This is the first report of a comprehensive exploration of the association of total number of non-motor symptoms and the quality of life for patients with PD quantitatively using a large smartphone-based prospective study to capture data in the UK. Non-motor symptoms have been recognized as an important part of PD, and appropriate therapy can considerably improve quality of life [25]. Many researchers have explored how some items of the NMS independently have impact on the health-related quality of life, such as depression, sleep, mood and attention [13, 26], less studied are the associations of the total number of non-motor symptoms and health-related quality of life. In this study, we explored the relationship between total number of non-motor symptoms and health-related quality of life for patients with PD and found that where the sum of NMS-30 questions is higher, PD patients’ health-related quality of life of is worse. The NMS could be related to degeneration of different brain structures and could exert a stronger influence on severe disability, impaired health-related quality of life and shortened life expectancy, which should be taken in account regarding treatment options [7].
Our study reveals that almost every patient with PD presented with more than one symptom. The relative NMS occurrence, as assessed by NMS-30, agrees with several other studies, showing sleep (difficulty getting to sleep or staying asleep), mood/cognition (sad or blue), memory/attention and urination (urgency, getting up regularly at night to pass urine) as the most prevalent symptoms both for PD patients with and without limited ability to work [27–29]. However, the absolute occurrence of the most frequent symptoms is lower than Paulo Bugalho’s study (Sleep/fatigue, affect/cognition, attention/memory were above 80%) [30], but higher than in most studies that used this same scale [28, 29], in which above 60% are rare, while in our sample sleep disturbances reached 70.90%, and urgency reached around 80%. This difference of results could be caused by cultural, geographic or pattern of health care particularities in our sample and deserves further investigation to verify.
Depression in PD is believed to be a common complication ranging in prevalence between 2.7% and 70% but is under self-reported[31]. Depression can precede the development of PD and has a major impact on a patient’s quality of life [31–34]. In our study, depression is one of the most prevalent symptoms both for patients with and without limited ability to work. Depression prior PD diagnosis was also a potential determinant of worse quality of life.
Age and education were found to be correlated with health-related quality of life of PD patients significantly. It is well known that age is the greatest risk factor for the development of PD. A previous study showed age-specific incidence rates of PD increased sharply beginning at age 60 years and peaking after 80 years [35]. In our study, patients aged 60–69 years had better quality of life than patients aged less than 59 years, which is inconsistent with previous research, further investigation is needed to verify. Compared with patients educated up to 16 years old, patients educated up to 18 years old is a potential determinant of better quality of life, which is consistent with the conclusion that higher education is associated with better health-related quality of life [36, 37].
Limitations
Our study presents some limitations. Firstly, the determinants of health-related quality of life are limited by the pre-specified questions in the surveys. There could be some potential unobserved confounding factors (such as individual exercise, occupation, income level or marital status) we did not control for in the generalised linear model. Secondly, this study conducted a brief question on whether patients’ motor symptoms limited their ability to work and divided the characteristic of motor symptoms into with or without limited ability to work, but lacked a detailed evaluation of Parkinson's patients' motor symptoms. More detailed study are needed to verify the association between non-motor symptoms and health-related quality of life for patients with Parkinson’s disease after controlling for their motor symptoms. A final limitation is our study is an observational study and reports the association between non-motor symptoms and health-related quality of life for patients with PD based on a quantitative study, more evidence based on qualitative studies and randomized controlled trials are needed to clarify the causal association in future research.