In this study, which aims to evaluate the relationship between IPD progression and RNFL, the average and superior quadrant RNFL thicknesses were significantly thinner at the 24th month in the IPD group compared to the control group.
OCT is a reliable and cost-effective technique used to evaluate RNFL thickness in the optic nerve head. It is often used in the diagnosis and follow-up of neurodegenerative diseases, such as multiple sclerosis, which leads to optic neuritis .
In IPD, another neurodegenerative disease that affects the elderly population, there may be difficulties in diagnosis by clinicians, especially due to the cardinal symptoms that do not appear in the early stage. Computed brain tomography or Magnetic Resonance Imaging images are insufficient to distinguish IPD from other neurodegenerative diseases and diagnosis is generally made by clinical examination. However, it is difficult to distinguish IPD with essential tremor and other types of tremor, which is similar to IPD, in cases where other cardinal symptoms of the disease are not dominant, and patients may receive incorrect treatments [9, 15].
Considering the related literature on OCT, which is seen as a potential candidate for IPD early diagnosis in recent years, the RNFL thickness examined with OCT was found to be reported as decreased in many studies.
Inzelberg et al. reported that RNFL thicknesses in the inferior quadrant in the IPD patients were thinner than in the control group . Moschos et al. reported that RNFL thicknesses in the inferior and temporal quadrants in the IPD patients were thinner than in the control group . Kırbaş et al. found thinning in RNFL values in average and temporal quadrants in newly diagnosed IPD patients . In a study conducted by Altıntaş et al. with 17 IPD patients and 11 controls, where RNFL and macular thickness were evaluated by OCT, macular thickness and RNFL in all quadrants and average RNFL values in the IPD group were found to be thinner than the control group . In a meta-analysis of 13 case-control studies conducted by Yu et al., eyes of 644 IPD patient and 604 healthy controls were examined by OCT, and average RNFL values and RNFL values in the temporal quadrant were found to be thinner in IPD patients compared to the control group .
However, in other studies, no significant difference was found between the IPD and control groups in terms of the RNFL thickness examined with OCT. In a study conducted in the U.S., IPD disease and the control group were assessed by OCT, and no significant difference was reported between average RNFL values, despite the decreased macular thickness in the IPD group . In a study conducted in the U.K., 51 IPD patients and 25 healthy controls were evaluated, and no difference was found between macular thickness and RNFL values measured with OCT, despite lower visual acuity and contrast sensitivity in the IPD group . Another study in Germany evaluated macular thickness and retinal layers by OCT in healthy controls and Parkinson's plus and IPD patients. And, no significant difference was found between IPD patients and the control group .
When we considered all these studies, we concluded that the different results might depend on the disease stage, the ages of the study participants, the OCT device used, and the fact that many of the studies were cross-sectional. In our study, average RNFL and superior quadrant RNFL thicknesses were found to be thinner in the 24th months in IPD patients compared to the controls. However, the inferior, nasal and temporal quadrants measured with OCT showed a thinning with the disease progression in certain periods, but there was no statistical significance.
It was reported that there was no significant relationship between UPDRS and H&Y scores and RNFL values in the studies that evaluated disease severity and RNFL thicknesses, similar to our study [6, 10, 19].
It has been reported that decreased dopamine levels in the retina in IPD patients may be the cause of retinal degeneration. However, in the study conducted by Şen et al., IPD patients who received and did not receive Levodopa therapy and healthy control groups were evaluated with OCT, and no significant difference was reported in the RNFL values in all groups . In our study, there was no significant relationship between RNFL values in the patients who received Levodopa and dopa agonist as monotherapy. Moreover, non-medicated IPD patients were not included in the study since the patients received treatment during the study.
In the study conducted by Satue et al., where IPD progression and retinal degeneration were evaluated in the similar patients, the IPD patient and control groups were re-evaluated 5 years after the first evaluation with OCT, and macular thickness and RNFL values in all quadrants in the IPD group were found to decrease significantly compared to the control group . However, in a study by Hasanov et al., early-stage IPD patient and healthy control groups were followed up with 6-month intervals at least 3 times by OCT in terms of their RNFL and macular thickness values. There was no significant reduction in the average RNFL and macular thickness values, but a thinning was present in RNFL value only in the superior quadrant in the patient group. In this study, we believe that the short duration of follow-up may be the reason why the results were unrelated .
In our study, however, superior quadrant and average RNFL thicknesses were significantly thinner in the 24th months, compared to the groups.
A small number of patients and the shorter observation period are among the limitations of our study. However, in other studies in the literature, the number of patients was not significantly higher than those included in our study. We also believe that our study may be valuable since there are not many studies investigating retinal abnormalities with disease progression.