We describe a patient affected by PD with a rapid progression of cognitive decline. This case could suggest the coexistence of many neurodegenerative diseases, which is a common condition in older patients. We propose an hypothetical trajectory of the cognitive impairment usually associated with motor symptoms in the later phase of Parkinsonian patients. The trajectory is almost linear in classical Parkinson's disease dementia (PDD), while a constant acceleration of the cognitive decline with a subsequent change of the slope of the direction could suggest the coexistence of PD with other neurodegenerative disease. Finally, if the cognitive decline in PD is comparable to a "stepped" decline, vascular lesions could be the cause of the change of the slope. This case could suggest to request an autopsy in all cases of unexplained PDD, for better understanding the mixture of non-motor symptoms in PD.