Interactions of visual hallucinations, rapid eye movement sleep behavior disorder and cognitive impairment in Parkinson's disease: A review

• Patients with Parkinson's disease (PD) may develop several non-motor symptoms.
• Rapid eye movement sleep behavior disorder (RBD) is commonly associated with PD.
• RBD may have associations with visual hallucinations (VH) and cognitive impairment (CI).
• Presence of either VH or CI may be a risk factor for development of the other.
• Interactions of VH, RBD and CI may have important clinical significance.

Patients with Parkinson's disease may develop various non-motor symptoms during the course of the illness. Visual hallucinations (VH) and cognitive impairment (CI) are two common non-motor symptoms of Parkinson's disease. Studies have reported association of both VH and CI with presence of rapid eye movement sleep behavior disorder (RBD). Presence of visual hallucinations and cognitive impairment has been described as risk factors for emergence of each other. There is marked overlap in the risk factors for development of RBD, VH and CI in patients with PD. Results of clinical and epidemiological studies as well as studies based on neuroimaging, electrophysiology especially transcranial magnetic stimulation and neuropsycholgical evaluations in PD patients have suggested presence of certain common neurobiological process leading to emergence of RBD, VH and CI. Structural neuroimaging studies using voxel-based morphometry have often reported grey matter atrophy of hippocampus and parahippocampal cortices in PD patients with RBD, VH and CI. Cholinergic dysfunction is common in PD patients with RBD, VH and CI. This review explores the complex interactions of RBD, VH and CI in patients with PD and their potential implications.