Why is this study important?
Many people with Parkinson’s disease (PD) are unusually sleepy during the day (termed excessive daytime sleepiness or EDS). Scientists do not know, however, whether the sleepiness has the same cause as the PD, is caused by the PD, or is caused by medicines that are typically used to treat PD. In an attempt to identify “which came first…the chicken or the egg,” researchers examined the medical records of patients who were taking part in an ongoing study of aging and heart disease.
 
Who were the participants and what did they do?
Participants in this study were men who took part in the Honolulu Asia Aging Study, which began in 1991. During the beginning assessment period from 1991 until 1993, researchers reviewed medical records from all participants, and the records of those men who had a diagnosis of PD were excluded. The remaining 3,078 men were between 71 and 93 years old. During this assessment period, participants completed several surveys, including one about sleep and usual sleep habits, which asked about EDS. Those men who reported being sleepy most of the day were said to have EDS. Between 1994 and 2001, the men returned to the clinic to answer more questions and to undergo an examination and evaluation for PD.
 
Who were the researchers and what did they do?
The researchers were from six different medical institutions and were supported by grants from the National Institutes of Health. They evaluated the questionnaires and examined the patients’ medical records to determine new diagnoses of PD or the patients’ use of typical PD medications. Neurologists examined the patients who had symptoms that suggested the presence of PD. The researchers reviewed the completed questionnaires and noted which subjects had EDS. They then divided the men into two groups, those with and without EDS, and checked to see if one group was more or less likely to develop PD.
 
What were the results of the study?
During the assessment period, 244 of the men (7.9%) were identified as having EDS. These men, as compared with those without EDS, were found to have drunk less coffee when they were between 30 and 50 years old, to nap more frequently, and to have a higher likelihood of being depressed, experiencing insomnia, or having difficulty thinking clearly (cognitive impairment).
 
During the follow-up period, 43 men had a new diagnosis of PD. The average age at diagnosis was 80 years. The men with EDS had a threefold increased risk of having a diagnosis of PD, as compared with those men without EDS. Even when all other risk factors were taken into account, the difference in the number of men who developed PD was higher in those who had previously had EDS than in those men who were not excessively sleepy.
 
What was the authors’ conclusion?
“. . . this report further suggests that EDS can predate clinical PD. The association between EDS and PD is also uninfluenced by medical intervention for PD and the effects of antidepressants, antipsychotics, and sedatives.”
 
Ross GW, White LR, Abbott RD, et al. Excessive daytime sleepiness and subsequent development of Parkinson disease. Neurology 2005;65:1442-1446.