## 4 Literature Review Sections  
### Exploring Retinal Findings in Patients with Typical (PD) and Atypical Parkinsonism (PSP, MSA, CBS)  
In an attempt to provide insight into the linkage between retinal parameters evaluated through OCT implementation and the differential diagnosis of PD versus atypical Parkinsonian syndromes, [PERSON] et al. examined a population of 84 patients presented both with PD and Parkinson-plus syndromes (PSP, MSA, and CBS) and recorded measures of peripapillary retinal nerve fiber layer (RNFL), paramacular thickness and volume, as well as the thickness of the different retinal layers. The researchers observed that the mean total macular thickness and volume of patients with PSP alone were significantly reduced when compared with patients with PD, in contrast to the decreased macular thickness and volume of patients with MSA and CBS versus PD, which did not reach statistical significance. As far as the mean peripapillary RNFL between groups is concerned, it was not proven to be significantly different. Regarding measurements of different retinal layers, it was demonstrated that not only the retinal ganglion cell and inner plexiform layer (RGC + IPL), but also the outer nuclear layer (ONL), was reduced in patients with PSP compared with PD. It was of great interest that the inner nuclear layer (INL) was reported to be thicker  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Authors,** & **Number of** & **Type of** & **Assessed Retinal** & **Cutoff Values;** & \\
**Year of** & **Participants** & **DCT** & **Parameters** & **(Specificity);** & **Main Results** \\ \hline  & & & & & & A significant thinning of both pRNFL and GCL, & \\  & & & pRNFL thickness, & & & IPL, INL was reported among patients with PSP when compared with a group with PD. \\ [PERSON] et al., & 33 healthy controls & Spectral domain & volume of retinal & & & \\
2018 [22] & 39 patients & OCT with & layers at the & & & Regarding levodopa use, \\
2018 [22] & - 29 PD & automatic & macula (mRNFL, & & & a decreased ONL/OPL \\  & - 10 PSP & segmentation & GCL, IPL, INL, & & & ratio and pRNFL \\  & & & OPL, ONL, PRs, & & & thinning were observed \\  & & & & RPE) & & & among levodopa \\  & & & & & & non-users compared \\  & & & & & & with levodopa users \\ \hline  & & & & & & A thicker mean average \\ [PERSON] & 53 patients & & & thickness cut-off & & RNFL and a higher \\  & - 38 PD & Spectral domain & pRNFL, macular & value of 69 \(\mu\)m for differentiating PSP & & mean macular volume \\
[23] & - 15 PSP & OCT & thickness, GCA & & & differentiating PSP & was found among patients with PD than \\  & & - 15 PSP & & & from PD (91.7\%), & & those patients of group \\  & & & & & & with PSP \\ \hline  & & & & & & A greater reduction in \\  & & & & & & RNFL thickness was \\  & & & & & & observed in patients \\ [PERSON] et al., & 30 patients & & & & & \\ [PERSON] et al., & - 10 AD & OCT & RNFL thickness & N/A & & \\
2013 [24] & - 10 LBD & & & & & \\  & - 10 PD & & & & & \\  & & & & & & \\  & & & & & & original parameters \\ \hline  & & & & & & \\ [PERSON] et al., & 30 patients & & & & & \\ [PERSON] et al., & - 10 AD & OCT & RNFL thickness & N/A & & \\
2013 [24] &  
in individuals presenting with PD than those with MSA and PSP. Taking into account the reverse alterations of ONL and outer plexiform layer (OPL) within the group of patients diagnosed with PSP, the ratio between the aforementioned layers was calculated in order to enhance the differentiation accuracy between disease entities. Notably, the ONL/OPL ratio of patients with PSP combined with INL changes was found to be significantly different compared with those of patients with PD; thus, it seems able to distinguish between these groups of patients with both high sensitivity and specificity [9].