Narcolepsy in Parkinson's disease

Non-motor symptoms in Parkinson's disease (PD), such as excessive daytime sleepiness, 'sleep attacks', insomnia, restless legs syndrome and rapid eye movement sleep behavior disorder, are common and provide a challenge to treatment. These sleep symptoms are also described in patients suffering from the sleep/wake disorder, narcolepsy. The International Classification of Sleep Disorders (ICSD-2) narcolepsy criteria uses a number of markers for diagnosis, of which lack or deficiency of cerebrospinal fluid (CSF) hypocretin-1 levels is a key marker. Hypocretin neurons prominently located in the lateral hypothalamus and perifornical nucleus have been proposed to interact with mechanisms involving sleep and arousal. Low hypocretin-1 levels in the CSF have been shown to correlate with hypothalamic hypocretin cell loss in narcolepsy and other forms of hypersomnia; therefore, it has been proposed that degenerative damage to hypocretin neurons (such as in PD) may be detected by low CSF hypocretin-1 concentrations, and may also explain the sleep symptoms experienced by some PD patients. To date, there is mixed conflicting data describing hypocretin-1 levels in the CSF of patients with parkinsonism associated with sleep symptoms, with most studies showing no significant decrease when compared with controls. However, hypocretin-1 CSF deficiency has been shown in some studies to be more prominent in PD patients with sleep symptoms versus those without. Notably, the hypocretin system has been shown not to be selectively disrupted, with one study showing melanin concentrating hormone cell loss in the same patients with hypocretin loss. It is likely that hypocretin deficiency in PD patients occurs secondary to collateral damage caused by the neurodegenerative process involving the hypothalamus. Awareness of narcoleptic events in PD is important for driving related advice, in addition to the possible use of dopamine D3 receptor active agonists.