As OCT technology has improved, it has become clearer that retinal thinning in PD is restricted to the dopamine-containing layers, the GCL and IPL, rather than the RNFL.12 Nuclei of dopaminergic amacrine interneurons lie adjacent to the IPL,24 with axons running horizontally across the IPL and GCL,28 and postmortem studies show that retinal alpha-synuclein accumulates at the interface with the IPL in PD.11 Our finding of a link between retinal thinning and dementia risk in these specific layers has important mechanistic implications for progression of PD dementia. This evidence concerns the gene SNCA and Parkinson disease.