Visual hallucinations in Parkinson's disease: Clues to separate origins

Our paper discusses two experimental studies suggesting that Visual Hallucinations (VH) in Parkinson's Disease (PD) may have separate origins. The first is a prospective 8 years study evaluating the appearance of VH, visual abnormalities assessed by Visual Evoked Potentials (VEPs) and REM sleep Behaviour Disorder (RBD), in 80 PD patients treated with l-Dopa and Dopaminoagonists (DA). In chronically treated, cognitively unimpaired, PD patients VH were statistically related (p = 0.001) to RBD occurrence and high DA doses. Visual abnormalities were significantly reduced by l-Dopa or DA intake, and were statistically unrelated to VH. The second study involved PD patients placed in a Virtual Reality Environment, to decontextualize visual input. When motor symptoms worsened and VEP abnormalities developed patients consistently described hallucinatory dysperceptions of the virtual environment. The two studies therefore show that VH can occur in two seemingly distinct conditions, one is related to chronic treatment and to a sleep disorder frequently observed in PD, the other is probably related to a hypodopaminergic state. Our studies support a recently proposed integrative model of VH, and show that the neural circuits purported to explain VH must include the retinal dopaminergic system and the REM sleep regulatory system.