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Chapter 22 Rating scales and clinical outcome measures in the evaluation of patients with Parkinson’s disease
Introduction
In recent years, the use of rating scales for Parkinson’s disease (PD) has been expanding in clinical practice and research, allowing clinicians and researchers to assess PD manifestations, the course of the disease and the response to treatment. However, the situation has been characterized by great variability in the content and quality of available scales, making comparisons between studies using different tools somewhat difficult.
As a whole, rating scales can be classified as: (i) rater based, used mainly by clinicians for assessment of motor and nonmotor manifestations, complications and treatment effects; and (ii) self-administered instruments commonly known as “patient-reported outcomes” (PROs) [1]. The use of PROs reflects a trend toward a more active role of patients in their treatment and highlights that interventions should be evaluated by using outcomes that are meaningful to patients. Patient-reported outcomes represent the patient’s report of a health condition and its treatment [1], and include a variety of subjective aspects of the disease (symptoms, functioning, quality of life, perceived change, satisfaction with treatment). They are essential for understanding the impact of the disease, evaluating treatment efficacy and interpreting the significance of clinical outcomes, and for evidence-based decision making.
Rating scales are very useful when they are developed following rigorous methodological standards and procedures that guarantee the appropriateness and quality of the data they provide. Development and validation of rating scales are based on two main approaches: the Classical Test Theory (CTT) and the Latent Trait Theories (LTT), which includes the Item Response Theory (IRT) and the Rasch measurement theory.
The development and validation of a rating scale is a complex task, involving several steps: (i) definition and operationalization of the concept to be measured, the target population and the purpose (evaluative, discriminative, predictive); (ii) components of the scale (items, subscales, response options); (iii) style and format; (iv) evaluation of a preliminary version (pilot testing); and finally (v) implementation of the first validation study. The main properties to be tested during the validation process are acceptability, reliability, validity, precision and responsiveness [2, 3]. Interpretability of scale scores is not a measurement property but is an important aspect of the scale usefulness. The definition and standard criteria for assessment of these attributes are described in Table 22.1. Acceptability, reliability, validity and responsiveness are especially relevant when rating scales are used as endpoints in clinical trials. The indexes related to the magnitude and meaningfulness of the change (responsiveness and interpretability) are of utmost importance in the context of intervention outcomes. Responsiveness or sensitivity to change is defined as “the ability of an instrument to detect true change” [2], and it is related to the presence of floor and ceiling effects, the reliability and the precision of the instrument. Several methods for assessing responsiveness exist but the most used are the effect size (ES) and the standardized response mean (SRM), both of them with standardized values for interpretation of change [4]. Interpretability is the extent to which a qualitative meaning can be assigned to quantitative scores. For health measures, it is represented by the minimally important change (MIC) or difference (MID), which can be ascertained by two groups of techniques: anchor-based and distribution-based approaches. The first operationalizes the change by assigning the subjects to groups (stable, worse or better) using an anchor (a clinical or patient-based measure of change). The distribution-based approaches use statistical tests to determine the ability of the scale to detect a clinically important change. There is no agreement about which approach is more suitable to calculate MIC or MID, so the use of several methods (triangulation) has been proposed to offer a range of figures that is likely to contain the real value [5].
Main psychometric attributes of rating scales
| Attribute and definition | Standard criteria (examples) |
|---|---|
| Feasibility: the extent to which the measure can be used in the intended context | Missing data <5–10% |
| Acceptability: how acceptable is the scale for the target population? | Observed vs theoretical range coincident Mean vs median coincident Floor and ceiling effects <15% Skewness –1 to +1 |
| Reliability: degree to which a scale is free of random error | |
| Internal consistency: the extent to which the items measure the same construct | Cronbach’s α >0.70 (group) Inter-item correlation: r >0.20 and r <0.75 Item-total correlation: r = 0.20–0.40 Item homogeneity coefficient >0.30 |
| Reproducibility: stability of scores over time or across different raters | Inter-observer and test-retest reliability: κ (nominal/ordinal data) >0.60 or >0.70 Intraclass correlation coefficient (continuous data) >0.70 |
| Validity: the extent to which an instrument measures the construct it is intended to measure | |
| Content validity: the degree to which the content of an instrument covers the main aspects of the construct to be measured | Expert opinion Lynn’s Content Validity Index |
| Construct validity (hypothesis testing): the extent to which the scale’s scores are consistent with hypothesis regarding the construct | Convergent validity: r > 0.60 Divergent validity: r < 0.30 Internal validity: r = 0.30–0.70 Known-groups validity: significant difference between groups |
| Criterion validity: the degree of relationship of a scale with a “gold standard” | R > 0.60 |
Rating scales for Parkinson’s disease
Clinician-based rating scales
In PD, clinicians need strategies and measures capable not only of detecting symptoms but also of identifying the progression of disability and the effects of treatment. For years, the main rating scales for assessing PD were rater based, focused on directly observable signs, such as the core PD manifestations (tremor, bradykinesia and rigidity), and functional ability, and most of them lacked full validation studies. At present, that situation has been deeply modified with the development of comprehensive rater-based scales and PROs for a wide array of motor and nonmotor manifestations, with formal validation studies.
The most frequently studied clinical outcomes include severity of motor manifestations, magnitude of disability, and long-term motor and nonmotor complications (e.g. fluctuations, dyskinesias, hallucinations), although nonmotor aspects of PD are currently receiving rapidly increasing attention. Some of the frequently used and tested rating scales in clinical practice and research are the Hoehn and Yahr staging (HY) for global disability appraisal; the Clinical Impression of Severity Index – Parkinson’s Disease (CISI-PD), for global severity appraisal; the Unified Parkinson’s Disease Rating Scale (UPDRS) and its clinimetrically enhanced form, the Movement Disorders Society-sponsored version of the UPDRS (MDS-UPDRS), for multidomain severity and disability evaluations; the Scales for Outcomes in Parkinson’s Disease (SCOPA) Motor, and the Schwab and England Scale (SES) to assess motor impairment and disability, respectively; the Abnormal Involuntary Movement Scale (AIMS), Rush Dyskinesia Rating Scale (RDRS) and Unified Dyskinesia Rating Scale (UDysRS), for evaluating motor complications and dyskinesias; and the Non-Motor Symptoms Scale (NMSS), SCOPA Cognition (SCOPA-Cog), and SCOPA Psychiatric Complications (SCOPA-PC), for assessing nonmotor symptoms.
Global severity and disability scales
The HY staging is the universally accepted system to classify and characterize the stages of disability of the disease [6]. It consists of a single item addressing the functional and clinical status of the patient on a five-point scale, based on the clinical judgment of the health professional. Although widely used, its properties have barely been assessed [7]. It captures PD progression and change due to treatment, and there is evidence that a change from one stage to another is clinically relevant. Its simplicity and ease of use have made it a common measure in clinical trials, mainly for selection of patients into relatively homogeneous disability groups [7].
The CISI-PD is a global severity/disability index formed by four items – motor signs, disability, cognitive status and motor complications – scored from 0 (not at all) to 6 (very severe or very disabled). The components of the CISI-PD explained 92% of the Clinical Global Impression of Severity variability [8]. It is able to discriminate between PD severity levels, and shows good psychometric properties [9]. As an approach to the minimal detectable change or MID, the standard error of measurement (SEM) resulted in a range between 3.8 and 5.4 [9]. However, there is still a lack of studies on the responsiveness and interpretability of the CISI-PD.
Multidomain scales
The UPDRS assesses motor impairment and complications, through 42 items grouped in four subscales: I, Mentation, Behavior and Mood (four items); II, Activities of Daily Living (ADLs) (13 items); III, Motor (14 items, 27 scores); and IV, Complications of Therapy (11 items) [10]. It also includes a modified HY and the SES. The UPDRS properties have been tested in several studies, applied in most studies on interventions with medication, surgery and other therapies, and it is the primary outcome measure for establishing the effectiveness of PD therapies for the European and US regulatory agencies [11]. The responsiveness of the UPDRS is well established, and its scores are considered for clinical decision making regarding therapy. In consequence, some responsiveness indexes of the UPDRS have been calculated, such as the minimally detectable change (MDC), which was 13 for total score [12]; the minimal clinically relevant incremental difference (MCRID), which was established in a range from 4 to 10 points for UPDRS motor section [13]; and the minimal clinically important change (MCIC) of the total score, which was –3.5 [14].
The MDS-UPDRS was designed to retain the UPDRS strengths and overcome some of its weaknesses [11, 15]. The MDS-UPDRS maintains the UPDRS sections, but they have been renamed: Part I: Non-Motor Experiences of Daily Living, with six rater-based items and seven for self-assessment; Part II: Motor Experiences of Daily Living, with 13 patient-rated items; Part III: Motor Examination, 18 items (33 scores); and Part IV: Motor Complications, with six items. Validation and translation programs have been carefully designed to ensure the instrument’s psychometric quality and its applicability across the spectrum of PD and in different cultural settings [16, 17]. The MDS-UPDRS has been replacing the UPDRS as reference scale for clinical trials and practice. It has been shown to be responsive to changes over time and to treatment, including surgery [18, 19], and minimal clinically relevant difference (MCRD) and minimal clinically important difference (MCID) have been calculated for Part III [20, 21]. However, to date no new treatment has been approved using the MDS-UPDRS as the primary outcome measure, and more research is needed to test its responsiveness. Some formulas have been elaborated to transform the scores from the UPDRS [22, 23] and SPES/SCOPA (SCOPA-Motor) [24] to corresponding MDS-UPDRS scores and vice versa.
Motor impairment and disability scales
The SCOPA-Motor assesses motor signs (10 items), impairments in activities of daily living (seven items) and motor complications (four items) [25]. Items are scored in a 4-point scale ranging from 0 (normal) to 3 (severe). The scale is short, easily administered, reliable and valid for PD patients at all stages of disease [25, 26]. The motor section scores can be converted to MDS-UPDRS scores and vice versa [24]. The SCOPA-Motor showed better precision than UPDRS, with SEMs ranging from 0.40 (dyskinesias) to 2.46 (motor examination) [26], but there have been no studies on the scale’s responsiveness and interpretability.
The SES has been the reference scale for assessing PD-related disability as it was included in the UPDRS, although it was not designed specifically for this disease [10]. It consists of a one-item scale, scored from 100% (completely independent) to 0% (vegetative functions, bedridden) [27]. Although it has several shortcomings such as the lack of standardization in its administration, its psychometric properties are satisfactory as a whole. The SES has proved to be sensitive to changes [28, 29].
Motor complications and dyskinesia scales
The AIMS is an extensively used scale for screening and rating the severity of dyskinetic complications in PD, although it was originally developed for psychiatric patients [30]. The AIMS counts with ten items scoring the presence and severity of abnormal movements in several parts of the body from 0 (absent) to 4 (severe). The scale includes specific instructions for the rater, its psychometric properties are adequate and it fulfills the criteria for a recommended scale by the MDS [31]. The AIMS has been widely used to assess the effects of interventions, although there is no firm evidence about its ability to detect changes in dyskinesias across the different stages of PD and it is not superior to other dyskinesia scales in responsiveness [31, 32].
The RDRS is a PD specifically designed scale that assesses the interference due to dyskinesias in three standardized motor tasks: walking, drinking from a cup and dressing [33]. It meets the criteria for a recommended scale by the MDS, as it has been applied extensively in PD patients in clinical trials and practice and has undergone some clinimetric testing [31]. However, its ability to detect response to treatment is limited [34].
The UDysRS is the most recent scale specifically designed to assess dyskinesias in PD patients, but it has been already used in numerous clinical trials and counts with excellent clinimetric properties and a teaching program and detailed instructions for standardized administration [35]. It has two primary sections: Historical, with Part 1 (On-Dyskinesia) and Part 2 (Off-Dystonia), whose items are answered by interview and self-report; and Objective, with Part 3 (Impairment) and Part 4 (Disability), answered by the clinician based on observation of the patient performing four tasks. Among the existing dyskinesia scales, the UDysRS is superior for detecting treatment effects and sets a clear standard for comparison for new dyskinesia agents, with an effect size of around 0.14, although more research is needed to confirm its properties [32]. It has been classified as “recommended” by the MDS [36].
Nonmotor symptoms scales
The NMSS is the first instrument developed to assess a wide range of nonmotor symptoms (NMSs) in PD [37]. It is composed of nine domains and 30 items: cardiovascular (two items); sleep/fatigue (four items); mood/apathy (six items); perceptual problems/hallucinations (three items); attention/memory (three items); gastrointestinal tract (three items); urinary (three items); sexual function (two items); and miscellaneous (four items). The score for each item is the result of the multiplication of severity (from 0 to 3) and frequency scores (from 1 to 4), with higher scores meaning more severity and frequency (burden) of NMSs. The NMSS psychometric properties are satisfactory, and it has been translated into and validated in several languages and used in several clinical trials [38]. Regarding responsiveness, the SEM has been deemed acceptable (13.91 for total score and 1.71–4.73 for domains), with effect size values confirming its ability to detect changes [38–40].
The SCOPA-Cog is a ten-item scale for assessing cognitive deficits in PD through tasks related to visual and verbal memory, delayed recall, executive and visuospatial functions, and attention [41]. The maximum score, indicative of good cognitive status, is 43. Its psychometric properties have been tested using both CTT and IRT approaches [42, 43] and it has been translated into several languages. The SEM and smallest real difference for SCOPA-Cog have been estimated, and its sensitivity to change has been tested in a cognitive training program, but information on the SCOPA-Cog responsiveness is still limited [42, 44].
The SCOPA-PC is a scale aimed at assessing psychotic and compulsive disorders in PD, developed from the modified Parkinson Psychosis Rating Scale (mPPRS) [45, 46]. It is composed of seven items, answered in a scale from 0 (absent) to 3 (severe), with a total score of 21 indicative of greater psychiatric complications. The SCOPA-PC has adequate psychometric properties, but its responsiveness needs additional testing.
Patient-reported outcome measures
In recent years, there have been significant efforts toward the development, validation and use of PRO measures in PD. As explained above, PROs measure a wide array of areas: symptoms, functional status, psychological well-being, quality of life, satisfaction with care, and others. Some of them (quality of life, psychological well-being and some NMSs such as sleeping disorders and pain) have been recognized as the most relevant for PD patients and therefore must be taken into account in clinical research and practice. The PRO measures used in PD can be classified into two main groups: generic and specific scales. For years, generic instruments have been used in PD, sometimes without a formal validation in this population. Only in the last decade have researchers assessed the validity of these measures or developed specific PROs for PD evaluation. The most-used PD-specific PRO measures, which are the scope of this section, are addressed to assess perceived motor manifestations, disability, NMSs and health-related quality of life (HRQoL) measures.
Motor manifestations and disability scales
Several scales can be considered in this section: the Motor Aspects of Experiences of Daily Living (M-EDL) section of the MDS-UPDRS, the Wearing-Off Questionnaires (WOQ), the Freezing of Gait Questionnaire (FOGQ), and the Parkinson Disease Dyskinesia Scale (PDYS-26).
The M-EDL section of the MDS-UPDRS is composed of 13 items, each ranging from 0 (normal) to 4 (severe), and assesses difficulties related to speech, saliva and drooling, chewing and swallowing, eating tasks, dressing, hygiene, handwriting, hobbies and other activities, turning in bed, tremor, getting out of bed, car seat or deep chair, walking and balance, and freezing of gait [16]. The M-EDL section is a valid, reliable and useful self-reported measure for PD-related disability, with better sensitivity than other disability instruments [47]. Its longitudinal metric properties have been tested, with good sensitivity to change that surpasses the responsiveness of the UPDRS-II [19].
The WOQ is a screening tool to identify patients with wearing off with several versions composed of 32 (WOQ-32), 19 (WOQ-19, Patient Card Questionnaire or Quick Questionnaire), ten (Q10) and nine (WOQ-9) items [48–52]. The WOQ-32 is a suggested screening tool for wearing off, while the WOQ-19 and WOQ-9 are recommended by the MDS [53]. These two latter questionnaires have been used in several studies as screening instruments, and the WOQ-9 was sensitive to dopaminergic treatment, but the information about their responsiveness is still scarce.
The FOGQ is a scale composed of six items, scored from 0 (normal) to 4 (inability to walk), for assessing the difficulties in walking, freezing-related circumstances and duration of blocks while walking [54]. There have been several validation studies and translation into several languages, and a revised version is available [55]. Both versions have been used as outcome measures in trials, but their sensitivity to change needs more research.
The PDYS-26 is a scale aimed at quantifying the impact of dyskinesias on daily living activities, designed following the Rasch analysis methodology and tested using both IRT and CTT approaches [56]. Items are scored from 0 (not at all) to 4 (activity impossible). Its clinimetric properties are excellent, the scale is easy to use, with clear and concise instructions to patients, and it deserves the status of a recommended scale by the MDS [36]. However, PDYS-26 responsiveness is not satisfactory [34].
NMS scales
The development and use of NMS scales have undergone considerable growth as a result of the increased awareness of the NMS consequences in PD patients. Nonmotor symptoms include neuropsychiatric and cognitive, sleep-related, autonomic, gastrointestinal, fatigue, sexual and sensory disturbances, contributing to disability and impacting on a patient’s social role, daily activities and HRQoL. In the last decade, several initiatives such as those carried out by the SCOPA Group, Non-Motor Symptoms Group and MDS-UPDRS Task Force have developed specific NMS measures for application in clinical practice and research.
Comprehensive questionnaires
The NMS Questionnaire (NMSQuest) is the first comprehensive, screening tool of NMSs in PD [57]. It is composed of 30 items, grouped into ten domains, with yes/no score options. The NMSQuest is useful for highlighting symptoms needing attention in clinical practice and for epidemiological studies [58], but its usefulness as an evaluative instrument has not been tested.
The Non-Motor Aspects of Experiences of Daily Living (nM-EDL) section of the MDS-UPDRS contains six rater-based items (cognitive impairment, hallucinations, anxiety, depression, apathy and dopamine dysregulation syndrome) and seven self-assessed items (sleep problems, daytime sleepiness, pain, urinary problems, constipation, light-headedness and fatigue) [16]. The nM-EDL is highly correlated with the NMSS [59] and is able to detect improvements in NMSs following deep-brain stimulation, particularly those that are self-assessed (constipation, light-headedness and fatigue), and changes over time [19, 60].
Neuropsychiatric symptom scales
The MDS Task Force has published reviews about the scales for neuropsychiatric disorders, including depression, anxiety, anhedonia, apathy and psychotic manifestations, used in studies on PD populations (available at http://www.movementdisorders.org/MDS/Resources/Publications-Reviews/Task-Force-Papers.htm).
Two other PD-specific instruments have appeared recently: the Questionnaire for Impulsive–Compulsive Disorders in Parkinson’s Disease – Rating Scale (QUIP-RS) [61] and the Scale for Evaluation of Neuropsychiatric Disorders in Parkinson’s Disease (SEND-PD) [62].
The QUIP has been available since 2012 [63], but as a screening instrument, it is not applicable for evaluative purposes. The QUIP-RS is aimed at assessing the severity of impulse control behaviors (compulsive gambling, buying, eating and sexual behavior) and related disorders (medication use, punding and hobbyism) for purposes of diagnosis and monitoring over time [61]. It consists of four core questions about thoughts, urges/desires, and difficulty controlling and engaging in behaviors for each of the seven ICDs, with responses scored in a Likert-type scale from 0 (never) to 4 (very often). It is a promising scale, due to its good psychometric properties, and it has been incorporated into ongoing clinical trials of symptomatic therapies. There is still limited information about its sensitivity to change.
The SEND-PD is an interview-based scale for assessment of neuropsychiatric symptoms that consists of 12 items, each one scoring for severity from 0 (no) to 4 (very severe) [62]. Items are grouped into three subscales: psychotic symptoms, mood/apathy, and impulse control disorders (ICDs). The SEND-PD showed good psychometric properties and potential responsiveness, with a precision (SEM) ranging from 0.71 (ICD subscales) to 1.43 (mood/apathy). Additional analyses are needed to confirm these results and confirm the validation data of the scale.
Sleep disorder scales
Two main PD-specific rating scales for sleep disorders are available: the Parkinson’s Disease Sleep Scale (PDSS) [64] and SCOPA-Sleep [65].
The PDSS contains 14 items measuring nocturnal sleep problems and one item addressed to excessive daytime sleepiness. Each item is rated on a visual analog scale from 0 (severe or always present) to 10 (never or not present). A revised version has recently been published, the PDSS-2, focused only on nocturnal sleep problems and with items scored on a Likert-type scale from 0 (never) to 4 (very frequent) [66]. The PDSS has good clinimetric properties and meets the criteria for a recommended scale for screening of sleep disorders [67]. It is also a precise and responsive scale. Regarding the responsiveness of the PDSS-2, there are contradictory results [68, 69].
The SCOPA-Sleep is composed by two parts: nocturnal sleep (NS), with five items, and daytime sleepiness (DS), with six items. Both parts are scored on a scale from 0 (not at all) to 3 (very much) [65]. The SCOPA-Sleep has been validated in several cultural settings with good results and has been listed as recommended by the MDS [67]. The SEM was calculated for both subscales, resulting in 1.4 for the NS and 1.5 for the DS subscale, and it seems to be responsive to treatment, although there is no further information about its responsiveness [70, 71].
Gastrointestinal and autonomic scales
Only two specific scales exist for gastrointestinal dysautonomic symptoms in PD: the Sialorrhea Clinical Scale for PD (SCS-PD) [72] and the Swallowing Disturbance Questionnaire (SDQ) [73]. However, neither meets the criteria for a recommended scale by the MDS and there is no information about their responsiveness [74].
The only specific scale for global assessment of dysautonomia in PD is the SCOPA-Autonomic (SCOPA-AUT) [75]. It is composed of 25 items assessing six dimensions: gastrointestinal, urinary, cardiovascular, thermoregulatory, pupillomotor and sexual dysfunction. Items are scored from 0 (never) to 3 (often), with a maximum possible score of 69. There are several validation studies of the SCOPA-AUT, including CTT and Rasch analysis approaches [76, 77], and the MDS has designed it as a recommended scale, although with some limitations [74]. The SEM has been determined and there is differential item functioning by sex in item 2 (sialorrhea) and by age in item 13 (nocturia) [76, 77]. The scale has been used in several clinical studies in parkinsonian patients but has not been tested for responsiveness.
Fatigue
The Parkinson Fatigue Scale (PFS-16) is the first PD-specific measure of fatigue [78], although other generic instruments, such as the Fatigue Severity Scale (FSS) and the Multidimensional Fatigue Inventory (MFI), have been validated in PD patients with good results [79]. The PFS-16 contains 16 items focused on physical fatigue and its impact on daily living activities. The MDS has classified it as a recommended instrument for fatigue screening and severity ratings [79]. The PFS-16 has demonstrated satisfactory responsiveness related to the effects of treatment [80].
HRQoL scales
The use of HRQoL measures, which rate the subjective perception of the impact of the disease and the treatment, is growing incessantly in clinical trials. Some generic HRQoL scales have been successfully tested and used in PD and therefore they are recommended by the MDS [112]: the Nottingham Health Profile (NHP), the EQ-5D, the 36-Item Short-Form Health Survey (SF-36) and the Sickness Impact Profile (SIP).
However, PD-specific instruments are more adequate and sensitive for the assessment of treatment effects. Several have been developed and validated in recent years and the following five are recommended by the MDS Task Force: Parkinson’s Disease Questionnaire, 39- and eight-item versions (PDQ-39 and PDQ-8), Parkinson’s Disease Quality of Life Questionnaire (PDQL), Parkinson’s Impact Scale (PIMS) and SCOPA-Psychosocial Questionnaire (SCOPA-PS) [81].
The SIP contains 136 items grouped into 12 categories and two dimensions (physical and psychosocial) [82]. The SIP covers areas of interest for PD patients and has good psychometric properties in this population. Responsiveness for PD treatment has been reported, although there is no information about its MID.
The NHP is a scale that contains 38 items addressed to measure emotional reactions, energy, pain, physical mobility, sleep and social isolation [83]. It has population normative values and is sensitive to changes due to PD treatment.
The EQ-5D is a short and useful scale widely used in clinical and economic evaluation and for comparing different health conditions and populations [84]. It provides a descriptive profile and a single index value for health status and is responsive to medical and surgical interventions in PD.
The SF-36 is a scale comprising 36 items grouped into eight domains: physical function, physical role functioning, bodily pain, general health, vitality, social role functioning, emotional role functioning and mental health [85]. Although it has good psychometric properties in PD, when used to measure change due to treatment it was less sensitive than specific instruments.
The PDQ-39 is the most-used HRQoL instrument in PD clinical trials, in research and in practice [81, 86, 87]. It is formed by 39 items grouped into eight domains: mobility, activities of daily living, emotional well-being, stigma, social support, cognitions, communication and bodily discomfort. Items are scored from 0 (never) to 4 (always) and a transformed total score ranging from 0 to 100 and a summary index can be obtained. The PDQ-39 has shown satisfactory metric properties and is sensitive to changes due to medical or surgical interventions [88, 89]. The ES, the MID and other interpretability parameters have been also calculated, varying across dimensions and between studies [90, 91].
The PDQ-8 is a shorter version of the PDQ-39, with one item from each of the PDQ-39 domains [92]. It has been widely used and tested in different PD settings, with good psychometric properties. It is sensitive to changes in health status and has several responsiveness indexes that can be calculated.
The PDQL has 37 items classified into four domains: parkinsonian symptoms, systemic symptoms, social function and emotional function [93]. Items are scored on a Likert-type scale from 1 (all the time) to 5 (never), and a summary index can be extracted from the sum of items. Data about its psychometric properties and responsiveness are satisfactory, revealing that PDQL scores showed significant changes following treatment [29, 88].
The PIMS contains ten items addressed to rate physical, psychological and social aspects of the disease and sexuality [94]. Although it has good psychometric properties and is moderately sensitive to changes, the scale has scarcely been used beyond its original developers.
The SCOPA-PS is a questionnaire developed to quantify the psychosocial impact of the disease, and is composed of 11 items [95]. It has good psychometric properties and has been tested in different cultural settings. The SCOPA-PS is sensitive to changes, and several responsiveness parameters have been calculated [96].
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