Hypocretin (orexin) loss and sleep disturbances in Parkinson's Disease

Extract

We would like to thank Baumann et al. for their interest in our paper (Fronczek et al., 2007). The questions raised are all relevant and mainly entail the relationship between the partial hypocretin loss in PD (Fronczek et al., 2007; Thannickal et al., 2007) and actual sleep disturbances. Furthermore, the selectivity of the hypocretin cell loss in PD is questioned. We fully agree with these remarks and therefore prominently discussed them in our original paper (Fronczek et al., 2007). However, two of the issues warrant a more detailed reply.

First, Baumann et al. state that the daytime sleep attacks in PD do not resemble those seen in narcolepsy. However, after listing those symptoms that are similar between the two disorders, the authors do not specify what the exact differences may be, and do not provide any studies on this topic. One of the few published studies that addressed this exact topic stated that the combination of excessive daytime sleepiness (sometimes culminating in sleep attacks), hypnagogic hallucinations, REM-sleep behaviour disorder (RBD) and daytime sleep-onset REM periods, is similar to narcolepsy (Arnulf et al., 2005). Note, that these are all symptoms that can occur without a complete loss of hypocretin.