Discussion

Our findings of the EEG waves slowing in frontal lobe areas and the association of this phenomenon with the executive function decline in PD patients (revealed with neuropsychological assessment using TMT-A and TMT-B) is in line with the previous literature data describing the mechanisms of PD-specific frontal dysfunction.1,7,8Furthermore, lower values of putamen and caudate nuclei [18F]Fluorodopa PET/CT uptake ratios in our PD patients are consistent with the current state of knowledge and describe PD-related dopaminergic deficits.4–8,19,20 Thus, taking into account the concept of the “dual hypothesis”,8 which assumes the dopaminergic-mediated striatofrontal executive dysfunction in early-stage PD (and cholinergic-based cognitive impairment, especially in later demented PD patients), we hypothesized a significant correlation of the frontal lobe EEG waves slowing with the dopaminergic deficiency and the executive dysfunction measures in mild non-demented PD patients tested in the present study. Indeed, our results confirmed almost completely this hypothesis as for the negative correlation between the PSD-EEG and striatal [18F]Fluorodopa PET/CT uptake ratios. However, with regard to consideration on the associations of the EEG waves slowing in frontal lobe and striatal dopaminergic deficiency with neuropsychological outcomes describing the executive dysfunction (i.e. TMT and and ST), one should point out that in majority of our results these associations were confirmed only with the Trail Making Test but not with the Stroop Test. The TMT consists of two parts (TMT-A and TMT-B). In TMT-A part a subject connects (by drawing a line) numbers in sequential order, but in the TMT-B the subject alternates between numbers and letters (1, A, 2, B, etc.). The former is used to examine cognitive processing speed, while the latter assesses executive function.15,16 The ST also consists of two parts: ST-I, in which a subject reads a list of 40 words, which are names of colors printed in the color itself; and ST-II, where color names are printed in an incongruent color and the subject is required to ignore the word and correctly name the ink color. The former is used as a measure of processing speed and the latter to test selective attention and inhibition.17,18 Thus, both of these tests (TMT and ST) evaluate an executive function, but the TMT is more related to motor skills (due to line’s hand drawing task), and therefore a disordered performance of this test in PD patients might be partly related to pathological neural networks causing bradykinesia. According to the triadic subdivision of the parallel striatal projections in the dopaminergic system (mesostriatal, mesocortical and mesolimbic pathways) the mesostriatal component is responsible for the bradykinesia in the hypodopaminergic conditions 21 as in our PD patients. At the same time, it is important to note that the mesocortical system is responsible for executive function.21 It is necessary take into account that the mesostriatal dopaminergic circuit is most prominently affected, also on the early stage of PD, whereas the mesocortical and mesolimbic dopaminergic circuits are relatively preserved in the early stage of the disease.22,23Therefore in our mild PD patients (being on early stage of disease) the mesostriatal component of the triadic striatal projections in the dopaminergic system might be more affected by stated in our study dopaminergic deficiency. In consequence, this may be a reason of stated by us significant correlations of the PD-specific slowing of brain waves and dopaminergic deficiency with the Trail Making Test, with simultaneous lack of significant correlations with the Stroop Test.
Our diagnostic accuracy analyses showed that some of the PSD-EEG parameters might be potentially a good alternative for expensive PET/CT diagnostics in PD. Particularly, the PSD-EEG theta-alpha maximal peak and lower-alpha band for the SMA and PFC regions seemed to be promising PD-related executive dysfunction biomarkers, since they presented high diagnostic sensitivity and specificity, significantly differentiated mild PD patients from healthy controls, as well as they were correlated with dopaminergic deficiency and executive dysfunction parameters.