The Low Serum Vitamin D Levels as a Risk Factor for Cognitive Impairment in Parkinson’s Disease: A Literature Review Ngakan Gde Aditya Permadi1*, A.A.A Meidiary2, A.A.A Putri Laksmidewi2 Abstract Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized not only by motor impairments but also by non-motor symptoms such as