Abstract
Cognitive problems are a major factor determining quality of life of patients with Parkinson's disease. These include deficits in inhibitory control, ranging from subclinical alterations in decision-making to severe impulse control disorders. Based on preclinical studies, we proposed that Parkinson's disease does not cause a unified disorder of inhibitory control, but rather a set of impulsivity factors with distinct psychological profiles, anatomy and pharmacology. We assessed a broad set of measures of the cognitive, behavioural and temperamental/trait aspects of impulsivity. Sixty adults, including 30 idiopathic Parkinson's disease patients (Hoehn and Yahr stage I–III) and 30 healthy controls, completed a neuropsychological battery, objective behavioural measures and self-report questionnaires. Univariate analyses of variance confirmed group differences in nine out of eleven metrics. We then used factor analysis (principal components method) to identify the structure of impulsivity in Parkinson's disease. Four principal factors were identified, consistent with four different mechanisms of impulsivity, explaining 60% of variance. The factors were related to (1) tests of response conflict, interference and self assessment of impulsive behaviours on the Barrett Impulsivity Scale, (2) tests of motor inhibitory control, and the self-report behavioural approach system, (3) time estimation and delay aversion, and (4) reflection in hypothetical scenarios including temporal discounting. The different test profiles of these four factors were consistent with human and comparative studies of the pharmacology and functional anatomy of impulsivity. Relationships between each factor and clinical and demographic features were examined by regression against factor loadings. Levodopa dose equivalent was associated only with factors (2) and (3). The results confirm that impulsivity is common in Parkinson's disease, even in the absence of impulse control disorders, and that it is not a unitary phenomenon. A better understanding of the structure of impulsivity in Parkinson's disease will support more evidence-based and effective strategies to treat impulsivity.
Highlights
Impulsivity is common in many developmental, psychiatric and neurological disorders, including Parkinson’s disease
For the eleven impulsivity tests included in the factor analysis, table 4 summaries the individual test differences between patients and controls
This study has shown that impulsivity in Parkinson’s disease is not a unitary phenomenon but multifactorial
Summary
Impulsivity is common in many developmental, psychiatric and neurological disorders, including Parkinson’s disease. Impulse control disorders affect up to one in seven people with Parkinson’s disease and are potentially worsened by common dopaminergic therapies [2,3]. Their importance has been emphasised by the operationalization of impulse control disorders in DSM-IV and the development of screening criteria for impulse control disorders in Parkinson’s disease. Such clinical diagnoses complement a translational cognitive neuroscience approach to impulsivity, identifying the mechanisms of behavioural control and inhibition of actions, the regulation of behavioural strategies and processing of risk or reward [4,5,6,7,8]. Neuropsychological studies of lesions and neuroimaging studies have identified critical anatomical substrates for such tests of impulse control [4,9,10,11], emphasising especially the inferior frontal gyri, medial frontal cortex and anterior cingulate cortex, as well as regions of the striatum
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