most common, with tactile and olfactory hallucinations occurring less commonly (18). Nonvisual hallucinations mainly occur in people who already suffer from visual ones (14,18,26,27).
Delusions, or beliefs that do not have a foundation in reality, occur less frequently in PD. The phenomenology of delusions in PD overlaps with those seen in other dementias (28) and usually carries a paranoid theme. However, most types of delusions occur with relatively equal frequency (18). Thought broadcasting, ideas of reference, loosened associations, and "negative" symptoms are generally uncommon in PD.
There is no gold standard rating scale for the severity of psychosis in PD. Many different scales have been used to assess psychosis in PD studies, including the Neu-ropsychiatric Inventory (NPI) (29), Brief Psychiatric Rating Scale (BPRS) (30), Behave-AD (31), Positive and Negative Symptom Scale (PANSS) (32), Scale for the Assessment of Positive Symptoms (33), Clinical Global Impression Scale (34), and the Parkinson Psychosis Rating Scale (PPRS) (35). Many of these scales were developed for use in Alzheimer's disease (AD) or schizophrenia. As psychosis in PD manifests generally as "positive" symptoms such as hallucinations and delusions (18), and "negative" symptoms such as conceptual disorganization are not present, scales such as the BPRS and the PANSS assess many nonrelevant items. Only one scale, the PPRS, has been validated in the PD population. However, some items on the PPRS, such as sexual preoccupation, may not be that prominent in PD psychosis. There is only one item each on hallucinations, delusions, and illusions, which may not fully explore and track the phenomenology of PD psychosis. It has also not been tested in a longitudinal fashion, and its ability to track changes due to treatment has not been studied. A scale to adequately assess psychosis in PD is warranted.
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