## 5 Discussion
According to a recent meta-analysis, the thickness of the combined ganglion cell layer and inner plexiform layer was significantly reduced among patients with PD when compared with healthy controls, whereas similar differences regarding the thickness of INL, OPL, and ONL were not detected [69]. Furthermore, thinning of the macula was reported in a setting of PD [59; 65; 70; 71; 72]. It is of great interest that the observed macular thinning is coupled with both disease severity and progression, as opposed to RNFL thickness reduction, where similar correlations are controversial [73]. However, a linkage between PD and retinal structure changes, including RNFL thinning, could not be elicited by others [9; 71; 74; 75; 76]. Researchers have highlighted the fact that patients with PD are accompanied by less profound retinal changes when compared with patients with PSP and a distinct retinal damage pattern from that observed among patients with MSA [47; 55], potentially facilitating differential diagnoses between PD and atypical Parkinsonian syndromes. In an attempt to efficiently differentiate between PD and Parkinson-plus syndromes by OCT implementation, OCT measures were compared within patients' groups. The most profound alterations were reported in cases of PSP and MSA, while the pattern of retinal involvement in PD remained rather heterogenous [9; 19]. Notably, the ONL to OPL ratio was found to be of great discriminatory value between PSP and MSA due to an increased OPL thickness in PSP and an increased ONL thickness in MSA, whereas patients with PD were not accompanied by significant changes when compared with healthy controls [19]. The aforementioned findings seem to be in accordance with brain MRI morphometry studies, where specific patterns of atrophy are observed in patients with atypical Parkinsonism with more pronounced retinal alterations than PD cases that are reflected in MRI morphometry, with rather minor changes [77]. Combining the ONL to OPL ratio with INL measures may serve as an accurate indicator of PSP versus PD and other Parkinsonian syndromes' patients (MSA, CBS), with a negative test almost certainly ruling out PSP [9]. With respect to the average pRNFL thickness and CMT, it was demonstrated that both measures were significantly thinner among patients with PSP and MSA than those with PD. Interestingly, MSA has been coupled with a distinct pattern of retinal thinning, capable of playing a crucial role in distinguishing between atypical Parkinsonism [26]. Reflecting the discriminatory value of specific OCT measures, pRNFL thinning was greater within a group of PSP patients than those with PD [21; 22]. On the contrary, pRNFL thickness was reported as significantly thinner in PD and MSA compared with PSP, while macular thickness and volume thinning were more marked in MSA than in PD. Thus, the diagnostic potential of pRNFL thickness in differentiating between PSP and PD, and between PSP and MSA, was pointed out, highlighting the fact that macular thickness and volume evaluation may enhance the discrimination between MSA and PD [20]. Additionally, the mean RNFL was not found to be significantly different between groups of PSP and PD patients in a study conducted by [PERSON] [9]. As far as the association between OCT parameters and duration of the disease is concerned, [PERSON] et al. could not elicit a link between retinal measures and both disease duration and severity, suggesting that observed retinal alterations occur early in the course of the disease, thus implying a potential role of OCT in the early differential diagnosis of Parkinsonism [9]. However, [PERSON] et al. supported the progression of retinal changes that are attributed to the progression of the disease itself, providing favorable relative evidence [78]. Similarly, [PERSON] et al. and [PERSON] et al. demonstrated a significant correlation between retinal measures and disease duration among patients with PD and PSP, as well as PD, PSP, and MSA [20; 22].