All patients with diagnosis of PD, consecutively admitted to the Neurology Outpatient Clinic of the Sarah Hospital in Brasilia DF, Brazil, from August 2006 to January 2007, were considered candidates for the study. Inclusion criteria were diagnosis of PD, as per the United Kingdom Parkinson's Disease Society Brain Bank Criteria11 and age ≥40 years at disease onset. Exclusion criteria were as follows: (1) PD patients with disability due to other neurological disorders; (2) concomitant severe illness, acute disorder or injury (e.g., advanced neoplasia, pneumonia, hip fracture), pharmacological effect (e.g., dopamine antagonists), sensorial deficit (e.g., blindness) or sequela status (e.g., hemiplegia) that could interfere with or significantly modify the evaluation of the effects caused by PD; (3) inability of any kind to read or understand Portuguese (e.g., illiteracy, severe dementia); and (4) refusal to participate in the study.
PD patients answered a demographic questionnaire that covered age, gender, marital status, and education, as well as historical information about PD (age at onset, duration of disease, and drug treatment).
Neurologist-based assessments included Hoehn and Yahr staging (HY),12 Scales for Outcomes in Parkinson's disease-Motor scale (SCOPA-MS),13 Clinical Impression of Severity Index-Parkinson's disease (CISI-PD),14 and Parkinson Psychosis Rating Scale (PPRS).15 Cognition was assessed using the Brazilian version16 of Folstein's Mini-Mental State Examination (MMSE),17 the Short Portable Mental Status Questionnaire (SPMSQ),18 and the SCOPA-COG.9 Patients also completed the Hospital Anxiety and Depression Scale (HADS).19
The assessments were conducted by neurologists with competence in movement disorders, during scheduled visits by patients to the clinic. In addition, the SCOPA-COG, CISI-PD, and PPRS were readministered to 52 patients who returned to the clinic ∼10 days after the first evaluation, in order to obtain data for a test–retest reliability analysis.
The HY is usually applied to establish the severity of PD,20 inasmuch as HY 1 is the mildest stage with only unilateral symptoms, and HY 5 is the most severe stage (wheelchair-bound or bedridden).
The SCOPA-MS13 contains three sections, namely, motor impairment (10 items), activities of daily living (ADL) (7 items), and motor complications (4 items). All items score in a range from 0 (normal) to 3 (severe); the higher the score, the greater the severity.
The neurologist-based CISI-PD score14 represents a subjective judgment of global PD severity, and reflects and summarizes the clinical evaluation. The CISI-PD contains the following 4 items: motor signs, disability, motor complications, and cognitive impairment. Overall evaluations of these four domains are then combined to provide the CISI-PD global score, which ranges from 0 (normal) to 24 (severe).
The Parkinson Psychosis Rating Scale15 was designed to assess the severity of psychosis in PD patients. It is administered by the clinician, and consists of 6 items scored in each case from 1 (no symptoms) to 4 (extreme symptoms). The total score ranges from 6 to 24 points.
The MMSE17 measures certain areas of cognitive functioning, including memory, orientation, language, and the ability to follow command. The cutoff point for cognitive impairment was 24/23.
The SPMSQ18 is a useful screening test for moderate-to-severe dementia in both community and hospital settings.21 Levels of severity of intellectual impairment are assigned to the number of errors as follows: mild (3–4 errors), moderate (5–7 errors), and severe (8–10 errors). It allows for scores to be adjusted to subjects' educational level.
The HADS is a self-administered screening tool for mood disorders, which consists of 14 items. Individual item scores can either be summed to calculate a total score, or summed per subscale to produce separate anxiety (HADS-A) and depression (HADS-D) scores.22, 23
The SCOPA-COG9 consists of 10 items with a maximum total score of 43, with higher scores reflecting better performance. The SCOPA-COG has the following four domains: Memory, Attention, Executive functions, and Visuospatial functions. Memory domain items assess both visual and verbal memory, as well as immediate and delayed recall. In the Executive Function domain, items address semantic fluency, set shifting, and motor planning. Two tasks are included in the Attention domain (counting down by threes; and months backward), and one in the area of Visuospatial Function (figure assembly task).
The following psychometric attributes were explored for the SCOPA-COG: acceptability, scaling assumptions, reliability, precision, and construct validity. To establish acceptability, observed versus possible score ranges, mean score distance to the median, floor and ceiling effects (acceptable, <15%),24 and skewness (limits: −1 to +1)25 were determined.
Scaling assumptions refer to the correct grouping of items and the appropriateness of their summed score. These were checked using item-total correlation, corrected for overlapping. Values of 0.40 or higher were deemed appropriate.26
Cronbach's alpha27 was used to assess internal consistency, with this being considered acceptable where the alpha value was 0.70 or higher.28 Test–retest reliability was ascertained using weighted kappa (with quadratic weights) for individual items and the intraclass correlation coefficient (ICC) (model 2, single rating) for the total score. Kappa values >0.40 for items and an ICC ≥ 0.70 for total measure scores were taken as the criterion of acceptable stability.26, 29
To determine the scale's precision and potential responsiveness, the standard error of measurement (SEM) and smallest real difference (SRD) were calculated. The SEM, defined as “the standard error in an observed score that obscures the true score” [SEM = SD × √(1 − reliability coefficient)],31 reflects a measure's reliability and precision.28, 30, 31 The SRD indicates 95% confidence of real difference between the true scores of two successive measurements, capturing “the essence of the reproducibility of a measurement instrument” [SRD = 1.96 × √2 × SEM].32 The less a scale's reliability, the greater its SEM and SRD, and the lower its precision and responsiveness.
Convergent validity represents the extent to which a measure is related to other variables and measures for the same construct, and was explored by means of Spearman rank correlation coefficient (rS). It was a priori hypothesized that coefficient values would be high (rS ≥ 0.60) as between the SCOPA-COG and the other applied cognitive instruments (MMSE and SPMSQ), moderate (rS = 0.30–0.59) as between education, PD duration, and PD rating scales, and low (rS < 0.30) as between age and measures of noncognitive mental aspects (psychosis, depression).26, 33 To ascertain the internal validity of the Brazilian SCOPA-COG, inter-domain correlations were determined. Coefficient values of 0.30 to 0.70 were deemed satisfactory.34
Discriminant validity refers to a measure's ability to discriminate between groups at a given point in time. The Mann-Whitney test was used to compare SCOPA-COG scores between patients with an MMSE cutoff point of 23/24. The Kruskal-Wallis test was used to compare SCOPA-COG scores broken down by HY stage and by SPMSQ and CISI-PD Item 4 levels.
To identify predictors of the SCOPA-COG score, a multiple regression model was constructed, with age, years of education, and duration of PD as independent variables. Interaction, collinearity, or transgression of the mathematical assumptions impeded the introduction of other variables. The assumptions made by the model (linear relationship, normality, and homoscedasticity) were upheld.