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Parkinsonism in GNDs432DiscussionIn this first systematic literature review on parkinsonism in patients with GNDs, we provide a comprehensive overview of phenotypic characteristics and proposed pathophysiology in 69 different GNDs, based on a total of 422 patients. The main messages and implications of this review can be found in Figure 3.Parkinsonism has been reported in a growing number of GNDs. This number may be expected to further increase given advances in clinical genetic testing and an increase in life expectancy due to improved medical care for patients with GNDs. Parkinsonism in these populations however appears to be underrecognized in clinical practice and to be an understudied research topic.93-95 An explanation may be that, on the one hand, patients with GNDs may not always be aware of motor symptoms and may not be capable to express their symptoms adequately. Professionals on the other hand may attribute motor symptoms to manifestations of the GND, antipsychotic medication-induced parkinsonism or co-existing neurologic disorders, while in fact they may miss a treatable condition of parkinsonism.82Phenotypic characteristicsMany patients showed the typical phenotypic characteristics found for patients with Parkinson’s disease: presence of cardinal motor signs, a good response to antiparkinsonian medication and reduced DAT binding with dopaminergic imaging. However, there were some notable findings. For example, almost half of the patients included in our review were female, atypical for idiopathic Parkinson’s disease sex distribution,96 and the median age at motor onset was 26 years, much younger compared to age-related parkinsonism in the general population.97, 98 Also, many patients had coexisting neurologic disorders, of which dystonia, seizures, and ataxia were most common.99, 100 While previous research has suggested that patients with neurodevelopmental disorders may be susceptible to drug-induced parkinsonism,93, 101 drug-induced parkinsonism as the main cause was reported in only two patients.