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Robust association between narcolepsy and RBD
Following the pioneering work of Schenk and Mahowald,1 a robust association of idiopathic narcolepsy and rapid eye movement (REM) sleep behaviour disorder (RBD) has recently been confirmed, suggesting that around one third of patients with narcolepsy have concomitant RBD.2 Although particularly severe cases with frequently occurring cataplexies and other associated symptoms of narcolepsy seem to be affected, the pathophysiological link between narcolepsy and RBD remains to be elucidated.
Contributing to this topic, in this issue, Mathis et al3(see p 427) report on a HLA DR2‐negative patient who developed both narcolepsy and RBD due to an isolated inflammatory lesion of the dorsomedial pontine tegmentum. Their finding is very interesting as it suggests a common pathway for narcolepsy and RBD at the level where REM muscle atonia is regulated. Apart from a conclusive neuroanatomical and pathophysiological survey, the authors lent further support to their hypothesis by showing that the depicted anatomical region was also affected in published cases of either RBD or symptomatic narcolepsy due to circumscribed brain stem lesions.
Future research, of course, has to establish the presumed connection of narcolepsy and RBD in larger collectives of patients. Methodologically, such studies may apply advanced neuroimaging techniques, such as voxel‐based morphometry and diffusion tensor imaging, which have recently been introduced for structural evaluation of the brain in neurological disorders. It might be interesting to compare groups of narcoleptics with and without RBD with regard to the integrity of fibre tracts involved in the generation of muscle atonia—that is, those originating from the pontine tegmentum and descending via the medial medulla to motor neurones.4
Looking at the topic in question from a different angle, one may choose to study patients with Parkinson's disease, multiple system atrophy and dementia with Lewy bodies. All these synucleinopathys are known to be associated with RBD, with one third of patients with Parkinson's disease and about 80% of patients with multiple system atrophy and dementia with Lewy bodies being affected.5,6,7 Interestingly, apart from excessive daytime sleepiness, sleep episodes and hypnagogic hallucinations, sleep‐onset REM periods on the Multiple Sleep Latency Test have repeatedly been described in the neurodegenerative disorders mentioned, which were especially prevalent in patients with concomitant RBD.8 The remarkable association of these features, especially the finding of multiple sleep‐onset REM periods, is reminiscent of primary narcolepsy, and helped in coining the term “narcolepsy‐like phenotype”.8 In the future, using voxel‐based morphometry and diffusion tensor imaging, we could compare well‐characterised groups of patients with and without RBD, with and without a narcolepsy‐like phenotype, and with a combination of both. Although changes in different brain regions are to be expected because of the multifocal pathophysiology of the synucleinopathys, it may be speculated that fibre tracts and neurone populations located in the suggested “common pathway for narcolepsy and RBD” are exclusively affected in patients showing both syndromes.
Competing interests: None declared.