Differential Diagnoses

PDP is a clinical diagnosis; no laboratory test or imaging method can definitively diagnose PDP or its symptoms.1 Diagnostic criteria, described by the National Institute of Neurological Disorders and Stroke and National Institute of Mental Health work group, have helped categorize the clinical symptoms of PDP (Table 1).11

PDP was originally thought to be an adverse effect of chronic use of antiparkinson drugs. While these agents are still posited to exacerbate PDP, the condition is now considered to be a complication of the underlying disease process separate from the effects of the patient’s drug regimen.9 However, given that hallucinations and confusion are listed as potential adverse effects for most treatment options for PD (Table 2),12 thorough communication among all providers involved in a patient’s care is needed to accurately diagnose and treat PDP.

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A patient cannot be diagnosed with PDP if the patient has an infection, is intoxicated, or is experiencing delirium or substance-induced psychosis.11 Thus, these conditions must be ruled out or treated before proceeding with the diagnosis of PDP.

Other neurodegenerative disorders also must be excluded in the workup for PDP, including dementia with Lewy bodies (DLB), progressive supranuclear palsy, and corticobasal ganglionic degeneration.11 The chronologic course of disease progression distinguishes these disorders from PDP. For example, DLB is one of the primary imitators of PDP when it occurs with dementia, with the main difference being the temporal course of the disease.11 DLB is defined as dementia occurring before the onset of motor symptoms, whereas PDP with dementia is defined as motor symptoms occurring before the onset of dementia.11 However, discerning a timeline for neurodegenerative disorders can prove difficult due to their subjective and often obscure origins.

Comorbid psychotic disorders, including schizophrenia, schizoaffective disorder, brief psychotic disorder, delusional disorder, and mood disorders with psychotic features, also must be considered in the workup of PDP.11 Primary psychotic disorders generally can be distinguished from PDP by their clinical features, including early psychosis (before or within 3 months of PD diagnosis), prominent nonvisual hallucinations, prominent fearful and paranoid delusions, and lack of diurnal fluctuation of symptoms.11 Additional clinical symptoms of PDP used to delineate it from other psychotic disorders include a time frame of >1 month, older age at symptom onset, and lack of affective symptoms.11 If these alternate disorders and external causes are ruled out and psychotic symptoms persist, then a diagnosis of PDP should be considered.