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Phenoconversion from rapid eye movement sleep behaviour disorder to multiple system atrophy is predicted by abnormal dopamine transporter imaging combined with normal 123I‐metaiodobenzylguanidine myocardial scintigraphy: a case report

To the Editor: Most cases of idiopathic rapid eye movement sleep behaviour disorder (RBD) represent a prodromal phenotype of α-synucleinopathies, and almost all of them eventually phenoconvert to Parkinson’s disease, dementia with Lewy bodies, or much less frequently to multiple system atrophy (MSA). In support of this close linkage, a high proportion of RBD cases show decreased dopamine transporter (DAT) availability in the striatum, which is commonly observed in Parkinson’s disease, dementia with Lewy bodies, and MSA. However, Parkinson’s disease and dementia with Lewy bodies, but not MSA, exhibit decreased myocardial uptake of I-metaiodobenzylguanidine (I-MIBG), suggesting myocardial sympathetic denervation in the former disorders but not in the latter. Accordingly, it has been hypothesized that the combination of abnormal DAT imaging and normal I-MIBG myocardial scintigraphy predicts phenoconversion from RBD to MSA. Recently, we confirmed this hypothesis in a patient with RBD. The patient was a right-handed 68-year-old Japanese man. In January 2016, he visited our hospital because he had had dream-enacting behaviours, including speaking loudly and moving his arms and limbs during his dreams, for the last year. The patient scored 7 on the Japanese RBD screening questionnaire, which has high sensitivity and specificity for idiopathic RBD, and was diagnosed with probable idiopathic RBD. He scored 28/30 on the Mini-Mental State Examination, reflecting the absence of dementia. No abnormalities were detected on neurological examinations. Magnetic resonance imaging did not reveal any abnormal findings (Fig. 1a). I-N-omega-fluoropropyl-2-beta-carbomethoxy-3beta-(4-iodophenyl)nortropane single-photon emission computed tomography, an accurate and sensitive form of DAT imaging, showed reduced DAT availability in the bilateral striatum, especially in the left pallidum (Fig. 1b). In contrast, there was no decrease in the heart-to-mediastinum ratios in I-MIBG myocardial scintigraphy (early: 2.53, delayed: 3.37; institutional cut-off value: 2.2) (Fig. 1c). In September 2016, orthostatic hypotension was observed. In January 2017, the patient developed gait disturbance, postural instability, and frequent urination. However, he still scored 29/30 on the Mini-Mental State Examination, reflecting the absence of dementia. Even at this stage, the heart-to-mediastinum ratios of IMIBG myocardial scintigraphy had not decreased (early: 2.69, delayed: 3.02). In August 2017, his orthostatic hypotension and postural instability worsened. His gait was wide-based and ataxic, necessitating assistance to walk. He also had urinary incontinence and mild dysphagia. Eventually, he was diagnosed with probable MSA based on the published diagnostic criteria. In January 2018, he began to require a wheelchair to move. Computed tomography of the head showed atrophy of the middle cerebellar peduncles, pons, and mesencephalic tectum (Fig. 1d). The present report suggests the usefulness of combining DAT imaging and I-MIBG myocardial scintigraphy to predict phenoconversion from RBD to MSA. The presence of reduced striatal DAT availability and the absence of myocardial sympathetic denervation— a pathophysiological characteristic of MSA, which is a specific type of α-synucleinopathy—predicts future progression to this neurodegenerative disorder. There are two limitations in the present report. Firstly, our patient did not receive polysomnography, and therefore, his diagnosis of RBD remained probable. Secondly, his diagnosis of MSA was not autopsy-proven.