Biomarker sources for Parkinson's disease: Time to shed tears?

Parkinson's disease (PD) is the most frequent neurodegenerative movement disorder. In nearby future its importance will increase, especially due to changes in age structure. There are still substantial problems in terms of early diagnosis of the disease. In clinical practice, the diagnosis of PD is still based on the evaluation of clinical presentation and is supported by various ancillary tests with heterogeneous predictive value. In regard to diagnosis and disease monitoring, body fluid-based biomarkers have become an indispensable tool in today's medicine. In this Current Opinion we briefly address the present approaches to biomarker development for PD, including clinical trials on blood-based, cerebrospinal fluid-based and other body fluid-based biomarkers. We then review the knowledge on tear fluid-based biomarkers in general and discuss its usability as a source for biomarkers in PD patients. The close spatial relation of the lacrimal glands to the cranial nerves may predispose tear fluid to mirror pathophysiological changes in the central nervous system and argues for its validation as a biomarker in clinical studies. Based on the requirements for clinically useful biomarkers, we propose that tear fluid may serve as an easily accessible source for biomarkers in patients with PD and other neurodegenerative movement disorders.