Retinal Thickness in Patients with Parkinson’s Disease and Dopa Responsive Dystonia—Is There Any Difference?

<b>Background/Objectives</b>: Certain aspects of retinal thickness assessed by optical coherence tomography (OCT) in patients with Parkinson's disease (PD) require additional clarification. It is supposed that attributing reduced retinal thickness in PD to dopaminergic loss may not be acceptable as it also happens in diseases where dopaminergic loss does not occur. The objective of our study is to compare the ganglion cell/inner plexiform layer (GCIPL), peripapillary retinal nerve fiber layer (pRNFL), and macular thickness of PD and dopa responsive dystonia (DRD) patients with healthy controls (HC), to investigate whether DRD patients, as a distinctive model of genetically induced dopamine deficiency, have reduced retinal thickness in comparison with PD, and to analyze correlation between retinal thickness and various PD clinical parameters. <b>Methods</b>: We analyzed 86 patients with PD, 10 patients with DRD, and 96 age- and sex-matched HC. <b>Results</b>: GCIPL, pRNFL, and central macula thickness (CMT) are statistically significantly thinner in PD patients compared to HC (<i>p</i> < 0.001, all). GCIPL and CMT are also statistically significantly thinner in DRD patients compared to HC (<i>p</i> = 0.012, <i>p</i> = 0.001, respectively). GCIPL thickness correlates positively with the daily dose of levodopa (r = 0.244, <i>p</i> < 0.01). The thickness of GCIPL and pRNFL correlate negatively with current age (r = -0.219; <i>p</i> < 0.01 and r = -0.358; <i>p</i> < 0.05, respectively). All retinal parameters are statistically significantly thinner in females than in males (<i>p</i> < 0.05). <b>Conclusions</b>: Patients with PD and DRD did not differ in GCIPL and pRNFL thickness when compared to one another. These results, supported by positive correlation of levodopa dose and GCIPL thickness in PD patients, emphasize the importance of dopamine in maintaining retinal thickness.