TABLE 3.

Methodological characteristics of included articles

Study	Study Aim	Sample	Study design	Sample size (PWP)	Sample Size (CG)	Caregiver Measures	Quality of Life Measures	Analysis Method	Relationship between CG factor and PWP QOL
Bartolomei 2018	To examine the relationship of sleep quality and depression in patients with PD with burden, mood, and quality of life of their caregivers.	Outpatient clinic	Cross sectional study	57	57	CBI, BDI	PDQ-39	Pearson correlation and Spearman correlation	Caregivers' burden and depression were interrelated (Pearson's correlation p < 0.001) and both correlated with PD quality of life.
Caap-Ahlgren 2002	To identify various factors in PD patients and their caregivers that are of supposed importance with respect to caregiver burden.	Outpatient clinic	Longitudinal study	65	65	CBI	PDQ-39	Spearman correlation	The correlation between PDQ-39 and caregiver burden: 0.48 (p < 0.005)
Chu 2019	To determine the relationships between QOL and life satisfaction among PD patients and their caregiver.	Active members of the Malaysia Parkinson's Disease Association (MPDA)	Cross sectional study	20	20	Life satisfaction Scale	PDQ-39	Pearson Correlation	Correlation between PDQ-39 and life satisfaction scale was 0.46 (significant 2-tailed)
Dahodwala 2018	To compare access to caregiving between men and women with Parkinson's disease (PD).	Participants enrolled in the National Parkinson Foundation Parkinson's Outcomes Project	Cross-sectional	6154	7209	Caregiver attending visit MCSI	PDQ-39	Linear regression (Cross-sectional)	Higher PDQ-39 score was related to higher odds of having a caregiver accompany to baseline visit (1.02; 95% CI: 1.01–1.02) and MCSI score (0.34; 95% CI: 0.30–0.37).
daSilva 2008	To evaluate the quality of life of patients with PD; measure the caregivers stress level; and to confront the caregivers stress levels with the patient's quality of life, relating them to the disease gradation.	Outpatient clinic	Cross-sectional	43	43	CSS	PDQ-39	Chi-Square test Spearman Correlation	PDQ-39 versus CSS (p = 0.0017, r = 0.4646).
Demeulemeester 2015	To investigate the perception of patients and partners of their own as well as of each other present and former quality of life and to explore a possible correlation of these estimates with measures of disease burden.	Outpatient clinic	Cross sectional study	50	50	VAS	VAS	Pearson correlation	Quality of life of the PD patient was correlated with caregiver QOL (0.395, p = 0.005)
Hand 2018	To examine the baseline care needs of the cohort living at home and detail the levels of formal and informal care required to meet these needs.	Participants in the Northumbria Care Needs Project.	Cross-sectional	132	115	Presence of caregiver Duration of caregiving Tasks performed Current health problems they had which they felt negatively affected their ability to fulfill their caring role.	PDQ-39	Chi-squared Test	There was a significant difference in PDQ-39 score for those with informal caregiver (32.97; 22.71–42.86 compared to those without an informal caregiver (18.7; 14.455–30.295), p < 0.001.
Henry 2020	To assess the relationship among a range of PD symptoms and four measures of caregiver QOL.	Outpatient clinic	Cross sectional study	181	181	PDQ-Carer Demographic information (age, sex, relationship to care recipient, how often they see the care recipient, and how many hours per week they spend with the care recipient)	PDQ-39	Correlation	All subscales of the PDQ-39 were correlated with PDQ-Carer subscales (p < 0.01 or lower).
Karlstedt 2017	To identify factors associated with mutuality, HRQOL, and caregiver burden.	Baseline data from a longitudinal study.	Cross sectional study	51	51	CBI MS Caregiver gender and age	PDQ-8	Multiple linear regression	The significant predictors of PDQ-8 Total score were MS score (beta = 433, = 0.001) and NMSQuest score (beta = 0.498, p < 0.001) contributed significantly to the explained variance of PDQ8SI scores. Caregiver gender (−4.329; 95% CI −11.565–2.907) and caregiver age (−0.190; 95% CI: −0.551–0.171) were not significant predictors of PDQ-8 total score.
Karlstedt 2018	To disentangle different pathways that could explain the effect of PD specific symptoms on patients' HRQOL.	Outpatient clinic	Cross sectional study	51	51	CBI MS Caregiver gender and age	PDQ-8	Structural equation modeling	With increasing severity of motor and nonmotor symptoms, patients' mutuality decreased which leads to worse PDQ-8.
Kelly 2012	To consider the relationship between HRQOL in people with PD and their caregivers.	Participants were drawn from general medical practitioners, neurologists, Parkinson's Victoria network and advertisements in local papers.	Cross sectional study	97	97	MCSI	EQ-5D PDQ-39 VAS	Spearman rank correlations coefficient	A small but significant negative relationship was found using the EQ-5D and VAS, but not the unweighted VAS, suggesting that poorer HRQOL in people with PD is associated with higher caregiver strain experienced by their caregivers. The correlation of PDQ-39 with MCSI scores (rs = 0.3 at p < 0.001) confirmed that poorer self-rated health in people with PD was associated with increased strain in their caregivers.
Kudlicka 2014	To evaluate how EF contributes to QOL and health status for the PwPD and caregiver burden.	Outpatient clinic	Cross sectional study	50	65	CBI caregiver rating of BRIEF-A	LSS PDQ-39	Multiple regression analyses	The BRIEF-A caregiver rating was the strongest and the only individually significant predictor of QOL-health, whereas the depression rating only approached significance. The model explained 32% of the variance in QOL-health. The BRIEF-A caregiver rating was the strongest and the only individually significant predictor of QOL movement disorders. The model explained 26% of the variance in QOL-movement disorders. The BRIEF-A self-rating was the strongest and the only individually significant predictor of PDQ-39. The model explained 34% of the variance in health status.
Lubomski 2021	To examine MCI in PD patients and their caregivers to determine if caregiver cognition affected their PD relative.	Outpatient clinic	Cross-sectional study	103	81	MoCA	SF-36	T-tests	PD patients without MCI were noted to experience an even lower QOL when their caregiver had MCI, compared to a caregiver with no MCI. This difference was observed on the physical component of the SF-36 [PCS 50.9 (SD 22.3) versus 55.8 (SD 16.1), t = −2.5, p = 0.015 and the mental components 60.5 (SD 18.4) versus 64.8 (SD 19.3), t = −2.3, p = 0.028].
Ma 2018	To explore neuroticism and conscientiousness personality factors of the patient and family caregiver associated with quality of life (QOL) of PD patients	Outpatient clinic	Cross sectional study	134	134	NEO-FFI	PDQ-39	Multivariate stepwise linear regression analysis with a forward selection.	The neuroticism of the caregiver was not significantly related to the three domains of QOL, while conscientiousness of the caregiver was added to the predictor of PDQ-39 (beta = −0.22).
Martinez-Martin 2005	To assess the impact of PD on informal caregivers of patients and identify the main factors related to caregiver strain.	Outpatient Clinic	Cross sectional study	64	64		PDQ-39	Spearman Correlation	Caregiver quality of life was correlated with patient quality of life (r = −0.600, p < 0.001).
Miyashita 2011	To clarify the QOL of patients with intractable neurological diseases and their caregivers.	Survey sent to homes	Cross sectional study	418	418	SF-8	SF-36	correlation and regression	The patients' mental health and caregivers' mental component summary were moderately correlated (standardized beta [SB] = 0.34, p < 0.001).
Muller 2010	To report the analysis of the survey outcomes on QOL, insurance, disability and caregiver burden in patients with PD and their carers.	Not described.	Cross sectional study	2603	2603	BSFC	PDQ-8	Spearman Correlation	Correlation between PDQ-8 & BSFC = 0.25 (p < 0.0001)
Navarta-Sanchez 2016	To explore potential clinical, social and attitudinal determinants of psychosocial adjustment and QOL in PD patients and informal caregivers	Three settings in a community context: The Navarre association of Parkinson's patients, a Neurology outpatient clinic and a primary care practice in Pamplona (Spain)	Cross sectional study	83	91	SQLC	PDQ-39	Multiple linear regression	The presence of a caregiver was not a significant predictor of PDQ-39 (−1.61 (−9.14,5.90) p = 0.67)) after controlling for age, gender, brief coping, benefit finding, disease severity, psychosocial adjustment, and resources.
Oguh 2013	To determine what measures of PD disability, demographics, and patient quality of life are associated with caregiver strain among caregivers of patients with PD.	Data was obtained from the National Parkinson Foundation's quality Improvement Registry (NPF-QII)	Cross sectional study	826	826	MCSI	PDQ-39	Spearman Correlation	The total PDQ-39 score was correlated with MCSI score (r = 0.567, p < 0.00).
Peters 2011	To explore to what extent patient self-reported health, as measured on the disease-specific Parkinson's Disease Questionnaire (PDQ-39), is associated with carer strain and self-reported quality of life.	A postal survey was carried out of both patients and caregivers through local branches of Parkinson's UK.	Cross sectional study	704	901	CSI	PDQ-39	Correlation	The PD Index was found to be highly associated with caregiver burden as measured on the CSI (r = 0.56, p < 0.001, n = 422).
Prizer 2020	To assess whether caregiver presence was associated with variations in patient presentation and outcomes in a palliative PD and atypical PD population.	Baseline data from a 3-site randomized controlled trial	Cross-sectional	210	175	Presence of a caregiver	PDQ-39, PROMIS-29, McGill QOL, QOL-AD	Regression	Quality of life as measured by the QOL-AD questionnaire was better for individuals with caregivers compared to those without caregivers (p < 0.0001). Differences in QOL between people with caregivers and those without caregivers as measured by other scales did not reach significance.
Rajiah 2017	To examine the impact of clinical features on PD patients' QOL, and the impact of PD patients' QOL on their caregivers' burden.	Malaysian Parkinson's Disease Association	Cross-sectional	122	122	ZBI	PDQ-39	Correlation	ZBI score was correlated with PDQ-39 score (r = 0.76, p < 0.005).
Ricciardi 2015	To evaluate the relationship quality and satisfaction in couples, in which one partner had PD, obtaining the perspective of both patients and partner and examine the impact of alexithymia, empathy, depression, and anxiety on ratings of relationship quality and satisfaction.	PD patients and their spousal caregivers were recruited at “Centro Studi Lorenzon” in Treviso, Italy, in the context of a free workshop series	Cross sectional study	15	15	DAS,CoSI	PDQ-39	Backward Step-wise regression	CoSI was a significant predictor of PDQ-39 social support subscale (beta = −0.606, p = 0.004).
Rodriguez-Violante 2015	To analyze the relationship between caregiver burden and PD patients' quality of life.	Outpatient clinic	Cross sectional study	201	250	ZBI	PDQ-8	t-test correlation	PDQ-8 score was higher for patients who had caregivers with burden (44.3+/−31) compared to those without burden (28.3 +/−20.6), p = 0.004. A moderate correlation was observed between PDQ-8 and ZBI (r = 0.35, p < 0.001)
Rosqvist 2019	To describe and assess satisfaction with care in a sample of patients with late stage PD in Sweden and to identify factors associated with patient satisfaction with care as well as to describe and assess their informal caregivers' satisfaction with support and to identify factors associated with caregivers' satisfaction with support.	Outpatient clinic	Cross sectional study	76	107	Caregiver satisfaction with support	SEIQOL	logistic regression	PWP quality of life was not significantly associated with caregiver satisfaction with support (p = 0.623).
Tan 2019	To investigate the characteristics of PD patients related to different levels of caregiver burden.	Outpatient clinic	Cross sectional study	104	104	ZBI	PDQ-39	ANOVA	The median PDQ-39 scores was significantly higher in the high caregiver burden group compared to no-or little group (30.7 vs. 16.2, p-value 0.002). The high caregiver burden group had significantly higher median scores for domain 2 (ADL, p-value 0.005), domain 4 (stigma, p-value 0.005), and domain 6 (cognition, p-value 0.002) compared to no-or little group.
Tanji 2008	To assess the association between mutuality of the marital relationship in Parkinson's disease with disease severity, disability, mental health, quality of life, and caregiver burden.	Outpatient clinic	Cross sectional study	96	96	MS	SF-12v2	Correlation	Spouse reported mutuality was not significantly correlated with physical health QOL (p = 0.20) or mental health QOL (p = 0.10).
Torny 2018	To determine correlations between the intensity of disease burden and characteristics of patients and their spouses.	Outpatient clinic	Cross sectional study	38	38	ZBI	PDQ-8	Correlation	PDQ-8 was correlated with ZBI score (r = 0.27, p = 0.007).
Trang 2020	To determine predictors of general QOL in a population of PDRD patients with high needs and advanced disease and to compare patient ratings of general QOL to health-related QOL and general patient QOL as reported by caregivers.	Baseline data obtained from a multisite randomized controlled trial of outpatient palliative care	Cross-sectional	210	175	HADS, FACIT, QOL-AD, ZBI	QOL-AD, PDQ-39	Regression (Elastic Nets Model)	Caregiver burden (ZBI) was associated with PDQ-39 (r = 0.1105, p = 0.0394).
Valldeoriola 2021	To assess, the effect of 6-month treatment with LCIG on the QOL of patients with APD using the self-reported PDQ-39.	Outpatient clinics	Longitudinal	62	62	SQLC, ZBI, CSI, GAS, GDS	PDQ-39	Regression	Improvements in patients' QOL (PDQ-39) did not correspond with improvements in caregivers' QOL (SQLC) or caregiver burden (ZBI).
Viwattanakulvanid 2014	To determine and analyze the main determinants of caregiver burden and examine the impact of nocturnal dis-abilities of PD patients on their caregivers.	Multicenter, national outpatient centers	Cross sectional study	89	89	ZBI	PDQ-8	Correlation	PDQ-8 was correlated with ZBI score (r = 0.38, p < 0.001).
Yuksel 2018	To assess the impact of early-stage PD on caregiver burden with disease severity, duration, disability and psychiatric symptoms.	Outpatient clinic	Cross sectional study	30	30	ZBI	SF-36	Student t test, Mann Whitney U test	Lower caregiver education was related to SF-36 subdomains of the patients (emotional role, social functioning, and pain), p < 0.05).

Abbreviations: BDI, Beck Depression Inventory; BRIEF-A, Behavior Rating Inventory of EF-Adult Version; BSFC, Burden Scale for Family Caregivers; CBI, Caregiver Burden Inventory; CoSI, Couple Satisfaction Index; CSI, Caregiver Strain Index; CSS, Caregiver Stress Scale; DAS, Dyadic Adjustment Scale; EQ-5D, EuroQOL; FACIT, Functional Assessment of Chronic Illness Therapy; GAS, Goldberg Anxiety Scale; GDS, Goldberg Depression Scale; HADS, Hospital Anxiety and Depression Scale; HRQOL, Health Related Quality of Life; LCIG, Levodopa-Carbidopa Intestinal Gel; LSS, Life Satisfaction Scale; MCI, Mild Cognitive Impairment; MCSI, Multidimensional Caregiver Strain Index; McGill QOL, McGill Quality of Life Questionnaire; MoCA, Montreal Cognitive Assessment; MS, Mutuality Scale; NEO-FFI, Neuroticism Extraversion Openness Five-Factor Inventory; Outpatient clinics, Clinic that provides care for patients without requiring a hospital stay; PDQ-Carer, Parkinson's disease Questionnaire for Caregivers; PDQ-39, Parkinson's disease Questionnaire-39; PDQ-8, Parkinson's disease Questionnaire-8; PDRD, Parkinson's Disease and Related Disorders; PROMIS-29, Patient-Reported Outcomes Measurement Information System; QOL-AD, Quality of Life in Alzheimer's Disease; SEIQOL, Schedule for the Evaluation of Individual QOL Questionnaire; SF-36, Short Form-36 Health Survey; SF-8, Short Form-8 Health Survey; SF-12v2, Short Form-12 Health Survey; SQLC, Scale of Quality of Life of Caregivers; VAS, Visual Analog Scale; WPAI, Work Productivity and Activity Impairment.