Factors Influencing Quality of Life in Parkinson's Disease
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BACKGROUND: Parkinson’s Disease (PD) is a progressive neurodegenerative disease, that encompasses a broad range of motor and non-motor symptoms, each of which has the potential to negatively impact health-related quality of life. Depression, disease duration, and level of disability have been found to significantly influence quality of life in PD. However, there is a paucity of research examining the combined influences of depression, PD motor symptom severity and cognition on overall HRQoL, as well as various domains of HRQoL, in a PD population. SUBJECTS: This study used data collected from 124 participants with PD, aged 50 to 85, who were enrolled in a larger study, and gave informed consent for the parent study, with no knowledge of the current study. Inclusion criteria consisted of a PD diagnosis and a response to levodopa treatment for at least 30 days. Participants meeting DSM-IV-TR criteria for Axis I disorders other than MDD were excluded. METHOD: The current study utilized linear and multiple regression analyses to explore the relationships between HRQoL and depression severity, PD motor symptom severity and global cognitive ability. HRQoL was measured by the Parkinson’s Disease Questionnaire – 39 Item, a PD specific disease questionnaire designed to assess HRQoL in a variety of domains. Measures included the Quick Inventory of Depressive Symptomatology-Clinician Version, the United Parkinson’s Disease Rating Scale Part III, selected disease characteristics, and a battery of cognitive tests. RESULTS: Depression severity (QIDS-C16) and PD motor symptom severity (UPDRS) and global cognitive ability accounted for approximately 30% of the variance in PDQ-39 Single Index scores. Depression severity and motor symptom severity were the most significant predictors of HRQoL (PDQ-39). The multiple regression analysis results aligned closely with separate, linear regression analyses designed to control for the redundancy in the independent and dependent variables, which showed that depression severity accounted for 18.7% of the variance in HRQoL, motor severity accounted for 12%, and that global cognitive ability accounted for 3.8%. Depression severity accounted for the greatest amount of variance in all domain scores comprising the PDQ-39 except for Mobility and Activities of Daily Living, of which PD motor symptom severity accounted for the largest amount of variance. Depression severity was significantly correlated with all PDQ-39 domains, (rs=0.22 to 0.48), while PD symptom severity was significantly correlated (rs=0.23 to 0.51) with Mobility, ADL, Cognition, Communication and Bodily Discomfort domains. Global cognitive functioning did not significantly predict overall HRQoL but did significantly influence the Communication domain. The PDQ-39 Single Index score was significantly correlated with measures visual learning and memory (BVMT-R) and processing speed (Trail Making Test Part A and Part B, and Symbol Search). Of all the significant Pearson correlations, Symbol Search scaled scores had the strongest correlation (r=0.29). In an ANCOVA analysis using QIDS-C16 scores as a covariate, verbal learning and memory (RAVLT) achieved statistical significance. When controlling for depression severity and age, no significant differences in HRQoL were found between individuals classified as bradykinetic/rigid motor subtype and those classified as tremor dominant motor subtype. DISCUSSION: The primary aim of the study was to examine the relative influence of depressive symptoms, PD motor symptoms and cognition on HRQoL in individuals with PD. Both depression severity and motor symptom severity significantly influenced HRQoL, while global cognitive ability did not. The influence of depression severity in the sample is impressive, given the low prevalence and severity of depression present. This suggests that although depression severity may be subclinical, early identification and management of these symptoms may positively influence HRQoL. The most frequently reported depressive symptoms included difficulties staying asleep, sad mood and low energy, which indicate that targeting these specific symptoms, even when subclinical, holds potential for improving the HRQoL of individuals with PD. In exploring the relationship between cognition and HRQoL in PD further, Pearson correlational analyses indicated that HRQoL was significantly correlated with measures of visual and verbal learning and memory, working memory and processing speed, even though a global cognition screening measure was not predictive of overall HRQoL. In summary, the collective findings from this study support the influence of motor severity and even subclinical depression on HRQoL in PD, suggesting that interventions designed to improve mood and the capacity to complete activities of daily living may positively impact quality of life.