Performance of a shortened Scale for Assessment of Positive Symptoms for Parkinson's disease psychosis.

BACKGROUND

Parkinson's disease psychosis is a frequent and serious complication of advanced disease, but few disease-specific outcome measures exist.

METHODS

Using baseline scores from 4 clinical trials, we identified relevant items that assessed Parkinson's disease psychosis to create a shortened version of the Scale for Assessment of Positive Symptoms. We then analyzed the validity and treatment sensitivity of the shortened scale. Principal component analyses evaluated the underlying structure. Scores were compared across age, gender, trial, cognition, and country of origin. Sensitivity to change was assessed by comparing change in psychosis scores to the clinical global impression of improvement score, and effect sizes were calculated to evaluate treatment response.

RESULTS

Nine items were selected based on face-validity and symptom frequency. Principal component analysis yielded a 4-factor structure and identified delusions and visual, auditory, and somatic hallucinations as distinct constructs. Baseline total scores were similar across study, gender, region, and age group. The clinically meaningful change in the shortened scale, defined as a 1-unit change in clinical global impression, was 2.33 points, and the effect size was -0.722. The change in scores did not significantly differ between those with cognitive impairment and those without.

CONCLUSIONS

The shortened Scale for Assessment of Positive Symptoms for Parkinson's disease retains the reliability, sensitivity to change, and effect size of the larger scale while reducing administration time and, more importantly, score variability. The scale is an effective outcome measure for use in clinical trials.