiopatic RBD/prodromal PD.

Purpose

To investigate the presence of microstructural brainstem changes in RBD by the use of: (i) high resolution (.75 mm isotropic) multi-contrast (T₁ and T₂*-weighted) MRI at 7 Tesla; (ii) our recently developed [10] stereotaxic probabilistic structural atlas in MNI space of several Bn of the arousal and motor systems (including: two subregions of the SN, i.e. subregion 1 and 2, compatible with SN pars reticulata –SNPR– and compacta –SNPC, respectively; two subregions of the red nucleus and of the subthalamic nucleus; median-raphe; dorsal-raphe; raphe-magnus, periaqueductal gray, and inferior olivary nucleus).

Methods

Data acquisition: Six patients with idiopathic-RBD (5m/1f, age 70 ± 1) underwent 7 Tesla MRI under IRB-approval. M2PRAGE image: echo-time (TE) = 2.39 ms, repetition-time (TR) = 5000 ms, inversion-times = [900 3200] ms, flip angles (FA) = [4° 5°], FOV = 156 ×158×180 mm³, 208×210×240 matrix, bandwidth = 330 Hz/pixel, GRAPPA-factor = 3, acquisition time: 9’32”. Multi-echo GRE image: TEs = [6.4 9.8 13.1 16.4 19.7 23.0] ms, TR = 3660 ms, FA = 55°, FOV = 82 ×158×180 mm³, 110×210×240 matrix, bandwidth = 330 Hz/pixel, GRAPPA-factor = 2, acquisition time: 9’38”. Data analysis: T₁ maps were computed from the MP2RAGE. Regions of interest (ROI) displaying T₁ hyperintensity (i.e. microstructural changes) within a brainstem mask were automatically detected by thresholding T₁ maps at 2500 ms (threshold determined considering that intact gray-matter at 7 Tesla has a T₁ value of ~1700-1900 ms [11]). To better evaluate the possible source of contrast in these ROIs, multi-echo GRE-signals were aligned to T₁ maps by affine coregistration [12] and T₂*-weighted signals changes in these ROIs inspected. To determine the location of these ROIs with respect to Bn of the arousal and motor system, we precisely coregistered T₁ maps to the Bn atlas [10] (i.e. MNI) space through high-dimensional non-linear transformations [12]; an average map of these ROIs in MNI space across subjects was computed and overlaid to the Bn atlas.

Results

Microstructural changes (i.e. ROIs with T₁ hyperintensity) in the brainstem were detected in 5 out of 6 RBD patients (Figure 1). The volume of the ROIs with T₁ hyperintensity ranged between .4 mm³ and 1.2 mm³. T₁ hyperintensities colocalized with T₂*-weighted signal increases (Figure 1). The average map of these ROIs in MNI space across subject is shown in a midbrain slice in Figure 2, superimposed on the atlas label of the SN-subregion1 [10].

Discussion

Our findings demonstrated –consistently across RBD patients- the presence of microstructural changes in the right ventro-caudal part of the right SN-subregion1 (compatible with SNPR) and in a region rostro-medial to the caudal part of SN-subregion1. These results provide compelling empirical evidence for the hypothesized role of specific Bn nuclei in the pathogenesis of RBD (1,3). Interestingly, the observed contrast (hyperintensities in T₁ and T₂*-weighted MRI) in the detected ROIs is compatible with the presence of small lacunar infarcts (e.g. parenchymal spaces filled with CSF/interstitial-fluid) in the nigral and peri-nigral region of RBD patients. This original result differs from recent findings of iron accumulation in the SN of RBD patients [9].

Conclusion

We foresee that ultra-high-field multi-contrast-MRI and our novel methods to localize Bn involved in arousal and motor functions [10] might bring new insight in the understanding of RBD mechanisms, by allowing a precise quantification of Bn microstructural damage and network disruption.

Acknowledgements

NIH NIBIB K01EB019474; NIH NIBIB P41-RR014075.

References

1. Boeve BF, Silber MH, Saper CB, et al. Pathophysiology of REM sleep behavior disorder and relevance to neurodegenerative disease. Brain. 2007;130:2770-2788. 2. Postuma RB. Prodromal Parkinson’s disease – using REM sleep behavior disorder as a window. Parkinsonism Relat Disord. 2014;Suppl1:S1-4. 3. Braak H, Del Tredici K, Rub U, et al. Staging of brain pathology related to sporadic Parkinson’s disease. Neurobiol Aging. 2003;24:197-211. 4. Kimura K, Tachibana N, Kohyama J, et al. A discrete pontine ischemic lesion could cause REM sleep behavior disorder. Neurology. 2000;55:894-895. 5. Plazzi G, Montagna P. Remitting REM sleep behavior disorder as the initial sign of multiple sclerosis. Sleep Medicine. 2002;3:437–439. 6. Provini F, Vetrugno R, Pastorelli F, et al. Status dissociatus after surgery for tegmental ponto-mesencephalic cavernoma: a state-dependent disorder of motor control during sleep. Movement Disorders. 2004;19:719-723. 7. Tippmann-Peikert M, Boeve BF, Keegan BM. REM sleep behavior disorder initiated by acute brainstem multiple sclerosis. Neurology. 2006;66:1277-1279. 8. Zambelis T, Paparrigopoulos T, Soldatos CR. REM sleep behavior disorder associated with a neurinoma of the left pontocerebellar angle. J Neurol Neurosurg Psychiatry. 2002;72:821-822. 9. De Marzi R, Seppi K, Hogl B, et al. Loss of dorsolateral nigral hyperintensity on 3.0 Tesla susceptibility-weighted imaging in idiopathic rapid eye movement sleep behavior disorder. Ann Neurol. 2016;79:1026-1030. 10. Bianciardi M, Toschi N, Edlow BE, et al. Toward an in vivo neuroimaging template of human brainstem nuclei of the ascending arousal, autonomic, and motor systems. Brain Connect. 2015;5:597-607. 11. Rooney WD, Johnson G, Li X, et al. Magnetic field and tissue dependencies of human brain longitudinal 1H2O relaxation in vivo. Magn Reson Med. 2007;57:308–318. 12. Avants BB, Tustison NJ, Song G, et al. A reproducible evaluation of ANTs similarity metric performance in brain image registration. Neuroimage. 2011;54:2033-2044.