Original Article The Longitudinal Impact of Excessive Daytime Somnolence on Motor and Nonmotor Symptoms of Parkinson’s Disease in a Southeast Asian Cohort Lai Gwen Chan, MBBS, MRCP, MRCPsych 1 , Kimberly Sim Soon Siang, MBBS 1 , Ting Ting Yong, BA 2 , Russell Chander, BA 2 , Louis Tan, MBBS, MRCP, FAMS, FRCP 2,3 , and Nagaendran Kandiah, MBBS, FAMS, FRCP 2,3 Abstract Objective: Sleep–wake disturbances, such as excessive daytime somnolence (EDS), are nonmotor symptoms of Parkinson’s disease (PD) and significantly affect the quality of life. This study aimed to examine the relationship between EDS and motor and nonmotor symptoms of PD. Methods: Eighty-two patients with idiopathic mild PD were followed up twice a year for 2 years and assessed on sleep, mood, anxiety, cognition, function, and disease severity. Data were analyzed retrospectively, comparing motor and nonmotor outcomes between those with EDS and those without. Results: At baseline, 27.9% had EDS and were similar in demographic and clinical characteristics to those without; 10% had persistent EDS during the first year of follow-up. Excessive daytime somnolence had a significant main effect on mood and anxiety as shown by consistently higher scores on the Geriatric Depression Scale (P ¼ .022) and Hospital Anxiety and Depression Scale-Anxiety subscale scores (P ¼ .011) throughout duration of follow-up. The group with persistent EDS showed greater rate of worsening Frontal Assessment Battery scores by the end of first- year follow-up (P ¼ .025) and greater rate of worsening Apathy Scale scores by the end of 2-year follow-up (P ¼ .002). No significant effects of EDS on motor symptoms and disease severity were found. Conclusions: In a Southeast Asian cohort of patients with PD, EDS had a negative longitudinal impact on mood, anxiety, apathy, and cognitive function but no impact on motor symptoms and disease severity. Excessive daytime somnolence is thus a potential therapeutic target to improve nonmotor outcomes. Keywords anxiety, cognitive impairment, depression, Parkinson’s disease, movement disorder, neurodegeneration Background Disorders of sleep and wakefulness are common and important nonmotor symptoms of Parkinson’s disease (PD) that have a significant impact on quality of life. A recent qualitative systematic review also showed that sleep distur- bances are associated with and influence other motor and nonmotor symptoms in PD. 1 Excessive daytime somnolence (EDS) is a disabling tendency to fall asleep during normal daytime activities 2 and affects up to 60% of patients with PD. 3 Excessive daytime somnolence has been found to be of higher prevalence in both drug-naive patients with PD and in patients with advanced PD when compared to controls. 4 Suzuki et al reported that patients having PD with Hoehn & Yahr stage 4 disease had significantly more EDS than patients with stages 1, 2, or 3 without group differences for dopami- nergic therapy. 5 A study by Verbaan et al showed that EDS was more common in patients with PD of higher age, higher dopamine agonist dose, autonomic dysfunction, and psychia- tric symptoms in addition to advanced disease. 6 A variety of cross-sectional studies have reported the asso- ciation between EDS and other nonmotor symptoms. For example, EDS has been found to be a predictor of depres- sion. 7,8 A greater prevalence of fatigue, 9 anxiety, 10 and impul- sive behavior 11 have been found in patients having PD with EDS. Patients with PD having EDS have also been found to 1 Department of Psychological Medicine, Tan Tock Seng Hospital, Singapore, 2 Department of Neurology, National Neuroscience Institute, Singapore 3 Duke-NUS Medical School, Singapore Received 6/5/2019. Received revised 9/25/2019. Accepted 9/29/2019. Corresponding Author: Kimberly Sim Soon Siang, Department of Psychological Medicine, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore. Email: kimberly.sim@mohh.com.sg Journal of Geriatric Psychiatry and Neurology 1-7 ª The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0891988719892326 journals.sagepub.com/home/jgp