AI Summary
The text discusses the loss of dopaminergic neurons in Parkinson's disease and different studies that have examined changes in the retina of patients with Parkinson's disease. The results of these studies have been inconsistent and the implications for clinical practice are unclear.
Parkinson disease (PD) neuropathology is characterized by a loss of nigrostriatal dopaminergic neurons. Dopaminergic cells are also located in the retina, specifically in the macular ganglion cell inner plexiform layer (mGCIPL) complex, where they mediate intercellular coupling.1 Both retinal dopaminergic cell populations and synaptic contacts were found to be reduced.1 Postmortem analyses revealed a reduced dopamine content in retinae of patients with PD.2 In vivo, studies using optical coherence tomography (OCT) have reported changes in retinal layer thickness in patients with PD. These changes primarily consider the mGCIPL and retinal nerve fiber layer (mRNFL).3 However, results remained inconsistent between studies. Some studies showed a thicker inner nuclear layer (INL) in patients with PD4-6 while the mRNFL, GCIPL, and outer plexiform layer were thinner.5 Others detected no differences between patients with PD and healthy controls.7 A reduced GCIPL thickness was reported to correlate with a higher Movement Disorder Society—Unified Parkinson’s Disease Rating Scale III (MDS-UPDRS III) score.8 Another study reported an altered shape of the foveal pit, which correlated with a reduced contrast sensitivity.9 However, all these studies were cross-sectional with small patient numbers, and therefore, the implications for clinical practice and patients’ care remained unclear.