The underlying cause of RBD is not well understood, but it is likely that RBD is an early symptom of synucleinopathy rather than a separate disorder. Brainstem circuits that control atonia during REM sleep may be damaged, including those in the pontomedullary brainstem. REM sleep circuits are located in caudal brainstem structures—the same structures that are known to lead to be implicated in the synucleinopathies. Motor deficits like those seen in RBD are known to result from lesions in those circuits.