Biomarkers for the early detection of Parkinson's and Alzheimer's disease.

In the aging population of many countries in the world, neurodegenerative diseases like Parkinson's disease (PD) are becoming an increasing burden. Therefore, early therapy and ultimately disease prevention is essential, which is only possible with an early diagnosis. Besides a genetic predisposition, a number of biomarkers are being discussed to indicate vulnerability to PD, some of them many years before disease manifestation. These include hyperechogenicity of the substantia nigra as well as premotor symptoms like olfactory and autonomic dysfunction, depression, REM sleep behavior disorder, and neuropsychological impairment. Moreover, first signs of affection of the substantia nigra like PET and SPECT abnormalities and slight motor signs can be included, as they may be detected before a definite diagnosis according to motor symptoms can be made. Interestingly, other frequent neurodegenerative disorders like Alzheimer's disease (AD) are also characterized by a long preclinical period, with several biomarkers discussed as indicative for disease vulnerability including cerebrospinal fluid, serum, and neuroimaging biomarkers, olfactory dysfunction as well as subtle neuropsychological deficits. However, future studies are necessary, which establish the predictive value of these markers singularly and in combination to detect a subgroup of the population at risk for PD and AD not only to accelerate research on etiology and pathophysiology but also to promote testing for neuroprotective strategies.