Independent patterns of damage within magno-, parvo- and koniocellular pathways in Parkinson’s disease
Annals of Neurology
Silva M.F., Faria P., Regateiro F.S., Forjaz V., Januario C., Freire A., Castelo-Branco M. (2005) Independent patterns of damage within magno-, parvo- and koniocellular pathways in Parkinson’s disease Annals of Neurology, Brain. Oct;128(Pt 10):2260-71
This was a very detailed article discussing the literature for visual disturbances in patients with PD as well as a study looking at different patterns of visual dysfunction. They began by discussing that the measurement of contrast sensitivity in patients with PD needs to be precise and some studies have used imprecise measures, which contributes to the variability amongst studies in the literature. They described the various methods to measure contrast sensitivity and the pros and cons of each. They noted that visual tests also need to be as free as possible from other cognitive skills (e.g. visuospatial), that may be mediated by other areas in the brain that may have dopamine receptors or depletion. They added that studies should also take age of one’s brain, retina, and other ocular structures into consideration when doing this type of research. These authors attempted to look at visual performance in the fovea (A part of the eye responsible for sharp central vision) and the periphery (A part of vision that occurs outside the very center of gaze). Doing this, also allowed them to look at multiple visual layers associated with the visual system including; magnocellular layers (associated with achromatic vision [not color] and luminence), parvocellular layers (associated with chromatic [color] vision, specifically the colors red-green) and koniocellular (associated with chromatic vision, specifically the colors yellow-blue) in order to specify areas affected by PD. They noted that in this type of research it is important to exclude common age-related visual difficulties, such as glaucoma, cataracts, maculopathy, and diabetic retinopathy as all of the above can affect the multiple visual pathways. This study was conducted in Portugal and looked at thirty patients (16 female, 14 male, mean duration 4.6 years, age 61.1, Hoehn and Yahr 1.9, UPDRS motor 25.0) and 32 controls (19 female, 13 males, mean age 57.9). There were no educational differences between groups. Notably 10 of the PD patients were newly diagnosed and were not receiving medication at the time of visual testing. Other patients receiving medication for PD were tested in the ON state. They found that patients with PD had deficits in all layers of the visual pathway (magno-, parvo-, and koniocellular) using different methods, suggesting pathways were affected independently not as a result of each other. Duration and age were not found to fully explain these findings, as those recently diagnosed also showed chromatic difficulties. The authors also found predominantly more differences in parvocellular (red-green) pathways between groups. They suggested that prior research that suggested more difficulties with koniocellular (yellow-blue) pathways may have been related to age. They also found that magnocellular pathway dysfunction was related to Hoehn and Yahr stage, suggesting deterioration with stage, as well as age. Additionally they found, as seen in previous literature, that there was reduced dopamine supply around the fovea that resulted in poor contrast sensitivity in patients with PD. The authors also cited a series of studies that discussed what causes the reduction in dopamine at the retina, including ganglion cell (a type of neuron located in the retina of the eye that receives visual information) changes and thinning of the retinal nerve fiber. They suggested that future studies evaluate the role of medications, disease duration, Hoehn and Yahr stage, and specific tests to measure the different visual pathways in patients with PD.