Nonmotor Signs in Genetic Forms of Parkinson's Disease.

Although only a minority (i.e., ~5%) of Parkinson's disease (PD) cases is due to well-defined genetic causes, important clues about the common, "idiopathic" PD (iPD) can be garnered from monogenic model diseases. Nonmotor signs (NMS) are also present in monogenic PD and reviewed in this chapter for the confirmed PD genes SNCA, LRRK2, VPS35, Parkin, PINK1, DJ-1, and the risk factor gene GBA. Within the context of the MDSGene database (www.mdsgene.org), we performed a systematic literature search and extracted information on cognitive decline, depression, psychotic signs and symptoms, autonomic signs and symptoms, anxiety, sleep disorder, and olfactory impairment. Notably, relatively few studies specifically addressed NMS in genetic PD and missing data ranged from 42% to 100%. Diagnostic criteria and examination methods were variable and cases differed widely for age at onset, disease duration, ethnicity, treatment, and comorbidity. Although in comparison to IPD, SNCA duplication carriers have the most similar course of disease, even for duplication carriers the frequencies of dementia, hallucinations, and depression seem higher than in IPD. Supporting the notion that LRRK2-linked PD has a similar course to iPD but is slightly more benign, the frequency of dementia is below that of iPD. For Parkin, the frequency of cognitive decline falls within the range of the general population above the age of 65 years. GBA mutations are associated with a distinct profile of cognitive impairment and a greater prevalence of depression. Despite the current data gaps, NMS should be considered as an important and often treatable concomitant feature of genetic parkinsonism.