Does REM sleep behavior disorder have the guts to be Parkinson disease?

Almost 200 years ago, James Parkinson1 described constipation as a frequent and clinically relevant nonmotor symptom in shaking palsy. Recent studies have shown that constipation can precede the motor manifestation of Parkinson disease (PD)2,3 and that constipation is more frequent in idiopathic REM sleep behavior disorder (iRBD) (a potential prodromal stage of PD) compared to controls.4 Even though the causal relationship between constipation and the neurodegenerative process that underlies PD remains debatable, constipation is considered as a risk marker for a subsequent diagnosis of PD.3 One potential pathoanatomical correlate for constipation in PD is Lewy bodies, the pathohistologic hallmark of PD, in the enteric nervous system (ENS). Lewy body pathology is found throughout the gastrointestinal tract with a rostral to caudal gradient. While early work on Lewy body pathology in the ENS was postmortem,5 recent studies have shown that the ENS is also easily amenable to in vivo pathohistologic assessment using endoscopy.6,7 The presence of ENS Lewy body pathology in PD led to the hypothesis that the ENS might be one of the first sites where Lewy body pathology occurs in PD.8 Few in vivo data exist on ENS pathology in early-stage PD.