This was a prospective case-control and hospital-based evaluation of PD patients and age- and sex-matched healthy controls. To control for and minimize the effects of medication involvement and disease severity, we recruited early-stage, medication-naïve PD patients. Specifically, de novo PD patients who had visited the Movement Disorder Clinic at Hallym University Dongtan Sacred Heart Hospital were consecutively recruited. Diagnosis of PD was made by a movement-disorders specialist on the basis of the clinical diagnostic criteria of the UK PD Brain Bank Society.12 For every participant with PD, information on age, sex, age at PD onset, and Unified Parkinson's Disease Rating Scale part III (UPDRS-III) was collected at the time of diagnosis. The controls were voluntarily recruited from either healthy volunteers who had visited for routine health checkups or patients from a General Ophthalmology Clinic of Hallym University Sacred Heart Hospital.

The exclusion criteria were the presence of comorbid ophthalmic pathologies capable of affecting retinal thickness, such as glaucoma, age-related macular degeneration, diabetic retinopathy, retinal vein or artery occlusion, epiretinal membrane, macular hole, high myopia (defined as spherical equivalent > −6.0 diopters or glaucomatous optic neuropathies), presence of media opacity impeding acquisition of high-quality OCT images, and inability to undergo OCT examination. Eyes that had undergone ocular surgery other than cataract extraction were excluded as well.

All of the participants underwent complete ophthalmologic examinations, including best-corrected visual acuity, slit-lamp examination, intraocular pressure measurement by non-contact tonometry, refractive error with an autorefractor (KR-890; Topcon Corporation, Tokyo, Japan), central corneal thickness measurement (Pocket II Pachymeter Echo graph; Quantel Medical, Clermont-Ferrand, France), axial length measurement (Axis II PR; Quantel Medical, Inc., Bozeman, MT, USA), dilated fundus examination, and a central 24-2 threshold test of the Humphrey Visual Field (HVF, HFA II; Humphrey Instruments Inc., Dublin, CA, USA). All also underwent retinal and optic nerve head imaging by spectral-domain optical coherence tomography (SD-OCT, Spectralis; Heidelberg Engineering, Heidelberg, Germany).

High-resolution retinal imaging was performed by SD-OCT. The automatic eye-tracking technology maintains fixation on the retina. Only well-centered images with a signal strength of greater than 20 dB were used for the analysis. The peripapillary RNFL thickness (pRNFLT) was measured around the optic disc with 16 averaged consecutive circular B-scans (diameter of 3.5 mm, 768 A-scans). The pRNFLT (from the inner margin of the internal limiting membrane to the outer margin of the RNFL layer) was automatically segmented using Spectralis software version 5.3.3.0 (Fig. 1A). pRNFLT data were collected on the RNFL in each of the superior, inferior, temporal, and nasal sectors.

The mRT topographic maps were acquired via raster images consisting of 31 high-resolution B scans covering a 30° × 25° area centered on the fovea. The mRT was defined as the distance between the surface line of the NFL and the hyporeflective line above the retinal pigment epithelium, as measured via the automatic segmentation algorithms of the Spectralis software. Spectralis SD-OCT provides the results of macular thickness measurements in circles of 1, 3 and 6 mm. These are divided into 9 regions: a center circle of 500 mm radius and inner and outer rings, each divided into four quadrants according to the Early Treatment of Diabetic Retinopathy Study (Fig. 1B). mRT values are reported numerically for the central, superior inner, inferior inner, temporal inner, nasal inner, superior outer, inferior outer, temporal outer, and nasal outer regions. The mRT of each superior, inferior, nasal, and temporal area was applied to the analysis of the average value of the measured mRT at the circles of 3 and 6 mm, as follows: Superior 3 mm + Superior 6 mm = Mean superior mRT. The same method was used to calculate the mean inferior, temporal, and nasal mRT, respectively.

Each sector and average value of pRNFLT and mRT, in both eyes, were obtained. Then, each patient's interocular asymmetry in the four directional quadrants and the overall average were calculated. Interocular asymmetry was the absolute value of thickness difference defined as follows: (More affected side of eye − Less affected side of eye).1617 Additionally, in order to correct the baseline thickness in the thickness-difference calculation, the asymmetry index for retinal thickness (AIRT) was applied. The AIRT is defined as: (Retinal thickness differences between ‘more affected side’ and ‘less affected side’/Average retinal thickness of ‘more affected side’ and ‘less affected side’) × 100.18 For comparison of the interocular difference at distinct perifoveolar radial distances, the thickness difference between the more and less affected sides for each corresponding sector, of each subject, was calculated.

For all of the patients, a detailed clinical history was taken and a neurological examination was carried out by a neurologic specialist. The motor assessment was performed based on UPDRS-III. The scores for tremor, rigidity of upper and lower extremities, finger taps, hand movements, and leg agility were calculated separately for each side (7 items, each item scored from 0 to 4).

For the PD patients, the laterality of motor symptoms was defined as the right or left side according to which was more affected by parkinsonism symptoms. In detail, the sum of the UPDRS-III scores for each side, left and right, was analyzed, and the higher score was determined as the more affected side. If lateralized UPDRS-III scores were equal for both sides, the more affected side was determined using the historical data on the disease onset side. If the onset symptoms were axial, the first lateralized symptom was used.19

Further, motor-symptom laterality in the PD patients was evaluated according to the Symmetric index. The Symmetric index parameter is the more and less affected side asymmetry of the UPDRS-III result, as calculated according to the following formula: |More affected − Less affected side of UPDRS score|/(More affected + Less affected side of UPDRS score).2021 Then, the correlation between the Symmetric index of UPDRS-III and the interocular asymmetry of the OCT parameters was analyzed. The symmetry index ranged from 0 to 1, higher values indicating higher degrees of asymmetry. A cut-off value of 0.20 or greater was used to estimate the frequency of asymmetric parkinsonism.22 Motor-symptom laterality in the PD patients was evaluated according to the Symmetric index.

For comparison of the baseline demographics and clinical characteristics between the PD patients and the controls, the independent t-test and χ² test were used. During the evaluation of pRNFLT and mRT, the more affected and less affected side eyes were separately compared between PD and the controls, as was the interocular difference. Additionally, the relationships between the UPDRS-III (including Symmetric index) and the OCT parameters were studied using Spearman ranked correlation. A P value < 0.01 (0.05/5) was considered significant for statistical adjustments of multiple comparisons of the segments (superior, inferior, nasal, temporal, average) for correlation analysis with pRNFLT or mRT. Otherwise, a P value of < 0.05 was considered to be statistically significant. All of the statistical analyses were carried out using SPSS version 21.0 (SPSS, Chicago, IL, USA). All of the tests were 2-tailed.

The present study protocol was reviewed and approved by the Institutional Review Board of Hallym University Sacred Heart Hospital (approval No. 2019-05-024-002), and informed consent was obtained from the participate in the research study.

There were no differences between the PD patients and the healthy controls in the average or any of the sectoral pRNFLT in either eye. Additionally, interocular asymmetry (More affected side – Less affected side) of pRNFLT did not differ between the two groups (all P > 0.05) (Table 2). Similarly, there was no difference in the average or sectoral mRT in either eye (all P > 0.05) (Table 3). However, the interocular asymmetry of average mRT and AIRT of temporal mRT showed significant differences between the two groups (P = 0.026 and 0.044).

For all of the PD patients, the motor functions of the upper and lower extremities on both sides were analyzed using UPDRS-III (Table 4). The sum of UPDRS-III showed a discrepancy between more-affected- and less-affected-side symptoms, the overall scores being 12.10 ± 4.36 and 8.71 ± 4.04, respectively (P = 0.002). In detail, the components for rigidity in upper extremity, finger taps, and rapid alternating hand movement showed significant asymmetry between the more affected and less affected sides (P = 0.001, 0.006 and 0.006). The calculated Symmetric index was 0.21 ± 0.19, which suggested asymmetric parkinsonism.

The results of the analysis of correlation between the interocular differences (More affected side − Less affected side) of the OCT thickness parameters and motor-symptom laterality are presented in Table 5. The calculated Symmetric index of UPDRS-III showed significant relations for interocular asymmetry of superior mRT and AIRT of average mRT (P = 0.001 and 0.008). By contrast, between asymmetry of pRNFLT and the Symmetric index, there was no association (P ≥ 0.01 for all) (Table 5).

Additionally, the correspondence of directionality between the laterality of motor symptoms and the OCT thickness parameters was analyzed (Table 6). There was no definite correspondence or reverse-correspondence of directionality between the affected body side and the same side of pRNFLT or mRT.