Advances in the role of neuroimaging to monitor disease progression in Parkinson's disease

Abstract

Since structural imaging has generally failed to demonstrate consistent abnormalities in Parkinson's disease (PD), from an imaging perspective, the diagnosis has typically been based upon the demonstration of impaired striatal dopamine (DA) function. Radiotracer imaging techniques such as positron emission tomography (PET) and single photon emission computerised tomography (SPECT) allow the in vivo assessment of nigrostriatal DA function as well as regional cerebral blood flow, glucose metabolism, and functional connectivity, and changes in these measures have been used to infer disease progression. Pre-synaptic radiotracer imaging (RTI) has shown that striatal dopaminergic hypofunction follows a negative exponential pattern with the fastest rate of decline in early disease. Moreover, while striatal subregions remain differentially affected throughout the course of disease, with the posterior putamen affected more than anterior structures, the rate of deterioration is similar in all subregions. However, although functional imaging is undoubtedly a very useful tool in the assessment of PD progression, various studies have shown discordance between clinical progression of PD and nigrostriatal degeneration estimated by PET or SPECT. Therefore, considerable caution is warranted in the interpretation of imaging findings. While a potentially invaluable complement in assessing the severity of dopaminergic dysfunction, functional imaging is not a substitute for clinical assessment and other objective measures of PD severity, and cannot be currently considered a biomarker for progression of PD.