With great interest we read the recent article by Garcia-Martin and associates. In this article, the authors suggested that in Parkinson disease (PD) “a reduction of retinal ganglion cells may lead to corresponding decrease in retinal and retinal nerve fiber layer thickness.” The underlying mechanism is interpreted as, at least in part, the result of retrograde trans-synaptic axonal degeneration. These findings are potentially important, but we have some concerns about the validity and the interpretation of the data.
In Table 1, the authors reported peripapillary retinal nerve fiber layer thicknesses (in μm) for PD patients (98.41 μm [range 45–150]) and healthy controls (100.78 μm [range 61–158]), which correspond well with previously reported data. Subsequently, automated retinal layer segmentation of the macular volume scans was performed, of which the results are shown in Table 2. The dimensionless values given in Table 2 caught our attention as they are not within the ranges reported in other papers using similar optical coherence tomography devices and segmentation algorithms. While the authors reported a mean thickness of the inner nuclear layer (INL) of 7.39 (SD 1.94) in PD and 7.14 (SD 1.90) in healthy controls, others have consistently reported values in the range of 35–50 μm in multiple sclerosis patients, PD patients, and healthy controls. Moreover, Figure 2 shows the gradually increasing thickness for the layers, which seems to correspond to the anatomic position of the layer (from innermost to outermost layers, see Figure 2) rather than the thickness of the actual layers. This appearing linear increase is, however, highly unlikely when one inspects Figure 1, where the retinal anatomy of PD patients and controls is depicted without this phenomenon of increasing layer thickness. Figure 3 raised further concerns as retinal layers were incorrectly labeled. The figure shows segmentation lines for the ganglion cell layer, the inner plexiform layer, and the INL which were erroneously labeled as retinal nerve fiber layer, ganglion cell layer, and inner plexiform layer.
We are worried that these discrepancies prohibit a valid interpretation of the data and lead to confusion in the literature on retinal morphology in PD patients.